Zinc recovery from baghouse dust generated at ferrochrome foundries

During the production of ferrochrome in electric arc furnaces, a dust is captured in baghouse filters as a means of air pollution control. This dust contains various metals such as chromium, zinc, iron, aluminium and magnesium. Due to the presence of hexavalent chromium in the dust, which is both toxic and carcinogenic, the dust requires disposal at a hazardous waste disposal site, entailing high cost. The possibility exists that these costs could be reduced if metals such as zinc can be recovered from the dust through, inter alia, leaching. Experiments were performed to determine suitable conditions for the leaching of zinc from such a baghouse dust. A two-step leaching procedure was followed which prevented the formation of silica gel. The most favourable conditions for zinc extraction was with a sulphuric acid concentration of 336 g/L and an acid to dust ratio of 0.56. Under these conditions, zinc was selectively leached relative to iron and aluminium. The percentage recovery for zinc was 71.2%, aluminium 1.8% and iron 0.1%. It was concluded that the second step (dilution) of the leachate did not have a significant influence on the percentage extraction of metals from the dust.