Fundamental studies of heap leaching hydrology using magnetic resonance imaging

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The recovery of mineral from ore in the heap leaching process requires the mineral to come into contact with the leaching solution. However, heap hydrology is prone to non-uniform behaviour due to the inhomogeneity of the ore particles and the heap structure. The primary aim of this thesis was therefore to develop a magnetic resonance imaging (MRI) technique which would allow for the novel non-invasive imaging of the liquid hold-up in representative laboratory-scale heap leaching systems. The ferro- and paramagnetic species in the ore were found to cause significant distortions in frequency encoded MRI acquisitions. These distortions were mitigated through the application of single point imaging techniques. Comparison with equivalent X-ray CT acquisitions, which are immune to magnetic susceptibility distortions, confirmed the accuracy of the single point acquisitions. Spin echo single point imaging (SESPI) was demonstrated to be more robust than the simple single point imaging (SPI) technique because the effect of T2 weighting on the MRI signal was less significant than that of T2*. Mapping of the gas, liquid and solid distributions in unsaturated leaching columns using SESPI allowed for the quantification of the hold-up and voidage values. These compared favourably to gravimetric measurements and literature values, thereby confirming their validity. Novel measurement of the liquid film thickness and the interfacial areas between the three phases was achieved and insight into the liquid behaviour was obtained through different flow rate experiments as well as MRI tracer experiments. Drip irrigation of the ore was considered for beds of large, fine and agglomerated ore. Liquid distribution in the fine ore closely resembled that of irrigation of clay soils whereas flow through the large ore was gravity dominated. Slumping of the agglomerated ore caused permanent limitation of the liquid distribution and the effect of flow rate changes was observed to be limited to the region below the irrigation point. In both the column and drip irrigation experiments, increases in the liquid flow rate caused small increases in the liquid hold-up in the form of new, relatively thicker rivulets, thereby improving the liquid-solid interfacial area. However it did not affect capillary held liquid volume so the effect of flow rate changes decreased as the lateral distance from the irrigation point increased. Finally, X-ray CT images of a long term ferric leach were analysed to assess the factors affecting the mineral leaching. Proximity of the mineral to the ore surface was found to be the critical factor, as only mineral located within 2 mm of the surface was recovered, though liquid distribution effects were also observed.

Sederman, Andrew
Johns, Mike
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge