Graphite slag resource utilization method and modified graphite slag

Abstract

The invention provides a graphite slag resource utilization method and modified graphite slag, and the method comprises the following steps: impregnating graphite slag in a pretreatment solution, and washing and drying after impregnation to obtain pretreated graphite slag; the pretreatment solution comprises metal salt and / or metal hydroxide; roasting the pretreated graphite slag in an oxygen-containing atmosphere, quenching the roasted material in a solution containing an oxidizing agent after roasting is finished, and then carrying out solid-liquid separation to obtain modified graphite slag; in the oxygen-containing atmosphere, the volume concentration of oxygen is smaller than or equal to 30%; and immersing the modified graphite slag into wastewater containing COD (Chemical Oxygen Demand) to reduce the content of COD in the wastewater. According to the method, through low-oxygen-concentration roasting and quenching in the solution containing the oxidizing agent, the graphite slag obtains rich defects and oxygen-containing functional groups, the modified graphite slag is obtained, the modified graphite slag is applied to treatment of COD-containing wastewater, and resource utilization of the graphite slag can be achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion battery recovery and resource utilization, and relates to a method for resource utilization of graphite slag and modified graphite slag. Background Art

[0002] With the rapid development of new energy technologies and the widespread use of lithium-ion batteries, lithium-ion batteries, after capacity decay, are gradually entering the recycling stage. During the battery recycling process, the recycling and reuse of lithium-ion battery positive electrode materials, which contain high-value-added metals such as Ni and Co, has attracted widespread attention and has achieved relatively good industrial recycling. However, graphite, the main component of lithium-ion battery negative electrode materials, is often stored as graphite slag due to its high impurity content, stable structure, and poor reactivity, and has not yet been fully utilized as a resource.

[0003] Graphite slag typically contains high levels of impurities. This is because during the wet leaching process, materials such as the cathode material and residual copper and aluminum foil from the pretreatment process are converted from compounds or elements into metal ions in the acidic solution. These metal ions, driven by concentration differences, intercalate into the interlayers of the graphite and form strong adsorption bonds with the graphite surface. After low-concentration acid leaching, the ash content in graphite slag from ternary lithium battery recycling lines reaches approximately 16%, and from lithium iron phosphate battery recycling lines, it reaches approximately 18%. Furthermore, due to the small interlayer spacing of graphite and its hydrophobic and oleophilic properties, surface tension forces on the surface. Conventional water washing or acid leaching methods can only remove metal ions bound to the edges of the graphite. Metal ions embedded within the graphite layers are difficult to remove effectively due to the difficulty of contact between the interlayers and the leachate. After high-concentration acid leaching, the ash content is generally reduced to around 5%.

[0004] The preparation of recycled graphite from graphite slag is currently the most researched area. Because graphite slag has undergone multiple charge-discharge cycles and has a large interlayer spacing, it needs to be doped with asphalt and subjected to a high-temperature graphitization process at 2600°C to ensure that its interlayer spacing meets the requirements for recycled graphite. Due to the high energy consumption of the graphite slag regeneration process and the impurity content of the regenerated graphite slag failing to meet the requirements for graphite negative electrodes, it is difficult to achieve reliability and cost advantages compared to current artificial graphite. At the same time, in order to improve the capacity and cycle stability of graphite negative electrodes, elements such as Si and Ti are often added to graphite negative electrodes. This will lead to an increasingly complex composition of graphite slag in the future, making its recycling increasingly difficult. Therefore, there is an urgent need to develop methods for the resource utilization of graphite slag.

[0005] During the repeated charge-discharge cycles of lithium-ion batteries, graphite slag develops defects on the graphite surface or between graphite layers due to contact between the graphite anode and the electrolyte, and the repeated insertion and extraction of lithium ions from the graphite anode. This increases the interlayer spacing, resulting in a lower structural stability of the graphite crystal after cycling than that of freshly synthesized graphite. Furthermore, during battery pretreatment (such as immersion discharge and mechanical crushing) and acid leaching, excessive discharge, mechanical damage, and the introduction of impurity ions further disrupt the graphite crystal structure. Consequently, compared to freshly synthesized graphite, graphite slag exhibits a more unstable crystal structure and exhibits more defects and impurities. Furthermore, during battery pretreatment, nitrogen is often used as a shielding gas during the calcination process for process safety, resulting in incomplete reaction of some binders and coating of the graphite slag surface with amorphous (non-graphitized) organic matter. This indicates that, compared to conventional synthetic graphite, graphite slag exhibits more crystal defects, a surface coating of some amorphous organic matter, and a higher reactivity.

