Mitigation of chromite ore processing residue hazards by reaction with calcium polysulfide, ferrous sulfate and ferrous chloride

Chromite Ore Processing Residue (COPR) is an industrial refuse that was used as fill material at various sites in the US and elsewhere. It was produced during the extraction of chromium from its ore using the high lime process. COPR is highly alkaline and it contains unreacted chromite ore and un-extracted chromate where chromium concentrations are present at up to 5% including hexavalent chromium at up to 2%. Most remediation methods of Cr(VI) contaminated soils involve the chemical reduction of Cr(VI) to Cr(III) and adjustment of pH to precipitate Cr(III) as Cr(OH)3. The remediation of the COPR is complicated by the high alkaline content of the COPR matrix, the slow release of chromium and alkalinity from the COPR minerals, and the potential swell of COPR pre- and post-treatment. Chromium in the COPR material may be sequestered in small nodules formed during the clinker formation process. The reaction between chromium and the chemical reagents is inhibited by the slow release of chromium and alkalinity bound in the COPR minerals. In addition, the delayed alkalinity release may prove detrimental to the remediation process due to the potential for future swell of the treated COPR material. In this work, the effects of particle size reduction and acid addition on the remediation of COPR using three reducing agents, calcium polysulfide, ferrous sulfate and ferrous chloride were investigated with a view to mitigate COPR hazards. Geochemical modeling showed that the COPR system is not at equilibrium, and hydration reactions are still ongoing some 80 years after deposition. Regulatory analytical methods, spectroscopic methods, diffractomerty, and microscopy methods were used to assess the treatment results of COPR. Regulatory methods were shown not suitable for the quantification of Cr(VI) in COPR. Particle size reduction concomitant with acid addition were shown efficient at destabilizing recalcitrant COPR minerals making Cr(VI) reaction with the chemical reductants feasible. The use of sulfate based reductant resulted in the formation of the expansive ettringte mineral. Conversely, ettringite was absent in the non sulfate based reductant treated COPR.