Method for recovering iron from lithium extraction slag

Abstract

A method for recycling iron from lithium extraction slag comprises the following steps that S1, the lithium extraction slag is added into alkali liquor to be soaked, and alkali leaching liquor is obtained; s2, a carbon-silicon composite reagent is added into the alkaline leaching solution for a reaction, and after the reaction is completed, solid-liquid separation is conducted to obtain filter residues; and S3, the filter residues obtained in the step S2 are subjected to anaerobic roasting, and the roasted product is subjected to magnetic separation to obtain iron powder. According to the method for recycling the iron from the lithium extraction slag, the modes of alkaline leaching, carbon-silicon composite reagent, anaerobic roasting and the like are adopted, the problems that the iron recycled from the lithium extraction slag is high in phosphorus content, low in grade, low in additional value and the like are solved, the utilization rate of the lithium extraction slag is increased, and obtained iron powder is high in grade, few in phosphorus impurity and greatly improved in additional value.

Description

Technical Field

[0001] The present invention relates to a method for recovering metals, in particular to a method for recovering iron from lithium extraction slag, belonging to the technical field of solid waste recovery. Background Art

[0002] Lithium iron phosphate (LiFePO4) is one of the most promising cathode materials for power batteries. With its high specific capacity, safe performance, long cycle life, and low production cost, it has recently been widely used in electric vehicles, electric buses, communication base stations, and other fields. With the rapid development of the new energy industry, the use of LiFePO4 batteries has increased annually, and its market share has also continued to grow, from 34% in 2019 to 51% in 2021. The average lifespan of LiFePO4 batteries is 5 to 8 years. my country is currently facing a large-scale retirement and a sharp increase in the scrapping of LiFePO4 batteries. It is estimated that by 2030, the annual recycling volume of retired LiFePO4 batteries will reach 1.5 million tons. Currently, the mainstream technology for recycling retired LiFePO4 battery cathode materials is the selective recovery of lithium through a wet process, which has achieved industrialized operation. However, the process of lithium extraction leaves behind and accumulates large amounts of LiFePO4 slag. This slag has a complex composition and high impurity content, making it an urgent resource for recycling. Processing one ton of retired lithium iron phosphate batteries generates approximately 1.0 to 1.2 tons of lithium iron phosphate extraction slag, a significant amount of waste. If this slag is not properly disposed of, its long-term storage will not only pollute the environment and impact the sustainable development of the industry, but also waste phosphorus and iron resources. Therefore, the need for resourceful processing of this lithium iron phosphate extraction slag is extremely urgent.

[0003] The main components of the lithium slag after lithium extraction from waste lithium iron phosphate are iron phosphate, graphite carbon and some impurities. SP The iron phosphate content is higher than that of iron phosphate. Under highly alkaline conditions, iron phosphate will convert to iron hydroxide. Some processes using strong alkali to extract iron have been proposed. However, due to the strong affinity of iron hydroxide for phosphate, phosphate will remain on the surface of the iron hydroxide in a strongly adsorbed state and is difficult to remove. After direct roasting and decarburization, phosphorus will enter the interior of the iron oxide lattice. The resulting iron oxide has a high phosphorus content and does not meet steelmaking requirements. Therefore, deep phosphorus reduction technology for alkaline leaching of iron from lithium iron phosphate extraction residues has yet to be developed. Summary of the Invention

[0004] To address the problem of high phosphorus content in iron oxides recovered from lithium iron phosphate extraction slag, the present invention proposes a method for recovering iron from lithium iron phosphate extraction slag. After alkaline leaching, iron hydroxide and sodium phosphate are separated using a carbon-silicon composite reagent to reduce the phosphorus content in the product, ensuring that the product meets steelmaking requirements.

[0005] According to an embodiment of the present invention, a method for recovering iron from lithium extraction slag is provided.

