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WO2025134007A1 - Process for the purification of graphite - Google Patents

Process for the purification of graphite Download PDF

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Publication number
WO2025134007A1
WO2025134007A1 PCT/IB2024/062946 IB2024062946W WO2025134007A1 WO 2025134007 A1 WO2025134007 A1 WO 2025134007A1 IB 2024062946 W IB2024062946 W IB 2024062946W WO 2025134007 A1 WO2025134007 A1 WO 2025134007A1
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WO
WIPO (PCT)
Prior art keywords
leach
impure
caustic
graphite
hsa
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
PCT/IB2024/062946
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French (fr)
Inventor
Karanveer Singh ANEJA
Anna Moisala MOTTA
Duncan William Turner
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Talga Technologies Ltd
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Talga Technologies Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from AU2023904209A external-priority patent/AU2023904209A0/en
Application filed by Talga Technologies Ltd filed Critical Talga Technologies Ltd
Publication of WO2025134007A1 publication Critical patent/WO2025134007A1/en
Pending legal-status Critical Current
Anticipated expiration legal-status Critical

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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/20—Graphite
    • C01B32/21—After-treatment
    • C01B32/215—Purification; Recovery or purification of graphite formed in iron making, e.g. kish graphite

Definitions

  • the present invention relates to a process for the production of purified high surface area (HSA) graphite materials.
  • the process of the present invention produces purified HSA graphite materials from impure graphite concentrates.
  • the present invention further relates to the use of the purified HSA graphite material in the production of anode and cathode materials.
  • the purified HSA graphite material is used as an additive for anode and cathode materials typically having a purity requirement of >99.5%w/w LOI depending on the application.
  • Graphite has a layered crystal structure consisting of stacks of parallel two- dimensional graphene sheets. Each graphene sheet has a two-dimensional hexagonal lattice of carbon atoms covalently bound. The graphene layers are held together by van der Waals-forces.
  • HSA graphite materials contain a mixture of single layer graphene particles and multi-layer graphite particles, having only a few layers of graphene. HSA graphite materials possess unique mechanical, thermal, optical and electrical properties. These materials have the potential for use in a wide range of applications, including battery components. [0004] Many different methods to prepare HSA graphite materials from graphite have been proposed.
  • the graphite starting material is treated with an expandable intercalation agent, followed by thermally expanding the intercalant to exfoliate flakes of HSA graphite materials.
  • the resulting material is then typically treated in a mechanical grinding process for further flake separation and size reduction.
  • the graphite starting material is subjected to a fine mechanical grinding process to produce HSA graphite materials.
  • the graphite starting material has a high graphite purity, typically >99% Cg.
  • Impure graphite sources such as graphite ores or graphite waste materials, require chemical purification to remove impurities prior to the production of HSA graphite materials.
  • HSA graphite materials Even with a high starting purity, the processing steps used to prepare the HSA graphite materials have been found to result in the introduction of impurities into the produced HSA graphite material. For example, residues from the grinding media may be introduced or the exfoliation process may introduce other compounds or reaction products. The HSA graphite material must then be subjected to further purification to remove these impurities.
  • the preparation of HSA graphite materials from impure graphite sources therefore requires an initial purification process before processing and a subsequent purification process to remove any additional impurities introduced by the processing.
  • the purified HSA graphite material production process and the purified HSA graphite material product of the present invention have as one object thereof to overcome substantially one or more of the abovementioned problems associated with prior art processes, or to at least provide a useful alternative thereto.
  • the preceding discussion of the background art is intended to facilitate an understanding of the present invention only. This discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
  • a range from about 1 micrometer ( ⁇ m) to about 2 ⁇ m should be interpreted to include not only the explicitly recited limits of from between from about 1 ⁇ m to about 2 ⁇ m, but also to include individual values, such as about 1.2 ⁇ m, about 1.5 ⁇ m, about 1.8 ⁇ m, etc., and sub-ranges, such as from about 1.1 ⁇ m to about 1.9 ⁇ m, from about 1.25 ⁇ m to about 1.75 ⁇ m, etc.
  • “about” and/or “substantially” are/is utilised to describe a value, they are meant to encompass minor variations (up to +/- 10%) from the stated value.
  • references to the % recovery or % removal of an element or mineral, or similar are, unless the context demands otherwise, a reference to the % of that component recovered or removed relative to the original content of the feed to the described process.
  • Other references to % are, again as long as the context does not require otherwise, to be taken as references to weight or wt % (rather than volume or v %).
  • a process for the production of a purified high surface area (HSA) graphite material comprising: treating an impure graphite concentrate in a mechanical exfoliation step to produce an impure high surface area (HSA) material; and treating the impure HSA material in a purification process to produce a purified HSA graphite material.
  • the impure graphite concentrate is recovered from a natural graphitic ore.
  • the impure graphite concentrate is recovered from an impure synthetic graphite material.
  • the impure graphite concentrate is recovered from an impure recycled graphite material.
  • the impure recycled graphite material is a waste battery material or a scrap anode material.
  • the impure graphite concentrate is subjected to one or more pre-treatment steps prior to the mechanical exfoliation step.
  • the one or more pre-treatment steps may be selected from size reduction, size classification, physical separation, chemical separation, drying, heat treatment or combinations thereof.
  • Suitable physical separation steps include gravity separation, flotation, and magnetic separation.
  • Suitable chemical separation steps include froth flotation and chemical leaching.
  • the impure graphite concentrate has a carbon concentration of less than 96%.
  • the impure graphite concentrate has a carbon concentration of between 60 and 96%.
  • the impure graphite concentrate has a carbon concentration of between 75 and 96%.
  • Impurities encountered in concentrate prepared from natural graphite sources may include silicate gangue, base-metal sulphide and oxide minerals, and various carbonate mineral phases. The type and quantity of these depends on the original graphite ore mineralogy and host rock geology.
  • the mechanical exfoliation step is carried out in a grinding apparatus.
  • the grinding apparatus imparts a shearing force on the impure graphite concentrate.
  • the mechanical exfoliation step is a dry mechanical exfoliation step.
  • a dry mechanical exfoliation step refers to a mechanical exfoliation step carried out in the absence of a liquid medium. Suitable dry mechanical exfoliation processes include jet milling.
  • the mechanical exfoliation step is conducted in the presence of a liquid medium.
  • the mechanical exfoliation step comprises: pulping of the impure graphite concentrate with a liquid medium to prepare an impure graphite slurry; and subjecting the impure graphite slurry to a mechanical exfoliation step.
  • the specific milling energy of the mechanical exfoliation step is greater than 50 kWh/t.
  • the mechanical exfoliation step will produce a slurry containing an impure HSA material dispersed in the liquid medium.
  • the slurry is subjected to a dewatering step to remove at least a portion of the liquid medium.
  • the impure HSA material is treated in a drying step.
  • the drying step is an air drying step.
  • the drying step is a cryogenic drying step.
  • the impure HSA material is treated in a beneficiation step prior to being treated in the purification process. Suitable beneficiation processes include size classification, liquid-liquid separation, gravity separation, flotation, froth flotation and magnetic separation.
  • the graphitic particles of the impure HSA material have a surface area of greater than 10 m 2 /g.
  • the graphitic particles of the impure HSA material have a surface area of greater than 20 m 2 /g. In one form of the present invention, the graphitic particles of the impure HSA material have a surface area in the range of 20 to 40 m 2 /g. In one form of the present invention, the graphitic particles of the impure HSA material have a surface area in the range of 25 to 35 m 2 /g. In one form of the present invention, the graphitic particles of the impure HSA material have a surface area of greater than 40 m 2 /g. In one form of the present invention, the graphitic particles of the impure HSA material have a surface area in the range of 40 to 80 m 2 /g.
  • the graphitic particles of the impure HSA material have a surface area in the range of 40 to 50 m 2 /g.
  • the particle size distribution of the impure HSA material is such that the D50 is lower than 30 ⁇ m. In one form of the present invention, the particle size distribution of the impure HSA material is such that the D50 is lower than 20 ⁇ m. In one form of the present invention, the particle size distribution of the impure HSA material is such that the D50 is between 10 and 20 ⁇ m. In one form of the present invention, the particle size distribution of the impure HSA material is such that the D50 is between 10 and 15 ⁇ m.
  • the purification process produces a purified HSA graphite material with a carbon concentration of at least 99% cg.
  • the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.9% cg.
  • the purification process comprises one or more impurity removal steps selected from: acid leach steps, caustic leach steps, oxidative leach steps, caustic bake steps or combinations thereof.
  • the purification process comprises one or more caustic impurity removal steps, followed by one or more acid leach steps.
  • the purification process comprises: treating the impure HSA material in a caustic impurity removal step, and recovering a solid caustic leach product; and treating the solid caustic leach product in an acid leach step, and recovering a solid acid leach product, wherein the solid acid leach product comprises a purified HSA graphite material.
  • the solid acid leach product is subjected to a washing step to recover the purified HSA graphite material.
  • the washing step will remove soluble impurities from the purified HSA graphite material.
  • the caustic impurity removal step comprises a caustic leach step.
  • the caustic leach step comprises the contact of the impure HSA material with a sodium hydroxide solution, and recovering a solid caustic leach product.
  • the caustic impurity removal step comprises a caustic bake step.
  • the caustic bake step comprises baking the impure HSA material under alkaline conditions, subjecting the product to a water leach, and recovering a solid caustic leach product.
  • the caustic impurity removal step comprises a caustic bake step followed by a caustic leach step.
  • the acid leach step comprises one or more separate acid leach stages.
  • each acid leach stage comprises the contact of the solid caustic leach product with an acidic solution, thereby forming an acidic leach slurry and recovering a solid acid leach product from the acidic leach slurry. It is envisaged that different acidic solutions may be used at different acid leach stages to target different impurities remaining in the solid caustic leach product.
  • the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution.
  • the acid leach step comprises two or more acid leach stages, where at least one of the acid leach stages comprises the contact of the solid caustic leach product with a hydrofluoric acid solution.
  • the acid leach step preferably comprises: treating the solid caustic leach product in a first sulphuric acid leach stage comprising the contact of the solid caustic leach product with a sulphuric acid solution and separating undissolved leach solids; treating the leach solids of the first sulphuric acid leach stage in a hydrofluoric acid leach stage comprising the contact of the leach solids with a hydrofluoric acid solution and separating undissolved leach solids; treating the leach solids of the hydrofluoric acid leach stage in a second sulphuric acid leach stage comprising the contact of the leach solids with a sulphuric acid solution and separating undissolved leach solids; and treating the leach solids of the second sulphuric acid leach stage to one or more washing stages, and recovering a solid acid leach product comprising a
  • the caustic impurity removal step comprises at least one of a caustic bake step or a caustic leach step conducted at a temperature of at least 200°C.
  • the acid leach step does not comprise the contact of the solid caustic leach product with a hydrofluoric acid solution.
  • the acid leach step preferably only comprises the contact of the solid caustic leach product with an acidic solution selected from a hydrochloric acid solution and a sulphuric acid solution.
  • the acid leach step only comprises the contact of the solid caustic leach product with a hydrochloric acid solution.
  • the process preferably comprises treating the impure HSA material in an oxidative leach step. More preferably, the impure HSA material is treated in an oxidative leach step prior to the caustic leach step.
  • the oxidative leach step comprises a first oxidative leach stage comprising the contact of the impure HSA material with an oxidant solution to form a first oxidative leach slurry.
  • the oxidative leach step further comprises a second oxidative leach stage.
  • the second oxidative leach stage comprises the contact of the first oxidative leach slurry with a hydrochloric acid solution.
  • the leach slurry is subjected to a solid liquid separation step to recover an oxidative leach residue.
  • the oxidative leach residue is subjected to the caustic leach step as discussed above.
  • the solid caustic leach product recovered from the caustic leach step is subjected to a secondary caustic leach step, prior to the acid leach step.
  • the secondary caustic leach step is operated at same conditions as the caustic leach step.
  • the purification process comprises one or more acid leach steps.
  • the purification process comprises: treating the impure HSA material in an acid leach step, and recovering a solid acid leach product, wherein the solid acid leach product comprises a purified HSA graphite material.
  • the acid leach step comprises one or more separate acid leach stages.
  • each acid leach stage comprises the contact of the impure HSA material with an acidic solution, thereby forming an acidic leach slurry and recovering a solid acid leach product from the acidic leach slurry.
  • different acidic solutions may be used at different acid leach stages to target different impurities remaining in the impure HSA material.
  • the solid leach product from each acid leach stage is washed before advancing to downstream acid leach stages or further processing steps. It is understood that washing may minimise soluble impurity carry over and help reduce reagent consumption.
  • the acid leach step comprises the contact of the impure HSA material with a hydrofluoric acid solution.
  • the acid leach step comprises two or more acid leach stages, where at least one of the acid leach stages comprises the contact of the impure HSA material with a hydrofluoric acid solution.
  • the acid leach step preferably comprises: treating the impure HSA material in a first sulphuric acid leach stage comprising the contact of the impure HSA material with a sulphuric acid solution and separating undissolved leach solids; treating the leach solids of the first sulphuric acid leach stage in a hydrofluoric acid leach stage comprising the contact of the leach solids with a hydrofluoric acid solution and separating undissolved leach solids; treating the leach solids of the hydrofluoric acid leach stage in a second sulphuric acid leach stage comprising the contact of the leach solids with a sulphuric acid solution and separating undissolved leach
  • the undissolved leach solids from the first sulphuric acid leach stage are washed prior to advancing to the hydrofluoric acid leach stage.
  • the undissolved leach solids from the hydrofluoric acid leach stage are washed prior to advancing to the second sulphuric acid leach stage.
  • the purification process does not comprise any caustic impurity removal steps.
  • the process further comprises a drying step in which the purified HSA graphite material is dried, providing a dried purified HSA graphite material.
  • the dried purified HSA graphite material preferably contains between 0 to 2.5% moisture, for example, less than about 1% and further preferably 0.1% moisture. In one form of the invention, the purified HSA graphite material will preferably contain about 40% moisture prior to the drying step. [0052] Preferably, the dried purified HSA graphite material has a pH of 7 ⁇ 2.5. [0053] Preferably, the carbon as graphite content of the dried purified HSA graphite material is greater than 99.5% w/w. [0054] In one form of the present invention, the graphitic particles of the purified HSA graphite material have a surface area of greater than 20 m 2 /g.
  • the graphitic particles of the purified HSA graphite material have a surface area in the range of 20 to 40 m 2 /g. In one form of the present invention, the graphitic particles of the purified HSA graphite material have a surface area in the range of 25 to 35 m 2 /g. In one form of the present invention, the graphitic particles of the purified HSA graphite material have a surface area of greater than 40 m 2 /g. In one form of the present invention, the graphitic particles of the purified HSA graphite material have a surface area in the range of 40 to 80 m 2 /g.
  • the graphitic particles of the purified HSA graphite material have a surface area in the range of 40 to 50 m 2 /g.
  • the particle size distribution of the purified HSA graphite material is such that the D50 is lower than 30 ⁇ m. In one form of the present invention, the particle size distribution of the purified HSA graphite material is such that the D50 is lower than 20 ⁇ m.
  • the dried purified HSA graphite material is classified in or after the drying step. The dried purified HSA graphite material is preferably classified in the drying step into multiple products having different particle size and surface area properties, for example into at least two fractions.
  • the classification utilises a dry powder classification method, for example cyclone classification.
  • a purified HSA graphite material produced in accordance with the process described hereinabove.
  • the process further comprises the step of processing the purified HSA graphite material to prepare an electrode material.
  • the purified HSA graphite material is used as an additive in an electrode material.
  • the Applicant s International Patent Application PCT/IB2020/056050 (WO 2020/261194), the entire content of which is incorporated herein by reference, describes a silicon and graphite containing composite material comprising a plurality of silicon nanoparticles coated with graphite particles, few-layer graphene particles, graphite nanoparticles, a carbon matrix, and an amorphous carbon external shell, wherein each of the graphite particle coated silicon nanoparticles, the few-layer graphene particles, and the graphite nanoparticles are held within the carbon matrix.
  • FIG. 1 is a schematic representation of a flow chart depicting a process for the production of a purified HSA graphite material in accordance with a first embodiment of the present invention
  • Figure 2 is a schematic representation of a flow chart depicting a process for the production of a purified HSA graphite material in accordance with a second embodiment of the present invention
  • Figure 3 is a schematic representation of a flow chart depicting a process for the production of a purified HSA graphite material in accordance with a third embodiment of the present invention
  • Figure 4 shows the results of trial comparing the milling energy required to produce HSA graphite materials.
  • the process of the present invention is used to produce purified HSA graphite materials from impure graphite concentrates.
  • the process comprises the general steps of: treating an impure graphite concentrate in a mechanical exfoliation step to produce an impure HSA material; and treating the impure HSA material in purification process to produce a purified HSA graphite material.
  • the mechanical exfoliation step of the present invention is used to exfoliate HSA graphite particles from the impure graphite concentrate. As the impure graphite concentrate is treated directly in the mechanical exfoliation step, the impurities contained within the impure graphite concentrate are also subjected to the mechanical exfoliation step.
  • the impure HSA material produced in the mechanical exfoliation step therefore comprises a mixture of HSA graphitic particles and impurity particles.
  • the impure HSA material is then subjected to a purification process to remove these impurities.
  • the inventors have found that the direct treatment of the impure graphite concentrate in the mechanical exfoliation process can offer certain advantages over processes that first purify impure graphite concentrates before mechanical exfoliation.
  • the mechanical exfoliation process itself can introduce impurities into the produced HSA graphite material.
  • the process of the present invention allows the impurities of the impure graphite concentrate and any impurities introduced in the mechanical exfoliation process to be removed simultaneously in a single purification process.
  • the mechanical exfoliation process will liberate impurities disposed within the impure graphite concentrate.
  • the liberation of such particles prior to purification can improve the efficiency of the purification process and/or increase the overall carbon content of the purified HSA graphite material.
  • the overall efficiency of the mechanical exfoliation step may be improved by treating an impure graphite concentrate instead of a purified graphite concentrate.
  • impure graphite concentrates will be understood to refer to graphite-containing materials that contain impurities.
  • the impure graphite concentrate may be recovered from an impure graphite precursor.
  • the impure graphite precursor may be subjected to one or more concentration steps to produce the impure graphite concentrate.
  • the impure graphite concentrate may be recovered from an impure natural graphite precursor.
