Precipitation of Ru, Rh and Ir with iron ions from synthetic nickel sulphate leach solutions

Abstract

The nickel sulphate leach solution produced in the first leaching stage of base metal refineries contains impurities such as iron as well as other precious metals (OPMs, i.e. ruthenium, rhodium and iridium). Precipitation of the dissolved iron in downstream unit operations leads to sludge formation. Some of the other precious metals have been reported to co-precipitate in the sludge; this could be used as a mechanism for OPM recovery. The objective of this study was to investigate the precipitation behaviour of OPMs during iron sludge formation from the nickel sulphate leach solution.Precipitation tests were performed on synthetic leach solutions to investigate the effect of solution pH, temperature, metal content, iron valence state and seeding on the iron and OPM precipitation behaviour. Typical iron precipitation via hydrolysis of ferric ions was observed. The primary mechanism of OPM precipitation also involved hydrolysis reactions. The solution pH had the most significant effect on OPM precipitation. Average ruthenium precipitation and rhodium precipitation of 98.1% and 92.2%, respectively, were achieved at pH4; the iron valence state and total metal concentration had no significant effect on the OPM precipitation. More basic conditions were required for thermodynamically stable iridium species to precipitate than for ruthenium and rhodium. Seeding did not enhance OPM scavenging from the leach solution.Tests performed on plant solution confirmed that OPM precipitation by neutralisation is possible. At high nickel concentrations, nickel sulphate crystallisation contributed to sludge formation and resulted in significant iridium co-precipitation due to entrainment.