Browsing by Author "Charikinya, Edson"
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- ItemCharacterising 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.
- ItemA modelling framework to determine the value proposition of microwave treatment of mineral ores(2011-03) Charikinya, Edson; Bradshaw, S. M.; University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: The extraction of mineral values from ore requires liberation of the minerals followed by separation. Liberation is achieved by size reduction operations which are inefficient processes typically accounting for up to 70% of the energy consumption in a mineral concentrator (Tromans, 2008). As the grade of ores reserves declines, future viability of mineral operations will be determined by the costs of comminution. Recent work has shown that microwave treatment of secondary crusher product at specific microwave energy consumption of the order of 1 kWh/t reduces the work index of the ore and increases grade and recovery in batch flotation tests. Improved liberation at coarse sizes was also demonstrated (Kingman, 2006). Based on these findings work is ongoing to commercialise the technology. The objective of this study is to develop a modelling framework to determine the value proposition of microwave treatment of ore. It was noted that various models exist in literature for modelling of mineral processing flotation flow sheets, but these models do not incorporate the feed ore liberation property as an input variable in their calculations. Thus, a fundamentally derived property based model was identified as appropriate for flow sheet modelling of microwave treated ore, as it utilised liberation as an indirect variable in calculating the flotation rate constant through the use of contact angle to describe particle surface hydrophobicity. The model was successfully incorporated into the flotation flow sheet units developed in HSC Chemistry and used with Mineral Liberation Analyser (MLA) data to investigate the effects of changes in feed ore liberation on rougher cell flotation recovery. Different liberation scenarios based around modification of porphyry copper flotation feed were created. A sensitivity analysis of the various feed stream liberation scenarios was carried out to test the ability of the model to effectively model the differences in downstream processing of microwave treated and untreated ores. For a single flotation cell of size 85 m3 with a solids feed flow rate of 890 tph, it was observed that below a certain size (120 μm in the case of the porphyry copper ore) changes in flotation feed liberation had no significant effect on value mineral recovery. Significant differences in value mineral recovery were observed only at coarser sizes above 120 μm. The results indicated that improvement in recovery of value minerals due to improved liberation from applying microwave technology has size limits and is significantly dependent on the feed grind size. Feed grind size sensitivity analysis was then carried on the same single cell flow sheet utilising feeds with the same mineralogy but with different grind sizes. The results indicated that maximum benefits from the application of microwave technology would be best obtained by utilising coarse grinding at sizes between P70 = 200 μm and P70 = 300 μm for the porphyry copper ores considered in this study. Coarse grinding appears to be the best way to exploit improved liberation in downstream processing of microwave treated ores. Investigations similar to those carried out on the single cell flotation flow sheet were then carried out on a continuous plant rougher flotation flow sheet. The flow sheet consisted of nine rougher cells in series each with a volume of 85 m3 with a solids feed flow rate of 890 tph into the bank of rougher cells. The results indicated that there was no significant difference in final rougher bank overall cumulative recovery at fine grind sizes below a P70 grind size of 120 μm with improvements in feed ore liberation. Feed grind size sensitivity analysis showed a significant variation in cumulative recovery at coarse grind sizes of above P70 = 129 μm .This variation was attributed to improvements in flotation feed ore value mineral liberation from locked composite particles to the maximum possible theoretical liberation scenario of fully liberated value mineral particles. A 7.2 percentage point improvement in cumulative value mineral overall recovery and a 2 to 3 percentage point improvement in enrichment ratio was also observed above the P70 = 250 μm grind sizes after improving the flotation feed ore value mineral particle liberation of a typical flotation plant feed to a maximum. The increases in grade and cumulative recovery at coarse sizes were attributed to improvements to the flotation plant feed ore value mineral particle liberation. From the results, it was concluded that microwave technology application will offer greater benefits in downstream processing of coarse ground ores.