Simple gold recovery from e-waste leachate by selective precipitation using a quaternary ammonium salt

Precipitation processes, if selective, present a simple and economical alternative for the recovery of critical metals from primary and secondary ores including electronic wastes. In this work, the recovery of gold by precipitation from both mono-elemental solutions and real CPU leach solution was demonstrated using hydro-philic quaternary ammonium salts. The gold precipitation yield is shown to be dependent on the apolar volume of the precipitant, with the addition of tetrabutylammonium-based salts resulting in the recovery of over 90 % of gold from synthetic solutions. The origin of gold precipitation selectivity relative to common metal ions upon addition of tetrabutylammonium nitrate ([N4444][NO3]) was assigned by X-ray crystal structure to the formation of size selective apolar cavity between neighbouring [N4444]+ cation and the [AuCl4]- anion. Following opti-misation as a function of the gold to precipitant molar ratio, aqua regia concentration and time, approximately 70 % of gold could be recovered from waste CPU leach solution with a final purity of 91.4 % (mol/mol). [N4444] [NO3] proved to be a versatile gold extractant and could be further applied as part of an acidic aqueous biphasic system at higher aqua regia concentrations were precipitation yields decreased, ensuring a selective gold recovery across a range of leachate conditions. The disclosed results improve the circularity of gold by providing a new avenue for its simple recycling.