Process for converting a solid carbon source into graphite

Abstract

The present invention provides a method and apparatus for converting a solid carbon source to graphite by precipitation from molten metal (e.g., molten iron), and the graphite prepared from the method and apparatus.

Description

[Technical Field]

[0001] The present invention relates to a process for converting a solid carbon source into graphite.

[0002] Priority This application claims priority to U.S. Provisional Application No. 63 / 389,300, filed July 14, 2022, the entire contents of which are incorporated herein by reference. [Background technology]

[0003] The increasing sales of electric vehicles (2.6% market share in 2019) has led to a significant increase in global lithium-ion battery production capacity, resulting in major disruptions to the supply chain of the traditional graphite market. Current lithium-ion batteries use approximately 1.2 kg / kWh of coated spherical graphite per kWh of storage capacity, meaning that a typical 75 kWh electric vehicle contains approximately 90 kg of graphite. In 2019, the production of 2.1 million electric vehicles required 190,000 tons of spherical coated graphite, 480,000 tons of large-scale exfoliated natural graphite (approximately 2.5 times spherical), and 960,000 tons of mined natural graphite (approximately 2 times flake). Approximately 80% of the mined natural graphite is ultimately used as secondary graphite fine powder. The reason for natural graphite is its lower cost (8,000 $ / t vs. 13,000 $ / t for synthetic graphite) and superior performance (high capacity retention). As a result, battery manufacturers have transitioned from using approximately 75% synthetic graphite to approximately 65% ​​natural graphite over the past 15 years. This transition has been fueled by the rapid development of over 40 new natural graphite mines worldwide, adding 1.9 Mt of new graphite capacity and 1.5 Mt of secondary graphite fines, cannibalizing the existing market. This means that secondary natural graphite fines are forcing synthetic graphite from pet coke out of the market and into use as a low-value heating fuel, resulting in an annual increase in greenhouse gas emissions of up to 10 Mt of CO2. Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional synthetic graphite production suffers from high activation barriers and slow reorganization of solid-phase atomic carbon (E at 2350°C). a is approximately 680 kJ / mol, and D is approximately 1.0 × 10 -12 cm 2 To overcome this problem, compressed pet coke compacts are electrically heated to over 2800°C for 2-3 weeks to form thermodynamically favorable crystalline graphite, resulting in energy consumption of approximately 22 GJ / t, CO2 emissions of 4.4 t / t, and a production cost of over $13,000 / t for synthetic graphite.

[0005] Petroleum coke is currently refined by calcination to remove moisture, remove volatiles, and produce anode-grade coke with the highest purity, high physical strength and conductivity, minimal porosity, and desired actual density. The calcination process is carried out at temperatures up to 1200–1400°C with contact times ranging from 0.5–48 hours, depending on the petroleum coke properties and process. Currently, there are three types of continuous pet coke calcination processes: rotary kiln, rotary hearth, and vertical shaft furnace. Even at high temperatures, these processes still produce 1 wt% sulfur, which dramatically hinders performance. Gases evolved during these processes produce "puffing," which increases porosity, lowers density, increases air reactivity, and reduces mechanical strength, electrical conductivity, and thermal conductivity. Improved methods for producing graphite are currently needed. In particular, methods are needed to economically produce graphite that meets market specifications for purity, crystallinity, and electrical conductivity. [Means for solving the problem]

[0006] Applicants have discovered an improved, more environmentally friendly method for economically preparing graphite that meets market specifications for purity, crystallinity, and electrical conductivity. Accordingly, in one embodiment, the present invention provides a process for producing graphite that: a) adding a solid carbon source to a molten metal to provide a solution containing dissolved carbon at a first temperature; b) reducing the temperature of all or a portion of the solution under conditions that allow graphite to form; c) isolating the graphite.

[0007] In another embodiment, the present invention provides providing a molten metal having a temperature gradient including a hot zone having a first temperature and a cold zone having a second temperature lower than the temperature of the hot zone; adding a solid carbon source to the hot zone; A method for preparing graphite is provided, which includes separating the graphite formed in a low temperature zone.

[0008] In another embodiment, the present invention provides maintaining the first chamber at a first temperature; maintaining a second chamber having a path to the first chamber at a second temperature lower than the first temperature; adding a solid carbon source to the molten metal in the first chamber to provide a solution; allowing a solution to pass through a pathway from the first chamber to the second chamber; A method for preparing graphite is provided, comprising separating the graphite from the second chamber.

[0009] In some embodiments of the present invention, molten metal (e.g., iron, nickel, and their alloys) alloys are used as effective separators to remove impurities from pet coke to yield high purity graphite, in contrast to processes where graphite is a by-product (waste) of steelmaking rather than an intended product. Molten iron (7.01 g / cm 3 ) to graphite (2.27g / cm 3 ) is low density, for example, by introducing an inert carrier gas (N2 or Ar) at a superficial velocity (e.g., 20–30 m / s) to disperse the graphite particles (d p <10 mm) to facilitate in situ separation.

