Thermal oxidation of covellite (CuS)

The thermal oxidation of covellite of particle size 45–90 μm was studied by heating 5–6 mg samples at 20°C 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°C, 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°C 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°C, 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°C. 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°C CuO·CuSO4 was detected, and continued to form up to 653°C. Finally an endothermic event and mass loss observed in the temperature range 653–820°C 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.