Nickel sulphide concentrate processing via low-temperature calcination with sodium chloride: Part 2 – Chemistry and mechanism of interaction

Abstract

Abstract The low-temperature calcination of nickel sulphide concentrates with NaCl (at 400–450\u202f°C) and subsequent hydrometallurgical processing provide a promising alternative to existing nickel sulphide concentrate processing technologies. The advantages of this processing approach arise from the formation of water-soluble compounds of the metals extracted as well as Na2SO4 generation during calcination, which contributes to significant (up to 75%) reduction in SO2 emission. The influences of the calcination conditions on the recoveries of Ni, Cu, and Fe from the calcine via water leaching are investigated. The optimal conditions for the samples investigated are as follows: calcination temperature of 400\u202f°C, NaCl content of 50–100%, and calcination duration of 90\u202fmin. Based on these results as well as the data published in Part 1 of this study (thermodynamic, X-ray diffraction, and scanning electron microscopic analyses of the interaction products), the chemistry and mechanism of the calcination of the nickel sulphide concentrate with NaCl are suggested. The proposed mechanism entails the oxidation of sulphides to sulphates (at 325–400\u202f°C) followed by partial decomposition of the latter at higher temperatures (400–450\u202f°C). During calcination, NaCl interacts with the SO2 emitted owing to sulphide oxidation. This interaction further promotes sulphide oxidation at 350–450\u202f°C via the consumption of the product of the reaction, i.e., SO2. Additionally, owing to the removal of SO2 from the system, the interaction of NaCl with SO2 decreases the partial pressure of S to values at which Ni remains in the sulphate form, whereas Fe has already transitioned to the oxide form. Calcine leaching provides the opportunity to extract up to 95% of Cu and Ni using water while simultaneously leaving a significant part of Fe in the cake, and this is very important from a technological perspective.