WO2025219174A1

WO2025219174A1 - Purification of carbon contained in black mass from lithium-ion batteries

Info

Publication number: WO2025219174A1
Application number: PCT/EP2025/059704
Authority: WO
Inventors: Wolfgang Rohde; Dieter ETTMUELLER
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2024-04-16
Filing date: 2025-04-09
Publication date: 2025-10-23
Anticipated expiration: 2026-10-16

Abstract

The present disclosure relates to the purification of carbon contained in black mass from lithium-ion batteries (LIB) or in leach residues obtained by acid treatment of black mass from lithium-ion batteries (LIB).

Description

Purification of carbon contained in black mass from lithium-ion batteries

The project leading to this application has received funding from Bundesministerium fur Wirtschaft und Energie (DE; FKZ:16BZF101A); the applicant bears responsibility for all disclosures herein.

Field of the invention

Background

In the recycling of lithium-ion batteries (LIB), hydrometallurgical processes for the recovery of valuable metals become more and more common, as they allow the facile recovery of lithium. The graphite - which is considered a critical raw material in the European Union - from the anodes will also be recovered and may be re-utilized which is not possible in pyrometallurgical processes.

A typical hydrometallurgical LIB recycling process comprises first a mechanical treatment of discharged batteries under inert gas to separate the active materials from anodes and cathodes as so-called black mass. This material contains graphite, the anode active material, and all valuable metals from the cathodes, namely Li, Ni, Co, Mn, and, as an impurity, also the valuable Cu from anode current collector foils and cables. Other impurity metals found in the black mass may comprise Al, P, Si, Sn, Ti, W, Zn and Zr. These impurities stem from the aluminum current collector foils of the cathodes, from silicon-containing anodes and fiber-reinforced plastics found, e. g., in circuit boards, solder both from electronic parts, e. g., from the battery management system, and different electrode material coatings and dopings. In some cases, the comminution of the batteries may also take place under aqueous conditions, and the water may react with some reactive components in the batteries, e. g., LiPF₆. This makes the comminution process safer with the disadvantage of producing wastewater.

As the black mass obtained by purely mechanical treatment of LIB also contains electrolyte and polymers, e. g., as binder in the electrodes, the black mass is often dried to evaporate the electrolyte solvents and pyrolyzed to decompose high-boiling organic components and polymer residues including binder polymers like the fluorine containing PVDF. Volatile fluorine-containing compounds like LiP F₆ will also be evaporated or decomposed by these treatments.

In still another process, batteries, battery modules, or cells are pyrolyzed, optionally after discharging, and subsequently comminuted and separated into metal residues and the active materials constituting the black mass.

It is also known in the art to apply solid-solid separation techniques to separate carbon particles like graphite from the anodes. Typical solid-solid separation techniques include froth flotation, magnetic separation or carrier magnetic separation, gravity separation like dense-media separation, sieving and sifting.

The black mass obtained is leached, typically with sulfuric acid, with careful control of the pH- and redox conditions, allowing for the dissolution of more noble metals like Cu under oxidizing conditions, and the dissolution of cathode active materials containing metals in high oxidation states under reducing conditions. After the acid treatment, the solution obtained which contains the dissolved metals is separated from the insoluble leach residue by solid-liquid separation. This leach residue contains carbon contained in the black mass, but also some impurities insoluble in acid and comprising compounds of Al, P, Si, Sn, Ti, W, Zn and/or Zr, mostly of oxidic nature. In certain applications, the impurities mentioned above may be not tolerable, especially when the carbonaceous residue is recycled to produce graphite for new LIB-anodes. Therefore, purification is required to remove these impurities.

H. Wang et al: "Preparation of high purity graphite from a fine microcrystalline graphite concentrate: Effect of alkali roasting pre-treatment and leaching process", Separation Science and Technology 51 (2016) 2465 discloses a purification process for natural graphite ore concentrate starting with mixing an ore concentrate containing 90% C, 2.7% SiO2 and 2.9% AI2O3 with solid sodium hydroxide and baking this mixture for 1 h at 500°C. Subsequently, the cold mixture is washed neutral with water and then subjected to an acid leach with 3 M hydrochloric acid at 40°C. A similar procedure is described in E. Forssberg et al.: "Preparation of high-purity and low-sulfur graphite from Woxna fine graphite concentrate by alkali roasting", Minerals Eng. 15 (2002) 755.

