Cr (VI)-Containing electri furnace dust and filter cake: characteristics, formation, leachability and stabilisation

Abstract

In South Africa, the ferrochromium industry produces approximately 100,000 t bag house filter dust and slurry, while the stainless steel industry produces 24,000 t of dust annually [17,39]. The toxic substances in these wastes potentially pose a threat to the environment and human health, especially Cr (VI) due to its toxic, carcinogenic, highly soluble and strongly oxidizing properties. Therefore, the existence and treatment of wastes from stainless steel and ferrochrome production remain a challenge and an issue of concern. The increase of environmental legislation globally and the trend towards sustainable development are drives for alternatives to landfill. In the present thesis, the characteristics, formation mechanisms, leachability and stabilisation of the Cr (VI)-containing electric furnace dust and filter cake were investigated using various techniques such as XRD, XRF, TG/DTA, XPS, SEM-EDS, FT-IR, Raman spectrometer and UV/Vis spectrometer. The electric furnace dust and filter cake are very fine particles. Stainless steel dust forms by the entrainment of charge materials, evaporation or volatilisation of elements and ejection of slag and metal by spitting or the bursting of gas bubbles. It was found that ferrochrome dust is formed by the ejection of slag and metals droplets from the electrode hole, the entrainment of charge materials, vaporisation as well as the formation and precipitation of compounds from vaporised species in the off-gas duct. Filter cake contains crystal phases (CaF2 and CaSO4 ) and metal rich amorphous phases. It is formed due to super saturation and precipitation. Leaching experiments on the wastes showed that Cr (VI) rapidly leaches out by distilled water. Bricks were produced by mixing wastes (stainless steel plant dust, ferrochrome dust and filter cake) and clay. The optimum sinter parameter was found to be 1100oC and 5 hours for a 50wt% SPD-50wt% AS mixture in the brick. The leachability of Cr(VI) is strongly influenced by the mass%CaO/mass%SiO2 ratio and alkali metal oxides content in the wastes. The emission factors from the stabilised wastes (SPD, FCD1, FCD2 and FC) are similar to those reported for the cement industry. Semi-dynamic leaching tests indicated that the predominant leaching mechanisms of chromium species are initial surface wash-off followed by matrix diffusion.