Thermal oxidation of covellite (CuS)

The thermal oxidation of covellite of particle size 45-90 mum war; studied by heating 5-6 mg samples at 20 degreesC min(-1) in dry air in a simultaneous thermogravimetry-differential thermal analysis (TG-DTA) apparatus. Evolved gases were analysed by coupled FTIR equipment. The unreacted and the partially oxidised covellite samples were characterised for phase composition by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and optical microscopy (OM). By 330 degreesC, a small amount of decomposition of covellite had occurred with the formation of cubic digenite (Cu1.8S). A mass loss was evident between 330 and 422 degreesC associated with an exothermic peak. This event was assigned to the decomposition of CuS to Cu2S followed by oxidation of the evolved sulfur. Between 422 and 474 degreesC, an exothermic peak accompanied by a mass gain was observed. The exothermic peak was attributed to the oxidation of Cu2S, and the mass gain to the formation of CuSO4. This was followed by a further exothermic peak and mass gain in the temperature range 474-585 degreesC. This event was attributed to a solid-solid reaction between Cu2S and CuSO4 which caused a melt to form. Until this stage, the reaction had proceeded by a shrinking core mechanism inhibited by oxide and sulfate coatings. However, the formation of a melt effectively destroyed this inhibition and permitted further oxidation of the sulfide and formation of sulfate. At 583 degreesC CuOCuSO4 was detected, and continued to form up to 653 degreesC. Finally an endothermic event and mass loss observed in the temperature range 653-820 degreesC were due to the decomposition of CuO . CuSO4 into CuO. Various other reactions were identified which did not produce significant events on the TG-DTA record. (C) 2001 Elsevier Science B.V. All rights reserved.