THE HYDROMETALLURGICAL TREATMENT OF CARBON-STEEL ELECTRIC-ARC FURNACE DUSTS BY THE UBC-CHAPARRAL PROCESS

Carbon steel electric arc furnace dusts are quite variable in composition depending upon the steelmaking practice and the source of scrap used in the furnace. Generally the dusts contain significant amounts (> 10%) of iron, calcium and zinc and lesser amounts (< 10%) of cadmium, lead, nickel, copper, magnesium, manganese, sodium, potassium, chromium and chloride. The dusts are generally regarded as toxic waste because of the leachability of toxic elements such as lead, cadmium and chromium from landfill sites. For this reason, under environmental pressure, the secondary steel industry has been seeking process alternatives for treating the dust. The objectives of most of the proposed processes are: (1) to recover valuable metals such as zinc, cadmium and lead from the dust; (2) to render the dust non-toxic according to current environmental legislation (always changing); and (3) to process the dust with the minimum economic penalty (or maximum benefit) to the steelmaker. The UBC-Chaparral process consists of a novel series of hydrometallurgical treatment steps designed to (1) selectively remove chlorides from the dust as a CaCl2NaClKClH2O solution, (2) remove lime as a soluble calcium salt with subsequent gypsum precipitation, (3) recover zinc as a pure zinc hydroxide-zinc carbonate material, and (4) to recover lead, cadmium and copper as a metallic cement. The residue from the process passes the July 1985 U.S. Environmental Protection Agency regulations for determining the toxicity of a solid waste. Additional process steps have been designed and evaluated to allow the residue to meet even more stringent environmental regulations. A preliminary capital cost estimate of $4.5-5.5 million (U.S.) for a 24,000 tonne per year plant and an operating cost estimate (including amortization of capital) or $140-160 (U.S.) per tonne have been made. The expected gross revenue from the process was $100-$125 (U.S.) per tonne based on a dust containing 15% zinc and minor amounts of cadmium and lead. The net treatment cost would therefore be $15-$60 (U.S.) per tonne. These values compare favourably with other proposed or operating process options. © 1990.