WO2021097539A1

WO2021097539A1 - Process and apparatus for producing high purity graphite

Info

Publication number: WO2021097539A1
Application number: PCT/AU2020/051268
Authority: WO
Inventors: Jasbir Khosa; Dave PASS
Original assignee: International Graphite Ltd
Current assignee: International Graphite Ltd
Priority date: 2019-11-21
Filing date: 2020-11-23
Publication date: 2021-05-27
Anticipated expiration: 2022-05-21

Abstract

A process for producing purified graphite wherein a source of graphite is contacted with a mixture of acids including hydrofluoric acid (HF) and at least one other mineral acid.

Description

PROCESS AND APPARATUS FOR PRODUCING HIGH PURITY GRAPHITE

Field of the Invention

[0001] This invention relates to a process and apparatus for producing high purity graphite from natural graphite or a natural graphite concentrate.

Backqround to the Invention

[0002] Demand for high purity graphite and graphite products for the expanding battery and electronics industries is predicted to grow significantly over the next decade. These graphite-based products include, but are not limited to high purity graphite, battery electrode feed graphite, foil graphite, sheet graphite, expanded graphite, micronized graphite and graphene.

[0003] A large portion of the high purity graphite currently produced comes from manufacturing synthetic graphite rather than from natural graphite, i.e. graphite that is mined and then optionally upgraded to produce a graphite concentrate. The synthetic graphite industry currently cannot meet this increased demand for high purity graphite.

[0004] Natural graphite concentrate produced from new mines may be able to meet this increased demand. However, without upgrading, mined graphite does not meet the specifications required for high purity graphite or the manufacture of commercial products therefrom. This is because silicates and other minerals are significant impurities in natural graphite, and either chemical or thermal processing must be undertaken to upgrade the natural graphite to the required purity.

[0005] A chemical processing route to remove silicate and other minerals involves leaching the natural graphite concentrate in hydrofluoric acid (HF). HF is the only acid that has been commercially proven to sufficiently and consistently dissolve and remove silicate minerals from the natural graphite, across a wide range of compositions and mineralogies, so that the leached graphite product meets the standards required for high purity graphite, at least 99.99% total graphitic carbon (TGC) content. However, such processes involving HF leaching, have to date generally been restricted due to the environmental problems caused by the treatment and disposal of wastewater from the process. This is because the fluoride in the wastewater cannot readily be reduced in concentration sufficiently to allow its discharge into waterways where it has been found to be harmful to marine life at levels above 5 mg/I (IPCS report EHC227).

[0006] Chemical purification routes that do not use HF are limited to selective natural flake compositions and mineralogies, in particular, those that do not contain silicate minerals. As such, few graphite concentrates can be treated without HF for high purity applications as the products are less likely to meet the high processing standards demanded by the battery and electronics markets.

[0007] Currently, the only alternative to chemical processing is the very high temperature thermal processing route using conventional batch furnaces that heat the natural graphite to temperatures above 2,500°C to vaporise and remove the impurities including Si, Al, Ca, Fe and S and minerals containing these. While achieving the high purity required, this process does not have the capacity to meet the predicted demand using conventional batch furnaces. For example, it would require 28 conventional batch furnaces to produce 1,500 tonnes per annum (tpa) of high purity graphite. High purity graphite plants are being designed to produce 20,000 tpa to 50,000 tpa, requiring about 430 and 1080 commercial furnaces respectively. However, with current technology, these plants are commercially not viable. Using the largest furnaces now available, that are only manufactured on demand, the number of required units can be reduced to about 70 and 180 respectively. However, the premium price associated with these furnaces also make the process unviable as a commercial operation.

[0008] There is currently no commercial reactor in the graphite processing industry using continuous thermal technology. Continuous thermal furnaces are still at the laboratory or pilot stage of development, and many years of research and development are still required before they are proven to be economic and become commercially available with current extreme cost addressed. This is due to the operating challenges required for sustained commercial operation above 2,500°C.

[0009] In summary, the development of new electronic technologies, and the demand for increasing efficiencies and effectiveness of batteries and electronic components, have created an increasing demand for the very high purity components required to manufacture these new batteries and electronic components, including high purity graphite.

[00010] The current processing technologies cannot readily meet this increased market demand while also meeting strict environmental guidelines for very low concentrations of contaminants in wastewater for disposal. Chemical purification technologies produce wastewaters that are difficult to dispose to the environment while thermal processing technologies are not yet sufficiently advanced to be economically viable.

