Deriving the ideal ore texture for microwave treatment of metalliferous ores

High power density microwave treatments on metalliferous ores have historically been shown to reduce ore competency prior to beneficiation at economically feasible energy inputs. However, the relationship between mineralogical textural features and the extent of the microwave-induced fracturing had previously been limited to qualitative descriptions or simplistic two-phase numerical models, which could not account for the complex mineral assemblages in real ores. In this paper, mineralogy, grain size, dissemination, textural consistency and mineral associations were determined for 13 commercially exploited nickel, copper and lead-zinc ores using a Mineral Liberation Analyser (MLA). The ores were subjected to high power density microwave treatments at up to 25kW in a single mode cavity with microwave energy inputs of approximately 0.5-10kWh/t, and the subsequent reductions in ore competency were measured by the Point Load Test. The ores that demonstrated the greatest reductions in strength typically contained between approximately 2%wt to 20%wt of highly microwave-absorbing minerals, with a native grain size d50 greater than approximately 500µm, constrained by hard matrix minerals such as quartz and feldspar. Texturally consistent ores with a high proportion of amenable textures also demonstrated the highest average reductions in strength. These findings support the qualitative descriptions and numerical modelling results available in the literature and provide a baseline for selecting likely candidate ores for microwave treatments in the future

Increasing the grind size for effective liberation and flotation of a porphyry copper ore by microwave treatment

In this paper, mineralogy, grain size, dissemination, textural consistency and mineral associations were determined for a commercially exploited porphyry copper ore using a Mineral Liberation Analyser (MLA). The ore was subjected to high power density microwave treatments in a single mode cavity at 15 kW and approximately 2 kW h/t. The untreated and microwave-treated samples were subsequently milled to two grind sizes near the nominal plant grind size and a size-by-liberation analysis performed. The analysis revealed that equivalent liberation could be obtained at a grind size approximately 50–60 μm coarser than the nominal plant grind due to the microwave treatment. Flotation testing indicated that an increase in copper recovery of approximately 1% could be achieved, or that a grind size increase of approximately 30 μm may potentially yield equivalent copper recovery due to the microwave-enhanced liberation observed. However, statistical analyses demonstrated that it is difficult to attain confidence in recovery increases of approximately 1% even when conducting batch flotation tests in triplicate. The ore under investigation had previously been shown to produce only modest average reductions in strength (∼8%) under similar microwave treatment conditions due to a prevalence of many unfavourable textures. However, the preferential association of copper minerals with a hard matrix mineral (quartz) and a hard microwave-absorbent mineral (pyrite) resulted in a significant change in liberation behaviour

Understanding selectivity in radio frequency and microwave sorting of porphyry copper ores

Continuous high-throughput microwave treatment followed by infrared thermal imaging (MW-IR) has previously been shown to provide attractive separations for a number of porphyry copper ores, leading to rejection of a large proportion of barren fragments from ore-grade material or concentration of copper values from waste-grade material. However, the efficacy of the sorting process is reduced by the presence of hydrated clays and pyrite. Literature measurements have shown differences in the conductivity of pyrite and copper sulphides such as chalcopyrite at radio frequencies. In this work the potential of using radio frequency (RF) heating to exploit these differences and achieve improved selectivity between copper and iron sulphides, is investigated. For the first time a novel bulk materials handling and presentation method that facilitates even heating of angular ore fragments in parallel plate RF systems is discussed. The fragment-by-fragment thermal response of five ore samples under equivalent pilot MW-IR and RF-IR processing conditions is evaluated, showing that there is an increase in selectivity in the heating of hydrated clay minerals in RF compared to microwave. It is suggested, again for the first time, that selectivity in the microwave processing of ores containing semi-conducting minerals is due predominantly to magnetic absorption (induction heating) caused by eddy currents associated with the magnetic field component of electromagnetic energy. In radio frequency processing, where electric field is the dominant component, heating of semi-conducting minerals is limited by the electric field screening effect. This effect is demonstrated using synthetic fragments. Thermal response profiles of synthetic fragments show that approximately 2.5 times the mass of sulphide minerals to hydrated clay minerals would result in an equal temperature increase for microwave heated fragments in which the microwave-heating minerals are evenly disseminated throughout the matrix. This understanding provides the foundations for development of models incorporating different thermal responses for individual heated phases, alongside other textural and treatment variables, that can be used to predict how close to intrinsic sortability ores will perform in MW-IR and RF-IR without the need for extensive processing trials

