Treatment of soda ash chromite ore processing residue by Waste-Molasses-Based Ball Milling: A new strategy for disposal of waste with waste

Abstract

The current technology of soda ash Chromium Ore Processing Residue (COPR) detoxification consumes a large amount of acids (as in the wet reduction process) and requires high temperature, along with the generation of carbon dioxide (as in the dry reduction process), and thus, is not cost-effective and environmentally-friendly. Waste molasses (WM) is waste biomass consisting of various reducing sugars, a cost-effective reducing agent. According to the idea of treating waste with waste, this study put forward a ball milling + WM method to detoxify leached-soda ash COPR (e.g., dried COPR containing 1622 ± 28 Cr(VI)/kg). Through response surface experiments, it was determined that under the best conditions (i.e., the milling speed = 600 rpm, the milling time = 2 h, WM dosage = 7.5% of the dry mass of COPR, and the ball-to-powder weight ratio, BPR = 12), the reduction efficiency of Cr(VI) reached 98.35%. The Toxicity Characteristic Leaching Procedure (TCLP) total Cr concentration reached 1.82 mg/L, meeting the standard (total Cr < 5 mg/L) of the US Environmental Protection Agency (USEPA). The associated reaction mechanism was determined as: under the action of mechanical energy, the structure of Cr(VI)-embedded matrixes was destroyed, and the internal, otherwise non-exchangeable, Cr(VI) was exposed and underwent redox reactions with WM under alkaline conditions. In brief, WM-based ball milling to detoxify COPR is cost-effective and provides a new reference for the disposal of waste with waste.