[0006] A method for recovering iron from lithium extraction slag, the method comprising the following steps:

[0007] S1: adding the lithium extraction residue into an alkaline solution and soaking it to obtain an alkaline leaching solution;

[0008] S2: adding a carbon-silicon composite reagent to the alkaline leaching solution to react, and after the reaction is completed, performing solid-liquid separation to obtain a filter residue;

[0009] S3: The filter residue obtained in step S2 is subjected to oxygen-free roasting, and the roasted product is subjected to magnetic separation to obtain iron powder.

[0010] Preferably, the alkali solution in step S1 is an aqueous solution of any one or more of sodium carbonate, sodium hydroxide, and potassium hydroxide; the liquid-to-solid ratio of the alkali solution to the lithium extraction slag is 5-10 ml / g, preferably 6-9 ml / g;

[0011] Preferably, the amount of alkali solution used is such that the mass ratio of alkali to lithium extraction slag is 1 to 2:1, preferably 1.2 to 1.8:1.

[0012] Preferably, the lithium extraction slag is soaked in the alkaline solution for 2 to 6 hours, preferably 3 to 5 hours.

[0013] Preferably, the temperature of the lithium extraction slag soaked in the alkaline solution is 40-80°C, preferably 50-70°C.

[0014] Preferably, the carbon-silicon composite reagent is a mixture of silicon dioxide and carbon powder;

[0015] Preferably, the amount of the carbon-silicon composite reagent added is 10% to 20% of the total mass of the alkali in the alkali solution, preferably 12% to 18%;

[0016] Preferably, the mixing mass ratio of carbon powder to silicon dioxide in the carbon-silicon composite reagent is 5 to 20:1, preferably 8 to 15:1.

[0017] Preferably, the calcination temperature of the oxygen-free calcination is 800 to 1200°C, more preferably 900 to 1100°C.

[0018] Preferably, the heating rate of the oxygen-free calcination is 5 to 10° C. / min, preferably 6 to 9° C. / min.

[0019] Preferably, the oxygen-free calcination time is 1 to 2 hours, preferably 1.2 to 1.8 hours.

[0020] Preferably, the calcination atmosphere for oxygen-free calcination is nitrogen or argon.

[0021] Preferably, the magnetic field strength of the magnetic separation in step S3 is 1000-1500 Gauss, preferably 1100-1400 Gauss.

[0022] Preferably, the method further comprises: grinding the calcined product before magnetic separation; preferably, the mesh size of the grinding is 200 to 400 meshes, preferably 250 to 350 meshes.

[0023] Preferably, step S1 is specifically: adding the lithium extraction slag to the alkali solution at a liquid-solid ratio of 5-10 ml / g (preferably 6-9 ml / g), wherein the ratio of the amount of alkali added to the lithium extraction slag mass in the alkali solution is 1-2:1 (preferably 1.2-1.8:1), and soaking in the alkali solution at 40-80° C. (preferably 50-70° C.) for 2-6 h (preferably 3-5 h) to obtain an alkali leaching solution.

[0024] Preferably, step S2 is specifically as follows: adding a carbon-silicon composite reagent to the alkaline leaching solution obtained in step S1, obtaining a solid-liquid mixture after the reaction is completed, wherein the amount of the carbon-silicon composite reagent added is 10% to 20% (preferably 12% to 18%) of the amount of alkali added in the alkaline solution, wherein the mass ratio of carbon powder to silicon dioxide is 5 to 20:1, preferably 8 to 15:1; and then stirring and cooling the obtained solid-liquid mixture at a stirring speed of 200 to 1000 r / min (preferably 400 to 800 r / min), filtering after cooling, washing the obtained solid with water 1 to 2 times, and then drying to obtain a filter residue.

[0025] Preferably, step S3 is specifically as follows: placing the filter residue obtained in step S2 in a nitrogen or argon atmosphere, heating it to 800-1200°C (preferably 900-1100°C) at a heating rate of 5-10°C / min (preferably 6-9°C / min), and calcining it at this temperature for 1-2h (preferably 1.2-1.8h) to obtain a calcined product, then grinding the calcined product to 200-400 mesh (preferably 250-350 mesh), and then using a magnetic field of 1000-1500 Gauss (preferably 1100-1400 Gauss) for magnetic separation to obtain iron powder.

