PASSIVATION OF CHALCOPYRITE DURING OXIDATIVE LEACHING IN SULFATE MEDIA

Sulfate-based leaching processes for chalcopyrite (CuFeS2) are attractive because of their inherent simplicity, Unfortunately, high copper extractions are not attainable in a reasonable residence time unless the leaching temperature exceeds 200 degrees C (oxygen pressure leaching) or chalcopyrite is 'activated' by a pretreatment method prior to leaching. In the present work, the oxygen pressure leaching behaviour of chalcopyrite in the temperature range 110-220 degrees C was studied to shed new light on the reasons for the slow leaching. Mineral surfaces were examined by Auger electron spectroscopy and X-ray photoelectron spectroscopy to identify the presence of any passivating layers that may form during leaching. The results suggest that chalcopyrite is passivated by a thin (<1 mu m) copper-rich surface layer which forms as a result of solid state changes that occur in the mineral during leaching. This layer is thought to be a copper polysulfide, CuSn, where n>2, The leaching kinetics at low temperature (110 degrees C) can be explained in terms of a mixed diffusion/chemical reaction model where the reaction rate is ultimately controlled by the rate at which the copper polysulfide leaches.