Process for converting solid carbon source to graphite

Abstract

Methods and apparatus are provided for converting a solid carbon source to graphite by precipitation from a molten metal (e.g., molten iron), as well as the graphite prepared by the methods and apparatus.

Description

[0001] Priority

[0002] This application claims priority to U.S. Provisional Application No. 63 / 389,300, filed on July 14, 2022. The entire content of the U.S. Provisional Patent Application is hereby incorporated by reference. Background Art

[0003] The growing sales of electric vehicles (2.6% market share in 2019) have driven a significant increase in global lithium-ion battery production capacity, which has severely disrupted the traditional graphite market supply chain. Currently, coated spherical graphite is used at a rate of approximately 1.2 kg / kWh of storage capacity in lithium-ion batteries. Therefore, a typical 75 kWh electric vehicle will contain approximately 90 kg of graphite. In 2019, the production of 2.1 million electric vehicles required 190,000 t of spherical coated graphite, 480,000 t of large flake natural graphite (∼2.5× spherical), and 960,000 t of mined natural graphite (∼2× flake)—note that approximately 80% of the mined natural graphite ultimately ends up as secondary graphite fines. Why natural graphite? Because it is cheaper ($8,000 / t compared to $13,000 / t for synthetic graphite) and has better performance (higher capacity retention). Thus, in the past 15 years, battery manufacturers have shifted from using approximately 75% synthetic graphite to 65% natural graphite. This shift has spurred a global rush to develop over 40 new natural graphite mines, adding 1.9 Mt of new graphite production capacity and 1.5 Mt of secondary graphite fines production capacity, attempting to capture the existing market. All this means that secondary natural graphite fines are expected to displace petroleum coke-derived synthetic graphite from the market and become a low-value heating fuel—resulting in up to 10 Mt of additional CO2 greenhouse gas emissions per year.

[0004] Conventional synthetic graphite production uses electricity to heat pressed petroleum coke briquettes to temperatures above 2800°C for 2 to 3 weeks to overcome the high activation barrier and the slow rate of solid-state atomic carbon recombination (at 2350°C, E a ∼680 kJ / mol and D ∼ 1.0×10 -12 cm 2 / s), thus forming thermodynamically favorable crystalline graphite—resulting in an energy consumption of approximately 22 GJ / t, 4.4 t / t of CO2 emissions, and a production cost of synthetic graphite of over $13,000 / t.

[0005] Currently, petroleum coke is purified by calcination to remove moisture, drive off volatile substances and produce anode-grade coke with the required actual density level of the highest purity, high physical strength and electrical conductivity, and the lowest porosity and reactivity. The calcination process is carried out at temperatures up to 1200 °C to 1400 °C with a contact time of 0.5 to 48 hours, depending on the nature of the petroleum coke and the process. There are three continuous calcination processes for petroleum coke: rotary kilns, rotary hearth furnaces and shaft furnaces. Even at high temperatures, these processes still produce 1 wt% sulfur, which greatly hinders performance. The gases produced by these processes cause "puffing", i.e., increased porosity, decreased density, increased air reactivity, and decreased mechanical strength, electrical conductivity and thermal conductivity. There is a need for an improved method of producing graphite. In particular, there is a need for a method of economically producing graphite that meets market specifications for purity, crystallinity and electrical conductivity. Summary of the Invention

[0006] The applicant has discovered a more environmentally friendly improved method for economically preparing graphite that meets market specifications for purity, crystallinity and electrical conductivity. Thus, in one embodiment, the present invention provides a method for preparing graphite, comprising:

[0007] a) adding a solid carbon source to a molten metal at a first temperature to provide a solution containing dissolved carbon;

[0008] b) lowering the temperature of all or part of the solution under conditions that allow graphite to form; and

[0009] c) separating the graphite.

[0010] In another embodiment, the present invention provides a method for preparing graphite, comprising:

[0011] providing a molten metal having a temperature gradient that includes a hot zone having a first temperature and a cooler zone having a second temperature lower than the hot zone;

[0012] adding a solid carbon source to the hot zone; and

[0013] separating the graphite formed in the cooler zone.

[0014] In another embodiment, the present invention provides a method for preparing graphite, comprising:

[0015] maintaining a first chamber at a first temperature;

[0016] maintaining a second chamber at a second temperature lower than the first temperature, the second chamber having a path leading to the first chamber;

[0017] adding a solid carbon source to the molten metal in the first chamber to provide a solution;

[0018] Permit the solution to pass through a path from a first chamber to a second chamber; and

[0019] Separate graphite from the second chamber.

