A New Method (Ball Milling and Sodium Sulfide) for Mechanochemical Treatment of Soda Ash Chromite Ore Processing Residue

Hexavalent chromium in soda ash Chromite Ore Processing Residue (COPR) is harmful to the environment, it is imperative to develop a low cost, efficient, and effective treatment. Herein, a new method (ball milling + sodium sulfide) was developed for mechanochemical treatment of water-leached COPR (W-COPR, about 900 mg/kg non-exchangeble Cr(VI) and mostly chromite bounded). Under a stoichiometric ratio of S2- to Cr(VI) of 5, milling speed of 200 rpm, milling time of 30 min, ball-to-powder weight ratios of 8.5, and, a total Cr(VI) of 76.88 mg/kg and a Toxicity Characteristic Leaching Procedure (TCLP) total Cr value of 1.15 mg/L were achieved after treatment. Results of the mineral liberation analyser (MLA) analyses showed 10% increase of chromite grains liberation and grain size reduction were beneficial to the chromite-bound Cr(VI) reduction. Similar effects were also observed on magnesioferrite encapsulated Cr(VI). Particle aggregation and formation of glass phase colloid precipitation could potentially impede Cr(VI) reduction. Results of X-ray absorption near edge structure (XANES) analyses indicated that the treatment method reduced the actual Cr(VI) concentration to 312 mg/kg with Cr(VI) reduction efficiency of 98% being achieved. Overall, the new method is simple and efficient, and provides a guidance for future industrial application.