The Rate of Reaction between Silica and Sodium Carbonate at Different Temperatures and the Process of Vitrification

JSGT 1933 V17 T025-T049

Mixtures of quartz grains lying between I.M.M. sieve sizes 150–180 mesh and of sodium carbonate in the proportions of Na₂CO₃+SiO₂, Na₂CO₃+2SiO₂,Na₂CO₃+3Si0₂, and Na₂CO₃+4SiO₂ have been heated in air under carefully controlled conditions. Appreciable reaction was noted at 390°, and the rate increased rapidly with rise of temperature. For Na₂CO₃+3Si0₂practically complete decomposition took place in 35 hours at 650°, in 15 hours at 700°, in 90 minutes at 800°, and in 14 minutes at 820°, whilst on fusion, which occurred at approximately 825° the reaction was very rapid. In general there was very slight residual carbon dioxide in the reaction product even in that at 1200°. The results of the rate of reaction can be represented by the formula, for a monomolecular reaction. Up to nearly 700°, the products of complete reaction (i.e. complete decomposition of sodium carbonate) are powders. The temperature of incipient sintering was found to be influenced by the composition of the mixture being slightly below 700° for Na₂CO₃+SiO₂ and slightly above 750° for Na₂CO₃+4SiO₂. On plotting the values of k (derived from the monomolecular formula for reaction rate) against 1/T, graphs were obtained consisting of two straight lines intersecting at the incipient sintering temperature. Determinations were made of the silica which had entered into reaction with the sodium carbonate in each mixture, and it was shown that at temperatures up to 820° the reaction took place at the surface of the quartz, the sodium carbonate ultimately disappearing leaving a granular product corresponding in shape to the quartz grains, but translucent. The compound or compounds produced formed an envelope round each residual quartz grain, and only slowly penetrated the latter.

J. T. Howarth, W. Maskill & W. E. S. Turner