Selective leaching of ecotoxic metals from lime dosing plant metalliferous ochre using acid mine drainage and organic acids

Abstract

Lime dosing in a high-density sludge (LD-HDS) process is amongst the most widely applied acid mine drainage (AMD) treatment processes worldwide. It is effective and reliable, however, an intrinsic issue for metalliferous mining derived AMD is the formation of an ecotoxic metal(loid)-bearing (e.g. As, Pb, Ni, Cd and Sb) ochre waste product. Herein the efficacy and selectivity of AMD (pH 3.5; 1–500 g/L), citric and oxalic acid (0.01–1 M) for ecotoxic metal(loid) dissolution from such ochre has been investigated. When AMD is applied at a low solid-liquid ratio (namely 1 g/L) or citric and oxalic acids are at low concentrations (namely ≤0.1 M) such lixiviants are selective for certain ecotoxic metals which have been designated by BCR 4-step sequential extraction as “exchangeable” (namely: Sb, Cd, Cu and Ni) in preference to Fe. This suggests that they could potentially be integrated into the LD-HDS process in order to lower the ecotoxicity of the final ochre product and thereby valorise it for use in a wide range of commercial applications. Conversely when AMD is applied at a higher solid-liquid ratio (namely ≥100 g/L) ecotoxic metal precipitation occurred (due to the alkaline pH of the ochre). This suggests that the ochre could also be reused for AMD remediation.