Sustainable recovery of valuable metals from high-grade nickel matte residue via sulfur dioxide pressure leaching and freeze crystallization

Abstract In the hydrometallurgical processing of high-grade nickel matte, oxygen pressure sulfuric acid leaching produces substantial residues containing hematite precipitates. In this study, high-grade nickel matte oxygen pressure leaching residue, generated during the hydrometallurgical processing of high-grade nickel, was used as the research subject. The investigation focused on the effects of various factors in the reduction leaching process on the behavior of copper (Cu) and iron (Fe), as well as on the enrichment of precious metals. A comprehensive recovery process was designed for Cu and Fe recovery. Under the conditions of an initial sulfuric acid (H 2 SO 4 ) concentration of 100 g/L, a liquid–solid ratio of 6 mL/g, a reaction time of 3 h, a temperature of 90 °C, and a sulfur dioxide (SO 2 ) partial pressure of 0.15 MPa, high leaching efficiencies were achieved: 99.35 % for Fe and 77.46 % for Cu. Concurrently, silver content was enriched from 1,200.1 g/t to 36273.5 g/t (a 30.2-fold increase). Notably, for Cu recovery, when the H 2 SO 4 concentration was 20 g/L–30 g/L at 70 °C, with 5.7 g/L of Fe powder and a reaction time of 40 min, displacement precipitation achieved 99.70 % Cu recovery, with the precipitate containing 67.91 % Cu. Subsequently, freeze crystallization of the post-Cu solution at −10 °C, with a holding time of 30 min and an initial H 2 SO 4 concentration of 150 g/L, yielded ferrous sulfate heptahydrate with 72.6 % Fe precipitation and > 99 % purity. The crystallization mother liquor was recycled into the reduction leaching process for recovery. This integrated process exhibited high metal recovery efficiency, significant precious metal enrichment, and zero wastewater discharge, representing an environmentally friendly and sustainable approach to nickel matte residue processing.