Steel mill residue recycling processes

Abstract

The American steel industry has a resource available to it, viz iron and steel making dust, sludges and mill scale, that could assist in energy conservation. Arthur D. Little, Inc. was commissioned by the Department of Energy to undertake a study to estimate the potential for energy conservation by the recycling of steel mill dusts and sludges generated in the United States. Included in the analysis was an evaluation of several conventional and non-conventional recycling processes, their technical and economic feasibility, and requirements for further development within the United States. Flue dusts, mill scale, and various in-plant particulates are generated in significant quantities in the iron and steelmaking operation. Blast furnace dusts have been returned to the blast furnace by recycling than through a sinter strand. Mill scale has been treated the same way. Steelmaking dusts typically have not been recycled. These dusts and sludges contain not only iron but also such potentially valuable constituents as carbon, oxygen, zinc, lead and occasionally small quantities of cadmium. The main contaminants are sulfur, phosphorus, alkalies, and gangue materials (CaO, SiO,, A1203, etc.). The proper approach to recover these metallic and carbon values involves two kinds of effort: (1) sound housekeeping and (2) sound recycling processes. Recycled materials can enter the steelmaking sequence at four points: (1) At the agglomeration facility, e.g. the sinter strand; (2) In the blast furnace, as pellets or briquets; (3) In the steelmaking furnace, after proper preparation; (4) In an iron-smelting electric furnace. Many processes have been developed to process iron and steel mill dusts and sludges and recover iron and/or other metallic values. All of these processes fit into one of the following four categories: (1) High temperature reduction (dezincing processes, direct reduction based, sponge iron product); (2) Processes with a molten iron product; (3) Low or moderate temperature agglomeration (non-dezincing processes); (4) Hydrometallurgical processes. High temperature reduction processes are capable of removing zinc from iron and steelmaking dusts. Seven of the ten processes shown in Table 1 are rotary kiln-based processes. A rotary kiln is a cylindrical furnace, the axis of which makes a slight angle with the horizontal level. The kiln rotates slowly, so that the feed moves regularly from the higher end to the lower end. The units are fired .with solid fuels. The atmosphere in the bed is reducing, so the zinc and lead are vaporized and iron oxides are partially reduced. The atmosphere above the bed is oxidizing, so that the metalic vapors in the off-gas oxidize. An hour or two at temperatures of around 1800 2000°F (980 109O’C) is usually sufficient to volatilize 95% of the zinc present. The vaporised zinc and lead can be recovered as contaminated oxides or by more sophisticated processing, they can be recovered in their metallic form. Most low or moderate temperature processes simply physically bond the dust or sludge together without any chemical action. They do not change the chemical characteristics of the recycled materials,but rather add a binder, such as cement clinker, calcium carbonate, or