A method for recycling black powder from phosphorus-iron batteries

Abstract

This invention provides a method for recycling black powder from phosphorus-iron batteries, relating to the field of battery recycling technology. The invention uses acid leaching to effectively dissolve the five elements—copper, aluminum, phosphorus, lithium, and iron—in the black powder. After removing copper and aluminum, ferrous oxalate precipitate is formed and recovered. Lithium phosphate salt precipitate is formed and converted for recovery of lithium dihydrogen phosphate. This method effectively converts phosphorus-iron battery black powder into ferrous oxalate and lithium dihydrogen phosphate, achieving high-value recycling of the black powder.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a method for recycling black powder from phosphorus-iron batteries. Background Technology

[0002] In the recycling of waste lithium iron phosphate (LFP) and lithium manganese iron phosphate (LFP) batteries, after removing non-metallic components such as separators, binders, conductive agents, and negative electrodes, the resulting metallic components, including aluminum foil, copper foil, and positive electrode materials, are commonly known as "phosphorus iron battery black powder." The recycling and processing of phosphorus iron battery black powder has become a key task and a research hotspot in the new energy battery industry. Different recycling processes yield different recycled products, such as lithium carbonate, iron phosphate, and lithium iron phosphate. The production processes and raw materials for high-compaction lithium iron phosphate and lithium manganese iron phosphate positive electrode materials are constantly being iterated and optimized. Processes using lithium dihydrogen phosphate and ferrous oxalate as raw materials are more efficient, economical, and environmentally friendly, and have gradually become mainstream, offering higher compaction density, better rate performance, and longer service life. Furthermore, using lithium dihydrogen phosphate instead of monoammonium phosphate is more environmentally friendly, as it does not generate ammonia or ammonium sulfate mother liquor during positive electrode material production, eliminating the need for high-concentration ammonia and ammonium sulfate recovery.

[0003] Therefore, it is necessary to develop a process for recovering lithium dihydrogen phosphate and ferrous oxalate from iron phosphate battery black powder. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for recovering black powder from phosphorus-iron batteries.

[0005] The technical solution of the present invention is as follows:

[0006] A method for recovering black powder from phosphorus-iron batteries includes the following steps:

[0007] S1. Phosphorus iron battery black powder is leached with acid to obtain leachate and leaching residue; the leachate is purified after removing copper and aluminum.

[0008] The acid is selected from sulfuric acid or a mixed acid, wherein the mixed acid is composed of sulfuric acid and phosphoric acid in a weight ratio of 1:0.2-10;

[0009] S2. A precipitant is added to the purified liquid in step S1 to precipitate ferrous oxalate precipitate and a first precipitate solution.

[0010] The ferrous oxalate precipitate was obtained by washing and drying.

[0011] S3. In step S2, the pH of the first precipitate is adjusted to 6-7. After removing impurities, at least one of sodium hydroxide and lithium hydroxide is added to adjust the pH to 7.5-12.6 to obtain lithium phosphate precipitate and second precipitate.

[0012] The lithium phosphate salt precipitate is dissolved in refined phosphoric acid to obtain a lithium dihydrogen phosphate solution. The lithium dihydrogen phosphate solution is then concentrated, crystallized, and dried to obtain lithium dihydrogen phosphate.

[0013] Preferably, in the leaching operation described in step S1, a reducing agent is also added to reduce Fe. 3+ Reduced to Fe 2+ ;

[0014] The reducing agent is selected from sulfites and / or sulfur dioxide gas.

[0015] Preferably, the copper removal in step S1 involves adding iron powder to replace copper ions with elemental copper and then filtering.

[0016] The aluminum removal process involves adding alkali to adjust the pH to 2.9-4.5, converting aluminum ions into aluminum hydroxide precipitate, and then filtering.

[0017] Preferably, the precipitant in step S2 is selected from a combination of oxalic acid and sodium oxalate in a molar ratio of 1:1-30;

[0018] The Fe / C molar ratio in the purified liquid and the precipitant is 1.01-1.1:2.

[0019] Preferably, in step S2, the pH of the purified solution after adding the precipitant is 1-2.5, and the temperature of the purified solution is 40-80℃.

[0020] Preferably, after adjusting the pH of the first precipitate to 6-7 in step S3, the molar ratio of Li / P in the first precipitate is 2.02-3:1.

