Microwave processing of ores

Giyani, Baker (2023) Microwave processing of ores. PhD thesis, University of Nottingham.

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Copper is a widely used metal in applications ranging from electronics to building and construction. The demand for copper has increased more rapidly in recent years due to technological advancements, industrial development, and population growth. This is set against a backdrop where the global copper head grade is dropping because higher-grade ore bodies are already mined or becoming exhausted. This means that larger volumes of low-grade ores are being processed in plant operations to keep up with the growing demand. However, processing these low-grade ores using existing flowsheets is unlikely to be sustainable due to enhanced energy consumption and low process efficiency. Thus, innovative processing technologies must be explored to treat such ores if mineral processing is ever to become sustainable.

Microwave treatment of ores is one such processing technology that has been demonstrated to reduce ore competency and improve mineral liberation by inducing intergranular and transgranular fractures. Both these outputs, of course, leading to reduced energy consumption and higher process efficiency. The principal mechanism of microwave-induced fractures in ores is based on differential thermal stresses generated through rapid and selective heating of microwave-absorbent phases (e.g., sulphide minerals) within a microwave-transparent gangue matrix (rock-forming minerals). In hydrometallurgical processes, the induced fractures expose more sulphide grains to the leach solution, improving mass transfer and enhancing metal leach recovery. It has also been shown that a candidate ore should have suitable mineralogy and texture properties in order to support the creation and the effective transfer of induced stresses to enable crack propagation.

This study provides new findings that both support and extend the current understanding of microwave ore pre-treatment in leaching systems. The findings of leaching works reported by previous researchers were derived based on one ore type, and thus the influence of mineralogy and texture on the leaching performance of microwave ore pre-treatment is in no way explored. This research investigates five porphyry copper ores with varying mineralogy and texture, assessing for the first time the impact of texture and mineralogy on leachability following microwave exposure.

The results showed a significant ore strength reduction following point load testing, significant ultrasonic velocity reductions, coupled with a large fracture volume increase of over 500% and fracture openings of up to 1.0-2.5 mm wide as determined by X-ray CT imaging following microwave exposure. This proves that microwave treatment has induced fractures in treated ore samples, and the magnitude of these fractures was found to be dependent on ore mineralogy and texture, as well as microwave energy input (although microwave energy input optimisation was not an objective of this study and is reported elsewhere). Ores with a higher modal abundance of microwave heaters, coarse and/or clustered grains of microwave heaters, stiffer microwave heaters (e.g., pyrite), consistent texture of microwave heaters, and hard/brittle microwave-transparent gangue matrix (e.g., quartz) exhibited significant fractures.

In terms of leaching performance, ore samples with a large magnitude of microwave-induced fractures (i.e., those with amenable mineralogy and texture) achieved significant copper recovery enhancements of up to 28% (absolute) and faster leaching kinetics (by a factor of four or more) to achieve equivalent metal extraction at a microwave energy input of about 10-20 kWh/t. This demonstrates that mineralogy and texture have a significant impact on the leaching performance of microwave-treated ores. The leaching recovery improvement observed through assay results by ICP analysis is complemented by 3D X-ray Computed Tomography (XRCT) imaging, which revealed that microwave ore pre-treatment induces fractures in the direct vicinity of sulphide mineralisation, and these fractures serve as leaching pathways to the interior of ore fragments, thereby improving metal leach extraction.

Furthermore, results showed that the effects of microwave treatment on ore fracturing and leaching improvement increase with particle size. These results suggest that the benefit of microwave treatment of ores can potentially be realised in leaching coarse ore fragments (e.g., heap leaching), where leaching is performed in coarse particles, but metal leach recoveries are typically low and leach durations are usually long. Several conceptual flowsheets incorporating microwave heating technology to assist mineral dissolution in heap leaching operations have been proposed, based on treating the whole ore or a portion of the feed with amenable mineralogy and texture for microwave treatment. To ensure effective treatment and avoid spread of power, a gravity-fed system was proposed in which ore fragments flow through a vertically aligned tube as a packed bed to interact with the high-intensity and well-confined microwave fields in a TE10 single mode cavity. This treatment system may require several applicators (connected in series) coupled to 100 kW generators to ensure homogeneous treatment of materials at higher throughput rates (e.g., >100 t/hr). A thorough techno-economic analysis should be performed to assess the economic viability of this technology in industrial heap leaching applications.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Kingman, Samuel
Batchelor, Andrew
Keywords: Microwave, ore pre-treatment, microwave-assisted leaching, microwave-induced damage, ore fracturing, ore strength reduction, microwave assisted heap leaching, copper sulphide ores
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
T Technology > TP Chemical technology
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 73949
Depositing User: Giyani, Baker
Date Deposited: 21 Jul 2023 04:40
Last Modified: 31 Dec 2023 04:30
URI: https://eprints.nottingham.ac.uk/id/eprint/73949

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