Recovery Of Valuable Elements From Lithium Iron Phosphate By Coupled Leaching-Extraction Technology

With the rapid development of economic,the new energy automobile industry has developed rapidly.As the main power source of new energy automobiles,lithium iron phosphate（LFP）batteries have also been produced and applied in large quantities.Due to high assurance factor,long cycle life and other excellent performance,it was widely used in the fields of initial power source,energy storage market,military industry,navigation aids and so on.The extensive application of LFP battery has led to scrap quantity increasing yearly,resulting in waste of resources,pollution of the environment,harm to human health and other serious problems.Due to the low content of valuable metal lithium in waste LFP batteries and poor economic recovery,low-cost recovery has become a research hotspot.Although achieved some results and applications,there were still problems such as low recovery efficiency and high cost of valuable elements.Therefore,the higher-value all-element recycling and reuse of spent LFP batteries has become the key and difficult point to solve the above problems,as great significance.Based on the isomorphic induced substitution leaching method,the inexpensive and environmentally-friendly iron salts FeCl₃ and Fe₂（SO₄）₃ as leaching agents,was used to single leach LFP and co-leach with H₂O₂ in a solid-liquid reaction system.The factors of the lithium leaching efficiency have been verified.With high selectivity and efficient,Li⁺has been leached and Fe PO₄ has been recovered.Using the unreacted nuclear shrinkage model,the kinetics of Fe³⁺to leach Li⁺from LFP was explored in FeCl₃ solution with isomorphic induced substitution.The main control step of lithium leaching efficiency was verified,and the kinetic equation and reaction activation energy were obtained.Based on the solvent extraction method,the TBP-sulfonated kerosene（SK）extraction system was used to simultaneously extract iron and lithium from the lixivium of FeCl₃ single leaching LFP.The separation of iron and lithium was realized through stepwise stripping,and the FeCl₃ was recycled in the leaching and extraction process.Li₂CO₃ was successfully prepared from the lithium stripping liquor.The N235-TBP-SK system was used to extract iron from the lixivium of Fe₂（SO₄）₃-H₂O₂ co-leaching LFP for achieving purification.After stripping process,the Fe₂（SO₄）₃ was recycled to leaching LFP.The organic phase was non-destructively regenerated through washing acid and saponification process,and Li₂CO₃ was successfully prepared from the raffinate.The main results of this study are as follows:（1）Iron salt FeCl₃ and Fe₂（SO₄）₃ were used for single leaching and co-leaching LFP with H₂O₂.It was found that the material ratio,solid-liquid ratio,acidity,and the amount of H₂O₂ had a significant effect on the leaching efficiency of lithium and phosphorus,thereby effecting the recovery efficiency of Fe PO₄.Under the optimal leaching conditions,as FeCl₃ leaching and FeCl₃-H₂O₂ co-leaching LFP,the lithium leaching efficiency were 99.00%and 99.01%,and the Fe PO₄ recovery efficiency were 98%and 96%,respectively.As Fe₂（SO₄）₃ leaching and Fe₂（SO₄）₃-H₂O₂co-leaching,the lithium leaching efficiency were 95.43%and 99.41%,and the Fe PO₄recovery efficiency were 92%and 96%,respectively.The Li⁺was leached from LFP with highly selective by Fe³⁺,avoiding the use of a large number of acids,bases and precipitators.（2）FeCl₃ as the leaching agent,the influencing factors on the leaching lithium from LFP and the macro kinetics of the two leaching stages were explored.The results show that increasing of the FeCl₃/LFP molar ratio and the reaction temperature,decreasing of the solid-to-liquid ratio can significantly promote the leaching of lithium.Within 4 minutes,the lithium leaching efficiency was mainly controlled by the diffusion inside product layer,and the activation energy was 6.68 k J/mol.After 4minutes,it was controlled by the mixing mechanism of diffusion outside the fluid film,the diffusion inside the product layer and the chemical reaction.（3）Based on the lixivium of FeCl₃ single-leaching LFP,the TBP-SK system as extractant,the single-stage extraction conditions were determined as follows:TBP concentration of 3 mol/L,phase ratio（O/A）of 5/1,HCl concentration of 0.05～0.1mol/L,and the Fe/Li molar ratio of 1.3.As optimal extraction conditions,the lithium extraction efficiency can reach 80.81%through three-stage counter-current extraction from the feed liquid containing 1.984 mol/L Li⁺.At the phase ratio O/A of 10/1,90.23%of lithium was stripped form loaded organic phase by 6 mol/L HCl through five-stage counter-current stripping.At the phase ratio O/A of 1/1,the iron stripping efficiency can reach 99.87%by 0.05 mol/L HCl through four-stage counter-current stripping of loaded iron organic phase.The high efficiency separation of iron and lithium was achieved by stepwise stripping.Raffinate and lixivium were concentrated and entered into extraction process to accomplished cycle.The closed-loop recycling LFP process was coupled of the leaching,the oxidation concentration,the extraction and the stripping process.In each cycle,the Li and Fe PO₄ recovery efficiency can reach 99%and 98%,respectively.Li₂CO₃ with a purity of 99.54%was prepared from the lithium stripping liquor of 3.059 mol/L.In the entire recovery process,FeCl₃ in iron stripping liquor was regenerated directly and used circularly for the leaching and extraction process.（4）Based on lixivium of the Fe₂（SO₄）₃-H₂O₂ co-leaching LFP,the N235-TBP-SK system as the extractant,the single-stage extraction conditions were determined as follows:the organic phase composition of 40vt.%N235+15vt.%TBP+SK,phase ratio O/A of 1/1,and the initial p H of the feed liquid of 0.95.As the total phase ratio O/A of 3.5/1,the 97.22%of iron was removed from the feed liquid containing 21.735 g/L Fe³⁺through four-stage cross-flow extraction.Using 1.4 mol/L H₂SO₄ solution to carry out three-stage countercurrent stripping of the loaded organic phase,the iron stripping efficiency can reach 99.83%at phase ratio O/A of 1/1.The organic phase was non-destructively regenerated by washed acid and saponified.The closed-loop recycling LFP process was coupled of leaching,extraction,stripping and regeneration process.In each cycle,the recovery efficiency of lithium and Fe PO₄were 99%and 96%,respectively.Li₂CO₃ with a purity of 99.51%was prepared from raffinate of 10.1 g/L.Fe₂（SO₄）₃ in stripping liquor was regenerated directly and used circularly for the leaching section in the whole process.In short,the coupling process of isomorphic induced leaching and solvent extraction was carried out higher-value all-element and efficient recycling of spent LFP for the first time,which provided new research ideas and technical support for the recycling of spent LFP batteries.And the research direction of the comprehensive recycle-utilization of spent lithium batteries and the application field of solvent extraction were expanded.