[0020] In some aspects of the present invention, molten metal (such as iron, nickel, and their alloys) alloys are used as effective separating agents to remove impurities from petroleum coke, thereby obtaining high-purity graphite, which is in contrast to methods in which graphite is a by-product (waste) of steelmaking rather than a dedicated product. Relative to molten iron (7.01 g / cm 3 ), the low density of graphite (2.27 g / cm 3 ) makes it easy to separate in situ. For example, by introducing an inert carrier gas (N2 or Ar) at an apparent velocity (e.g., 20 m / s to 30 m / s), which can separate, fluidize, and transport graphite particles (d p <10 mm), as shown in Figure 1b.

[0021] According to the disclosed embodiments, a device for converting a solid carbon source into highly crystalline graphite is disclosed. The device includes a tube having a horizontal tube axis, a crucible, and a heating unit. The crucible has a horizontal crucible axis that is substantially aligned with the horizontal tube axis. The crucible has a first crucible end and a second crucible end. The crucible is substantially positioned within the tube. The crucible is for containing a metal and a solid carbon source. The heating unit generates a temperature gradient along the horizontal crucible axis. The temperature gradient decreases from the first crucible end to the second crucible end. And highly crystalline graphite is produced at the second crucible end.

[0022] In some embodiments, the device further includes a gas source coupled to the tube for supplying an inert gas to the tube. In some embodiments, the tube comprises quartz. In some embodiments, the heating unit includes a refractory brick thermally coupled to the crucible and an induction heating coil inductively coupled to the refractory brick. The tube is substantially located within the induction heating coil.

[0023] According to the disclosed embodiments, a device for converting a solid carbon source into highly crystalline graphite is disclosed. The device includes a reactor that includes a container and a carbon heating element. The container has a first input port for receiving nitrogen gas and includes a carbon heating element to provide energy for liquefying a solid metal to a solid carbon source. A second input port is provided to receive the solid carbon source, and an output port is provided to deliver nitrogen gas and highly crystalline graphite as an output. The container also houses a solid metal to be liquefied. The device further includes a container heating unit to provide a temperature gradient within the container.

[0024] In some embodiments, the container heating unit can provide a temperature gradient that has a temperature ranging from about 1700 degrees Celsius at the high-temperature section of the container to about 1300 degrees Celsius at the low-temperature section of the container. In some embodiments, the low-temperature section of the container is closer to the output port than the high-temperature section of the container. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figures 1A to 1B Shows: ( Figure 1A ) a carbon-iron binary phase diagram, which represents a process path; and ( Figure 1B ) a conceptual diagram of the process.

[0026] FIGS. 2A through 2D show a flow-through reactor that can be used to implement the method of the present invention.

[0027] Figure 3 Shows a diagram of an apparatus for converting a solid carbon source into highly crystalline graphite according to some embodiments of the present disclosure.

[0028] Figure 4 Shows a diagram of an apparatus for converting a solid carbon source into highly crystalline graphite according to some embodiments of the present disclosure.

[0029] Figures 5A to 5B Compares the X-ray diffraction patterns of graphite raw materials ( Figure 5A commercial graphite and Figure 5B amorphous carbon) with the highly crystalline graphite produced by the present invention.

[0030] Figure 6 Shows a photograph of a graphite foil produced on the surface of a nickel melt using the furnace shown in FIGS. 2A through 2D.

[0031] Figure 7 Shows a diagram of an apparatus for converting a solid carbon source into highly crystalline graphite according to some embodiments of the present disclosure. DETAILED DESCRIPTION

[0032] As used herein, the term "solid carbon source" includes any suitable carbon source for the method or apparatus of the present invention. In one embodiment, the solid carbon source includes petroleum coke (e.g., petcoke), coal, charcoal, graphite fines, or amorphous carbon or vitreous carbon. The term includes solid and liquid carbon sources, but does not include gaseous carbon sources.

[0033] In one embodiment, the graphite product is at least 99% carbon. In one embodiment, the graphite comprises at least 95% graphite crystals. In one embodiment, the graphite comprises at least 98% graphite crystals. In one embodiment, the graphite comprises at least 99% graphite crystals. In one embodiment, the graphite is at least 99% carbon and comprises at least 95% graphite crystals. In one embodiment, the graphite is at least 99% carbon and comprises at least 98% graphite crystals. In one embodiment, the graphite is at least 99% crystalline graphite.

