Method for recycling waste lithium iron phosphate powder

Abstract

The invention provides a method for recovering waste lithium iron phosphate powder, and relates to the technical field of solid waste resource recovery. The method comprises the following steps: mixing waste lithium iron phosphate powder with silicon powder, and carrying out silicothermic reduction under a vacuum condition to obtain steam (lithium and phosphorus steam) and solid residues (comprising silicon dioxide and silicon iron); carrying out condensation separation on the steam to respectively obtain lithium and phosphorus; and the solid residues are subjected to magnetic attraction separation, and ferrosilicon is obtained. According to the method, the waste lithium iron phosphate powder is recovered by adopting a method of combining vacuum metallurgy silicothermic reduction with condensation separation and magnetic attraction, no harmful gas and waste liquid are generated, the method is safe and environment-friendly, comprehensive recovery of Li, P and Fe is realized, the process is simple, and the metal recovery rate is high. Results of the embodiment show that when the waste lithium iron phosphate powder is recycled through the method, the reduction rate of lithium ranges from 98.67% to 99.97%, and the reduction rate of phosphorus ranges from 99.87% to 99.91%.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solid waste resource recycling, and particularly relates to a method for recycling waste lithium iron phosphate powder. BACKGROUND

[0002] Lithium iron phosphate (LiFePO4, LFP) batteries are widely used in electric vehicles and energy storage fields due to their high safety, long cycle life and low cost. With the rapid growth of retired LFP batteries, recycling technology has become a key link for sustainable development of new energy industry chain. LFP cathode material contains lithium, phosphorus and iron, and if not recycled, it will lead to resource loss.

[0003] Currently, the recycling methods of waste lithium iron phosphate are divided into pyrometallurgy and hydrometallurgy. Pyrometallurgy is to carry out high-temperature treatment (700-1200℃) on lithium iron phosphate obtained by pretreatment such as crushing and sorting of waste lithium iron phosphate batteries, and the metal components are reduced to alloy or oxide; iron and other metals are separated by magnetic separation; and the remaining residue is extracted by leaching to extract lithium and phosphorus. This method is simple, but the lithium recovery rate is low (part of the lithium is lost by volatilization, and the lithium recovery rate is less than 50%), and harmful gases are easily produced. Hydrometallurgy is to dissolve the metals (Li, Fe, P) in the waste lithium iron phosphate cathode material with sulfuric acid, hydrochloric acid or nitric acid; and then separate lithium, iron and phosphorus by chemical precipitation, solvent extraction or ion exchange method. This method has high metal recovery rate (lithium recovery rate can reach more than 90%), but uses strong acid and alkali, produces a large amount of wastewater, and the process is complex. SUMMARY

[0004] Therefore, the present application provides a method for recycling waste lithium iron phosphate powder. The method provided by the present application can comprehensively recover lithium, phosphorus and iron from waste lithium iron phosphate powder, and has simple process, no waste gas and wastewater production, and high metal recovery rate.

[0005] In order to achieve the above-mentioned application purpose, the present application provides the following technical scheme:

[0006] The present application provides a method for recycling waste lithium iron phosphate powder, comprising the following steps:

[0007] Mixing the waste lithium iron phosphate powder with silicon powder under vacuum condition to carry out silicon thermal reduction, to obtain steam and solid residue; the vacuum degree of the vacuum condition is 1-10 Pa, and the temperature of the silicon thermal reduction is 1300-1500℃; the steam is lithium steam and phosphorus steam, and the solid residue includes silicon dioxide and silicon iron;

[0008] Condensing and separating the steam to obtain lithium and phosphorus respectively;

[0009] Magnetic separation of the solid residue to obtain silicon iron.

[0010] Preferably, the waste lithium iron phosphate powder is from a waste lithium iron phosphate battery.

[0011] Preferably, the mass content of lithium in the waste lithium iron phosphate powder is 4-5%, the mass content of phosphorus is 18-20%, and the mass content of iron is 34-36%.

