Pressure oxidative leaching of copper cake from nickel refining plant in sulfuric acid media

Manufacturing of nickel by hydrometallurgical technology at Harjavalta plant (Finland) had been caring out since 1960 by Outokumpu. Later on, the technology was modernized and currently Harjavalta plant belongs to PJSC “MMC Nornickel”. The plant is specializing on processing of copper-nickel mattes, converter mattes and by-products, originated from “Kola MMC” and third-party enterprises. Currently, the technology for the materials processing includes oxidative autoclave leaching (POX), solvent extraction, nickel electrowinning as well as solutions purification and metal salts obtaining stages. Selective POX produces nickel solution and copper cake (Cu-cake), which is a valuable by-product of the plant. The article presents the results on investigating the possibility of Cu-cake processing using POX in sulfate media at different temperature regimes. Low-temperature POX (105 oC) allowed to extract more than 95% Cu into solution within 30-60 min, while the dominating compound of solid residue was elemental sulfur; mass output of the solid residue was 20-25%. High-temperature POX (190 oC) allowed to extract more than 98% Cu in 60-80 min; mass output of the solid residue was 2.5-5%, which significantly increased a concentration of precious metals. Mid-temperature POX (150 oC) is characterized by increased duration. Copper extraction level of 95-98% is achieved at least after 120-180 minutes of leaching. Mass output of the solid residue was 6-10%. A detailed analysis of Cu-cake and the resulting products is provided. The behavior of precious metals was considered.