WO2023081979A1

WO2023081979A1 - Improved method of producing purified graphite

Info

Publication number: WO2023081979A1
Application number: PCT/AU2022/051353
Authority: WO
Inventors: Michael Chan
Original assignee: Ecograf Limited
Priority date: 2021-11-15
Filing date: 2022-11-11
Publication date: 2023-05-19
Also published as: AU2022387279A1; CA3237999A1

Abstract

An improved method of producing purified graphite is described in which graphite material is mixed with a solid phase alkali to form a solid alkali-graphite mixture. The solid alkali-graphite mixture is subject to an alkali bake, and then any remaining alkali is released using water. The graphite material is then subject to a first acid wash followed by neutralising and washing the acid washed graphite material to deliver an intermediate purified graphite product. The intermediate purified graphite product is then subject to an alkali leach, and then any remaining alkali in the intermediate purified graphite product is released using water. The intermediate purified graphite product is then subject to an acid wash, which is followed by neutralising and washing the intermediate purified graphite product to deliver a final purified graphite product.

Description

“IMPROVED METHOD OF PRODUCING PURIFIED GRAPHITE”

Field of the Invention

The present invention relates to an improved method of producing purified graphite and relates particularly, though not exclusively, to such a process for producing battery-grade spherical purified graphite (SPG).

Background to the Invention

Spherical graphite is manufactured from flake graphite concentrates in which the flakes have to be rounded to produce rounded particles of graphite. The spherical graphite can be spread thinly and uniformly during the manufacturing process and is used for the anode material in lithium ion batteries. Spherical purified graphite (SPG) is sold as either a coated or uncoated product. Uncoated SPG is manufactured by micronizing, rounding and purifying flake graphite. The bulk of natural, uncoated SPG is currently manufactured in China due low labour costs and weak environmental regulations. The micronized, rounded graphite is chemically purified from 94% to 99.95% C using strong hydrofluoric and other acids. Impurities will have a deleterious effect on battery performance.

Apart from the environmental concerns, the hydrofluoric acid typically employed in the prior art purification process is also highly toxic and hazardous to use. A great deal of effort is therefore being expended to find a more cost-effective, non-toxic and environmentally sustainable process for the purification of spherical graphite.

With the widespread use of lithium-ion batteries, there is now also a significant market for the recycling of these kinds of batteries. In typical prior art recycling plants, the lithium-ion batteries are first made safe for further treatment, by separating the plastics, aluminium and copper components and directing them to their own recycling processes. The remaining components of the battery after these processes are the chemical and mineral components. “Black mass” is the residue remaining from existing hydrometallurgical processes that recover the valuable metals. The black mass typically consists of a mixture of carbon, lithium, manganese, cobalt and nickel in various ratios, including anode material which consists of natural battery graphite, synthetic graphite and silicon. As graphite comprises almost 50% of the mass of a typical lithium-ion battery, there is a significant benefit to be gained in being able to purify the black mass material for the recovery of high purity graphite.

Co-pending International Application No. PCT/AU2021/050453 describes a cost-effective and environmentally sustainable method of producing purified graphite with carbon content higher than 99%C. In that method, a front-end step of subjecting the spherical graphite material to a caustic bake typically comprises mixing the spherical graphite material with liquid NaOH (50% w/w) and heating the mixture in a furnace or kiln. The present invention was developed with a view to providing an improved method by incorporating a more efficient solid-solid phase front-end alkali bake.

Although the present invention is described with particular reference to producing battery-grade SPG it will be understood that the method of producing purified graphite may also have wider applications.

References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.

Summary of the Invention

According to one aspect of the present invention there is provided a method of producing purified graphite, the method comprising the steps of: mixing graphite material with a solid phase alkali to form a homogenous solid alkali-graphite mixture; subjecting the solid alkali-graphite mixture to an alkali bake; releasing any remaining alkali 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 an alkali leach; releasing any remaining alkali in the intermediate purified graphite product using water; subjecting the intermediate purified graphite product to an acid wash; and, neutralising and washing the intermediate purified graphite product to deliver a final purified graphite product.