[0026] In the present invention, the lithium extraction slag is first soaked in alkaline solution. Since the Ksp of ferric hydroxide is higher than that of ferric phosphate, the ferric phosphate is converted to ferric hydroxide under highly alkaline conditions, resulting in an alkaline leaching solution, i.e., a mixture of phosphorus-containing ferric hydroxide, carbon, and alkali solution. A carbon-silicon composite reagent is then added to the alkaline leaching solution to improve the separation between the ferric hydroxide and trisodium phosphate. The filter residue is then extracted from the solid-liquid mixture and calcined in the absence of oxygen. The calcined product is then subjected to magnetic separation to obtain iron powder. This method solves the problems of high phosphorus content, low grade, and low added value of iron recovered from lithium extraction slag, improves the utilization rate of the lithium extraction slag, and produces high-grade iron powder with low phosphorus impurities and significantly increased added value.

[0027] In the present invention, the carbon powder and silicon dioxide in the carbon-silicon composite reagent are mixed by physical mixing (such as grinding or mechanical stirring) until they are uniformly dispersed. The carbon powder reduces iron oxides and, during the oxygen-free roasting stage, can also reduce some phosphates (such as Na3PO4) to gaseous phosphorus (P4) or low-melting-point phosphides, reducing the adsorption of phosphate on the surface of ferric hydroxide and improving the yield and grade of iron powder. The silicon dioxide reacts with excess alkali solution to form silicates (such as sodium silicate), which preferentially bind to free phosphates (phosphates) to form a stable silicon-phosphate complex, inhibiting phosphorus from re-binding with iron during the roasting stage, thereby achieving efficient separation of iron and phosphorus, reducing the phosphorus content in the final iron powder, and improving the purity of the iron powder. Furthermore, carbon powder and silica exhibit a synergistic effect: during the calcination process, carbon powder directionally reduces iron hydroxide or iron oxides to magnetic Fe / Fe3O4 in the absence of oxygen, while phosphorus is fixed in the silicophosphate slag phase. Silicon dioxide reacts with excess alkali to form silicates, reducing system viscosity and preventing high-temperature sintering, thereby improving iron recovery. Compared to the high-phosphorus iron hydroxide produced in existing technologies, the iron hydroxide obtained in this invention is of higher quality and phosphorus is easily separated, which helps improve the quality of subsequent iron powder products.

[0028] In the present invention, the lithium extraction slag is alkali-leached using an aqueous solution of any one or more alkalis selected from sodium carbonate, potassium hydroxide, and sodium hydroxide, which can effectively convert the iron phosphate in the lithium extraction slag into iron hydroxide to obtain a mixture of high-phosphorus iron hydroxide and carbon. It is preferred to control parameters such as the solid-liquid mass ratio and the alkali leaching time during the alkali leaching process, such as controlling the mass ratio of the amount of alkali added in the alkali leaching to the lithium extraction slag to be 1 to 2:1 (e.g., 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1), so that the lithium extraction slag is at a suitable pH and the iron phosphate is fully converted into iron hydroxide. The liquid-solid ratio is controlled to be 5-10 ml / g (e.g., 5 ml / g, 5.1 ml / g, 5.3 ml / g, 5.5 ml / g, 5.8 ml / g, 6 ml / g, 6.3 ml / g, 6.6 ml / g, 6.9 ml / g, 7 ml / g, 7.2 ml / g, 7.5 ml / g, 7.8 ml / g, 8 ml / g, 8.3 ml / g, 8.6 ml / g, 9 ml / g, 9.3 ml / g, 9.5 ml / g, 9.7 ml / g, 9.9 ml / g, 10 ml / g) to avoid crystallization of sodium phosphate due to too low liquid-solid ratio, and the solution becoming lumpy and unable to be separated, or waste of water resources due to too high liquid-solid ratio, which is not conducive to the environment and control costs. In addition, the alkali leaching time is controlled to 2 to 6 hours to avoid the adsorption of phosphorus on the generated iron hydroxide due to too long alkali leaching time, and the appropriate alkali leaching temperature (40 to 80°C) is conducive to the good separation of iron and phosphorus.

