Arsenic removal from industrial effluent: In-situ ferric sulfate production and arsenic partitioning in the residues

Abstract

Ferric sulfate solution was producedin situby sulfating arsenic-bearing, hematite-rich calcines (56–62% Fe2O3) with sulfuric acid, both produced by roasting refractory gold ore. The solution was then used for arsenic precipitation in the treatment of effluents from the cyanidation plant. The conversion of hematite to ferric sulfate increased as the acid/calcine ratio increased and reached 60.7%, with 0.87 of the stoichiometric acid amounts. An iron sulfate solution (13.4 g.L−1 Fe and 2.32 g.L−1 As) was produced at the end of the calcine sulfation/dissolution. Arsenic removal varied between 99.4 and 99.8%. Arsenic concentration dropped from 1583.4 mg.L−1 to 5.31 mg.L−1 As, which is far less than the industrial circuit's typical concentration (15 mg.L−1). Hydrated calcium sulfates were the principal constituents in the arsenical residues (i.e., 42–50%). Its removal allowed the identification of ferric arsenate/arsenite coprecipitates, arsenical ferrihydrite, and hydrated calcium-iron hydroxy-arsenate and hydrated calcium using X-ray diffraction analyses and Raman spectroscopy. The sequential extraction protocol and microscope automatic system confirmed the arsenic carriers. These are ferric (aluminum) or calcium arsenate/arsenites, accounting for 71.2 and 72.8% total As, and minor arsenic adsorbed on iron oxyhydroxides. Arsenic was primarily found (63.3%) in the Fe,Al-AsOx coprecipitates in the industrial residue. A 17% As-richer sludge, as achieved here (4.18% As), would result in a lower sludge volume for disposal. Transportation and disposal costs would then drop. These benefits were confirmed along one year of industrial operation. The projected reduction in effluent treatment costs reaches 60%.