[0006] The prior art has disclosed the use of graphite slag as an adsorbent, but it only utilizes the oleophilic and hydrophobic properties of graphite slag to remove large molecular organic matter (oil) with weak polarity. However, it is not effective in removing small molecular COD. The reason is that small molecular COD has a shorter carbon chain and contains oxygen-containing functional groups such as hydroxyl and carboxyl groups, and the proportion of polar molecules is relatively high.

[0007] Based on the above research, it is necessary to provide a method for resource utilization of graphite slag, which can use graphite slag for the treatment of COD-containing wastewater, especially the treatment of wastewater containing small molecular organic matter, to achieve resource utilization of graphite slag. Summary of the Invention

[0008] The object of the present invention is to provide a method for resource utilization of graphite slag and modified graphite slag. The method comprises the following steps: first loading a metal salt and / or a metal hydroxide, then calcining the graphite slag under a low oxygen concentration, performing a preliminary oxidation on the graphite slag, so that the graphite slag obtains higher reaction activity and carries some oxygen-containing functional groups; then quenching the high-temperature graphite slag with a solution containing an oxidant, so that the graphite slag with higher activity reacts with the oxidant, achieving deep oxidation of the graphite slag, and obtaining rich defects and oxygen-containing functional groups in the graphite slag; finally, using the modified graphite slag for treating COD-containing wastewater, thereby realizing resource utilization of the graphite slag.

[0009] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present invention provides a method for resource utilization of graphite slag, the method comprising the following steps:

[0011] (1) impregnating the graphite slag in a pretreatment solution, washing and drying after the impregnation is completed to obtain the pretreated graphite slag;

[0012] The pretreatment solution includes metal salts and / or metal hydroxides;

[0013] (2) calcining the pretreated graphite slag described in step (1) in an oxygen-containing atmosphere, quenching the calcined material in a solution containing an oxidant after the calcination, and then performing solid-liquid separation to obtain modified graphite slag;

[0014] (3) Immersing the modified graphite slag in step (2) into wastewater containing COD to reduce the COD content in the wastewater and realize resource utilization of the graphite slag.

[0015] When using graphite slag to remove COD from wastewater, if you want to achieve a better adsorption effect, you need to use carbon-containing substances with rich oxygen-containing functional groups on the surface of the graphite slag. The functional groups adsorb polar COD, and the active sites on the surface of the carbon-containing substances adsorb non-polar COD. The two work together. In other words, if you want to use graphite slag to efficiently adsorb COD, you need to modify its surface so that its surface is loaded with rich oxygen-containing functional groups and active sites.

[0016] Therefore, the present invention uses an immersion pretreatment to load the metal salt and / or metal hydroxide in the pretreatment solution onto the surface of the graphite slag. Due to the large specific surface area of organic matter on the graphite slag surface, the metal salt and / or metal hydroxide are easily strongly adsorbed at the defects of the graphite slag, and can remain on the surface of the graphite slag after washing. Due to the presence of the metal salt and / or metal hydroxide, the organic matter on the surface of the graphite slag and the graphite slag defects are highly reactive. When calcined under low oxygen concentration conditions, the surface organic matter and defect locations can be fully activated, thereby improving the reactivity of the graphite slag. The high-temperature graphite slag is then quickly placed in a solution containing an oxidant for quenching to achieve redox reaction and heat exchange. The oxidant is used to further deeply oxidize the pores, defect sites, partially oxidized functional groups, and highly active surfaces of the graphite generated during the roasting process, so that abundant oxygen-containing functional groups or crystal defects are generated on the surface and in the pore structure of the graphite slag. At the same time, the oxidized graphite slag is quickly cooled by utilizing the intense heat exchange process to prevent excessive oxidation of the oxygen-containing functional groups and avoid the problem of a reduction in oxygen-containing functional groups due to high-temperature decomposition of the oxygen-containing functional groups, which affects the adsorption effect on COD. When the surface and interlayers of the modified graphite slag obtained by the present invention are loaded with abundant oxygen-containing functional groups, the adsorption of small-molecule organic matter by the graphite slag is improved, and the adsorption effect of the modified graphite slag on COD in wastewater is enhanced.

