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 generation from sulphidic wastes containing 1–10% S. In the present paper, the effectiveness of small amounts, lower than the stoichiometric requirement, of this alkaline additive in inhibiting acid generation from a pyrite concentrate material is studied. Long term laboratory column tests were conducted on a partially oxidised pyrite concentrate, where limestone was added by thoroughly mixing. The amount of alkaline additive ranged from 6.4 to 29% wt. corresponding to 5–30% of the stoichiometric quantity. The performance of the pyrite-limestone mixtures was evaluated by monitoring the drainage quality of the columns. Furthermore, a detailed geochemical characterisation of the column solid residues was performed after a monitoring period of 270 days. The effect of secondary oxidation-neutralisation products on the hydraulic conductivity of material was also examined. Dissolution of previously formed oxidation products occurred in the control column during the monitoring 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 secondary products, further oxidation of pyrite particles was delayed. The experimental results showed that homogeneous mixing of pyrite with limestone amounting to only a fraction of the contained acidity inhibited the generation of acidic drainage and significantly reduced the dissolved amount of metals and sulphates for a test period of 270 days. Under the alkaline conditions 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, corresponding to 15% of the contained acidity.