N.A. Rowson

Dielectric properties of coal

Dielectric measurements of UK coals have shown that electric permittivity decreases with coal rank (dry, mineral free basis), however, the moisture and mineral matter content may increase the bulk coal electric permittivity. Electric permittivity increases with low rank coals as a result of increasing moisture levels associated with low rank coals. The relative real permittivity of coal is greater than most composite mineral matter with the exception of pyrite, which may increase the bulk coal electric permittivity if present in sufficient quantity. No significant changes in electric permittivity were detected in the frequency range tested.

Microwave treatment of minerals-a review

This paper presents a review of the advances in the microwave treatment of minerals from the early stages of development to possibilities for future utilisation. Many different applications are considered, including fundamental heating rate studies, microwave assisted grinding, possible exploitation in the area of extractive metallurgy and also microwave treatment of coal. Conclusions are presented regarding the need for further fundamental and pilot scale data. Further indications of the potential for the commercial exploitation of microwaves within the mineral processing and extractive metallurgical industries have been included.

The influence of mineralogy on microwave assisted grinding

The influence of mineralogy on microwave assisted grinding is elucidated. Detailed mineralogical analysis of various commercially exploited ores was carried out and the textural relationships between the individual mineral phases established. Microwave heating profiles were determined for each constituent mineral. Each ore was then subjected to microwave radiation for varying time periods, and change in work index with microwave exposure time quantified. It is suggested that increases in grindability of ores after microwave treatment are related to the specific mineral species present, particle size of the specific mineral and also the degree of dissemination. Conclusions are made regarding the economic implementation of this technology.

A novel electrobiotechnology for the recovery of precious metals from spent automotive catalysts

Abstract Platinum group metals are routinely used in automotive catalysts but recycle technology lags behind demand. There is no available ‘clean technology’ and leach solutions (e.g. aqua regia) to solubilise the metals from scrap are highly aggressive. A microwave‐assisted leaching method was developed which gave 80% metals recovery, with the leach time reduced from 2 h to 15 min using 50% (aq.) diluted aqua regia to give potentially a more biocompatible leachate. Desulfovibrio desulfuricans reduces soluble platinum group metals to cell‐bound insoluble base metals (e.g. Pd(II) ? Pd(0)). For use, biofilm was immobilised on a Pd‐23% Ag solid alloy membrane which delivered H” to the cells via an electrochemical chamber at the back‐side. The biomass‐coated Pd‐Ag alloy electrode was used in a flow‐through reactor for recovery of Pd, Pt and Rh from aqua regia leachates (pH 2.5) of spent automotive catalysts with up to 90% efficiency at a flow residence time of 15 minutes. Free cells did not reduce platinum group metals from the leachates but the electrobioreactor did so using biofilm‐cells preloaded with Pd(0). Reactors lacking biomass or reactors with heat‐killed biofilm removed less platinum group metals, via electrochemically‐synthesised H’ reductant alone. The use of an active biofilm layer in a flow‐through electrobioreactor provides a simple, clean and rapid potential recycle technology.

The effect of microwave radiation upon the processing of Neves Corvo copper ore

An investigation into the effect of microwave radiation on the processing of a massive sulphide copper ore has been carried out. It is shown that significant reductions in the Bond work index can be achieved without any discernible adverse effect on the subsequent flotation process. A maximum reduction in work index of 70% was achieved after microwave exposure for 90 s. Microwave exposure followed by water quenching is shown to reduce the work index by up to 15% more than for unquenched samples. Initial trials on the effect of mass loading on the reduction in work index indicate that there exists an optimum mass of material that can be pretreated effectively by microwave radiation in any microwave cavity. Process simulation on the package USIMPAC showed significant flowsheet changes are possible as a result of exposure to microwave radiation and the subsequent reduction in work index. Conclusions are made regarding the economic implications of microwave pretreatment on the flowsheet.

The effect of Pb(NO3)2 on ilmenite flotation

Abstract A novel activation flotation method for ilmenite was developed in this study. Pb(NO3)2 was used as an activator to effectively and selectively improve ilmenite flotability. The results of the flotation tests for a massive ilmenite ore sample indicated that, when the dosage of Pb(NO3)2 was 60 g/tonne, ilmenite recovery increased from 65% during conventional flotation to 83% during Pb(NO3)2 activated flotation. The ilmenite concentrate contained 36.6% TiO2.