[0029] In the present invention, a mixture of silicon dioxide and carbon powder is used to form a carbon-silicon composite reagent, which promotes the conversion of phosphate adsorbed on the iron oxide into phosphorus, improves the separation degree of iron hydroxide and trisodium phosphate, reduces the phenomenon of phosphate adsorption on the surface of iron hydroxide, and improves the purity of the iron oxide. In addition, the amount of the carbon-silicon composite reagent added is controlled to be 10% to 20% of the amount of alkali added (for example, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, 18.5%, 19%, 19.5%, 20%). On the one hand, it ensures that there is enough silicate generated to bind most of the free phosphate and prevent phosphorus from being re-adsorbed on the iron phase; on the other hand, it avoids adding too much inert silicate slag phase, which reduces the efficiency of the roasting reaction and reduces the economic efficiency. The mass ratio of carbon powder to silica in the carbon-silicon composite reagent is controlled to be between 5 and 20:1 (e.g., 5:1, 5.5:1, 6:1, 7:1, 8:1, 8.5:1, 9:1, 9.5:1, 10:1, 10.5:1, 11:1, 12:1, 12.5:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, and 20:1). This is done to avoid excessive carbon powder ratios, which lead to excessive reducing power and the destruction of the silicate structure, releasing bound phosphorus. It is also important to avoid excessive carbon powder ratios, which lead to insufficient reducing agent, incomplete iron oxide reduction, and reduced phosphorus removal efficiency. The 8-15:1 range is particularly important to balance the synergistic effects of reducing power and silicate stability, ensuring efficient iron reduction and stable phosphorus removal. Excess carbon powder can participate in subsequent calcination and dephosphorization reactions.

[0030] In the present invention, after the filter residue is separated from the solid-liquid mixture, it is washed with water to remove the weakly adsorbed phosphorus on the surface of the iron hydroxide. In addition, the iron hydroxide is reduced to iron powder by oxygen-free roasting. The carbon powder of the carbon-silicon composite reagent can also migrate O from the iron oxide (hydroxide) to generate CO to accelerate the reduction of the iron oxide (iron hydroxide), ultimately achieving deep removal of phosphorus and effective recovery of iron. Among them, the specific path of carbon powder migrating O from the iron oxide (hydroxide) to generate CO is as follows:

[0031] Decomposition of ferric hydroxide: Fe(OH)3 decomposes into Fe2O3 (or Fe3O4) when heated and releases H2O (O2 is not generated under anaerobic conditions);

[0032] Carbon reduces iron oxides: Carbon powder (C) reacts with Fe2O3, capturing its lattice oxygen to generate CO (reaction formula: Fe2O3+3C→2Fe+3CO↑);

[0033] Deep removal of phosphorus: Part of the CO reacts with residual phosphate (such as 3Na3PO4+5CO→5Na2CO3+P2↑), further reducing the phosphorus content.

[0034] In the present invention, the calcined product can be ground before magnetic separation to facilitate the subsequent magnetic separation step.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] 1. The present invention provides a method for recovering iron from lithium extraction slag, which adopts alkaline leaching, carbon-silicon composite reagent, oxygen-free roasting and other methods to solve the problems of high phosphorus content, low grade and low added value of iron recovered from lithium extraction slag, improve the utilization rate of lithium extraction slag, and obtain iron powder with high grade, less phosphorus impurities and greatly improved added value.

[0037] 2. The present invention provides a method for recovering iron from lithium extraction slag, which limits the amount of carbon-silicon composite reagent added and the ratio of carbon powder and silicon dioxide in the carbon-silicon composite reagent, maximizes the separation degree of ferric hydroxide and trisodium phosphate, and reduces the adsorption of phosphate on the surface of ferric hydroxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is the XRD pattern of the lithium extraction slag in Example 1.

[0039] Figure 2 This is the XRD pattern of the filter residue in Example 1.

[0040] Figure 3 This is the XRD pattern of the filter residue after calcination and reduction in Example 1. DETAILED DESCRIPTION

[0041] The technical solutions of the present invention are illustrated below with examples, and the scope of protection requested by the present invention includes but is not limited to the following embodiments.

