A method for preparing porous graphite from waste carbon slag of lithium-ion batteries

Abstract

The present invention relates to a method for preparing porous graphite from waste carbon slag of lithium-ion batteries. The waste carbon slag of lithium-ion batteries to be treated is dried, crushed, and screened to obtain fine carbon slag; the fine carbon slag is mixed uniformly with calcium fluoride and concentrated sulfuric acid to obtain a slurry; after the slurry is calcined, sintered carbon slag and flue gas are obtained; the sintered carbon slag is mixed with water, and after leaching, solid-liquid separation is carried out to obtain crude porous graphite; then, the crude porous graphite is washed with water until neutral and then dried to obtain the finished porous graphite. The method of the present invention has a short process flow, and porous graphite is successfully prepared from waste carbon slag of lithium-ion batteries, which helps to improve the treatment efficiency of waste carbon slag of lithium-ion batteries.

Description

Technical Field

[0001] The invention relates to a method for preparing porous graphite by utilizing waste lithium ion battery carbon residue, and belongs to the field of resource recovery and utilization of industrial hazardous waste. Background Art

[0002] As new energy occupies more and more markets, the demand for lithium-ion batteries in the automotive, battery products and other industries continues to increase. However, the number of scrapped batteries is also growing exponentially. In addition, the graphite in waste batteries is currently treated by stacking, burying or incineration, which increases environmental pollution. In addition, due to the high requirements for graphitization of carbon materials, the current market for graphite resources is in short supply. Therefore, waste graphite has a high degree of graphitization and contains many valuable metal elements. Recycling and reusing graphite in waste lithium-ion batteries is an important way.

[0003] At present, the focus of recycling of waste lithium-ion batteries is mainly on the recovery of valuable metal elements in positive electrode materials, while there are few methods for recycling and utilizing graphite negative electrode materials. This is mainly due to the mainstream recycling methods of lithium-ion batteries. The first is the pyrometallurgical process, in which carbon is mainly used as a reducing agent and there is almost no residue. The second method is the wet leaching process, which will undergo a mechanical crushing step in the previous processing process to prepare the positive and negative electrode mixed powders and then enter the wet process. After the leaching and extraction process of valuable metals, the remaining slag (i.e., the waste lithium-ion battery carbon-containing waste slag described in the present invention) is mainly graphite, a complex mixed system of various metals, non-metals and impurities, resulting in the difficulty of handling waste lithium-ion battery carbon-containing waste slag, low economic benefits, and poor reusability. Therefore, many companies adopt methods such as stacking and landfilling to handle, which causes serious pollution to the environment. Therefore, for the industry problem that it is difficult to economically and efficiently recycle the negative electrode of waste lithium-ion batteries, researchers have conducted a series of experimental explorations.

[0004] Xiao Hougui et al. conducted research on the purification, recycling, and reuse of the graphite anode of waste lithium-ion batteries. They treated the impurities and structure of the graphite anode through acid leaching. In a 0.8M ammonium persulfate solution, with a solid-liquid ratio of 60g / L, after leaching at 80°C for 1h, the leaching rates of lithium, iron, and aluminum impurities in the waste graphite all reached over 98%. Moreover, the defects of the recovered graphite decreased, and its graphitization degree was close to that of commercial graphite. Subsequently, expanded graphite was prepared from the recovered graphite through an oxidation intercalation method. 4ml of concentrated phosphoric acid, 12ml of concentrated sulfuric acid, and 1g of potassium permanganate were added to 1g of recovered graphite respectively, and they were stirred and reacted for 1h. After liquid-solid separation, the graphite was placed in a muffle furnace at 900°C for 15s to obtain expanded graphite. After oxidation expansion, the contents of impurities such as copper and lithium in the graphite decreased to below 0.1ppm, and the interlayer spacing of the expanded graphite increased by 0.054nm. However, a large amount of acidic wastewater was also generated during the recycling process. Nevertheless, this process still provides a promising path for the recycling and regeneration of waste graphite (Xiao Hougui. Recycling and Reuse of the Graphite Anode of Waste Lithium-Ion Batteries [D]. Central South University, 2022).

