Kinetics of dissolution of synthetic heazlewoodite (Ni 3S 2) in nitric acid solutions

Abstract

The dissolution rate of heazlewoodite in nitric acid solution has been determined. The effects of nitric acid concentration, temperature, particle size, stirring intensity and addition of Cu 2+ ions have been investigated. Solid residues after leaching were examined by SEM, X-ray diffraction and chemical analysis. In the solutions containing less than 2.0 M HNO 3, dissolution was observed to be completely inhibited after 30 min leaching time, and the rate of hydrogen sulphide production was faster than its oxidation to S 0 and HSO 4 −. In 3 M HNO 3, an abrupt increase in dissolution rate of Ni 3S 2 was found. Two different regions of the dissolution of heazlewoodite were observed below and above 50°C. At temperatures below 50°C, the dissolution rate was very slow, even in 3.0 M HNO 3 solution, and H 2S gas was evolved. Above 50°C, the dissolution rate rapidly increased. Over the temperature interval 60–90°C in 3.0 M HNO 3 dissolution followed a linear rate law, and the activation energy was found to be 42.1 kJ mol −1. Most of the oxidized sulphide ion was found in the solution as sulphate. The leaching rate was independent of stirring speed. The rate-controlling step of the Ni 3S 2 dissolution is the oxidation of hydrogen sulphide to elemental sulphur or sulphate ions on the Ni 3S 2 surface. Addition of small amounts of Cu 2+ ions to the nitric acid acted as catalyst for the dissolution of Ni 3S 2. Bubbling air through the leach suspension increased the dissolution rate of Ni 3S 2 in solutions containing less than 2.0 M HNO 3.