CN 117 374 446 A discloses a method for recovering and regenerating a graphite negative electrode material of a waste lithium ion battery. The method comprises uniformly mixing the graphite negative electrode material of the waste lithium ion battery with an alkaline solution, drying, and carrying out alkali melting and calcining treatment in an inert atmosphere at 500-900°C; washing the calcined alkali molten slag with deionized water, then adding the washed alkali molten slag into an acid solution, carrying out an acid dissolution reaction at 25-100°C, and washing the reaction material to be neutral to obtain impurity- removed graphite; and mixing the impurity-removed graphite with petroleum asphalt, and coating and granulating in an inert atmosphere at 300-800°C to obtain the recycled graphite.

WO 2021/232090 A1 (US 11 702 342 B2) discloses a method of producing purified graphite. The method comprises subjecting graphite material to a NaOH bake by mixing the graphite material with liquid NaOH (50%) and heating the mixture in a furnace to 500° C. for 30 minutes; releasing any remaining NaOH using water; subjecting the graphite material to a first acid wash; neutralising and washing the acid washed graphite material to deliver an intermediate purified graphite product; subjecting the intermediate purified graphite product to a NaOH leach; releasing any remaining NaOH in the intermediate purified graphite product using water; subjecting the intermediate purified graphite product to a second acid wash; and neutralising and washing the intermediate purified graphite product to deliver a purified graphite product.

WO 2023/081979 A1 (CA 3 237 999 A1 ) describes a similar process, except that sodium hydroxide is added as solid material instead of a 50% aqueous solution..

CN 112 086 703 A discloses a treatment method of retired battery carbon residues, which comprises the following steps: crushing and drying retired battery carbon residues to be treated to obtain fine carbon residues; uniformly mixing the fine carbon residues with villiaumite to obtain a mixture; enabling the mixture to be subjected to heat preservation for 0.5-4 h at the temperature of 100-400 °C in the protective atmosphere, and obtaining slag and smoke; and leaching the obtained slag in water or an acidic aqueous solution, then, carrying out solid-liquid separation, and washingthe solid-phase substance with water to obtain leachate and graphite.

CN 116 040626 A discloses a method for purifying alkali-fused graphite under reduced pressure, comprising the steps of: (1) adding graphite powder to a sodium hydroxide solution, heating and ultrasonically stirring to make the graphite fully contact sodium hydroxide to obtain mixture 1 ; (2) putting mixture 1 into a high-temperature furnace to extract air and depressurize it, and carrying out reduced-pressure alkali leaching; after alkali leaching, heating and drying until the water is completely volatilized; washing with deionized water to neutrality to obtain mixture 2; (3) mixing mixture 2 with acid solution, carrying out acid leaching with heating and stirring, suction filtering and recovering the acid solution after the acid leaching, washing the filter cake with water until it is neutral, and then drying it to obtain purified graphite. Summary of the invention

The present disclosure provides an improved process for the purification of carbon contained in black mass from lithium-ion batteries or in leach residues obtained by acid treatment of black mass from lithium-ion batteries. The process comprises a caustic leach, followed by caustic baking, followed by washing with water, and subsequent acid leaching. By using this particular sequence of steps, extraction of silicon and refractory metals like titanium from the black mass or the leach residues is substantially improved in comparison to prior art processes.

It has been found that the sequence of steps of the process of the present disclosure is crucial for efficient removal of impurities such as silicon or titanium from the carbon material. Silicon only is efficiently removed when a caustic leaching step is performed before the caustic baking. Without prior caustic leaching, acid leaching directly after the baking step does not remove silicon. After the baking step, silicon is not removed by a subsequent caustic leach unless an intermediate acid leach step is added directly after the baking step. Titanium is more efficiently removed from the feed by the process of the present disclosure than by the prior art processes

Detailed description

The black mass and the acid leach residue from the hydrometallurgical treatment of black mass contain Al, Si and other metals, e.g., Sn, Zr, W, and/or Ti. For the recycling of the graphite contained therein, it is important to reduce these metal contaminants., because Si, Zr, W, Ti are high-boiling metals that will not evaporate under graphitization conditions; and Al and Si have a high specific evaporation energy.

