Studies of Alkali Attack by Molten Salts on Alumino-Silicate Refractory Compositions: I—Preparation and Properties of Sintered Refractory Samples

Abstract

Alkali attack, a vital factor contributing to the failure of alumino-silicate refractory materials, has been studied using molten alkaline salts, and laboratory prepared and field samples of refractory compositions. This study deals with among other topics, the optimum temperatures for the sintering of various alumino-silicate compositions, prepared from mixtures of pure alumina and silica, as well as naturally occurring silica sand or bauxite and china clay, to obtain dense and impervious refractory samples. It has been observed that the ratio of optimum sintering temperature with the fusion temperature were in the range 0.87–0.95. Mullite contents as represented by hydrofluoric acid insoluble residues, were 30–84% for sintered silica sand or bauxite and china clay mixtures, compared with 30–77% for sintered pure alumina and silica mixes. It was difficult to obtain by sintering, dense mullite samples using pure alumina and pure silica, as this temperature (1770°C) was very sharp and even then the samples had some porosity (14.7%). Large number of porous samples having alumina contents in the range of 28.8 to 68.12% and porosities in the range of 2.5 to 31.4% were prepared to study the effect of porosity on the rate of alkali attack. A field sample of blast furnace stack brick of around 14% porosity, containing predominantly alumino-silicates with 43.1% alumina was also collected for the study. The microstructural studies were also done, using X-ray diffraction and microscopic methods.