[0034] "Molten metal" includes any molten metal applicable to the methods or apparatuses of the present invention. In one embodiment, the molten metal includes iron, silicon, nickel, copper, germanium, manganese, bismuth, or silver, or a mixture thereof. In one embodiment, the molten metal is a molten ferroalloy. In one embodiment, the molten metal includes iron, silicon, or nickel, or a mixture thereof.

[0035] In one embodiment, a solid carbon source is added to the molten metal at a temperature below about 2000°C. In one embodiment, a solid carbon source is added to the molten metal at a temperature below about 1700°C. In one embodiment, graphite is formed at a temperature below about 1500°C. In one embodiment, graphite is formed at a temperature below about 1300°C. In one embodiment, graphite is formed at a temperature below about 1200°C.

[0036] In one embodiment, separating the graphite includes collecting the graphite for future sale.

[0037] In one embodiment, the solid carbon is added in an oxygen-free environment. "Oxygen-free environment" means an environment containing less than about 5%, 4%, 3%, 2%, or 1% oxygen. In one embodiment, "oxygen-free environment" means an environment that does not contain measurable oxygen.

[0038] In one embodiment, the method does not produce steel.

[0039] In one embodiment, the method is carried out in a phosphorus-free environment. "Phosphorus-free environment" means an environment containing less than about 5%, 4%, 3%, 2%, or 1% phosphorus. In one embodiment, "phosphorus-free environment" means an environment that does not contain added phosphorus.

[0040] Figure 3 A diagram of an apparatus 300 for converting a solid carbon source 302 into highly crystalline graphite 304 according to some embodiments of the present disclosure is shown. The apparatus 300 includes a tube 306 having a horizontal tube axis 308, a crucible 310, and a heating unit 320.

[0041] The crucible 310 has a horizontal crucible axis 312 that is substantially aligned with the horizontal tube axis 308. The crucible 310 has a first crucible end 314 and a second crucible end 316. The crucible 310 is substantially positioned within the tube 306. In some embodiments, the tube 306 comprises quartz.

[0042] In operation, the crucible 310 contains a metal 318 and a solid carbon source 302. The metal 318 is not limited to a particular metal. In some embodiments, the metal 318 is nickel. The solid carbon source 302 is not limited to a particular type of carbon. Slag, solid carbon, and charcoal are all suitable as the solid carbon source 302. The heating unit 320 generates a temperature gradient 322 along the horizontal crucible axis 312. In some embodiments, the temperature gradient 322 decreases from the first crucible end 314 towards the second crucible end 316. The temperature gradient 322 is not limited to a particular mathematical function or shape. In some embodiments, the temperature gradient is linear. Highly crystalline graphite 304 is produced near the second crucible end 316. In some embodiments, the highly crystalline graphite 304 is scooped out from the crucible 310.

[0043] In some embodiments, the apparatus 300 further includes a gas source 324 that is coupled to the tube 306 for supplying an inert gas to the tube 306 and preventing the highly crystalline graphite 304 from reacting with oxygen or other elements. The apparatus 300 is not limited to use with a particular inert gas. In some embodiments, the inert gas is nitrogen N2.

[0044] The heating unit 320 is not limited to a particular type of heating unit. In some embodiments, the heating unit 320 includes a refractory brick 326 that is thermally coupled to the crucible 310 and an induction heating coil 328 that is inductively coupled to the refractory brick 326. The tube 306 is substantially located within the induction heating coil 328.

[0045] Figure 4 A diagram of an apparatus 400 for converting a solid carbon source 302 into highly crystalline graphite 304 in accordance with some embodiments of the present disclosure is shown. The apparatus 400 includes a reactor 402 that includes a vessel 404 and a carbon heating element 406. The vessel 404 has a first input port 408 for receiving nitrogen and includes a carbon heating element 406 to provide energy for liquefying the solid carbon source 302. A second input port 410 is provided to receive the solid carbon source 302, and an output port 412 is provided to deliver nitrogen and highly crystalline graphite 304 as the output of the apparatus 400. The vessel 404 is provided to contain a solid metal 414 to be liquefied. A heating unit 416 is included to provide a temperature gradient within the vessel 404. The heating unit 416 is not limited to a particular type of heating unit. In some embodiments, the heating unit 416 provides energy to heat the contents of the vessel 404 by induction.

