Decomposition of Na2CO3 by interaction with SiO2 in mold flux of steel continuous casting

Abstract

The effect of SiO2 addition on the decomposition of Na2CO3 was investigated using the thermo-gravimetric and differential scanning calorimetric method (TG-DSC). Addition of SiO2 greatly enhanced the decomposition of Na2CO3. The main decomposition reaction began to take place at a eutectic temperature of the Na2O-SiO2 system (800 degreesC). The initial decomposition product was identified as Na2SiO3, irrespective of the mixing ratio of Na2CO3 to SiO2. The governing reaction of the decomposition was concluded to be Na2CO3+SiO2=Na2SiO3(s)+CO2(g). The decomposition rate was independent of the Na2CO3/SiO2 mixing ratio until either one has completely been exhausted and hence was not available any more for the above reaction. If surplus Na2CO3 exists after formation of Na2SiO3 by reacting with all SiO2, the remaining Na2CO3 decomposes partly by reacting with Na2SiO3 to form Na6Si2O7 and partly by thermal self-decomposition. A liquid layer that was formed at the carbonate/additive interface facilitated the decomposition of the carbonates. The melting behavior of a mold flux was greatly influenced by the lowest eutectic temperature that a mold flux system can exhibit. The apparent activation energy of the decomposition of Na2CO3 in existence with SiO2 was estimated to be 426 kJ (.) mol(-1).