[0017] The present invention makes full use of the non-graphitized organic matter wrapped on the surface of the graphite slag, and uses metal salts and / or metal hydroxides for impregnation. During the roasting process, the reaction activity at the sites where the metal salts and / or metal hydroxides are combined with the organic matter is high. Under low oxygen concentration conditions, the organic matter at the metal salt and / or metal hydroxide binding sites is broken, and the sites not combined with the metal salts and / or metal hydroxides retain the original structure. Through the differential oxidation process, the long-chain organic matter can be converted into short-chain organic matter. At the same time, the short-chain organic matter undergoes a moderate oxidation reaction with oxygen to generate oxygen-containing functional groups, so that the long-chain organic matter on the surface of the graphite slag is converted into short-chain organic matter rich in oxygen-containing functional groups, enriching the oxygen-containing functional groups of the graphite slag. The low oxygen concentration condition avoids the problem that when there is more oxygen, the organic matter on the surface of the graphite slag is completely oxidized and removed, and when reacting with the oxidizing solution, fewer functional groups are formed on the surface of the graphite slag.

[0018] In the oxygen-containing atmosphere, the volume concentration of oxygen is ≤30%, for example, it can be 30%, 25%, 20%, 15%, 10% or 5%, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0019] Preferably, the metal salt in step (1) comprises sodium carbonate and / or potassium carbonate.

[0020] Preferably, the metal hydroxide in step (1) comprises sodium hydroxide and / or potassium hydroxide.

[0021] Preferably, the concentration of the pretreatment solution in step (1) is 0.5 mol / L-1.0 mol / L, for example, it can be 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L or 1.0 mol / L, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0022] Preferably, the liquid-to-solid ratio of the pretreatment solution to the graphite slag in step (1) is 2 mL / g-10 mL / g, for example, 2 mL / g, 4 mL / g, 6 mL / g, 8 mL / g or 10 mL / g, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

[0023] Preferably, the graphite slag in step (1) includes graphite having a mass percentage of ≥75%, for example, ≥75%, 80%, 85% or 90%, ash having a mass percentage of 3%-17%, for example, 3%, 5%, 7%, 9%, 11%, 13%, 15% or 17%, and residual organic matter having a mass percentage of 5%-10%, for example, 5%, 6%, 7%, 8%, 9% or 10%, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0024] The graphite slag described in the present invention is obtained by crushing and sorting black powder during the lithium-ion battery recycling process. The black powder is then acid-leached to separate the positive electrode material, resulting in a graphite slag with graphite as its main component. The ash is primarily composed of minerals, including compounds containing Ni, Co, and Mn, FePO4 and its hydrates, and residual sulfates. Residual organic matter primarily consists of unreacted binder and unvolatile electrolyte from the pretreatment process.

[0025] Preferably, the washing method in step (1) includes pulping or rinsing.

[0026] Preferably, the liquid-to-solid ratio of the washing in step (1) is 2 mL / g-5 mL / g, for example, 2 mL / g, 3 mL / g, 4 mL / g or 5 mL / g, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

[0027] Preferably, the volume concentration of oxygen in the oxygen-containing atmosphere in step (2) is 5%-10%, for example, it can be 5%, 6%, 7%, 8%, 9% or 10%, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0028] Preferably, the calcination temperature in step (2) is 300°C-550°C, for example, 300°C, 400°C, 450°C, 500°C or 550°C, and the calcination time is 5 min-30 min, for example, 5 min, 10 min, 15 min, 20 min, 25 min or 30 min, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0029] The calcination conditions of the present invention affect the content of oxygen-containing functional groups. If the calcination time is too long, the temperature is too high, or the volume concentration of oxygen is too high, the graphite slag will be over-oxidized during the calcination process, causing the organic matter on the graphite surface to decompose, affecting the number of oxygen-containing functional groups, and it is impossible to obtain graphite slag rich in oxygen-containing functional groups.

[0030] Preferably, the quenching time in step (2) is ≤5 min, for example, it can be 5 min, 4 min, 3 min, 2 min, 1 min or 0.5 min, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0031] The quenching in the present invention refers to quickly placing the high-temperature graphite slag after roasting into a solution containing an oxidant, rapidly cooling the high-temperature graphite slag, and further performing a deep oxidation reaction.