Typically the step of subjecting the solid alkali-graphite mixture to an alkali bake comprises heating the mixture in a furnace. Preferably the alkali is sodium hydroxide (NaOH). Preferably the solid phase NaOH is >99.0% pure and is in the form of solid flake, powder or mini pearl sized between 0.5- 0.9mm. Advantageously the graphite material is mixed with the pure NaOH in a high shear mixer to provide homogeneity in mixing. Typically the high shear mixer is a planetary type mixer or conical screw type mixer. Preferably the mixture is mixed in the high shear mixer for a period of 30 to 60 minutes and then fed into the furnace. Typically the ratio of the solid NaOH to graphite material is 0.33kg of pure NaOH per 1 .0 kg of graphite material. Preferably the mixture is heated to a temperature of between 450° C and 600° C. Advantageously the mixture is heated for between approximately 30 to 60 minutes. Preferably the mixture is heated to 500°C for 30 minutes.

Preferably the step of releasing any remaining alkali comprises immersing the material in water. Advantageously the step of releasing any remaining NaOH also releases any soluble sodium silicates (Na2SiOa) and sodium aluminates (NaAIOs) that are formed during the NaOH bake. Preferably the step of releasing any remaining NaOH and soluble silicates (Na2SiOa) and aluminates (NaAIC ) formed during the NaOH bake comprises immersing the baked material in a hot water quench.

Preferably after the step of releasing the remaining NaOH and soluble silicates and aluminates, the material is washed and filtered. Typically, the material is washed and filtered in a wash neutral step at ambient temperature to 80°C, for 5-10 minutes, using 2.0 to 7.0 parts by volume of H₂O.

Preferably the step of subjecting the graphite material to a first acid wash comprises washing the material with a diluted acid mixture comprising water and H2SO4. Typically, the diluted acid mixture comprises 2.0 to 5.0 parts by volume of H2O and between 0.15 and 0.25 parts by volume H2SO4 96%. Advantageously the diluted acid mixture comprises 4 parts by volume H2O and 0.17 parts by volume H2SO4 96%. Preferably the first acid wash is performed at elevated temperatures in the range of approximately 70°C to 90°C to increase reactivity. More preferably the first acid wash is performed at an elevated temperature of 80°C. Typically the graphite material is subjected to an acid wash for 25 to 45 minutes.

Preferably the step of neutralizing and washing the acid washed graphite material comprises the filtering the material at ambient temperature and final washing with water at elevated temperatures respectively. Typically the final washing with water occurs at 85°C for 30 minutes using 20 parts by volume H2O. Preferably the step of neutralizing and washing the material comprises washing the acid washed graphite material in 3 to 7 parts by volume H2O for 3 to 7 minutes while stirring at ambient temperature to generate an intermediate purified graphite product with a purity of +99.4%C.

Preferably the step of subjecting the intermediate purified graphite product to an alkali leach comprises immersing the intermediate purified graphite product in 0.05 to 0.15 parts parts by volume alkali (99%) with 2.0 to 4.0 parts by volume H2O at low temperature. Preferably the alkali is sodium hydroxide (NaOH). Typically the NaOH leach takes place at temperatures in the range of 72°C to 88°C. Preferably the residence time for the NaOH leach is 1 .5 to 2.5 hours.

Afterwards the intermediate purified graphite product is preferably neutralized and washed, before it is subject to an acid wash step in a diluted acid mixture, where a very low concentration of H2SO4 is sufficient. Preferably the acid wash of the intermediate purified graphite product is performed with a diluted acid mixture comprising between 2.0 and 4.0 parts by volume H2O and 0.01 to 0.04 parts by volume H2SO4 96%. More preferably the diluted acid mixture comprises 2.5 parts by volume H2O and 0.03 parts by volume H2SO4 96%, and the acid wash is performed at elevated temperatures in the range of approximately 77°C to 93°C to increase reactivity. Preferably the acid wash of the intermediate purified graphite product is performed at an elevated temperature of 80°C for approximately 35 minutes.

