Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone

Limestone addition is a commonly applied technique to prevent acid generati on from sulphidic wastes containing 1-10% S. In the present paper, the effe ctiveness of small amounts, lower than the stoichiometric requirement, of t his alkaline additive in inhibiting acid generation from a pyrite concentra te material is studied. Long term laboratory column tests were conducted on a partially oxidised pyrite concentrate, where limestone was added by thor oughly mixing. The amount of alkaline additive ranged front 6.4 to 29% wt. corresponding to 5-30% of the stoichiometric quantity. The performance of t he pyrite-limestone mixtures was evaluated by monitoring the drainage quali ty of the columns, Furthermore, a detailed geochemical characterisation of the column solid residues was performed after a monitoring period of 270 da ys. The effect of secondary oxidation-neutralisation products on the hydrau lic conductivity of material was also examined. Dissolution of previously f ormed oxidation products occurred in the control column during the monitori ng period, resulting in the release of a significant amount of Fe, Zn, Mn, Cd, As and SO4 and to a lesser extent Pb. However, due to the presence of s econdary products, further oxidation of pyrite particles was delayed, The e xperimental results slowed that homogeneous mixing of pyrite with limestone amounting to only a fraction of the contained acidity inhibited the genera tion of acidic drainage and significantly reduced the dissolved amount of m etals and sulphates for a test period of 270 days. Under the alkaline condi tions prevailing in the limestone amended columns, secondary precipitation of ferric hydroxides and gypsum occurred. A ten-fold decrease of hydraulic conductivity was observed for the material amended with 207 kg CaCO3/t, cor responding to 15% of the contained acidity. (C) 2000 Elsevier Science Ltd. All rights reserved.