[0032] Preferably, in the oxidant-containing solution in step (2), the mass concentration of the oxidant is 10%-70%, for example, it can be 10%, 20%, 30%, 40%, 50%, 60% or 70%, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0033] Preferably, the temperature of the oxidant-containing solution in step (2) is ≤50°C, for example, it can be 50°C, 40°C, 30°C, 20°C or 10°C, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0034] Preferably, the oxidant-containing solution in step (2) includes any one of hydrogen peroxide, concentrated sulfuric acid, nitric acid, NaClO, Na2S2O8, KMnO4, K2Cr2O7 or Na2Cr2O7, or a combination of at least two thereof.

[0035] Preferably, the roasted material in step (2) is completely immersed in a solution containing an oxidant.

[0036] The present invention does not impose any specific limitation on the volume of the solution containing the oxidant, and the roasted material can be completely immersed in the solution containing the oxidant.

[0037] Preferably, a water washing step is performed after the solid-liquid separation in step (2).

[0038] Preferably, the COD-containing wastewater in step (3) includes wastewater containing small molecule organic matter, and the number of carbon atoms of the small molecule organic matter is ≤10, for example, it can be 10, 9, 8, 7, 6, 5 or 4, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

[0039] In a second aspect, the present invention provides a modified graphite slag, wherein the modified graphite slag comprises the modified graphite slag prepared in step (2) of the method according to the first aspect.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] (1) The present invention makes full use of the characteristics that graphite slag has many surface defects and surface impurities enhance the reactivity of graphite slag. The reactivity of graphite slag is improved by impregnation in a pretreatment solution and calcination under low oxygen concentration conditions. The graphite slag is further quenched in a solution containing an oxidant and oxidized with an oxidant to generate rich oxygen-containing functional groups on the surface and defect sites of the graphite slag.

[0042] (2) The present invention fully utilizes the fact that after charge and discharge cycles, the interlayer spacing of graphite slag is wider than that of natural graphite. During the acid leaching and purification process, some impurities are inserted into the interlayers of graphite, further expanding the interlayer spacing. After the interlayer spacing of graphite slag is expanded, it is easier to fully activate the graphite slag during the roasting process. During the reaction with the oxidant, the graphite interlayers are more likely to produce abundant oxygen-containing functional groups. When the graphite slag surface and interlayers are loaded with abundant oxygen-containing functional groups, it helps to improve the adsorption of small molecular organic matter by the graphite slag and enhance the adsorption effect of the modified graphite slag on COD in wastewater.

[0043] (3) In the prior art, methods for preparing recycled graphite using graphite slag all involve removing organic / inorganic impurities and oxygen-containing functional groups from the graphite slag, restoring the graphite slag's complete crystal structure and restoring its lithium ion deintercalation and recycling stability. The present invention, however, fully retains the organic matter on the graphite slag surface and the impurities between the graphite slag layers, utilizing the characteristics of the organic matter and impurities to increase the number of defects and oxygen-containing functional groups in the graphite slag. This is contrary to the overall modification approach of the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a flow chart of the method described in Example 1 of the present invention. DETAILED DESCRIPTION

[0045] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0046] Example 1

[0047] This embodiment provides a method for resource utilization of graphite slag, the method is as follows Figure 1 As shown, the graphite slag is first impregnated with a sodium carbonate solution, then washed and dried, and then roasted under a low oxygen concentration gas, quenched in a solution containing an oxidant, and finally filtered and washed with water to obtain modified graphite slag, which is then used to adsorb COD.

[0048] The specific steps include:

[0049] Graphite slag (ash mass proportion of 5%, residual organic matter mass proportion of 8%, graphite mass proportion of 87%) was placed in a 0.5 mol / L sodium carbonate solution with a liquid-solid ratio of sodium carbonate solution and graphite slag of 2 mL / g. After stirring and mixing for 1 hour, solid-liquid separation was performed, and the solid was rinsed with 2 mL / g of spray water. After drying in a 100-degree oven, it was placed in a tubular furnace, and the oxygen volume concentration in the reaction gas was adjusted to 8% (the rest was nitrogen). The tubular furnace was heated from room temperature to 450°C, and the graphite slag was calcined at 450°C for 10 minutes and then taken out. The high-temperature graphite slag was quickly placed in a hydrogen peroxide solution at room temperature with a mass concentration of 30%. After reacting for 5 minutes, solid-liquid separation and water washing were performed to obtain modified graphite slag.