[0046] In some embodiments, heating unit 416 provides a temperature gradient having a temperature from about 1700 degrees Celsius at the high temperature section of vessel 404 to about 1300 degrees Celsius at the low temperature section 420 of vessel 404. In some embodiments, the low temperature section 420 of vessel 404 is closer to the output port 412 than the high temperature section 418 of vessel 404.

[0047] In operation, nitrogen is delivered to vessel 404 at input port 408. Solid carbon source 302 is delivered to vessel 404 through second input port 410. Carbon heating element 406 liquefies solid carbon source 302. Heating unit 416 provides a temperature gradient within vessel 404. The liquefied carbon source 302 and solid metal 414 are heated within vessel 404 along the temperature gradient. Highly crystalline graphite 304 and nitrogen are output through output port 412.

[0048] The present invention will now be illustrated by the following non-limiting examples.

[0049] Examples

[0050] Example 1.

[0051] Graphite with high purity (99.99 at% C) and high crystallinity (99% graphite) was prepared from low grade petroleum coke (petcoke) by dissolving carbon in a molten ferroalloy containing silicon in a chamber at a temperature above the liquidus (~1700 °C) and providing a path for it to diffuse into a second chamber maintained at a lower temperature (~1300 °C), where the carbon would precipitate out of the melt as high purity graphite crystals, leaving petcoke impurities as slag (Figure 1b). This process takes advantage of the low activation barrier and carbon mobility in the molten iron solvent that is over a million times faster (at 1550 °C, E a ~41 kJ / mol and D ~ 6.0×10 -5 cm 2 / s), allowing the production of graphite from petcoke using less than 5 GJ / t of energy, emitting less than 0.5 t / t CO2 (assuming the US grid electricity mix) and costing less than $500 / t of graphite.

[0052] Example 2.

[0053] The semi - continuous process can be carried out in a system that is capable of intermittently receiving a carbon source at the feed end of a crucible and simultaneously capable of removing purified graphite from the product end of the crucible. The temperature gradient in the reactor is important. In order for carbon to dissolve rapidly and then diffuse into the melt, a high temperature is required at the feed end. The temperature at the carbon - saturated production end of the crucible needs to be maintained between the liquidus and eutectic solidification temperature of the binary system. Therefore, it is necessary to control the feed - end and production - end temperatures at specific and different values. This can be achieved by the coil design shown in Figure 2. The magnetic field strength within the coil and the heat induced in the melt are proportional to the coil diameter and pitch. Various coil designs, their temperature gradients, and their magnetic field strengths within the coil are shown below. They include A) coils with constant pitch and constant diameter, B) coils with constant pitch and varying diameter, C) coils with varying pitch and varying diameter, and D) coils with varying pitch and constant diameter.

[0054] Example 3.

[0055] A horizontal furnace is used to increase the graphite yield (Figures 2 to Figure 4 and Figure 7 ). The horizontal configuration of the sample within the induction coil allows for a larger collection surface area to be obtained on the product side of the reactor. The horizontal design also allows for better control of the temperature gradient compared to the previous vertical systems. The sample can be moved into or out of the hot zone. It was observed that the position of the sample within the coil and the effect of its external magnetic field are directly related to the overall effect of graphite formation in the melt.

[0056] All publications, patents, and patent documents are incorporated herein by reference as if individually incorporated by reference. The present invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications can be made while remaining within the spirit and scope of the present invention.

Claims

1. A method for preparing graphite, comprising: a) adding a solid carbon source to a molten metal at a first temperature to provide a solution containing dissolved carbon; b) reducing the temperature of all or part of the solution under conditions allowing graphite to form; and c) separating the graphite.

2. A method for preparing graphite, comprising: providing a molten metal having a temperature gradient, the temperature gradient including a hot zone having a first temperature and a cooler zone having a second temperature lower than that of the hot zone; adding a solid carbon source to the hot zone; and separating the graphite formed in the cooler zone.

3. A method for preparing graphite, comprising: maintaining a first chamber at a first temperature; maintaining a second chamber at a second temperature lower than the first temperature, the second chamber having a path leading to the first chamber; adding a solid carbon source to the molten metal in the first chamber to provide a solution; allowing the solution to pass through the path from the first chamber to the second chamber; and separating graphite from the second chamber.