  • Impure natural graphite precursors include natural graphitic ores. Graphitic ores contain a certain amount of other mineral phases as impurities. Common impurities found in graphite mainly include potassium, sodium, aluminium, calcium, magnesium and other silicate minerals. The process of the present invention is intended to remove a substantial amount of these impurities to allow for the recovery of a purified HSA graphite material.
  • the impure graphite concentrate may be recovered from an impure synthetic graphite precursor.
  • the impure graphite concentrate may be recovered from an impure recycled graphite precursor.
  • Impure recycled graphite precursors include graphite- containing materials recovered from recycled, scrap, spent or discarded materials. The graphite materials contained in the impure recycled graphite precursor contain natural or synthetic graphite.
  • the impure graphite concentrate is recovered from waste battery materials. In one embodiment, the impure graphite concentrate is recovered from waste lithium-ion battery materials.
  • Batteries including lithium-ion batteries, contain anode materials that have a high graphite content, together with cathode material, electrolyte and separator material.
  • the graphite contained in the waste battery materials may be a synthetic graphite or a natural graphite.
  • the batteries may be subjected to a physical separation method to separate the anode materials from the other components.
  • the physical separation method may include dismantling, crushing, screening, and other mechanical processes.
  • the recovered anode material is suitable for use as the impure graphite concentrate of the present invention.
  • the impure graphite concentrate may be recovered from battery black mass. Black mass refers to a concentrated mixture of valuable metals that is recovered from waste batteries.
  • Black mass is typically prepared by shredding waste batteries, followed by the beneficiation of the shredded material to remove unwanted components such as casings, electrolyte materials and plastic wrappers.
  • the black mass contains a mixture of anode and cathode materials.
  • the black mass may first be treated in a hydrometallurgical process to recover valuable metals such as lithium, manganese, cobalt and nickel.
  • Such hydrometallurgical processes typically involve the leaching of these valuable metals in an aqueous solution.
  • the leach residues typically have a high concentration of graphite and may be used as the impure graphite concentrate of the present invention. It is further envisaged that the products resulting from the pyrometallurgical treatment of the waste battery materials may be used as the impure graphite concentrate of the present invention.
  • the impure graphite concentrate is recovered from scrap anode materials.
  • scrap anode materials refer to scraps or waste materials generated during the manufacture of battery components, including lithium-ion battery components.
  • Such anode scraps comprise the graphite anode film adhered to a current collector foil with a polyvinylidene fluoride binder.
  • the anode scraps have a high concentration of graphite and may be used as the impure graphite concentrate of the present invention.
  • the graphite contained in the scrap anode material may be a synthetic graphite or a natural graphite.
  • the scrap anode materials are subjected to one or more graphite concentration steps.
  • the graphite concentration steps comprise the separation of anode materials comprising a recycled graphite concentrate from waste electrode foils.
  • the graphite concentration steps comprise communition of the scrap anode materials and classification of the comminuted material.
  • the graphite concentration steps comprise a heat treatment step.
  • the heat treatment step will remove carbon additives and/or binders from the scrap anode materials.
  • Graphite materials found within waste battery materials and scrap anode material contain a certain amount of organic and inorganic impurities, mainly including organic binders, solid electrolyte interphase films and current collectors.
  • the impure graphite concentrate has a carbon content of less than 96% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 95% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 94% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 93% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 92% Cg.
  • the impure graphite concentrate has a carbon content of less than 91% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 90% Cg. [0070] In one embodiment, the impure graphite concentrate has a carbon content of at least 60% Cg. In one embodiment, the impure graphite concentrate has a carbon content of at least 65% Cg. In one embodiment, the impure graphite concentrate has a carbon content of at least 70% Cg. In one embodiment, the impure graphite concentrate has a carbon content of at least 75% Cg. [0071] In one embodiment, the impure graphite concentrate has a carbon content of between 60 and 96% Cg.
  • the impure graphite concentrate has a carbon content of between 65 and 96% Cg. In one embodiment, the impure graphite concentrate has a carbon content of between 70 and 96% Cg. In one embodiment, the impure graphite concentrate has a carbon content of between 75 and 96% Cg. [0072] In one embodiment, the particle size of the impure graphite concentrate has a D50 of less than about 30 microns. In one embodiment, the particle size of the impure graphite concentrate has a D50 of less than about 15 microns. [0073] In one embodiment, the particle size of the impure graphite concentrate has a D50 of between about 5 to 30 microns.
  • the particle size of the impure graphite concentrate has a D50 of between about 5 to 15 microns.
  • the impure graphite concentrate is subjected to one or more pre-treatment steps prior to the mechanical exfoliation step.
  • the one or more pre-treatment steps may be selected from size reduction, size classification, physical separation, chemical separation, drying, heat treatment or combinations thereof.
  • Suitable physical separation steps include gravity separation, flotation, and magnetic separation.
  • Suitable chemical separation steps include froth flotation and chemical leaching.
  • Mechanical Exfoliation [0075] The mechanical exfoliation step is used to exfoliate HSA graphite particles from the impure graphite concentrate. [0076] The mechanical exfoliation step is conducted in a grinding apparatus.
  • grinding apparatus will be understood to refer to an apparatus that relies on mechanical forces to reduce the particle size of the impure graphite concentrate. Any grinding apparatus known in the art to be suitable to exfoliate HSA graphite particles from the impure graphite concentrate may be used. It is preferred that the grinding apparatus imparts a substantial shearing force on the impure graphite concentrate. Those skilled in the art would be able to readily determine whether a particular grinding apparatus known in the art would be suitable for this purpose. Suitable grinding apparatus include, bead mills, ball mills, rod mills, jet mills, hammer mills, attrition mills vibratory mills and rotating disc mills.
  • the grinding apparatus includes an agitated grinding media.
  • Such grinding media is typically in the form of multiple grinding media ‘beads’ that are held within a vessel. A rotor is then used to generate bead motion within the vessel.
  • Such grinding apparatus rely on the motion of the grinding media to impart a mechanical shear force on the feed material.
  • the size of the grinding media, the agitation speed and the grinding time may be used to influence the properties of the impure HSA material produced.
  • Suitable grinding apparatus that include an agitated grinding media include, for example, bead mills and ball mills.
  • the grinding apparatus may include one or more rotating discs.
  • the diameter of the grinding media is between 0.1 and 3 mm.
  • the diameter of the grinding media is between 0.1 and 1 mm.
  • the size of the beads required may be calculated from the particle size of the raw material and the intended final particle size of the ground material. Generally, the size of the beads required for milling particles is 10 to 30 times the maximum particle size of the raw material and 1,000 to 3,000 times the mean particle size after milling. While smaller bead size will generally allow better energy efficiency, hence lower energy to reach the same size/surface area, this should be balanced with the higher cost of small size beads.
  • the mechanical exfoliation step is a dry mechanical exfoliation step. In a preferred form of the present invention, the mechanical exfoliation step is conducted in the presence of a liquid medium.
  • the liquid medium is an aqueous solution, preferably deionised water.
  • the liquid medium is an organic medium.
  • Example organic liquids include alcohols, ketones, hydrocarbons or solvents.
  • the impure graphite concentrate is pulped with the liquid medium to prepare an impure graphite slurry and the impure graphite slurry is treated in the mechanical exfoliation step.
  • the pulp density of the impure graphite slurry is at least 10 wt%. In one embodiment, the pulp density of the impure graphite slurry is at least 15 wt%.
  • the pulp density of the impure graphite slurry is between 10 and 50 wt%. In one embodiment, the pulp density of the impure graphite slurry is between 10 and 30 wt%. In one embodiment, the pulp density of the impure graphite slurry is between 15 and 20 wt%.
  • the liquid medium may include one or more additives, such as surfactants and dispersing agents, to assist with the exfoliation of the impure graphite concentrate.
  • the mechanical exfoliation step is conducted at a temperature below 100°C. Preferably, the mechanical exfoliation step is conducted at ambient temperature.
  • the surface area of the impure HSA material is directly related to the energy input of the mechanical exfoliation step. Higher energy inputs will result in particles having a greater surface area. The energy may then be used to control the surface area of the impure HSA material.
  • the specific milling energy of the mechanical exfoliation step is greater than 50 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is greater than 100 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is greater than 200 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 50 to 400 kWh/t.
  • the specific milling energy of the mechanical exfoliation step is in the range of 100 to 400 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 50 to 500 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 100 to 500 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 200 to 500 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is greater than 400 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 400 to 500 kWh/t.
  • the specific milling energy of the mechanical exfoliation step is greater than 700 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 700 to 1200 kWh/t. In one embodiment, the mechanical exfoliation step is conducted in the range of 1000 to 1200 kWh/t. Throughout this specification, the specific milling energy (kWh/t) is expressed in terms of milling material weight. In embodiments where the mechanical exfoliation step is conducted in the presence of a liquid medium, the specific milling energy (kWh/t) is expressed in terms of the milling material weight at 15 wt% solids. [0086] In one embodiment, the mechanical exfoliation step may comprise multiple grinding stages, preferably with beneficial treatment steps being conducted between successive grinding stages.
  • the slurry is subjected to a dewatering step to remove at least a portion of the liquid medium.
  • the impure HSA material may be further treated in a drying step to remove substantially all the residual liquid medium. Any suitable drying method may be used, for example, an air-drying step conducted at ambient temperature or above-ambient temperature. In one embodiment of the present invention, a cryogenic drying step may be used. Cryogenic drying methods include the freezing of liquids present in the impure HSA material and lowering the pressure to remove the ice by sublimation. These methods may be used to substantially prevent the agglomeration of any HSA particles.
  • An example of suitable process conditions includes the freezing of the impure HSA material, followed by subjecting the frozen material to: (i) ⁇ 6 mbar vacuum, >0 °C drying temperature, and condenser temperature of ⁇ 60- 70 °C; or (ii) ⁇ 1mbar vacuum, >30-40 °C drying temperature, and condenser temperature of 60–70 °C.
  • a drying step is not essential.
  • the impure HSA material recovered from the dewatering step may be treated directly in the purification process without an intermediate drying step.
  • the impure HSA material is treated in a beneficiation step prior to being treated in the purification process.
  • the inventors have identified that the fine grinding of the mechanical exfoliation step can result in the production of impurity-containing fines. These fines can be removed using beneficiation techniques to remove these impurities prior to the purification process. Suitable beneficiation processes include size classification, liquid-liquid separation, gravity separation, flotation, froth flotation and magnetic separation. Purification Process [0091]
  • the impure HSA material comprises a mixture of graphite particles and impurity particles. These impurities include those present in the impure graphite concentrate, together with any impurities introduced in the mechanical exfoliation step. Such introduced impurities may include, for example, residues from the grinding media itself.
  • the purification process of the present invention is used to remove a substantial portion of the impurities present in the impure HSA material, thereby producing a purified HSA graphite material.
  • the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.0% Cg. In one embodiment, the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.5% Cg. In one embodiment, the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.9% Cg. In one embodiment, the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.92% Cg.
  • the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.95% Cg.
  • the purification process may include one or more separate impurity removal steps. These impurity removal steps may be selected from one or more acid leach steps, caustic leach steps, oxidative leach steps, caustic bake steps or combinations thereof. The impurity removal steps used will depend largely on the impurities present in the impure graphite concentrate.
  • the purification process comprises one or more caustic impurity removal steps, followed by one or more acid leach steps.
  • the process comprises the steps of: treating the impure HSA material in a caustic impurity removal step, and recovering a solid caustic leach product; and treating the solid caustic leach product in an acid leach step, and recovering a solid acid leach product, wherein the solid acid leach product comprises a purified HSA graphite material.
  • Caustic Leach Step [0095]
  • the caustic impurity removal step comprises a caustic leach step.
  • the caustic leach step comprises the contact of the impure HSA material with a sodium hydroxide solution, and recovering a solid caustic leach product.
  • the sodium hydroxide solution is a concentrated sodium hydroxide solution.
  • the concentration of the sodium hydroxide solution is at least 100 g/L. In one embodiment, the concentration of the sodium hydroxide solution is at least 200 g/L. In one embodiment, the concentration of the sodium hydroxide solution is at least 400 g/L. In one embodiment, the concentration of the sodium hydroxide solution is at least 600 g/L. In one embodiment, the concentration of the sodium hydroxide solution is at least 800 g/L.
  • the caustic leach step is conducted at a temperature of at least 50°C. In one embodiment of the present invention, the caustic leach step is conducted at a temperature between about 50 to 200°C. More preferably, the caustic leach step is conducted at a temperature of at least 100°C.
  • the caustic leach step is conducted at a temperature between about 100 to 200°C. In an alternative embodiment, the caustic leach step is conducted at a temperature of at least 200°C. Preferably, the caustic leach step is conducted at a temperature of at least 220°C. The inventors have found that higher temperature caustic leach steps may be required to solubilise certain impurities, for example, crystalline Al2O3, should they be present in the impure HSA material. [0098] In one form of the present invention, the starting slurry density of the caustic leach step is 150 to 250 g/L solids.
  • the residence time of the caustic leach step is 2 to 16 hours.
  • the caustic leach step is conducted at a pressure of at least 2 bar.
  • the caustic leach step comprises a two- stage leach. Preferably, both stages are operated at a temperature of at least 100°C and elevated pressure. More preferably, the two-stage leach operates in a counter-current manner.
  • the caustic leach step comprises the contact of the impure HSA material with a sodium hydroxide solution at a temperature of at least 100°C and elevated pressure, to form a caustic leach slurry and recovering a solid caustic leach product from the caustic leach slurry.
  • the caustic leach slurry is subjected to a solid liquid separation step to recover a solid caustic leach product and a caustic leachate solution.
  • the solid caustic leach product is treated in one or more re-pulp stages and/or washing stages.
  • the solid caustic leach product is re-pulped with a sodium hydroxide solution.
  • the solid caustic leach product is subjected to a drying step.
  • the solid caustic leach product from the caustic leach step is subjected to a secondary caustic leach step prior to the acid leach step.
  • the secondary caustic leach step may be used to remove further impurities that may be left in the solid caustic leach product.
  • Caustic Bake Step comprises a caustic bake step.
  • the caustic bake step comprises baking the impure HSA material in caustic conditions and subjecting the product to a water leach step, and recovering a solid caustic leach product.
  • the caustic bake step causes the caustic soda and impurities, in particular silicon impurities, to react, rendering these impurities soluble in water and mild acid conditions.
  • the caustic bake step comprises: preparing a mixture of the impure graphite concentrate and caustic soda; subjecting the mixture to a baking step at a temperature between 150°C and 300°C to prepare a product; and subjecting the product to a water leach step.
  • the amount of caustic soda in the mixture is based on the silicon content of the impure graphite concentrate.
  • the molar ratio of caustic to silicon is at least 1 : 1.
  • the caustic bake step is undertaken in a rotating kiln.
  • the water leach step is preferably undertaken at between about 5-60°C, for example about 35°C ⁇ 5°C.
  • the water leach step is undertaken in a single stage.
  • the water leach step may be undertaken in multiple, for example three, counter-current leach stages.
  • the water leach step has a retention time of between about 30 to 240 minutes.
  • the caustic impurity removal step comprises a caustic bake step followed by a caustic leach step.
  • the caustic impurity removal step comprises baking the impure HSA material in caustic conditions, subjecting the product to a caustic leach step, and recovering a solid caustic leach product.
  • the above discussion of the caustic bake step and the caustic leach step equally applies to the combined caustic bake step and caustic leach step. The inventors have found that the inclusion of a caustic bake step prior to the caustic leach step can increase the overall amount of impurities removed from the impure HSA material.
  • the caustic bake step is included when treating recycled graphite concentrates that contain refractory aluminium oxide.
  • the combination of the caustic bake step and the caustic leach step will render the refractory aluminium oxide phases more amenable to leaching in the downstream acid leach step. This increases the amount of aluminium removed from the impure HSA material when compared to the caustic bake step or the caustic leach step in isolation.
  • the inclusion of the caustic bake step will reduce the valence of certain metals in the impure HSA material, such as cobalt and manganese, assisting in their subsequent leaching in the downstream acid leach steps.
  • the acid leach step comprises one or more separate acid leach stages.
  • each acid leach stage comprises the contact of the solid caustic leach product with an acidic solution, thereby forming an acidic leach slurry and recovering a solid acid leach product from the acidic leach slurry.
  • the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution.
  • the acid leach step comprises two or more acid leach stages, where at least one of the acid leach stages comprises the contact of the solid caustic leach product with a hydrofluoric acid solution.
  • the acid leach step preferably comprises: treating the solid caustic leach product in a first sulphuric acid leach stage comprising the contact of the solid caustic leach product with a sulphuric acid solution and separating undissolved leach solids; treating the leach solids of the first sulphuric acid leach stage in a hydrofluoric acid leach stage comprising the contact of the leach solids with a hydrofluoric acid solution and separating undissolved leach solids; treating the leach solids of the hydrofluoric acid leach stage in a second sulphuric acid leach stage comprising the contact of the leach solids with a sulphuric acid solution and separating undissolved leach solids; and treating the leach solids of the second sulphuric acid leach stage to one or more washing stages, and recovering a solid acid leach product comprising a
  • the first sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ⁇ 5°C. Still preferably, the first sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes.
  • concentrated sulphuric acid is added in the first sulphuric acid leach stage. Still preferably, the residual free acid at the end of the first sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ⁇ 5 g/L H2SO4.
  • the first sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids.
  • the hydrofluoric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ⁇ 5°C.
  • the residual free acid at the end of the hydrofluoric acid leach stage is in the range of about 15-75 g/L HF, for example about 60 g/L ⁇ 5 g/L HF.
  • the hydrofluoric acid leach stage operates with between about 5 to 25% solids, for example about 10% solids.
  • the hydrofluoric acid added to the hydrofluoric acid leach stage is in the range of about 20 to 70% concentration.
  • the hydrofluoric acid concentration in the hydrofluoric acid leach stage is preferably in the range of about 15-50 g/L.
  • the leach solids from the hydrofluoric acid leach stage have no remaining silicon therein, or only trace amounts thereof.
  • the second sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ⁇ 5°C.
  • the second sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids.
  • the second sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes.
  • concentrated sulphuric acid is added in the second sulphuric acid leach stage.
  • the residual free acid at the end of the second sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ⁇ 5 g/L H2SO4.
  • Acid solutions from the second sulphuric acid leach stage are preferably recovered and recycled to the first sulphuric acid leach stage and the second sulphuric acid leach stage.
  • the washing stages preferably comprise a single repulp-filtration stage using deionised water, multiple counter-current repulp-filtration stages, or, for example, five multiple counter-current repulp-filtration stages, using deionised water.