Advantageously the only reagents used are NaOH in the alkali bake and alkali leach steps, and H2SO4 in the acid wash steps.

Preferably the intermediate purified graphite product is finally washed in a water bath and neutralized with water after the acid wash step.

By incorporating an alkali leach step (such as an NaOH leach step), with low consumption of chemicals, carbon content of the final purified graphite product can be increased to 99.97%.

The term ‘alkali’ as used throughout the specification refers to any of the soluble hydroxides of the alkali metals, including lithium, sodium and potassium, suitable for carrying out the alkali bake or alkali leach.

Throughout the specification, 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. Likewise the word “preferably” or variations such as “preferred”, will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention.

Brief Description of the Drawings

The nature of the invention will be better understood from the following detailed description of several specific embodiments of the method of producing purified graphite, given by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a process flow diagram illustrating an embodiment of the method of producing purified spherical graphite according to the present invention.

Detailed Description of Preferred Embodiments

One embodiment of the method 10 of producing purified graphite in accordance with the invention, as illustrated in Figure 1 , comprises the step of subjecting unpurified spherical graphite (SPG) 12 to a molten-salt solidsolid phase alkali bake, as shown at 18. The starting material 12 is typically 1 part w/w SPG to 20 parts w/w un-purified spherical graphite material. Preferably the step of subjecting the spherical graphite material to an alkali bake comprises premixing the graphite material at 14 with pure alkali to form a homogenous mixture and heating the mixture in a furnace or kiln. In this embodiment the alkali is sodium hydroxide (NaOH). Preferably the solid phase NaOH is >99.0% pure and is in the form of solid flake, powder or mini pearl sized between 0.5-0.9mm. Advantageously the graphite material is premixed with the pure NaOH in a planetary type, or conical screw, high shear mixer to provide homogeneity in mixing. Preferably the mixture is premixed in the high shear mixer for a period of 30 to 60 minutes and then fed via a screw feeder at 16 into the furnace or kiln. Typically the ratio of the solid NaOH to graphite material is 0.33kg of pure NaOH per kg of graphite material. The solid mixture is observed to be thoroughly homogeneous as the NaOH solid appeared to be well integrated with the graphite. There was no stratification of the solids at the bottom of the mixer.

Preferably the mixture is heated to a temperature of between 450° C and 600° C. Advantageously the mixture is heated for between approximately 30 to 60 minutes. The weight of the material was checked before and after the heat treatment at different temperatures. The results indicate that no graphite burns at a process temperature of 500°C if the residence time does not exceed 45 minutes. More preferably the solid mixture is heated to 500°C for 30 minutes. After the alkali bake at 18 the material has a homogenous solid but friable consistency.

NaOH melts at around 318°C with the NaOH mixing within the kiln to initiate the removal of impurities in the graphite, in the bed of the kiln, which is at a temperature of 500°C. Mixing of the dry solids enables better introduction to the kiln of the graphite and alkali mixture with the mixing of the molten alkali and graphite promoted by the temperature and residence time in the kiln.

The method further comprises the step 20 of releasing any remaining NaOH and soluble sodium silicates (Na2SiOa) and sodium aluminates (NaAIOs) that were formed during the baking process, using water. Preferably the step 20 of releasing any remaining NaOH and soluble silicates (Na2SiOa) and aluminates (NaAIO2) formed during the NaOH bake comprises immersing the solid baked material in water for about 10-45 minutes. Typically this is done in warm water, for example, water heated to between 40°C to 85°C. Typically 1 part baked spherical graphite material is immersed in from 3 to 20 parts by volume H2O. In this embodiment 1 part baked spherical graphite material is immersed in 17 parts by volume H2O at about 85°C, for 30 minutes.

Preferably after the step of releasing the remaining NaOH and soluble silicates and aluminates, the material is washed and filtered. Typically, the material is washed and filtered in a wash neutral step 22 at ambient temperature to 80°C, stirring for about 3-10 minutes, using 2.0 to 7.0 parts by volume of H2O. More preferably the wash neutral step 22 is performed at ambient temperature, stirring for about 5 minutes, using 2.3 parts by volume of H₂O.

