The effect of microwave heating on ore sorting

Today the Mining industry is being challenged to develop methodologies and technology to process the lower grade and mineralogically complex ore types using ore sorting. The potential of microwave driven selective heating as an excitation tool to underpin sorting is possibly not well known in the mining and mineral industries due to very few applications and lack of awareness of the potential users. This thesis investigates the conditions under which this process is technically effective and can be utilised. A detailed investigation was conducted to understand the reasons for selective heating of specific mineral phases and how infrared sensing can be used as an identification technique to discriminate certain particles from others. This thesis also quantifies the impact of other important factors on the sorting process including; particle shape and size, mineral composition and most importantly the textures of the mineral bearing particles which have a tendency to heat quickly when exposed to microwave energy. An extensive assembly of analytical techniques such as optical microscopy, high-resolution X-ray computed tomography and XL Scanning Electron Microscopy (used by the Mineral Liberation Analyser) were utilised to obtain a mineralogical characterisation of the tested ores. The choice of microwave applicators enabled heating to be carried out in multimode and single mode types of cavity. By engineering synthetic samples a more comprehensive investigation was carried out which enabled some focusing questions from the thesis hypothesis to be addressed. The synthetic samples were used to experimentally validate an adopted theoretical approach of investigating the influence of mineral texture upon selective heating. The supplied ore from the Bingham Canyon Mine, USA (operated by Rio Tinto’s Kennecott Utah Copper Corporation) was experimentally tested in two steps of investigations. The first step involved the approach of an “ideal”, theoretical sorter for which rock particles had to be destroyed (necessary to perform assaying analysis). The temperature threshold for economically justifiable sorting was determined from a temperature difference and assayed metal content of heated particles. In the second step, samples were analysed by heating them in two applicators and the temperature threshold was determined as a function of mineral texture which caused selective heating as in contrast to assayed metal content. The results showed from the exposure of synthetic particles (with designed textures of microwave more responsive minerals) that it is advantageous to use both multimode and single mode cavities for better understanding of microwave heating of the ore. It was also shown that the texture of microwave responsive minerals has a significant effect on the formation of the temperature profiles which are used to evaluate selectivity and potential for the separation as opposed to only mineral composition of the ore particles. It was demonstrated that the types of ores studied in this work, will respond to microwave selective heating to the extent that infrared detection can be applied to perform selection between cold and hot particles defined by a set threshold

Understanding microwave induced sorting of porphyry copper ores

Global demand for minerals and metals is increasing. It has been established that the impact of mining and mineral processing operations must be reduced to sustainably meet the demands of a low grade future. Successful incorporation of ore sorting in flow sheets has the potential to improve energy efficiency by rejecting non-economic material before grinding. Microwave heating combined with infra-red temperature measurement has been shown to distinguish low and high grade ore fragments from each other. In this work, experimentally validated 2-D finite difference models of a theoretical two phase ore, representing typical fragment textures and grades, are constructed. Microwave heating is applied at economically viable energy inputs and the resultant surface thermal profiles analysed up to 2 minutes after microwave heating. It is shown that the size and location of grains can dramatically alter surface temperature rise at short thermal measurement delay times and that the range of temperatures increases with increasing fragment grade. For the first time, it is suggested that increasing the delay time between microwave heating and thermal measurement can reduce the variation seen for fragments of the same grade but different textures, improving overall differentiation between high and low grade fragments

Pilot scale microwave sorting of porphyry copper ores: Part 1: laboratory investigations

Microwave treatment followed by infrared thermal imaging (MW–IRT) has been proposed as a potential excitation-discrimination technique to facilitate sorting of porphyry copper ores. A continuous, high throughput (up to 100t/h), belt–based microwave cavity operating at up to 100kW has been designed to interface directly with commercially available sorters at industrially relevant scales. In this paper, the fragment-by-fragment thermal response of 16 porphyry copper ore samples following microwave treatment in the bespoke system is evaluated to elucidate key performance criteria and identify likely candidate ores for microwave sorting. Microwave treatment energy dose was found to be the driving force behind the ultimate average temperature fragments experience, with other process variables (e.g. belt speed, power, belt mass loading, thermal equilibration time) having little effect on sortability performance. While fragment mineralogical texture and ore textural heterogeneity were shown to influence the average temperature rise of the fragment surface presented to the thermal camera, in most cases this variability did not adversely affect sortability performance. An abundance of microwave-heating gangue minerals (e.g. iron sulphides, iron oxides and hydrated clays) was shown to be the dominant source of deviation from intrinsic sortability. However, low average moisture content and co-mineralisation of copper and iron sulphides (or bulk sulphide sorting) was found to mitigate the deviation and provide better sortability performance. An attractive separation could be proposed for many of the ores tested, either to remove a large proportion of barren fragments from ore-grade material or concentrate a large proportion of copper values from waste-grade material

Re-print of : FDTD simulation of microwave heating of variable feed.

Microwave heating inside a cavity is not uniform and depends on various factors such as: cavity geometry, properties of treated material and frequency. In most applications of microwave (MW) heating there is only a small variation in the nature of the feed. At present, the assessment of uniformity of heating is possible only for the case of batch processing cavities. Therefore, it is desirable to have a parameter that will quantify uniformity of heating for continuously operated (non-batch) applicators. This paper has given an account of and the reasons for the widespread use of a new smoothness parameter (SP). This parameter is also compared to the already available parameter (designed for batch processes) through Finite-difference time-domain (FDTD) simulations. Assuming a uniform load, it was demonstrated that heating time and applied microwave power have only a small influence on the proposed uniformity parameter. Simulations of heating were carried out using a variable feed inside two microwave applicators with different uniformity parameters. The results are presented in this paper

FDTD simulation of microwave heating of variable feed

Microwave heating inside a cavity is not uniform and depends on various factors such as: cavity geometry, properties of treated material and frequency. In most applications of microwave (MW) heating there is only a small variation in the nature of the feed. At present, the assessment of uniformity of heating is possible only for the case of batch processing cavities. Therefore, it is desirable to have a parameter that will quantify uniformity of heating for continuously operated (non-batch) applicators. This paper has given an account of and the reasons for the widespread use of a new smoothness parameter (SP). This parameter is also compared to the already available parameter (designed for batch processes) through Finite-difference time-domain (FDTD) simulations. Assuming a uniform load, it was demonstrated that heating time and applied microwave power have only a small influence on the proposed uniformity parameter. Simulations of heating were carried out using a variable feed inside two microwave applicators with different uniformity parameters. The results are presented in this paper