[0005] The Chinese invention patent specification CN115353100A discloses a method for recycling and utilizing cathode graphite of waste batteries. The waste lithium-ion batteries are successively crushed, ground, and sieved to obtain waste graphite, then purified by acid leaching to remove impurities, followed by solid-liquid separation, washing with deionized water until neutral, and drying to obtain purified graphite. Then, the purified graphite is mixed and reacted with potassium permanganate, sodium nitrate, and concentrated sulfuric acid to prepare graphene oxide, and then the graphene oxide is subjected to precipitation loading of metal salts and high-temperature roasting to obtain graphene loaded with multi-metal oxides.

[0006] The Chinese invention patent specification CN110589812A discloses a method for preparing porous graphene by recycling the graphite anode material of waste power batteries. The negative electrode sheet obtained by disassembling the waste battery is ultrasonically treated with water and then dried to obtain the graphite anode material. Porous graphite material is prepared by heating in an inert gas and water vapor, and then a mixed solution of sulfuric acid and phosphoric acid is added to the porous graphite and potassium permanganate, and hydrogen peroxide is added during the heating process to prepare porous graphene, and porous graphene is obtained after heating.

[0007] The Chinese invention patent specification CN114890414A discloses a method for recycling and utilizing graphite materials in waste batteries. After the waste graphite is acid-leached, washed, and dried, it is immersed in a metal salt solution and dried, then immersed in an alkali solution, washed and dried, and then subjected to a hydrothermal reaction in an organic carbon solution to form a double-layer coated graphite material A, and a double-layer coated graphite material B is obtained after sintering, and a porous C-coated membrane material is obtained after acid treatment.

[0008] Porous graphite is a kind of graphite material formed by the internal interconnection of pores in graphite. Porous graphite has a large specific surface area and a small particle size, and is often used in the preparation of adsorption materials, catalyst carriers, anodes for fast charging of lithium-ion batteries, supercapacitors, etc. Studying its preparation method is of great significance. Summary of the Invention

[0009] In view of the deficiencies of the prior art, the present invention provides a method for preparing porous graphite using waste carbon slag from lithium-ion batteries.

[0010] To solve the above technical problems, the technical solution of the present invention is as follows:

[0011] A method for preparing porous graphite using waste carbon slag from lithium-ion batteries, comprising the following steps:

[0012] S1. Dry, crush, and screen the waste carbon slag of the lithium-ion battery to be processed to obtain fine carbon slag;

[0013] S2. Mix the fine carbon slag, calcium fluoride, and concentrated sulfuric acid evenly according to the ratio of 100 g: 1 - 15 g: 10 - 150 mL to obtain a slurry;

[0014] Among them, the addition amount of calcium fluoride is 1 - 10 wt% of the fine carbon slag,

[0015] S3. Keep the slurry at 230 - 300 °C for 0.5 - 4 h to obtain sintered carbon slag and flue gas;

[0016] S4. Mix the sintered carbon slag and water according to a liquid-solid ratio of 3 - 15 mL: 1 g, and under the condition of 30 - 80 °C, leach for 0.5 - 4 h, then separate the solid and liquid to obtain crude porous graphite; then, wash the crude porous graphite to neutral and dry it to obtain the finished porous graphite.

[0017] Further, in S1, the particle size of the fine carbon slag is less than 150 μm.

[0018] Further, in S2, the concentration of concentrated sulfuric acid is 10 - 18.4 mol / L, preferably 16 - 18 mol / L.

[0019] Further, in S2, the ratio of fine carbon slag, calcium fluoride, and concentrated sulfuric acid is 100 g: 2 - 10 g: 30 - 120 mL, and more preferably 100 g: 4 - 8 g: 50 - 100 mL.

[0020] The applicant has found through research that controlling the ratio of fine carbon slag to calcium fluoride under certain conditions not only helps to smoothly obtain porous graphite, but also helps to improve the purity of porous graphite, and can also avoid the generation of excessive fluorine-containing flue gas and wastewater.

