Method for synergistically removing nickel impurity in ternary impurity-containing lithium iron phosphate positive electrode powder by using mixed acid

Abstract

The invention discloses a method for synergistically removing a nickel impurity in lithium iron phosphate positive electrode powder containing ternary impurities by using a mixed acid, which comprises the following steps of: taking the mixed acid of reducing phosphoric acid and citric acid as an impurity removing agent for the recycled lithium iron phosphate positive electrode powder containing the ternary impurities; the Ni impurity in the lithium iron phosphate positive electrode powder is effectively removed, about 35% of Al in the positive electrode powder can be removed at the same time, and the Fe loss in the process is not higher than 7%. The impurity-removed slag is leached and neutralized to obtain a pure lithium solution which can be used for preparing Li2CO3; and removing calcium by using a chemical precipitation method to obtain a pure lithium solution after the complexing action of citric acid radicals in Li in the impurity removal liquid is broken by CaO, dissolving the lithium extraction slag by using sulfuric acid, and adding ammonia water to adjust the pH value to prepare an iron phosphate finished product.

Description

Technical Field

[0001] The invention belongs to the field of resource recovery and utilization of solid waste, and specifically relates to a method for removing nickel impurities in lithium iron phosphate positive electrode powder containing ternary impurities by using a mixed acid in a coordinated manner. Background Art

[0002] Manufacturers who recycle lithium iron phosphate positive plates will inevitably mix a small amount of ternary plates in the process of recycling the plates. The recycled lithium iron phosphate positive plates are converted into lithium iron phosphate positive powder through sorting equipment, and the Ni content is 0.1~1%. At present, lithium iron phosphate positive powder usually selectively recovers Li as Li2CO3, and the remaining ferrophosphorus slag is prepared into iron phosphate and reused in the preparation process of lithium iron phosphate to achieve the recycling of lithium iron phosphate positive plates. Whether in the preparation of Li2CO3 or FePO4, Ni will exist in the form of impurities, and as a magnetic metal impurity, it will first be oxidized at the positive electrode and then reduced at the negative electrode during the battery formation stage. When the metal element at the negative electrode accumulates to a certain extent, dendrites will be formed, resulting in diaphragm perforation, causing internal short circuit of the battery, and increasing the self-discharge rate of the battery. In severe cases, it may even cause the battery to catch fire and explode, affecting the safety performance of the battery. Therefore, it is necessary to remove the lithium iron phosphate positive powder containing ternary impurities.

[0003] At present, the research on nickel is mainly focused on ternary positive electrode materials, including hydrometallurgy, pyrometallurgy, etc. Although these methods have achieved the recovery of valuable metals to a certain extent, they are often energy-intensive and polluting, and it is difficult to meet the growing environmental protection requirements and industrial standards. Summary of the invention

[0004] Based on the above background, the present invention introduces a new method for removing ternary impurities in lithium iron phosphate positive electrode powder by mixed acid synergistically.

[0005] The present invention is achieved through the following technical steps: (1) Add pure water to the lithium iron phosphate positive electrode powder containing ternary impurities to prepare a slurry with a liquid-to-solid ratio of 3-5:1, then add 1%-3% phosphoric acid and 1.5%-6% citric acid based on the mass of the positive electrode powder, react at 60-80°C for 2-4 hours, and filter to obtain the impurity removal liquid and impurity removal residue; (2) Add 1.5%-8% calcium oxide by mass of the solution to the obtained impurity-removing liquid, react at 60-80°C for 2-8h to remove metal impurities in the solution, then add oxalic acid to adjust the pH according to n(Ca)=1.05-1.2 to generate calcium oxalate precipitate, remove the calcium impurities introduced into the solution, and filter the residue obtained as calcium oxalate precipitate, and the filtrate is a pure lithium solution; (3) extracting lithium from the obtained impurity-removed slag with sulfuric acid and an oxidant, and filtering to obtain a lithium extraction liquid and a lithium extraction slag; (4) The lithium extraction solution is neutralized with sodium hydroxide solution to a pH of 9.0-9.5 to remove impurities such as iron, phosphorus, aluminum, and manganese in the solution to obtain a pure lithium solution; (5) The obtained pure lithium solution is added with sodium carbonate to prepare a finished lithium carbonate product, and the obtained lithium extraction slag is dissolved in sulfuric acid and then neutralized and precipitated by adjusting the pH with ammonia water to prepare a finished iron phosphate product.