[0042] Example 1

[0043] A method for recovering iron from lithium extraction slag, the method comprising the following steps:

[0044] S1. Add 50 g of lithium extraction slag to 400 mL of alkaline solution, wherein the amount of sodium hydroxide added to the alkaline solution is 75 g. Soak the lithium extraction slag in the alkaline solution at 60° C. for 4 h to obtain 430 mL of alkaline leaching solution.

[0045] S2. Add 11.3 g of a carbon-silicon composite reagent to the alkaline leaching solution obtained in step S1, wherein the mass ratio of carbon powder to silicon dioxide is 12:1, to obtain a solid-liquid mixture. The obtained solid-liquid mixture is stirred and cooled at a stirring speed of 600 r / min, and filtered after cooling to obtain 19.3 g of a solid. The obtained solid is washed 1 to 2 times with 50 mL of water, and then dried to obtain 12.1 g of a filter residue.

[0046] S3. Place 12.1 g of the filter residue obtained in step S2 in a nitrogen atmosphere, heat it to 1000 ° C at a heating rate of 7 ° C / min, and calcine it at this temperature for 1.6 h to obtain a calcined product, then grind it to 300 mesh, and then perform magnetic separation in a magnetic field of 1200 Gauss to obtain 7.3 g of iron powder.

[0047] The obtained 7.3 g iron powder was detected to have an Fe grade of 99.1% and a P grade of 0.05%.

[0048] Example 1 was repeated, but some parameters were changed to conduct comparative experiments. The specific parameter settings are shown in Table 1.

[0049] Table 1

[0050]

[0051]

[0052] Comparative Example 1

[0053] A method for recovering iron from lithium extraction slag, the method comprising the following steps:

[0054] S1. Add 50 g of lithium extraction slag to 400 mL of alkaline solution, wherein the amount of sodium hydroxide added to the alkaline solution is 75 g. Soak the lithium extraction slag in the alkaline solution at 60° C. for 4 h to obtain 430 mL of alkaline leaching solution.

[0055] S2. The alkaline leaching solution was stirred and cooled at a stirring speed of 600 r / min, and filtered after cooling to obtain 16.2 g of solid. The obtained solid was washed 1 to 2 times with 50 mL of water, and then dried to obtain 10.6 g of filter residue.

[0056] S3. Place 10.6 g of the filter residue obtained in step S2 in a nitrogen atmosphere, heat it to 1000 ° C at a heating rate of 7 ° C / min, and calcine it at this temperature for 1.6 h to obtain a calcined product, then grind it to 300 mesh, and then perform magnetic separation in a magnetic field of 1200 Gauss to obtain 4.1 g of iron powder.

[0057] The 4.1g iron powder obtained was tested, with an Fe grade of 73.6% and a P grade of 5.35%. This indicates that the iron oxides were not reduced to metallic iron during calcination due to the lack of carbon powder reduction, resulting in low magnetic separation efficiency. Furthermore, there was no SiO2 to form a SiO3-P2O5 slag phase with phosphorus, and phosphorus could not be effectively removed.

[0058] Comparative Example 2

[0059] A method for recovering iron from lithium extraction slag, the method comprising the following steps:

[0060] 50g of lithium-extracted slag was mixed with 75g of sodium carbonate. The mixture was placed in a nitrogen atmosphere and heated to 1000°C at a heating rate of 7°C / min. The mixture was calcined at this temperature for 1.6h to obtain a calcined product. The calcined product was then ground to 300 mesh and magnetically separated using a 1200 gauss magnetic field to obtain 2.5g of iron powder.

[0061] The 2.5g iron powder obtained by testing had an Fe grade of 68.73% and a P grade of 6.88%, indicating that the impurities such as Al and Si encapsulating the iron phase were not removed by alkaline leaching, and no effective slag phase was formed.