[0021] Further, in S2, mechanical grinding mixing method and / or impregnation method are adopted for mixing.

[0022] Further, in S2, first, fine carbon slag and calcium fluoride are mixed evenly to obtain a mixture; then the mixture is mixed evenly with concentrated sulfuric acid to obtain a slurry.

[0023] Further, in S2, the fine carbon slag, calcium fluoride and concentrated sulfuric acid are added into a graphite crucible, and stirred with a glass rod until it becomes slurry-like. The fluidity of the concentrated sulfuric acid liquid helps the calcium fluoride to disperse evenly.

[0024] Further, in S3, the slurry is kept at 240 - 290 °C for 1 - 3 h.

[0025] Further, in S3, the flue gas is introduced into a calcium salt solution. After S3 ends, silicon tetrafluoride in the flue gas hydrolyzes when passing through the solution and reacts with water to generate silica sol and hydrogen fluoride. Silica can be removed by solid-liquid separation. Subsequently, the pH value of the calcium salt solution (optionally, the temperature is controlled at 10 - 60 °C) is adjusted to 1 - 7, and stirred at a rate of 1 - 30 r / min (further 10 - 20 r / min) for 0.2 - 3 h (further 0.5 - 1 h), then left to stand. After solid-liquid separation (optionally by suction filtration), it is dried to obtain calcium fluoride; the obtained calcium fluoride is returned to S2. In this way, not only the treatment of the flue gas is realized, but also the recycling of calcium fluoride can be achieved, reducing the treatment cost.

[0026] Furthermore, the standing time is 0.5 - 50 h, further 1 - 45 h, and still further 1.5 - 2 h.

[0027] Further, the molar amount of calcium ions in the calcium salt solution is 0.5 - 5 times, further 1 - 3 times, and still further 1.5 - 2.5 times the molar amount of calcium fluoride added in S2.

[0028] Further, in S4, the sintered carbon slag and water are mixed at a liquid-solid ratio of 5 - 10 mL:1 g, and leached at 40 - 80 °C for 1 - 3 h, then solid-liquid separation is carried out to obtain a crude porous graphite product.

[0029] Further, the waste lithium-ion battery carbon slag to be treated is the slag remaining after the extraction process of valuable metals from one or several of cobalt acid lithium batteries, nickel acid lithium batteries, manganese acid lithium batteries, and lithium iron phosphate batteries.

[0030] Further, in the waste carbon slag of the waste lithium-ion battery to be treated, the fixed carbon content is 60 - 95 wt%, the volatile content is 2 - 14 wt%, and the ash content is 2 - 15 wt%; the ash contains one or several of Si, Ni, O, Mn, Co, Al, Na, Fe, P.

[0031] Furthermore, in the waste lithium-ion battery carbon residue to be processed, the fixed carbon content is 70 - 85 wt%, the volatile content is 5 - 10 wt%, and the ash content is 5 - 15 wt%; among them, the main element contents (wt%) in the ash are: Si 5 - 10, Ni 2 - 6, O 10 - 18, Mn 4 - 10, Co 2 - 8, Al 10 - 15, Fe 25 - 30, P 1 - 6, Na 1 - 4.

[0032] In the present invention, by mixing calcium fluoride, concentrated sulfuric acid with the waste lithium-ion battery carbon residue and then performing low-temperature roasting, elements such as Ni, Co, Mn, Al, Fe, Na, etc. can react with sulfuric acid to form soluble sulfates, which can be separated and removed during the subsequent water leaching process. At the same time, calcium fluoride reacts with sulfuric acid to form HF, and HF can react with silicon-containing substances, converting silicon in the carbon residue into fluoride gas and removing it; under the temperature conditions of the present invention, the generated calcium sulfate can react with carbon to form calcium sulfide and carbon dioxide, creating pores in the carbon residue; calcium sulfide can further react with sulfuric acid to form calcium sulfate and hydrogen sulfide gas. Since calcium sulfate is slightly soluble in water and the addition amount of calcium fluoride is not too large, calcium sulfate can be basically dissolved and removed during the subsequent water leaching and washing processes. Therefore, the present invention can finally obtain a porous graphite product with relatively high purity.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] (1) The method process of the present invention is short. Porous graphite is successfully prepared using waste lithium-ion battery carbon residue. The specific surface area of the porous graphite can reach more than 440 m 2 / g, which helps to improve the treatment efficiency of waste lithium-ion battery carbon residue.