[0006] Compared with the prior art, the present invention has the following advantages: 1. The process of removing lithium iron phosphate cathode powder containing ternary impurities by mixed acid is simple, the amount of acid consumed in the impurity removal process is small, the nickel removal rate is high and the impurity removal cost is extremely low; 2. The citric acid and phosphoric acid in the impurity remover have little impact on the environment, making the entire leaching and impurity removal process environmentally friendly; 3. Phosphoric acid is a non-oxidizing acid that can inhibit the increase in the valence of Ni. At the same time, citric acid has a good chelating effect with Ni ions. The mixed use of phosphoric acid and citric acid promotes the dissolution of impurities in the acidic solution, so that the purpose of selective Ni removal can be achieved without the need to add additional reducing agents during the raw material impurity removal process, and it also has the advantage of low Fe loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0008] 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.

[0009] A method for removing nickel impurities from lithium iron phosphate positive electrode powder containing ternary impurities by mixed acid synergistically comprises the following steps: (1) Add pure water to the lithium iron phosphate positive electrode powder containing ternary impurities to prepare a slurry with a liquid-to-solid ratio of 3-5:1, then add 1%-3% phosphoric acid and 1.5%-6% citric acid based on the mass of the positive electrode powder, react at 60-80°C for 2-4 hours, and filter to obtain the impurity removal liquid and impurity removal residue; (2) Add 1.5%-8% calcium oxide by mass of the solution to the obtained impurity-removing liquid, react at 60-80°C for 2-8h to remove metal impurities in the solution, then add oxalic acid to adjust the pH according to n(Ca)=1.05-1.2 to generate calcium oxalate precipitate, remove the calcium impurities introduced into the solution, and filter the residue obtained as calcium oxalate precipitate, and the filtrate is a pure lithium solution; (3) extracting lithium from the obtained impurity-removed slag with sulfuric acid and an oxidant, and filtering to obtain a lithium extraction liquid and a lithium extraction slag; (4) The lithium extraction solution is neutralized with sodium hydroxide solution to a pH of 9.0-9.5 to remove impurities such as iron, phosphorus, aluminum, and manganese in the solution to obtain a pure lithium solution; (5) The obtained pure lithium solution is added with sodium carbonate to prepare a finished lithium carbonate product, and the obtained lithium extraction slag is dissolved in sulfuric acid and then neutralized and precipitated by adjusting the pH with ammonia water to prepare a finished iron phosphate product.

[0010] Phosphoric acid and citric acid are two common chemical substances. Phosphoric acid, as a non-oxidizing inorganic acid, is often used as a leaching agent. Ni in ternary materials usually exists in a mixture of +2 / +3 valences. Phosphoric acid, as a non-oxidizing acid, is used in combination with citric acid, which can provide acidity without affecting the low-valent nickel element. Under the action of the dual acid system, all the nickel elements in the lithium iron phosphate positive electrode powder containing the ternary material are reduced to +2 valence and leached into the solution, realizing the effective removal of impurity nickel elements.

[0011] The present invention optimizes the previous research which mainly focused on using H2O2 as a reducing agent and one or more organic acids / inorganic acids as leaching agents, without adding additional reducing agents; After the mixed acid is used to remove nickel from the lithium iron phosphate positive electrode powder containing ternary impurities, 10% of the lithium enters the impurity removal liquid, and this part of the lithium element needs to be recovered. It mainly contains citric acid-M (citrate metal salt). Due to the strong complexation of citric acid, the conventional neutralization precipitation method cannot remove impurities, resulting in the inability to recover the Li in the pickling liquid. The present invention uses calcium oxide to break the complex at high temperature and then neutralize and remove impurities to obtain a pure Li solution, which provides a new idea for the neutralization and impurity removal of this type of solution.

