The oxidation of violarite

A sample of violarite was synthesized from pure components and characterized. Samples of particle size 45–75 μm were oxidized in a TG-DTA apparatus at a heating rate of 10°C min ^‑1 and the products characterized at various temperatures by X-ray diffraction (XRD), backscattered electron (BSE) images obtained on a Scanning Electron Microscope (SEM), Fourier transform infrared (FT-IR) spectroscopy and electronprobe microanalysis (EPMA). Only minor sulfation reactions occurred up to 405°C, but above this temperature the violarite decomposed to form a monosulfide solid solution (mss, (Fe,Ni) _{1 ‑ x} S) and sulfur. The first reaction produced a mass loss, and oxidation of the evolved sulfur produced an exotherm. From 470°C, there was a mass gain due to the continued formation of sulfate species and an exotherm caused by the conversion of iron sulfide to hematite. From 585°C, a mass loss occurred as the iron sulfates decomposed. At 670°C decomposition of the mss took place, with the formation of heazlewoodite, (Ni, Fe) _{3 ± x} S ₂, and sulfur. The first reaction produced a mass loss, and oxidation of the evolved sulfur gave an exotherm. Trevorite, (Fe, Ni) ₃O ₄, was also formed above this temperature, from the oxidation of either mss or heazlewoodite, and this reaction also contributed to the mass loss. Above 725°C a mass loss and sulfur dioxide evolution was associated with the decomposition of nickel sulfate. This was followed by an endotherm at 785°C caused by the melting of the heazlewoodite, which immediately oxidized to produce a mass loss and exotherm. The final product above 860°C contained trevorite, nickel oxide and hematite. Extensive migration of iron occurred during the oxidation process.