[0021] Preferably, the impurity removal in step S3 is one or a combination of two or more of the following: fluorine removal, oil removal, iron removal, and heavy metal removal.

[0022] The defluorination process involves adding a defluorinating agent to generate a precipitate, which is then removed.

[0023] The degreasing process involves adding hydrogen peroxide for oxidation and degreasing.

[0024] The iron and heavy metal removal processes include dual-alkali precipitation and resin adsorption.

[0025] Preferably, in step S3, lime is added to the second precipitate to obtain a crude sodium hydroxide solution and precipitate residue;

[0026] Concentrated sulfuric acid was added to the precipitate residue, and the Ca / S molar ratio was controlled to be 0.95-1.05:1 to obtain crude phosphoric acid and phosphogypsum.

[0027] The crude phosphoric acid is purified to obtain refined phosphoric acid, which is used to dissolve the lithium hydrogen phosphate precipitate.

[0028] More preferably, the phosphogypsum is calcined to obtain lime and tail gas, and the tail gas is used to prepare sulfuric acid.

[0029] Preferably, the Li / P molar ratio in the lithium dihydrogen phosphate solution in step S3 is 0.99-1.01:1.

[0030] The beneficial effects of this invention are:

[0031] (1) The present invention uses a relatively simple recycling process to recover high-quality ferrous oxalate and lithium hydrogen phosphate from iron phosphate battery black powder, which can be reused as raw materials to prepare positive electrode materials such as lithium iron phosphate and / or lithium manganese iron phosphate, thereby realizing the high-value recycling of iron phosphate battery black powder.

[0032] (2) Using a combination of oxalic acid and oxalate as a precipitant can effectively precipitate iron and recover ferrous oxalate. It carries less Li and P, which is beneficial to obtain high-quality ferrous oxalate and achieve effective separation from Li and P.

[0033] (3) The present invention has a high recovery rate of Li, P and iron in phosphorus-iron battery black powder. Adding a reducing agent during black powder leaching can further improve the recovery rate. Detailed Implementation

[0034] The technical solution of the present invention will be further explained and described below through specific embodiments.

[0035] This invention proposes a method for recovering black powder from phosphorus-iron batteries, comprising the following steps:

[0036] S1. Phosphorus iron battery black powder is leached with acid to obtain leachate and leaching residue; after removing copper and aluminum from the leachate, a purified solution is obtained.

[0037] The acid is selected from sulfuric acid or a mixed acid, wherein the mixed acid is composed of sulfuric acid and phosphoric acid in a weight ratio of 1:0.2-10;

[0038] S2. In step S1, a precipitant is added to the purified liquid to precipitate ferrous oxalate precipitate and the first precipitate.

[0039] Ferrous oxalate precipitate was obtained by washing and drying.

[0040] S3. In step S2, the pH of the first precipitate is adjusted to 6-7. After removing impurities, at least one of sodium hydroxide and lithium hydroxide is added to adjust the pH to 7.5-12.6 to obtain lithium phosphate precipitate and second precipitate.

[0041] Lithium phosphate salt precipitate is dissolved in refined phosphoric acid to obtain lithium dihydrogen phosphate solution. The lithium dihydrogen phosphate solution is then concentrated, crystallized, and dried to obtain lithium dihydrogen phosphate.

[0042] The black powder from phosphorus-iron batteries mainly contains five elements: copper, aluminum, phosphorus, lithium, and iron. It may also contain organic colloids, separators, etc. Sulfuric acid or mixed acids can dissolve all five elements in the solution, while organic colloids and separators cannot be dissolved. Solid-liquid separation removes the organic matter. To improve leaching efficiency and reduce the adsorption of these five elements by the leaching residue, a multi-stage countercurrent leaching method can be used, which is well-known to those skilled in the art. For mixed acids and sulfuric acid, there are no particular limitations on the concentration, which can be 5-20 wt%, and the solid-liquid ratio during leaching can be 1:3-8. Taking mixed acid as an example, the leachate after leaching contains inorganic salts such as copper sulfate, lithium sulfate, ferrous sulfate, lithium dihydrogen phosphate, ferrous dihydrogen phosphate, aluminum sulfate, and ferric sulfate. After removing copper and aluminum, the leachate still contains elements such as P, Li, and Fe awaiting recovery. For Fe, it is converted into ferrous oxalate for precipitation and recovery; for P and Li, it is converted into lithium dihydrogen phosphate and recovered. This enables the effective recovery of iron oxalate and lithium dihydrogen phosphate from iron phosphate battery black powder, with recovery rates of Li, Fe, and P all exceeding 90%.

