Measuring the dielectric properties of crushed copper ore

[摘要] ENGLISH ABSTRACT: Previous work has shown that microwave heating of mineral ores induces micro cracks within the orestructure, which can be attributed to the difference in the adsorption of microwaves amongst the differentmineral phases. This reduces the energy required during subsequent grinding and enhances the liberationof valuable minerals. In order to design microwave applicators for this purpose, knowledge of theeffective dielectric properties of the crushed ore is required. Of particular interest is the effective complexpermittivity of the bulk crushed ore. The measurement of the effective permittivity of a large volume ofcrushed ore is most readily accomplished using the waveguide measurement technique. In this method arepresentative sample of the material is placed in a defined and fixed volume in a standard sizerectangular section metallic waveguide. The magnitude and phase angle of the transmitted and reflectedlow power microwaves through and from the sample are measured. The complex permittivity can beextracted from these so-called scattering, or Sij parameters.In this study the effective complex permittivities for two porphyry copper ores and a copper carbonatiteore were determined as a function of particle size distribution (-26.5+2mm) using two sizes of waveguide(WR284 and WR340). The sample holders incorporate dielectric windows for the location of the materialunder test. The extraction of dielectric properties from Sij parameter measurements is problematic usingstandard algorithms in such cases. Accordingly a new Database Extraction (DBE) Algorithm has beendeveloped. In this method, a database of scattering parameters is established through electromagneticmodelling of the measurement system. A search algorithm is used to determine the effective complexpermittivity of the modelled load whose scattering parameters provide the best fit to the experimentaldata. The goodness of the experimental fit of the simulated to the measured Sij parameters is determinedby a root mean squared deviation minimisation metric. Results show that the method can be used successfully to determine an effective complex permittivity for abulk volume of the crushed material. It is concluded that the dielectric property extraction over the fulloperational frequency interval (2.3-3 GHz) is preferred as it has a larger degree of extraction confidenceand hence reliability.Results show that with increasing particle size, the experimental fit between the simulated and measuredSij parameters becomes increasingly poor, as wall effect become more prominent. The effect is mostprominent for the smaller WR284 waveguide size. It is shown that for a waveguide size of similar size tothe particle size, the Sij parameter fitting is poorer compared to when a larger waveguide size is used. Theextracted complex permittivity reproducibility between repeated dielectric property measurements isimproved for the WR340 waveguide size, as the extractions in the WR284 waveguide is dominated by thecombined particle size and wall-effects of the sample holder.Ore mineralogy is identified as a key parameter that influences the dielectric properties of the crushedore. For ores with a dominant microwave absorbent mineral phase, the dielectric constant and loss factoris found to be larger, compared with ores with a more dominant microwave transparent gangue mineralphase.

[发布机构] Stellenbosch University