Treatability of chromite ore processing waste by leaching.

Developing treatment and disposal strategies and health-based clean-up standards for chromium containing wastes continues to be an important environmental regulatory issue because of the opposing solubility and toxicity characteristics of chromium species under diverse environmental conditions. In this study, leaching characteristics of total Cr and Cr(VI) were investigated using laboratory column studies. The data obtained from the experimental studies were analysed to assess the treatability of chromite ore processing waste (COPW) by leaching and to identify the leaching strategies that enhance mass removal rates of chromium species. COPW used for laboratory soil column studies was obtained from an industrial plant producing sodium chromate in Mersin, Turkey. Laboratory investigations involved chemical characterisation of waste material and column studies. For waste characterisation, U.S. EPA toxicity characterisation leaching procedure (TCLP) was performed on COPW to determine the concentrations of metal species in the TCLP extract. For column studies, various laboratory columns containing plain COPW material, 1:1 COPW/reducing agent (elemental iron or manure) mixture and different type soils (sand, loam and clay) overlain by COPW were subjected to leaching tests using acidic, neutral and alkaline influent water to determine Cr mass leaching efficiencies. Based on the TCLP analyses, COPW is classified as hazardous waste. As a result of comparing the leaching efficiency data from twelve leaching columns, the maximum removal of total Cr was achieved by leaching COPW/manure mixture using acidic (pH 4.78) influent water. The highest Cr(VI) leaching efficiency was achieved in the columns of plain COPW and COPW/manure mixture using highly alkaline (pH 12.0) influent water. The least effective leaching efficiency for both total Cr and Cr (VI) was obtained by leaching plain COPW with neutral (pH 7.0) influent water. Land-disposal of the treated COPW material by mixing with clayey soils seems to be a viable alternative.