[0050] 2g of modified graphite slag was placed in 1L of wastewater containing COD (the number of carbon atoms of organic matter was ≤10). After adsorption for 1h, the COD content in the water decreased from 2000mg / L to 200mg / L.

[0051] Examples 2-4

[0052] Example 2-4 provides a method for resource utilization of graphite slag. The method is the same as Example 1 except that the type of pretreatment solution, the concentration of the pretreatment solution, and the liquid-solid ratio of the pretreatment solution to the graphite slag are different, as shown in Table 1, so that the COD concentration after adsorption changes.

[0053] Table 1

[0054]

[0055] Examples 5-7

[0056] Example 5-7 provides a method for resource utilization of graphite slag, which is the same as Example 1 except that the volume concentration of oxygen during roasting is different, as shown in Table 2, and the COD concentration changes after adsorption.

[0057] Table 2

[0058]

[0059] Examples 8-13

[0060] Examples 8-13 provide a method for resource utilization of graphite slag, which is the same as Example 1 except that the type and concentration of the oxidant-containing solution are changed, as shown in Table 3, to change the COD concentration after adsorption.

[0061] Table 3

[0062]

[0063] Examples 14-17

[0064] Examples 14-17 provide a method for resource utilization of graphite slag, which is the same as Example 1 except that the roasting temperature and time are changed, as shown in Table 4, to change the COD concentration after adsorption.

[0065] Table 4

[0066]

[0067]

[0068] Comparative Example 1

[0069] This comparative example provides a method for resource utilization of graphite slag, which is the same as Example 1 except that the graphite slag is not placed in a sodium carbonate solution, but is directly roasted to change the COD concentration after adsorption.

[0070] Comparative Example 2

[0071] This comparative example provides a method for resource utilization of graphite slag, which is the same as Example 1 except that the roasting step is not performed and the dried graphite slag is directly placed in a hydrogen peroxide solution at room temperature and with a mass concentration of 30% to react so that the COD concentration changes after adsorption.

[0072] Comparative Example 3

[0073] This comparative example provides a method for resource utilization of graphite slag. The method is the same as Example 1 except that the high-temperature graphite slag is not quickly placed in a hydrogen peroxide solution at room temperature and with a mass concentration of 30% for reaction, but the high-temperature graphite slag is slowly cooled in air to change the COD concentration after adsorption.

[0074] Comparative Example 4

[0075] This comparative example provides a method for resource utilization of graphite slag, which is the same as Example 1 except that the roasting is carried out in a pure nitrogen atmosphere to change the COD concentration after adsorption.

[0076] Comparative Example 5

[0077] This comparative example provides a method for resource utilization of graphite slag, which is the same as Example 1 except that the roasting is carried out in a pure oxygen atmosphere to change the COD concentration after adsorption.

[0078] Comparative Example 6

[0079] This comparative example provides a method for resource utilization of graphite slag, which is the same as Example 1 except that the high-temperature graphite slag is quickly placed in water instead of a solution containing an oxidant to change the COD concentration after adsorption.

[0080] The initial COD content in the COD-containing wastewater of Example 1 and Comparative Examples 1-6, and the COD content in the wastewater after adding modified graphite slag for adsorption are shown in Table 5:

[0081] Table 5

[0082]

[0083] From Tables 1 to 5, we can see that:

[0084] (1) It can be seen from Example 1 and Comparative Example 1 that the graphite slag in Comparative Example 1 was not immersed in the pretreatment solution before calcination, while the present invention first performed an immersion pretreatment, which enabled the metal salt and / or metal hydroxide to be adsorbed on the surface of the graphite slag, thereby improving the reaction activity of the graphite slag and thus enhancing the adsorption effect of the modified graphite slag; it can be seen from Example 1 and Comparative Example 2 that the graphite slag in Comparative Example 2 was not subjected to calcination treatment, and the metal salt and / or metal hydroxide adsorbed by the immersion pretreatment could not play a role, the functional groups on the surface of the graphite slag were reduced, and the oxidation effect was poor when it directly reacted with the solution containing the oxidant at room temperature. Therefore, the adsorption effect of the modified graphite slag was greatly reduced; it can be seen from Example 1 and Comparative Example 3 that the graphite slag in Comparative Example 3 was not further oxidized by the oxidant solution, and the adsorption effect was poor; it can be seen from Example 1 and Comparative Example 4 that the graphite slag in Comparative Example 4 was calcined in a pure nitrogen atmosphere, which would affect the number of oxygen-containing functional groups, thereby reducing the adsorption effect of the modified graphite slag.

