ARSENIC REMOVAL FROM METAL ARSENIDES BY SULFIDIZATION

The kinetics of the reaction between Co2As, CoAs, and Cu3As alloys and a gaseous sulphur atmosphere were studied in the temperature range 87 3-1173 K. Sulphur partial pressures ranged from 0.05 to 0.2 atm and we re generated by saturating an argon carrier gas with sulphur vapour. C obalt arsenide samples were radially sectioned and analysed topographi cally by optical and scanning electron microscopy with the results int erpreted in terms of a shrinking core model. The rate determing step w as found to change from chemical to diffusion control through the coba lt sulphide product layer. The rate of arsenic removal was found to in crease with increasing temperature and sulphur partial pressure. The r ate of elimination of arsenic was largely under chemical reaction cont rol, with diffusion becoming important at high P(S)x, possibly due to the diffusion and desorption of the As-S product species. Reaction kin etics for the conversion of Cu3As samples to sulphides were rapid and found to increase with increasing temperature and decreasing sulphur p artial pressure.