Importance of the Atmosphere on the Mechanisms and Kinetics of Reactions Between Silica and Solid Sodium Carbonate

Using thermogravimetric analysis, the reaction kinetics between powders of silica (SiO2) and sodium carbonate (Na2CO3) are investigated below the melting point of sodium carbonate, under different atmospheres. These experiments show that the reaction kinetics critically depend on the partial pressure of CO2 in the surrounding atmosphere. Under a flow of nitrogen, the reaction rate is constant with time and follows an Arrhenian law, with a weak influence of the grain sizes of the two reactants. Under a flow of carbon dioxide, the reaction rate is found to be much slower, to have nonlinear time dependence and to have a weaker temperature dependence. The influence of grain size is also found to be more significant in this case. Two different reaction mechanisms between silica and sodium carbonate may account for these results: (i) sodium carbonate dissociates at a rate depending on the partial pressure of CO2, releasing sodium to the vapor, that may then react with the surface of silica grains, or (ii) silica and sodium carbonate grains react at granular contacts between individual grains. A quantitative equation is proposed that accounts for the influence of temperature, pCO(2), and grain size on the reaction rate between silica and sodium carbonate.