The oxidation process has been analysed from an equilibrium viewpoint in order to provide a theoretical basis for experimental observations. The published thermodynamic data for the Cu–S-0 system have been critically reviewed and regions of stability of the phases present have been calculated.
Single crystal specimens of synthetic cuprous sulphide were oxidised in a vertical tube furnace to determine the products formed under various conditions of temperature, time and partial pressures of oxygen, nitrogen and sulphur dioxide. The reaction products were identified by X-ray diffraction, using the powder method. The products observed experimentally could be predicted from the thermodynamic stability diagrams. It was found that CU₂O was formed as the primary oxidation product within the temperature range 200° to 1000°C. Any sulphates present in the "oxide" were produced by subsequent sulphation of the primary Cu₂0 phase. The presence of sulphate in the oxidation product was controlled largely by a balance between oxidation and decomposition reactions. Under optimum conditions, sulphates were found to be stable up to 750°C. Reduction reactions involving Cu₂S vapour have no marked effect up to 800°C, but above 850°C they influence the proportion of CuO present. Above 850°C, metallic Cu was formed by a direct oxidation mechanism from the Cu₂S.
A modified differential thermal analysis technique was used to measure the true reaction temperature of the specimen. The observed deviations from the furnace temperature were explained in terms of the exothermic and endothermic reactions taking place within the "oxide".
The overall kinetics of the oxidation process were investigated by means of metallographic, thermogravimetric and gas analysis techniques. The metallographic investigation provided the most reliable quantitative data for calculation of empirical rate equations and the results indicated a parabolic oxidation law was obeyed over the range of conditions investigated. The oxidation of Cu₂S within the temperature range 500° to 750°C and under partial pressures of 0.05 to 1.00 atmospheres of oxygen obeyed the relationship
K = A exp (- Q/RT)
where K is the parabolic rate constant, Q is the activation energy in K. cal, R the gas constant in cals deg-¹ mol-¹ and T the temperature in degrees absolute. Both A and Q decreased when the partial pressure of oxygen was increased beyond 0.21 atmospheres.
An activation energy of 31.4 ± 3 k cal per mole was obtained for the oxidation of Cu₂S in air over the temperature range 550° to 750°C. The thermogravimetric results showed an activation energy of 25.5 ± 3 k cal per mole for the initial weight losses associated with Cu₂0 formation, under similar conditions. The metallographic results enabled a detailed study to be made of the rate of growth of individual phases within the 'oxide" and the effect of experimental variables upon the composition of the "oxide."
The parabolic nature of the kinetic curves indicated a diffusion-controlled process. The experimental results suggested that for the oxidation of CU₂S in air, the inward diffusion of 0= ions within the Cu₂0 lattice was rate controlling over the temperature range 550° to 750°C. Above 830°C, the rate of formation of metallic Cu became the most important factor.