Method for recovering elemental phosphorus through vacuum silicothermic reduction of iron phosphate

Abstract

The invention discloses a method for recovering simple substance phosphorus through vacuum silicothermic reduction of iron phosphate, and belongs to the technical field of resource circulation. The method comprises the following steps: firstly, crushing and sorting iron phosphate generated after lithium extraction of lithium iron phosphate to realize separation of iron phosphate and impurities; secondly, the separated iron phosphate is placed in a vacuum smelting furnace, silicon thermal reduction roasting is carried out with silicon powder as a reducing agent, reduction residues are ferric oxide (Fe2O3) and silicon dioxide (SiO2), and elemental phosphorus (P) is generated after a reduction gas phase is subjected to shock cooling. According to the method, iron phosphate generated in the lithium iron phosphate battery recycling process is used as a raw material to prepare a high-added-value product, separation and recycling of valuable elements such as phosphorus and iron in the phosphorus iron 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 invention belongs to the technical field of recycling of waste resources and lithium-ion battery materials, and in particular relates to a method for recovering elemental phosphorus through vacuum silicon thermal reduction of iron phosphate. Background Art

[0002] At present, the market demand for lithium iron phosphate batteries, which is a core component of my country's new energy industry, has shown explosive growth. 4 , LFP) and ternary materials (LiNi x Co y Mn 1 - x-y O 2 , NCM) are the two most widely used positive electrode materials. They each have their own advantages and disadvantages, so electric vehicle manufacturers need to make a choice based on the comprehensive performance of the battery. Although NCM materials have high energy density, electric vehicles using NCM as positive electrodes have a higher range under the same volume or the same mass. However, due to the structural characteristics of ternary materials, they are prone to out of control and fire and explosion under extreme conditions. Compared with NCM, LiFePO 4 The positive electrode material performs well in terms of safety performance. Its unique olivine structure gives LiFePO 4 Excellent thermal stability enables the battery to maintain stable operation under extreme conditions. In some application scenarios with extremely high safety requirements, such as large public transportation, LiFePO 4 Batteries still have a place due to their excellent safety performance. In addition, LiFePO 4 The material also has the advantages of low price and environmental protection, which further enhances its competitiveness in the new energy vehicle market. 4 After a long period of use, the battery can no longer meet the requirements of electric vehicles due to problems such as aging of the positive electrode material, decomposition of the electrolyte, and passivation of the interface side reactions. Depending on the actual use conditions and environment, the service life of LiFePO4 power batteries is generally 5-8 years. This means that in the context of the rapid development of the electric vehicle industry, a large number of batteries will reach retirement age in the next few years. In addition, the battery still contains some chemical substances, which may cause serious pollution to the environment if not handled properly, so the treatment of retired batteries needs to be given enough attention.

[0003] At present, lithium iron phosphate battery recycling technology is mainly divided into two categories: hydrometallurgy and pyrometallurgy. Hydrometallurgical processes usually use an acid / alkali leaching system to separate and recover valuable components through a chemical dissolution-precipitation process. For example, patent CN113772649A discloses a method for recycling and regenerating waste lithium iron phosphate positive electrode materials to prepare battery-grade iron phosphate, and its process route includes acid leaching of lithium, sulfuric acid conversion to obtain hydrated iron phosphate, and calcination at 600°C to prepare orthophosphate iron; patent CN113044824A proposes a method for recycling waste iron phosphate, using acid and homemade iron-phosphorus solution as precipitants, and preparing anhydrous iron phosphate through heat treatment at 80~100°C. The pyrometallurgical process mainly uses the differences in the physical properties of materials for separation and recovery. For example, researchers such as Yang Qiuju successfully prepared repaired lithium iron phosphate by adding lithium carbonate, ammonium dihydrogen phosphate and ferrous oxalate to the recycled lithium iron phosphate black powder, and calcined it in an argon-hydrogen mixed atmosphere after dry ball milling. Bian Ducheng et al. used air calcination to remove the binder in the positive electrode sheet, then added glucose and lithium carbonate, and used ethanol as a dispersant for wet ball milling, and finally obtained regenerated LiFePO 4 Positive electrode material.