[0012] The technical points of the present invention are: 1. Lithium iron phosphate cathode powder containing ternary impurities is selectively leached with a mixture of phosphoric acid and citric acid to remove nickel impurities. The concentration of phosphoric acid is 1%-3%, and the concentration of citric acid is 1.5%-6%. The appropriate concentration of the mixed acid is adjusted according to the nickel content in the cathode powder. At the same time, the Fe loss in this process is not higher than 7%; 2. The impurity removal liquid obtained after mixed acid impurity removal contains citric acid metal salts, which need to be neutralized and impurized after the complex is broken by calcium oxide. The amount of calcium oxide added is 1.5%-8% of the mass of the impurity removal liquid. After the complex is broken, the Li yield is 75%-90%; 3. After calcium oxide is broken and impurities are removed, the neutralized solution will introduce impurities of calcium, which needs to be further decalcified with oxalic acid. The amount of oxalic acid added is n(Ca)=1.05-1.2, that is, the molar ratio of oxalic acid to calcium is 1.05-1.2; 4. Remove impurities from the slag and extract the lithium element through acid leaching. This process uses sulfuric acid + hydrogen peroxide leaching, with almost no Fe loss; Example 1

[0013] Add pure water to the lithium iron phosphate positive electrode powder containing ternary impurities (Ni=500~1000ppm) to prepare the slurry, with a liquid-to-solid ratio of 3.5:1, then add 1.5% phosphoric acid and 2% citric acid based on the mass of the positive electrode powder, react at 80°C for 3 h, and filter to obtain the impurity removal liquid and impurity removal residue; Add 2% calcium oxide by mass of the solution to the impurity removal solution obtained in step (1), react at 60°C for 4 hours to remove the impurity metals in the solution, then add oxalic acid according to n(Ca)=1.2 to generate calcium oxalate precipitate, remove the calcium impurities introduced into the solution, and filter to obtain the calcium oxalate precipitate as the filter residue, and the filtrate is a pure lithium solution; Extracting lithium from the impurity-removed slag obtained in step (1) with sulfuric acid and an oxidant, and filtering to obtain a lithium extraction liquid and a lithium extraction slag; The lithium extraction solution obtained in step (3) is neutralized to pH = 9.0-9.5 with sodium hydroxide solution to remove impurities such as iron, phosphorus, aluminum, manganese, nickel, etc. in the solution to obtain a pure lithium solution; The lithium solution obtained in steps (2) and (4) is added with sodium carbonate to prepare a lithium carbonate product, and the lithium extraction slag obtained in step (3) is dissolved in sulfuric acid and the pH is adjusted with ammonia water to prepare an iron phosphate product; In the process of step (1), the lithium iron phosphate positive electrode powder containing ternary impurities contains Ni=1500ppm; In the process of step (1), the Ni removal rate is ≥95%, the Li loss rate is ≤2%, the Fe loss rate is ≤3.6%, and the P loss rate is ≤4%; In the process of step (5), the Li extraction rate is ≥98%, the Fe recovery rate is ≥95%, and the P recovery rate is ≥95%. Example 2

[0014] Add pure water to the lithium iron phosphate positive electrode powder containing ternary impurities (Ni=about 1000-2000ppm) to prepare the slurry, with a liquid-to-solid ratio of 3-5:1, and then add 3.0% phosphoric acid and 4% citric acid based on the mass of the positive electrode powder, react at 60°C for 4 hours, and filter to obtain the impurity removal liquid and impurity removal residue; The impurity removal solution obtained in step (1) is added with 4% calcium oxide by mass of the solution, and reacted at 60° C. for 6 hours to remove the impurity metals in the solution, and then oxalic acid is added according to n(Ca)=1.2 to generate calcium oxalate precipitate, and the calcium impurities introduced into the solution are removed. The filter residue obtained by filtration is the calcium oxalate precipitate, and the filtrate is a pure lithium solution; Extracting lithium from the impurity-removed slag obtained in step (1) with sulfuric acid and an oxidant, and filtering to obtain a lithium extraction liquid and a lithium extraction slag; The lithium extraction solution obtained in step (3) is neutralized to pH = 9.0-9.5 with sodium hydroxide solution to remove impurities such as iron, phosphorus, aluminum, manganese, nickel, etc. in the solution to obtain a pure lithium solution; The lithium solution obtained in steps (2) and (4) is added with sodium carbonate to prepare a lithium carbonate product, and the lithium extraction slag obtained in step (3) is dissolved in sulfuric acid and the pH is adjusted with ammonia water to prepare an iron phosphate product; In the process of step (1), the lithium iron phosphate positive electrode powder containing ternary impurities contains Ni=2000ppm; In the process of step (1), the Ni removal rate is ≥97%, the Li loss rate is ≤1.2%, the Fe loss rate is ≤4.2%, and the P loss rate is ≤3.8%; In the process of step (5), the Li extraction rate is ≥98.5%, the Fe recovery rate is ≥95.1%, and the P recovery rate is ≥95.5%. Example 3

