Study On Development And Application Of New Co-extraction Reagent And Diluent For Co-extraction Of Lithium Ions From Salt Lake Brines With High Magnesium-lithium Ratio

Lithium is the lightest metal in nature and plays a critical role in various fields.Among them,lithium-ion batteries are most closely related to our lives.With the currently gradual depletion of fossil fuels,the pace of electric-powered and hybrid-powered vehicles replacing fossil-fueled vehicles has gradually accelerated,leading to a rapid increase in demand for lithium.Salt lake brine is rich in lithium ions.Extraction of lithium ions from brines gradually becomes the main route of obtaining lithium resources.Most of salt lake brines in China have the characteristics of high magnesium-lithium ratio,and the properties of magnesium ions and lithium ions are similar.Thus,selective extraction of lithium ions from salt lake brines with high magnesium-lithium ratio is the focus of current researches.Among the existing methods for extracting lithium ions from salt lake brines,the liquid-liquid extraction method has the characteristics of low energy consumption,high extraction efficiency and selectivity,and simple operation,which is considered as the most potential method to accomplish industrialization.The most common extraction system consists of tributyl phosphate(TBP)and ferric chloride(FeC13)as extractant and co-extracting reagent,respectively.Kerosene or ketone compounds like methyl isobutyl ketone(MIBK)are usually used as the diluent.The former is a non-polar diluent and the third phase is easy to form during the extraction process.Although the latter can effectively eliminate the formation of the third phase,the low flash point and strong volatility of ketone compounds make it easy to cause hidden safety hazards in industrial production.In addition,the co-extraction reagent will play the co-extraction effect just by forming FeC14’between FeC13 and high concentration of Cl-.In order to prevent the loss of co-extraction reagent,a certain concentration of Cl’ in all the aqueous solutions during the whole extraction process will be required,which not only limits the selection ranges of washing,stripping and regeneration reagents,but also makes this extraction technology only suitable for high chlorine type salt lake brine.At the same time,a large amount of strong acids and alkalis is needed for stripping and regeneration process,which causes potential harm to the equipments and operating workers.In order to solve the deluent problems of TBP-FeC13 extraction system,a new diluent and a new extraction process were developed in this paper.Diethyl succinate was used as diluent and the effects of TBP volume concentraion,Fe/Li molar ratio and O/A ratio on the extraction performance were studied.The results showed that the extraction efficiency of lithium ions and Li/Mg separation factor were 65%and 350,respectively.No appearance of the third phase was observed in the entire process flow.HCl+MgCl2 and HCl+NaCl were used as washing and stripping agents,respectively.Effects of washing and stripping conditions on washing and stripping performances were investigated too.Mg(OH)2 and MgCO3 were used as regeneration agents to regenerate the organic phase,and the regenerated organic phase was used for extraction again.The extraction efficiency of lithium ions was maintained at about 53%during repeating the whole extraction process for ten times,demonstrating that the selected extraction system had good stability and reusability.The residual MgC12 in the raffinate was used as a chlorine source in the new technological process,achieving the purpose of maximizing the use of resources.In order to solve the co-extraction reagent problem of TBP-FeCl3 extraction system,1-methylimidazole silicotungstate ionic liquid([Mmim]4SiWi2O40)formed by reaction between 1-methylimidazole(1-Mmim)and silicotungstic acid(H4SiWi2O40)was used as co-extraction reagent to replace FeCl3 for co-extraction of lithium ions.TBP and selected heptanolide were used as extractant and diluent to investigate the extraction conditions.The results showed that the extraction efficiency of lithium ions,Li/Mg and Li/K separation facors were approximately 60%,40 and 190,respectively.Then two different processes were used in the washing,stripping and regeneration processes.In the first process,HCl was used as stripping reagent,and the auqeous solution after stripping contained only HCl and LiCl.Then part of the aqueous solution was used as washing reagents to wash off the Mg2+,Na+and K+extracted to the organic phase in the extraction process and good washing results were obtianed.The innovation in this process was that only HCl needed to be added to the system with no addition of other reagents,which greatly reduced production costs.In the other process,NaCl+LiCl was used as the washing reagents and Na2CO3 was used as both the stripping and regeneration reagents,shortening the whole process with no addiiton of strong acids and alkalis,which was more environmentally friendly.There were no significant decrese of the extraction efficiencies of these two extraction systems after repeating the whole extraction processes for ten times,indicating that both the selected extraction systems were stable and suitable for recovering of lithium ions from all types of aqueous solution containing lithium resources.