  • the washing stages operate with between about 5 to 25% solids, for example about 10% solids using three stages of counter-current repulp-filtration stages.
  • liquid, residual salts, and/or acidity from the residual solids of the second sulphuric acid leach stage are recovered in the washing stages and returned to one or both of the first sulphuric acid leach stage and the second sulphuric acid leach stage.
  • Acid Leach Step – Non-HF Option In accordance with an alternative embodiment of the present invention, the acid leach step does not comprise the contact of the solid caustic leach product with a hydrofluoric acid solution.
  • the acidic solution used in the acid leach step is selected from only a hydrochloric acid solution and a sulphuric acid solution. More preferably, the only acidic solution used in the acid leach step is a hydrochloric acid solution.
  • the hydrochloric acid concentration is between about 30 to 60 g/L.
  • Hydrochloric acid is preferably added to the acid leach step at a rate of between about 150 to 500 kg/t feed, preferably 150 to 350 kg/t feed, to acidify residual alkalinity. The rate of addition of hydrochloric acid is preferably undertaken with consideration of background acid requirements.
  • the acid leach step is operated at a temperature of between about 60 to 100°C.
  • the acid leach step is operated with a slurry density of between about 10 to 35% w/w.
  • the acid leach step is operated with a slurry density of between about 15 to 35% w/w.
  • the acid leach step is operated with a slurry density of about20% w/w.
  • the acid leach step is operated with a total residence time of between about 2 to 6 hours.
  • the acid leach step is conducted: (i) at a temperature of about 80 o C; and (ii) with a total residence time of between about 2 to 4 hours.
  • the process preferably comprises treating the impure HSA material in an oxidative leach step. More preferably, the impure HSA material is treated in an oxidative leach step prior to the caustic leach step.
  • the oxidative leach step comprises a first oxidative leach stage comprising the contact of the impure HSA material with an oxidant solution to form a first oxidative leach slurry. At least one of sodium hypochlorite, sodium chlorite, hydrogen peroxide, sodium meta bisulphite or sodium chlorate is utilised as the oxidant in the first oxidative leach stage.
  • the concentration of the oxidant in the first oxidative leach stage is based on: (i) about 125 to 200% of the stoichiometric amount based on sulphide sulphur content; (ii) a mole ratio of 7:2; and/or (iii) 50 to 150 kg/t feed material based on sulphide sulphur content.
  • the first oxidative leach step operates with a target redox potential of > about 425 mV (versus Ag/AgCl), preferably > about 550 mV (versus Ag/AgCl).
  • the first oxidative leach step operates with a pH of > 10.
  • the first oxidative leach step operates with a slurry density of between about 10 to 35% w/w. In one form of the present invention, the first oxidative leach step operates with a slurry density of between about 15 to 35% w/w.
  • the first oxidative leach step operates with a slurry density of about 20% w/w [00133] In one form of the present invention, the first oxidative leach step operates at a temperature of between about 30 to 60°C. [00134] In one form of the present invention, the first oxidative leach step operates with a residence time of between about 30 to 60 minutes. [00135] In one form of the present invention, the target redox potential of the first oxidative leach step is in the range of about 800 to 1200 mV (versus Ag/AgCl). More preferably, the target redox potential of the first oxidative leach step is in the range of about 950 to 1000 mV (versus Ag/AgCl).
  • the target redox potential of the first oxidative leach step is in the range of about 750 to 1500 mV (versus Ag/AgCl). More preferably, the target redox potential of the first oxidative leach step is in the range of about 850 to 1150 mV (versus Ag/AgCl).
  • the oxidative leach step further comprises a second oxidative leach stage.
  • the second oxidative leach stage comprises the contact of the first oxidative leach slurry with a hydrochloric acid solution.
  • the second oxidative leach stage targets a background HCl acidity of between about 30 to 60 g/L.
  • the second oxidative leach stage targets a background HCl acidity of at least 60 g/L.
  • the second oxidative leach stage operates at a temperature of about 40 to 100°C.
  • the second oxidative leach stage operates with a slurry density of between about 15 to 35% w/w, for example about 20% w/w.
  • the second oxidative leach stage operates with a residence time in the range of about 2 to 6 hours.
  • the second oxidative leach stage operates with an HCl addition rate of between about 150 to 500 kg/t feed.
  • the second oxidative leach stage operates with an HCl addition rate of between about 150 to 350 kg/t feed.
  • the second oxidative leach stage may be conducted in multiple leach vessels arranged in counter current operation.
  • the second oxidative leach stage operates: (i) with a residence time of between about 2 to 4 hours; and (ii) with an HCl addition rate of about 300 kg/t feed.
  • Aluminium and/or calcium minerals are at least partially leached in the second oxidative leach stage.
  • the leach slurry in subjected to a solid liquid separation step to recover an oxidative leach residue is subjected to the caustic leach step as discussed above.
  • the solid caustic leach product recovered from the caustic leach step is subjected to a secondary caustic leach step, prior to the acid leach step.
  • Purification Process – Acid Leach [00147] In an alternative form of the present invention, the purification process comprises one or more acid leach steps.
  • the purification process comprises: treating the impure HSA material in an acid leach step, and recovering a solid acid leach product, wherein the solid acid leach product comprises a purified HSA graphite material.
  • the acid leach step comprises one or more separate acid leach stages.
  • each acid leach stage comprises the contact of the impure HSA material with an acidic solution, thereby forming an acidic leach slurry and recovering a solid acid leach product from the acidic leach slurry. It is envisaged that different acidic solutions may be used at different acid leach stages to target different impurities remaining in the impure HSA material.
  • the acid leach step comprises the contact of the impure HSA material with a hydrofluoric acid solution.
  • the acid leach step comprises two or more acid leach stages, where at least one of the acid leach stages comprises the contact of the impure HSA material with a hydrofluoric acid solution.
  • the acid leach step preferably comprises: treating the impure HSA material in a first sulphuric acid leach stage comprising the contact of the impure HSA material with a sulphuric acid solution and separating undissolved leach solids; treating the leach solids of the first sulphuric acid leach stage in a hydrofluoric acid leach stage comprising the contact of the leach solids with a hydrofluoric acid solution and separating undissolved leach solids; treating the leach solids of the hydrofluoric acid leach stage in a second sulphuric acid leach stage comprising the contact of the leach solids with a sulphuric acid solution and separating undissolved leach solids; and treating the leach solids of the second sulphuric acid leach stage to one or more washing stages, and recovering a solid acid leach product comprising a purified graphite material
  • the purification process does not comprise any caustic impurity removal steps.
  • the inventors have found that the treatment of the impure graphite concentrate in the mechanical exfoliation step may avoid the need to treat the impure HSA material in a caustic impurity removal step prior to the acid leach step.
  • the first sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ⁇ 5°C. Still preferably, the first sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes.
  • concentrated sulphuric acid is added in the first sulphuric acid leach stage.
  • the residual free acid at the end of the first sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ⁇ 5 g/L H2SO4.
  • the first sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids.
  • the hydrofluoric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ⁇ 5°C.
  • the residual free acid at the end of the hydrofluoric acid leach stage is in the range of about 15-75 g/L HF, for example about 60 g/L ⁇ 5 g/L HF.
  • the hydrofluoric acid leach stage operates with between about 5 to 25% solids, for example about 10% solids.
  • the hydrofluoric acid added to the hydrofluoric acid leach stage is in the range of about 20 to 70% concentration.
  • the hydrofluoric acid concentration in the hydrofluoric acid leach stage is preferably in the range of about 15-50 g/L.
  • the leach solids from the hydrofluoric acid leach stage have no remaining silicon therein, or only trace amounts thereof.
  • the second sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ⁇ 5°C.
  • the second sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids.
  • the second sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes.
  • concentrated sulphuric acid is added in the second sulphuric acid leach stage.
  • the residual free acid at the end of the second sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ⁇ 5 g/L H2SO4.
  • Acid solutions from the second sulphuric acid leach stage are preferably recovered and recycled to the first sulphuric acid leach stage and the second sulphuric acid leach stage.
  • the washing stages preferably comprise a single repulp-filtration stage using deionised water, multiple counter-current repulp-filtration stages, or, for example, five multiple counter-current repulp-filtration stages, using deionised water.
  • the washing stages operate with between about 5 to 25% solids, for example about 10% solids using three stages of counter-current repulp-filtration stages.
  • liquid, residual salts, and/or acidity from the residual solids of the second sulphuric acid leach stage are recovered in the washing stages and returned to one or both of the first sulphuric acid leach stage and the second sulphuric acid leach stage.
  • Drying Step [00162]
  • the purified HSA graphite material may be further treated in a drying step to remove substantially all residual liquids. Any suitable drying method may be used, for example, an air-drying step conducted at increased temperatures. In one embodiment of the present invention, a cryogenic drying step may be used.
  • Cryogenic drying methods include the freezing of liquids present in the purified HSA graphite material and lowering the pressure to remove the ice by sublimation. These methods may be used to substantially prevent the agglomeration of any HSA graphite particles.
  • An example of suitable process conditions includes the freezing of the purified HSA graphite material, followed by subjecting the frozen material to: (i) ⁇ 6 mbar vacuum, >0 °C drying temperature, and condenser temperature of ⁇ 60-70 °C; or (ii) ⁇ 1mbar vacuum, >30-40 °C drying temperature, and condenser temperature of 60– 70 °C.
  • Impurity Removal Options [00163] The particular impurity removal steps incorporated into the purification process of the present invention will depend on several factors.
  • Impure graphite concentrates typically comprise silicon containing species.
  • a caustic impurity removal step for example, the caustic bake step or the caustic leach step, is used to solubilise the silicate species.
  • the caustic impurity removal step should comprise a caustic leach step conducted at a temperature of at least 200°C and a pressure of 2 bar.
  • the caustic impurity removal step should include a combination of a caustic bake step and a caustic leach step.
  • the inclusion of the caustic bake step may avoid the need for the caustic leach step to be conducted at a temperature of at least 200°C, however it is envisaged that higher purities may be achieved by incorporating a high temperature caustic leach.
  • the remaining impurities in the impure HSA material are targeted in one or more acid leach steps.
  • the choice of the acid leach steps is influenced by the remaining impurities present in the impure HSA material. Those skilled in the art would be able to determine appropriate acid leach steps to remove the remaining impurities.
  • a further factor to consider when selecting the acid leach steps is the acidic solutions available, and in particular, whether hydrofluoric acid solutions are to be incorporated.
  • the acid leach step includes the use of a hydrofluoric acid solution.
  • the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution
  • the acid leach step preferably comprises several acid leach stages.
  • the acid leach stages comprise a first sulphuric acid leach stage, a hydrofluoric acid leach stage, a second sulphuric acid leach stage and a washing stage.
  • the caustic leach step 28 is operated over multiple stages, at least one of the stages is operated at a temperature of at least 200°C and elevated pressure. However, it is preferable to operate all stages at a temperature of at least 200°C and elevated pressure.
  • the caustic leach slurry is subjected to a solid liquid separation step (not shown) to recover a solid caustic leach product 36 and a caustic leachate solution 38.
  • the solid material recovered in the solid liquid separation step is subjected to one or repulp steps to remove residual or entrained soluble species.
  • the solid material is repulped in a sodium hydroxide solution and the slurry is subjected to a further solid liquid separation step to recover a solid material.
  • the solid material may be repulped into a 200 g/l NaOH solution at 80°C for 30 minutes.
  • the solid material is repulped in water and the slurry is subjected to a further solid liquid separation step to recover a solid material.
  • the solid material may be repulped into water at 80°C for 30 minutes.
  • the solid caustic leach product 20 is preferably dried at elevated temperature, for example 105°C, prior to further processing.
  • the caustic leach step 28 Caustic containing leachate 38 from the caustic leach step 28 is treated in a caustic regeneration step 34 to which lime and/or hydrated/slaked lime is fed, and from which regenerated caustic 35 is recovered, optionally concentrated, and recycled to the caustic leach step 28.
  • the solid caustic leach product 36 is then passed to acid leach step 30.
  • the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution
  • the acid leach step 30 preferably comprises several acid leach stages.
  • the acid leach step 30 comprises a first sulphuric acid leach stage 40, a hydrofluoric acid leach stage 42, a second sulphuric acid leach stage 44 and a washing stage 46.
  • the embodiment shown in Figure 1 includes a caustic leach step, it is envisaged that in certain embodiments where the acid leach step comprises the use of a hydrofluoric acid solution, the caustic leach step may be omitted.
  • the impure HSA material 26 is treated directly in the first sulphuric acid leach stage 40.
  • the treatment of the impure graphite concentrate in the mechanical exfoliation step 20 may avoid the need for the caustic leach step.
  • the first sulphuric acid leach stage 40 comprises the contact of the solid caustic leach product 36 with a sulphuric acid solution 48 to form a leach slurry and the separation of an acidic leachate 50 from undissolved leach solids 52.
  • the first sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ⁇ 5°C. Still preferably, the first sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes.
  • the residual free acid at the end of the first sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ⁇ 5 g/L H2SO4. Concentrated sulphuric acid is added to the slurry to maintain target residual free acid.
  • the first sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids.
  • the first sulphuric acid leach step may be conducted in a single leach vessel or may be conducted across multiple leach vessels arranged in series. [00180]
  • the leach slurry is directed to a solid liquid separation step, for example a filter, to recover acidic leachate 50.
  • the hydrofluoric acid leach stage 42 comprises the contact of the undissolved leach solids 52 with a hydrofluoric acid solution 54 to form a leach slurry and the separation of an acidic leachate 56 from undissolved leach solids 58.
  • the hydrofluoric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ⁇ 5°C.
  • the residual free acid at the end of the hydrofluoric acid leach stage is in the range of about 15-75 g/L HF, for example about 60 g/L ⁇ 5 g/L HF.
  • the hydrofluoric acid leach stage operates with between about 5 to 25% solids, for example about 10% solids.
  • Hydrofluoric acid added to the hydrofluoric acid leach stage is in the range of about 20 to 70% concentration.
  • the hydrofluoric acid concentration in the hydrofluoric acid leach stage is preferably in the range of about 15-50 g/L.
  • the hydrofluoric acid leach stage may be conducted in a single leach vessel or may be conducted across multiple leach vessels arranged in series.
  • the leach slurry is directed to a solid liquid separation step, for example a filter, to recover acidic leachate 56.
  • Filter cake is washed and repulped with water at 40°C during 30 minutes at a solid/liquid ratio of about 165 g/l and the repulp slurry is directed to a solid liquid separation step, for example a filter, to recover a wash solution and undissolved leach solids 58.
  • the second sulphuric acid leach stage 44 comprises the contact of the undissolved leach solids 58 with a sulphuric acid solution 60 to form a leach slurry and the separation of an acidic leachate 62 from undissolved leach solids 64.
  • the second sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ⁇ 5°C.
  • the second sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids.
  • the second sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes.
  • Concentrated sulphuric acid is added in the second sulphuric acid leach stage.
  • the residual free acid at the end of the second sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ⁇ 5 g/L H2SO4.
  • Acid solutions from the second sulphuric acid leach stage are preferably recovered and recycled to the first sulphuric acid leach stage and the second sulphuric acid leach stage.
  • the leach slurry is directed to a solid liquid separation step, for example a filter, to recover acidic leachate 62.
  • Filter cake is washed and repulped with water at 40°C during 30 minutes at a solid/liquid ratio of about 165 g/l and the repulp slurry is directed to a solid liquid separation step, for example a filter, to recover a wash solution and undissolved leach solids 64.
  • the washing stage 46 comprises the contact of the undissolved leach solids 64 with a wash solution 66 and the separation of a purified HSA material 68.
  • the washing stage 46 comprises a single repulp-filtration stage using deionised water, multiple counter- current repulp-filtration stages, or, for example, five multiple counter-current repulp- filtration stages, using deionised water.
  • the washing stages operate with between about 5 to 25% solids, for example about 10% solids using three stages of counter-current repulp-filtration stages.
  • Water, residual salts, and/or acidity from the residual solids of the second sulphuric acid leach stage are recovered in the washing stages and returned to one or both of the first sulphuric acid leach stage and the second sulphuric acid leach stage. It is envisaged that carbonation may be incorporated during the washing stages 46, through the addition of sodium bicarbonate or the bubbling of carbon dioxide with caustic soda for pH control. This will help to neutralise carry-over acid from the second sulphuric acid leach stage and reduce the number of counter current washing steps required.
  • the purified HSA material 68 may be further treated in a drying step (not shown) to remove residual liquids.
  • a first effluent treatment plant 70 receives leachate streams 50 and 62 from the two sulphuric acid leach steps 40 and 44, respectively.
  • the first effluent treatment plant 70 contacts the leachates with lime and iron sulphate, for example ferric sulphate, producing a neutralised solution 72 and a gypsum product 74.
  • the volume of gypsum precipitation, or residue, may be minimised through use of caustic soda rather than lime in this step.
  • a second effluent treatment plant 76 receives leachate 56 from the hydrofluoric leach step 56.
  • the second effluent treatment plant 76 also receives lime and iron sulphate, for example ferric sulphate, producing a neutralised solution 78 and a calcium fluoride product 80. Alternatively, the second effluent treatment plant receives aluminium hydroxide, thereby producing an aluminium fluoride product.
  • a process 100 for the recovery of purified graphite materials from an impure graphite concentrate 12 in accordance with an alternative embodiment of the present invention.
  • the embodiment shown in Figure 2 shares many similarities with the embodiment shown in Figure 1 and like numerals denote like parts.
  • the impure graphite concentrate 12 may be passed to a pre-treatment circuit (not shown).
  • the impure graphite concentrate 12 is subjected to a pulping step 14 in which the impure graphite concentrate 12 in mixed with a liquid medium 16 to prepare an impure graphite slurry 18.
  • the impure graphite slurry 18 is directed to a mechanical exfoliation step 20.
  • mechanical exfoliation step 20 the impure graphite slurry 18 is subjected to mechanical shearing forces to exfoliate HSA graphite particles from the impure graphite concentrate.
  • Mechanical exfoliation step 20 results in the production of a slurry 22 comprising impure HSA graphite particles dispersed through the liquid medium.
  • the slurry 22 is directed to a drying step 24 to remove the majority of the liquid medium and recover an impure HSA material 26.
  • the impure HSA material 26 is then treated in a purification process to remove a substantial amount of the impurities present in the impure HSA material 26.
  • the purification process comprises an oxidative leach step 102, followed by a caustic leach step 28 and an acid leach step 104.
