Thermal and carbothermic decomposition of Na₂CO₃ and Li₂CO₃

Abstract

In order to elucidate the decomposition mechanism of Na₂CO₃ and Li₂CO₃ in mold-powder systems employed in the continuous casting of steel, decompositions of Na₂CO₃ and Li₂CO₃ were investigated using thermogravimetric (TG) and differential scanning calorimetric (DSC) methods at temperatures up to 1200 °C, under a flow of argon gas. For the case of pure Na₂CO₃, the thermal decomposition started from its melting point and continued as the temperature was increased, but at a very slow rate. For Li₂CO₃, however, the decomposition occurred at much faster rates than that for Na₂CO₃. When carbon black was added to the carbonate particles, the decomposition rates of both Na₂CO₃ and Li₂CO₃ were significantly enhanced. From mass-balanced calulations and X-ray diffraction (XRD) analyses of the reaction products, it is concluded that decompositions of Na₂CO₃ and Li₂CO₃ with carbon black take place according to the respective reactions of Na₂CO₃ (1) + 2C (s) = 2Na (g) + 3CO (g) and Li₂CO₃ (l) + C (s) = Li₂O (s) + 2CO (g). It was found that liquid droplets of Na₂CO₃ were initially isolated due to carbon particles surrounding them, but, as the carbon particles were consumed, the liquid droplets were gradually agglomerated. This effected a reduction of the total surface area of the carbonate, resulting in a dependence of the decomposition rate on the amount of carbon black. For the case of Li₂CO₃, on the other hand, hardly any agglomeration occurred up to the completion of decomposition, and, hence, the rate was almost independent of the amount of carbon black mixed. The apparent activation energies for the decomposition of Na₂CO₃ and Li₂CO₃ with carbon black were found to be similar and were estimated to be 180 to 223 kJ mole⁻¹.