THERMAL-ANALYSIS OF CHALCOPYRITE ROASTING REACTIONS

Roasting of chalcopyrite and of the binary copper and iron sulphides covellite, chalcocite, pyrrhotite and pyrite has been the subject of many investigations. The addition of salts to such reactions is of practical interest because it lowers the decomposition temperature and leads to water-leachable sulphates and chlorides. The salt roasting of copper bearing sulphides is a very complex process involving numerous reaction steps which have not been adequately characterized. The aim of the present investigations was to study the reaction mechanism of sulphide roasting in more detail. The combination of thermo-analytical methods (Mettler Thermosystem TA 4000) and X-ray diffraction (XRD) proved to be very useful for such studies. Mixtures with different ratios of CuFeS2 (and also CuS and FeS1-x) and NaCl, KCl or NH4Cl were heated in an oxidizing atmosphere either continuously (heating rates from 0.2-10-degrees-C min-1) or stepwise isothermally. Pure chalcopyrite starts to oxidize at about 400-degrees-C. Addition of alkali chlorides to chalcopyrite lowers the temperature decomposition and oxidation quite drastically, especially in the case of NH4Cl and KCl. A strongly exothermic reaction with corresponding weight increase had already begun at about 200-degrees-C. Slow heating rates throughout the decomposition region (not faster than 0.2-degrees-C min-1), covered crucibles and addition of excess salt (preferably 4-5 moles of salt to 1 mole of chalcopyrite) are important for the completeness of the reaction in mixtures with NH4Cl.