4. The method according to any one of claims 1 to 3, wherein the graphite is at least 99% carbon.

5. The method according to any one of claims 1 to 3, further comprising removing one or more impurities from the graphite to provide graphite that is at least 99% carbon.

6. The method according to claim 4 or 5, wherein the graphite is at least 99% carbon and contains graphite crystals.

7. The method according to claim 4 or 5, wherein the graphite that is at least 99% carbon is at least 99% graphite.

8. The method according to any one of claims 1 to 7, wherein the solid carbon source includes petroleum coke (such as petcoke), coal, or charcoal.

9. The method according to any one of claims 1 to 7, wherein the solid carbon source includes petroleum coke.

10. The method according to any one of claims 1 to 9, wherein the molten metal includes iron, silicon, nickel, copper, germanium, manganese, bismuth, or silver, or a mixture thereof.

11. The method according to any one of claims 1 to 9, wherein the molten metal is a molten ferroalloy.

12. The method according to any one of claims 1 to 9, wherein the molten metal includes iron, silicon, or nickel, or a mixture thereof.

13. The method according to any one of claims 1 to 12, wherein the solid carbon source is added to the molten metal at a temperature below about 2000 °C.

14. The method according to any one of claims 1 to 12, wherein the solid carbon source is added to the molten metal at a temperature below about 1700 °C.

15. The method according to any one of claims 1 to 14, wherein the graphite is formed at a temperature below about 1500 °C.

16. The method according to any one of claims 1 to 14, wherein the graphite is formed at a temperature below about 1300 °C.

17. The method according to any one of claims 1 to 16, wherein the method is carried out in the presence of an inert gas.

18. The method according to claim 17, wherein the inert gas is nitrogen.

19. The method according to any one of claims 1 to 18 further comprises packaging the graphite for transportation or sale.

20. The method according to any one of claims 1 to 19 further comprises incorporating the graphite into a battery.

21. An apparatus for converting a solid carbon source into highly crystalline graphite, the apparatus comprising: a tube having a horizontal tube axis; a crucible having a horizontal crucible axis substantially aligned with the horizontal tube axis, the crucible having a first crucible end and a second crucible end, the crucible being substantially positioned within the tube and the crucible being adapted to contain a metal and the solid carbon source; and a heating unit that generates a temperature gradient along the horizontal crucible axis, the temperature gradient decreasing from the first crucible end towards the second crucible end, the highly crystalline graphite being produced at the second crucible end.

22. The apparatus according to claim 21 further comprising a gas source coupled to the tube to supply an inert gas to the tube.

23. The apparatus according to claim 22, wherein the tube comprises quartz.

24. The apparatus according to claim 23, wherein the heating unit comprises a refractory brick thermally coupled to the crucible and an induction heating coil inductively coupled to the refractory brick, the tube being substantially located within the induction heating coil.

25. An apparatus for converting a solid carbon source into highly crystalline graphite, the apparatus comprising: a reactor comprising a vessel and a carbon heating element, the vessel having: a first input port for receiving nitrogen, and the carbon heating element being adapted to provide energy for liquefying the solid carbon source; a second input port for receiving the solid carbon source; and an output port for delivering nitrogen and the highly crystalline graphite, the vessel being adapted to contain a solid metal to be liquefied; and a vessel heating unit adapted to provide a temperature gradient within the vessel.

26. The apparatus according to claim 25, wherein the vessel heating unit is adapted to provide the temperature gradient having a temperature from about 1700 degrees Celsius at a high temperature section of the vessel to about 1300 degrees Celsius at a low temperature section of the vessel.

27. The apparatus according to claim 26, wherein the low temperature section of the vessel is closer to the output port than the high temperature section of the vessel.

28. The method according to claim 20 further comprises incorporating the battery into an automobile, a hand tool or a generator.

29. The method according to any one of claims 1 to 20, wherein separating the graphite comprises collecting the graphite for future sale.

30. The method according to any one of claims 1 to 20, wherein the solid carbon is added in an oxygen-free environment.

31. The method according to any one of claims 1 to 20, the method being carried out in an oxygen-free environment.

32. The method according to any one of claims 1 to 20, the method not producing steel.

33. The method according to any one of claims 1 to 20, the method being carried out in a phosphorus-free environment.

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