A Model for Dissolution of Lime in Steelmaking Slags
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ODUCTION
IN their recent publication,[1] the authors developed a dynamic model for predicting the bath and slag compositions during blowing in a 160-ton LD converter. While model predictions for bath compositions appeared to be in relatively good agreement with measurements in an actual LD converter by Cicutti et al.,[2] those for the slag compositions did not correspond well to the actual values. Predictions for percent CaO in slag were too high and consequently those for percent FeO were too low. Among the assumptions made in the model,[1] one was the uniform dissolution of lime during the blowing period. The model thus lacked a proper submodel for lime dissolution which might have resulted in the observed incongruity with the real values. The current study aims to develop a submodel for lime dissolution in steelmaking slags and study the effects of different process variables on dissolution rates. Finally, this submodel would be
RAHUL SARKAR and USHASI ROY, Researchers, are with Research and Development, Tata Steel Limited, Jamshedpur, India. DINABANDHU GHOSH, Professor, is with the Department of Metallurgical and Materials Engineering, Jadavpur University, Kolkata, 700032, India. Conatct e-mail: [email protected] Manuscript submitted August 10, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
incorporated into the global model for dynamically calculating lime dissolution rates with the aim of improving upon the predictions for slag compositions. In the context of LD steelmaking, understanding dissolution kinetics of lime is imperative for several reasons. Lime constitutes a significant part of the total expenses for raw materials, and cutting down on lime consumption would considerably reduce the overall cost of steelmaking. A far more important reason is to get rid of free lime which is a perennial problem for operating steel plants. For conditions pertinent to Tata Steel, the amount of free lime in slags varies in the range from 5 to 10 pct,[3] and this limits its usage as a suitable building material. Another key motivation to study lime dissolution is to produce low-P steels with low-basicity slags. LD shops in Tata Steel typically operate in the basicity range from 3.2 to 3.8, and recently, there has been an increasing drive to reduce the basicity of LD slags without compromising on turndown phosphorus. Process conditions, sequence of lime addition, lime particle size, etc. may be controlled within permissible limits to achieve this goal, but first, the effects of these parameters on turndown slag conditions must be understood. A fundamental study on lime dissolution in steelmaking slags is necessary because it plays a very important role in achieving the desired slag composition at turndown.
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MODEL FORMULATION
Reaction between solid CaO and SiO2 (as SiO44 ions) in slags results in the formation of calcium silicates, the experimental evidence for which may be found in the studies of several researchers.[4–7] The type of silicates formed depends on slag conditions (primarily slag basicity and
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