Selective leaching of zinc from electric arc furnace dust by a hydrothermal reduction method in a sodium hydroxide system

Abstract

Electric arc furnace (EAF) dust is a hazardous waste material containing valuable elements, such as zinc (Zn) and iron (Fe), as well as other environmentally harmful elements. In the conventional process, the existence of zinc ferrite (ZnFe2O4) in EAF dust results in a low recovery ratio of zinc and can induce high energy consumption or difficulties utilizing iron resources. In this article, a hydrothermal reduction method in a NaOH system is proposed to recover Zn selectively from EAF dust. First, when pure ZnFe2O4 was used as the input material to test the feasibility of the new method, 70% Zn leaching efficiencies and the isolation of Fe in the residue through the formation of ferroferric oxide (Fe3O4) were achieved at the optimal conditions of 260 °C, 6 M [NaOH], 10 g/L [C6H12O6]n and 50 mL/g of L/S. Second, a leaching process of EAF dust was proposed using the following two procedures: (i) first leaching under normal pressure to dissolve zinc oxide (ZnO); and (ii) second leaching of the first leach residue under hydrothermal reduction conditions to dissolve ZnFe2O4. The leach liquor from the second step was recycled to the first step. Under optimum conditions, the leaching efficiencies of Zn during the first and second leaching were 66.4% and 69.3%, respectively. The total Zn leaching efficiency was calculated to be 89.7%, producing Zn-impregnated leach solution and a leach residue rich in Fe (up to 39.4 wt%) that are suitable as resources for a steel-making plant. However, this process also has its shortcomings owning to larger liquid-solid ratio in the leaching process. Overall, the study technically provides an alternative approach to improve the recycling of zinc and iron and effectively reduce environmental risks from EAF dust.