  • the acid leach step 104 comprises the use of a hydrofluoric acid solution.
  • the oxidative leach step 102 comprises a first oxidative leach stage comprising the contact of the graphite concentrate with an oxidant solution 106 to form a first oxidative leach slurry.
  • the impure HSA material 26 is repulped, for example using recycled process water, adjusted to a pH in the range of 10-11 with caustic soda.
  • the first oxidative leach stage operates with a target redox potential of > about 425 mV (versus Ag/AgCl), with a pH of > 10, with a slurry density of between about 15 to 35% w/w solids, for example about 20% w/w, at a temperature of between about 30 to 60°C, and with a residence time of between about 30 to 60 minutes.
  • the target redox potential is in the range of about 800 to 1200 mV (versus Ag/AgCl), for example in the range of about 950 to 1000 mV (versus Ag/AgCl).
  • the oxidative leach step 102 further comprises a second oxidative leach stage (not shown) that comprises the contact of the first oxidative leach slurry with a hydrochloric acid solution.
  • the second oxidative leach stages operates at a temperature of about 40 to 100°C, over multiple leach stages, with a slurry density of between about 15 to 35% w/w, for example about 20% w/w, with a residence time in the range of about 2 to 6 hours, with an HCl addition rate of between about 150 to 350 kg/t feed, and with a background HCl acidity of between about 30 to 60 g/L.
  • the second oxidative leach portion operates with a residence time of between about 2 to 4 hours, and with an HCl addition rate of about 320 kg/t feed.
  • the slurry product of the oxidative leach step 102 is passed to a solid liquid separation step (not shown) to recover an oxidative leach residue 110 and a liquid product 112.
  • the oxidative leach residue 110 is directed to a caustic leach step 28 as described above to produce a solid caustic leach product 36.
  • the solid caustic leach product may be directed to a secondary caustic leach step (not shown) conducted under the same conditions as the caustic leach step 28.
  • the secondary caustic leach step may be used in circumstances where residual impurities remain in the solid caustic leach product 36.
  • the solid caustic leach product 36 is directed to an acid leach step 104.
  • the acid leach step 104 comprises a single acid leach stage 114.
  • Acid leach stage 114 comprises the contact of the solid caustic leach product 36 with an acidic solution 116 selected from hydrochloric acid and sulphuric acid to form a leach slurry.
  • Acid leach stage 114 Is conducted at a temperature of between about 60 to 100°C, with a slurry density of between about 15 to 35% w/w, for example about 20% w/w, with a total residence time of between about 2 to 6 hours.
  • the background HCl acidity is between about 30 to 60 g/L.
  • the hydrochloric acid solution is added to acid leach stage 114 at a rate of between about 150 to 500 kg/t feed, for example about 320 kg/t feed. The rate of addition of hydrochloric acid to the acid leach stage 114 is undertaken with consideration of background acid requirements.
  • Acid leach stage 114 may be conducted in a single reactor or multiple reactors arranged in series.
  • the leach slurry is directed to a solid liquid separation step, for example a filter, to recover acidic leachate 117.
  • Filter cake 118 is washed and directed to a water repulping step 120 in which it is repulped with water 121 at 40°C during 30 minutes at a solid/liquid ratio of about 165 g/l and the repulp slurry is directed to a solid liquid separation step, for example a filter, to recover a wash solution and purified HSA material 68.
  • the purified HSA material 68 may be further treated in a drying step (not shown) to remove residual liquids.
  • the process further comprises a first effluent treatment stage 122 in which liquid products 112 and 117 from the oxidative leach step 102 and the acid leach step 114 are combined and treated.
  • the treatment may comprise neutralisation by addition of an alkali and/or calcium precipitation by addition of sodium sulphate.
  • the liquid product 124 is directed to a reagent recovery step 126 to recover sodium hydroxide, hydrochloric acid or sodium hypochlorite reagents to be recycled or reused within the process.
  • Reagent recovery step 126 may include, for example, the recovery of a concentrated sodium chloride solution and a chlor-alkali electrolysis step to generate chlorine gas.
  • FIG 3 there is shown a process 200 for the recovery of purified graphite materials from a impure graphite concentrate 12 in accordance with an alternative embodiment of the present invention.
  • the embodiment shown in Figure 3 shares many similarities with the embodiment shown in Figure 1 and like numerals denote like parts.
  • the impure graphite concentrate 12 may be passed to a pre-treatment circuit (not shown).
  • the impure graphite concentrate 12 is subjected to a pulping step 14 in which the impure graphite concentrate 12 in mixed with a liquid medium 16 to prepare an impure graphite slurry 18.
  • the impure graphite slurry 18 is directed to a mechanical exfoliation step 20.
  • mechanical exfoliation step 20 the impure graphite slurry 18 is subjected to mechanical shearing forces to exfoliate HSA graphite particles from the impure graphite concentrate.
  • Mechanical exfoliation step 20 results in the production of a slurry 22 comprising impure HSA graphite particles dispersed through the liquid medium.
  • the slurry 22 is directed to a drying step 24 to remove the majority of the liquid medium and recover an impure HSA material 26.
  • the impure HSA material 26 is then treated in a purification process to remove a substantial amount of the impurities present in the impure HSA material 26.
  • the purification process comprises pelletisation step 202, a caustic bake step 204, a water leach step 206 followed by an acid leach step 30.
  • the acid leach step 30 comprises the use of a hydrofluoric acid solution.
  • the pelletisation step 202 comprises the addition, in stepwise fashion, of caustic soda, in the form of caustic pill, and water to the impure HSA material 26.
  • the pellets produced in the pelletisation step 202 are micro-pellets of about 2-10 mm in diameter, for example 5mm ⁇ 2mm.
  • the pellets produced in the pelletisation step 202 have a moisture content of about 13 to 24%w/w, for example about 20%w/w.
  • the pellets are then treated in caustic bake step 204 in which the pellets are subjected to temperatures of between about 150-300°C, resulting in the formation of a product 208.
  • the caustic bake step 204 has a residence time of greater than about 60 minutes, for example about 120 minutes.
  • the molar ratio of caustic to silicon between 2.5- 5.5 : 1.
  • the caustic bake step 204 is undertaken, for example, in a rotating kiln.
  • the water leach step 206 comprises the contact of the product 208 with an aqueous solution 210 to form a leach slurry.
  • the water leach step 206 is undertaken at between about 5-60°C, for example about 35°C.
  • the water leach step 206 is undertaken in either a single or in multiple stages, for example up to three, counter-current leach stages and has a retention time of between about 30 to 240 minutes.
  • the leach slurry is directed to a solid liquid separation step, for example a filter. Filter cake is washed and repulped with water at 40°C during 30 minutes at a solid/liquid ratio of about 165 g/l and the repulp slurry is directed to a solid liquid separation step, for example a filter, to recover a wash solution and a solid caustic leach product 36.
  • the solid caustic leach product 36 is then passed to acid leach step 30, comprising a first sulphuric acid leach stage 40, a hydrofluoric acid leach stage 42, a second sulphuric acid leach stage 44 and a washing stage 48 as described above to recover a purified HSA material 68.
  • acid leach step comprises the use of a hydrofluoric acid solution
  • the pelletisation step 202, caustic bake step 204 and water leach step 206 may be omitted.
  • the impure HSA material 26 is treated directly in the first sulphuric acid leach stage 40.
  • the treatment of the impure graphite concentrate in the mechanical exfoliation step 20 may avoid the need for the caustic bake step 204.
  • a first effluent treatment plant 70 and a second effluent treatment plant 76 are similarly used to treat recovered streams from the process.
  • the purified HSA material 68 may be further treated in a drying step (not shown) to remove residual liquids.
  • Example 1 An impure natural graphite concentrate sample with a carbon content of 86.8 CT% and a surface area of 8.6 m2/g was subjected to a mechanical exfoliation step conducted at a specific milling energy of about 340 kWh/t. The product was further treated in a LLS beneficiation step to remove a portion of fine silicate minerals. The resulting impure HSA material had a surface area of 25.1 m2/g and an elemental composition as summarised in Table 1 Table 1: Mechanically Exfoliated Material Head Assay (M10843) [00212] The impure HSA material was subjected to two separate purification processes.
  • the first purification process included caustic baking the impure HSA material and then treating the material in a first sulphuric acid leach stage, a hydrofluoric acid leach stage, a second sulphuric acid leach stage and a washing stage.
  • the process conditions are summarised in Table 2: Table 2: Test 1 Roast-Leach Process (HF) [00213]
  • the second purification process included the caustic leaching of the impure HSA material at a temperature of about 140°C followed by hydrochloric acid leaching and water washing.
  • Table 3 Test 2 MLP Process (non-HF) [00214] The results of the two trials are shown in Table 4 and show that the impurities present in the graphite concentrate can be removed following the mechanical exfoliation step.
  • Table 5 Sample [00216] The samples were treated in a mechanical exfoliation step conducted in a grinding apparatus containing a grinding media having a ceramic bead size of 0.7-0.8mm. The change in surface area of the samples was measured against the milling energy. The results of this test are shown in Figure 3. It can be seen that lower milling energies were required for both impure graphite concentrate samples as compared to the purified graphite sample. The treatment of impure recycled graphite concentrates, in particular, exhibited significant reduction in milling energy. The milling energy required to reach a nominal particle size of 25 m2/g when treating the purified natural graphite concentrate was ⁇ 570 kWh/t.
  • the milling energy to achieve the same particle size when treating the impure graphite concentrates was ⁇ 300 kWh/t and ⁇ 77 kWh/t for the impure natural and impure recycled concentrates respectively.
  • the purity of the HSA material recovered from the purified graphite concentrate was analysed.
  • the purified graphite starting purity was 99.92%Cg and it reduced to 99.87%Cg after the exfoliation step. This indicates that the exfoliation step introduced impurities, likely associated with the milling media used, that must be removed from the HSA material.
  • Example 3 An impure natural graphite concentrate sample with a carbon content of 95.5 CT% and a surface area of 2.5 m2/g was subjected to a mechanical exfoliation step as a dry powder using a jet mill.
  • the resulting impure HSA material had a surface area of 13.8 m2/g and an elemental composition as summarised in Table 6 below.
  • M11628 Mechanically Exfoliated Material Head Assay
  • Test A conditions incorporated caustic baking, caustic leaching, HF leaching, two stages of H2SO4 leaching and several stages of residue rinsing.
  • the caustic baking step was dropped in Test B, whereas the HF leaching step was removed in Test C.
  • the results of the trials are summarised Table 8 below showing that high purity material can be produced from dry milled mechanically exfoliated HSA graphite feedstock under a range of purification conditions.
  • Table 9 [00222] The impure HSA material was subjected to purification incorporating caustic baking, caustic leaching, HF leaching, two stages of H2SO4 leaching and several stages of residue rinsing with key process conditions summarised in Table 10 below.
  • Table 10 Purification Parameters [00223] The results of the trial are summarised in the Table 11 below showing that high purity material can be produced mechanically exfoliated HSA graphite feedstock produced from beneficiation methods, under purification conditions identified in the present invention.
  • Table 11 HSA Beneficiated Concentrated - Purification Summary

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Abstract

The present invention relates to a process for the production of a purified high surface area (HSA) graphite material, the process comprising treating an impure graphite concentrate in a mechanical exfoliation step to produce an impure high surface area (HSA) material; and treating the impure HSA material in a purification process to produce a purified HSA graphite material.

Description

Process for the Production of High Surface Area Graphite Field of the Invention [0001] The present invention relates to a process for the production of purified high surface area (HSA) graphite materials. In particular, the process of the present invention produces purified HSA graphite materials from impure graphite concentrates. [0002] The present invention further relates to the use of the purified HSA graphite material in the production of anode and cathode materials. In particular, the purified HSA graphite material is used as an additive for anode and cathode materials typically having a purity requirement of >99.5%w/w LOI depending on the application. Background Art [0003] Graphite has a layered crystal structure consisting of stacks of parallel two- dimensional graphene sheets. Each graphene sheet has a two-dimensional hexagonal lattice of carbon atoms covalently bound. The graphene layers are held together by van der Waals-forces. HSA graphite materials contain a mixture of single layer graphene particles and multi-layer graphite particles, having only a few layers of graphene. HSA graphite materials possess unique mechanical, thermal, optical and electrical properties. These materials have the potential for use in a wide range of applications, including battery components. [0004] Many different methods to prepare HSA graphite materials from graphite have been proposed. In most prior art methods, the graphite starting material is treated with an expandable intercalation agent, followed by thermally expanding the intercalant to exfoliate flakes of HSA graphite materials. The resulting material is then typically treated in a mechanical grinding process for further flake separation and size reduction. In some methods, the graphite starting material is subjected to a fine mechanical grinding process to produce HSA graphite materials. [0005] In typical production methods, the graphite starting material has a high graphite purity, typically >99% Cg. Impure graphite sources, such as graphite ores or graphite waste materials, require chemical purification to remove impurities prior to the production of HSA graphite materials. Even with a high starting purity, the processing steps used to prepare the HSA graphite materials have been found to result in the introduction of impurities into the produced HSA graphite material. For example, residues from the grinding media may be introduced or the exfoliation process may introduce other compounds or reaction products. The HSA graphite material must then be subjected to further purification to remove these impurities. The preparation of HSA graphite materials from impure graphite sources therefore requires an initial purification process before processing and a subsequent purification process to remove any additional impurities introduced by the processing. [0006] The purified HSA graphite material production process and the purified HSA graphite material product of the present invention have as one object thereof to overcome substantially one or more of the abovementioned problems associated with prior art processes, or to at least provide a useful alternative thereto. [0007] The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. This discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application. [0008] Throughout the specification and claims, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. [0009] It is to be understood that the ranges provided herein include the stated range and any value or sub-range within the stated range. For example, a range from about 1 micrometer (μm) to about 2 μm should be interpreted to include not only the explicitly recited limits of from between from about 1 μm to about 2 μm, but also to include individual values, such as about 1.2 μm, about 1.5 μm, about 1.8 μm, etc., and sub-ranges, such as from about 1.1 μm to about 1.9 μm, from about 1.25 μm to about 1.75 μm, etc. Furthermore, when “about” and/or “substantially” are/is utilised to describe a value, they are meant to encompass minor variations (up to +/- 10%) from the stated value. Still further, when reference is made to a “trace amount” of something it is a reference to a concentration of less than about 100 micrograms per gram, this corresponds to less than 100 ppm in analytical terms. [0010] It is to be further understood that references to the % recovery or % removal of an element or mineral, or similar, are, unless the context demands otherwise, a reference to the % of that component recovered or removed relative to the original content of the feed to the described process. Other references to % are, again as long as the context does not require otherwise, to be taken as references to weight or wt % (rather than volume or v %). Disclosure of the Invention [0011] In accordance with a first aspect of the present invention there is provided a process for the production of a purified high surface area (HSA) graphite material, the process comprising: treating an impure graphite concentrate in a mechanical exfoliation step to produce an impure high surface area (HSA) material; and treating the impure HSA material in a purification process to produce a purified HSA graphite material. [0012] In one form of the present invention, the impure graphite concentrate is recovered from a natural graphitic ore. [0013] In one form of the present invention, the impure graphite concentrate is recovered from an impure synthetic graphite material. [0014] In one form of the present invention, the impure graphite concentrate is recovered from an impure recycled graphite material. Preferably, the impure recycled graphite material is a waste battery material or a scrap anode material. [0015] In one form of the present invention, the impure graphite concentrate is subjected to one or more pre-treatment steps prior to the mechanical exfoliation step. The one or more pre-treatment steps may be selected from size reduction, size classification, physical separation, chemical separation, drying, heat treatment or combinations thereof. Suitable physical separation steps include gravity separation, flotation, and magnetic separation. Suitable chemical separation steps include froth flotation and chemical leaching. [0016] In one form of the present invention, the impure graphite concentrate has a carbon concentration of less than 96%. As would be appreciated by a person skilled in the art, such impure graphite materials are unsuitable for direct use in battery and graphene applications owing to their purity level, particle size and surface area properties. [0017] In one form of the present invention, the impure graphite concentrate has a carbon concentration of between 60 and 96%. Preferably, the impure graphite concentrate has a carbon concentration of between 75 and 96%. Impurities encountered in concentrate prepared from natural graphite sources may include silicate gangue, base-metal sulphide and oxide minerals, and various carbonate mineral phases. The type and quantity of these depends on the original graphite ore mineralogy and host rock geology. [0018] In one form of the present invention, the mechanical exfoliation step is carried out in a grinding apparatus. Preferably, the grinding apparatus imparts a shearing force on the impure graphite concentrate. [0019] In one form of the present invention, the mechanical exfoliation step is a dry mechanical exfoliation step. In this context, a dry mechanical exfoliation step refers to a mechanical exfoliation step carried out in the absence of a liquid medium. Suitable dry mechanical exfoliation processes include jet milling. [0020] In an alternative form of the present invention, the mechanical exfoliation step is conducted in the presence of a liquid medium. [0021] In one form of the present invention, the mechanical exfoliation step comprises: pulping of the impure graphite concentrate with a liquid medium to prepare an impure graphite slurry; and subjecting the impure graphite slurry to a mechanical exfoliation step. [0022] In one form of the present invention, the specific milling energy of the mechanical exfoliation step is greater than 50 kWh/t. [0023] In forms of the invention where the mechanical exfoliation step is conducted in the presence of a liquid medium, the mechanical exfoliation step will produce a slurry containing an impure HSA material dispersed in the liquid medium. Preferably, the slurry is subjected to a dewatering step to remove at least a portion of the liquid medium. [0024] In one form of the present invention, the impure HSA material is treated in a drying step. In one form of the present invention, the drying step is an air drying step. In an alternative form of the present invention, the drying step is a cryogenic drying step. [0025] In one form of the present invention, the impure HSA material is treated in a beneficiation step prior to being treated in the purification process. Suitable beneficiation processes include size classification, liquid-liquid separation, gravity separation, flotation, froth flotation and magnetic separation. [0026] In one form of the present invention, the graphitic particles of the impure HSA material have a surface area of greater than 10 m2/g. In one form of the present invention, the graphitic particles of the impure HSA material have a surface area of greater than 20 m2/g. In one form of the present invention, the graphitic particles of the impure HSA material have a surface area in the range of 20 to 40 m2/g. In one form of the present invention, the graphitic particles of the impure HSA material have a surface area in the range of 25 to 35 m2/g. In one form of the present invention, the graphitic particles of the impure HSA material have a surface area of greater than 40 m2/g. In one form of the present invention, the graphitic particles of the impure HSA material have a surface area in the range of 40 to 80 m2/g. In one form of the present invention, the graphitic particles of the impure HSA material have a surface area in the range of 40 to 50 m2/g. [0027] In one form of the present invention, the particle size distribution of the impure HSA material is such that the D50 is lower than 30 µm. In one form of the present invention, the particle size distribution of the impure HSA material is such that the D50 is lower than 20 µm. In