[0035] (2) The present invention can realize the recycling of calcium fluoride. The roasting temperature is low, the energy consumption is low, and the process is short, which helps to reduce the treatment cost and has good prospects for industrial application.

[0036] (3) After the waste carbon residue of waste lithium-ion batteries to be treated is processed by improved sulfation roasting in the present invention, the purification and resource utilization process of the waste residue can be realized in one step, without the need for segmented treatment, which can effectively shorten the treatment cycle.

[0037] (4) The method of the present invention can obtain porous graphite with a purity of up to 99%, and can effectively remove impurities such as non-carbon elements, silicon dioxide, and aluminosilicates.

[0038] (5) The porous graphite obtained in the present invention provides a promising way to solve the problem of single graphite recycling and resource utilization products, and improves the added value of waste carbon residue treatment.

[0039] (6) When the present invention processes the waste carbon slag of waste lithium-ion batteries, the fluorine in the flue gas and waste water is recovered and reused by the precipitation method, which can avoid the pollution of the environment by fluorine, is green and environmentally friendly, and reduces costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a process flow chart of a process for preparing porous graphite using the carbon slag of waste lithium-ion batteries according to the present invention.

[0041] Figure 2 is a scanning electron micrograph of the porous graphite material obtained in Comparative Example 1.

[0042] Figure 3 is a scanning electron micrograph of the porous graphite material obtained in Comparative Example 2.

[0043] Figure 4 is a scanning electron micrograph of the porous graphite material obtained in Example 1.

[0044] Figure 5 is a high-magnification scanning electron micrograph of the porous graphite material obtained in Example 1.

[0045] Figure 6 is a scanning electron micrograph of the porous graphite material obtained in Example 2.

[0046] Figure 7 is a high-magnification scanning electron micrograph of the porous graphite material obtained in Example 2.

[0047] Figure 8 is a scanning electron micrograph of the porous graphite material obtained in Example 3.

[0048] Figure 9 is a scanning electron micrograph of the graphite material obtained in Comparative Example 3.

[0049] Figure 10 is a scanning electron micrograph of the graphite material of Comparative Example 6.

[0050] Figure 11 is a scanning electron micrograph of the graphite material obtained in Example 4.

[0051] Figure 12 is a scanning electron micrograph of the graphite material obtained in Example 5.

[0052] Figure 13 is a high-magnification scanning electron micrograph of the graphite material obtained in Example 5.

[0053] Figure 14 is a scanning electron micrograph of the graphite material obtained in Example 5.

[0054] Figure 15 is a high-magnification scanning electron micrograph of the graphite material obtained in Example 5.

[0055] Figure 16 It is the SEM image of the graphite material obtained in Comparative Example 7.

[0056] Figure 17 It is the SEM image of the graphite material obtained in Example 6.

[0057] Figure 18 It is the SEM image of the graphite material obtained in Example 7.

[0058] Figure 19 It is the SEM image of the graphite material obtained in Example 8.

[0059] Figure 20 It is the SEM image of the graphite material obtained in Comparative Example 8.

[0060] Figure 21 It is the XRD pattern of the porous graphite powder of Example 3. Detailed Description of the Invention

[0061] The present invention will be described in detail below with reference to the embodiments. It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other. Unless otherwise specified, the relevant percentages refer to mass percentages.

