Buse, R; Mombelli, D; Mapelli, C
Electric arc furnace dust (or powders) are considered hazardous waste due to their high heavy metals content (zinc, lead, etc.). The Waelz process is one of the most efficient technologies, in terms of capacity and quality, able to recover nearly 90% of zinc content from such powders. The Waelz process is applied on industrial scale in the metallurgy of zinc and it is designed to the enrichment of materials with low metal content to produce a concentrated zinc oxide (Waelz oxide). The process consists in a series of chemical and physical operations (drying, reduction, vaporization, oxidation, etc...) carried out in a rotary kiln whose dimensions vary with the required capacity. The charge consists in fuel and reducing agent, provided by the coal (anthracite or pulverized coke) and correctors, such as lime or silica. The raw materials mainly used are the electric arc furnace powders, hazardous waste normally destined for disposal in special landfills, that are produced during the melting processes inside the electric arc furnace, picked up by the fourth hole, slaughtered and stored in appropriated filtration plants. The charge, in forms of pellets, that moves along the rotary kiln, is passed through by the reducing agent (CO) in countercurrent. The high temperatures reached in the furnace allow the reduction and the vaporization of zinc and other volatile metals (lead and cadmium), that are aspired to an exhaust chimney and subsequently oxidized and condensed to recover the final Waelz oxide. Before transforming Waelz oxide in metallic zinc through pyro-metallurgical or hydrometallurgical way, the oxide is washed to remove the residual chlorides fraction. Several auxiliary systems, such as the post-regenerative combustor for the elimination of unburned organic compounds or chlorides crystallization plant for the cleaning of Waelz oxide washing water, complete the system by significantly reducing the environmental impact of the entire process. The Waelz process enables the recycling of steel mill dusts leading to the production of a secondary raw material widely used in the zinc industry. The European steel industry produces more than 1,000,000 tons/year of steel powders and their recycling could lead to the generation of nearly 250,000 tons/year of zinc.