one form of the present invention, the particle size distribution of the impure HSA material is such that the D50 is between 10 and 20 µm. In one form of the present invention, the particle size distribution of the impure HSA material is such that the D50 is between 10 and 15 µm. [0028] In one form of the present invention, the purification process produces a purified HSA graphite material with a carbon concentration of at least 99% cg. Preferably, the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.9% cg. [0029] In one form of the present invention, the purification process comprises one or more impurity removal steps selected from: acid leach steps, caustic leach steps, oxidative leach steps, caustic bake steps or combinations thereof. [0030] In a one form of the present invention, the purification process comprises one or more caustic impurity removal steps, followed by one or more acid leach steps. In one form of the present invention, the purification process comprises: treating the impure HSA material in a caustic impurity removal step, and recovering a solid caustic leach product; and treating the solid caustic leach product in an acid leach step, and recovering a solid acid leach product, wherein the solid acid leach product comprises a purified HSA graphite material. [0031] In one form of the present invention, the solid acid leach product is subjected to a washing step to recover the purified HSA graphite material. Preferably, the washing step will remove soluble impurities from the purified HSA graphite material. [0032] In one form of the present invention, the caustic impurity removal step comprises a caustic leach step. Preferably, the caustic leach step comprises the contact of the impure HSA material with a sodium hydroxide solution, and recovering a solid caustic leach product. [0033] In an alternative form of the present invention, the caustic impurity removal step comprises a caustic bake step. Preferably, the caustic bake step comprises baking the impure HSA material under alkaline conditions, subjecting the product to a water leach, and recovering a solid caustic leach product. [0034] In one form of the present invention, the caustic impurity removal step comprises a caustic bake step followed by a caustic leach step. [0035] In one form of the present invention, the acid leach step comprises one or more separate acid leach stages. Preferably, each acid leach stage comprises the contact of the solid caustic leach product with an acidic solution, thereby forming an acidic leach slurry and recovering a solid acid leach product from the acidic leach slurry. It is envisaged that different acidic solutions may be used at different acid leach stages to target different impurities remaining in the solid caustic leach product. [0036] In one form of the present invention, the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution. Preferably, the acid leach step comprises two or more acid leach stages, where at least one of the acid leach stages comprises the contact of the solid caustic leach product with a hydrofluoric acid solution. [0037] In embodiments where the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution, the acid leach step preferably comprises: treating the solid caustic leach product in a first sulphuric acid leach stage comprising the contact of the solid caustic leach product with a sulphuric acid solution and separating undissolved leach solids; treating the leach solids of the first sulphuric acid leach stage in a hydrofluoric acid leach stage comprising the contact of the leach solids with a hydrofluoric acid solution and separating undissolved leach solids; treating the leach solids of the hydrofluoric acid leach stage in a second sulphuric acid leach stage comprising the contact of the leach solids with a sulphuric acid solution and separating undissolved leach solids; and treating the leach solids of the second sulphuric acid leach stage to one or more washing stages, and recovering a solid acid leach product comprising a purified graphite material. [0038] In embodiments where the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution, the caustic impurity removal step comprises at least one of a caustic bake step or a caustic leach step conducted at a temperature of at least 200°C. [0039] In accordance with an alternative embodiment of the present invention, the acid leach step does not comprise the contact of the solid caustic leach product with a hydrofluoric acid solution. In this embodiment, the acid leach step preferably only comprises the contact of the solid caustic leach product with an acidic solution selected from a hydrochloric acid solution and a sulphuric acid solution. More preferably, the acid leach step only comprises the contact of the solid caustic leach product with a hydrochloric acid solution. [0040] In embodiments where the acid leach step does not comprise the contact of the solid caustic leach product with a hydrofluoric acid solution, the process preferably comprises treating the impure HSA material in an oxidative leach step. More preferably, the impure HSA material is treated in an oxidative leach step prior to the caustic leach step. [0041] In one form of the present invention, the oxidative leach step comprises a first oxidative leach stage comprising the contact of the impure HSA material with an oxidant solution to form a first oxidative leach slurry. At least one of sodium hypochlorite or sodium chlorate is utilised as the oxidant in the first oxidative leach stage. [0042] In one form of the present invention, the oxidative leach step further comprises a second oxidative leach stage. Preferably, the second oxidative leach stage comprises the contact of the first oxidative leach slurry with a hydrochloric acid solution. [0043] Following the completion of the second oxidative leach stage, the leach slurry is subjected to a solid liquid separation step to recover an oxidative leach residue. In this embodiment, the oxidative leach residue is subjected to the caustic leach step as discussed above. [0044] In this embodiment of the present invention, the solid caustic leach product recovered from the caustic leach step is subjected to a secondary caustic leach step, prior to the acid leach step. Preferably, the secondary caustic leach step is operated at same conditions as the caustic leach step. [0045] In an alternative form of the present invention, the purification process comprises one or more acid leach steps. In this form of the present invention, the purification process comprises: treating the impure HSA material in an acid leach step, and recovering a solid acid leach product, wherein the solid acid leach product comprises a purified HSA graphite material. [0046] In one form of the present invention, the acid leach step comprises one or more separate acid leach stages. Preferably, each acid leach stage comprises the contact of the impure HSA material with an acidic solution, thereby forming an acidic leach slurry and recovering a solid acid leach product from the acidic leach slurry. It is envisaged that different acidic solutions may be used at different acid leach stages to target different impurities remaining in the impure HSA material. Preferably, the solid leach product from each acid leach stage is washed before advancing to downstream acid leach stages or further processing steps. It is understood that washing may minimise soluble impurity carry over and help reduce reagent consumption. [0047] In one form of the present invention, the acid leach step comprises the contact of the impure HSA material with a hydrofluoric acid solution. Preferably, the acid leach step comprises two or more acid leach stages, where at least one of the acid leach stages comprises the contact of the impure HSA material with a hydrofluoric acid solution. [0048] In embodiments where the acid leach step comprises the contact of the impure HSA material with a hydrofluoric acid solution, the acid leach step preferably comprises: treating the impure HSA material in a first sulphuric acid leach stage comprising the contact of the impure HSA material with a sulphuric acid solution and separating undissolved leach solids; treating the leach solids of the first sulphuric acid leach stage in a hydrofluoric acid leach stage comprising the contact of the leach solids with a hydrofluoric acid solution and separating undissolved leach solids; treating the leach solids of the hydrofluoric acid leach stage in a second sulphuric acid leach stage comprising the contact of the leach solids with a sulphuric acid solution and separating undissolved leach solids; and treating the leach solids of the second sulphuric acid leach stage to one or more washing stages, and recovering a solid acid leach product comprising a purified graphite material. [0049] Preferably, the undissolved leach solids from the first sulphuric acid leach stage are washed prior to advancing to the hydrofluoric acid leach stage. Preferably, the undissolved leach solids from the hydrofluoric acid leach stage are washed prior to advancing to the second sulphuric acid leach stage. [0050] In some embodiments where the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution, the purification process does not comprise any caustic impurity removal steps. [0051] Preferably, the process further comprises a drying step in which the purified HSA graphite material is dried, providing a dried purified HSA graphite material. The dried purified HSA graphite material preferably contains between 0 to 2.5% moisture, for example, less than about 1% and further preferably 0.1% moisture. In one form of the invention, the purified HSA graphite material will preferably contain about 40% moisture prior to the drying step. [0052] Preferably, the dried purified HSA graphite material has a pH of 7 ± 2.5. [0053] Preferably, the carbon as graphite content of the dried purified HSA graphite material is greater than 99.5% w/w. [0054] In one form of the present invention, the graphitic particles of the purified HSA graphite material have a surface area of greater than 20 m2/g. In one form of the present invention, the graphitic particles of the purified HSA graphite material have a surface area in the range of 20 to 40 m2/g. In one form of the present invention, the graphitic particles of the purified HSA graphite material have a surface area in the range of 25 to 35 m2/g. In one form of the present invention, the graphitic particles of the purified HSA graphite material have a surface area of greater than 40 m2/g. In one form of the present invention, the graphitic particles of the purified HSA graphite material have a surface area in the range of 40 to 80 m2/g. In one form of the present invention, the graphitic particles of the purified HSA graphite material have a surface area in the range of 40 to 50 m2/g. [0055] In one form of the present invention, the particle size distribution of the purified HSA graphite material is such that the D50 is lower than 30 µm. In one form of the present invention, the particle size distribution of the purified HSA graphite material is such that the D50 is lower than 20 µm. [0056] In one form of the present invention, the dried purified HSA graphite material is classified in or after the drying step. The dried purified HSA graphite material is preferably classified in the drying step into multiple products having different particle size and surface area properties, for example into at least two fractions. In one form the classification utilises a dry powder classification method, for example cyclone classification. [0057] In accordance with the present invention there is further provided a purified HSA graphite material produced in accordance with the process described hereinabove. [0058] In one form of the present invention, the process further comprises the step of processing the purified HSA graphite material to prepare an electrode material. Preferably, the purified HSA graphite material is used as an additive in an electrode material. Those skilled in the art would be aware of electrode production processes that incorporate purified HSA graphite materials. For example, the Applicant’s International Patent Application PCT/IB2020/056050 (WO 2020/261194), the entire content of which is incorporated herein by reference, describes a silicon and graphite containing composite material comprising a plurality of silicon nanoparticles coated with graphite particles, few-layer graphene particles, graphite nanoparticles, a carbon matrix, and an amorphous carbon external shell, wherein each of the graphite particle coated silicon nanoparticles, the few-layer graphene particles, and the graphite nanoparticles are held within the carbon matrix. Brief Description of the Drawings [0059] The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawing, in which:- Figure 1 is a schematic representation of a flow chart depicting a process for the production of a purified HSA graphite material in accordance with a first embodiment of the present invention; Figure 2 is a schematic representation of a flow chart depicting a process for the production of a purified HSA graphite material in accordance with a second embodiment of the present invention; Figure 3 is a schematic representation of a flow chart depicting a process for the production of a purified HSA graphite material in accordance with a third embodiment of the present invention; and Figure 4 shows the results of trial comparing the milling energy required to produce HSA graphite materials. Best Mode(s) for Carrying Out the Invention [0060] The process of the present invention is used to produce purified HSA graphite materials from impure graphite concentrates. The process comprises the general steps of: treating an impure graphite concentrate in a mechanical exfoliation step to produce an impure HSA material; and treating the impure HSA material in purification process to produce a purified HSA graphite material. [0061] The mechanical exfoliation step of the present invention is used to exfoliate HSA graphite particles from the impure graphite concentrate. As the impure graphite concentrate is treated directly in the mechanical exfoliation step, the impurities contained within the impure graphite concentrate are also subjected to the mechanical exfoliation step. The impure HSA material produced in the mechanical exfoliation step therefore comprises a mixture of HSA graphitic particles and impurity particles. The impure HSA material is then subjected to a purification process to remove these impurities. The inventors have found that the direct treatment of the impure graphite concentrate in the mechanical exfoliation process can offer certain advantages over processes that first purify impure graphite concentrates before mechanical exfoliation. In some instances, the mechanical exfoliation process itself can introduce impurities into the produced HSA graphite material. The process of the present invention allows the impurities of the impure graphite concentrate and any impurities introduced in the mechanical exfoliation process to be removed simultaneously in a single purification process. In some instances, the mechanical exfoliation process will liberate impurities disposed within the impure graphite concentrate. The liberation of such particles prior to purification can improve the efficiency of the purification process and/or increase the overall carbon content of the purified HSA graphite material. In some instances, the overall efficiency of the mechanical exfoliation step may be improved by treating an impure graphite concentrate instead of a purified graphite concentrate. Without wishing to be bound by theory, the inventors believe that some impurities may impart shearing forces on the graphite particles, therefore contributing to the mechanical exfoliation. [0062] Throughout the specification and claims, unless the context requires otherwise, impure graphite concentrates, will be understood to refer to graphite-containing materials that contain impurities. The impure graphite concentrate may be recovered from an impure graphite precursor. The impure graphite precursor may be subjected to one or more concentration steps to produce the impure graphite concentrate. [0063] In one embodiment, the impure graphite concentrate may be recovered from an impure natural graphite precursor. Impure natural graphite precursors include natural graphitic ores. Graphitic ores contain a certain amount of other mineral phases as impurities. Common impurities found in graphite mainly include potassium, sodium, aluminium, calcium, magnesium and other silicate minerals. The process of the present invention is intended to remove a substantial amount of these impurities to allow for the recovery of a purified HSA graphite material. [0064] In one embodiment, the impure graphite concentrate may be recovered from an impure synthetic graphite precursor. [0065] In one embodiment, the impure graphite concentrate may be recovered from an impure recycled graphite precursor. Impure recycled graphite precursors include graphite- containing materials recovered from recycled, scrap, spent or discarded materials. The graphite materials contained in the impure recycled graphite precursor contain natural or synthetic graphite. [0066] In one embodiment, the impure graphite concentrate is recovered from waste battery materials. In one embodiment, the impure graphite concentrate is recovered from waste lithium-ion battery materials. Batteries, including lithium-ion batteries, contain anode materials that have a high graphite content, together with cathode material, electrolyte and separator material. The graphite contained in the waste battery materials may be a synthetic graphite or a natural graphite. The batteries may be subjected to a physical separation method to separate the anode materials from the other components. The physical separation method may include dismantling, crushing, screening, and other mechanical processes. The recovered anode material is suitable for use as the impure graphite concentrate of the present invention. Alternatively, the impure graphite concentrate may be recovered from battery black mass. Black mass refers to a concentrated mixture of valuable metals that is recovered from waste batteries. Black mass is typically prepared by shredding waste batteries, followed by the beneficiation of the shredded material to remove unwanted components such as casings, electrolyte materials and plastic wrappers. The black mass contains a mixture of anode and cathode materials. The black mass may first be treated in a hydrometallurgical process to recover valuable metals such as lithium, manganese, cobalt and nickel. Such hydrometallurgical processes typically involve the leaching of these valuable metals in an aqueous solution. The leach residues typically have a high concentration of graphite and may be used as the impure graphite concentrate of the present invention. It is further envisaged that the products resulting from the pyrometallurgical treatment of the waste battery materials may be used as the impure graphite concentrate of the present invention. [0067] In an alternative embodiment, the impure graphite concentrate is recovered from scrap anode materials. In this context, scrap anode materials refer to scraps or waste materials generated during the manufacture of battery components, including lithium-ion battery components. Such anode scraps comprise the graphite anode film adhered to a current collector foil with a polyvinylidene fluoride binder. The anode scraps have a high concentration of graphite and may be used as the impure graphite concentrate of the present invention. The graphite contained in the scrap anode material may be a synthetic graphite or a natural graphite. Preferably, the scrap anode materials are subjected to one or more graphite concentration steps. In one form of the present invention, the graphite concentration steps comprise the separation of anode materials comprising a recycled graphite concentrate from waste electrode foils. Preferably, the graphite concentration steps comprise communition of the scrap anode materials and classification of the comminuted material. In one form of the present invention, the graphite concentration steps comprise a heat treatment step. Preferably, the heat treatment step will remove carbon additives and/or binders from the scrap anode materials. [0068] Graphite materials found within waste battery materials and scrap anode material contain a certain amount of organic and inorganic impurities, mainly including organic binders, solid electrolyte interphase films and current collectors. Graphite materials recovered from waste battery materials also contain residual electrolytes, intercalated ions and metals from other battery components. The process of the present invention is intended to remove a substantial amount of these impurities to allow for the recovery of a purified HSA graphite material. [0069] In one embodiment, the impure graphite concentrate has a carbon content of less than 96% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 95% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 94% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 93% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 92% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 91% Cg. In one embodiment, the impure graphite concentrate has a carbon content of less than 90% Cg. [0070] In one embodiment, the impure graphite concentrate has a carbon content of at least 60% Cg. In one embodiment, the impure graphite concentrate has a carbon content of at least 65% Cg. In one embodiment, the impure graphite concentrate has a carbon content of at least 70% Cg. In one embodiment, the impure graphite concentrate has a carbon content of at least 75% Cg. [0071] In one embodiment, the impure graphite concentrate has a carbon content of between 60 and 96% Cg. In one embodiment, the impure graphite concentrate has a carbon content of between 65 and 96% Cg. In one embodiment, the impure graphite concentrate has a carbon content of between 70 and 96% Cg. In one embodiment, the impure graphite concentrate has a carbon content of between 75 and 96% Cg. [0072] In one embodiment, the particle size of the impure graphite concentrate has a D50 of less than about 30 microns. In one embodiment, the particle size of the impure graphite concentrate has a D50 of less than about 15 microns. [0073] In one embodiment, the particle size of the impure graphite concentrate has a D50 of between about 5 to 30 microns. In one embodiment, the particle size of the impure graphite concentrate has a D50 of between about 5 to 15 microns. [0074] In one embodiment of the present invention, the impure graphite concentrate is subjected to one or more pre-treatment steps prior to the mechanical exfoliation step. The one or more pre-treatment steps may be selected from size reduction, size classification, physical separation, chemical separation, drying, heat treatment or combinations thereof. Suitable physical separation steps include gravity separation, flotation, and magnetic separation. Suitable chemical separation steps include froth flotation and chemical leaching. Mechanical Exfoliation [0075] The mechanical exfoliation step is used to exfoliate HSA graphite particles from the impure graphite concentrate. [0076] The mechanical exfoliation step is conducted in a grinding apparatus. Throughout