[0062] Comparative Example 1

[0063] Take 50 g of waste lithium-ion battery carbon slag from a domestic factory. The fixed carbon content is 82.15 wt%, the volatile matter is 8.12 wt%, and the ash content is 9.73 wt%. The main element content (wt%) of the ash: Si 6.18, Ni 3.84, O 14.19, Mn 5.32, Co 3.29, Al 13.88, Na 1.15, Fe 26.30, P 1.27. After drying the carbon slag, it is crushed to -150 um (-100 mesh), 2% of CaF2 by mass of the carbon slag is added, and mechanically mixed evenly. Then the mixture is poured into a graphite crucible. Then, 18.4 mol / L sulfuric acid with a liquid-solid ratio of 1.2 mL / g to the carbon slag is added to the mixture and stirred evenly. After keeping it warm at 200 °C for 3 h in a well-type furnace and cooling, the sintered carbon slag is subjected to water leaching treatment. Leaching temperature: 60 °C, leaching time: 1 h, liquid-solid ratio: 10 ml:1 g. After water leaching, the carbon slag is washed with water until neutral, filtered by suction, dried, and tested according to the graphite chemical analysis method (GB / T - 2008) to obtain graphite powder with a purity of 98.8%. The effect diagram of the graphite powder observed under a scanning electron microscope is as Figure 2 shown. It can be seen that the pores of the graphite are not obvious, and there are still some white substances on the surface. The specific surface area of the porous graphite in this example was tested by a fully automatic specific surface area and pore size analyzer of Micromeritics Instrument Corporation model ASAP2020HD88, and the result was 125.87 m2 / g。

[0064] During roasting, a calcium chloride solution is prepared with 0.5 times the molar amount of the added CaF2. The flue gas is passed into the calcium chloride solution, the pH is controlled to be 2, and it is stirred evenly at a stirring rate of 10 r / min. After stirring for 30 min at 40 °C, it is allowed to stand and precipitate for 30 min. The calcium fluoride crystals are separated by suction filtration, and then dried to obtain calcium fluoride with a purity of 90%, which is used to treat the next batch of waste lithium-ion battery carbon slag.

[0065] Comparative Example 2

[0066] Take 50 g of waste lithium-ion battery carbon slag from a domestic factory. The fixed carbon content is 82.15 wt%, the volatile matter is 8.12 wt%, and the ash content is 9.73 wt%. The main element content (wt%) of the ash: Si 6.18, Ni 3.84, O 14.19, Mn 5.32, Co 3.29, Al 13.88, Na 1.15, Fe 26.30, P 1.27. After drying the carbon slag, it is crushed to -150 um (-100 mesh), 5% of CaF2 by mass of the carbon slag is added, and it is mechanically mixed evenly. Then the mixture is poured into a graphite crucible. Then, 17 mol / L sulfuric acid with a liquid-solid ratio of 1.5 ml / g to the carbon slag is added, and it is stirred evenly. After holding at 220 °C for 3 h in a pit furnace and cooling, the sintered carbon slag is subjected to water leaching treatment. Leaching temperature: 60 °C, leaching time: 1 h, liquid-solid ratio: 10 ml:1 g. After water leaching, the carbon slag is washed with water until neutral, filtered by suction, dried, and tested according to the graphite chemical analysis method (GB / T - 2008) to obtain porous graphite powder with a purity of 99.5%. The effect diagram of the porous graphite observed under a scanning electron microscope is as Figure 3 shown. The porous graphite has a certain number of pores, but they are not particularly obvious. Compared with Example 1, the white substances on the graphite surface are reduced.

[0067] During roasting, a calcium chloride solution is prepared with 0.8 times the molar amount of the added CaF2. The flue gas is passed into the calcium chloride solution, the pH is controlled to be 3, and it is stirred evenly at a stirring rate of 10 r / min. The stirring time is 30 min, the stirring temperature is 50 °C. After stirring, it is allowed to stand and precipitate for 50 min. The calcium fluoride crystals are separated by suction filtration and dried to obtain calcium fluoride with a purity of 91.2%, which is used to treat the next batch of waste lithium-ion battery carbon slag.