[0043] In some embodiments, a reducing agent is also added during the leaching process in step S1 to reduce Fe. 3+ Reduced to Fe 2+ ;

[0044] The reducing agent is selected from sulfites and / or sulfur dioxide gas. Further, the sulfite can be sodium sulfite, lithium sulfite, etc.

[0045] Typically, 10-30 mol% of the iron in black powder is ferric iron (Fe3+). When treating black powder with mixed acids, the ferric iron forms insoluble ferric phosphate, resulting in some iron being unrecoverable. Adding a reducing agent during leaching allows the iron to be completely converted into more soluble ferrous phosphate and / or ferrous sulfate, essentially dissolving all the iron in the leachate and improving the iron recovery rate. Furthermore, ferric oxalate is even less soluble than ferrous oxalate; if a small amount of iron ions are present in the leachate, ferric oxalate will form, affecting the purity of the ferrous oxalate.

[0046] In some embodiments, copper removal in step S1 involves adding iron powder to replace copper ions with elemental copper and then filtering.

[0047] To remove aluminum, alkali is added to adjust the pH to 2.9-4.5, converting aluminum ions into aluminum hydroxide precipitate, which is then filtered.

[0048] Adding excess iron powder during copper removal can completely displace and recover the copper. The molar ratio of added iron powder to copper ions in the leachate can be 1.05-1.1:1, with a slight excess of iron powder to facilitate the complete displacement and precipitation of copper ions. The pH of the leachate for copper removal can be 1.5-2.5.

[0049] For aluminum ions, the pH can be adjusted to convert them into aluminum hydroxide and cause them to precipitate. For the added alkali, NaOH and / or LiOH can be used to avoid introducing new impurities.

[0050] In some embodiments, the precipitant in step S2 is selected from a combination of oxalic acid and sodium oxalate in a molar ratio of 1:1-30;

[0051] The molar ratio of Fe / C in the purification solution and the precipitant is 1.01-1.1:2.

[0052] The combination of oxalic acid and sodium oxalate not only introduces oxalate ions to precipitate ferrous ions to form ferrous oxalate, but also allows for adjustment of the pH of the purification solution system within a suitable range by adjusting the amount of oxalic acid, avoiding excessively low or high pH. For the Fe / C molar ratio in the purification solution and precipitant, a slight excess of Fe allows for the recovery of as much Fe as possible while ensuring the purity of ferrous oxalate. For example, the molar ratio of oxalic acid to sodium oxalate can be any value or any value between 1:1, 1:3, 1:5, 1:8, 1:10, 1:12, 1:15, 1:18, 1:20, 1:22, 1:25, 1:27, 1:30, etc., without any particular restriction; the molar ratio of Fe / C in the purification solution and the precipitant can be any value or any value between 1.01:2, 1.02:2, 1.03:2, 1.04:2, 1.05:2, 1.06:2, 1.07:2, 1.08:2, 1.09:2, 1.1:2, etc., without any particular restriction. If the oxalate ions in the precipitant are in excess relative to the Fe ions in the purification solution, for example, if the Fe / C molar ratio is 1:2.1, the excess oxalate ions will precipitate with other metal ions (such as calcium and magnesium ions) to form other precipitates or leave residual oxalate ions.

[0053] In some embodiments, after adding the precipitant in step S2, the pH of the purification solution is 1-2.5, and the temperature of the purification solution is 40-80°C. By adjusting the molar ratio of oxalic acid and oxalate and the temperature of the purification solution, the pH at which ferrous oxalate precipitates in the purification solution can be controlled at 1-2.5, avoiding the formation of other oxalate precipitates. This ensures that ferrous ions precipitate out as much as possible while obtaining ferrous oxalate with high purity. Simultaneously, maintaining a pH of 1-2.5 after adding the precipitant in the purification solution also prevents the formation of other oxalate precipitates, such as calcium oxalate and magnesium oxalate, which would otherwise reduce the purity of ferrous oxalate.

