Evaluating the effects of radio-frequency treatment of rocks: textural changes and implications for rock comminution

Abstract

Ore, from a mining operation, goes through a process that separates the valuable minerals from the gangue (waste material). This process usually involves crushing, milling, separation and extraction where the gangue is usually discarded in tailings piles. Current physical methods used for crushing of rocks in the mineral processing industry result in erratic breakages that do not efficiently liberate the economically valuable minerals. Research studies have found that the rock comminution and mineral liberation can be enhanced through various electrical treatment techniques, including pulsed power, ultrasound and microwave. These electrical treatment techniques each have their own advantages and disadvantages which are discussed in this dissertation. However, this research proposes a new technique in an attempt to improve the rock comminution process. The main purpose of this research is to evaluate the effect that RF power exerts on rock samples, with particular focus on textural changes. Four valuable scientific contributions to the fields of metallurgical and electrical engineering were made in this regard. Firstly, a new technique for the treatment of rock samples using RF heating is substantiated. The effect of RF power on textural changes of the rocks is evident in their surface temperature rise, where the RF heating of dole-rite (JSA) and marble (JSB, JS 1 and JS2) resulted in surface temperatures of approximately 100 °C within two minutes of treatment. A particle screening analysis of particles obtained form a swing-pot mill of both the untreated (not exposed to RF power) and treated (exposed to RF power) rock samples were performed to ascertain if the treated samples' size had changed. Two samples (JSA and JSD) revealed a notable change in their particle size distribution. The fact that the percentage of larger sized particles increased (from 38 J..Lm to 90 J..Lffi as seen in Chapter 6) suggests that the rock was strengthened rather than weakened. Secondly, an innovative coupling technique (using a parallel-plate capacitor with dimensions of 28 x 47 mm) to connect rock samples to high powered RF electronic equipment is described. The feasibility of this technique is confirmed by repeated correlated measurements taken on a vector voltmeter and network analyser. Low SWR readings obtained from an inline RF Wattmeter in a practical setup also proves the viability of the matching network used in the coupling technique. Thirdly, anoriginal coupling coefficient (81.58 x 10-3) for the parallel-plate capacitor is presented. This value may be used in similar sized capacitors to determine the specific heat capacity of dielectric materials. However, the value of the coupling coefficient was only verified for seven (relatively dark in surface colour) out of the ten rock samples. Therefore, this coupling coefficient may hold true for all dark coloured rock samples, as it represents the coupling of energy between the parallel-plate capacitor and the rock sample. Finally,this research defines the mathematical models for 10 rock samples for the VHF range of frequencies (30 - 300 MHz), providing unique phase angle to resonance equations for each sample. These equations can be used with each specific rock to determine the resonating frequency where the maximum current flows and the minimum resistance is present. Evaluating the effects of RF power treatment on rocks has brought to light that mineral grain boundaries within specified rock samples are not significantly weakened by RF treatment. This was firstly confirmed by the similar electrical properties of the untreated and treated samples, where consistent values for the resonating frequency were obtained from the network analyser. Secondly, the SEM analysis of the untreated and treated rock samples revealed no significant changes in the form of fractures or breakages along the mineral grain boundaries. Photomicrographs of the thin sections of all ten rock samples were used in this analysis. The particle size distribution of both samples further revealed no weakening or softening of the rock, as the percentage of smaller sized particles did not increase in the treated samples. It may therefore be stated that treating rock samples with RF power within the VHF range will not significantly improve rock comminution and mineral liberation.