[0068] Example 1

[0069] Take 50 g of waste lithium-ion battery carbon slag from a domestic factory. The fixed carbon content is 82.15 wt%, the volatile content is 8.12 wt%, and the ash content is 9.73 wt%. The main element content (wt%) of the ash: Si 6.18, Ni 3.84, O 14.19, Mn 5.32, Co 3.29, Al 13.88, Na 1.15, Fe 26.30, P 1.27. After drying the carbon slag, it is crushed to -150 um (-100 mesh), 10% of CaF2 by mass of the carbon slag is added, and they are mechanically mixed evenly. Then the mixture is poured into a graphite crucible. Then, 18.4 mol / L sulfuric acid with a liquid-solid ratio of 1.5 ml / g to the carbon slag is taken and added to the mixture and stirred evenly. After keeping it warm at 250 °C for 3 h in a well-type furnace and cooling, the sintered carbon slag is subjected to water leaching treatment. Leaching temperature: 60 °C, leaching time: 6 h, liquid-solid ratio: 10 ml:1 g. After water leaching, the carbon slag is washed with water until neutral, filtered by suction, dried, and tested according to the graphite chemical analysis method (GB / T - 2008) to obtain high-purity porous graphite powder with a purity of 99.95%. The effect diagram of the porous graphite observed under a scanning electron microscope is as Figure 4-5 shown. The specific surface area of the porous graphite in this example was tested by a fully automatic specific surface area and pore size analyzer of model ASAP2020HD88 from Micromeritics Instrument Corporation, and the result was 651.55 m 2 / g.

[0070] During roasting, a calcium chloride solution is prepared with 1.2 times the molar amount of the added CaF2. The flue gas is passed into the calcium chloride solution, the pH is controlled to be 3, stirred evenly, the stirring rate is 15 r / min, the stirring time is 30 min, the stirring temperature is 50 °C. After stirring, it is left to stand and precipitate for 1 h, and calcium fluoride crystals are separated by suction filtration and dried to obtain calcium fluoride with a purity of 90.5% for treating the next batch of waste lithium-ion battery carbon slag.

[0071] Example 2

[0072] Take 50 g of waste lithium-ion battery carbon slag from a domestic factory. The fixed carbon content is 82.15 wt%, the volatile matter is 8.12 wt%, and the ash content is 9.73 wt%. The main element content (wt%) of the ash: Si 6.18, Ni 3.84, O 14.19, Mn 5.32, Co 3.29, Al 13.88, Na 1.15, Fe 26.30, P 1.27. After drying the carbon slag, it is crushed to -150 um (-100 mesh), 6% of CaF2 by mass of the carbon slag is added, and mechanically mixed evenly. Then the mixture is poured into a graphite crucible. Then, 18 mol / L sulfuric acid with a liquid-solid ratio of 0.8 ml / g to the carbon slag is taken and added to the mixture and stirred evenly. After being kept warm at 280 °C for 3 h in a well-type furnace and cooled, the sintered carbon slag is subjected to water leaching treatment. Leaching temperature: 80 °C, leaching time: 1 h, liquid-solid ratio: 8 ml:1 g. After water leaching, the carbon slag is washed with water until neutral, filtered by suction, dried, and tested according to the graphite chemical analysis method (GB / T-2008) to obtain high-purity porous graphite powder with a purity of 99.78%. The effect diagram of the porous graphite observed under a scanning electron microscope is as Figure 6-7 shown. The pore formation of the graphite is not yet complete. The specific surface area of the porous graphite in this example was measured by a fully automatic specific surface area and pore size analyzer of model ASAP2020HD88 from Micromeritics Instrument Corporation, and the result was 448.12 m 2 / g.

[0073] During roasting, a solution is prepared with calcium chloride in a molar amount 1.0 times that of the added CaF2. The flue gas is passed into the calcium chloride solution, the pH is controlled to be 3, stirred evenly, the stirring rate is 15 r / min, the stirring time is 1 h, the stirring temperature is 60 °C. After stirring, it is left to stand and precipitate for 45 h, and calcium fluoride crystals are separated by suction filtration and dried to obtain calcium fluoride with a purity of 92.5% for treating the next batch of waste lithium-ion battery carbon slag.