the specification and claims, unless the context requires otherwise, grinding apparatus, will be understood to refer to an apparatus that relies on mechanical forces to reduce the particle size of the impure graphite concentrate. Any grinding apparatus known in the art to be suitable to exfoliate HSA graphite particles from the impure graphite concentrate may be used. It is preferred that the grinding apparatus imparts a substantial shearing force on the impure graphite concentrate. Those skilled in the art would be able to readily determine whether a particular grinding apparatus known in the art would be suitable for this purpose. Suitable grinding apparatus include, bead mills, ball mills, rod mills, jet mills, hammer mills, attrition mills vibratory mills and rotating disc mills. [0077] In a preferred embodiment of the present invention, the grinding apparatus includes an agitated grinding media. Such grinding media is typically in the form of multiple grinding media ‘beads’ that are held within a vessel. A rotor is then used to generate bead motion within the vessel. Such grinding apparatus rely on the motion of the grinding media to impart a mechanical shear force on the feed material. The size of the grinding media, the agitation speed and the grinding time may be used to influence the properties of the impure HSA material produced. Suitable grinding apparatus that include an agitated grinding media include, for example, bead mills and ball mills. Alternatively, the grinding apparatus may include one or more rotating discs. [0078] In one embodiment, the diameter of the grinding media is between 0.1 and 3 mm. In one embodiment, the diameter of the grinding media is between 0.1 and 1 mm. As would be appreciated by a person skilled in the art, the size of the beads required may be calculated from the particle size of the raw material and the intended final particle size of the ground material. Generally, the size of the beads required for milling particles is 10 to 30 times the maximum particle size of the raw material and 1,000 to 3,000 times the mean particle size after milling. While smaller bead size will generally allow better energy efficiency, hence lower energy to reach the same size/surface area, this should be balanced with the higher cost of small size beads. [0079] In one embodiment, the mechanical exfoliation step is a dry mechanical exfoliation step. In a preferred form of the present invention, the mechanical exfoliation step is conducted in the presence of a liquid medium. In one embodiment, the liquid medium is an aqueous solution, preferably deionised water. In an alternative embodiment, the liquid medium is an organic medium. Example organic liquids include alcohols, ketones, hydrocarbons or solvents. [0080] Preferably, the impure graphite concentrate is pulped with the liquid medium to prepare an impure graphite slurry and the impure graphite slurry is treated in the mechanical exfoliation step. [0081] Those skilled in the art would recognise that increasing the solid content will increase the grinding time. In one embodiment, the pulp density of the impure graphite slurry is at least 10 wt%. In one embodiment, the pulp density of the impure graphite slurry is at least 15 wt%. [0082] In one embodiment, the pulp density of the impure graphite slurry is between 10 and 50 wt%. In one embodiment, the pulp density of the impure graphite slurry is between 10 and 30 wt%. In one embodiment, the pulp density of the impure graphite slurry is between 15 and 20 wt%. [0083] The liquid medium may include one or more additives, such as surfactants and dispersing agents, to assist with the exfoliation of the impure graphite concentrate. [0084] In one embodiment, the mechanical exfoliation step is conducted at a temperature below 100°C. Preferably, the mechanical exfoliation step is conducted at ambient temperature. [0085] Those skilled in the art would appreciate that the surface area of the impure HSA material is directly related to the energy input of the mechanical exfoliation step. Higher energy inputs will result in particles having a greater surface area. The energy may then be used to control the surface area of the impure HSA material. In one embodiment, the specific milling energy of the mechanical exfoliation step is greater than 50 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is greater than 100 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is greater than 200 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 50 to 400 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 100 to 400 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 50 to 500 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 100 to 500 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 200 to 500 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is greater than 400 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 400 to 500 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is greater than 700 kWh/t. In one embodiment, the specific milling energy of the mechanical exfoliation step is in the range of 700 to 1200 kWh/t. In one embodiment, the mechanical exfoliation step is conducted in the range of 1000 to 1200 kWh/t. Throughout this specification, the specific milling energy (kWh/t) is expressed in terms of milling material weight. In embodiments where the mechanical exfoliation step is conducted in the presence of a liquid medium, the specific milling energy (kWh/t) is expressed in terms of the milling material weight at 15 wt% solids. [0086] In one embodiment, the mechanical exfoliation step may comprise multiple grinding stages, preferably with beneficial treatment steps being conducted between successive grinding stages. It is envisaged that the impure HSA material may be generated in the course of the beneficiation of graphite materials. The graphite material may first be treated in a rough grinding stage followed by a first beneficiation stage. The resulting concentrate may be subjected to a fine grinding stage followed by a second beneficiation stage to produce the impure HSA material. Suitable beneficiation processes include size classification, liquid-liquid separation, gravity separation, flotation, froth flotation and magnetic separation. [0087] In embodiments where the mechanical exfoliation step is conducted in the presence of a liquid medium, the mechanical exfoliation step will produce a slurry containing an impure HSA material dispersed in the liquid medium. Preferably, the slurry is subjected to a dewatering step to remove at least a portion of the liquid medium. [0088] The impure HSA material may be further treated in a drying step to remove substantially all the residual liquid medium. Any suitable drying method may be used, for example, an air-drying step conducted at ambient temperature or above-ambient temperature. In one embodiment of the present invention, a cryogenic drying step may be used. Cryogenic drying methods include the freezing of liquids present in the impure HSA material and lowering the pressure to remove the ice by sublimation. These methods may be used to substantially prevent the agglomeration of any HSA particles. An example of suitable process conditions includes the freezing of the impure HSA material, followed by subjecting the frozen material to: (i) <6 mbar vacuum, >0 °C drying temperature, and condenser temperature of <60- 70 °C; or (ii) <1mbar vacuum, >30-40 °C drying temperature, and condenser temperature of 60–70 °C. [0089] It should be noted that the inclusion of a drying step is not essential. In an alternative embodiment, the impure HSA material recovered from the dewatering step may be treated directly in the purification process without an intermediate drying step. [0090] In one form of the present invention, the impure HSA material is treated in a beneficiation step prior to being treated in the purification process. The inventors have identified that the fine grinding of the mechanical exfoliation step can result in the production of impurity-containing fines. These fines can be removed using beneficiation techniques to remove these impurities prior to the purification process. Suitable beneficiation processes include size classification, liquid-liquid separation, gravity separation, flotation, froth flotation and magnetic separation. Purification Process [0091] The impure HSA material comprises a mixture of graphite particles and impurity particles. These impurities include those present in the impure graphite concentrate, together with any impurities introduced in the mechanical exfoliation step. Such introduced impurities may include, for example, residues from the grinding media itself. The purification process of the present invention is used to remove a substantial portion of the impurities present in the impure HSA material, thereby producing a purified HSA graphite material. [0092] In one embodiment, the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.0% Cg. In one embodiment, the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.5% Cg. In one embodiment, the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.9% Cg. In one embodiment, the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.92% Cg. In one embodiment, the purification process produces a purified HSA graphite material with a carbon concentration of at least 99.95% Cg. [0093] The purification process may include one or more separate impurity removal steps. These impurity removal steps may be selected from one or more acid leach steps, caustic leach steps, oxidative leach steps, caustic bake steps or combinations thereof. The impurity removal steps used will depend largely on the impurities present in the impure graphite concentrate. [0094] In one embodiment, the purification process comprises one or more caustic impurity removal steps, followed by one or more acid leach steps. In this embodiment, the process comprises the steps of: treating the impure HSA material in a caustic impurity removal step, and recovering a solid caustic leach product; and treating the solid caustic leach product in an acid leach step, and recovering a solid acid leach product, wherein the solid acid leach product comprises a purified HSA graphite material. Caustic Leach Step [0095] In one embodiment of the present invention, the caustic impurity removal step comprises a caustic leach step. Preferably, the caustic leach step comprises the contact of the impure HSA material with a sodium hydroxide solution, and recovering a solid caustic leach product. [0096] Preferably, the sodium hydroxide solution is a concentrated sodium hydroxide solution. In one embodiment, the concentration of the sodium hydroxide solution is at least 100 g/L. In one embodiment, the concentration of the sodium hydroxide solution is at least 200 g/L. In one embodiment, the concentration of the sodium hydroxide solution is at least 400 g/L. In one embodiment, the concentration of the sodium hydroxide solution is at least 600 g/L. In one embodiment, the concentration of the sodium hydroxide solution is at least 800 g/L. [0097] Preferably, the caustic leach step is conducted at a temperature of at least 50°C. In one embodiment of the present invention, the caustic leach step is conducted at a temperature between about 50 to 200°C. More preferably, the caustic leach step is conducted at a temperature of at least 100°C. In one embodiment of the present invention, the caustic leach step is conducted at a temperature between about 100 to 200°C. In an alternative embodiment, the caustic leach step is conducted at a temperature of at least 200°C. Preferably, the caustic leach step is conducted at a temperature of at least 220°C. The inventors have found that higher temperature caustic leach steps may be required to solubilise certain impurities, for example, crystalline Al2O3, should they be present in the impure HSA material. [0098] In one form of the present invention, the starting slurry density of the caustic leach step is 150 to 250 g/L solids. [0099] In one form of the present invention, the residence time of the caustic leach step is 2 to 16 hours. [00100] In one form of the present invention, the caustic leach step is conducted at a pressure of at least 2 bar. [00101] In one form of the present invention, the caustic leach step comprises a two- stage leach. Preferably, both stages are operated at a temperature of at least 100°C and elevated pressure. More preferably, the two-stage leach operates in a counter-current manner. [00102] In one form of the present invention, the caustic leach step comprises the contact of the impure HSA material with a sodium hydroxide solution at a temperature of at least 100°C and elevated pressure, to form a caustic leach slurry and recovering a solid caustic leach product from the caustic leach slurry. Preferably, the caustic leach slurry is subjected to a solid liquid separation step to recover a solid caustic leach product and a caustic leachate solution. In one form of the present invention, the solid caustic leach product is treated in one or more re-pulp stages and/or washing stages. Preferably, the solid caustic leach product is re-pulped with a sodium hydroxide solution. In one form of the present invention, the solid caustic leach product is subjected to a drying step. [00103] In one form of the present invention, the solid caustic leach product from the caustic leach step is subjected to a secondary caustic leach step prior to the acid leach step. The secondary caustic leach step may be used to remove further impurities that may be left in the solid caustic leach product. Caustic Bake Step [00104] In an alternative form of the present invention, the caustic impurity removal step comprises a caustic bake step. Preferably, the caustic bake step comprises baking the impure HSA material in caustic conditions and subjecting the product to a water leach step, and recovering a solid caustic leach product. The caustic bake step causes the caustic soda and impurities, in particular silicon impurities, to react, rendering these impurities soluble in water and mild acid conditions. [00105] In one embodiment, the caustic bake step comprises: preparing a mixture of the impure graphite concentrate and caustic soda; subjecting the mixture to a baking step at a temperature between 150°C and 300°C to prepare a product; and subjecting the product to a water leach step. [00106] Preferably, the amount of caustic soda in the mixture is based on the silicon content of the impure graphite concentrate. In a preferred embodiment, the molar ratio of caustic to silicon is at least 1 : 1. In one embodiment, the molar ratio of caustic to silicon between 2.5-5.5 : 1. In one embodiment, the molar ratio of caustic to silicon is 3.2 : 1. [00107] In a preferred embodiment, the caustic bake step is undertaken in a rotating kiln. [00108] The water leach step is preferably undertaken at between about 5-60°C, for example about 35°C±5°C. Preferably, the water leach step is undertaken in a single stage. In certain circumstances, the water leach step may be undertaken in multiple, for example three, counter-current leach stages. Still further preferably, the water leach step has a retention time of between about 30 to 240 minutes. [00109] In an alternative form of the present invention, the caustic impurity removal step comprises a caustic bake step followed by a caustic leach step. Preferably, the caustic impurity removal step comprises baking the impure HSA material in caustic conditions, subjecting the product to a caustic leach step, and recovering a solid caustic leach product. The above discussion of the caustic bake step and the caustic leach step equally applies to the combined caustic bake step and caustic leach step. The inventors have found that the inclusion of a caustic bake step prior to the caustic leach step can increase the overall amount of impurities removed from the impure HSA material. Preferably, the caustic bake step is included when treating recycled graphite concentrates that contain refractory aluminium oxide. Without wishing to be bound by theory, it is understood that the combination of the caustic bake step and the caustic leach step will render the refractory aluminium oxide phases more amenable to leaching in the downstream acid leach step. This increases the amount of aluminium removed from the impure HSA material when compared to the caustic bake step or the caustic leach step in isolation. Furthermore, it has been found that the inclusion of the caustic bake step will reduce the valence of certain metals in the impure HSA material, such as cobalt and manganese, assisting in their subsequent leaching in the downstream acid leach steps. The inclusion of a caustic bake step has also been found to assist in the removal of fluorine components found in the binders and plastics of waste battery materials. It should be understood that the combined use of both the caustic bake step and the caustic leach step is not essential to the purification of the impure HSA material, but may improve the purity of the graphite recovered from certain recycled graphite feeds. Acid Leach Step [00110] In one form of the present invention, the acid leach step comprises one or more separate acid leach stages. Preferably, each acid leach stage comprises the contact of the solid caustic leach product with an acidic solution, thereby forming an acidic leach slurry and recovering a solid acid leach product from the acidic leach slurry. It is envisaged that different acidic solutions may be used at different acid leach stages to target different impurities remaining in the solid caustic leach product. Acid Leach Step – HF Option [00111] In one form of the present invention, the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution. Preferably, the acid leach step comprises two or more acid leach stages, where at least one of the acid leach stages comprises the contact of the solid caustic leach product with a hydrofluoric acid solution. [00112] In embodiments where the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution, the acid leach step preferably comprises: treating the solid caustic leach product in a first sulphuric acid leach stage comprising the contact of the solid caustic leach product with a sulphuric acid solution and separating undissolved leach solids; treating the leach solids of the first sulphuric acid leach stage in a hydrofluoric acid leach stage comprising the contact of the leach solids with a hydrofluoric acid solution and separating undissolved leach solids; treating the leach solids of the hydrofluoric acid leach stage in a second sulphuric acid leach stage comprising the contact of the leach solids with a sulphuric acid solution and separating undissolved leach solids; and treating the leach solids of the second sulphuric acid leach stage to one or more washing stages, and recovering a solid acid leach product comprising a purified graphite material. The first sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ±5°C. Still preferably, the first sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes. [00113] Preferably, concentrated sulphuric acid is added in the first sulphuric acid leach stage. Still preferably, the residual free acid at the end of the first sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ±5 g/L H2SO4. The first sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids. [00114] The hydrofluoric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ±5°C. Preferably, the residual free acid at the end of the hydrofluoric acid leach stage is in the range of about 15-75 g/L HF, for example about 60 g/L ±5 g/L HF. The hydrofluoric acid leach stage operates with between about 5 to 25% solids, for example about 10% solids. [00115] Preferably, the hydrofluoric acid added to the hydrofluoric acid leach stage is in the range of about 20 to 70% concentration. The hydrofluoric acid concentration in the hydrofluoric acid leach stage is preferably in the range of about 15-50 g/L. [00116] Still preferably, the leach solids from the hydrofluoric acid leach stage have no remaining silicon therein, or only trace amounts thereof. [00117] The second sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ±5°C. The second sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids. [00118] Preferably, the second sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes. [00119] Preferably, concentrated sulphuric acid is added in the second sulphuric acid leach stage. Still preferably, the residual free acid at the end of the second sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ±5 g/L H2SO4. Acid solutions from the second sulphuric acid leach stage are preferably recovered and recycled to the first sulphuric acid leach stage and the second sulphuric acid leach stage. [00120] The washing stages preferably comprise a single repulp-filtration stage using deionised water, multiple counter-current repulp-filtration stages, or, for example, five multiple counter-current repulp-filtration stages, using deionised water. Preferably, the washing stages operate with between about 5 to 25% solids, for example about 10% solids using three stages of counter-current repulp-filtration stages. [00121] Preferably, liquid, residual salts, and/or acidity from the residual solids of the second sulphuric acid leach stage are recovered in the washing stages and returned to one or both of the first sulphuric acid leach stage and the second sulphuric acid leach stage. Acid Leach Step – Non-HF Option [00122] In accordance with an alternative embodiment of the present invention, the acid leach step does not comprise the contact of the solid caustic leach product with a hydrofluoric acid solution. In this embodiment, the acidic solution used in the acid leach step is selected from only a hydrochloric acid solution and a sulphuric acid solution. More preferably, the only acidic solution used in the acid leach step is a hydrochloric acid solution. [00123] In embodiments where the acid leach step comprises a hydrochloric acid leach solution, the hydrochloric acid concentration is between about 30 to 60 g/L. Hydrochloric acid is preferably added to the acid leach step at a rate of between about 150 to 500 kg/t feed, preferably 150 to 350 kg/t feed, to acidify residual alkalinity. The rate of addition of hydrochloric acid is preferably undertaken with consideration of background acid requirements. [00124] In one form of the present invention, the acid leach step is operated at a temperature of between about 60 to 100°C. [00125] In one form of the present invention, the acid leach step is operated with a slurry density of between about 10 to 35% w/w. In one form of the present invention, the acid leach step is operated with a slurry density of between about 15 to 35% w/w. In one form of the present invention, the acid leach step is operated with a slurry density of about20% w/w. [00126] In one form of the present invention, the acid leach step is operated with a total residence time of between about 2 to 6 hours. [00127] In a preferred form of the present invention, the acid leach step is conducted: (i) at a temperature of about 80oC; and (ii) with a total residence time of between about 2 to 4 hours. [00128] In embodiments where the acid leach step does not comprise the contact of the solid caustic leach product with a hydrofluoric acid solution, the process preferably comprises treating the impure HSA material in an oxidative leach