Platinum Group Metals (PGM) are usually recovered from pyrometallurgically concentrated and refined PGM concentrate. PGM concentrate is dissolved with highly concentrated hydrochloric acid with oxidants. Therefore PGM form the maximum coordinated chloro-complex, such as [MCl6]2-, [MCl6]3- (M: PGM) and they are easily extracted with some extractants1) or adsorbed with some ion exchange resins2) except for rhodium.
However, concentration of chloride ion in leach solution of copper anode slime with chlorine gas is kept relative low level, around 3kmol/m3, to prevent dissolution of harmful impurities (ex: antimony, bismuth, lead, and tin). Therefore some parts of PGM exist as aquo- (or hydroxo-) chloro-complex, such as [MClx (H2O) 6-x]2-, [MClx (H2O) 6-x]3-. Since aquo chloro-complexes are difficult to extract or adsorb, co-extractant with anion exchange extractant is tested. Addition of TBP is successfully performs on extraction of PGM ions by synergic effect. Even chloro-complexes of rhodium which are not extracted with the conventional extractants can be extracted to the mixture of TOMAC and TBP.
Since both PGM aquo-chloro complexes and are fully stable in the room temperature, it is important to change the all complexes to maximum coordinated chloro-complexes in chlorine leach step of copper anode slime with high concentrated chloride ion (≥7.3kmol/m3). Chloride ion concentration under low temperature (25°C), ORP, pH, both temperature and retention time under low concentrated chloride ion (≤2.8kmol/m3) do not affect the distribution coefficient of PGM.
Under optimum condition, the extraction rates of iridium, ruthenium, rhodium are 92, 95, 85 (%) respectively with TOMAC and TBP (1:1) in five stages of extraction.
