Modulating the carbon chain length of organic phosphine for the selective Au(I) recovery from a copper-ammonia-thiosulfate leaching solution

The selective recovery of gold ions from gold leaching solutions limits the industrial application of the copper-ammonia (Cu 2+ –NH 3 )-thiosulfate system. Here, the influence of the carbon chain length of organic phosphines on the selective recovery of gold ions was studied, and an extractant (trihexylphosphine, THP) that is suitable for the selective recovery of gold ions from a Cu 2+ –NH 3 -thiosulfate leaching solution was screened. The selectivity of organic phosphines with different carbon chain lengths for gold ions was C 8  = C 6  > C 4  > C 3  > C 2 , and when the number of carbon atoms in a single carbon chain was at least six, the selective recovery of gold ions from the Cu 2+ –NH 3 -thiosulfate solution was realized. THP completely recovered low concentration gold ions (6.73 mg/L) in the Cu 2+ –NH 3 -thiosulfate gold leaching solution regardless of the pH. The mechanism of gold ion extraction into the organic phase mainly involved ligand exchange between THP and [Au(S 2 O 3 ) 2 ] 3− . Job’s method and density functional theory calculations confirmed that the composition ratio of the complex formed by THP and Au was 2:1 at the practical and theoretical levels, respectively. Finally, Multiwfn and VMD were used to visually predict the possible reaction sites in THP. This study provides an exact method for the selective recovery of gold ions from thiosulfate-based gold leaching solutions, which will promote the development of thiosulfate-based gold leaching systems.