The present disclosure provides a process for the purification of carbon contained in leach residues obtained by acid treatment of black mass from lithium-ion batteries, comprising a) providing a leach residue obtained by acid treatment of black mass from lithium-ion batteries, b) treating the leach residue with aqueous alkali hydroxide or carbonate solution at a temperature in the range of from 20°C to 200°C, at ambient pressure or elevated pressure, for a period of time in the range of from 1 hr to 20 hrs, c) recovering first solids from the mixture obtained in step b) by solid/liquid separation, d) optionally, washing the first solids, e) optionally, reducing the water content of the first solids obtained in step d), f) adding alkali hydroxide as a solid or as an aqueous solution to the first solids obtained in step c), d) or e) in a mass ratio of solids (taken as dry matter) to alkali hydroxide (taken as dry matter) in the range of from 0.1 to 4 to obtain a first mixture, g) heating the first mixture obtained in step f) to a temperature of from 350°C to 900°C to obtain a baked material, h) suspending the baked material obtained in step g) in an aqueous medium to obtain a second mixture, i) recovering second solids from the second mixture obtained in step h) by solid/liquid separation, j) washing the second solids obtained in step i) with water, k) leaching the second solids obtained in step j) with an acid to obtain a third mixture, l) recovering third solids from the third mixture obtained in step k) by solid/liquid separation, m) washing the third solids obtained in step I) with water to obtain a purified carbon material.

The present disclosure also provides a process for the purification of carbon contained in black mass from lithium-ion batteries, comprising a) providing a black mass from lithium-ion batteries, b) treating the black mass with aqueous alkali hydroxide or carbonate solution at a temperature in the range of from 20°C to 200°C, at ambient pressure or elevated pressure, for a period of time in the range of from 1 hr to 20 hrs, c) recovering first solids from the mixture obtained in step b) by solid/liquid separation, d) optionally, washing the first solids, e) optionally, reducing the water content of the first solids obtained in step d), f) adding alkali hydroxide as a solid or as an aqueous solution to the first solids obtained in step c), d) or e) in a mass ratio of solids (taken as dry matter) to alkali hydroxide (taken as dry matter) in the range of from 0.1 to 10 to obtain a first mixture, g) heating the first mixture obtained in step f) to a temperature of from 350°C to 900°C to obtain a baked material, h) suspending the baked material obtained in step g) in an aqueous medium to obtain a second mixture, i) recovering second solids from the second mixture obtained in step h) by solid/liquid separation, j) washing the second solids obtained in step i) with water, k) leaching the second solids obtained in step j) with an acid to obtain a third mixture, l) recovering third solids from the third mixture obtained in step k) by solid/liquid separation, m) washing the third solids obtained in step I) with water to obtain a purified carbon material.

In both cases, the treatment of acid leach residues and the treatment of black mass, it may be favorable to separate carbon by a solid-solid separation method like flotation, magnetic separation, carrier magnetic separation, gravity separation, sieving or sifting prior to the treatments described above. This will reduce the amount of material that has to be treated. Typically, a flotation is performed by adding collectors and foam-stabilizing compounds (frother). In some cases, depressants also are added to reduce entrainment of unwanted particles in the concentrate. Typical collectors in coal and graphite flotation are kerosine or other hydrocarbons. Typical-foam stabilizing compounds (frothers) are alcohols like methyl isobutyl carbinol (MIBC). A depressant often used is waterglass, i.e. , sodium silicate.

The first solids obtained in step c) are optionally washed in step d) with water, or first with an aqueous solution of the alkali hydroxide and subsequently with water, before they are fed to step e) or f).

Optionally, the water content of the washed first solids obtained in step d) is reduced in step e) before they are fed to step f). Optionally, the washed first solids obtained in step d) are dried in step e) to reduce their water content.

In some embodiments, the process of the present disclosure uses a leach residue obtained by acid treatment of black mass from lithium-ion batteries as a starting material.

In other embodiments, the process of the present disclosure uses black mass from lithium-ion batteries as a starting material.

