Characterization and leaching options of a Zn and Ni-containing secondary sulfide

A sulfidic secondary raw material rich in Ni (millerite) and Zn (sphalerite) as well as mixed Ni-Fe-S sulfides was characterized and investigated for its leaching behavior under atmospheric direct acid leaching system utilizing air or oxygen, H₂SO₄, temperature ranges, and gas flow rates. The goal was to investigate extractability of the main elements (Ni, 15 wt% and Zn, 16 wt%) in the raw material whether both Ni and Zn could be extract simultaneously, or whether the material is more amenable to selective leaching, impacting the subsequent process options. Batch leaching experiments were performed in a 1 L and 0.5 L systems in two complementary leaching series. Ni was not appreciably extracted apart from the Ni sulfates present in the raw material apart from the experiments utilizing O₂(g). The Ni dissolution did not readily occur until Zn extraction was finished and solution oxidation–reduction potential (ORP) increased to ca. 450 mV (vs. Ag/AgCl), and only reached ca. 50 % extraction at t = 24 h in T=90 °C and pH=1. The leach residue Zn content closely followed the ORP, achieving a minimum (0.1 wt% Zn) at E=471 mV (vs. Ag/AgCl). The most rapid leaching kinetics for ZnS was with excess acid, followed by experiments with O₂(g). Decent kinetics were observed in response surface design of experiments (T=30, 50, 70 °C; m_(H2SO4)/m_{(Raw material)} = 0.6, 0.9, 1.2 g/g), allowing Zn extraction in < 6h. as it was possible to produce upgraded NiS (20 wt%) while extracting most of the Zn (99 %) under several experimental conditions, ranging from high T (90 °C) and limited constant acidity (pH 1) to low temperature (T=50 °C) and high excess acidity. It was shown that it is possible in an atmospheric process to selectively extract Zn while keeping majority of Ni in the leach residue. Selective extraction of Zn allows separation of Ni and Zn into separate materials feeds to be subsequently processed: Zn stream in a Zn process, and Ni-containing residue in a Ni smelter.