Dissolution of minor sulphides present in a pyritic sludge at pH 3 and 25º C

The steady-state dissolution rates of galena, sphalerite and chalcopyrite at pH 3 under oxygen saturated atmosphere and at 25ºC are obtained by means of non-stirred flow-through experiments. These dissolution rates are compared with those estimated by dissolving pyritic sludge from the Aznalcollar mining tailings composed of pyrite and minor sulphides galena, sphalerite and chalcopyrite.Based on the respective release of Fe, Pb, Zn and Cu, the steady-state dissolution rates of pyrite (RateFe), galena (RatePb), sphalerite (RateZn) and chalcopyrite (RateCu) are 6.33 ± 0.95 x 10-11, 1.2 ± 0.18x10-10, 1.3 ± 0.20x10-11 and 1.71 ± 0.25x10-11 mol m-2 s-1, respectively, yielding RatePb > RateFe > RateZn = RateCu. Based on the release of metal and sulphur to solution, the stoichiometric ratios Pb/S, Zn/S and Cu/S are 4 ± 0.25, 1.2 ± 0.1 and 0.90 ± 0.05 for the respective dissolution reactions of galena, sphalerite and chalcopyrite, which are higher than the ideal ones. These high values result from a sulphur deficit in the output solutions attributed to the loss of H2S(aq) via gasification by which H2S(aq) partially converts to H2S(g). Nevertheless, the Cu/Fe ratio is 0.95 ± 0.05 during chalcopyrite dissolution at steady state, suggesting that chalcopyrite dissolves stoichiometrically.