Method for stepwise recovering phosphorus, iron and carbon from ferrophosphorus slag

Abstract

The invention relates to a method for recovering phosphoric acid and ferric oxide from ferrophosphorus slag, and belongs to the technical field of resource circulation. Firstly, ferrophosphorus slag generated after lithium iron phosphate is extracted is crushed and sorted, separation of iron phosphate and graphite in the ferrophosphorus slag is achieved, and the graphite can be used as a lithium iron phosphate battery negative electrode preparation material. Secondly, the separated iron phosphate is placed in a smelting furnace to be roasted, decomposition products are separated to form iron oxide (Fe2O3) and phosphorus pentoxide (P2O5), and after being purified, the iron oxide can be used as a precursor for preparing a lithium iron phosphate battery. And thirdly, washing phosphorus pentoxide with water to prepare phosphoric acid. According to the method, the ferrophosphorus slag in the lithium iron phosphate battery recycling process is used as the raw material to prepare the high-added-value product, enrichment, separation and recycling of valuable elements such as phosphorus, iron and carbon in the ferrophosphorus slag are achieved, and the method has important practical significance on promoting resource circulation in the lithium iron phosphate battery recycling industry.

Description

Technical Field

[0001] The present invention belongs to the technical field of recycling of waste resources and lithium-ion battery materials, and particularly relates to a method for stepwise recovering phosphorus, iron, and carbon from phosphorus-iron slag. Background Art

[0002] With the rapid development of the new energy vehicle industry, the demand for lithium-ion power batteries has increased year by year. When their service life ends, a large number of waste LiFePO4 batteries will inevitably be generated. If the scrapped lithium iron phosphate batteries are not reasonably utilized, it will not only cause serious waste of metal resources, but also endanger the environment and human health. Therefore, recycling lithium iron phosphate batteries has great economic value and social value; during the recovery and refining process of lithium iron phosphate, after the lithium carbonate product is refined, a large amount of phosphorus-iron slag ( Figure 1 ) will be produced. The main components of the phosphorus-iron slag are iron phosphate, carbon slag, and a small amount of impurities, among which the iron element content is 28% - 52%, the phosphorus element content is 17% - 19%, and the carbon element content is 0.5% - 3%. The phosphorus-iron residue contains rich iron and phosphorus resources, and its effective recovery can reduce the dependence on primary ore sources.

[0003] At present, the recovery process of phosphorus-iron slag is not yet mature, and there are relatively few enterprises with the ability to process phosphorus-iron slag. At present, a mature process of an enterprise in Hubei can simultaneously recover and produce lithium carbonate and iron phosphate to maximize resource utilization. The general process is to acid-leach lithium iron phosphate waste to obtain leachate and tailings, and then perform subsequent treatment to obtain lithium carbonate and anhydrous ferric sulfate. There are also some enterprises that have mastered the integrated recovery process from iron-lithium waste to lithium iron phosphate materials, that is, omitting the intermediate products - lithium carbonate and iron phosphate, and directly making lithium iron phosphate materials. From waste to the raw materials of usable iron-lithium materials, this measure improves the utilization rate of waste resources and also reduces the production cost of lithium iron phosphate materials. However, although some scholars have conducted research on the resource recovery of phosphorus-iron slag, no economically feasible large-scale utilization method has been found yet.

