Glass ceramics from a South African pulverised fuel ash
Master Thesis
1991
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University of Cape Town
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Abstract
The generation of electricity by the combustion of pulverised coal produces large quantities of coal ash (PF A). The disposal of this ash lias become a matter of concern due to the unsightly and hazardous nature of the PF A, and it has been the subject of intense investigations into its suitability as a raw material. Many uses have been proposed for the PF A. When used as landfill or mining backfill, the attraction is the low cost of the material. Other uses, as in the concrete industry, use PF A because of the improvements in quality of the resultant product. PF A has been suggested as a raw material for the production of wear resistant materials. The PF A is composed in the main of SiO₂ and AI₂O₃, and is a suitable material for the production of alumino-silicate ceramic materials, which are known to be tough and wear resistant. To establish the suitability of PF A from the Lethabo Power Station as a raw material, a project to prepare glass ceramic materials from the PF A was started. The conversion of the PF A to a glass ceramic material is a complex process involving many stages, and the processing at each stage will affects the final properties of the material. It is not possible in a short project such as this to examine all the factors which exert some control on the process, and so a small subset of these parameters was selected for study, namely the effect of added oxides on the crystallisation behaviour. Glass items which crystallise on holding at high temperatures commonly do so by growth of crystals from the surface of the item. This results in a material that is mechanically weak, due to the highly oriented microstructure that results. Nucleating agents can be used to obviate this. By providing sites for crystal growth in the bulk of the sample, they induce the crystallisation of fine grained ceramics with good mechanical properties. This study examines the effect of TiO₂, P₂O₅, and a mixture of iron and chrome oxides on the crystallisation of the glass prepared using PF A. The effect of these oxides was evaluated by examination of the microstructure of the crystalline specimens, and the kinetics of crystallisation were analysed by fitting data obtained by isothermal crystallisation of the specimens to the Avrami equation. Finally, the mechanical properties of the materials were tested by solid particle erosion, and the materials ranked against a selection of other materials used for their wear resistance.
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Bibliography: pages 92-101.
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Kirby, M. 1991. Glass ceramics from a South African pulverised fuel ash. University of Cape Town.