Microwave processing of minerals

The present growth in population drives a need for minerals resources and this coupled with the fact that the quality of mineral deposits are rapidly decreasing makes such minerals harder to obtain in the future. Therefore, society is facing the problem of how to efficiently mine (reduce energy consumption) and process low-grade deposits that are current un-economic. To overcome these issues, researchers have focussed on mineral sorting. Mineral sorting is a method of particle separation, where the desired metals (gold...) or ore minerals (chalcopyrite...) are separated from gangue or waste minerals and unwanted rocks. In this way, the efficiency of the downstream processes (grinding and flotation) is increased by reducing the amount of material to be processed and increasing its grade. The ultimate economic benefit of ore sorting is making un-economic low-grade deposits exploitable. The main idea of microwave/infra-red (MW/IR) ore sorting is to use MW heating to selectively heat ore particles and separate particles based on their temperatures. This is based on the assumption that during the MW exposure some metal rich minerals tend to heat more than other minerals. A great challenge in this approach however, is to adapt microwave technology which has traditionally been used for low throughputs of materials to the demands of the mining industry where hundreds of thousands of tonnes of material are processed each day. Microwave heating inside a multimode cavity is not uniform and depends on various factors such as, a cavity design, dielectric properties of treated material and etc. Therefore, in applications such as MW/IR ore sorting it is desirable to have a parameter that will quantify non-uniform heating for continuously operated cavities MW heating in context of MW/IR ore sorting, is not used to heat the rock particles in order to initiate a chemical process, but for selective heating so the sorting of ore particles based on their temperature can be completed. Therefore, non-uniform heating caused by the design of a MW applicator can be considered as a systematic error. Therefore, it is of interest to find a way to separate the effect of the instrument (MW applicator) from the material response. Based on the above overview, several focusing questions were defined for the project: • Is it possible to quantify the heating uniformity of continuous microwave cavities using software which simulates MW heating of batch cavities? • Is there a way to take into account of non-uniform heating into decision making process (of a MW/IR ore sorting), which would reduce the impact on sorting performance for any particular ore type? • Design continuously processing multimode cavity, capable to selectively heat ore particles in a uniform manner for a frequency of 922MHz These are the conclusions from this investigation: • A new parameter for quantifying the heating uniformity for continuous microwave cavities has been defined. The parameter can be calculated based on the experimental or the simulation results and it is used in this thesis to compare different cavity designs • Introducing the correction method (by using the information about particle’s position on the conveyor belt or the particle’s path) for reducing the influence of non-uniform heating of the continuous processing MW cavities upon MW/IR ore sorting • A novel design of MW cavity (rectangular cavity), which has improved performance (in terms of heating uniformity, MW efficiency and material throughput capacity) compared to an existing pentagonal cavity. The correction method is also applicable in other industries, such as the food industry where the treated material has the same size and shape. The simulation results for various shaped particles, indicate that different shaped particles heat differently and that cylindrical particles heat more compared to cylindroid particles of the same volume; therefore, the tracking of particles shape need to be implemented in order to reduce the influence of non-uniform heating into MW/IR sorting. MW/IR ore sorting will have a significant influence upon the mining industry. Further research should be focussed on a design of MW cavities with no conveyor belt, which will further reduce the operational cost of MW/IR ore sorting and improve accuracy of MW/IR ore sorting.