Integrated management of ferrochrome slag: Metal recovery, Cr(VI) stabilization, and sustainable reuse in construction materials

Abstract

This study presents an integrated approach to valorize ferrochrome slag (FCS), a hazardous byproduct generated during chromite ore smelting in submerged arc furnaces. Approximately 1.1–1.2 tons of FCS, containing 10–12 % metal values alongside non-metallic slag-forming phases is produced per ton of ferrochrome alloy, which poses serious environmental concerns due to the presence of hexavalent chromium Cr(VI). The research encompasses three interconnected objectives: metal recovery, Cr(VI) leachability assessment, and beneficial applications. Alloy similar to charge chrome (58–60 % Cr) was successfully produced through arc smelting of FCS with more than 90 % recovery of metals. Subsequent extensive leachability studies were conducted, evaluating parameters such as: leaching time, liquid/solid (L/S) ratio, and pH, which revealed leachable Cr(VI) concentrations up to 0.6 ppm from the slag matrix, and were further reduced to permissible limits using suitable additives. The residual FCS was then evaluated as a geopolymer binder for construction applications, demonstrating a compressive strength of approximately 30 MPa in addition with silica fume (SF) as an additive. While the initial FCS-based geopolymer exhibited significant Cr(VI) leaching, incorporation of SF (1–2 %) as an additive substantially mitigated this concern. Furthermore, the compressive strength of the SF-modified mixtures increased from 18 MPa to 30 MPa with just 1 % addition of SF, after 28 days of curing. This comprehensive investigation demonstrates a holistic strategy for enhancing sustainability in the ferrochrome industry through metal recovery coupled with development of environmentally viable construction materials.