Department of Chemical Engineering

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Department Process Engineering now has a new name, and will be known from March 2023, as Department of Chemical Engineering.

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Browsing Department of Chemical Engineering by Author "Ali, Abubeker Yimam"

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    Understanding the effects of mineralogy, ore texture and microwave power delivery on microwave treatment of ores.
    (Stellenbosch : University of Stellenbosch, 2010-03) Ali, Abubeker Yimam; Bradshaw, S. M.; University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Previous work has shown that microwave heating of mineral ores induces fractures around grain boundaries due to the differences in absorption of microwaves and the resulting differential thermal expansion among the various mineral phases in the ore particles. As a consequence, this reduces the energy required in subsequent grinding and enhances liberation of valuable minerals. In this study, first, the influences of different variables on bulk strength reduction of microwave treated ores have been investigated. Nine different binary ore models were constructed by randomly disseminating 10 vol.% microwave absorbing minerals in transparent matrices. Computational simulations of heating, thermal damage and unconfined compressive strength (UCS) tests on the conceptual binary ores have been undertaken by using finite-difference modelling techniques. The influence of thermo-mechanical properties of minerals on strength reduction of microwave treated ores was examined. It was shown that in general the thermal properties of the microwave absorbing mineral and the mechanical properties of the transparent matrix have the most significant effect on the strength reduction. Binary ores containing a microwave absorbing mineral that has a high thermal expansion coefficient in a strong transparent matrix achieved higher reductions in strength. The influence of absorbent phase grain size on strength reduction of ores was also quantified. It was shown that for the same energy inputs and mineral types, the reductions in strength were much higher in coarse-grained ores. It has also been shown that for the same mineralogy and treatment condition, ores with poorly disseminated heated phase achieved much higher strength reduction. The effect of microwave treatment on the mechanical state of an ore sample was also examined. It was demonstrated that unconfined compressive strength is less sensitive to microwave-induced micro-fractures and found to be a poor descriptor of liberation behaviour. A new method of characterizing damage in microwave treated ore using a continuum approach was developed. The method measures the damage around the grain boundary regions during the heating process. Using the method, it was possible to elucidate in detail the influences of power density, mineralogy, ore texture on microwave treatment of ore. It was shown that the amount of grain boundary damage incurred at a specific power density and energy input is dependent both on the ore mineralogy and its texture. The energy inputs that were required for significant (> 10%) grain boundary damage in the ores range from 0.09 to 7.06 kWh/t depending on the power density applied, the ore mineralogy and its texture. It was also shown that for a given mineralogy and ore texture there is a power density level below which no further increase in grain boundary damage is possible by increasing exposure time. The effect of pulse repetition frequency on grain boundary damage was also elucidated using the method. It was found that high pulse repetition frequencies (³ 50 Hz) resulted in an amount of grain boundary damage that was indistinguishable from that caused by continuous wave operation for a fixed energy input. It has also been shown that for a fixed microwave energy input the best result would be obtained by using the lowest possible pulse repetition frequency and highest peak pulse power. The effect of microwave treatment of ores at different treatment conditions on the extent of damage and crack pattern was also investigated in detail using bondedparticle model (BPM). It has been shown that the amount of micro-cracks and also the cracks pattern in an ore sample after microwave treatment significantly depend on its mineralogy, microwave treatment condition (power density) and absorbent phase grain size. It has also been shown that a minimum power density is required to localize damage around the grain boundary in an ore sample. This minimum power density was found to strongly depend on the ore mineralogy and its texture. Initial simulation test work concerning the effect of microwave treatment on liberation of minerals is also presented. It has been shown that microwave irradiation considerably changed the fracture pattern of an ore in simulated single particle crushing. The fracture pattern of the ore treated at high power density (Pd = 0.1 kW /mm3abs for 1 ms) was along the grain boundary and the absorbent mineral was intact. In the ore treated at lower power density for the same energy input (Pd = 1 W/mm3abs for 0.1 s) both intergranular and transgranular fractures were observed. However, in all cases the fracture patterns were preferentially localized around the grain boundary compared to that of the untreated ore.