In the present disclosure, the term black mass (BM) means a particulate material comprised of particles passing a sieve having a mesh width of 1 mm, obtained by mechanical comminution of at least one battery material chosen from a lithium ion battery, lithium ion battery waste, lithium ion battery production scrap, lithium ion cell production scrap, lithium ion cathode active material, and combinations thereof, drying the comminuted battery material at a temperature below 200°C, and sifting the comminuted and dried battery material to obtain a fine fraction of particles having a size of less than 1 mm (oxidic black mass), or obtained by mechanical comminution of at least one battery material chosen from a lithium ion battery, lithium ion battery waste, lithium ion battery production scrap, lithium ion cell production scrap, lithium ion cathode active material, and combinations thereof, and subsequent heat treatment of the comminuted battery material at a temperature in the range of from 350°C to 900°C under an inert or reducing atmosphere and sifting to obtain a fine fraction of particles having a size of less than 1 mm (reduced black mass).

The black mass contains valuable metals from the cathodes, namely Li, Ni, Co, Mn, and, as an impurity, also valuable Cu from anode current collector foils and cables. Other impurity metals found in the black mass may comprise Al, P, Si, Sn, Ti, W, Zn, and/or Zr.

In the present disclosure, acid treatment of the black mass involves leaching with aqueous acid, typically with sulfuric acid, under oxidizing and/or reducing conditions, and dissolving noble metals like Cu under oxidizing conditions, and dissolving cathode active materials containing metals in high oxidation states under reducing conditions. After the acid treatment, the solution obtained which contains the dissolved metals is separated from the insoluble leach residue by solid-liquid separation. This leach residue contains carbon contained in the black mass, but also some insoluble impurities comprising compounds of Al, P, Si, Sn, Ti, W, Zn and/or Zr, mostly of oxidic nature. In some embodiments, carbon content of the leach residue is in the range of from 75 to 95 wt%, e.g., from 80 to 90 wt%, relative to the total weight of the leach residue. In some embodiments, aluminum content of the leach residue is in the range of from 2 to 5 wt%, e.g., from 3 to 4 wt%, relative to the total weight of the leach residue. In some embodiments, silicon content of the leach residue is in the range of from 0.5 to 2 wt%, e.g., from 0.8 to 1 .5 wt%, relative to the total weight of the leach residue.

The leach residue or black mass is b) treated with aqueous alkali hydroxide or carbonate solution, preferably a solution of sodium hydroxide or potassium hydroxide or sodium carbonate or potassium carbonate, at a temperature in the range of from 20°C to 200°C at ambient pressure or elevated pressure. For temperatures up to 100°C, ambient pressure will usually be used. At higher temperatures, leaching will be performed at elevated pressures according to the vapor pressure of the solution. Leaching is performed for a period of time in the range of from 1 hour to 20 hours ("caustic leaching"). In some embodiments of the process, the concentration of the alkali hydroxide or carbonate in the aqueous alkali hydroxide or carbonate solution is in the range of from 1 to 13 mol/l, for instance, from 2 to 3 mol/l. In some embodiments of the process, the aqueous alkali hydroxide or carbonate solution comprises from 3 to 50 wt% alkali hydroxide or carbonate, relative to the total weight of the aqueous alkali hydroxide or carbonate solution. In some embodiments of the process, the aqueous alkali hydroxide or carbonate solution used in step b) comprises from 8 to 15 wt% NaOH, relative to the total weight of the aqueous alkali hydroxide or carbonate solution. In some embodiments of the process, the mass ratio of leach residue (dry mass) to alkali hydroxide or carbonate (dry mass) is in the range of from 0.2 to 6.5.

Caustic leaching can be performed in a heated stirred vessel in cases of solid contents up to 30%. At higher solid contents, heated kneaders may be employed. Kneaders may be advantageous in cases where the impurity content is low, avoiding high stoichiometric excess of alkali hydroxide or carbonate. The amount of caustic in the caustic leaching step is mainly governed by the amount of Si and W. Thus, without wishing to be bound by theory, the amount of caustic should be at least stoichiometric with respect to the formation of the corresponding alkali silicates and tungstates.

After caustic leaching b), first solids are recovered from the mixture obtained in step b) by solid/liquid separation in a subsequent step c). In some embodiments of the process, solid/liquid separation involves filtration. In other embodiments of the process, solid/liquid separation involves sedimentation. In other embodiments of the process, solid/liquid separation involves centrifugation. In other embodiments, combinations of these methods are employed.