[0054] In some embodiments, after adjusting the pH of the first precipitate to 6-7 in step S3, the Li / P molar ratio in the first precipitate is 2.02-3:1. The main components of the first precipitate include lithium sulfate, sodium sulfate, lithium dihydrogen phosphate, sodium dihydrogen phosphate, etc., and may also contain small amounts of impurities such as F, heavy metal ions, iron ions, and organic components. The pH can be adjusted to neutral or near-neutral by adding LiOH, sodium phosphate, lithium phosphate, etc., based on the pH and Li / P molar ratio requirements, which is beneficial for impurity removal. For example, the Li / P molar ratio can be any value or any value between 2.02:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1, 3:1, etc. Within the above range, the Li / P molar ratio can be a mixture of lithium dihydrogen phosphate and lithium phosphate or lithium phosphate alone. For example, when the Li / P molar ratio is 2.02:1, it is a mixture of lithium dihydrogen phosphate and lithium phosphate; when the Li / P molar ratio is 2.9:1, it is a mixture of lithium dihydrogen phosphate and lithium phosphate; and when the Li / P molar ratio is 3:1, it is lithium phosphate.

[0055] When the first precipitate contains small amounts of impurities such as F, Fe, heavy metal ions, calcium ions, magnesium ions, and oil, further impurity removal is required. In some embodiments, impurity removal in step S3 is one or a combination of two or more of the following: fluoride removal, oil removal, iron removal, and heavy metal removal.

[0056] Defluorination involves adding a defluorinating agent to generate a precipitate and remove it; the defluorinating agent can be an adsorption-type defluorinating agent, such as activated alumina, activated carbon, etc.

[0057] The degreasing process involves adding hydrogen peroxide for oxidation; adding hydrogen peroxide utilizes the residual ferrous ions in the first precipitate to generate Fenton's reagent, which oxidizes and removes oily substances, followed by adsorption and degreasing using activated carbon and / or ceramic membranes.

[0058] Iron and heavy metal removal includes double alkali precipitation and resin adsorption, which can remove about 90% of iron and heavy metals and 70-85% of calcium and magnesium, so that the sum of heavy metal ions and calcium and magnesium content in the first precipitation solution is ≤40ppm.

[0059] After removing impurities from the first precipitate and adjusting the pH to 7.5-12.6, lithium phosphate precipitate and a second precipitate are obtained, thereby achieving the purpose of recovering dilithium hydrogen phosphate. The second precipitate contains sodium sulfate, sodium phosphate, disodium hydrogen phosphate, etc. In some embodiments, lime is added to the second precipitate in step S3 to obtain a crude sodium hydroxide solution and precipitate residue; lime reacts with sodium sulfate, sodium phosphate, disodium hydrogen phosphate, etc. to generate calcium sulfate and calcium phosphate precipitate (precipitate residue), and sodium hydroxide is generated;

[0060] Concentrated sulfuric acid was added to the precipitate residue, and the Ca / S molar ratio was controlled at 0.95-1.05:1 to obtain crude phosphoric acid and phosphogypsum.

[0061] Crude phosphoric acid is purified to obtain refined phosphoric acid, which is used to dissolve lithium hydrogen phosphate precipitate.

[0062] For example, the Ca / S molar ratio can be 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1, 1:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.05:1, etc. Furthermore, the Ca / S molar ratio can be between 0.98 and 1.02:1.

[0063] Phosphogypsum contains calcium sulfate. In some embodiments, calcination of phosphogypsum yields lime and tail gas, which is used to prepare sulfuric acid. The sulfuric acid can be used directly or as a mixed acid in step S1 for the leaching of black powder.

[0064] Typically, lithium phosphate precipitates consist mainly of lithium dihydrogen phosphate and lithium phosphate. When refined phosphoric acid is added to the lithium phosphate precipitate and the Li / P molar ratio is controlled to be close to 1:1, both lithium dihydrogen phosphate and lithium phosphate can be converted to lithium dihydrogen phosphate, as shown in equations (1) and (2) below.

