Application of the Waelz Technology on Resource - Recycling of Steel Mill Dust

摘要

The Waelz process is the oldest and most often applied technology for the recycling of dusty steel mill residues world-wide, especially for EAF dust. It is a pyro-metallurgical process characterized by the volatilization of non-ferrous metals like zinc, lead and cadmium out of an oxidised solid mixture by means of reduction by coke in a rotary kiln without generating a liquid slag. The recycling of EAF dust closes the material loop between the steel and the zinc industry. Roughly 80 percent of the produced zinc metal is used as corrosion protection agent on steel plates or structures. The steel at the end of its use is recycled as scrap in electric arc furnaces for producing new steel. During this process the zinc coating is going to the dust. Nowadays still more than 60 percent of the generated dust is lost by sending it to landfill, which means an annual loss of 600 to 800,000 t of zinc. The paper gives a short overview about all known recycling techniques for EAF dust before describing in detail the Waelz process. A Waelz plant consists of normally two, sometimes three parts: the raw material preparation, the Waelz kiln with off-gas and slag treatment and eventually the washing of the crude ZnO dust for de-halogenation. All parts are described by their targets and main process units. A state-of-the-art Waelz plant fulfils all requirements for environmental protection. It is listed in the BREF-Notes of the European Community. It can be operated free of waste water, with low diffuse dust emission and with very low atmospheric emission. The release of heavy metals or dioxin is well controlled by appropriate techniques. At the end of the paper some estimative figures are presented concerning capital expenditure and operating costs. For more precise figures a detailed study is required taking into account the size of the plant and the local conditions. Successful operating plants have a treatment capacity which ranges from 40,000 tons per year to 160,000 in one kiln. Some installations operate up to 5 kilns in parallel.