The solids are optionally washed in step d) with water, or first with alkali hydroxide solution and subsequently with water. In an optional step e), the water content of the washed first solids obtained in step d) is reduced, e.g., by drying the washed solids. In some embodiments, the water content of the washed first solids obtained in step d) is reduced to a value in the range of from 1 to 30 wt%.

In a subsequent step f), alkali hydroxide, e.g., sodium or potassium hydroxide, is added to the first solids obtained in step c), d), or e), in a mass ratio (dry mass) of 0.1 to 4, or from 0.1 to 10 in the case of black mass. The alkali hydroxide can be added as a solid or as an aqueous solution. The materials are thoroughly mixed to obtain a first mixture, which then is transferred to an oven for baking. When adding the alkali hydroxide as dry matter, the residue should preferably contain water to allow for at least partly dissolution of the alkali hydroxide. Otherwise, some water may be added. The mixing can be performed in a stirred vessel or in a kneader, the latter being preferred to limit the amount of water in the first mixture that is transferred to the oven. Alternatively, the mixture can be produced in a stirred vessel. Excess water may be evaporated afterwards, e.g., in drying ovens such as horde ovens, or in the baking oven as a pretreatment before the baking. The first mixture may be obtained as a paste with solid contents above 30%, or as powder with residual water contents of 3% or less.

In a subsequent step g), the first mixture obtained in step f) is heated to a temperature of from 350°C to 900°C, e.g., from 350°C to 550°C, and kept at the temperature for a period of time in the range of from 0.5 hrs to 2 hrs. It is preferred to heat the first mixture to temperatures above the melting point of the alkali hydroxide used. The mixture is then allowed to cool down, e.g., to ambient temperature, or to a temperature in the range of from 20°C to 100°C. Heating preferably is performed under inert atmosphere, e.g., an atmosphere of nitrogen or argon or other inert gases or steam. The residual oxygen content should be below the explosion limit of the carbon material present in the solid mixture.

To achieve thorough mixing during the baking process, rotary kilns are preferred. Directly heated or indirectly heated kilns may be used for the baking process.

In a subsequent step h), the baked material obtained in step g) is suspended in an aqueous medium comprising water or a solution of alkali hydroxide or alkali carbonate in water to obtain a second mixture. The second mixture is stirred or kneaded at temperatures in the range of from 20°C to 100°C for a duration of from 0.5 hrs to 10 hrs.

The (second) solids contained in the second mixture obtained in step h) are recovered by solid/liquid separation in step i) and washed with water in step j). Optionally, the second solids are washed first with alkali hydroxide or alkali carbonate solution and finally with water.

In a subsequent step k), the second solids obtained in step j) are leached with an aqueous acid to obtain a third mixture. In some embodiments of the process, the concentration of the aqueous acid is in the range of from 5 to 45 wt%, relative to the total weight of the aqueous acid. In some embodiments of the process, the concentration of the aqueous acid is in the range of from 35 to 45 wt% acid, relative to the total weight of the aqueous acid. In some embodiments of the process, the mass ratio of the second solids (dry mass) obtained in step j) to the aqueous acid is in the range of from 0.1 to 0.5, e.g., from 0.2 to 0.3. In some embodiments of the process, acid leaching is performed at a temperature in the range of from 50°C to 100°C, for instance, from 60°C to 80°C. The acid leaching may also be performed at temperatures above 100°C e. g. between 110 to 200°C in an autoclave. In some embodiments of the process, the second solids obtained in step j) are leached with acid for a period of time in the range of from 1 hour to 10 hours, for instance, from 2 hours to 5 hours.

In general, all strong acids like hydrochloric acid, nitric acid, methane sulfonic acid, and sulfuric acid, or organic acids like carboxylic acids may be employed. Mixtures of at least two of these acids may also be employed. However, sulfuric acid is often preferred mainly for cost reasons. In integrated plants comprising the acid treatment of black mass and the treatment of the carbon leach residue, it is preferred that the same type of acid is used in both stages. This is advantageous not only for limiting storage facilities, but also for the re-utilization of acids from one stage in the other.

In a subsequent step I), third solids are recovered from the third mixture obtained in step k) by solid/liquid separation. In some embodiments of the process, solid/liquid separation involves filtration. In other embodiments of the process, solid/liquid separation involves sedimentation. In other embodiments of the process, solid/liquid separation involves centrifugation. In other embodiments, combinations of these methods are involved.

