Browsing by Author "Heckroodt, R O"

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    Crystallisation of PFA glasses
    (1994) Emms, S; Heckroodt, R O
    Glasses with various compositions, falling in the CaO-AI20rSi02 and MgO-CaOAI20rSi02 systems were made, using pulverised fuel ash and silica, hydrated lime, kaolin and magnesium carbonate. Titania or ferric oxide and chromia were used as nucleants. Various crystallisation heat treatments were carried out and the nucleation and crystallisation behaviour was studied. A minimum MgO:CaO was found to be necessary for bulk nucleation to occur. The activation energy for viscous flow decreased with increased MgO:CaO ratios. This was accompanied by an increase in the surface crystal growth rates and a decrease in the activation energy for surface crystal growth. Titania also lowered the activation energies for viscous flow and surface crystal growth and caused an increase in the surface crystal growth rates.
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    Glass ceramics from a South African pulverised fuel ash
    (1991) Kirby, M J; Heckroodt, R O
    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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    Non-silicate porous glasses obtained by the leaching of borate-rich glasses
    (1985) De Villiers, Daniel Robert; Heckroodt, R O
    Glasses of the composition Na₂O-B₂O₃-X-Y and Na₂O-B₂O₃-Al₂o₃-X-Y were made where X and Y are two of the oxides CeO₂, HfO₂, ThO₂,Y₂O₃,ZrO₂ or Ga₂O₃. The glasses were either quenched or heat treated to promote phase separation and/or crystallization. The materials were subsequently leached in distilled water for periods of up to 96 hours. Leaching resulted in porous glasses or porous crystalline materials. The porous materials had high surface areas and good alkali resistance. A wide variety of compositions of these porous materials had BET surface areas between 100 and 413 m²/g. Selected porous glasses were chemically analysed. The heat resistances of two porous glasses were evaluated by observing the temperatures necessary for densification. Electron micrographs and X-ray diffraction scans were taken where necessary. Part of the glass forming regions for quenched samples of the system Na₂O-B₂O₃-Ga₂O₃-Y₂O₃ having Ga₂O₃ : Y₂O₃ ratios of either 3 : 1 or 3 : 2 were investigated. Part of the glass forming region for quenched samples of the Na₂O-B₂O₃-Y₂O₃ system was also investigated.
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    The wear of materials in hydraulic transport pipelines
    (1988) Steward, N R; Heckroodt, R O
    The hydraulic transportation of particulate solids through pipelines results in wear of the pipeline walls. The lifetime of the pipeline is determined by this rate of material loss and is therefore critical to the designer. Due to the small amounts of material lost in in-situ tests, requiring in many cases in excess of 1000 hours testing, an accelerated test procedure is necessary. This work introduces an accelerated method of evaluating materials under simulated pipeline wear conditions. The solids in the slurry and the materials were closely monitored to attain an understanding of their interaction. The wear rate of the materials tested was found to decrease with a decrease in the average rounding of the particles. These changes in particle characteristics occur with time due to comminution within the pump and pipeline. The mechanical properties and wear rates of the materials evaluated were examined to determine whether any relationships existed.