Thermal and carbothermic decomposition of Na 2 CO 3 and Li 2 CO 3

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IN the continuous casting of steel, mold powder plays an important role in the lubrication and control of heat transfer between the cast strand and the mold, the insulation of the surface of the molten steel pool in the mold, the protection of liquid steel against reoxidation, and the absorption of inclusions. Tight control of the melting behavior and physico-chemical properties of the mold powder is, therefore, essential for the production of quality cast products. Recently, it has become common practice to add 1 to 20 wt pct Na2O to control the melting behavior and viscosity of the mold powder.[1] Li2O is also added (particularly in high-speed casting[2]), because of its ability to reduce both the viscosity and solidification temperature. In general, these oxides are added in the form of Na2CO3 and Li2CO3, which are subsequently decomposed during the melting of the mold powder. Therefore, it is important to clarify the decomposition and melting mechanisms of these carbonates in moldpowder mixtures. It was reported that an increase in the melting rate of mold powder by the addition of carbonates is due to the increase of the thermal conductivity of the mold powder by gases generated during the decomposition of the carbonates.[3] In the present study, the kinetics of the thermal decomposition of Na2CO3 and Li2CO3 and the carbothermic decomposition of these carbonates with carbon

black have been investigated by employing thermogravimetric (TG) and differential manning calorimetric (DSC) techniques. II. EXPERIMENTAL The chemical compositions of the materials used in the present study are given in Table I. The mean particle sizes of Na2CO3, Li2CO3, and the carbon black were 200, 20, and 0.11 ␮m in diameter, respectively. A thermal analyzer equipped with both TG and DSC functions was employed. The analyzer enabled simultaneous analysis of the TG and DSC measurements with a detection accuracy of 1 ␮g. Experiments on the decomposition of pure Na2CO3, Li2CO3, and carbonates with carbon black were conducted in an inert atmosphere under a flow of argon gas. The gaseous flow rate was maintained at 5 ⫻ 10⫺5 m3 min⫺1. The sample mass was in the range of 15 to 25 mg, depending on experimental conditions, and Al2O3 powder was used as a reference material. For the decomposition of pure carbonates, both the samples and references were contained in a platinum crucible, whereas the carbonates mixed with carbon black were contained in an alumina crucible. Both crucibles had a 5 mm i.d. and 6 mm height. The samples were heated to the desired temperature at a heating rate of 10 K min⫺1. III. RESULTS

JONG-WAN KIM, Researcher, Technical Research Laboratories, POSCO, Pohang, Gyungbuk, 790-785 Korea, is Research Assistant, Department of Materials Science and Engineering, Pohang University of Science and Technology. HAE-GEON LEE, Professor, is with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea. Manuscript submitted January 5, 2000. METALLURGICAL AND MATERIALS TRANS