Summary of the Invention

[00011] It is an object of the present invention to provide a process for producing graphite of high purity, such as from 80% TGC to 99.5% TGC, by a chemical purification process involving treating a source of graphite.

[00012] With this object in view, the present invention provides - in one aspect - a process for producing purified graphite comprising leaching a source of graphite with an acid mixture including hydrofluoric acid (HF).

[00013] Conveniently, the acid mixture includes HF and at least one mineral acid selected from the group consisting of nitric, sulphuric and hydrochloric acids though other mineral acids may also be used.

[00014] Preferably, HF is generated for the acid mixture by reacting sodium fluoride with hydrochloric acid. [00015] A preferred source of sodium fluoride is process wastewater from leaching and/or washing after neutralisation with sodium hydroxide, as described below.

[00016] Preferably, the mixture of acids used, and their rates of addition, will depend on the nature of the impurities in the graphite feed materials. This may require addition of two or more mineral acids. For example, while hydrofluoric acid is required to dissolve silicate minerals, nitric acid may be added to oxidise and dissolve sulphide minerals, and hydrochloric acid may be used to increase the rates of dissolution of the impurities.

[00017] The acid addition rates to leaching are desirably adjusted and optimised for each graphite feed material.

[00018] The amount of the acids added for leaching the source of graphite would preferably be at least the stoichiometric amount required to dissolve all the impurities, and more preferably will be added in excess to improve the rates and extents of dissolution of the impurity minerals. At the same time, exfoliation can be avoided.

[00019] Preferably, acid addition rates for leaching include, but are not in any way limited to, 400 to 800 kg of HF, 300 to 800 kg hydrochloric acid (HCI) and 200 to 500 kg nitric acid (FINO3) per tonne of graphite feed material. In total, these preferred acid addition rate ranges equate to weight ratios of between 0.9 and 2.1 times the weight of the graphite feed material, optionally 1.2 and 1.6 times the weight of the graphite feed material.

[00020] Preferably the source of graphite, or raw graphite, is leached in the acid mixture under effective operating conditions including at a temperature between 20°C and the acid mixture boiling point at atmospheric pressure, preferably between 60°C and 90°C, more preferably between 80°C and 90°C. Preferred particle size range is dependent on final graphite-based product to be produced with preferred range (on an average particle size, D50, falling below 50 microns, preferably 10 to 25 microns, optionally 15 to 50 microns. [00021] A preferred retention time of the raw graphite during leaching in the acid mixture is between 1 and 24 hours (h), preferably between 4 and 12 h. Preferably, leaching of a raw graphite slurry is conducted in agitated tanks, for example acid resistant lined glass reinforced plastic tanks.

[00022] While the process may be conducted in a single step or stage, a multi-step or multi-stage leaching process, preferably involving separation of leached graphite solids from a first leaching step and leaching the separated graphite solids from the first leaching step with a fresh batch of acid mixture in a second or subsequent leaching step(s), is preferred to obtain higher graphite purity.

[00023] The operating conditions of the second or subsequent leaching steps may be the same or different from the operating conditions in the first leaching step.

[00024] The leaching step may be repeated as many times as desired to obtain a required graphite purity subject to acceptable capital and operating costs. Two leaching steps are typically sufficient.

[00025] Where the process includes a plurality of leaching steps, desirably two leaching steps, leaching is desirably conducted in a counter-current fashion, with spent acid mixture from the first (or preceding) leaching step being sent to neutralisation, and the acid mixture from the second leaching step being re-used to contact a fresh batch of graphite.

[00026] Following leaching, whether single or multi-step, leached graphite is separated from the acid mixture and washed, desirably with water and more desirably with ultra-pure water, desirably in a plurality of washing stages, for example two to three washing stages.

[00027] The amount of washing is desirably adjusted depending on the impurity and acid concentrations in the leach solution. Wash water should be treated, preferably with limestone and/or lime and/or caustic soda, to ensure the neutralisation and precipitation of the majority of the soluble impurities dissolved from the graphite during leaching.

[00028] In a preferred embodiment, the process involves directing neutralised wash water to settling to allow settling and dewatering of the precipitated solids, before drying, recovery and storage. Dewatering may involve evaporation, preferably solar evaporation.

