Access creation and its measurement in impermeable rock mass for the in situ recovery of metals from ore bodies

Sahar Kafashi

Current mining methods, aimed at sustaining production, are often energy-intensive, environmentally harmful, and costly. These challenges are amplified in deep construction areas with high in situ stresses, leading to a higher carbon footprint, increased waste, and additional tailings. Moreover, rising mining costs and scarcity of ore deposits, have further driven the need for alternative metal recovery techniques from deposits that are no longer economically or environmentally exploitable by conventional mining. In Situ Recovery (ISR) method has the potential to tackle these challenges by injecting fluids (i.e., lixiviant) into deep ore formations to react with targeted minerals, making them extractable from the surface. Although ISR has typically been used for mining uranium in sediments, it has also been used less frequently for treating low permeability rocks, such as hard rocks containing copper, nickel, and gold. The limited uptake of the technology for hard rock mineralisations is primarily due to the low natural rock porosity and permeability, which hinder lixiviant infiltration and its contact with mineral surfaces. This study investigates the effects of various fracturing techniques on rock integrity to enhance fluid/lixiviant flow in hard rock formations, focusing on both micro-fracturing and fragmentation. Three techniques are tested: (i) microwave energy (MW), (ii) high voltage pulse (HV), and (iii) cryogenic fluid (CF) to generate cracks that would enable greater penetration of the leaching fluid. Each technique is assessed for its effectiveness in weakening rock structures and promoting permeability. MW induces electric and magnetic waves that penetrate all minerals composing rock materials, creating thermal gradients between MW-absorbing and less-absorbing minerals in the rock matrix. This gradient leads to decoupled volumetric mineral changes, generating various local mechanical stresses around each mineral, which result in detachment of grain contacts and/or micro-fractures inside minerals under mechanical stresses. The HV method generates short electric pulses at very high voltages, creating a thermal gradient that leads to extensive micro-fracturing (weakening) and/or complete fragmentation, depending on the applied energy. Finally, CF creates cracks through thermal gradient by lowering the temperature of the minerals (frost heave stress), expanding evaporated liquid, and resulting in tensile stresses. The network of micro- and macro-cracks can lead to increased mass transfer between the solid and solution, which can also increase the penetration of leaching solution exposing more solid mineral surfaces. This study investigates the viability of these three methods for ISR, divided into five main parts. The first part develops a standardised setup and methodology for each method that could be applied to ore samples and yield reproducible results when different parameters were varied. Synthetic rocks with consistent and reproducible known parameters were produced, allowing for the evaluation and comparison of different methods while eliminating the effects of more complex factors present in real rocks. The synthetic sample collection underwent thorough petrophysical and mineralogical characterisation before and after applying ISR methods using several laboratory apparatuses to compare the results and determine the effectiveness of each ISR method to enhance the sample permeability. In the second part, MW was used to enhance the permeability inside the synthetic rock samples. Experiments were designed to evaluate the effect of water content inside rock and consequently how MW power level and time of exposure affected the permeability and crack propagation inside synthetic and real spodumene core samples inside MW. The third part evaluates the effects of HV pulses on rock fragmentation by testing different voltage levels and number of pulses, as well as the gap between the voltage inducer (or electrode) of the apparatus and the sample. The fourth part focuses on the CF method, examining the time of exposure of the sample to liquid nitrogen (LN2), which is correlated to the amount of LN2 and immersion time of each sample. During the final phase of the project, MW and CF were employed on natural hard rock of spodumene (a rock rich in lithium), and these two methodologies were meticulously assessed using various techniques such as NMR (Nuclear Magnetic Resonance), gas porosity and permeability, SEM imaging, P-wave, and S-wave ultrasonic velocities. The aim was to identify the optimal conditions, or a synergistic combination thereof, for achieving maximum access creation through a synthetic core sample. Key findings demonstrate that HV has a potential for complete fragmentation, which supports fluid mobility, while MW demonstrated notable permeability enhancement in initiating micro-fracturing under wet conditions, resulting in more visible cracks confirming its efficiency due to higher dielectric factor of water. At both moderate (20%) and (100%) water content, permeability substantially increased 233-47286% respectively, which was consistent with the observation of multiple cracks in SEM images. The results indicate a hybrid hydraulic-MW crack promotion solution might be needed for improved access creation in ISR since water existence is a necessary part of the pretreatment using MW. MW technique has potential applications beyond rock fracturing; it could also be effectively utilised in comminution and preconditioning processes to enhance mineral liberation. Additionally, CF fracturing revealed promising results in stimulating fluid flow paths in otherwise impermeable rock, especially in the presence of water within the pore space of the rock. At (100%) water content, permeability increased up to 350%. Dry hard rock might not work with CF to enhance sufficiently their permeabilities. These outcomes contribute valuable insights to the field, advancing our understanding of rock-fluid interactions and offering innovative methods for permeability improvement. The results suggest further exploration into these techniques could significantly impact fluid dynamics within geological formations. Nevertheless, all these three methods can complement conventional mining techniques, including blasting, and have significant potential for their application in downstream processes and the comminution section, offering the prospect of reduced costs and increased efficiency.

Access creation and its measurement in impermeable rock mass for the in situ recovery of metals from ore bodies

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