A specially designed mill which allowed the control of pH throughout grinding was used to study the effect of grinding conditions on chalcopyrite flotation and chalcopyrite separation from pyrite. The mechanism of galvanic interaction between minerals and grinding media was investigated by ethylene diamine-tetra acetic acid disodium salt (EDTA) extraction and X-ray photoelectron spectroscopy (XPS) measurements. Chalcopyrite flotation was strongly dependent on both iron oxidation species and metal deficiency on the chalcopyrite surface. Iron oxidation species from grinding media played a dominant role in depressing chalcopyrite flotation, while metal deficiency from chalcopyrite oxidation improved chalcopyrite flotation. Therefore, chromium grinding medium produced a higher chalcopyrite recovery than mild steel grinding medium while gas purging during grinding had little effect on chalcopyrite flotation. Chalcopyrite separation from pyrite was affected by the activation of pyrite flotation by copper species dissolved from chalcopyrite. Grinding media had a large effect on the reduction of copper(II) to copper(I) on the pyrite surface. The reducing grinding condition generated by mild steel medium favoured formation of copper(I) sulphide phase, which resulted in high pyrite activation. Thus, chromium medium produced better chalcopyrite selectivity against pyrite than the mild steel medium. (C) 2003 Elsevier Science B.V. All rights reserved.