step. More preferably, the impure HSA material is treated in an oxidative leach step prior to the caustic leach step. [00129] In one form of the present invention, the oxidative leach step comprises a first oxidative leach stage comprising the contact of the impure HSA material with an oxidant solution to form a first oxidative leach slurry. At least one of sodium hypochlorite, sodium chlorite, hydrogen peroxide, sodium meta bisulphite or sodium chlorate is utilised as the oxidant in the first oxidative leach stage. Preferably, the concentration of the oxidant in the first oxidative leach stage is based on: (i) about 125 to 200% of the stoichiometric amount based on sulphide sulphur content; (ii) a mole ratio of 7:2; and/or (iii) 50 to 150 kg/t feed material based on sulphide sulphur content. [00130] In one form of the present invention, the first oxidative leach step operates with a target redox potential of > about 425 mV (versus Ag/AgCl), preferably > about 550 mV (versus Ag/AgCl). [00131] In one form of the present invention, the first oxidative leach step operates with a pH of > 10. Preferably, adjustment of the pH occurs first in the first oxidative leach stage. NaOH is added to adjust the pH, in the range of about 10 to 50 kg/t feed, for example in the range of about 20 to 25 kg/t feed, dependent upon specific process requirements related to impurity load. [00132] In one form of the present invention, the first oxidative leach step operates with a slurry density of between about 10 to 35% w/w. In one form of the present invention, the first oxidative leach step operates with a slurry density of between about 15 to 35% w/w. In one form of the present invention, the first oxidative leach step operates with a slurry density of about 20% w/w [00133] In one form of the present invention, the first oxidative leach step operates at a temperature of between about 30 to 60°C. [00134] In one form of the present invention, the first oxidative leach step operates with a residence time of between about 30 to 60 minutes. [00135] In one form of the present invention, the target redox potential of the first oxidative leach step is in the range of about 800 to 1200 mV (versus Ag/AgCl). More preferably, the target redox potential of the first oxidative leach step is in the range of about 950 to 1000 mV (versus Ag/AgCl). [00136] In one form of the present invention, the target redox potential of the first oxidative leach step is in the range of about 750 to 1500 mV (versus Ag/AgCl). More preferably, the target redox potential of the first oxidative leach step is in the range of about 850 to 1150 mV (versus Ag/AgCl). [00137] In one form of the present invention, the oxidative leach step further comprises a second oxidative leach stage. Preferably, the second oxidative leach stage comprises the contact of the first oxidative leach slurry with a hydrochloric acid solution. Preferably, the second oxidative leach stage targets a background HCl acidity of between about 30 to 60 g/L. More preferably, the second oxidative leach stage targets a background HCl acidity of at least 60 g/L. [00138] In one form of the present invention, the second oxidative leach stage operates at a temperature of about 40 to 100°C. [00139] In one form of the present invention, the second oxidative leach stage operates with a slurry density of between about 15 to 35% w/w, for example about 20% w/w. [00140] In one form of the present invention, the second oxidative leach stage operates with a residence time in the range of about 2 to 6 hours. [00141] In one form of the present invention, the second oxidative leach stage operates with an HCl addition rate of between about 150 to 500 kg/t feed. In one form of the present invention, the second oxidative leach stage operates with an HCl addition rate of between about 150 to 350 kg/t feed. [00142] In one form of the present invention, the second oxidative leach stage may be conducted in multiple leach vessels arranged in counter current operation. [00143] In a preferred embodiment, the second oxidative leach stage operates: (i) with a residence time of between about 2 to 4 hours; and (ii) with an HCl addition rate of about 300 kg/t feed. [00144] Aluminium and/or calcium minerals are at least partially leached in the second oxidative leach stage. [00145] Following the completion of the second oxidative leach stage, the leach slurry in subjected to a solid liquid separation step to recover an oxidative leach residue. In this embodiment, the oxidative leach residue is subjected to the caustic leach step as discussed above. [00146] In some embodiments, the solid caustic leach product recovered from the caustic leach step is subjected to a secondary caustic leach step, prior to the acid leach step. Purification Process – Acid Leach [00147] In an alternative form of the present invention, the purification process comprises one or more acid leach steps. In this form of the present invention, the purification process comprises: treating the impure HSA material in an acid leach step, and recovering a solid acid leach product, wherein the solid acid leach product comprises a purified HSA graphite material. [00148] In one form of the present invention, the acid leach step comprises one or more separate acid leach stages. Preferably, each acid leach stage comprises the contact of the impure HSA material with an acidic solution, thereby forming an acidic leach slurry and recovering a solid acid leach product from the acidic leach slurry. It is envisaged that different acidic solutions may be used at different acid leach stages to target different impurities remaining in the impure HSA material. [00149] In one form of the present invention, the acid leach step comprises the contact of the impure HSA material with a hydrofluoric acid solution. Preferably, the acid leach step comprises two or more acid leach stages, where at least one of the acid leach stages comprises the contact of the impure HSA material with a hydrofluoric acid solution. [00150] In embodiments where the acid leach step comprises the contact of the impure HSA material with a hydrofluoric acid solution, the acid leach step preferably comprises: treating the impure HSA material in a first sulphuric acid leach stage comprising the contact of the impure HSA material with a sulphuric acid solution and separating undissolved leach solids; treating the leach solids of the first sulphuric acid leach stage in a hydrofluoric acid leach stage comprising the contact of the leach solids with a hydrofluoric acid solution and separating undissolved leach solids; treating the leach solids of the hydrofluoric acid leach stage in a second sulphuric acid leach stage comprising the contact of the leach solids with a sulphuric acid solution and separating undissolved leach solids; and treating the leach solids of the second sulphuric acid leach stage to one or more washing stages, and recovering a solid acid leach product comprising a purified graphite material. [00151] In some embodiments where the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution, the purification process does not comprise any caustic impurity removal steps. The inventors have found that the treatment of the impure graphite concentrate in the mechanical exfoliation step may avoid the need to treat the impure HSA material in a caustic impurity removal step prior to the acid leach step. [00152] The first sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ±5°C. Still preferably, the first sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes. [00153] Preferably, concentrated sulphuric acid is added in the first sulphuric acid leach stage. Still preferably, the residual free acid at the end of the first sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ±5 g/L H2SO4. The first sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids. [00154] The hydrofluoric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ±5°C. Preferably, the residual free acid at the end of the hydrofluoric acid leach stage is in the range of about 15-75 g/L HF, for example about 60 g/L ±5 g/L HF. The hydrofluoric acid leach stage operates with between about 5 to 25% solids, for example about 10% solids. [00155] Preferably, the hydrofluoric acid added to the hydrofluoric acid leach stage is in the range of about 20 to 70% concentration. The hydrofluoric acid concentration in the hydrofluoric acid leach stage is preferably in the range of about 15-50 g/L. [00156] Still preferably, the leach solids from the hydrofluoric acid leach stage have no remaining silicon therein, or only trace amounts thereof. [00157] The second sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ±5°C. The second sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids. [00158] Preferably, the second sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes. [00159] Preferably, concentrated sulphuric acid is added in the second sulphuric acid leach stage. Still preferably, the residual free acid at the end of the second sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ±5 g/L H2SO4. Acid solutions from the second sulphuric acid leach stage are preferably recovered and recycled to the first sulphuric acid leach stage and the second sulphuric acid leach stage. [00160] The washing stages preferably comprise a single repulp-filtration stage using deionised water, multiple counter-current repulp-filtration stages, or, for example, five multiple counter-current repulp-filtration stages, using deionised water. Preferably, the washing stages operate with between about 5 to 25% solids, for example about 10% solids using three stages of counter-current repulp-filtration stages. [00161] Preferably, liquid, residual salts, and/or acidity from the residual solids of the second sulphuric acid leach stage are recovered in the washing stages and returned to one or both of the first sulphuric acid leach stage and the second sulphuric acid leach stage. Drying Step [00162] The purified HSA graphite material may be further treated in a drying step to remove substantially all residual liquids. Any suitable drying method may be used, for example, an air-drying step conducted at increased temperatures. In one embodiment of the present invention, a cryogenic drying step may be used. Cryogenic drying methods include the freezing of liquids present in the purified HSA graphite material and lowering the pressure to remove the ice by sublimation. These methods may be used to substantially prevent the agglomeration of any HSA graphite particles. An example of suitable process conditions includes the freezing of the purified HSA graphite material, followed by subjecting the frozen material to: (i) <6 mbar vacuum, >0 °C drying temperature, and condenser temperature of <60-70 °C; or (ii) <1mbar vacuum, >30-40 °C drying temperature, and condenser temperature of 60– 70 °C. Impurity Removal Options [00163] The particular impurity removal steps incorporated into the purification process of the present invention will depend on several factors. The first factor to consider is the impurities present in the impure HSA material. Impure graphite concentrates typically comprise silicon containing species. A caustic impurity removal step, for example, the caustic bake step or the caustic leach step, is used to solubilise the silicate species. When treating impure graphite concentrates that contain crystalline Al2O3, such as recycled graphite materials, the inventors have found that the caustic impurity removal step should comprise a caustic leach step conducted at a temperature of at least 200°C and a pressure of 2 bar. Alternatively, the caustic impurity removal step should include a combination of a caustic bake step and a caustic leach step. The inclusion of the caustic bake step may avoid the need for the caustic leach step to be conducted at a temperature of at least 200°C, however it is envisaged that higher purities may be achieved by incorporating a high temperature caustic leach. The remaining impurities in the impure HSA material are targeted in one or more acid leach steps. The choice of the acid leach steps is influenced by the remaining impurities present in the impure HSA material. Those skilled in the art would be able to determine appropriate acid leach steps to remove the remaining impurities. [00164] A further factor to consider when selecting the acid leach steps is the acidic solutions available, and in particular, whether hydrofluoric acid solutions are to be incorporated. As would be appreciated by a person skilled in the art, the use of hydrofluoric acid brings with it significant environmental and occupational health and safety concerns. This makes the use of hydrofluoric acid unviable in certain settings. [00165] In one embodiment, the acid leach step includes the use of a hydrofluoric acid solution. In embodiments where the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution, the acid leach step preferably comprises several acid leach stages. Preferably, the acid leach stages comprise a first sulphuric acid leach stage, a hydrofluoric acid leach stage, a second sulphuric acid leach stage and a washing stage. [00166] In an alternative embodiment, the acid leach step does not include the use of hydrofluoric acid solutions. In such embodiments, the impure HSA material is directed to an oxidative leach step prior to the caustic impurity removal step. In this embodiment, the caustic removal step is preferably a caustic leach step. In this embodiment, the subsequent acid leach step preferably comprises a single acid leach stage in which the solid caustic leach product is contacted with an acidic solution selected from hydrochloric acid and sulphuric acid. [00167] In Figures 1 to 3 there are shown processes for the production of a purified HSA graphite material in accordance with separate embodiments of the present invention. In each of these embodiments, an impure graphite concentrate is treated in a mechanical exfoliation step to produce an impure HSA material. A purification process is then used to treat the impure HSA material to produce a purified HSA graphite material. The embodiments shown in Figures 1 to 3 each show examples of different purification processes that might be employed. [00168] In Figure 1, there is shown a process 10 for the recovery of purified graphite materials from an impure graphite concentrate 12 in accordance with the present invention. If required, the impure graphite concentrate 12 may be passed to a pre-treatment circuit (not shown) in which the impure graphite concentrate 12 is subjected to one or more pre- treatment steps. As detailed above, the pre-treatment steps required will depend on the nature of the impure graphite concentrate to be treated. The particle size of the impure graphite concentrate 12 should be less than 50 microns and preferably less than 30 microns. It should be noted that the pre-treatment circuit may not be required for all incoming feeds, in which case the impure graphite concentrate 12 may be processed directly. [00169] The impure graphite concentrate 12 is subjected to a pulping step 14 in which the impure graphite concentrate 12 is mixed with a liquid medium 16 to prepare an impure graphite slurry 18. [00170] The impure graphite slurry 18 is directed to a mechanical exfoliation step 20. In mechanical exfoliation step 20, the impure graphite slurry 18 is subjected to mechanical shearing forces to exfoliate HSA graphite particles from the impure graphite concentrate. [00171] Mechanical exfoliation step 20 results in the production of a slurry 22 comprising impure HSA graphite particles dispersed through the liquid medium. The slurry 22 is directed to a drying step 24 to remove the majority of the liquid medium and recover an impure HSA material 26. Drying step 24 may include a dewatering step (not shown) to first remove the bulk of the liquid medium prior to further drying. [00172] The impure HSA material 26 is then treated in a purification process to remove a substantial amount of the impurities present in the impure HSA material 26. In the embodiment shown in Figure 1, the purification process comprises a caustic leach step 28, followed by an acid leach step 30. In this embodiment, the acid leach step 30 comprises the use of a hydrofluoric acid solution. [00173] The caustic leach step 28 comprises the contact of the impure HSA material 26 with a sodium hydroxide solution 32 to form a caustic leach slurry. In this embodiment, the caustic leach step 28 is conducted at a temperature of at least 200°C and elevated pressure. As detailed above, conducting the caustic leach step 28 at this temperature will solubilise silica, amorphous Al2O3 and crystalline Al2O3 present in the impure HSA material 26. [00174] In a preferred embodiment, the concentration of the sodium hydroxide solution is at least 400 g/L, the starting slurry density of the caustic leach step 28 is 150 to 250 g/L solids, and, the residence time of the caustic leach step 28 is 2 to 16 hours. The caustic leach step 28 is conducted in a suitable pressurised leach reactor constructed from a material that exhibits sufficient resistance to the caustic leach step 28. The caustic leach step 28 may comprise a single leach stage conducted in a single leach reactor or may comprise two or more leach stages conducted across multiple leach reactors. It is preferred that the leach reactors are operated in a countercurrent manner. In embodiments where the caustic leach step 28 is operated over multiple stages, at least one of the stages is operated at a temperature of at least 200°C and elevated pressure. However, it is preferable to operate all stages at a temperature of at least 200°C and elevated pressure. [00175] The caustic leach slurry is subjected to a solid liquid separation step (not shown) to recover a solid caustic leach product 36 and a caustic leachate solution 38. In one embodiment, the solid material recovered in the solid liquid separation step is subjected to one or repulp steps to remove residual or entrained soluble species. In one embodiment, the solid material is repulped in a sodium hydroxide solution and the slurry is subjected to a further solid liquid separation step to recover a solid material. For example, the solid material may be repulped into a 200 g/l NaOH solution at 80°C for 30 minutes. In one embodiment, the solid material is repulped in water and the slurry is subjected to a further solid liquid separation step to recover a solid material. For example, the solid material may be repulped into water at 80°C for 30 minutes. The solid caustic leach product 20 is preferably dried at elevated temperature, for example 105°C, prior to further processing. [00176] Caustic containing leachate 38 from the caustic leach step 28 is treated in a caustic regeneration step 34 to which lime and/or hydrated/slaked lime is fed, and from which regenerated caustic 35 is recovered, optionally concentrated, and recycled to the caustic leach step 28. [00177] The solid caustic leach product 36 is then passed to acid leach step 30. In embodiments where the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution, the acid leach step 30 preferably comprises several acid leach stages. In the embodiment shown in Figure 1, the acid leach step 30 comprises a first sulphuric acid leach stage 40, a hydrofluoric acid leach stage 42, a second sulphuric acid leach stage 44 and a washing stage 46. While the embodiment shown in Figure 1 includes a caustic leach step, it is envisaged that in certain embodiments where the acid leach step comprises the use of a hydrofluoric acid solution, the caustic leach step may be omitted. In such an embodiment, the impure HSA material 26 is treated directly in the first sulphuric acid leach stage 40. Without wishing to be bound by theory, the inventors understand that the treatment of the impure graphite concentrate in the mechanical exfoliation step 20 may avoid the need for the caustic leach step. [00178] The first sulphuric acid leach stage 40 comprises the contact of the solid caustic leach product 36 with a sulphuric acid solution 48 to form a leach slurry and the separation of an acidic leachate 50 from undissolved leach solids 52. [00179] The first sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ±5°C. Still preferably, the first sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes. The residual free acid at the end of the first sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ±5 g/L H2SO4. Concentrated sulphuric acid is added to the slurry to maintain target residual free acid. The first sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids. The first sulphuric acid leach step may be conducted in a single leach vessel or may be conducted across multiple leach vessels arranged in series. [00180] The leach slurry is directed to a solid liquid separation step, for example a filter, to recover acidic leachate 50. Filter cake is washed and repulped with water at 40°C during 30 minutes at a solid/liquid ratio of about 165 g/l and the repulp slurry is directed to a solid liquid separation step, for example a filter, to recover a wash solution and undissolved leach solids 52. [00181] The hydrofluoric acid leach stage 42 comprises the contact of the undissolved leach solids 52 with a hydrofluoric acid solution 54 to form a leach slurry and the separation of an acidic leachate 56 from undissolved leach solids 58. [00182] The hydrofluoric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ±5°C. Preferably, the residual free acid at the end of the hydrofluoric acid leach stage is in the range of about 15-75 g/L HF, for example about 60 g/L ±5 g/L HF. The hydrofluoric acid leach stage operates with between about 5 to 25% solids, for example about 10% solids. Hydrofluoric acid added to the hydrofluoric acid leach stage is in the range of about 20 to 70% concentration. The hydrofluoric acid concentration in the hydrofluoric acid leach stage is preferably in the range of about 15-50 g/L. The hydrofluoric acid leach stage may be conducted in a single leach vessel or may be conducted across multiple leach vessels arranged in series. [00183] The leach slurry is directed to a solid liquid separation step, for example a filter, to recover acidic leachate 56. Filter cake is washed and repulped with water at 40°C during 30 minutes at a solid/liquid ratio of about 165 g/l and the repulp slurry is directed to a solid liquid separation step, for example a filter, to recover a wash solution and undissolved leach solids 58. [00184] The second sulphuric acid leach stage 44 comprises the contact of the undissolved leach solids 58 with a sulphuric acid solution 60 to form a leach slurry and the separation of an acidic leachate 62 from undissolved leach solids 64. [00185] The second sulphuric acid leach stage is preferably undertaken at between about 5 to 60°C, for example about 40°C ±5°C. The second sulphuric acid leach stage preferably operates with between about 5 to 25% solids, for example about 10% solids. Preferably, the second sulphuric acid leach stage has a retention time of between about 30 to 240 minutes, for example about 120 minutes. Concentrated sulphuric acid is added in the second sulphuric acid leach stage. Still preferably, the residual free acid at the end of the second sulphuric leach stage is in the range of about 5-75 g/L H2SO4, for example about 50 g/L ±5 g/L H2SO4. Acid solutions from the second sulphuric acid leach stage are preferably recovered and