[00029] A preferred settling system includes pond(s) or drain(s) though clarifier vessels could possibly be used as an alternative, if desired, subject to economic considerations. Supernatant liquor from ponds or drains is desirably directed during operation, optionally through overflow over a weir or weir system, to evaporation ponds for storage and evaporation, typically solar evaporation, of excess water so that no process liquors are discharged to the environment.

[00030] If caustic soda is used as neutralising agent, the soluble fluoride in the wastewater may be recovered, for example by thermal evaporation or ion exchange, and re-used in a leaching step.

[00031] The final pH of the wastewater will preferably be maintained in the range 7.5 to 9.0 to ensure precipitation of substantially all deleterious metal contaminants.

[00032] Settled solids from the settling step may have commercial value and may be recovered as desired following evaporation, if practised, to enhance the process economics. For example, the solids may include gypsum and/or calcium fluoride.

[00033] Preferably, the gypsum and calcium fluoride dried solids are collected in separate stages - in a pond or dam system, collection of each product is desirably from separate ponds - and utilised as by-products, for example in the manufacture of plasterboard and cement respectively.

[00034] At completion of an evaporation stage, the former base of a pond or dam becomes a pad allowing storage and management by appropriate equipment prior to direction to commercialisation or disposal, for example by burial. [00035] Preferably, the source of graphite is mined natural graphite such as a natural flake graphite. However, other sources of raw graphite may be used, these sources typically having a cost advantage over synthetic graphites which, while not typically requiring purification, can be expensive to produce.

[00036] The source of graphite may be a blend of graphitic materials sourced from different mining operations or otherwise. The source of graphite may be a natural flake graphite concentrate.

[00037] In a preferred variation of the invention, the graphite concentrate may be produced by a beneficiation step, such as flotation, to improve the graphite grade prior to leaching. Use of a higher grade graphite concentrate may advantageously reduce use of chemical reagents, particularly acids, in the process.

[00038] The process conveniently allows upgrading and processing of the source of graphite, and desirably the natural flake graphite concentrate across a range of compositions and mineralogies for the production of intermediate graphite products. These products include, but are not limited to, purified graphite, battery electrode feed, foil graphite, sheet graphite, expanded graphite, micronized graphite and graphene.

[00039] In a further embodiment, the present invention provides an apparatus being a graphite purification plant comprising:

(a) at least one leaching step for leaching a source of graphite with an acid mixture comprising hydrofluoric acid and at least one other mineral acid;

(b) a washing stage for washing graphite produced by leaching step (a) with wash water to produce purified graphite;

(c) a neutralising stage for treating wash water from step (b) with an alkali to produce neutralised wash water; and

(d) a settling and dewatering stage for settling and dewatering solids from the neutralised wash water. [00040] The settling stage may include a series of ponds and/or drains. Dewatering may involve evaporation, preferably solar evaporation, saving costs through avoiding energy generation plant for evaporation.

[00041] The process and plant may advantageously enable natural graphite flake concentrate to be commercially purified at high rates while meeting strict environmental targets to sustainably meet demand.

Short Description of the Drawinq

[00042] Figure 1 is a block diagram for a process for graphite treatment according to one embodiment of the present invention.

Description of Preferred Embodiments

[00043] In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

[00044] Embodiments of the invention are described below to assist understanding of the process for graphite treatment. The embodiments and examples are purely illustrative of the invention and are not intended to limit its scope in any way. Example 1

[00045] A raw graphite in the form of a graphite concentrate was analysed as follows:

Natural graphite flake (95% TGC)

Contained moisture (1%)

Major gangue: Kaolinite (1%), Mica (1%) and quartz (1%)

Minor gangue: Pyrite, Plagioclase, Fe oxide, chlorite and K-feldspar (each 0.2%)

Trace gangue: chalcopyrite, Talc, zircon, rutile.

I. This graphite concentrate, having a particle size greater than 10 microns and less than 50 microns, was treated according to the block diagram of Fig. 1.

II. In first leaching step or stage 10, the graphite concentrate was leached as a slurry with an acid mixture comprising HF, HCI and FINO3 to solubilise the gangue and notably the major silicate gangue. The calculated stoichiometric acid requirements were as follows:

163 kg/t of 40% HF for dissolution of the silicate minerals;

1.6 kg/t of 32% HCI for dissolution of oxide minerals; and

21 kg/t of 68% HNCtefor dissolution of sulphide minerals.