[0004] In the research on ferrophosphorus slag, Patent US20240182304A1 involves a method for the efficient recycling and utilization of black phosphoric iron powder slag. This method extracts aluminum phosphate, iron phosphate, and lithium phosphate from the waste slag. By adding sodium hydroxide to extract aluminum phosphate and sulfuric acid to solvate lithium, iron, and phosphate compounds separately, carbon additives, graphite, and other organic compounds are removed, hydrogen peroxide is used to precipitate iron phosphate, lithium phosphate is extracted from the mother liquor, and the mother liquor is recycled into water and sodium sulfate. This method still has a high recovery efficiency in terms of purity and quantity. Also, due to its relatively low cost, the profit margin of this process is good. However, its recovery process is complex and the recovery time is long. Patent CN110683528A involves a method for the regeneration of iron phosphate waste. After acid-dissolving the iron phosphate waste and filtering, a solution containing iron and phosphorus is obtained. After controlling the iron-phosphorus ratio in the solution to be 1:(0.9 - 1.02), an alkaline solution is added, the pH value is adjusted to 2.0 - 2.5, and then the reaction is carried out at (85 - 100)°C for (0.2 - 2) h followed by solid-liquid separation. After washing the filter cake, an amorphous iron phosphate intermediate filter cake is obtained; after pulping the filter cake and adding phosphoric acid, the reaction is carried out under stirring and heating conditions, followed by solid-liquid separation. After washing the filter cake, a crystalline iron phosphate filter cake is obtained, and after calcination, battery-grade anhydrous iron phosphate is obtained. This method can not only achieve the recycling and reuse of resources but also improve the economic benefits of enterprises; however, this method only targets pure iron phosphate waste and has poor adaptability to raw materials. Patent CN111646447A involves a method for recovering iron phosphate from the iron-phosphorus slag after lithium extraction from lithium iron phosphate batteries. First, the iron-phosphorus slag after lithium extraction from lithium iron phosphate batteries is mixed with water to form a slurry, reacted with an acid, and after solid-liquid separation, a leaching solution containing iron and phosphorus ions is obtained. Then, after removing copper by iron replacement and removing aluminum by resin, a purified solution is obtained. By adding ferric phosphate heptahydrate or phosphoric acid to adjust the phosphorus-iron ratio, a synthetic stock solution with a certain P:Fe is obtained. Hydrogen peroxide and ammonia water are added to adjust the pH to obtain a precipitate of iron phosphate precursor. After post-treatment, a battery-grade iron phosphate precursor product is obtained. This method has good adaptability to raw materials, and iron-phosphorus slag with or without positive and negative carbon powder and copper-aluminum impurities can be recycled. Moreover, the recovery rates of iron and phosphorus are high, the purity of the obtained iron phosphate precursor material is high. In addition, this recovery method has a simple process, low production cost, and is environmentally friendly. This technology requires a particularly large amount of acid, and a large amount of alkali is consumed during the precipitation of iron phosphate. At the same time, a large amount of high-salt wastewater is also generated. Patent CN117460694A involves a method for removing carbon from iron-phosphorus slag after lithium extraction and a method for preparing iron phosphate from iron-phosphorus slag after lithium extraction. The iron-phosphorus slag after lithium extraction is dispersed in water to obtain an iron-phosphorus slag dispersion. A magnetic field is applied below the iron-phosphorus slag dispersion, and a strong oxidation activator is added to the dispersion for oxidation activation reaction. After the reaction is complete, an aromatic ether, a foaming agent, and an inhibitor are added and stirred evenly to obtain an activated dispersion; a collector and air are introduced into the activated dispersion together for flotation, and solid-liquid separation is carried out to obtain carbon-removed iron-phosphorus slag. This method solves the problem that the impurity content is high and affects the product performance when the iron-phosphorus slag generated during the recycling of waste lithium iron phosphate batteries is reused to prepare battery-grade iron phosphate.However, its cost performance is low, and the dosage of the inhibitor is excessive, which not only corrodes the experimental equipment but also generates waste water that needs to be further treated. Summary of the Invention

[0005] Aiming at the problems of high impurity content, complex recovery process, and large amount of waste water existing in the existing treatment methods of phosphorus-iron slag, the present invention aims to provide a method for stepwise recovering phosphorus, iron, and carbon by pyrolysis decomposition-wet separation. This process has the advantages of short process flow and high resource utilization rate.

[0006] 1. A method for stepwise recovering phosphorus, iron, and carbon from phosphorus-iron slag. First, the phosphorus-iron slag is placed in a drying oven for drying to remove the water in the phosphorus-iron slag, which is beneficial to the crushing treatment. 2. The method for wet separation of graphite and iron phosphate in phosphorus-iron slag. According to the density difference between graphite and iron phosphate, through wet separation, the graphite floats on the surface of the separation liquid, and the iron phosphate settles at the bottom of the separation liquid, thereby realizing the separation of iron phosphate and graphite. 3. The method for high-temperature roasting of the decarbonized iron phosphate material. Control the decomposition roasting temperature at 1000-1200 °C, the heat preservation time at 30 min-120 min, and the heating rate at 15-20 °C / min to realize the decomposition of iron phosphate. 4. The method for washing phosphorus pentoxide to prepare phosphoric acid. Control the washing temperature at 60-80 °C, and the H2O / P2O5 at 1.32-1.2 to realize the washing and acid preparation of P2O5. Brief Description of the Drawings

[0007] Figure 1 It is a flow chart for lithium extraction from retired lithium iron phosphate batteries.

[0008] Figure 2 It is a schematic diagram of the process flow of the present invention. Detailed Embodiments

[0009] The following is a further detailed description of the present invention through examples, but the protection scope of the present invention is not limited to the content described.