The method preferably further comprises the step of subjecting the baked graphite material to a first acid wash at step 24. In this embodiment the step of subjecting the graphite material to an acid wash comprises washing the material with a diluted acid mixture comprising water and H2SO4. Typically, the diluted acid mixture comprises 2.0 to 5.0 parts by volume of H2O and between 0.15 and 0.25 parts by volume H2SO4 96%. Advantageously the diluted acid mixture comprises 4 parts by volume H2O and 0.17 parts by volume H2SO4 96%. Preferably the first acid wash is performed at elevated temperatures in the range of approximately 70°C to 90°C to increase reactivity. More preferably the first acid wash is performed at an elevated temperature of 80°C. Typically the graphite material is subjected to an acid wash for about 25 to 45 minutes.

Preferably the first acid wash step 24 is followed by another wash neutral step 26. The step 26 of neutralizing and washing the acid washed graphite material comprises the filtering the material at ambient temperature and final washing with water at elevated temperatures respectively. Preferably the step 26 of neutralizing and washing the material comprises washing the acid washed graphite material in 3 to 7 parts by volume H2O for 3 to 7 minutes while stirring at ambient temperature. More preferably the wash neutral step 26 comprises stirring the acid washed graphite material in 3 parts by volume H2O at ambient temperature for 5 minutes while stirring. Typically this is followed by the final washing with water in water bath 28 at ambient temperature for 20 minutes using 3.3 parts by volume H2O to generate an intermediate purified graphite product with a purity of +99.4%C.

All wet process steps in the above-described method were made under constant stirring.

The method 10 further comprises a second stage which begins with the step 30 of subjecting the intermediate purified graphite product to an alkali leach. Preferably the alkali leach step 30 comprises immersing the intermediate purified graphite product in 0.05 to 0.15 parts by volume NaOH (99%) with 2.0 to 4.0 parts by volume H2O at low temperature. Typically the alkali leach takes place at temperatures in the range of 72°C to 88°C. Preferably the residence time for the NaOH leach is about 1.5 to 2.5 hours. More preferably the NaOH leach step 30 is performed at 80°C for about 120 minutes.

Afterwards the intermediate purified graphite product is preferably neutralized and washed in wash neutral step 32, water bath 34 and wash neutral step 36, before it is subject to a second acid wash step 38. The acid wash step 38 is performed in a diluted acid mixture, where a very low concentration of H2SO4 is sufficient. Typically, the diluted acid mixture comprises between 2.0 and 4.0 parts by volume H2O and 0.01 to 0.04 parts by volume H2SO4 96%. More preferably the diluted acid mixture comprises 2.5 parts by volume H2O and 0.03 parts by volume H2SO4 96%. Typically, the acid wash step 38 is performed at elevated temperatures in the range of approximately 77° C to 93° C to increase reactivity. Preferably the acid wash of the intermediate purified graphite product is performed at an elevated temperature of 80°C for approximately 35 minutes.

Advantageously the only reagents used are NaOH in the alkali bake and alkali leach steps, and H2SO4 in the acid wash step.

Preferably the intermediate purified graphite product is finally neutralized and washed with water after the acid wash step in wash neutral steps 40 and 44 and a water bath 42. Preferably wash neutral step 40 comprises washing the acid washed intermediate purified graphite product with 3.5 parts by volume H2O at ambient temperature for 5 minutes with stirring. The acid washed intermediate purified graphite product is then preferably subject to washing with water in water bath 42 using 3.3 parts by volume H2O at 80°C for 30 minutes. Final wash neutralisation in preferably occurs in wash neutral step 44 using 5 parts by volume H2O, 2 times 10/2 each, for 5 minutes with stirring. By incorporating an alkali (caustic) leach step 30, and low consumption of chemicals, carbon content of the final purified graphite product can be increased to 99.97%.

The above method 10 involves a relatively simple process, and the carbon content of 99.97% achieved by this process indicates that the purification is very intensive, with only 300 ppm of remaining impurities.