Production of high purity silica by microfluidic-inclusion fracture using microwave pre-treatment

© 2018 Demand for high purity silica used in component manufacture is set to outstrip current supply in the near future. As such, alternative processing routes to feed-stock materials suitable for use in lighting and solar cell fabrication are required, without having to rely on reject material from semi-conductor manufacture. In this work, we report a facile, environmentally friendly method of producing quartz powder with a total residual impurity level of 30 ± 3 ppm from whole pebbles having an initial impurity level of 158 ± 22 ppm. This has been achieved using a metallurgical upgrading process incorporating microwave pre-treatment, crushing and milling, High Intensity Wet Magnetic Separation (HIWMS) and acid leaching. This process yielded a quartz powder having an 80% reduction in residual impurities compared to the untreated quartz pebbles. Pre-treatment of whole quartz pebbles in a multimode microwave cavity for 10 min yielded a reduction of the residual elemental impurity content associated with micro-fluidic inclusion sites containing calcium, potassium and sodium of 84, 78, and 50% respectively. Statistically significant reduction in residual aluminium phases was also observed (83%) compared to the as received material to below the IOTA® specification for Ultra High Pure Quartz produced by Sibleco. Mechanistically, this has been achieved by selectively heating impurity containing micro-fluidic inclusion sites. Resulting in their explosive decrepitation and enabling removal of the impurities in subsequent processing steps. It has been concluded that natural quartz pebbles can be upgraded through a combination of microwave treatment, magnetic and chemical refinement to produce a viable feedstock for the subsequent production of solar grade silicon

A tool for predicting heating uniformity in industrial radio frequency processing

Radio frequency energy is utilised for heating in a wide range of applications, particularly in the food industry. A major challenge of RF processing is non-uniform heating in loads of variable and angular geometry, leading to reduced quality and product damage. In the study, the specific effects of geometry on the heating profiles of a range of geometrically variable loads in an industrial scale RF system are analysed, and the understanding used to derive a general tool to predict heating uniformity. Potato was selected as a test material for experimental work; dielectric properties were measured using a 44mm coaxial probe. Analysis of simulated and experimental surface temperature profiles and simulated power uniformity indices indicates that the presence of vertices and edges on angular particles, and their proximity to faces perpendicular to the RF electrodes increases localised heating; faces parallel to the electrodes heated less than those faces perpendicular to them. Comparison of the same geometrical shape in different orientations indicates that overall power absorption uniformity can be better even when localised heating of edges is greater. It is suggested, for the first time, that the rotation of angular shapes within a parallel plate electric field can improve heating uniformity, and that this can be achieved through the design of bespoke electrode systems. A Euler characteristic based shape factor is proposed, again for the first time, that can predict heating uniformity for solid, dielectrically homogenous shapes. This provides industry with a tool to quickly determine the feasibility for uniform RF heating of different three dimensional shapes based on geometry alone. This provides a screening method for food technologists developing new products, allowing rapid assessment of potential heating uniformity and reducing the need for early stage specialist computational modelling

Statistical description of inhomogeneous samples by scanning microwave microscopy

A quantitative analysis of the dielectric properties of a multiphase sample using a scanning microwave microscope (SMM) is proposed. The method is demonstrated using inhomogeneous samples composed of a resin containing micrometric inclusions of a known ceramic material. The SMM suitable for this task employs relatively large tips (tens of micrometers in diameter). Additionally, in order to make the instrument more suitable for high-throughput analysis, an original design for rapid tip changes is implemented. Single-point measurements of dielectric constant at random locations on the sample were performed, leading to histograms of dielectric constant values. These are related to the dielectric constants of the two phases using Maxwell–Garnett effective medium theory, taking into account the volume-of-interaction in the sample beneath the tip

