Cloud point extraction of platinum group elements and gold: elimination of nitric acid-related problems with sulphamic acid

Abstract

Microwave-assisted cloud point extraction (CPE) of platinum group elements (PGEs) and Au in solutions containing aqua regia using 2-mercaptobenzothiazole (2-MBT) was studied. For comparison, the extractions were also performed in hydrochloric acid and nitric acid solutions. A previously published CPE method, employing 2-MBT in hydrochloric acid solutions, was used as a reference method in order to investigate the effect of added nitric acid on the recoveries of Ru, Rh, Pd, Ir, Pt and Au. The results indicated that the presence of nitric acid did not cause problems to the cloud point phenomenon, but had an effect on the complexation processes of 2-MBT, thus leading to low recoveries. Therefore, a novel approach of adding sulphamic acid to enhance the extraction efficiencies of the PGEs and Au in the CPE performed in solutions containing aqua regia was investigated. A clear increase in the recoveries of PGEs and Au in solutions containing aqua regia or nitric acid was observed when sulphamic acid was used. The recoveries for Ru, Rh, Pd, Pt and Au obtained in optimized conditions were between 80 and 100%. However, the recoveries of Ir remained below 40%. The accuracy of the developed method was evaluated by analysing a certified reference material NIST SRM 2557 (used automotive catalyst).