Test Work

Test work is in progress.

Although the above-described embodiments describe a method of purifying graphite using unpurified spherical graphite (SPG) as the starting material, the same chemical purification process can also be used for purifying black mass, a residue produced in the recycling of Lithium-ion batteries. Black mass is the residue remaining from existing hydrometallurgical processes that recover the valuable metals. The purification process increases the grade from 30-50% carbon to +99% carbon to re-use in graphite markets.

Now that a preferred embodiment of the method of purifying graphite material has been described in detail, it will be apparent that the described embodiment provides a number of advantages over the prior art, including the following:

(i) The preferred method has the advantage of overcoming feed problems by conveying a dry mix of alkali (NaOH) and graphite to the furnace or kiln. The mixing of dry materials in a high shear mixer also provides ease of material flow. (ii) It has lower energy requirements than when using liquid NaOH

(50%) and provides more efficient contact between the molten NaOH and graphite in the furnace or kiln.

(iii) The chemicals used in the method are more environmentally sustainable than prior art methods and do not harm the spherical graphite product.

It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. For example, the method comprises a multi-step and multi-parameter (temperatures, residence time, concentration of acids and caustic, volume of wash water, etc.) process, and therefore sophisticated optimization will undoubtedly yield further improvements in purity and cost savings. Furthermore although the process described is applied to unpurified SPG, it is not limited to spherical graphite but could also be applied using other types of flake graphite as a precursor material, such as screened fractions of a flake graphite concentrate or a by-product from spherical graphite production. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described.

Claims

1 . A method of producing purified graphite, the method comprising the steps of: mixing graphite material with a solid phase alkali to form a homogenous solid alkali-graphite mixture; subjecting the solid alkali-graphite mixture to an alkali bake; releasing any remaining alkali 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 an alkali leach; releasing any remaining alkali in the intermediate purified graphite product using water; subjecting the intermediate purified graphite product to an acid wash; and, neutralising and washing the intermediate purified graphite product to deliver a final purified graphite product.

2. A method of producing purified graphite as defined in claim 1 , wherein the step of subjecting the solid alkali-graphite mixture to an alkali bake comprises heating the mixture in a furnace.

3. A method of producing purified graphite as defined in claim 2, wherein the alkali is sodium hydroxide (NaOH).

4. A method of producing purified graphite as defined in claim 3, wherein the solid phase NaOH is >99.0% pure and is in the form of solid flake, powder or mini pearl sized between 0.5-0.9mm.

5. A method of producing purified graphite as defined in claim 3 or claim 4, wherein the graphite material is mixed with pure NaOH in a high shear mixer to provide homogeneity in mixing.

6. A method of producing purified graphite as defined in claim 5, wherein the high shear mixer is a planetary type mixer or conical screw type mixer.

7. A method of producing purified graphite as defined in claim 5 or claim 6, wherein the mixture is mixed in the high shear mixer for a period of 30 to 60 minutes and then fed into the furnace.

8. A method of producing purified graphite as defined in claim 3, wherein the ratio of the solid NaOH to graphite material is 0.33kg of pure NaOH per 1 .0 kg of graphite material.

9. A method of producing purified graphite as defined in claim 2, wherein the mixture is heated to a temperature of between 450° C and 600° C.

10. A method of producing purified graphite as defined in claim 9, wherein the mixture is heated for between approximately 30 to 60 minutes.

11 . A method of producing purified graphite as defined in claim 10, wherein the mixture is heated to 500°C for 30 minutes.

12. A method of producing purified graphite as defined in claim 3, wherein the step of releasing any remaining alkali comprises immersing the material in water.

13. A method of producing purified graphite as defined in claim 12, wherein the step of releasing any remaining NaOH also releases any soluble sodium silicates (Na2SiOa) and sodium aluminates (NaAIO2) that are formed during the NaOH bake.

14. A method of producing purified graphite as defined in claim 13, wherein the step of releasing any remaining NaOH and soluble silicates (Na2SiOa) and aluminates (NaAIO2) formed during the NaOH bake comprises immersing the baked material in a hot water quench.