[0065] Li2HPO4+H3PO4 2LiH2PO4 (1)

[0066] Li3PO4 + 2H3PO4 3LiH2PO4(2)

[0067] If the lithium phosphate precipitate contains trace amounts or small amounts of insoluble impurities such as iron phosphate, calcium phosphate, and / or sodium and potassium ions, the following methods can be used to remove them: For insoluble impurities such as iron phosphate and calcium phosphate, they can be removed by gradually adding refined phosphoric acid. For example, first add 30-50% by weight of refined phosphoric acid to dissolve the lithium phosphate precipitate. The lithium phosphate and dilithium hydrogen phosphate in the lithium phosphate precipitate will dissolve, and the insoluble impurities such as iron phosphate and calcium phosphate will be removed by filtration. Then add the remaining refined phosphoric acid to the filtrate and adjust the Li / P molar ratio. For sodium and potassium ions, the lithium dihydrogen phosphate solution can be further removed by passing it through an ion exchange resin.

[0068] In some embodiments, the Li / P molar ratio in the lithium dihydrogen phosphate solution in step S3 is 0.99-1.01:1. For example, the Li / P molar ratio can be any value or any value between 0.99:1, 0.992:1, 0.995:1, 0.997:1, 1:1, 1.002:1, 1.005:1, 1.007:1, 1.008:1, and 1.01:1, without particular limitation. Further, the Li / P molar ratio in the lithium dihydrogen phosphate solution can be 0.992-1.003:1. The closer the Li / P molar ratio in the lithium dihydrogen phosphate solution is to 1:1, the higher the purity of the obtained lithium dihydrogen phosphate.

[0069] For the crystallization of lithium dihydrogen phosphate solution, the concentration temperature before crystallization can be 80-103℃, and the solid content can be concentrated to 10-30wt%. The negative pressure evaporation crystallization method can be used, and the absolute pressure can be 6-45kPa. For drying, centrifugal drying can be used.

[0070] The technical solution of the present invention will be further described and illustrated below with reference to various embodiments. Unless otherwise specified, the parts mentioned in the following embodiments are parts by weight.

[0071] Example 1

[0072] The recycling process for phosphorus-iron battery black powder in this embodiment includes the following steps:

[0073] Phosphorus iron battery black powder is added to sulfuric acid with a concentration of 20wt% at a liquid-to-solid ratio of 3:1 and leached in two countercurrent stages with SO2 gas continuously introduced during leaching. The final leaching stage is filtered by pressure to obtain leachate and leach residue. The leach residue is then washed in two stages and combined with the leachate.

[0074] The above leachate was treated with iron powder to remove copper (pH 1.5), and then sodium hydroxide was added to adjust the pH to 3.5. The precipitate was removed and aluminum was removed by filtration to obtain a purified solution. The molar ratio of copper ions in the iron powder and the leachate was 1.06:1.

[0075] The purified solution was heated to 50°C, and a precipitant consisting of oxalic acid and sodium oxalate in a molar ratio of 1:1.4 was added. The molar ratio of Fe in the purified solution to C in the precipitant was adjusted to 1.01:2. Solid-liquid separation was performed to obtain ferrous oxalate precipitate and a first precipitate solution with a pH of 1.2. The ferrous oxalate precipitate was washed three times with water and dried overnight at 60°C to obtain ferrous oxalate with a purity of 99.2%.

[0076] The first precipitate solution was adjusted to pH 6.5 with sodium phosphate. It then underwent further purification processes, including defluorination with activated alumina, oil removal with hydrogen peroxide and activated carbon adsorption, precipitation using a double-alkali method, and iron, heavy metal, and calcium and magnesium ion removal via resin adsorption. The combined content of heavy metal ions and calcium and magnesium ions in the purified solution was 32 ppm. Lithium hydroxide and sodium hydroxide were then added to the purified solution to adjust the pH to 7.5, resulting in a Li / P molar ratio of 2.02:1, yielding a lithium phosphate precipitate and a second precipitate solution.

[0077] Lithium phosphate precipitate was dissolved in refined phosphoric acid, and the Li / P molar ratio was controlled at 0.992:1 to obtain a lithium dihydrogen phosphate solution. The lithium dihydrogen phosphate solution was concentrated at 80°C to a solid content of 30wt%, evaporated and crystallized under negative pressure at an absolute pressure of 6kPa, and then dried by centrifugation to obtain battery-grade lithium dihydrogen phosphate.

