Cr(VI) formation in ferrochrome-smelter dusts

Dusts generated during ferrochrome smelting in a pilot-scale DC arc furnace were sampled from the furnace freeboard and from the off-gas handling stream (cyclone and baghouse). The dusts comprise fine-grained feed material (chromite, flux, and reductant); compositionally heterogeneous, glassy micro-spherules, interpreted as gas condensates and/or aerosolized melt droplets; and trace amounts of ferrochrome. Synchrotron-based micro-X-ray-fluorescence and micro-X-ray absorption near-edge structure around the chromium (Cr) K-edge was used to identify the distribution and speciation of Cr in dust particulates from the freeboard and the cyclone. The dust samples contain Cr in multiple oxidation states, including Cr(0) (hosted by ferrochrome), Cr(III), and Cr(VI). The majority of Cr occurs as Cr(III) in chromite. In both the furnace-freeboard and the cyclone dusts, Cr(VI) was consistently associated with the Si-Ca-Mg-rich micro-spherules. A major finding of this study is that Cr oxidation and Cr(VI) formation can occur in aerosolized dusts within the closed DC-arc furnace during ferrochrome smelting under conditions that are deemed to be essentially reducing. The association of Cr(VI) with the micro-spherules forwards the hypothesis that the high temperature of the furnace off-gasses and the flux composition influence the likelihood of Cr(VI) formation within a closed furnace under open-bath smelting conditions.