Study On Recovery Of Lithium And Comprehensive Utilization Of Iron And Phosphorus From Lithium Iron Phosphate Scrap

With the rapid development of new energy vehicles,the application of LiFePO₄ batteries has become more and more extensive,which means that a large number of spent LiFePO₄batteries will be generated in the next few years.Aiming at the defects of excessive consumption of chemical reagents and long process in the current hydrometallurgical recovery technology for spent LiFePO₄ batteries,this paper focus on the recovery of spent LiFePO₄batteries.Firstly,the process of selectively leaching lithium via oxidation pressure leaching was studied.Then,the kinetics and leaching mechanism of the lithium leaching process were analyzed.Subsequently,the leaching solution was purified and recovery of lithium via chemical precipitation was investigated.Finally,the adsorption performance of the hydroxy iron phosphate leaching residues obtained under different conditions for the heavy metal ions was studied.The specific contents are as follows:The process of selectively leaching lithium from spent LiFePO₄ cathode material via oxidation pressure leaching was studied,while O₂ was used as an oxidant and near lithium stoichiometric dilute sulfuric acid was used as the leaching agent.The results indicate that under the optimized conditions(O₂ partial pressure of 1.3 MPa,H₂SO₄ concentration of 0.6mol·L^-1,leaching temperature of 120°C,H₂SO₄/Li molar ratio of 0.525,and leaching time of90 min),the leaching rates of Li,Fe and P are 97.82%,0.022%,and 22.48%,respectively.The leaching selectivity reaches an ultrahigh level with 99.83%,accessing to a one-step separation of Li and Fe.The unreacted shrinking core model was used to fit the leaching data at different temperatures to study the kinetics of the lithium leaching process.The results illustrate that the selective leaching of lithium via oxidation pressure leaching is controlled by the interface chemical reaction.To study the mechanism of the selective lithium leaching process,XRD,SEM,XPS and HR-TEM characterization were conducted to analyze the raw material and leaching residues obtained under different leaching conditions,then the leaching mechanism was further confirmed through the simulation experiments.The results show that it is easier to form Fe PO₄ leaching residue with an olivine structure during the low-temperature leaching process.The Fe PO₄ formation process is similar to the phase transformation during the LiFePO₄ charging process,where the Fe²⁺present in LiFePO₄ is directly oxidized to form Fe PO₄,such that Li⁺is then released out.Differently,it is easier to form octahedral Fe₅（PO₄）₄（OH）₃·2H₂O leaching residue when leaching at high temperature under an identical O₂ partial pressure.The formation process is similar to the dissolution-precipitation process,the Fe²⁺present in LiFePO₄ is leached out and oxidized to Fe³⁺by O₂,then the Fe³⁺combines with PO₄^3-in the solution to gradually form Fe₅（PO₄）₄（OH）₃·2H₂O at high temperature.Impurity in the leaching solution was removed by adjusting the pH,and the results show that the concentration of Fe³⁺in the solution was reduced to 0.327 mg·L^-1 at pH=5.The effects of sodium carbonate,sodium phosphate,and sodium fluoride on lithium precipitation were compared by investigating the effects of precipitant dosage,temperature,reaction time,and pH on the lithium precipitation rate.The results demonstrate that the recovery rate of lithium is the highest when sodium phosphate is used as precipitant.Moreover,the PO₄^3-in the purified solution can theoretically precipitate about 60%of Li⁺,which means that only 50%of the theoretical dosage of sodium phosphate should be used to precipitate lithium with a precipitation rate of 97.97%under the optimized conditions,and the global recovery rate of lithium reaches 95.74%.The adsorption performance of the hydroxy iron phosphate leaching residues obtained under different leaching conditions was compared through the adsorption experiments,and it was found that the FPOH-ML leaching residue obtained after adding mannitol during the leaching process had the best comprehensive adsorption performance for heavy metals.Then the effects of the initial concentration and pH of the solution on the adsorption performance of FPOH-ML for Pb²⁺,Cu²⁺,Zn²⁺,and Ni²⁺were investigated.Under the optimized adsorption conditions,the adsorption capacities of FPOH-ML for Pb²⁺,Cu²⁺,Zn²⁺,and Ni²⁺were 18.26,17.53,9.60,and 6.24 mg·g^-1,respectively.The FPOH-ML before and after adsorption were analyzed by XRD,FTIR characterization to study the adsorption mechanism.It was found that no new products were formed during the adsorption process,and the main structure of FPOH-ML did not change significantly,indicating that the adsorption process belongs to physical adsorption.This paper proposes a feasible method for recovering spent LiFePO₄ cathode material via oxidation pressure leaching and chemical precipitation.Valuable metal lithium is effectively recovered,and comprehensive utilization of iron and phosphorus is also realized,which is expected to provide reference and guidance for the optimization and improvement of current process technology.