One-step synthesis of heteropolyacid ionic liquid as co-extraction agent for recovery of lithium from aqueous solution with high magnesium/lithium ratio

The novel heteropolyacid ionic liquid 1-methylimidazolium silicotungstate ([Mmim]4SiW12O40) was synthesized using a one-step method and subsequently characterized. This compound was then employed as a co-extractant for the removal of lithium from salt lake brine having a high Mg/Li molar ratio, in conjunction with tributyl phosphate (TBP) and γ-heptanolide as the extractant and diluent, respectively. The optimal extraction conditions gave a single-stage lithium ion extraction efficiency of approximately 60 %. More environmentally friendly and efficient processes were also developed. In one approach, a portion of the aqueous phase containing only HCl and LiCl after stripping was used for washing along with an aqueous HCl solution as the stripping agent and Mg(OH)2 for regeneration. In the alternative method, NaCl and LiCl were used as washing agents while Na2CO3 was employed for stripping and regeneration, reducing the time required without the need for strong acids or bases. This system showed good extraction efficiency stability over 10 consecutive cycles. The mechanism of this process was explored using Fourier transform infrared spectroscopy and 31P nuclear magnetic resonance spectroscopy and the binding capacity of TBP for the metals was found to decrease in the order of Li+ > Na+ > Mg2+ > K+.