The Effects of Various Mineralising Agents in Promoting Recrystallisation in Mixtures of Clay and Alumina during Firing
JSGT 1955 V39 T314-T350
The effects of various "mineralising agents" in promoting mullite formation in mixtures of china clay and alumina have been investigated by determining the amounts of mullite, corundum and glass in specimens which had been fired at temperatures between 1300° and 1600° for periods of 24 hours. A specially pure china clay was used, and the alumina was added as calcined alumina (corundum) in proportions to give overall compositions corresponding with mullite, 3Al2O3.2SiO2, and with sillimanite, Al2O3.SiO2 • The mullite and corundum contents of the fired specimens were determined by adding a known proportion of fluorite to the powdered specimen and then obtaining the X-ray diffraction pattern of the mixture. The relative proportions of mullite and fluorite and of corundum arid fluorite could then be determined by comparing the photographic densities of lines characteristic of these three types of crystal in the X-ray-diffraction patterns obtained. The "mineralising agents" investigated included six sodium salts, the fluorides and carbonates of lithium, potassium, magnesium and calcium, and also titania, manganese dioxide and ferric oxide. When small amounts of these materials were used, in most cases the amount of mullite formed was greater than in the original mixture without any additions, fired for 24 hours under the same conditions. If the proportion of mineralising agent was increased above some quite small "optimum" amount, however, mullite formation was reduced and eventually was less than in the original mixture. Approximate values are given for the optimum additions of the different materials; these depend on the temperature of firing. "Permissible" additions in excess of the optimum amounts are suggested, having regard to the extent of mullite breakdown which occurs when the optimum amounts are exceeded. The "permissible limits" for different mineralising agents differ quite considerably. In order to obtain porosities which might be regarded as acceptable in commercial refractories of the "sillimanite" composition the permissible limits would have to be exceeded with all the mineralising agents investigated, used singly, unless the refractory mixtures were fired at temperatures approaching 1500°. In the specimens of the "mullite" composition the porosities are higher than in the corresponding "sillimanite" specimens.
H. Moore & M. R. Prasad