[0078] Lime was added to the second precipitate to obtain a crude sodium hydroxide solution and a precipitate residue. Concentrated sulfuric acid was added to the precipitate residue, and the Ca / S molar ratio was controlled at 0.98:1 to obtain crude phosphoric acid and phosphogypsum. The crude phosphoric acid was purified to obtain refined phosphoric acid, which was used to dissolve the above-mentioned crude lithium hydrogen phosphate precipitate. The phosphogypsum was calcined with lime, and the tail gas was collected to produce sulfuric acid with a concentration of 20 wt%, which was used to leach phosphorus iron battery black powder.

[0079] In this embodiment, the processing capacity of phosphorus-iron battery black powder is 1250 kg / h. The phosphorus-iron battery black powder contains 2.51% lithium, 10.59% phosphorus, and 19.43% iron by mass. The recovery rate of battery-grade lithium dihydrogen phosphate is 433.16 kg / h, with a lithium recovery rate of 92.5% and a phosphorus recovery rate of 91.3% (with an external sodium phosphate supplement of 44.08 kg / h). The recovery rate of battery-grade ferrous oxalate is 573.9 kg / h, with an iron recovery rate of 91.16%.

[0080] Comparative Example 1

[0081] The difference between this comparative example and Example 1 is that the molar ratio of Fe in the purified solution to C in the precipitant was adjusted from 1.01:2 to 0.95:2. All other steps remained unchanged. The purity of the obtained ferrous oxalate was 97.4%.

[0082] Comparative Example 2

[0083] The difference between this comparative example and Example 1 is that the precipitant was adjusted to an equal weight of sodium oxalate, and the pH of the first precipitate was 3.7. The remaining steps remained unchanged. The purity of the obtained ferrous oxalate was 98.1%.

[0084] Example 2

[0085] The recycling process for phosphorus-iron battery black powder in this embodiment includes the following steps:

[0086] The black powder of phosphorus iron battery is added to a mixed acid with a concentration of 5 wt% consisting of sulfuric acid and phosphoric acid in a weight ratio of 1:1, with a liquid-to-solid ratio of 8:1. The process involves two-stage countercurrent leaching, during which SO2 gas is continuously introduced. The final leaching stage is filtered by pressure to obtain leachate and leach residue. The leach residue is then washed with water in two stages and combined with the leachate.

[0087] The above leachate was treated with iron powder to remove copper (pH 1.5), and then sodium hydroxide was added to adjust the pH to 4.0. The precipitate was removed and aluminum was removed by filtration to obtain a purified solution. The molar ratio of copper ions in the iron powder and the leachate was 1.05:1.

[0088] The purified solution was heated to 50°C, and a precipitant consisting of oxalic acid and sodium oxalate in a molar ratio of 1:10 was added. The molar ratio of Fe in the purified solution to C in the precipitant was adjusted to 1.1:2. Solid-liquid separation was performed to obtain ferrous oxalate precipitate and a first precipitate solution with a pH of 2.0. The ferrous oxalate precipitate was washed three times with water and dried overnight at 60°C to obtain ferrous oxalate with a purity of 99.1%.

[0089] The first precipitate solution was adjusted to pH 6.8 with sodium phosphate. It then underwent further purification processes, including defluorination with activated alumina, oil removal with hydrogen peroxide and activated carbon adsorption, precipitation using a double-alkali method, and iron, heavy metal, and calcium and magnesium ion removal via resin adsorption. The combined content of heavy metal ions and calcium and magnesium ions in the purified solution was 30 ppm. Lithium hydroxide was then added to the purified solution to adjust the pH to 12.6, resulting in a Li / P molar ratio of 2.95:1, yielding a lithium phosphate precipitate and a second precipitate solution.

[0090] Lithium phosphate precipitate was dissolved in refined phosphoric acid, controlling the Li / P molar ratio at 1.002:1 to obtain a lithium dihydrogen phosphate solution. When adding refined phosphoric acid, 40% by weight of the phosphoric acid was first added to dissolve the lithium phosphate precipitate, and the insoluble matter was removed by filtration. The remaining refined phosphoric acid was then added to form the lithium dihydrogen phosphate solution. The lithium dihydrogen phosphate solution was concentrated at 80°C to a solid content of 30 wt%, evaporated under negative pressure at 6 kPa, and then centrifuged and dried to obtain battery-grade lithium dihydrogen phosphate.