[0010] Example 1: A method for stepwise recovering phosphorus, iron, and carbon from phosphorus-iron slag is as follows: Using the phosphorus-iron slag generated during the lithium iron phosphate recovery process in a domestic factory (Fe: 34.74%; P: 18.84%; H2O: 39.77%; Li: 0.12%), it is dried, crushed and ground to about 8 mm. The ground material is placed in a spiral centrifugal classifier for separation, and the recovery rate of graphite reaches 95.35%. The separated phosphorus-iron slag is placed in a high-temperature roasting furnace and heated from room temperature to 1100 °C at a heating rate of 10 K / min, and held for 120 min. The decomposed product is separated by wet method again. The recovery rate of Fe2O3 reaches 96.48%, and the recovery rate of P2O5 reaches 98.31%. For the separated P2O5, by controlling the water washing temperature at 60 °C and H2O / P2O5 at 1:1.8, after being treated by the above method, the concentration of P2O5 in phosphoric acid is 48.2% (mass fraction).

[0011] Example 2: A method for the cascade recovery of phosphorus, iron and carbon from phosphorus-iron slag is as follows: Using the phosphorus-iron slag generated during the lithium iron phosphate recovery process in a domestic factory (Fe: 34.50%; P: 18.92%; H2O: 41.66%; Li: 0.13%), it is dried, crushed and ground to about 7 mm. The ground material is placed in a spiral centrifugal classifier for separation, and the recovery rate of graphite reaches 98.67%. The separated phosphorus-iron slag is placed in a high-temperature roasting furnace and heated from room temperature to 1150 °C at a heating rate of 15 K / min, and held for 120 min. The decomposed product is separated by wet method again. The recovery rate of Fe2O3 reaches 98.97%, and the recovery rate of P2O5 reaches 99.43%. For the separated P2O5, by controlling the water washing temperature at 70 °C and H2O / P2O5 at 1:1.9, after being treated by the above method, the concentration of P2O5 in phosphoric acid is 50.27% (mass fraction).

[0012] Example 3: A method for the cascade recovery of phosphorus, iron and carbon from phosphorus-iron slag is as follows: Using the phosphorus-iron slag generated during the lithium iron phosphate recovery process in a domestic factory (Fe: 21%; P: 10.5%; H2O: 37%; Li: 0.18%), it is dried, crushed and ground to about 6 mm. The ground material is placed in a spiral centrifugal classifier for separation, and the recovery rate of graphite reaches 98.87%. The separated phosphorus-iron slag is placed in a high-temperature roasting furnace and heated from room temperature to 1200 °C at a heating rate of 15 K / min, and held for 120 min. The decomposed product is separated by wet method again. The recovery rate of Fe2O3 reaches 99.12%, and the recovery rate of P2O5 reaches 99.54%. For the separated P2O5, by controlling the water washing temperature at 75 °C and H2O / P2O5 at 1:2, after being treated by the above method, the concentration of P2O5 in phosphoric acid is 50.33% (mass fraction).

Claims

1. A method for recovering phosphoric acid and iron oxide from ferrophosphorus slag, characterized in that Recycling of lithium iron phosphate batteries Recycling of valuable elements in iron phosphate slag, the specific steps include: (1) Drying the ferrophosphorus slag after lithium extraction in a drying oven; (2) crushing and screening the material obtained in step (1); (3) separating and decarbonizing the material obtained in step (2); (4) calcining the decarbonized iron phosphate material obtained in step (3) at high temperature; (5) separating the roasting product obtained in step (4); (6) The phosphorus pentoxide obtained in step (5) is subjected to acid washing to prepare phosphoric acid; and the iron oxide is used to prepare a lithium iron phosphate precursor.

2. The method for recovering phosphoric acid and iron oxide from ferrophosphorus slag according to claim 1, characterized in that: The separation of graphite and ferric phosphate in ferrophosphorus slag is achieved by water washing.

3. The method for recovering phosphoric acid and iron oxide from ferrophosphorus slag according to claim 1, characterized in that: Iron phosphate decomposes in roasting equipment at temperatures greater than 1000°C.

4. The method for recovering phosphoric acid and iron oxide from ferrophosphorus slag according to claim 1, characterized in that: The temperature of the drying oven is 110-130℃, the insulation time is 2-5h, and the moisture content of the material is ≤1% (implemented according to industrial production data).

5. The method for recovering phosphoric acid and iron oxide from ferrophosphorus slag according to claim 1, characterized in that: During the roasting process, the temperature is raised from room temperature to 1000-1200°C at a heating rate of 15-20°C / min and kept at this temperature for 30-120 minutes to cause the iron phosphate in the material to decompose.

6. The method for recovering phosphoric acid and iron oxide from ferrophosphorus slag according to claim 1, characterized in that: The roasted product phosphorus pentoxide is washed with high-temperature water to prepare phosphoric acid, wherein the washing temperature is not higher than 80°C.

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