In a final step m), the third solids obtained in step I) are washed with water to obtain a purified carbon material. The final product obtained is high-purity carbon, e.g., graphite, containing very low amounts of residual impurities. In some embodiments, the contents of tungsten and titanium in the final product each are below 10 ppm. In some embodiments, the content of zirconium in the final product is less than 150 ppm, e.g., less than 25 ppm. In some embodiments, the content of silicon in the final product is less than 100 ppm, e.g., less than 75 ppm. In some embodiments, the content of aluminum in the final product is less than 0.25 wt%. Element contents in the final product are measured using X-ray fluorescence spectroscopy (XRF). The present disclosure also provides a carbon material obtained from black mass from lithium-ion batteries or from a leach residue obtained by acid treatment of black mass from lithium-ion batteries, the carbon material having a content of zirconium, measured by XRF, of less than 25 ppm.

In some embodiments, the carbon material has contents of tungsten and titanium below 10 ppm, and a content of silicon of less than 100 ppm, e.g., less than 75 ppm, for instance, not more than 20 ppm. In some embodiments, the content of aluminum in the carbon material is less than 0.25 wt%, and the content of zirconium is less than 150 ppm, e.g., less than 25 ppm. Element contents in the carbon material are measured using X-ray fluorescence spectroscopy (XRF).

The advantages of the process of the present disclosure over prior art processes like those described in WO 2021/232090 A1 and WO 2023/081979 A1 include the lower number of processing steps, as the prior art processes comprise an additional acid leach step. Fewer process steps translate into a simpler process, less hardware and equipment, and thus significant cost reductions. Further, silicon is much better removed by the process of the present disclosure which involves a caustic leaching prior to the baking and acid leaching. An additional advantage of the process of the present disclosure is that it requires only one pH change from basic pH to acidic pH after the baking, while in the prior art process two pH-changes, one after the baking step and one after the acid leaching, are required which results in higher consumption of water and higher amounts of wastewater. In addition, it may be possible to directly introduce the filter cake, or the washed filter cake, preferably after washing with alkali hydroxide, of the caustic leach residue into the baking stage, avoiding dry mixing of NaOH and leach residue in a separate step.

The process of the present disclosure still requires a high amount of alkali hydroxides or carbonates. It is therefore preferred to recycle these reagents by suitable purification techniques and/or to introduce the corresponding caustic solutions in processes able to digest such solutions. Such processes may be hydrometallurgical processes requiring caustic for the precipitation of metal hydroxides or carbonates. One hydrometallurgical process of this kind is the hydrometallurgical processing of black masses mentioned above. Here, the precipitation of impurities like Fe or Al is often required. In some of these processes, a mixed metal hydroxide precipitate is formed as intermediate product. This mixed metal hydroxide precipitate may also be obtained by using the caustic solutions of the present inventive process. It is also possible to use these caustic solutions within the steps of the process of the present disclosure. For instance, the solution obtained in steps i) and j) may be recycled into step b).

The carbon material obtained by the process of the present disclosure from an acid leaching residue or from a black mass has a high purity. It is thus suitable for applications of graphite/carbon where high purity is required. Examples include metallurgical applications like steel recarburization, electrodes for electric arc furnaces, and also anodes for lithium-ion-batteries.

In some embodiments, the carbon material obtained by the process of the present disclosure is subjected to a high temperature graphitization treatment at temperatures of from 2,500 to 3,500°C. Such a treatment may repair defects in the graphite structure, lower the BET surface area of the carbonaceous material, and further improve the quality of the material. For an application in anodes of lithium-ion batteries, a low BET surface area is required.

Examples

Caustic leaching (L)

In the caustic leaching step, the leach residue obtained from an acid leaching process of black mass obtained from LIB, or black mass obtained from LIB, respectively, was mixed with aqueous sodium hydroxide solution. The suspension was stirred at temperatures between 20 and 100°C at ambient pressure for a period of 1 to 20 h. Then the mixture was filtered and the filter residue was dried.

