Ore particle breakage behaviour in a pilot scale high voltage pulse machine
Introduction
Comminution remains by far the largest energy consumer in the mining industry. The high energy consumption implies both a high operational cost and a greenhouse footprint. As one of the potential technical means of reducing the energy consumption of comminution, pre-weakening of ore particles by high voltage pulse (HVP) has attracted the attention of researchers in the past few years (Usov and Tsukerman, 2006, Wang et al., 2011, van der Wielen et al., 2013, Razavian et al., 2014).
HVP breakage is a comminution method that uses high voltage pulses to initiate electrical breakdown inside ore particles that are immersed in water, generating a strong tensile force to disintegrate the particles. The research found that particle strength of a number of ores can be reduced significantly by HVP treatment using a low specific energy (Wang et al., 2011), thus reducing the energy consumption in the downstream comminution process. A number of potential applications of HVP technology in the mineral industry have been proposed, including pre-weakening the AG/SAG feed, hard particles treatment, ball mill feed pre-treatment, etc. (Shi et al., 2012). In addition, Shi et al. (2014a) demonstrated that, taking into account both HVP and mechanical breakage energy consumptions in a hybrid circuit simulation, the pre-weakening effect on energy reduction through changes in comminution circuit configurations is significant.
The initial study of pre-weakening by HVP breakage was performed with a selFrag Lab device manufactured by SELFRAG AG based in Switzerland (Wang et al., 2011). The selFrag Lab is primarily designed for the selective fragmentation of composite materials, mineralogical and geological samples in the kilogram range, rather than for pre-weakening study purposes. In order to demonstrate the benefits of HVP pre-weakening to the mineral industry, the equipment has to be scaled up to treat larger particles with more flexibility in operation settings. SELFRAG AG has recognised the need and developed a pilot scale Pre-Weakening Test Station (PWTS), which is a purpose-built R&D machine at the SELFRAG pilot plant (van der Wielen et al., 2014). In comparison with the selFrag Lab, the advantage of the PWTS is that it offers more flexibility in terms of generator setup, as well as the possibility to process continuously. Voltage and capacitance of pulse generator can be adjusted independently for the PWTS, allowing testing of same pulse energy but different voltages. Investigation of the ore particle breakage behaviour in the PWTS is helpful in further development of the HVP technology.
van der Wielen et al. (2014) conducted a detailed HVP breakage characterisation on a sample of granite with the PWTS. It was found that the relation of pre-weakening degree of the granite sample to specific energy is similar to the impact breakage product fineness-specific energy relationship described by JKMRC breakage model (Napier-Munn et al., 1996). The work helped to understand rock breakage behaviour in HVP breakage, but may not well represent ore breakage behaviour. The difference in breakage behaviour under HVP treatment between rocks and ores can be exemplified in a recent study that the content and location of the minerals with high permittivity/conductivity affect breakage behaviour of ore particles (Zuo et al., 2014, Zuo et al., 2015a). In order to obtain a thorough understanding of ore particle breakage behaviour in the PWTS, a joint campaign was conducted by the Julius Kruttschnitt Mineral Research Centre (JKMRC) and the PWTS manufacturer SELFRAG AG. This study investigated the breakage behaviour of three ore samples, two copper–gold ores and one iron ore, in the PWTS under different operating conditions. The major findings of ore particle breakage behaviour in the PWTS are presented in this paper.
Section snippets
HVP treatment with the PWTS
Ore particle behaviour in HVP breakage was investigated with the pilot scale PWTS machine installed in Kerzers, Switzerland. The structure of the PWTS is illustrated in Fig. 1. The PWTS machine consists of a pulse generator, a metal plate conveyor, a water vessel and a processing zone. The top electrode is connected to the pulse generator, just above the flat bottom section of the metal plate conveyor. The flat bottom section of the metal plate conveyor is guided by insulation material and
Sample analysis and data reduction
In this study, three criteria were used to describe and evaluate the ore particle breakage behaviour in the PWTS experiment: the body breakage probability, HVP product fineness and pre-weakening degree. In an ore particle subjected to pulse discharge, the breakdown channel grows preferentially along the enhanced electrical field on the boundary of minerals with different permittivities and conductivities. The body breakage and surface breakage that resulted from the HVP treatment are
The effects of specific energy and particle size on HVP breakage behaviour of ores
For all the ore samples used in this study, approximately 40–100% of initial feed particles were subjected to body breakage at the first pulse discharge. The number of particles remaining for the 2nd or 3rd pulse discharges was limited. Therefore the effects of specific energy and particle size on HVP breakage behaviour of the ores are discussed based on the body breakage product of the first pulse discharge in this section. The specific energy is calculated from the total generator energy
Conclusion
Investigation of particle breakage behaviour of three ore samples treated with HVP in a pilot scale PWTS was conducted. The investigation reveals that the mass-specific energy of HVP is one of the most significant factors affecting the breakage behaviour in the PWTS, with the larger specific energy producing the higher body breakage probability, the finer product size distribution and the more significant pre-weakening effect. The effect of particle size on HVP breakage behaviour is
Acknowledgement
The financial support from Newcrest Mining Ltd. for a PhD candidate in this study is gratefully acknowledged.
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