Effects of additives and temperature on dissolution rate and diffusivity of lime in Al 2 O 3 -CaO-SiO 2 based slags
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INTRODUCTION
IN secondary steelmaking, synthetic slag is used to influence the final chemistry and residual oxide inclusions in the steel. The most important functions of the ladle slag are to prevent direct contact between the liquid steel and oxidizing atmosphere, desulfurize the liquid steel, and absorb oxide inclusions formed as the result of deoxidation reactions. Lime based slags are used for aluminum-killed steels, to absorb the alumina and silicate inclusions and form a calcium aluminosilicate slag with a low level of other oxides. Fluorspar (CaF2) is commonly used as an additive to increase the dissolution rate of calcined lime into this ladle slag as well as to reduce the slag viscosity. While fluorspar is known to be an effective fluxing agent, there are some drawbacks associated with its use. These include volatilization of fluorine containing species into the atmosphere, leaching of the residual fluorine from the discard slag, higher refractory wear rate, and relatively high cost of fluorspar. Therefore, an alternative flux or slag practice, which will promote rapid and complete dissolution of lime into slag without adverse technical and environmental effects, is highly desirable. The effects of additives on the dissolution rate of lime in the slags have been investigated by a number of researchers. Matsushima et al.[1] used a rotating disk technique to measure the dissolution of lime in calcium silicate based slags with a CaO/SiO2 ratio of 1 containing about 20 wt pct Al2O3 or FeOx at temperatures between 1450 °C and 1600 °C. They found that the dissolution rate was several times S.H. AMINI, formerly Postgraduate Student, School of Chemical Sciences and Engineering, The University of New South Wales, and CSIRO Minerals, is Research Fellow, School of Chemical Engineering and Analytical Science, The University of Manchester, M60 1QD United Kingdom. Contact e-mail: [email protected] M.P. BRUNGS, Associate Professor, School of Chemical Sciences and Engineering, and O. OSTROVSKI, Professor, School of Material Science and Engineering, are with The University of New South Wales, Sydney, NSW, 2052, Australia. S. JAHANSHANI manages the High Temperature Processing research program at CSIRO Minerals, Melbourne, Victoria, 3169, Australia. Manuscript submitted December 1, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
greater in slags containing FeOx. Hamano et al.[2] also used the rotating disc technique to study the effects of various additives (CaF2, CaCl2, Al2O3, and B2O3) on the dissolution of lime in calcium-iron silicate slags with CaO/SiO2 ratio of 1 and 40 wt pct FeOx at temperatures between 1300 °C and 1400 °C. The dissolution rate was found to vary with the sample size. Hamano et al.[2] found that while addition of CaF2 and CaCl2 resulted in a higher dissolution rate of lime in the slag, the rate decreased with increasing slag basicity. They also found that the addition of FeOx increased the dissolution rate of CaO in the slag. While these findings suggest that additives such as iron oxide and
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