Treatment of Cr(VI) in COPR Using Ferrous Sulfate–Sulfuric Acid or Cationic Polysulfides

Abstract

Column tests were conducted to evaluate two treatment strategies for reducing and stabilizing hexavalent chromium, Cr(VI), in chromium ore processing residue (COPR): permeation with a FeSO4–H2SO4 solution and blending with a cationic polysulfide reagent (CaSX). Cr(VI) leached at concentrations exceeding 50mg∕L from untreated COPR permeated with synthetic groundwater for >20 pore volumes of flow (PVF), and concentrations of Cr(VI) in the solid phase remained high (6,600mg∕kg). Permeation with solutions containing FeSO4–H2SO4 eliminated Cr(VI) from the effluent after initial, elevated leaching of Cr(VI) (100–1,500mg∕kg); however, high solid-phase concentrations of Cr(VI) remained in the column residuals (>1,300mg∕kg). COPR treated with CaSX leached Cr at <0.33mg∕L for 23.5 PVF and had solid-phase concentrations of Cr(VI) <10mg∕kg, although mineralogical analyses of treated solids showed potential chromate-containing mineral phases. Mineralogical analyses showed that precipitation and cementation occurred in the pore space of the COPR permeated with FeSO4–H2SO4, initially lowering the hydraulic conductivity > two orders of magnitude. However, acid dissolution channels eventually formed, resulting in preferential flow. COPR permeated with FeSO4–H2SO4 contained less brownmillerite and Cr(VI)-bearing hydrocalumite and hydrogarnet relative to untreated COPR. For COPR treated with CaSx, S encapsulated the subparticles of COPR with some micropore penetration, suggesting permanence of excess reductant after leaching with 23.5 PVF of synthetic rainwater.