[0012] Preferably, the mass ratio of the waste lithium iron phosphate powder to the silicon powder is 1:(0.5-1).

[0013] Preferably, the vacuum degree is 5 Pa.

[0014] Preferably, the temperature of the silicon thermal reduction is 1400 DEG C.

[0015] Preferably, the holding time of the silicon thermal reduction is 1-5 h.

[0016] Preferably, the condensation separation comprises sequentially performing first condensation and second condensation, the temperature of the first condensation is 300-550 DEG C, and the temperature of the second condensation is 25-100 DEG C.

[0017] Preferably, the purity of lithium is above 99.1%.

[0018] Preferably, the purity of phosphorus is above 99.4%.

[0019] The present application provides a method for recycling waste lithium iron phosphate powder, compared with the prior art, the present application has the following beneficial effects:

[0020] The present application utilizes a vacuum metallurgy method, adds elemental silicon, and performs silicon thermal reduction on the waste lithium iron phosphate powder. In the process of silicon thermal reduction, first, the structure of lithium and iron phosphate is destroyed at high temperature, so that lithium is volatilized in the form of vapor; then, the structure of iron phosphate is destroyed by silicon, the phosphorus in the iron phosphate is volatilized in the form of vapor, the oxygen in the iron phosphate forms silicon dioxide, and the iron forms ferrosilicon. The lithium and phosphorus vapor are collected through condensation separation, and the mixed residue of silicon dioxide and ferrosilicon is separated from the silicon dioxide through magnetic attraction method by utilizing the magnetism of ferrosilicon. The present application can comprehensively recycle lithium, phosphorus and iron in the waste lithium iron phosphate powder through the vacuum metallurgy silicon thermal reduction process, without generating hazardous waste such as waste gas and waste water, and has the advantages of simple process and high metal recovery rate.

[0021] The results of the examples show that, by adopting the method provided by the present application to recycle the waste lithium iron phosphate powder, the reduction rate of lithium is 98.67-99.97%, the reduction rate of phosphorus is 99.87-99.91%, the purity of lithium after condensation separation is above 99.1%, and the purity of phosphorus is above 99.4%. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1XRD pattern of waste lithium iron phosphate powder in the example;

[0023] Figure 2 XRD pattern of silicon thermal reduction residue in example 1;

[0024] Figure 3 XRD pattern of silicon thermal reduction residue in example 2;

[0025] Figure 4 XRD pattern of silicon thermal reduction residue in example 3. DETAILED DESCRIPTION

[0026] The present application provides a method for recycling waste lithium iron phosphate powder, comprising the following steps:

[0027] Mixing the waste lithium iron phosphate powder with silicon powder under vacuum condition to carry out silicon thermal reduction, to obtain steam and solid residue; the vacuum degree of the vacuum condition is 1-10 Pa, the temperature of the silicon thermal reduction is 1300-1500 ℃; the steam is lithium steam and phosphorus steam, and the solid residue comprises silicon dioxide and ferrosilicon;

[0028] Carrying out condensation separation on the steam to obtain lithium and phosphorus respectively;

[0029] Carrying out magnetic separation on the solid residue to obtain ferrosilicon.

[0030] In the present application, the raw materials involved are all commercially available goods well known in the art, unless otherwise specified.

[0031] The present application mixes the waste lithium iron phosphate powder with silicon powder under vacuum condition to carry out silicon thermal reduction, to obtain steam and solid residue; the steam is lithium steam and phosphorus steam, and the solid residue comprises silicon dioxide and ferrosilicon.