[0004] A realistic problem is that lithium is the only high-value component in retired lithium iron phosphate battery materials. So there are many technologies that can efficiently extract lithium, but there is no better way to deal with the iron phosphate waste generated after lithium extraction. Therefore, exploring simpler, more efficient, and greener diversified upgrading and regeneration strategies is of great significance to the construction of a recycling system for retired lithium iron phosphate battery materials. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a method for recovering elemental phosphorus by vacuum silicon thermal reduction of iron phosphate. The method uses iron phosphate slag after lithium extraction from lithium iron phosphate as raw material, industrial silicon powder or silicon-containing waste as a reducing agent, and directly prepares elemental phosphorus by vacuum thermal reduction. The present invention has the advantages of economical and readily available raw materials, short process flow, easy operation, low overall cost, and environmental friendliness.

[0006] The technical solution of the present invention is as follows: (1) Using iron phosphate slag as raw material and industrial silicon powder or silicon-containing waste as reducing agent, the Si in the reducing agent (industrial silicon powder, silicon-containing waste, etc.) and the FePO4 in the iron phosphate slag are used as the raw material. 4 The ingredients are mixed in a molar ratio of 1.25 to 3:1 and mixed evenly; (2) Pressing the mixed material obtained in step (1) into a shape and placing it in a drying oven for drying; The drying oven temperature is 110-130℃, the holding time is 4-7h, and the moisture content of the material is ≤1% (according to industrial production data); (3) placing the material block obtained in step (2) into a vacuum furnace for reduction smelting; The reduction smelting is carried out under vacuum conditions, the vacuum furnace is heated from room temperature to 400-600°C at a heating rate of 15-20°C / min, and kept warm for 30-90min, so that the iron phosphate in the material undergoes a silicon thermal reduction reaction with the reducing agent; (4) After the reduction smelting product is cooled, reduction slag is obtained, and at the same time, the volatiles after reduction are condensed to obtain elemental phosphorus.

[0007] Preferably, the iron phosphate slag in step (1) is the iron phosphate slag produced after lithium is recovered from waste lithium iron phosphate batteries.

[0008] Preferably, the briquetting pressure of the mixed material in step (2) is 20 to 100 MPa.

[0009] Furthermore, the briquetting pressure of the mixed material in step (2) is 20 to 30 MPa.

[0010] Preferably, the smelting in step (3) is carried out under vacuum conditions, with the temperature rising from room temperature to 400-600°C at a heating rate of 15-20°C / min, and the temperature is kept for 30-90 minutes, so that the iron phosphate in the material undergoes a silicothermic reduction reaction with the reducing agent.

[0011] Preferably, the system pressure in step (3) is 1-1000 Pa.

[0012] Preferably, the condensation of the reduced volatiles in step (4) is performed by controlling the gas phase cooling temperature to 25-35°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic diagram of the process flow of the present invention. DETAILED DESCRIPTION

[0014] The present invention is further described below in conjunction with the embodiments, but the present invention is not limited in any way. Any changes or substitutions made based on the teachings of the present invention belong to the protection scope of the present invention.

[0015] Example 1: A method for recovering elemental phosphorus by vacuum silicothermal reduction of iron phosphate is as follows: The lithium iron phosphate slag (Fe: 34.74%; P: 18.84%; H 2O: 39.77%; Li: 0.12%), dried, crushed and ground to about 8 mm, mixed silicon powder and lithium iron phosphate slag according to the molar ratio of Si and FePO4 in the feed of 1.5:1, and after mixing evenly, pressed into a cylindrical material with a diameter of 30 mm and a height of 5-10 mm under a pressure of 20 MPa; put the pressed material block into a drying oven at a temperature of 110°C for drying for 4 hours; put the dried material block into a vacuum reduction furnace; turn on the vacuum pump to evacuate the system pressure to 20 Pa, and heat it from room temperature to 400°C at a heating rate of 15°C / min, keep warm for 30 minutes, and the condenser temperature is 25°C. The reduced product Fe 2 O 3 The recovery rate of P in the condenser reached 95.68%, and the recovery rate of P in the condenser reached 99.21%. After treatment by the above method, the purity of P was 99.95% (mass fraction).