Caustic Baking (B)

In the caustic baking step, the dried material was mixed with an aqueous solution of sodium hydroxide (50 wt% sodium hydroxide). The mass ratio of the solid to the solution was in the range of 1 to 0.5. This material was mixed in a rotating vessel equipped with baffles to ensure good mixing. The mixture was then transferred to a glassy carbon crucible placed in an oven and heated up to the desired temperature where it was kept for a defined period of time. After cooling down to ambient temperature, the crucible was removed from the oven and flushed with hot water (approx. 80°C). The slurry was then filtered and washed with sodium hydroxide solution and finally with water. The filter cake obtained was treated further without prior drying.

Acid Leaching (A)

In the acid leaching step, sulfuric acid was mixed with the wet filter cake obtained from the caustic baking step. The concentration of the sulfuric acid was varied between 5 wt% and 44 wt%. The mixture was heated to a temperature between 60 and 78°C under stirring and kept at the temperature for 168 to 200 min. Examples 1-12

Table 1 summarizes the reaction conditions used in the individual examples and the concentration of the impurity elements Al, Si, W, Ti, Zr in the feed and the final products obtained, as determined by XRF and ICP-OES.

The codes in the column "Sequence of steps" indicate the sequence of the steps L, B and A. The sequence BAL corresponds to the prior art process, the sequence LBA is the sequence presented in this disclosure. The sequence BLA also is comparative, as this example demonstrates that after the baking step, some elements cannot be removed any more by caustic leaching.

A leach residue obtained from an acid leaching process of black mass obtained from LIB containing 3.277 wt% Al, 0.988 wt% Si, 485 ppm W, 769 ppm Ti, 976 ppm Zr, (elemental composition measured by XRF) and 0.11 wt% F was used as starting material in Examples 1 to 10.

A black mass obtained from LIB containing 3.2 wt% Al, 0.17 wt% Si, 1 ,300 ppm W, 116 ppm Ti, 1 ,700 ppm Zr (elemental composition measured by XRF) , and 2.4 wt% F was used as starting material in Examples 11 and 12.

The results show that Si is efficiently removed only after a caustic leaching, either before baking (inventive) or after baking and acid leaching (comparative). Without prior caustic leaching, acid leaching after the baking step does not remove silicon. After the baking step, Si cannot be removed by a caustic leach. Si removal only is achieved by a prior acid leach directly after the baking step. Ti is more efficiently removed from the feed by the process of the present disclosure than by the comparative processes. Table 1 : Reaction conditions and composition of the final products obtained.

*LiOH instead of NaOH **Na₂CO₃ instead of NaOH

Example 13

An acid leach residue from an acid leaching process of black mass comprising 88.9 wt% C, 3.5 wt% Al, 1 .2 wt% Si, 0.06 wt% S, 0.02 wt% Li, 0.02 wt% Ni, 0.01 wt% Co, 0.01 wt% Mn, < 0.01 wt% Cu, and < 0.01 wt% Fe, and having a BET surface area of 10.3 m²/g, measured according to DIN ISO 9277:2014-01 (static volumetric method, 5 point determination), was treated in the following sequence of steps: i) Caustic leach (L)

The leach residue was mixed with a 11 wt% aqueous sodium hydroxide solution to form a slurry with a solid content of 24 wt%. This suspension was stirred at 90°C and ambient pressure for a period of 4 hrs. Then the mixture was filtered, and the filter residue was washed with water and dried. ii) Caustic Baking (B):

The filter residue from step (i) was mixed with a 50 wt% aqueous solution of sodium hydroxide in a mass ratio of solid to solution of 0.8. The material was mixed in a rotating vessel equipped with baffles to ensure good mixing. This mixture was then transferred to a glassy carbon crucible placed in an oven, heated to 500°C, and kept at 500°C for a period of 1 hr. After cooling down to ambient temperature, the crucible was removed from the oven and flushed with hot water. The slurry was then filtered and washed with water. The filter cake obtained was further treated in step iii) without prior drying. iii) Acid Leaching (A):

A 40 wt% sulfuric acid was mixed with the wet filter cake from step ii) to obtain an acidic slurry with an initial solid content of 9 wt%. The slurry was heated to 70°C under stirring and kept at this temperature for 3 hrs. The reaction mixture was then cooled to room temperature, filtered, washed with deionized water, and dried. The filter residue obtained in iii) contained 99.3 wt% C, 0.31 wt% Al, 0.03 wt% Si, < 0.05 wt% S, 0.002 wt% Li, 0.002 wt% Ni, < 0.001 wt% Co, 0.001 wt% Mn, < 0.001 wt% Cu, and < 0.001 wt% Fe, and had a BET surface area of 7.1 m²/g, measured according to DIN ISO 9277:2014-01 (static volumetric method, 5 point determination).