[0091] Lime was added to the second precipitate to obtain a crude sodium hydroxide solution and a precipitate residue. Concentrated sulfuric acid was added to the precipitate residue, and the Ca / S molar ratio was controlled at 0.99:1 to obtain crude phosphoric acid and phosphogypsum. The crude phosphoric acid was purified to obtain refined phosphoric acid, which was used to dissolve the above crude lithium hydrogen phosphate precipitate. The phosphogypsum was calcined with lime, and the tail gas was collected to produce sulfuric acid with a concentration of 20 wt%, which was used to leach phosphorus iron battery black powder.

[0092] In this embodiment, the processing capacity of phosphorus-iron battery black powder is 800 kg / h. The phosphorus-iron battery black powder contains 2.47% lithium, 10.86% phosphorus, and 19.23% iron by mass. The recovery rate of battery-grade lithium dihydrogen phosphate is 280.22 kg / h, with a lithium recovery rate of 93.3% and a phosphorus recovery rate of 92.7% (with an additional 85% phosphoric acid content of 13.56 kg / h). The recovery rate of battery-grade ferrous oxalate is 366 kg / h, with an iron recovery rate of 91.78%.

[0093] Example 3

[0094] The recycling process for phosphorus-iron battery black powder in this embodiment includes the following steps:

[0095] The black powder of phosphorus iron battery is added to a mixed acid with a concentration of 7.5 wt% consisting of sulfuric acid and phosphoric acid in a weight ratio of 20:1, with a liquid-to-solid ratio of 6:1. The mixture is leached in two countercurrent stages, with sodium sulfite added during leaching. The final stage of leaching is filtered by pressure to obtain leachate and leach residue. The leach residue is then washed in two stages and combined with the leachate.

[0096] The above leachate was treated with iron powder to remove copper (pH 2.0), and then sodium hydroxide was added to adjust the pH to 4.2. The precipitate was removed and aluminum was removed by filtration to obtain a purified solution. The molar ratio of copper ions in the iron powder and the leachate was 1.03:1.

[0097] The purified solution was heated to 40°C, and a precipitant consisting of oxalic acid and sodium oxalate in a molar ratio of 1:8 was added. The molar ratio of Fe in the purified solution to C in the precipitant was adjusted to 1.06:2. Solid-liquid separation was performed to obtain ferrous oxalate precipitate and a first precipitate solution with a pH of 1.7. The ferrous oxalate precipitate was washed three times with water and dried overnight at 60°C to obtain ferrous oxalate with a purity of 99.2%.

[0098] The first precipitate solution was adjusted to pH 6.5 with sodium phosphate. It then underwent further purification processes, including defluorination with activated alumina, oil removal with hydrogen peroxide and activated carbon adsorption, precipitation using a double-alkali method, and iron, heavy metal, and calcium and magnesium ion removal via resin adsorption. The combined content of heavy metal ions and calcium and magnesium ions in the purified solution was 36 ppm. Lithium hydroxide and sodium hydroxide were then added to the purified solution to adjust the pH to 11.2, resulting in a Li / P molar ratio of 2.47:1, yielding a lithium phosphate precipitate and a second precipitate solution.

[0099] Lithium phosphate precipitate was dissolved in refined phosphoric acid, and the Li / P molar ratio was controlled at 1:1 to obtain a lithium dihydrogen phosphate solution. The lithium dihydrogen phosphate solution contained trace amounts of sodium and potassium ions. After passing the solution through an ion exchange resin, the sodium and potassium ion contents were found to be below 1 ppm. The lithium dihydrogen phosphate solution was concentrated at 80°C to a solid content of 25 wt%, evaporated under negative pressure at 21 kPa, and then centrifuged and dried to obtain battery-grade lithium dihydrogen phosphate.

[0100] Lime was added to the second precipitate to obtain a crude sodium hydroxide solution and a precipitate residue. Concentrated sulfuric acid was added to the precipitate residue, and the Ca / S molar ratio was controlled at 1:1 to obtain crude phosphoric acid and phosphogypsum. The crude phosphoric acid was purified to obtain refined phosphoric acid, which was used to dissolve the above crude lithium hydrogen phosphate precipitate. The phosphogypsum was calcined with lime, and the tail gas was collected to produce sulfuric acid with a concentration of 20 wt%, which was used to leach phosphorus iron battery black powder.