[0016] Example 2: A method for recovering elemental phosphorus by vacuum silicothermal reduction of iron phosphate is as follows: The lithium iron phosphate slag (Fe: 34.50%; P: 18.92%; H 2 O: 41.66%; Li: 0.13%), dried, crushed and ground to about 7mm, mixed silicon powder and lithium iron phosphate slag according to the molar ratio of Si and FePO4 in the feed of 2:1, and after mixing evenly, pressed into a cylindrical material with a diameter of 30mm and a height of 5-10mm under a pressure of 25MPa; put the pressed material block into a drying oven at a temperature of 120℃ for drying for 5h; put the dried material block into a vacuum reduction furnace; turn on the vacuum pump to evacuate the system pressure to 10Pa, and heat it from room temperature to 500℃ at a heating rate of 15℃ / min, keep warm for 60min, and the condenser temperature is 25℃. The reduced product Fe 2 O 3 The recovery rate of P in the condenser reached 98.22%, and the recovery rate of P in the condenser reached 99.64%. After treatment by the above method, the purity of P was 99.97% (mass fraction).

Claims

1. A method for recovering elemental phosphorus by vacuum silicothermal reduction of iron phosphate, characterized in that The specific steps include: (1) mixing the reducing agent and the iron phosphate slag uniformly according to a molar ratio of Si in the reducing agent to FePO4 in the iron phosphate slag of 1.25 to 3:1; (2) Pressing the mixed material obtained in step (1) into a shape and placing it in a drying oven for drying; (3) placing the material block obtained in step (2) into a vacuum reduction furnace for reduction smelting; (4) After the reduction smelting product is cooled, reduction slag is obtained, and at the same time, the volatiles after reduction are condensed to obtain elemental phosphorus.

2. The method for recovering elemental phosphorus by vacuum silicothermal reduction of iron phosphate according to claim 1, characterized in that: The reducing agent in step (1) is industrial silicon powder and / or silicon-containing waste.

3. The method for recovering elemental phosphorus by vacuum silicothermal reduction of iron phosphate according to claim 1, characterized in that: The iron phosphate slag in step (1) is the iron phosphate slag produced after lithium is recovered from waste lithium iron phosphate batteries.

4. The method for recovering elemental phosphorus by vacuum silicothermal reduction of iron phosphate according to claim 1, characterized in that: The briquetting pressure of the mixed material in step (2) is 20 to 100 MPa.

5. The method for recovering elemental phosphorus by vacuum silicothermal reduction of iron phosphate according to claim 1, characterized in that: The drying oven temperature in step (2) is 110-130° C., the insulation time is 4-7 hours, and the moisture content of the material blocks is ≤1%.

6. The method for recovering elemental phosphorus by vacuum silicothermal reduction of iron phosphate according to claim 1, characterized in that: The smelting in step (3) is carried out under vacuum conditions, with the temperature rising from room temperature to 400-600°C at a heating rate of 15-20°C / min, and the temperature is kept for 30-90 minutes, so that the iron phosphate in the material undergoes a silicothermic reduction reaction with the reducing agent.

7. The method for recovering elemental phosphorus by vacuum silicothermal reduction of iron phosphate according to claim 1, characterized in that: The pressure of the reaction system in step (3) is 1-1000 Pa.

8. The method for recovering elemental phosphorus by vacuum silicothermal reduction of iron phosphate according to claim 1, characterized in that: The condensation of the reduced volatiles in step (4) is specifically performed by controlling the gas phase cooling temperature to 25-35°C.

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