[0010] Consistent with disclosed embodiments, an apparatus for converting a solid carbon source into highly crystalline graphite is disclosed. The apparatus includes a tube having a horizontal tube axis, a crucible, and a heating unit. The crucible has a horizontal crucible axis substantially aligned with the horizontal tube axis. The crucible has a first crucible end and a second crucible end. The crucible is disposed substantially within the tube. The crucible holds 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. Highly crystalline graphite is then produced at the second crucible end.

[0011] In some embodiments, the apparatus 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 disposed within the induction heating coil.

[0012] Consistent with the disclosed embodiments, an apparatus for converting a solid carbon source into highly crystalline graphite is disclosed. The apparatus includes a reactor including a vessel and a carbon heating element. The vessel has a first inlet port for receiving nitrogen, and the carbon heating element is included to provide energy for the solid carbon source to liquefy a metal. A second inlet port is provided for receiving the solid carbon source, and an outlet port is provided for delivering nitrogen and the highly crystalline graphite as an outlet. The vessel also holds the solid metal to be liquefied. The apparatus further includes a vessel heating unit for providing a temperature gradient within the vessel.

[0013] In some embodiments, the vessel heating unit provides a temperature gradient having a temperature from about 1700° C. in a hot section of the vessel to about 1300° C. in a cold section of the vessel. In some embodiments, the cold section of the vessel is closer to the exit port than the hot section of the vessel. [Brief explanation of the drawings]

[0014] [Figure 1] A shows a carbon-iron binary phase diagram illustrating the process pathway, and B shows a conceptual diagram of the process. [Figure 2] 1 illustrates a flow-through reactor that can be used to carry out the process of the present invention. [Figure 3] FIG. 1 shows a diagram of an apparatus for converting a solid carbon source into highly crystalline graphite, according to some embodiments of the present disclosure. [Figure 4] FIG. 1 shows a diagram of an apparatus for converting a solid carbon source into highly crystalline graphite, according to some embodiments of the present disclosure. [Figure 5A] The X-ray diffraction pattern of graphite feedstock (commercial graphite) is compared with the X-ray diffraction pattern of highly crystalline graphite produced according to the present invention. [Figure 5B] The X-ray diffraction pattern of graphite feedstock (amorphous carbon) is compared to the X-ray diffraction pattern of highly crystalline graphite produced according to the present invention. [Figure 6] 2A to 2D show images of graphite foil formed on the surface of a nickel melt using the furnace shown in FIGS. [Figure 7] FIG. 1 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 OF THE INVENTION

[0015] As used herein, the term "solid carbon source" includes any suitable carbon source for use in the method or apparatus of the present invention. In one embodiment, the solid carbon source includes petroleum coke (e.g., pet coke), coal, charcoal, fine graphite powder, or amorphous or glassy carbon. The term includes solid and liquid carbon sources, but does not include gaseous carbon sources.

[0016] In one embodiment, the graphite product is at least 99% carbon. In one embodiment, the graphite comprises at least 95% graphite crystallites. In one embodiment, the graphite comprises at least 98% graphite crystallites. In one embodiment, the graphite comprises at least 99% graphite crystallites. In one embodiment, the graphite is at least 99% carbon and comprises at least 95% graphite crystallites. In one embodiment, the graphite is at least 99% carbon and comprises at least 98% graphite crystallites. In one embodiment, the graphite is at least 99% crystalline graphite.

[0017] "Molten metal" includes any molten metal suitable for use in the method or apparatus 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 iron alloy. In one embodiment, the molten metal includes iron, silicon, or nickel, or a mixture thereof.

[0018] In one embodiment, the solid carbon source is added to the molten metal at a temperature less than about 2000°C. In one embodiment, the solid carbon source is added to the molten metal at a temperature less than about 1700°C. In one embodiment, the graphite is formed at a temperature less than about 1500°C. In one embodiment, the graphite is formed at a temperature less than about 1300°C. In one embodiment, the graphite is formed at a temperature less than about 1200°C.

[0019] In one embodiment, separating the graphite includes collecting the graphite for future sale.

[0020] In one embodiment, the solid carbon is added in an oxygen-free environment. "Anoxic environment" refers to an environment containing less than about 5%, 4%, 3%, 2%, or 1% oxygen. In one embodiment, "anoxic environment" refers to an environment that does not contain any measurable oxygen.

[0021] In one embodiment, the method does not produce steel.

[0022] In one embodiment, the method is carried out in a phosphorus-free environment. By "phosphorus-free environment" is meant an environment containing less than about 5%, 4%, 3%, 2%, or 1% phosphorus. In one embodiment, by "phosphorus-free environment" is meant an environment that does not contain added phosphorus.

[0023] 3 shows 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. The apparatus 300 includes a tube 306 having a horizontal tube axis 308, a crucible 310, and a heating unit 320.

