Water leaching and magnetic separation for decreasing the chloride level and upgrading the zinc content of EAF steelmaking baghouse dusts

Recycling scrap iron and steel in electric arc furnaces (EAFs) generates very fine metal-containing dusts which can present major environmental problems. This paper describes experimental work on a relatively novel, simple, and inexpensive process for decreasing the chloride content and upgrading the zinc level of dust from an Australian plant, to produce saleable products and environmentally safe waste products for disposal. The dust contained about 2.1% Cl as various chlorides, 23.1% Zn as zincite and franklinite, 27.1% Fe as magnetite, franklinite, and hematite, plus small quantities of lead, cadmium, chromium, and other materials. The dust was very fine (P-80 about 2 mum). Individual particles were commonly aggregates of fine spheres and other shapes. Simple water washing at ambient temperatures and natural pH (12) for 60 min extracted 99% of the chlorides, giving a residue assaying 200 ppm Cl. This residue was strongly coagulated by lime present in the dust and settled rapidly. The wash solution contained low levels of iron, zinc, lead, cadmium, and chromium, most of which could be removed by sulphide precipitation. Wet magnetic separation with a Davis tube was investigated. At the lowest field strength employed (0.6 A), 95% of the zinc was recovered in a non-magnetic product assaying 28% Zn and 24% Fe. At the highest field strength (1.6 A), 91% of the zinc was recovered in a non-magnetic product assaying 29% Zn and 25% Fe. Wet cycloning at a nominal separation size of 2-5 mum Gave a cyclone overflow product assaying 31% Zn and 26% Fe, and containing 85% of the zinc. All these results fit a typical zinc grade-recovery relationship. The proposed process of water washing followed by magnetic separation or cycloning would produce a relatively high-zinc, low-iron, low-chloride product suitable for treatment in a lead-zinc smelter or electrolytic zinc plant, a high-iron, low-zinc product suitable for land fill, and a treated waste water for discharge to a sewer. (C) 2004 Elsevier B.V. All rights reserved.