Doctoral Degrees (Chemical Engineering)

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Browsing Doctoral Degrees (Chemical Engineering) by Author "Charikinya, Edson"

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    Characterising the effect of microwave treatment on bio-leaching of coarse, massive sulphide ore particles
    (Stellenbosch : Stellenbosch University, 2015-12) Charikinya, Edson; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: The aim of this work was to determine if microwave treatment of a typical massive sulphide ore, prior to bio-leaching would induce micro-cracks and enhance value mineral exposure resulting in improved bio-leaching metal extraction and kinetics. Using X-ray Computed tomography (XCT) and Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN) image analysis techniques, the degree of microwave induced damage, and the effect of this damage on mineral exposure, was quantified directly for the first time, in this study. Ore sample preparation was carried out using a primary jaw crusher followed by secondary crushing by either high pressure grinding rolls (HPGR) or a cone crusher. Particles investigated consisted of small (-5+4.75) mm, medium (-16+9.5) mm, and large (-25+19) mm HPGR and cone crushed particles. XCT and QEMSCAN results showed the presence of microwave induced cracks within the cone and HPGR crushed particles, treated at a microwave power density of 1× 109 W/m3 abs and exposure time 1.00 s. A qualitative analysis of the cracks, showed that the cracks consisted of both interphase trans-granular and grain boundary cracks. Both XCT and QEMSCAN analysis results showed that microwave treatment resulted in a significant increase of over 500% in crack volume for both modes of prior comminution at all particle sizes. Measurements of specific interfacial areas of particles before and after microwave treatment using XCT showed average losses in interfacial area of 31%, 23% and 16% for small (-5+4.75) mm, medium (-16+9.5) mm, and large (-25+19) mm particles. This demonstrated quantitatively for the first time, that microwave treatment of sulphide ores results in both grain boundary and trans-granular fracture. Value mineral grain exposure analysis was carried out on the XCT 3D data of particles before and after microwave treatment. The results showed an increase in the degree of sulphide grain exposure of 28%, 26% and 15% for small medium and large particles respectively. This is the first time that microwave particle damage and enhanced mineral exposure has been successfully quantified experimentally using XCT and image analysis techniques. Column leaching experiments to simulate heap bio-leaching environment, were carried out over 350 days to assess the downstream benefits of microwave treatment of heap leaching feed. The results showed that microwave pre-treatment of crushed ore for bio-leaching, will lead to improved leaching recovery. Improvements in overall Zn metal recoveries of 26%, 24% and 23% were observed for small (-5+4.75) mm, medium (-16+9.5) mm, and large (-25+19) mm microwave treated particles. The enhanced metal recovery seen for microwave treated material correlates well with the crack volume measurements and mineral exposure results. The column leaching results showed that medium (-16+9.5) mm sized microwave treated particles had a 10.1% higher metal recovery compared to small (-5+4.75) mm untreated particles. A comparison of overall metal recoveries of microwave treated cone and HPGR crushed ore particles showed that the mode of prior comminution gave no significant difference in recoveries at all sizes. This suggests that microwave treatment reduces the influence of mode of comminution on bio-leaching recovery. An investigation of the dissolution of sulphide grains in selected particles, using XCT 3D image analysis techniques over the course of 350 days of leaching, showed greater sub-surface conversion of minerals in microwave treated particles compared to untreated. Analysis of the cracks over the period of leaching using XCT data, showed a growth in microwave induced crack networks over 350 days of leaching. This suggest that microwave induced cracks accelerate reagent diffusion into the particles resulting in sub-surface conversion of minerals, during bio-leaching. A bonded particle model (BPM) was developed to simulate a multiphase massive sulphide ore approximating the ore used in the physical experimental investigations. The developed ore model consisted of pyrite, sphalerite and quartz phases which were identified as the major phases in the ore used in the experiment. The resulting microwave induced crack patterns for different model resolution were compared against those obtained from physical experiments. The results showed that model resolution has a significant effect on observed microwave induced crack damage and patterns. It was observed that cracks in models with different resolution propagate in a different pattern despite having the same macro-mechanical properties. Crack patterns obtained for higher models were observed to compare well with crack patterns observed from physical microwave treatment experiments. It can be concluded that model calibration using the usual simulated UCS and Brazilian tests alone is not adequate to fix the model resolution, for simulations of thermal induced cracks. These results show that model specimen resolution has a significant effect on observed micro crack damage and that the minimum base material “particle” size is not a free parameter. The effect of absorbent phase content on microwave induced damage was investigated for the first time using a high resolution model. Damage maps which show the percentage of micro-cracks as a function of power density and exposure time for different ternary ores and absorbent phase content were constructed. It has been shown that for the same power density and energy input, the fraction of micro-fractures induced by microwave treatment considerably depends on absorbent phase grain content.