Ionic Liquids-Assisted Solvent Extraction of Precious Metals from Chloride Solutions

Increasing applications of precious metals (PMs: Au, Ag and platinum group metals) and the growing demand is a matter of great concern for supply security. The hydrometallurgical route through chloride-based leaching has been considered as the most promising method to recover almost all the metals. However, separation of metal-chloro complexes formed during the leaching is difficult to achieve because of similar properties of these complexes. Among the methods developed to extract and separate the metal-chloro complexes, ionic liquid (IL)-based solvent extraction has been recognized as one of the most promising approaches. The ionic liquids possess unique characteristics of 'greener,' 'designer' and 'highly effective' solvents. This paper reviews extraction and separation of PM-chloro complexes by ionic liquids. Mechanisms and IL role in the process, along with merits and drawbacks of IL-assisted solvent extraction are discussed comprehensively. Effects of operational conditions, stripping, and regeneration of the loaded ILs and possible use of ionic liquids in other separation methods are also presented. A flow-sheet for separation of PM-chloro complexes has been developed as a part of basic process approach using ionic liquids. In addition, future directions for the development and application of IL-assisted solvent extraction in separation of precious metals are provided.