The Formation of Silicate-Stabilized Passivating Layers on Pyrite for Reduced Acid Rock Drainage

Acid and metalliferous release occurring when sulfide (principally pyrite)-containing rock from mining activities and from natural environments is exposed to the elements is acknowledged as a major environmental problem. Acid rock drainage (ARD) management is both challenging and costly for operating and legacy mine sites. Current technological solutions are expensive and focused on treating ARD on release rather than preventing it at source. We describe here a viable, practical mechanism for reduced ARD through the formation of silicate-stabilized iron oxyhydroxide surface layers. Without silicate, oxidized pyrite particles form an overlayer of crystalline goethite or lepidocrocite with porous structure. With silicate addition, a smooth, continuous, coherent and apparently amorphous iron oxyhydroxide surface layer is observed, with consequent pyrite dissolution rates reduced by more than 90% at neutral pH. Silicate is structurally incorporated within this layer and inhibits the phase transformation from amorphous iron (oxy)hydroxide to goethite, resulting in pyrite surface passivation. This is confirmed by computational simulation, suggesting that silicate-doping of a pseudoamorphous iron oxyhydroxide (ferrihydrite structure) is thermodynamically more stable than the equivalent undoped structure. This mechanism and its controlling factors are described. As a consequence of the greatly reduced acid generation rate, neutralization from on-site available reactive silicate minerals may be used to maintain neutral pH, after initial limestone addition to achieve neutral pH, thus maintaining the integrity of these layers for effective ARD management.