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A. Objective

MOST of the iron presently is produced in blast furnaces using metallurgical coke. The final product is pig iron, which contains about 3 to 4.5 mass pct carbon. The carbon is picked up by the molten iron droplets when they permeate through the coke column in the furnace. The blast furnace will continue to be the major ironmaking reactor in the foreseeable future on account of its high productivity and energy efficiency. Over recent years, there has been considerable effort to develop alternative ironmaking routes, which use noncoking coal to reduce iron-ores to metallic iron. The rotary hearth furnace (RHF) ironmaking process is one such route, which was developed as a means to treat the fines and waste oxides generated in the steel plants.[1] The RHF is a donut-shaped refractory-lined vessel with a rotating bottom or rotating ‘‘hearth.’’ It makes use of composite pellets made of iron-ore and a carbonaceous reductant in the form of a single or a multilayer bed. The temperature and atmosphere inside the furnace is controlled by means of burners positioned along the walls and sometimes on the roof of the furnace. As the S. HALDER, formerly Graduate Student, Department of Materials Science and Engineering, Carnegie Mellon University, is with Praxair Technology Center, Praxair Inc., Tonawanda, NY 14150. Contact e-mail: [email protected] R.J. FRUEHAN, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted April 4, 2008. Article published online November 11, 2008. 784—VOLUME 39B, DECEMBER 2008

hearth rotates, the pellets are heated up to the reduction temperature followed by their subsequent reduction to metallic iron. The principal mode of heat transport to the pellet bed is by radiation from the walls and roof of the furnace. The productivity of the RHF is limited by heat transport. As the number of layers of the multilayer bed increases, the amount of radiation reaching the lower layers of the bed decreases because of shielding of radiation by the upper layers of the bed. As a result, the lower layers of the bed do not undergo complete reduction. Because of the limitations imposed by heat transfer, a new ironmaking process is being proposed along the lines of Fortini,[2] which involves the combination of a RHF with an iron bath smelter. Iron-oxide is reduced to about 70 to 75 pct metallization in the RHF followed by smelting of the direct reduced iron (DRI) in a bath smelter. Because of the lower target degree of metallization in the RHF, a multilayer bed of composite pellets can be employed. Using this combination of the RHF and the smelter can eliminate the individual shortcomings of the two reactors. The productivity of the RHF can be enhanced and the energy requirements of the smelter shall be substantially reduced because of the preheated and pre-reduced feed. The reduction in composites is known to be controlled by the kinetics of the surface-chemical reactions such as the carbon oxidation by CO2 and wu¨stite