[0085] (2) It can be seen from Example 1 and Comparative Example 5 that Comparative Example 5 is calcined in a pure oxygen atmosphere, and the volume concentration of oxygen is too high, so that the organic matter on the surface of the graphite slag is completely oxidized and removed. When reacting with a solution containing an oxidant, fewer functional groups are formed on the surface of the graphite slag, thereby significantly reducing the adsorption effect of the modified graphite slag; It can be seen from Example 1, Examples 8-13 and Comparative Example 6 that the present invention performs quenching in a solution containing an oxidant after calcination, which can further deepen the oxidation reaction and prevent excessive oxidation, thereby further improving the adsorption effect of the modified graphite slag; It can be seen from Example 1 and Examples 5-7 that the present invention is preferably calcined at a low volume concentration of oxygen. If the volume concentration of oxygen is too high, the organic matter on the surface of the graphite slag will decompose, affecting the number of oxygen-containing functional groups; It can be seen from Example 1 and Examples 14-17 that if the calcination temperature is too high, the graphite slag will be over-oxidized during the calcination process, causing the organic matter on the graphite surface to decompose, affecting the number of oxygen-containing functional groups, thereby affecting the adsorption effect of the modified graphite slag.

[0086] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for resource utilization of graphite slag, characterized in that: The method comprises the following steps: (1) impregnating the graphite slag in a pretreatment solution, washing and drying after the impregnation is completed to obtain the pretreated graphite slag; The pretreatment solution includes metal salts and / or metal hydroxides; (2) calcining the pretreated graphite slag described in step (1) in an oxygen-containing atmosphere, quenching the calcined material in a solution containing an oxidant after the calcination, and then performing solid-liquid separation to obtain modified graphite slag; In the oxygen-containing atmosphere, the volume concentration of oxygen is ≤30%; (3) Immersing the modified graphite slag in step (2) into wastewater containing COD to reduce the COD content in the wastewater and realize resource utilization of the graphite slag.

2. The method according to claim 1, characterized in that The metal salt in step (1) comprises sodium carbonate and / or potassium carbonate; Preferably, the metal hydroxide in step (1) comprises sodium hydroxide and / or potassium hydroxide; Preferably, the concentration of the pretreatment solution in step (1) is 0.5 mol / L-1.0 mol / L.

3. The method according to claim 1 or 2, characterized in that The liquid-to-solid ratio of the pretreatment solution to the graphite slag in step (1) is 2 mL / g-10 mL / g; Preferably, the graphite slag in step (1) comprises graphite accounting for ≥75% by mass, ash accounting for 3%-17% by mass, and residual organic matter accounting for 5%-10% by mass.

4. The method according to any one of claims 1 to 3, characterized in that The washing method in step (1) includes pulp washing or rinsing; Preferably, the liquid-to-solid ratio of the washing in step (1) is 2 mL / g-5 mL / g.

5. The method according to any one of claims 1 to 4, characterized in that In the oxygen-containing atmosphere of step (2), the volume concentration of oxygen is 5%-10%; Preferably, the calcination temperature in step (2) is 300° C.-550° C., and the calcination time is 5 min-30 min.

6. The method according to any one of claims 1 to 5, characterized in that The quenching time in step (2) is ≤ 5 min; Preferably, in the oxidant-containing solution in step (2), the mass concentration of the oxidant is 10%-70%; Preferably, the temperature of the solution containing the oxidant in step (2) is ≤50°C.

7. The method according to any one of claims 1 to 6, characterized in that The oxidant-containing solution in step (2) includes any one or a combination of at least two of hydrogen peroxide, concentrated sulfuric acid, nitric acid, NaClO, Na2S2O8, KMnO4, K2Cr2O7 or Na2Cr2O7; Preferably, the roasted material in step (2) is completely immersed in a solution containing an oxidant.

8. The method according to any one of claims 1 to 7, characterized in that After the solid-liquid separation in step (2), a water washing step is also performed.

9. The method according to any one of claims 1 to 8, characterized in that The COD-containing wastewater in step (3) includes wastewater containing small-molecule organic matter, and the number of carbon atoms of the small-molecule organic matter is ≤10.

10. A modified graphite slag, characterized in that: The modified graphite slag includes the modified graphite slag prepared in step (2) of the method according to any one of claims 1 to 9.

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