After graphitization of the filter residue at 2,800°C for 4 h, the BET surface area of the product obtained was 5. 1 m²/g, measured according to DIN ISO 9277:2014-01 (static volumetric method, 5 point determination).

Claims

1 . A process for the purification of carbon contained in i) black mass from lithium- ion batteries, or in ii) leach residues obtained by acid treatment of black mass from lithium-ion batteries, comprising a) providing a starting material selected from i) black mass from lithium-ion batteries or ii) a leach residue obtained by acid treatment of black mass from lithium-ion batteries, b) treating the starting material with aqueous alkali hydroxide or carbonate solution at a temperature in the range of from 20°C to 200°C at ambient or higher pressure for a period of time in the range of from 1 hour to 20 hours, c) recovering first solids from the mixture obtained in step b) by solid/liquid separation, d) optionally, washing the first solids obtained in step c), e) optionally, reducing the water content of the first solids obtained in step d), f) adding alkali hydroxide as a solid or as an aqueous solution to the first solids obtained in step c), d) or e) in a mass ratio of solids (taken as dry matter) to alkali hydroxide (taken as dry matter) in the range of from 0.1 to 10 to obtain a first mixture, g) heating the first mixture obtained in step f) to a temperature of from 350°C to 900°C to obtain a baked material, h) suspending the baked material obtained in step g) in an aqueous medium to obtain a second mixture, i) recovering second solids from the second mixture obtained in step h) by solid/liquid separation, j) washing the second solids obtained in step i) with water, k) leaching the second solids obtained in step j) with an aqueous acid to obtain a third mixture, l) recovering third solids from the third mixture obtained in step k) by solid/liquid separation, m) washing the third solids obtained in step I) with water to obtain a purified carbon material.

2. The process of claim 1 , wherein the starting material contains carbon contained in the black mass, and impurities insoluble in acid which comprise compounds of Al, P, Si, Sn, Ti, W, Zn and/or Zr.

3. The process of claim 1 or 2, wherein the alkali hydroxide or carbonate solution used in step b) comprises NaOH, Na₂CO3, KOH, or K2CO3.

4. The process of any one of claims 1 to 3, wherein the aqueous alkali hydroxide or carbonate solution used in step b) comprises from 8 to 15 wt% NaOH, relative to the total weight of the aqueous alkali hydroxide or carbonate solution.

5. The process of any one of claims 1 to 4, wherein in step e) the water content of the washed first solids obtained in step d) is reduced to a value in the range of from 1 to 30 wt%.

6. The process of any one of claims 1 to 5, wherein in step g) the first mixture obtained in step f) is heated and kept at a temperature of from 350°C to 550°C for a period of time in the range of from 0.5 hours to 2 hours.

7. The process of any one of claims 1 to 6, wherein the aqueous acid used in step k) is selected from hydrochloric acid, nitric acid, methane sulfonic acid or sulfuric acid or a mixture of at least two of these acids.

8. The process of any one of claims 1 to 7, wherein the concentration of the aqueous acid used in step k) is in the range of from 35 to 45 wt% acid, relative to the total weight of the aqueous acid.

9. The process of any one of claims 1 to 8, wherein the starting material is a carbon material obtained by solid-solid separation from black mass from lithium- ion batteries or from a leach residue obtained by acid treatment of black mass from lithium-ion batteries.

10. The process of any one of claims 1 to 9, wherein the purified carbon material obtained in step m) is further subjected to a graphitization step at a temperature in the range of from 2,500°C to 3,500°C.

11. A carbon material obtained from i) black mass from lithium-ion batteries or from ii) a leach residue obtained by acid treatment of black mass from lithium-ion batteries, wherein the content of zirconium in the carbon material, measured by XRF, is less than 25 ppm.

12. A carbon material obtained from i) black mass from lithium-ion batteries or from ii) a leach residue obtained by acid treatment of black mass from lithium-ion batteries, wherein the content of silicon in the carbon material, measured by XRF, is not more than 20 ppm.