[0074] Example 3

[0075] Take 50 g of waste lithium-ion battery carbon slag from a domestic factory. The fixed carbon content is 82.15 wt%, the volatile matter is 8.12 wt%, and the ash content is 9.73 wt%. The main element content (wt%) of the ash: Si 6.18, Ni 3.84, O 14.19, Mn 5.32, Co 3.29, Al 13.88, Na 1.15, Fe 26.30, P 1.27. After drying the carbon slag, it is crushed to -150 um (-100 mesh), 8% of CaF2 by mass of the carbon slag is added, and it is mechanically mixed evenly. Then the mixture is poured into a graphite crucible. Then, 18.4 mol / L sulfuric acid with a liquid-solid ratio of 1.0 ml / g to the carbon slag is taken and added to the mixture and stirred evenly. After holding at 300 °C for 2 h in a well-type furnace and cooling, the sintered carbon slag is subjected to water leaching treatment. Leaching temperature: 40 °C, leaching time: 1 h, liquid-solid ratio: 6 ml:1 g. After water leaching, the carbon slag is washed with water until neutral, filtered by suction, dried, and tested according to the graphite chemical analysis method (GB / T - 2008) to obtain porous graphite powder with a purity of 99.91%. The effect diagram of the porous graphite observed under a scanning electron microscope is as Figure 8 shown. The porous graphite has abundant pores. However, due to the high degree of pore formation, the graphite particle size is greatly reduced. The specific surface area of the porous graphite in this example was tested by a fully automatic specific surface and pore analyzer of model ASAP2020HD88 of Micromeritics Instrument Corporation, and the result is 624.35 m 2 / g.

[0076] During roasting, a solution is prepared with calcium chloride in an amount 1.5 times the molar amount of the added CaF2. The flue gas is passed into the calcium chloride solution, the pH is controlled to be 2, stirred evenly, the stirring rate is 20 r / min, the stirring time is 40 min, the stirring temperature is 60 °C. After stirring, it is allowed to stand and precipitate for 1 h, and calcium fluoride crystals are separated by suction filtration and dried to obtain calcium fluoride with a purity of 91.5% for treating the next batch of waste lithium-ion battery carbon slag.

[0077] Comparative Example 3

[0078] Repeat Example 3, with the only difference being that calcium fluoride is replaced by sodium fluoride, and 8% of sodium fluoride is added.

[0079] As Figure 9 shown, the graphite obtained after treatment with calcium fluoride did not form a porous structure, and the graphite particles were well preserved.

[0080] Comparative Example 4

[0081] Repeat Example 3, with the only difference being that calcium fluoride is replaced by potassium fluoride.

[0082] It was found that ( Figure 10) The resulting graphite did not form a porous structure either, and the graphite was broken into flaky structures.

[0083] Comparative Example 5

[0084] Repeat Example 3, with the only difference being that calcium fluoride was replaced with aluminum fluoride.

[0085] It was found that ( Figure 11 ) the resulting graphite did not form a porous structure either.

[0086] Comparative Example 6

[0087] Repeat Example 3, with the only difference being that the addition amount of calcium fluoride was changed to 0%.

[0088] As Figure 12 shown, the graphite structure remained intact, but there were more impurities on the surface.

[0089] Example 4

[0090] Repeat Example 3, with the only difference being that the addition amount of calcium fluoride was changed to 5%.

[0091] As Figure 13 shown, part of the original flaky structure of the graphite was damaged, and part of the graphite formed a porous structure.

[0092] Example 5

[0093] Repeat Example 3, with the only difference being that the addition amount of calcium fluoride was changed to 15%.

[0094] As Figure 14-15 shown, the graphite lost its flaky structure and formed a porous structure.

[0095] Comparative Example 7

[0096] Repeat Example 3, with the only difference being that it was heat-insulated at 200 °C in a pit furnace. The results are as Figure 16 shown. Since the pore-forming reaction conditions were not reached, the graphite was not damaged, and there were still many impurities remaining on its surface.

