Prevention of acid rock drainage formation through pyrite inhibition by silica coating

Passive treatment of acid rock drainage (ARD) is a sustainable approach to control ARD, with sulfide inhibition by silica being a promising alternative. In a small-scale column leaching, a total of four cells loaded with pyritic waste rock (11 wt% S) from an operating Cu mine in Sweden were kept in a climatic chamber at a controlled temperature and humidity. The waste rock was leached for 11 weeks before treatment using alkaline silicate solution was applied, without pH buffer and adjuster. One cell was left untreated, whereas the others were treated with silicate solution as a source of dissolved silica, with and without H₂O₂ pre-oxidation. The pH in silica-treated cells generated leachate with circumneutral pH until the end of the leaching cycle, whereas sulfide oxidation accelerated in the absence of treatment. Leachate quality in all Si-treated cells improved, as evidenced by the suppressed release of sulfur and other metals (e.g., Al, Fe, Cu, Co, Mn, and Ni). Upon treatment with a longer contact time, silica (SiO₂) layer developed on waste rock and inhibited pyrite. The layer remained stable upon extended exposure to air and water for up to 10 weeks after treatment. Despite forming a siliceous Fe-O phase, H₂O₂ pre-oxidation resulted in indirect oxidation of sulfides and other phases. With an excess of silicate solution and at alkaline pH, pyrite surfaces are devoid of coating and metal ions were mobilized. Finally, this study suggested that treatment of pyritic waste rock using silica can attenuate ARD formation and prevent metal leaching by pyrite inhibition and maintaining a circumneutral pH environment or both.