Understanding the scabbling of concrete using microwave energy

This paper reports on the use of microwave energy to scabble concrete. While the technique is not new, little information exists relating to the controllability of the process, the effect of different types of concrete and the performance and durability of the scabbled concrete post treatment. Concrete blocks supplied by the UK Sellafield nuclear site were treated with microwave energy using a 15kW system, operating at 2.45GHz. The effect of aggregate type (Whinstone, Gravel and Limestone); standoff distance; and effect of surface coating were studied to determine their influence on the systems performance, in terms of mass and area removal rates. Complimentary modelling studies were undertaken relating the power density of the microwave energy to observed scabbling profile. Mass and area removal rates averaged 11.3gs-1 and 3cms-1 respectively on treating large areas to a depth of 25mm. The process was shown to not adversely affect structural properties of the specimens after treatment

Microwave treatment of electric arc furnace dust with Tetrabromobisphenol A: Dielectric characterization and pyrolysis-leaching

In the present work microwave treatment of electric arc furnace dust (EAFD) mixed with tetrabromobisphenol A (TBBPA) was investigated. A range of characterization techniques were used to understand the thermal behaviour of TBBPA-EAFD mixtures under microwave pyrolysis conditions. Dielectric and thermal properties of EAFD, TBBPA and their mixtures were determined. Both the dielectric constant and loss factor of the mixture were found to vary considerably with temperature and subsequently it was found that the mixtures of these materials absorbed microwaves effectively, especially at temperatures above 170 °C. The high loss tangent of EAFD-TBBPA mixture above 170 °C resulted in fast heating and high temperatures (above 700 °C) resulting in reduction of Fe, Pb and Zn to their metallic form. This resulted in low recoveries of both Zn and Pb when the residue was leached in water. The recovery of Zn varied between 14 and 52 wt,%, while Pb recovery varied between 3 and 31 wt.% depending on microwave treatment efficiency. The low recovery of Zn and Pb could be ascribed by the reduction of metal oxides into their metallic form. More importantly this work has shown great selectivity in the leachability of both zinc and iron; with iron being left in the solid residue

Microwave pyrolysis of pecan nut shell and thermogravimetric, textural and spectroscopic characterization of carbonaceous products

In the present work, the pyrolysis of pecan nut shell was studied using microwave technology at different input power and exposure time. The carbonaceous products were characterized using elemental analysis, potentiometric titration, thermogravimetric analysis, N2 adsorption isotherms at −196 °C and FT-IR spectroscopy. The characteristics of microwave carbonaceous products were compared with carbons prepared by conventional heating and commercial carbons and finally, the interaction of textile dyes with the carbonaceous materials was studied. The results are indicating that it is possible to obtain carbonaceous materials with similar textural parameters in conventional and microwave systems, but for microwave heating the processing times are short (3 min). Also, the exposure time of 2 min of microwave are not enough for the complete depolymerization of the lignocellulosic matrix of the pecan nut shell and significant amount of cellulose and hemicelluloses remain in char after microwave treatment. Finally, all the carbons obtained by microwave heating are microporous materials with a high number of basic groups on their surface and the high molecular size of dyes is controlling the adsorption on these materials

High-resolution dielectric characterization of minerals: a step towards understanding the basic interactions between microwaves and rocks

Microwave energy was demonstrated to be potentially beneficial for reducing the cost of several steps of the mining process. Significant literature was developed about this topic but few studies are focused on understanding the interaction between microwaves and minerals at a fundamental level in order to elucidate the underlying physical processes that control the observed phenomena. This is ascribed to the complexity of such phenomena, related to chemical and physical transformations, where electrical, thermal and mechanical forces play concurrent roles. In this work a new characterization method for the dielectric properties of mineral samples at microwave frequencies is presented. The method is based upon the scanning microwave microscopy technique that enables measurement of the dielectric constant, loss factor and conductivity with extremely high spatial resolution and accuracy. As opposed to conventional dielectric techniques, the scanning microwave microscope can then access and measure the dielectric properties of micrometric-sized mineral inclusions within a complex structure of natural rock. In this work two micrometric hematite inclusions were characterized at a microwave frequency of 3 GHz. Scanning electron microscopy/energy-dispersive x-ray spectroscopy and confocal micro-Raman spectroscopy were used to determine the structural details and chemical and elemental composition of mineral sample on similar scale