[0062] Comparative Example 3

[0063] A method for recovering iron from lithium extraction slag, the method comprising the following steps:

[0064] S1. Add 50 g of lithium extraction slag to 400 mL of alkaline solution, wherein the amount of sodium hydroxide added to the alkaline solution is 75 g. Soak the lithium extraction slag in the alkaline solution at 60° C. for 4 h to obtain 430 mL of alkaline leaching solution.

[0065] S2. Add 10 g of carbon powder to the alkaline leaching solution obtained in step S1 to obtain a solid-liquid mixture. The obtained solid-liquid mixture is stirred and cooled at a stirring speed of 600 r / min. After cooling, it is filtered to obtain 16.6 g of solid. The obtained solid is washed 1-2 times with 50 mL of water and then dried to obtain 11.9 g of filter residue.

[0066] S3. 11.9 g of the filter residue obtained in step S2 was placed in a nitrogen atmosphere, heated to 1000° C. at a heating rate of 7° C. / min, and calcined at this temperature for 1.6 h to obtain a calcined product, which was then ground to 300 mesh and magnetically separated in a 1200 Gauss magnetic field to obtain 6.1 g of iron powder.

[0067] The 6.1g iron powder obtained was tested, with an Fe grade of 93.2% and a P grade of 4.72%. This indicates that the carbon provided a reducing atmosphere, but the lack of SiO2 slag formation prevented phosphorus from being fixed; the iron was fully reduced, but the residual phosphorus caused a decrease in purity.

[0068] Comparative Example 4

[0069] A method for recovering iron from lithium extraction slag, the method comprising the following steps:

[0070] S1. Add 50 g of lithium extraction slag to 400 mL of alkaline solution, wherein the amount of sodium hydroxide added to the alkaline solution is 75 g. Soak the lithium extraction slag in the alkaline solution at 60° C. for 4 h to obtain 430 mL of alkaline leaching solution.

[0071] S2. Add 1.3 g of silicon dioxide to the alkaline leaching solution obtained in step S1 to obtain a solid-liquid mixture. The obtained solid-liquid mixture is stirred and cooled at a stirring speed of 600 r / min. After cooling, it is filtered to obtain 15.9 g of solid. The obtained solid is washed 1-2 times with 40 mL of water and then dried to obtain 11.8 g of filter residue.

[0072] S3. Place 11.8 g of the filter residue obtained in step S2 in a nitrogen atmosphere, heat it to 1000 ° C at a heating rate of 7 ° C / min, and calcine it at this temperature for 1.6 h to obtain a calcined product, then grind it to 300 mesh, and then perform magnetic separation in a magnetic field of 1200 Gauss to obtain 3.4 g of iron powder.

[0073] The 3.4g iron powder obtained was tested, with an Fe grade of 79.4% and a P grade of 1.15%. This indicates that SiO2 can partially fix phosphorus, but the iron oxides are not reduced due to the absence of a reducing agent; magnetic separation only separates Fe3O4, and the impurity phase is not destroyed.

[0074] The quality of the iron powders obtained in Examples 1 to 23 and Comparative Examples 1 to 4 and the grades of iron and phosphorus therein were tested respectively. The results are shown in Table 2.

[0075] Table 2

[0076]

[0077]

[0078] Based on the experimental results of Examples 1-23 and Comparative Examples 1-4, the method for recovering iron from lithium extraction slag provided by the present invention can effectively solve the problem of high phosphorus content and low iron grade in the iron recovered from lithium extraction slag, thereby improving the utilization rate of the lithium extraction slag. In addition, experimental parameters such as the optimal addition amount of the carbon-silicon composite reagent and the optimal ratio of carbon powder to silicon dioxide in the carbon-silicon composite reagent were also determined, ensuring the production of higher-purity iron powder.

Claims

1. A method for recovering iron from lithium extraction slag, characterized in that: The method comprises the following steps: S1: adding the lithium extraction residue into an alkaline solution and soaking it to obtain an alkaline leaching solution; S2: adding a carbon-silicon composite reagent to the alkaline leaching solution to react, and after the reaction is completed, performing solid-liquid separation to obtain a filter residue; S3: The filter residue obtained in step S2 is subjected to oxygen-free roasting, and the roasted product is subjected to magnetic separation to obtain iron powder.