[0032] In the present application, the waste lithium iron phosphate powder is preferably from waste lithium iron phosphate battery, specifically, discharging the lithium iron phosphate battery, disassembling and separating the shell, electrode sheet, diaphragm and other components after discharging to obtain the waste lithium iron phosphate battery positive electrode material; crushing the waste lithium iron phosphate battery positive electrode material and then pyrolyzing to remove organic matter and residual carbon to obtain the waste lithium iron phosphate powder. The present application does not have special requirements on the particle size of the waste lithium iron phosphate powder. In the present application, the mass content of lithium in the waste lithium iron phosphate powder is preferably 4-5%, the mass content of phosphorus is preferably 18-20%, and the mass content of iron is preferably 34-36%; in the example of the present application, the mass content of lithium in the waste lithium iron phosphate powder is 4.07%, the mass content of phosphorus is 18.73%, and the mass content of iron is 35.32%.

[0033] In the present application, the mass ratio of the waste lithium iron phosphate powder to the silicon powder is preferably 1:(0.5-1), and can be 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9 or 1:1. In the present application, the mixing method is preferably mixing the lithium iron phosphate powder and the silicon powder in a mixer until they are uniformly mixed. In the present application, increasing the amount of the silicon powder added is beneficial to improving the reduction rate of lithium and phosphorus, but too much silicon powder added will result in a surplus of silicon.

[0034] In the present application, the vacuum degree of the vacuum condition is 1-10 Pa, and can be 1 Pa, 5 Pa or 10 Pa; the temperature of the silicon thermal reduction is 1300-1500°C, and can be 1300°C, 1400°C or 1500°C; and the holding time of the silicon thermal reduction is preferably 1-5 h, and can be 1 h, 2 h, 3 h, 4 h or 5 h. In the present application, low vacuum and high temperature are beneficial to destroying the structure of lithium iron phosphate, but economic benefits also need to be considered. In the present application, the silicon thermal reduction is carried out in a vacuum furnace; preferably, the mixed powder obtained by mixing the waste lithium iron phosphate powder and the silicon powder is placed in a graphite crucible, the graphite crucible is transferred into the vacuum furnace, and the silicon thermal reduction is carried out under the vacuum degree and the temperature condition. In the process of the silicon thermal reduction, first, the structure of lithium and iron phosphate is destroyed by high temperature, so that lithium is volatilized in the form of vapor and collected; then, the structure of iron phosphate is destroyed by silicon, so that phosphorus in the iron phosphate is volatilized in the form of vapor and collected, oxygen in the iron phosphate forms silicon dioxide, and iron forms ferrosilicon; and after the silicon thermal reduction is completed, the furnace is preferably cooled to room temperature. That is, after the silicon thermal reduction, vapor (lithium vapor and phosphorus vapor) and solid residues including silicon dioxide and ferrosilicon are obtained, and the solid residues are specifically residual in the graphite crucible. In the present application, the ferrosilicon is at least one of FeSi and FeSi2; and when the silicon powder is excessive, the solid residues further include silicon. In the present application, the solid residues are also referred to as silicon thermal reduction residues.

[0035] After the vapor is obtained, the vapor is subjected to condensation separation to obtain lithium and phosphorus respectively. In the present application, the condensation separation preferably includes sequentially performing first condensation and second condensation, the temperature of the first condensation is preferably 300-550°C, and can be 300°C, 400°C, 450°C, 500°C or 550°C, and the temperature of the second condensation is preferably 25-100°C, and can be 25°C, 50°C, 70°C, 75°C, 80°C, 90°C or 100°C. In the present application, the condensation separation is specifically carried out in different condensation zones of the vacuum furnace. Lithium and phosphorus vapors are collected by different condensation temperatures in stages, and lithium and phosphorus are obtained respectively, the purity of the lithium is above 99.1%, and the purity of the phosphorus is above 99.4%.

[0036] After obtaining the solid residue, the present invention performs magnetic separation on the solid residue to obtain ferrosilicon. In the present invention, the magnetic separation is specifically performed using a magnet. The present invention utilizes the magnetic properties of ferrosilicon to separate ferrosilicon from silicon dioxide, respectively obtaining ferrosilicon and silicon dioxide. When the solid residue also includes silicon, the obtained silicon dioxide also includes silicon.