[0015] Add pure water to the lithium iron phosphate positive electrode powder containing ternary impurities (Ni=about 3000~4500ppm) to prepare the slurry, with a liquid-to-solid ratio of 5:1, and then add 3.0% phosphoric acid and 6% citric acid based on the mass of the positive electrode powder, react at 80°C for 4 hours, and filter to obtain the impurity removal liquid and impurity removal residue; Add 4% calcium oxide by mass of the solution to the impurity removal solution obtained in step (1), react at 80°C for 8h to remove the impurity metals in the solution, then add oxalic acid according to n(Ca)=1.1 to generate calcium oxalate precipitate, remove the calcium impurities introduced into the solution, and filter to obtain the calcium oxalate precipitate as the filter residue, and the filtrate is a pure lithium solution; Extracting lithium from the impurity-removed slag obtained in step (1) with sulfuric acid and an oxidant, and filtering to obtain a lithium extraction liquid and a lithium extraction slag; The lithium extraction solution obtained in step (3) is neutralized to pH = 9.0-9.5 with sodium hydroxide solution to remove impurities such as iron, phosphorus, aluminum, manganese, nickel, etc. in the solution to obtain a pure lithium solution; The lithium solution obtained in steps (2) and (4) is added with sodium carbonate to prepare a lithium carbonate product, and the lithium extraction slag obtained in step (3) is dissolved in sulfuric acid and the pH is adjusted with ammonia water to prepare an iron phosphate product; In the process of step (1), the lithium iron phosphate positive electrode powder containing ternary impurities contains Ni=4400ppm; In the process of step (1), the Ni removal rate is ≥98.5%, the Li loss rate is ≤1.2%, the Fe loss rate is ≤3.1%, and the P loss rate is ≤3.5%; In the process of step (5), the Li extraction rate is ≥98.8%, the Fe recovery rate is ≥95.6%, and the P recovery rate is ≥95.2%.

Claims

1. A method for removing nickel impurities from lithium iron phosphate positive electrode powder containing ternary impurities by mixed acid synergistically, characterized in that The following steps are involved: (1) Add pure water to the lithium iron phosphate positive electrode powder containing ternary impurities to prepare a slurry with a liquid-to-solid ratio of 3-5:1, then add 1%-3% phosphoric acid and 1.5%-6% citric acid based on the mass of the positive electrode powder, react at 60-80°C for 2-4 hours, and filter to obtain the impurity removal liquid and impurity removal residue; (2) Add 1.5%-8% calcium oxide by mass of the solution to the obtained impurity-removing liquid, react at 60-80°C for 2-8h to remove metal impurities in the solution, then add oxalic acid to adjust the pH according to n(Ca)=1.05-1.2 to generate calcium oxalate precipitate, remove the calcium impurities introduced into the solution, and filter the residue obtained as calcium oxalate precipitate, and the filtrate is a pure lithium solution; (3) extracting lithium from the obtained impurity-removed slag with sulfuric acid and an oxidant, and filtering to obtain a lithium extraction liquid and a lithium extraction slag; (4) The lithium extraction solution is neutralized with sodium hydroxide solution to pH = 9.0-9.5 to obtain a pure lithium solution; (5) The obtained pure lithium solution is added with sodium carbonate to prepare a finished lithium carbonate product, and the obtained lithium extraction slag is dissolved in sulfuric acid and then neutralized and precipitated by adjusting the pH with ammonia water to prepare a finished iron phosphate product.

2. The method for removing nickel impurities from lithium iron phosphate positive electrode powder containing ternary impurities by mixed acid according to claim 1, characterized in that The oxidant in step (3) is hydrogen peroxide.

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