[0097] Example 6

[0098] Repeat Example 3, with the only difference being that it was heat-insulated at 230 °C in a pit furnace. The results are as Figure 17 shown. When the roasting temperature exceeded 220 °C, the graphite began to be damaged, but the reaction degree was not high, and part of the graphite still maintained its original morphology.

[0099] Example 7

[0100] Repeat Example 3, with the only difference being that it was heat-insulated at 280 °C in a pit furnace. The results are as Figure 18As shown. The overall structure of the graphite is basically broken to form a porous structure.

[0101] Example 8

[0102] Repeat Example 3, with the only difference being that it is kept warm at 300 °C in a pit furnace. The results are as Figure 19 shown. The graphite lamellae are broken and a porous structure is formed.

[0103] Comparative Example 8

[0104] Repeat Example 3, with the only difference being that it is kept warm at 310 °C in a pit furnace. The results are as Figure 20 shown. The graphite lamellae are damaged to form a porous structure, but the degree of damage does not increase with the increase in temperature.

[0105] The content clarified in the above embodiments should be understood that these embodiments are only used to illustrate the present invention more clearly, rather than to limit the scope of the present invention. After reading the present invention, various equivalent forms of modification by those skilled in the art fall within the scope defined by the appended claims of this application.

Claims

1. A method for preparing porous graphite from waste carbon slag of lithium-ion batteries, characterized in that, It includes the following steps: S1. Dry, crush, and screen the waste carbon slag of used lithium-ion batteries to be processed to obtain fine carbon slag; S2. Mix the fine carbon slag, calcium fluoride, and concentrated sulfuric acid evenly in a ratio of 100 g: 5 - 15 g: 10 - 150 mL to obtain a slurry; S3. Keep the slurry at 230 - 300 °C for 0.5 - 4 h to obtain sintered carbon slag and flue gas; S4. Mix the sintered carbon slag and water in a liquid-solid ratio of 3 - 15 mL: 1 g, leach for 0.5 - 4 h at 30 - 80 °C, then perform solid-liquid separation to obtain crude porous graphite; then, wash the crude porous graphite with water until neutral and dry it to obtain finished porous graphite.

2. The method according to claim 1, wherein In S1, the particle size of the fine carbon slag is less than 150 μm.

3. The method according to claim 1, characterized in that, In S2, the concentration of the concentrated sulfuric acid is 10 - 18.4 mol / L.

4. The method according to claim 1, wherein In S2, the ratio of the fine carbon slag, calcium fluoride, and concentrated sulfuric acid is 100 g: 5 - 10 g: 30 - 120 mL.

5. The method according to claim 1, wherein In S2, mechanical grinding mixing method and / or impregnation method are used for mixing.

6. The method according to claim 1, wherein In S2, first mix the fine carbon slag and calcium fluoride evenly to obtain a mixture; then mix the mixture and concentrated sulfuric acid evenly to obtain a slurry.

7. The method according to claim 1, characterized in that, In S3, keep the slurry at 240 - 290 °C for 1 - 3 h.

8. The method according to claim 1, wherein In S3, introduce the flue gas into a calcium salt solution. After S3 ends, adjust the pH value of the calcium salt solution to 1 - 7, stir at a rate of 1 - 30 r / min for 0.2 - 3 h, then let it stand, perform solid-liquid separation, and dry to obtain calcium fluoride; return the obtained calcium fluoride to S2.

9. The method according to claim 8, wherein The molar amount of calcium ions in the calcium salt solution is 0.5 - 5 times the molar amount of calcium fluoride added in S2.

10. The method according to any one of claims 1-9, characterized in that, In the waste carbon slag of used lithium-ion batteries to be processed, the fixed carbon content is 60 - 95 wt%, the volatile content is 2 - 14 wt%, and the ash content is 2 - 15 wt%; the ash contains one or more of Si, Ni, O, Mn, Co, Al, Na, Fe, P.

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