[0037] The lithium iron phosphate (LiFe(PO4)) phase in waste lithium iron phosphate powder has an olivine structure, which is stable. The LiFe(PO4) crystal structure is dense and chemically inert. In addition, the lithium content is low (only about 1 / 3 of the lithium content in ternary batteries (NCM / NCA), making economical recovery difficult. The present invention recycles waste lithium iron phosphate powder using a vacuum metallurgical silicon thermal reduction method combined with condensation separation and magnetic attraction. This method produces no harmful gases, is safe and environmentally friendly, does not use liquids such as acids and alkalis, and produces no waste liquid. It also synergistically achieves efficient recovery of Li, P, and Fe, collecting them separately: Li is collected as metallic lithium, P is collected as elemental phosphorus, and Fe is collected as ferrosilicon, achieving the purpose of comprehensive recovery.

[0038] In order to further illustrate the present invention, the method for recycling waste lithium iron phosphate powder provided by the present invention is described in detail below with reference to examples, but they should not be construed as limiting the scope of protection of the present invention.

[0039] In each embodiment, the waste lithium iron phosphate powder used is a powder obtained by crushing and thermally removing organic matter and residual carbon from waste lithium iron phosphate battery positive electrode materials, and its phase composition is LiFe(PO4), of which the Li mass content is 4.07%, the P mass content is 18.73%, and the Fe mass content is 35.32%. Figure 1 This is the XRD pattern of waste lithium iron phosphate powder.

[0040] Example 1

[0041] Waste lithium iron phosphate powder and silicon powder are evenly mixed in a mixer at a mass ratio of 1:0.6 (i.e., LiFe(PO4) / Si=1:0.6), and placed in a graphite crucible. The graphite crucible is placed in a vacuum furnace, and the vacuum pump is turned on to reduce the vacuum degree to 1 Pa. The temperature is raised to 1300°C and kept warm for 5 hours for silicon thermal reduction. After keeping warm, the furnace is cooled to room temperature; the vapor obtained from the silicon thermal reduction is collected in different condensation zones of the vacuum furnace at different condensation temperatures, with the condensation temperature of the lithium collection section being 500°C and the condensation temperature of the phosphorus collection section being 80°C; the remaining ferrosilicon and silicon dioxide (i.e., silicon thermal reduction residues) in the crucible are poured out, and the silicon dioxide and ferrosilicon are separated by a magnet.

[0042] Figure 2For the XRD pattern of the silicon thermal reduction residue in Example 1, the main phases in the silicon thermal reduction residue are SiO2, FeSi and FeSi2, indicating that LiFe(PO4) has been completely decomposed at this time, and the reduction rate of Li is 98.67% and the reduction rate of P is 99.87%. Li and P are volatilized in the form of vapor and condensed, and the purity of the Li condensation zone is 99.64% and the purity of the P condensation zone is 99.97%. The silicon dioxide and ferrosilicon are separated by a magnet, and the silicon dioxide and ferrosilicon can be separated, and the separation rate of ferrosilicon is more than 99%.

[0043] Example 2

[0044] The waste lithium iron phosphate powder and silicon powder are mixed in a mass ratio of 1:0.8 (i.e. LiFe(PO4) / Si = 1:0.8) in a mixer, placed in a graphite crucible, and placed in a vacuum furnace. The vacuum pump is turned on to a vacuum degree of 5 Pa, the temperature is raised to 1400°C, and the silicon thermal reduction is carried out for 3 h. After holding, the furnace is cooled to room temperature. The vapor obtained by silicon thermal reduction is collected in different condensation zones of the vacuum furnace by different condensation temperatures, the condensation temperature of the lithium collection section is 450°C, and the condensation temperature of the phosphorus collection section is 70°C. The silicon thermal reduction residue in the crucible is poured out, and the ferrosilicon therein is separated out by a magnet.