2. The method according to claim 1, wherein: The alkali solution in step S1 is an aqueous solution of any one or more of sodium carbonate, sodium hydroxide, and potassium hydroxide; the liquid-to-solid ratio of the alkali solution to the lithium extraction slag is 5-10 ml / g, preferably 6-9 ml / g.

3. The method according to claim 2, wherein: The amount of alkali solution used is such that the mass ratio of alkali to lithium extraction slag is 1 to 2:1, preferably 1.2 to 1.8:1; and / or The lithium extraction slag is soaked in the alkali solution for 2 to 6 hours, preferably 3 to 5 hours; and / or The temperature for soaking the lithium extraction slag in the alkaline solution is 40-80°C, preferably 50-70°C.

4. The method according to any one of claims 1 to 3, characterized in that: The carbon-silicon composite reagent is a mixture of silicon dioxide and carbon powder; Preferably, the amount of the carbon-silicon composite reagent added is 10% to 20% of the total mass of the alkali in the alkali solution, preferably 12% to 18%; Preferably, the mixing mass ratio of carbon powder to silicon dioxide in the carbon-silicon composite reagent is 5 to 20:1, preferably 8 to 15:

1.

5. The method according to any one of claims 1 to 4, characterized in that: The calcination temperature of the oxygen-free calcination is 800-1200° C., preferably 900-1100° C.; and / or The heating rate of oxygen-free calcination is 5 to 10°C / min, preferably 6 to 9°C / min; and / or The time of oxygen-free roasting is 1 to 2 hours, preferably 1.2 to 1.8 hours; and / or The calcination atmosphere for oxygen-free calcination is nitrogen or argon.

6. The method according to any one of claims 1 to 5, characterized in that: The magnetic field strength of the magnetic separation in step S3 is 1000-1500 Gauss, preferably 1100-1400 Gauss.

7. The method according to any one of claims 1 to 6, characterized in that: The method further comprises: grinding the roasted product before magnetic separation; preferably, the mesh size of the grinding is 200 to 400 meshes, and more preferably 250 to 350 meshes.

8. The method according to any one of claims 1 to 7, characterized in that: The step S1 specifically comprises: adding the lithium extraction slag to an alkaline solution at a liquid-solid ratio of 5 to 10 ml / g (preferably 6 to 9 ml / g), wherein the ratio of the amount of alkali added to the lithium extraction slag mass in the alkaline solution is 1 to 2:1 (preferably 1.2 to 1.8:1), and soaking the slag in the alkaline solution at 40 to 80° C. (preferably 50 to 70° C.) for 2 to 6 hours (preferably 3 to 5 hours) to obtain an alkaline leaching solution.

9. The method according to any one of claims 1 to 8, characterized in that: The step S2 specifically comprises: adding a carbon-silicon composite reagent to the alkaline leaching solution obtained in step S1, obtaining a solid-liquid mixture after the reaction is completed, wherein the amount of the carbon-silicon composite reagent added is 10% to 20% (preferably 12% to 18%) of the amount of alkali added in the alkaline solution, wherein the mass ratio of carbon powder to silicon dioxide is 5 to 20:1, preferably 8 to 15:1; stirring and cooling the obtained solid-liquid mixture at a stirring speed of 200 to 1000 r / min (preferably 400 to 800 r / min), filtering after cooling, washing the obtained solid with water 1 to 2 times, and then drying to obtain a filter residue.

10. The method according to any one of claims 1 to 9, characterized in that: The specific step S3 is: placing the filter residue obtained in step S2 in a nitrogen or argon atmosphere, heating it to 800-1200°C (preferably 900-1100°C) at a heating rate of 5-10°C / min (preferably 6-9°C / min), and calcining it at this temperature for 1-2 hours (preferably 1.2-1.8 hours) to obtain a calcined product, then grinding the calcined product to 200-400 mesh (preferably 250-350 mesh), and then using a magnetic field of 1000-1500 Gauss (preferably 1100-1400 Gauss) for magnetic separation to obtain iron powder.

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