[0024] Crucible 310 has a horizontal crucible axis 312 that is substantially aligned with horizontal tube axis 308. Crucible 310 has a first crucible end 314 and a second crucible end 316. Crucible 310 is disposed substantially within tube 306. In some embodiments, tube 306 comprises quartz.

[0025] During operation, the crucible 310 holds 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 suitable for use as the solid carbon source 302. A 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 to 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 skimmed from the crucible 310.

[0026] In some embodiments, the apparatus 300 further includes a gas source 324 coupled to the tube 306, which supplies an inert gas to the tube 306 to prevent the highly crystalline graphite 304 from reacting with oxygen or another element. The apparatus 300 is not limited to use with a particular inert gas. In some embodiments, the inert gas is nitrogen, i.e., N2.

[0027] Heating unit 320 is not limited to any particular type of heating unit. In some embodiments, heating unit 320 comprises refractory bricks 326 thermally coupled to crucible 310 and an induction heating coil 328 inductively coupled to refractory bricks 326. Tube 306 is substantially disposed within induction heating coil 328.

[0028] FIG. 4 shows a diagram of an apparatus 400 for converting a solid carbon source 302 into highly crystalline graphite 304, according to some embodiments of the present disclosure. The apparatus 400 includes a reactor 402, which includes a vessel 404 and a carbon heating element 406. The vessel 404 has a first inlet port 408 for receiving nitrogen, and the carbon heating element 406 is included to provide energy for liquefying the solid carbon source 302. A second inlet port 410 is provided for receiving the solid carbon source 302, and an outlet port 412 is provided for delivering nitrogen and the highly crystalline graphite 304 as an outlet of the apparatus 400. The vessel 404 is configured to hold 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 for heating the contents of the vessel 404 by induction.

[0029] In some embodiments, the heating unit 416 provides a temperature gradient having a temperature from about 1700° C. in a hot section of the vessel 404 to about 1300° C. in a cold section 420 of the vessel 404. In some embodiments, the cold section 420 of the vessel 404 is closer to the exit port 412 than the hot section 418 of the vessel 404.

[0030] During operation, nitrogen is delivered to the vessel 404 at the inlet port 408. The solid carbon source 302 is delivered to the vessel 404 through a second inlet port 410. The carbon heating element 406 liquefies the solid carbon source 302. The heating unit 416 provides a temperature gradient within the vessel 404. The liquefied carbon source 302 and the solid metal 414 are heated along the temperature gradient within the vessel 404. The highly crystalline graphite 304 and nitrogen exit through the outlet port 412. [Example]

[0031] The invention will now be illustrated by the following non-limiting examples.

[0032] Example 1 Graphite with high purity (99.99 at% C) and high crystallinity (99% graphite) was prepared from low-grade petroleum coke (pet coke) by dissolving carbon in a molten iron alloy containing silicon at a temperature above the liquidus (approximately 1700 °C) in one chamber and providing a pathway for carbon diffusion into a second chamber held at a lower temperature (approximately 1300 °C). In this case, carbon precipitates from the melt in the form of high-purity graphite crystals, while the pet coke impurities remain as a slag (Figure 1b). This process is facilitated by a low activation barrier in the molten iron solvent and a million times faster carbon mobility (E at 1550 °C). a is about 41 kJ / mol and D is about 6.0 × 10 -5 cm 2 / sec), enabling the production of graphite from pet coke at a cost of less than $500 / t graphite, using less than 5GJ / t of energy, and emitting less than 0.5g / t of CO2 (assuming a US power grid).

[0033] Example 2. A semi-continuous process can be implemented in a system that can intermittently accept a carbon source at the feed end of the crucible while removing refined graphite from the product end of the crucible. The temperature gradient within the reactor is important. A high temperature at the feed end is required for rapid dissolution of carbon and subsequent diffusion into the melt. The temperature at the carbon-saturated product end of the crucible must be maintained between the liquidus temperature and the eutectic freezing temperature of the binary system. Therefore, both the temperature at the feed end and the product end must be controlled at specific and distinct values. This can be achieved by the coil design shown in Figure 2. The strength of the magnetic field inside 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 visualized below. They include A) constant pitch and constant diameter, B) constant pitch and variable diameter, C) variable pitch and variable diameter, and D) variable pitch and constant diameter coils.

[0034] Example 3 To increase graphite yield, a horizontal furnace was implemented (Figures 2-4 and 7). Placing the sample horizontally within the induction coil allows for a larger surface area for collection on the product side of the reactor. The horizontal design also allows for greater control over temperature gradients compared to previous vertical systems. Samples can be moved into and out of the hot zone. A direct correlation was observed between the sample's position within the coil and its effect on the external magnetic field and the overall effect on graphite formation during melting.

[0035] All publications, patents, and patent applications are incorporated herein by reference, as if individually incorporated by reference. The invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope of the invention.

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