[0101] In this embodiment, the processing capacity of phosphorus-iron battery black powder is 640 kg / h. The phosphorus-iron battery black powder contains 2.55% lithium, 10.75% phosphorus, and 19.51% iron by mass. The recovery rate of battery-grade lithium dihydrogen phosphate is 225.62 kg / h, with a lithium recovery rate of 94.1% and a phosphorus recovery rate of 93.1% (with an additional 85% phosphoric acid content of 9.86 kg / h). The recovery rate of battery-grade ferrous oxalate is 294.7 kg / h, with an iron recovery rate of 92.35%.

[0102] As described above, the basic principles, main features, and advantages of the present invention have been shown and described. Those skilled in the art should understand that the present invention is not limited to the above embodiments, which are merely preferred embodiments and should not be construed as limiting the scope of the invention. All equivalent changes and modifications made in accordance with the scope of the patent and the description should still fall within the scope of the present invention. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for recovering black powder from phosphorus-iron batteries, characterized in that, Includes the following steps: S1. Phosphorus iron battery black powder is leached with acid to obtain leachate and leaching residue; the leachate is purified after removing copper and aluminum. The leaching process also involves adding a reducing agent to remove Fe. 3+ Reduced to Fe 2+ ; The reducing agent is selected from sulfites and / or sulfur dioxide gas; The acid is selected from sulfuric acid or a mixed acid, wherein the mixed acid is composed of sulfuric acid and phosphoric acid in a weight ratio of 1:0.2-10; S2. A precipitant is added to the purified liquid in step S1 to precipitate ferrous oxalate precipitate and a first precipitate solution. The ferrous oxalate precipitate was obtained by washing and drying. The precipitant is selected from a combination of oxalic acid and sodium oxalate in a molar ratio of 1:1-30; The molar ratio of Fe / C in the purified liquid and the precipitant is 1.01-1.1:2; S3. In step S2, the pH of the first precipitate is adjusted to 6-7. After removing impurities, at least one of sodium hydroxide and lithium hydroxide is added to adjust the pH to 7.5-12.6 to obtain lithium phosphate precipitate and second precipitate. The lithium phosphate salt precipitate is dissolved in refined phosphoric acid to obtain a lithium dihydrogen phosphate solution. The lithium dihydrogen phosphate solution is then concentrated, crystallized, and dried to obtain lithium dihydrogen phosphate. After adding lime to the second precipitate, a crude sodium hydroxide solution and precipitate residue are obtained; Concentrated sulfuric acid was added to the precipitate residue, and the Ca / S molar ratio was controlled to be 0.95-1.05:1 to obtain crude phosphoric acid and phosphogypsum. The crude phosphoric acid is purified to obtain refined phosphoric acid, which is used to dissolve the lithium phosphate precipitate.

2. The method for recovering phosphorus-iron battery black powder according to claim 1, characterized in that, The copper removal process in step S1 involves adding iron powder to replace copper ions with elemental copper and then filtering. The aluminum removal process involves adding alkali to adjust the pH to 2.9-4.5, converting aluminum ions into aluminum hydroxide precipitate, and then filtering.

3. The method for recovering phosphorus-iron battery black powder according to claim 1, characterized in that, In step S2, the pH of the purified solution after adding the precipitant is 1-2.5, and the temperature of the purified solution is 40-80℃.

4. The method for recovering phosphorus-iron battery black powder according to claim 1, characterized in that, After adjusting the pH of the first precipitate to 6-7 in step S3, the molar ratio of Li / P in the first precipitate is 2.02-3:

1.

5. The method for recovering phosphorus-iron battery black powder according to claim 1, characterized in that, The impurity removal mentioned in step S3 is one or a combination of two or more of the following: fluorine removal, oil removal, iron removal, and heavy metal removal; The defluorination process involves adding a defluorinating agent to generate a precipitate, which is then removed. The degreasing process involves adding hydrogen peroxide for oxidation and degreasing. The iron and heavy metal removal processes include dual-alkali precipitation and resin adsorption.

6. The method for recovering phosphorus-iron battery black powder according to claim 1, characterized in that, The phosphogypsum is calcined to obtain lime and tail gas, and the tail gas is used to prepare sulfuric acid.

7. The method for recovering phosphorus-iron battery black powder according to claim 1, characterized in that, The Li / P molar ratio in the lithium dihydrogen phosphate solution described in step S3 is 0.99-1.01:1.