These quantities do not include excess HF to drive the silicate dissolution reactions, additional HCI for increased acidity and catalysis of the aluminosilicate and quartz dissolution processes, nor excess HNO3 for oxidation of organic matter or other oxidisable components (excluding graphite). The actual added amounts of acid in the two leaching steps or stages 10 (first leaching stage) and 12 (second leaching stage) were much greater than these requirements, as described below, to ensure the mineral dissolution reactions proceeded to near completion in a reasonable time. Leaching is preferably to be conducted in agitated acid resistant lined glass reinforced plastic tanks. [00046] To the natural graphite concentrate (1.0 kg) was added the following mixture of acids in first and second leaching stages 10 and 12:

40% HF solution (700 g);

32% HCI solution (500 g)

68% HNO3 solution (250 g)

The resultant slurry was heated to 85°C and maintained at temperature for 6 hours (h). The solids were then separated from the acidic liquor, and the leaching process repeated in second leaching stage 12 with a fresh addition of the same acid mixture as described above.

[00047] The resultant graphite solids were separated from the acidic liquor from second leaching stage 12 by filtration stage 16 and washed with ultra-pure water three times in washing stage 20. The amount of wash water used was between 4 to 6 litres of water per kilogram of graphite with the amount determined by trial and error for the graphite purity required.

[00048] The purified graphite from washing stage 20 was separated by filtration in stage 28 and then dried in air in drying stage 30. The treated graphite product 70 was analysed as containing >99.95% TGC and may be used for various applications including purified graphite, battery electrode feed, foil graphite, sheet graphite, expanded graphite, micronized graphite and graphene.

[00049] Wastewater from the washing stage 20, and separated as filtrate from filtration stage 28, was then neutralised with alkali (limestone and/or lime and/or sodium hydroxide (caustic soda)) in neutralisation stage 60 using up to 120% of the stoichiometric amount required for neutralisation. The neutralised slurry was then directed to settling stage 40.

[00050] The spent acidic liquor from first and second leaching stages 10 and 12 was combined and treated with alkali (limestone and/or lime and/or caustic soda) to pH 9 in neutralisation stage 18 to neutralise the excess acid and precipitate dissolved impurities, and notably deleterious metal contaminants, from the graphite. In settling step 40, the “impurity” solids were allowed to settle for 24 h and then separated from the clear supernatant liquor. The clear supernatant liquor was composed mainly of calcium chloride and calcium nitrate with minor quantities of magnesium and sodium chloride (from the graphite impurities), as well as about 50 mg/L fluoride ions, which is in excess of the allowable discharge limits in most countries. The solid residue 50 consisted mainly of calcium sulphate (gypsum) and calcium fluoride with minor aluminium, and silicon and iron fluorides and hydroxides.

[00051] Working ponds and/or drains may be used for the dewatering and separation stage 40 for separating the solid and liquid waste streams, and the drying thereof. The slurry is allowed to settle in the ponds of stage 40, and the clear supernatant liquor 45 will overflow a weir into a series of evaporation ponds 48 for storage, and re-use or drying of the liquor as required.

[00052] The solid residues 50 will, when the storage ponds 48 are full, be dewatered and emptied. The solids will be transferred onto a pad for drying before being transferred to a holding facility, by suitable vehicles, for commercialisation or long term storage or burial. The residues 50 may be used as by-products in the manufacture of building products and cement and so have value. For example, calcium fluoride may be used as a mineraliser in cement production, and gypsum may be used in plasterboard manufacture.

Example 2

[00053] A graphite concentrate, as was used in Example 1 , was treated as follows. To the natural graphite concentrate (1.0 kg) was added the following mixture:

Solid NaF (590 g) mixed with 36% HCI solution (1 ,420 g) to form, following reaction, HF and NaCI solution.

36% HCI solution (500 g) 68% HNO3 solution (250 g)

[00054] The resultant slurry was heated to 85°C and maintained at that temperature for 6 h. The purified graphite solids were then separated from the acidic liquor, and the leaching process repeated with fresh addition of the acids as described above. The resultant graphite solids were separated from the acidic liquor and washed with ultra-pure water three times, then dried in air. The graphite product was analysed as containing >99.95% total graphitic carbon (TGC) and may be used for various applications including purified graphite, battery electrode feed, foil graphite, sheet graphite, expanded graphite, micronized graphite and graphene.