recycled to the first sulphuric acid leach stage and the second sulphuric acid leach stage. [00186] The leach slurry is directed to a solid liquid separation step, for example a filter, to recover acidic leachate 62. Filter cake is washed and repulped with water at 40°C during 30 minutes at a solid/liquid ratio of about 165 g/l and the repulp slurry is directed to a solid liquid separation step, for example a filter, to recover a wash solution and undissolved leach solids 64. [00187] The washing stage 46 comprises the contact of the undissolved leach solids 64 with a wash solution 66 and the separation of a purified HSA material 68. The washing stage 46 comprises a single repulp-filtration stage using deionised water, multiple counter- current repulp-filtration stages, or, for example, five multiple counter-current repulp- filtration stages, using deionised water. Preferably, the washing stages operate with between about 5 to 25% solids, for example about 10% solids using three stages of counter-current repulp-filtration stages. Water, residual salts, and/or acidity from the residual solids of the second sulphuric acid leach stage are recovered in the washing stages and returned to one or both of the first sulphuric acid leach stage and the second sulphuric acid leach stage. It is envisaged that carbonation may be incorporated during the washing stages 46, through the addition of sodium bicarbonate or the bubbling of carbon dioxide with caustic soda for pH control. This will help to neutralise carry-over acid from the second sulphuric acid leach stage and reduce the number of counter current washing steps required. [00188] The purified HSA material 68 may be further treated in a drying step (not shown) to remove residual liquids. [00189] A first effluent treatment plant 70 receives leachate streams 50 and 62 from the two sulphuric acid leach steps 40 and 44, respectively. The first effluent treatment plant 70 contacts the leachates with lime and iron sulphate, for example ferric sulphate, producing a neutralised solution 72 and a gypsum product 74. The volume of gypsum precipitation, or residue, may be minimised through use of caustic soda rather than lime in this step. [00190] A second effluent treatment plant 76 receives leachate 56 from the hydrofluoric leach step 56. The second effluent treatment plant 76 also receives lime and iron sulphate, for example ferric sulphate, producing a neutralised solution 78 and a calcium fluoride product 80. Alternatively, the second effluent treatment plant receives aluminium hydroxide, thereby producing an aluminium fluoride product. [00191] In Figure 2, there is shown a process 100 for the recovery of purified graphite materials from an impure graphite concentrate 12 in accordance with an alternative embodiment of the present invention. The embodiment shown in Figure 2 shares many similarities with the embodiment shown in Figure 1 and like numerals denote like parts. The impure graphite concentrate 12 may be passed to a pre-treatment circuit (not shown). The impure graphite concentrate 12 is subjected to a pulping step 14 in which the impure graphite concentrate 12 in mixed with a liquid medium 16 to prepare an impure graphite slurry 18. The impure graphite slurry 18 is directed to a mechanical exfoliation step 20. In mechanical exfoliation step 20, the impure graphite slurry 18 is subjected to mechanical shearing forces to exfoliate HSA graphite particles from the impure graphite concentrate. Mechanical exfoliation step 20 results in the production of a slurry 22 comprising impure HSA graphite particles dispersed through the liquid medium. The slurry 22 is directed to a drying step 24 to remove the majority of the liquid medium and recover an impure HSA material 26. [00192] The impure HSA material 26 is then treated in a purification process to remove a substantial amount of the impurities present in the impure HSA material 26. In the embodiment shown in Figure 2, the purification process comprises an oxidative leach step 102, followed by a caustic leach step 28 and an acid leach step 104. In this embodiment, the acid leach step 104 comprises the use of a hydrofluoric acid solution. [00193] The oxidative leach step 102 comprises a first oxidative leach stage comprising the contact of the graphite concentrate with an oxidant solution 106 to form a first oxidative leach slurry. The impure HSA material 26 is repulped, for example using recycled process water, adjusted to a pH in the range of 10-11 with caustic soda. The first oxidative leach stage operates with a target redox potential of > about 425 mV (versus Ag/AgCl), with a pH of > 10, with a slurry density of between about 15 to 35% w/w solids, for example about 20% w/w, at a temperature of between about 30 to 60°C, and with a residence time of between about 30 to 60 minutes. The target redox potential is in the range of about 800 to 1200 mV (versus Ag/AgCl), for example in the range of about 950 to 1000 mV (versus Ag/AgCl). [00194] At least one of sodium hypochlorite or sodium chlorate or a combination of both is utilised as the oxidant in the first oxidative leach stage. Added sodium hypochlorite is based on about 125 to 200% of the stoichiometric amount based on sulphide sulphur content, a mole ratio of 7:2, and/or 50 to 150 kg/t feed material based on sulphide sulphur content. [00195] The oxidative leach step 102 further comprises a second oxidative leach stage (not shown) that comprises the contact of the first oxidative leach slurry with a hydrochloric acid solution. The second oxidative leach stages operates at a temperature of about 40 to 100°C, over multiple leach stages, with a slurry density of between about 15 to 35% w/w, for example about 20% w/w, with a residence time in the range of about 2 to 6 hours, with an HCl addition rate of between about 150 to 350 kg/t feed, and with a background HCl acidity of between about 30 to 60 g/L. For example, the second oxidative leach portion operates with a residence time of between about 2 to 4 hours, and with an HCl addition rate of about 320 kg/t feed. [00196] The slurry product of the oxidative leach step 102 is passed to a solid liquid separation step (not shown) to recover an oxidative leach residue 110 and a liquid product 112. [00197] The oxidative leach residue 110 is directed to a caustic leach step 28 as described above to produce a solid caustic leach product 36. The solid caustic leach product may be directed to a secondary caustic leach step (not shown) conducted under the same conditions as the caustic leach step 28. The secondary caustic leach step may be used in circumstances where residual impurities remain in the solid caustic leach product 36. Caustic containing leachate 38 from the caustic leach step 28 is treated in a caustic regeneration step 34 from which regenerated caustic 35 is recovered, optionally concentrated, and recycled to the caustic leach step 28. [00198] The solid caustic leach product 36 is directed to an acid leach step 104. In the embodiment shown in Figure 2, the acid leach step 104 comprises a single acid leach stage 114. Acid leach stage 114 comprises the contact of the solid caustic leach product 36 with an acidic solution 116 selected from hydrochloric acid and sulphuric acid to form a leach slurry. [00199] Acid leach stage 114 Is conducted at a temperature of between about 60 to 100°C, with a slurry density of between about 15 to 35% w/w, for example about 20% w/w, with a total residence time of between about 2 to 6 hours. In embodiments where a hydraulic acid solution is used, the background HCl acidity is between about 30 to 60 g/L. The hydrochloric acid solution is added to acid leach stage 114 at a rate of between about 150 to 500 kg/t feed, for example about 320 kg/t feed. The rate of addition of hydrochloric acid to the acid leach stage 114 is undertaken with consideration of background acid requirements. [00200] Acid leach stage 114 may be conducted in a single reactor or multiple reactors arranged in series. [00201] The leach slurry is directed to a solid liquid separation step, for example a filter, to recover acidic leachate 117. Filter cake 118 is washed and directed to a water repulping step 120 in which it is repulped with water 121 at 40°C during 30 minutes at a solid/liquid ratio of about 165 g/l and the repulp slurry is directed to a solid liquid separation step, for example a filter, to recover a wash solution and purified HSA material 68. The purified HSA material 68 may be further treated in a drying step (not shown) to remove residual liquids. [00202] The process further comprises a first effluent treatment stage 122 in which liquid products 112 and 117 from the oxidative leach step 102 and the acid leach step 114 are combined and treated. The treatment may comprise neutralisation by addition of an alkali and/or calcium precipitation by addition of sodium sulphate. The liquid product 124 is directed to a reagent recovery step 126 to recover sodium hydroxide, hydrochloric acid or sodium hypochlorite reagents to be recycled or reused within the process. Reagent recovery step 126, may include, for example, the recovery of a concentrated sodium chloride solution and a chlor-alkali electrolysis step to generate chlorine gas. [00203] In Figure 3, there is shown a process 200 for the recovery of purified graphite materials from a impure graphite concentrate 12 in accordance with an alternative embodiment of the present invention. The embodiment shown in Figure 3 shares many similarities with the embodiment shown in Figure 1 and like numerals denote like parts. The impure graphite concentrate 12 may be passed to a pre-treatment circuit (not shown). The impure graphite concentrate 12 is subjected to a pulping step 14 in which the impure graphite concentrate 12 in mixed with a liquid medium 16 to prepare an impure graphite slurry 18. The impure graphite slurry 18 is directed to a mechanical exfoliation step 20. In mechanical exfoliation step 20, the impure graphite slurry 18 is subjected to mechanical shearing forces to exfoliate HSA graphite particles from the impure graphite concentrate. Mechanical exfoliation step 20 results in the production of a slurry 22 comprising impure HSA graphite particles dispersed through the liquid medium. The slurry 22 is directed to a drying step 24 to remove the majority of the liquid medium and recover an impure HSA material 26. [00204] The impure HSA material 26 is then treated in a purification process to remove a substantial amount of the impurities present in the impure HSA material 26. In the embodiment shown in Figure 3, the purification process comprises pelletisation step 202, a caustic bake step 204, a water leach step 206 followed by an acid leach step 30. In this embodiment, the acid leach step 30 comprises the use of a hydrofluoric acid solution. [00205] The pelletisation step 202 comprises the addition, in stepwise fashion, of caustic soda, in the form of caustic pill, and water to the impure HSA material 26. The pellets produced in the pelletisation step 202 are micro-pellets of about 2-10 mm in diameter, for example 5mm ± 2mm. The pellets produced in the pelletisation step 202 have a moisture content of about 13 to 24%w/w, for example about 20%w/w. [00206] The pellets are then treated in caustic bake step 204 in which the pellets are subjected to temperatures of between about 150-300°C, resulting in the formation of a product 208. The caustic bake step 204 has a residence time of greater than about 60 minutes, for example about 120 minutes. The molar ratio of caustic to silicon between 2.5- 5.5 : 1. The caustic bake step 204 is undertaken, for example, in a rotating kiln. [00207] The water leach step 206 comprises the contact of the product 208 with an aqueous solution 210 to form a leach slurry. The water leach step 206 is undertaken at between about 5-60°C, for example about 35°C. The water leach step 206 is undertaken in either a single or in multiple stages, for example up to three, counter-current leach stages and has a retention time of between about 30 to 240 minutes. [00208] The leach slurry is directed to a solid liquid separation step, for example a filter. Filter cake is washed and repulped with water at 40°C during 30 minutes at a solid/liquid ratio of about 165 g/l and the repulp slurry is directed to a solid liquid separation step, for example a filter, to recover a wash solution and a solid caustic leach product 36. The solid caustic leach product 36 is then passed to acid leach step 30, comprising a first sulphuric acid leach stage 40, a hydrofluoric acid leach stage 42, a second sulphuric acid leach stage 44 and a washing stage 48 as described above to recover a purified HSA material 68. While the embodiment shown in Figure 3 includes a pelletisation step 202, a caustic bake step 204 and a water leach step 206, it is envisaged that in certain embodiments where the acid leach step comprises the use of a hydrofluoric acid solution, the pelletisation step 202, caustic bake step 204 and water leach step 206 may be omitted. In such an embodiment, the impure HSA material 26 is treated directly in the first sulphuric acid leach stage 40. Without wishing to be bound by theory, the inventors understand that the treatment of the impure graphite concentrate in the mechanical exfoliation step 20 may avoid the need for the caustic bake step 204. [00209] A first effluent treatment plant 70 and a second effluent treatment plant 76 are similarly used to treat recovered streams from the process. [00210] The purified HSA material 68 may be further treated in a drying step (not shown) to remove residual liquids. Example 1 [00211] An impure natural graphite concentrate sample with a carbon content of 86.8 CT% and a surface area of 8.6 m2/g was subjected to a mechanical exfoliation step conducted at a specific milling energy of about 340 kWh/t. The product was further treated in a LLS beneficiation step to remove a portion of fine silicate minerals. The resulting impure HSA material had a surface area of 25.1 m2/g and an elemental composition as summarised in Table 1 Table 1: Mechanically Exfoliated Material Head Assay (M10843)
Figure imgf000048_0001
[00212] The impure HSA material was subjected to two separate purification processes. The first purification process (HF Type) included caustic baking the impure HSA material and then treating the material in a first sulphuric acid leach stage, a hydrofluoric acid leach stage, a second sulphuric acid leach stage and a washing stage. The process conditions are summarised in Table 2: Table 2: Test 1 Roast-Leach Process (HF)
Figure imgf000048_0002
Figure imgf000049_0001
[00213] The second purification process (MLP Type) included the caustic leaching of the impure HSA material at a temperature of about 140°C followed by hydrochloric acid leaching and water washing. The process conditions are summarised in Table 3: Table 3: Test 2 MLP Process (non-HF)
Figure imgf000049_0002
Figure imgf000050_0002
[00214] The results of the two trials are shown in Table 4 and show that the impurities present in the graphite concentrate can be removed following the mechanical exfoliation step. Table 4: Purification Summary
Figure imgf000050_0003
Example 2 [00215] A trial was conducted to investigate the milling energy required to produce HSA material from impure graphite concentrates in comparison to purified graphite concentrates. Two samples of impure graphite concentrates and one sample of purified graphite concentrate were used. The physical properties of these samples are shown in Table 5 below. Table 5: Sample
Figure imgf000050_0001
Figure imgf000050_0004
[00216] The samples were treated in a mechanical exfoliation step conducted in a grinding apparatus containing a grinding media having a ceramic bead size of 0.7-0.8mm. The change in surface area of the samples was measured against the milling energy. The results of this test are shown in Figure 3. It can be seen that lower milling energies were required for both impure graphite concentrate samples as compared to the purified graphite sample. The treatment of impure recycled graphite concentrates, in particular, exhibited significant reduction in milling energy. The milling energy required to reach a nominal particle size of 25 m2/g when treating the purified natural graphite concentrate was ~ 570 kWh/t. The milling energy to achieve the same particle size when treating the impure graphite concentrates was ~300 kWh/t and ~77 kWh/t for the impure natural and impure recycled concentrates respectively. [00217] The purity of the HSA material recovered from the purified graphite concentrate was analysed. The purified graphite starting purity was 99.92%Cg and it reduced to 99.87%Cg after the exfoliation step. This indicates that the exfoliation step introduced impurities, likely associated with the milling media used, that must be removed from the HSA material. Example 3 [00218] An impure natural graphite concentrate sample with a carbon content of 95.5 CT% and a surface area of 2.5 m2/g was subjected to a mechanical exfoliation step as a dry powder using a jet mill. The resulting impure HSA material had a surface area of 13.8 m2/g and an elemental composition as summarised in Table 6 below. Table 6: Mechanically Exfoliated Material Head Assay (M11628)
Figure imgf000051_0001
[00219] The impure HSA material was subjected to three purification processes with the process conditions summarised in Table 7. Table 7: Purification Process Conditions
Figure imgf000052_0001
[00220] Test A conditions incorporated caustic baking, caustic leaching, HF leaching, two stages of H2SO4 leaching and several stages of residue rinsing. The caustic baking step was dropped in Test B, whereas the HF leaching step was removed in Test C. The results of the trials are summarised Table 8 below showing that high purity material can be produced from dry milled mechanically exfoliated HSA graphite feedstock under a range of purification conditions. Table 8: Purification Summary
Figure imgf000052_0002
Example 4 [00221] An impure natural graphite concentrate sample with a carbon content of 89.3 CT% and a surface area of 22.9 m2/g was generated from graphite ore through beneficiation methods incorporating rougher flotation, concentrate regrinding with a horizontal bead mill and multiple stages of flotation cleaning and cleaner tails regrinding steps. The resulting impure HSA material had a surface area of 22.9 m2/g and an elemental composition as summarised in Table 9 below. Table 9:
Figure imgf000053_0001
Figure imgf000053_0002
[00222] The impure HSA material was subjected to purification incorporating caustic baking, caustic leaching, HF leaching, two stages of H2SO4 leaching and several stages of residue rinsing with key process conditions summarised in Table 10 below. Table 10: Purification Parameters
Figure imgf000053_0003
Figure imgf000054_0001
[00223] The results of the trial are summarised in the Table 11 below showing that high purity material can be produced mechanically exfoliated HSA graphite feedstock produced from beneficiation methods, under purification conditions identified in the present invention. Table 11: HSA Beneficiated Concentrated - Purification Summary
Figure imgf000054_0002

Claims

Claims 1. A process for the production of a purified high surface area (HSA) graphite material, the process comprising: treating an impure graphite concentrate in a mechanical exfoliation step to produce an impure high surface area (HSA) material; and treating the impure HSA material in purification process to produce a purified HSA graphite material.
2. A process according to claim 1, wherein the impure graphite concentrate has a carbon concentration of less than 96%.
3. A process according to claim 2, wherein the impure graphite concentrate has a carbon concentration of between 60 and 96%.
4. A process according to claim 2, wherein the impure graphite concentrate has a carbon concentration of between 75 and 96%.
5. A process according to any of preceding claims, wherein the mechanical exfoliation step is conducted in the presence of a liquid medium.
6. A process according to claim 5, wherein the mechanical exfoliation step comprises: pulping of the impure graphite concentrate with a liquid medium to prepare an impure graphite slurry; and subjecting the impure graphite slurry to a mechanical exfoliation step.
7. A process according to any of preceding claims, wherein the impure graphite concentrate is recovered from a natural graphitic ore.
8. A process according to any of preceding claims, wherein the purification process produces a purified HSA graphite material with a carbon concentration of at least 99% cg.
9. A process according to any of preceding claims, wherein the purification process comprises one or more impurity removal steps selected from: acid leach steps, caustic leach steps, oxidative leach steps, caustic bake steps or combinations thereof.
10. A process according to any of preceding claims, wherein the purification process comprises one or more acid leach steps.
11. A process according to claim 10, wherein the purification process comprises one or more caustic impurity removal steps, followed by one or more acid leach steps 12. A process according to claim 11, wherein the purification process comprises: treating the impure HSA material in a caustic impurity removal step, and recovering a solid caustic leach product; and treating the solid caustic leach product in an acid leach step, and recovering a solid acid leach product, wherein the solid acid leach product comprises a purified HSA graphite material. 13. A process according to claim 11 or 12, wherein the caustic impurity removal step comprises a caustic leach step. 14. A process according to claim 11 or 12, wherein the caustic impurity removal step comprises a caustic bake step. 15. A process according to any of claims 10 to 14, wherein the acid leach step comprises the contact of the solid caustic leach product with a hydrofluoric acid solution. 16. A process according to claim 15, wherein the acid leach step comprises: treating the solid caustic leach product in a first sulphuric acid leach stage comprising the contact of the solid caustic leach product with a sulphuric acid solution and separating undissolved leach solids; treating the leach solids of the first sulphuric acid leach stage in a hydrofluoric acid leach stage comprising the contact of the leach solids with a hydrofluoric acid solution and separating undissolved leach solids; treating the leach solids of the hydrofluoric acid leach stage in a second sulphuric acid leach stage comprising the contact of the leach solids with a sulphuric acid solution and separating undissolved leach solids; and treating the leach solids of the second sulphuric acid leach stage to one or more washing stages, and recovering a solid acid leach product comprising a purified graphite material. 17. A process according to any of claims 10 to 14, wherein the acid leach step only comprises the contact of the solid caustic leach product with an acidic solution selected from a hydrochloric acid solution and a sulphuric acid solution. 18. A process according to claim 17, wherein the process preferably comprises treating the impure HSA material in an oxidative leach step 19. A process according to claim 18, wherein the impure HSA material is treated in the oxidative leach step prior to the caustic leach step.
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