Desulfurization of bath smelter metal
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INTRODUCTION
THE majority of studies conducted on the desulfurization of molten iron have simulated the hot metal produced in the blast furnace ll-71 or cupola cast iron. tSl Hot metal desulfurization is reasonably well optimized, and it is possible to achieve 50-80 pct desulfurization, down to 0.005 pct S, using reagents such as CaC2, Mg, and Mg-CaO. The molten iron produced in the blast furnace is carbon saturated and contains 0.3 to 1.0 pct silicon. The metal produced by the bath smelting process, such as the AISI-Direct Steelmaking process 191 and the Japanese DIOS process I9~ would have carbon levels approaching saturation but does not contain significant levels of silicon. It has been estimated that bath smelter metal could contain significant levels of sulfur. It is anticipated that the AISI-Direct Steelmaking process will use a continuous method for desulfurization similar to the method practiced by General Motors Foundry Division and described in Reference 8. Sulfur is fed into the smelter as a constituent in the coal and leaves in the gas, dust, slag, and metal. The sulfur may be reintroduced as sulfidized dolomite from the prereducer or from other recycled materials. In traditional steelmaking processes, sulfur is brought into the steel from the coke and much of the sulfur in the coal is removed during coke making. Sulfur is also removed by hot metal desulfurization and during steelmaking. For bath smelting, there is no coke making, making the sulfur input greater, and the process is run under a higher oxygen potential than a blast furnace, decreasing the sulfur partition ratio between the slag and metal. Therefore, desulfurization during ironmaking is limited. Depending on the particular process flow sheet and coal used, the sulfur level could be as high as 0 . 1 - 0 . 2 pct. Furthermore, during steelmaking of bath smelting metal, sulfur removal may be less due to low slag volumes resulting L.B. McFEATERS, formerly Research Associate, Department of Materials Science and Engineering, Carnegie Mellon University, is Manager-Quality Assurance Primary Operations, Weirton Steel Company, Weirton, WV 26062. R.J. FRUEHAN, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted November 11, 1992. METALLURGICAL TRANSACTIONS B
from the absence of silicon. It is the purpose of the present research to explore methods of desulfurizing metal of a composition similar to that which will be produced during bath smelting. CaO is an effective and economical desulfurizing agent, and in melts containing silicon and carbon in addition to sulfur, the following reactions take place: CaO(s) + S + C = CaS (s) + CO (g) 4CaO (s) + 2S + Si = 2CaS (s) + Ca2SiO4 (s)
[1] [2]
When the melt contains aluminum, an equilibrium CaO A1203 liquid, or solid, is formed by the following reaction: 4CaO (s) + 3S + 2A1 = (CaO - A1203) + 3CaS (s)
[3]
The composition of the CaO-A1203 phase will depend on the metal chemistry. If this phase is liquid, the CaS
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