[00055] The spent acidic liquor was treated with caustic soda solution (2,600 ml_ 4M NaOH) to pH 9 to neutralise the excess acid and precipitate the dissolved impurities from the graphite. The solids were allowed to settle for 24 h and then separated from the clear supernatant liquor. The clear liquor was composed mainly of sodium fluoride, sodium chloride and sodium nitrate with minor quantities of magnesium and calcium chloride (from the graphite impurities). The minor amount of solid residue (70 g) consisted mainly of aluminium, and silicon and iron (probably as hydroxides).

[00056] Modifications and variations to the process and apparatus for producing high purity graphite described here may be apparent to the skilled reader of this disclosure. Such modifications and variations form part of the present invention.

[00057] It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms a part of the common general knowledge in the art, in Australia or any other country.

[00058] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

CLAIMS:

1. A process for producing purified graphite wherein a source of graphite is leached with a mixture of acids including hydrofluoric acid (HF) and at least one other mineral acid.

2. Process according to Claim 1 , wherein the graphite is leached with a mixture of acids including hydrofluoric acid (HF) and at least one other mineral acid selected from the group consisting of nitric, sulphuric and hydrochloric acids.

3. Process according to Claim 1 , wherein the graphite is leached with a mixture of acids including hydrofluoric acid (HF) and at least two other mineral acids selected from the group consisting of nitric, sulphuric and hydrochloric acids.

4. Process according to any one of claims 1 to 3, wherein the graphite is leached in the acid mixture, preferably in at least two leaching stages, at a temperature between 60°C and 90°C.

5. Process according to any one of claims 1 to 4, wherein acid addition rates range from 400 to 800 kg of HF, 300 to 800 kg hydrochloric acid (HCI) and 200 to 500 kg nitric acid (HNO3) per tonne of graphite feed material.

6. Process according to any one of claims 1 to 5, wherein the leaching retention time of the graphite in the acid mixture is between 1 and 24 hours, preferably between 4 and 12 hours.

7. Process according to any one of claims 1 to 6, wherein after separating the solids from the acid mixture, a second leaching step is conducted with the separated solids being re-contacted with a fresh batch of the acid mixture under the same or different conditions.

8. Process according to claim 7, wherein the two leaching steps are carried out in a counter-current fashion, with the spent acid mixture from the first leaching step being sent to neutralisation, and the acid mixture from the second leaching step being re-used to contact a fresh batch of raw graphite feed.

9. Process according to any one of claims 1 to 8, wherein the wash water is ultra- pure (deionised) water.

10. Process according to any one of claims 1 to 9, wherein the wash water is neutralised with limestone and/or lime, to precipitate soluble impurities leached from the graphite.

11. Process according to any one of claims 1 to 9, wherein the wash water is neutralised with caustic soda, to precipitate soluble impurities leached from the graphite, but retaining most of the fluoride ion in the supernatant solution.

12. Process according to claim 10 or 11 , wherein the supernatant wastewater containing fluoride ions is treated to recover the fluoride ions for recycle to the leaching process.

13. Process according to any one of claims 1 to 12, wherein the neutralised wash water is sent to a series of ponds or drains to allow settling and dewatering of the solids, before drying, recovery and storage.

14. Process according to claim 13, wherein the gypsum and calcium fluoride dried solids are collected from separate ponds and utilised as by-products, optionally in the manufacture of plasterboard and cement respectively.

15. Process according to any one of claims 1 to 14, wherein the ponds have a weir system to allow the supernatant wastewater to overflow into a series of evaporation ponds to allow storage and then evaporation of the wastewater.

16. Process according to any one of claims 1 to 15, wherein the graphite concentrate is produced by a beneficiation step, optionally flotation, to improve the grade prior to leaching.

17. A graphite purification plant comprising:

(a) at least one leaching stage for leaching a source of graphite with an acid mixture comprising hydrofluoric acid and at least one other mineral acid;

(b) a washing stage for washing graphite produced by step (a) with wash water to produce purified graphite;

(d) a settling and dewatering stage for settling and dewatering solids from the neutralised wash water.

18. Graphite purification plant as claimed in claim 17, wherein the settling and dewatering stage includes a series of ponds and/or drains.

19. Graphite purification plant as claimed in claim 18, wherein said solids include gypsum and calcium fluoride, each optionally being recovered from separate ponds.

20. Graphite purification plant as claimed in claim 18 or 19, wherein dewatering is by solar evaporation.