Study On The Process Of Preparing Iron Phosphate From Spent Lithium Iron Phosphate Cathode Powder By Phosphoric Acid Method

Lithium iron phosphate(LiFePO4,briefed as LFP)battery is the mainstream power battery in the early development of China's new energy automobile industry.It has always been the preferred power battery for electric commercial vehicles,special vehicles and energy storage due to its outstanding advantages such as good safety performance,long cycle life and low cost.The explosive growth of retired LFP batteries demands the urgent need for recycling.Lithium iron phosphate cathode material is the most valuable component of lithium iron phosphate batteries,but the value for recycling is still low because it contains only a small amount of lithium and cheap iron/phosphorus.Currently,there is no economical and effective recycling technology.This paper focuses on the utilization of iron and phosphorus resources in the spent lithium iron phosphate cathode powder.A new process for preparing iron phosphate from spent lithium iron phosphate cathode powder by phosphoric acid method was proposed in the paper.The paper focused on the fundamental and process optimization research of acid leaching of spent cathode powder phosphoric,removing aluminum from the leaching liquor by complexing method,and the precipitation of iron phosphate.The main findings are as follows:(1)Effects of main process parameters such as acid-to-material ratio,leaching temperature on the leaching process of spent LFP cathode powder was systematically studied by using aluminum-containing spent lithium iron phosphate cathode powder as raw material,and the macro-dynamics of the leaching process was also investigated.The research results show that the acid-to-material ratio and liquid-solid ratio have the most significant impact on the leaching rate of lithium iron phosphate and aluminum.The use of anionic surfactant SDS(sodium dodecyl sulfate)can effectively solve the problem of poor hydrophilicity of the spent cathode powder in leaching process.Under the optimized conditions of acid-to-material ratio of 1.1,temperature of 20?,liquid-to-solid ratio of 5:1,stirring speed of 400 rpm,and leaching time of 120 min,the leaching rate of lithium iron phosphate is greater than 93%,and the leaching rate of aluminum is less than 20%.The phosphoric acid leaching process of spent lithium iron phosphate cathode powder conforms to the shrinking core model without solid product layer,and the apparent activation energy is 24.62 kJ/mol.The leaching process is controlled by diffusion,and the kinetic equation of the leaching process is as follows:(?)(2)A new method for removing aluminum by complexation of fluoride in phosphoric acid leaching solution of spent cathode powder is proposed.Effects of the main process parameters on the removal efficiency of aluminum in the leaching solution when sodium fluoride and hydrofluoric acid are used as aluminum removal agents.By theoretical calculation,the relationship between the hydrogen ion concentration and total fluorine concentration of the solution on the aluminum removal effect when sodium fluoride is used as the aluminum removal agent is obtained.It was found that the total fluorine concentration has a more significant effect on the aluminum removal effect than the acidity,which was verified by the orthogonal experiment results.When sodium fluoride is used as the aluminum removal agent,under the optimized condition of the reaction temperature 50?,the initial pH of the solution 1.8,6 times the molar amount of sodium fluoride added that of aluminum,and the reaction time 30 minutes.the aluminum removal efficiency can reach more than 97%,and the aluminum concentration in the leaching solution can be reduced to 48 ppm with sodium hexafluoroaluminate as the aluminum removal product.When hydrofluoric acid is used as the aluminum removal agent,under the optimized condition of the reaction temperature 30?,the initial pH of the solution 2,6 times the molar amount of sodium fluoride added that of aluminum,and the reaction time 20 minutes,the aluminum removal efficiency can reach more than 95%,and the aluminum removal product is lithium hexafluoroaluminate.(3)A new method for direct precipitation of iron phosphate from iron-phosphorus solution under weak acid(pH<2)was proposed.Effects of main process parameters on the precipitation process of iron phosphate was studied,and the influence of impurity cations such as aluminum,copper and sodium on the composition of iron phosphate products was investigated.The research results show that the acidity of the solution as well as the precipitation temperature have significant effects on the precipitation rate of iron phosphate,and the initial iron concentration and seed crystals of the solution have little effect on the precipitation rate.Under optimized conditions of the pH value higher than 0.7,the precipitation temperature 95? and the initial iron concentration less than 35 g/L,the precipitation efficiency of iron phosphate is more than 88%.The content of aluminum and copper in iron phosphate is directly related to the content of aluminum and copper in the pregnant solution,while the sodium content in iron phosphate needs to be removed by washing with a large amount of water.(4)The principle process of preparing iron phosphate dihydrate from spent lithium iron phosphate cathode powder by phosphoric acid method was proposed in the paper,including multiple procedures of phosphoric acid leaching,aluminum removing,oxidation of ferrous,and precipitation of iron phosphate.The whole process has been experimental verified by using real aluminum-containing spent lithium iron phosphate cathode powder,and ferric phosphate dihydrate products with qualified Al,Fe and P components can be obtained.