Zinc metal recovery from electric arc furnace dust

Abstract

Electric arc furnace (EAF) dusts contain significant quantities of zinc, which is found almost entirely as either zinc oxide, or zinc ferrite. The dust has been classified as a hazardous waste due to the relative high lead, cadmium and hexavalent chromium contents. It is important that environmentally acceptable processes be developed to treat this waste. A major problem with the current carbothermic reduction processes, which have been designed to treat the dust, is that during the zinc condensation stage, the carbon dioxide off-gas back-reacts with the zinc vapour to form zinc oxide. One possible alternative process would involve reacting the zinc oxide in the dust with metallic iron and thus, the off-gas would mainly consist of zinc. In this study, the iron-reduction distillation process was used to extract zinc from EAF dust. This process consists of two reduction stages. The first stage is the reduction of iron oxide to iron and the second stage is the reduction of zinc oxide by the reduced metallic iron. The effects of operating variables such as temperature, CO/CO2 gas composition and sintering process on the reduction of iron oxide in the first stage and temperature and pressure (in a nitrogen atmosphere and under vacuum at 2*10 -3) atm) on the reduction of zinc oxide in the second stage from the EAF dust were investigated. It was concluded that in the first reduction stage, the reduction of iron oxide was promoted by increasing the temperature from 600 to 800 ํC, and by increasing CO/CO2 gas ratio from 3 to 9. For the second reduction stage, the reduction of zinc oxide with metallic iron obtained from the first reduction stage was promoted by increasing the temperature from 900 to 1100 ํC and under vacuum at 2*10 -3 atm. The reduction of zinc oxide with metallic iron is not topochemical and has no well-defined interface. The activation energies of the reduction processes in the second stage were 141+-8.2 kJ/mol and 70+-8.2 kJ/mol for the reduction of EAF dust in nitrogen and under vacuum respectively. It was found that reaction occurs throughout the whole briquette. The zinc recovery was 95%. Greater than 99% of cadmium, lead and chlorine were removed from the EAF dust.