15. A method of producing purified graphite as defined in claim 14, wherein after the step of releasing the remaining NaOH and soluble silicates and aluminates, the material is washed and filtered. 14

16. A method of producing purified graphite as defined in claim 15, wherein the material is washed and filtered in a wash neutral step at ambient temperature to 80°C, for 5-10 minutes, using 2.0 to 7.0 parts by volume of H₂O.

17. A method of producing purified graphite as defined in claim 1 , wherein the step of subjecting the graphite material to a first acid wash comprises washing the material with a diluted acid mixture comprising water and H2SO4.

18. A method of producing purified graphite as defined in claim 17, wherein the diluted acid mixture comprises 2.0 to 5.0 parts by volume H2O and between 0.15 and 0.25 parts by volume H2SO496%.

19. A method of producing purified graphite as defined in claim 18, wherein the diluted acid mixture comprises 4 parts by volume H2O and 0.17 parts by volume H2SO496%.

20. A method of producing purified graphite as defined in claim 1 , wherein the first acid wash is performed at elevated temperatures in the range of approximately 70°C to 90°C to increase reactivity.

21. A method of producing purified graphite as defined in claim 20, wherein the first acid wash is performed at an elevated temperature of 80°C.

22. A method of producing purified graphite as defined in claim 21 , wherein the graphite material is subjected to an acid wash for 25 to 45 minutes.

23. A method of producing purified graphite as defined in claim 1 , wherein the step of neutralizing and washing the acid washed graphite material comprises the filtering the material at ambient temperature and final washing with water at elevated temperatures respectively.

24. A method of producing purified graphite as defined in claim 23, wherein the final washing with water occurs at 85°C for 30 minutes using 20 parts by volume H2O.

25. A method of producing purified graphite as defined in claim 23, wherein the step of neutralizing and washing the material comprises washing the 15 acid washed graphite material in 3 to 7 parts by volume H2O for 3 to 7 minutes while stirring at ambient temperature to generate an intermediate purified graphite product with a purity of +99.4%C.

26. A method of producing purified graphite as defined in claim 1 , wherein the step of subjecting the intermediate purified graphite product to an alkali leach comprises immersing the intermediate purified graphite product in 0.05 to 0.15 parts by volume alkali (99%) with 2.0 to 4.0 parts by volume H2O at low temperature.

27. A method of producing purified graphite as defined in claim 26, wherein the alkali is sodium hydroxide (NaOH).

28. A method of producing purified graphite as defined in claim 27, wherein the NaOH leach takes place at temperatures in the range of 72°C to 88°C.

29. A method of producing purified graphite as defined in claim 28, wherein the residence time for the NaOH leach is 1 .5 to 2.5 hours.

30. A method of producing purified graphite as defined in claim 1 , wherein after the alkali leach the intermediate purified graphite product is neutralized and washed, before it is subject to an acid wash step in a diluted acid mixture, where a very low concentration of H2SO4 is sufficient.

31. A method of producing purified graphite as defined in claim 30, wherein the acid wash of the intermediate purified graphite product is performed with a diluted acid mixture comprising between 2.0 and 4.0 parts by volume H2O and 0.01 to 0.04 parts by volume H2SO4 96%.

32. A method of producing purified graphite as defined in claim 31 , wherein the diluted acid mixture comprises 2.5 parts by volume H2O and 0.03 parts by volume H2SO4 96%, and the acid wash is performed at elevated temperatures in the range of approximately 77°C to 93°C to increase reactivity.

16 33. A method of producing purified graphite as defined in claim 32, wherein the acid wash of the intermediate purified graphite product is performed at an elevated temperature of 80°C for approximately 35 minutes.

34. A method of producing purified graphite as defined in claim 1 , wherein the only reagents used are NaOH in the alkali bake and alkali leach steps, and H2SO4 in the acid wash steps.

35. A method of producing purified graphite as defined in claim 1 , wherein the intermediate purified graphite product is finally washed in a water bath and neutralized with water after the acid wash step.