Application Fundamental Research On Lithium Extraction From Salt Lake Brines With Novel Heteropoly Acid Co-extraction Agent

As an emerging strategic energy metal,lithium has been widely used in various fields such as batteries,ceramic glass,aerospace,medicine and other fields due to its excellent physical and chemical properties.It is imperative to develop high-purity lithium products with the short supply of high-energy-density lithium-ion batteries.Since the grade of lithium ore resources in China is low and about 80%of the lithium resources are stored in salt lake brine,lithium extraction from salt lake brines will become the main way to produce lithium products.Most of the brines in China has characteristic of high magnesium and low lithium,resulting in difficult separating of magnesium and lithium.Therefore,it is necessary to select a suitable and efficient lithium extraction technology to explore the lithium extraction process.Among them,the solvent extraction method has the advantages of high degree of automatic operation,high lithium-magnesium separation factor and mature industrial scale,which provides a choice for industrialized extraction of lithium.The extraction system used for the solvent extraction of lithium generally consists of three parts,namely the extractant,the co-extraction agent and the diluent.However,most of the co-extraction agents currently used contain iron or fluorine,resulting in poor circulation stability of the co-extraction agents.In addition,the fluorine-containing co-extraction agents are also prone to produce toxic gases.To solve this problem,in this work,two kinds of heteropolyacids ionic liquid were designed and synthesized as co-extraction agents for lithium extraction from salt lake brines.First,a ternary heteropolyacid,namely phosphomolybdovanadic acid(H₅PMo₁₀V₂O₄₀),was first synthesized,which was reacted with 1-methyl imidazole to synthesize the final heteropolyacid ionic liquid of[Mmim]₅PMo₁₀V₂O₄₀,and the successful synthesis was verified by 1H NMR and FT-IR characterizations.After screening the diluent,a new extraction system of TBP-[Mmim]₅PMo₁₀V₂O₄₀-acetylacetone was constructed,and the operation conditions of extraction,washing,stripping,regeneration and circulation of the organic phase were optimized.Under the optimal extraction conditions with the volume fraction of TBP as 50%,the molar ratio of co-extraction agent to Li⁺as 0.8,and the phase ratio as 1,the one-stage extraction efficiency of Li⁺was 51.25%,and the one-stage separation factor of lithium and magnesium was 62.10.After that,the organic phase was washed by using the mixed solution of HCl and Li Cl.The elution efficiency of Mg²⁺reached up to 100%,while the concentration of Na⁺and K⁺in the organic phase was lower than 5 and 0.5ppm,respectively.Then 0.2 mol/L HCl solution was used for two times stripping,and the stripping efficiency of lithium ion was 98.36%.The lithium-free brine was used for two times regenerations of the organic phase and 10 extraction cycles were conducted,the results of which showed that the extraction efficiency of lithium ions maintained at about 60%,indicating that the proposed extraction system had good stability.In order to increase the flash point of the diluent and the separation factor of lithium and magnesium,the heteropolyacid ionic liquid of[H（CL₂）]₃PMo₁₂O₄₀was synthesized by using phosphomolybdic acid and caprolactam.After screening the diluent,the extraction system of TBP-[H（CL₂）]₃PMo₁₂O₄₀-2-octanone was constructed and the volume fraction of TBP,the dosage of[H（CL₂）]₃PMo₁₂O₄₀and the extraction phase ratio were optimized.The single-stage extraction efficiency of Li⁺was 49.76%,and the separation factor of lithium and magnesium was 113.64.Then a mixed solution of 1.2 mol/L Na Cl and 0.3 mol/L Li Cl was used to wash the organic phase,and the one-stage washing efficiency of Mg²⁺was 97.80%.The stripping efficiency of Li⁺was about 100%after two times stripping by using0.48 mol/L HCl solution as stripping agent.After that,0.2 mol/L Na OH solution was used for regeneration experiments and 10 cycles of the whole extraction experiments were carried out,the results of which showed that the one-stage extraction efficiency of Li⁺maintained at 65%and the variation range wass within 3%,demonstrating that the extraction system had good recycling performance.The ³¹P NMR characterization was used to conduct a mechanism exploration for the two novel extraction systems constracted in this work.The results showed that the binding ability between the P=O functional groups in TBP and metal cations followed the sequence Li⁺>Na⁺>Mg²⁺,demonstrating the selective extraction abilities of Li⁺by the two extraction systems.