[0045] Figure 3 For the XRD pattern of the silicon thermal reduction residue in Example 2, the main phases in the silicon thermal reduction residue are SiO2, Si and FeSi2, indicating that LiFe(PO4) has been completely decomposed at this time, but the silicon is excessive, and elemental silicon remains. At this time, the reduction rate of Li is 99.97% and the reduction rate of P is 99.91%. Li and P are volatilized in the form of vapor and condensed, and the purity of the Li condensation zone is 99.13% and the purity of the P condensation zone is 99.47%. The ferrosilicon, silicon dioxide and silicon are separated by a magnet, and the separation rate of ferrosilicon is more than 99%.

[0046] Example 3

[0047] The waste lithium iron phosphate powder and silicon powder are mixed in a mass ratio of 1:1 (i.e. LiFe(PO4) / Si = 1:1) in a mixer, placed in a graphite crucible, and placed in a vacuum furnace. The vacuum pump is turned on to a vacuum degree of 10 Pa, the temperature is raised to 1500°C, and the silicon thermal reduction is carried out for 1 h. After holding, the furnace is cooled to room temperature. The vapor obtained by silicon thermal reduction is collected in different condensation zones of the vacuum furnace by different condensation temperatures, the condensation temperature of the lithium collection section is 550°C, and the condensation temperature of the phosphorus collection section is 90°C. The silicon thermal reduction residue in the crucible is poured out, and the ferrosilicon therein is separated out by a magnet.

[0048] Figure 4For the XRD pattern of the silicon thermal reduction residue in Example 3, the main phases in the silicon thermal reduction residue are SiO2, Si and FeSi2, which indicates that LiFe(PO4) has been completely decomposed at this time, but there is an excess of silicon, and elemental silicon appears, and the elemental silicon peak is increased. At this time, the reduction rate of Li is 99.94%, and the reduction rate of P is 99.90%. Li and P are volatilized in the form of vapor and condensed, and the purity of the Li condensation zone is 99.47%, and the purity of the P condensation zone is 99.54%. The silicon iron is separated from the silicon dioxide and silicon by a magnet, and the silicon iron separation rate is more than 99%.

[0049] The above only describes the preferred embodiments of the present application, and does not limit the present application in any form. It should be noted that for ordinary skilled persons in the art, several improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should be considered as the protection scope of the present application.

Claims

1. A method for recycling waste lithium iron phosphate powder, characterized in that: The following steps are involved: Mixing waste lithium iron phosphate powder and silicon powder and performing silicothermic reduction under vacuum conditions to obtain vapor and solid residue; the vacuum degree of the vacuum condition is 1-10 Pa, the temperature of the silicothermic reduction is 1300-1500° C.; the vapor is lithium vapor and phosphorus vapor, and the solid residue includes silicon dioxide and ferrosilicon; condensing and separating the steam to obtain lithium and phosphorus respectively; The solid residue is subjected to magnetic separation to obtain ferrosilicon.

2. The method according to claim 1, characterized in that The waste lithium iron phosphate powder comes from waste lithium iron phosphate batteries.

3. The method according to claim 1 or 2, characterized in that The waste lithium iron phosphate powder has a lithium content of 4-5% by mass, a phosphorus content of 18-20% by mass, and an iron content of 34-36% by mass.

4. The method according to claim 1, wherein The mass ratio of the waste lithium iron phosphate powder to the silicon powder is 1:(0.5-1).

5. The method according to claim 1, wherein The vacuum degree is 5Pa.

6. The method according to claim 1, characterized in that The temperature of the silicon thermal reduction is 1400°C.

7. The method according to claim 1, 5 or 6, characterized in that The heat preservation and pressure preservation time of the silicon thermal reduction is 1 to 5 hours.

8. The method according to claim 1, characterized in that The condensation separation comprises performing a first condensation and a second condensation in sequence, wherein the temperature of the first condensation is 300-550°C, and the temperature of the second condensation is 25-100°C.

9. The method according to claim 1, characterized in that The purity of the lithium is above 99.1%.

10. The method according to claim 1, characterized in that The purity of the phosphorus is above 99.4%.

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