Microwave reduction of oxidised ilmenite concentrates

Abstract This paper covers preliminary investigations into the novel use of microwave energy as a heat source for the partial reduction of oxidised ilmenite (FeTiO3) concentrates. Microwave energy has potential for the speedy and efficient heating of minerals and in a commerical context may provide savings in both time and energy. The oxidation and reduction of iron in ilmenite concentrates between the ferrous and ferric states has been found to greatly enhance its chemical activity. When reduced to the correct extent, iron can be preferentially extracted to yield a titanium rich beneficiate suitable for use as a feed for the production of titanium dioxide (TiO2) pigments via the Chloride Process. Two ilmenite concentrates were first oxidised in a conventional muffle furnace at 1000°C. The ferric iron was then reduced back to ferrous iron by heating with a fine carbon powder under an inert nitrogen atmosphere. The reduction process was carried out in a variable power (0 to 1500 kW), 2.45 GHz microwave oven. Duplicate control samples were reduced at temperatures over the range 700 to 1000°C in a conventional muffle furnace. The mineral phase changes occurring during the oxidation and reduction stages were detected by X-ray diffraction techniques. Changes in the surface morphology of the concentrate particles were detected by scanning electron microscopy and quantitatively described through changes in the specific surface area. Samples were leached with 25% hydrochloric acid to detect changes in the activity of both iron and titanium components.

Modification of ilmenite surface chemistry for enhancing surfactants adsorption and bubble attachment.

In this study, microwave irradiation is used to modify ilmenite surface chemistry to enhance the adsorption of surfactants and the air bubble attachment. The results indicate that microwave irradiation can increase ilmenite flotation recovery by 20%. A positron emission particle tracking technique is used to study the dynamic behaviour of ilmenite particles in a Denver cell. The data shows that the poor flotation recovery of ilmenite is not only due to the reduce probability of ilmenite being captured by air bubbles, but also the short residence time of the particles remaining in the froth phase. The ilmenite particles can be frequently captured by air bubbles, but dropped to the bulk liquid from the froth phase, normally over 15 s. Microwave irradiation changes the ilmenite flow pattern in the Denver cell. The average time of ilmenite remaining in froth phase is increased from 11.5 to 29.1 s.

Twin screw wet granulation: the study of a continuous twin screw granulator using Positron Emission Particle Tracking (PEPT) technique.

In this paper, Positron Emission Particle Tracking (PEPT) techniques are utilised to track the trajectory of single particles through the mixing and conveying zones of a Twin Screw Granulator (TSG). A TSG consisting of conveying zones and mixing zones is used in this study. The mixing zones are arranged with kneading discs at an angle of 30°, 60° or 90°. Experiments were carried out using different mixing configurations with various screw speed and total mass flow rate. The PEPT data obtained were then utilised to obtain the residence time distribution (RTD) and the Peclet number in an attempt to gain some insight into the mixing of the process. The fill level of the granulator was also estimated to study the mechanism of granulation. As might be expected, it was shown that the residence time of the granulation process increases with decreasing screw speed. It also increases with increasing angle of the arrangement of kneading blocks in the mixing zones, but will decreases when powder feed rate is increased. The fill level of the mixing zone in particular increases when the screw speed decreases or when powder feed rate increases. Furthermore, the fill level of the granulator will increase when the mixing zone configuration changes from 30° to 90°. It is shown that the granulator is never fully filled, even using 90° mixer elements implying limited compaction which may explain why the granules produced are porous compared with those from a high shear mixer. Interestingly, the RTD analysis reveals that the extent of axial mixing in the mixing zone of the granulator does not change significantly for different configurations and process conditions. There is evidence of a tail in the RTD which implies some material hold up and channelling.

A new method for determining the optimum dispersant concentration in aqueous grinding

The use of electroacoustics as a method for determining the optimum dispersant dosage for the ultrafine grinding of limestone is presented. The technique measures the zeta potential of concentrated suspensions, hence it was possible to study the adsorption of three commercially available polyelectrolyte dispersants onto the limestone. The optimum dosage of the dispersants was determined and the most suitable one chosen for the grinding experiments. By utilising this optimum dispersant dosage, the productivity and throughput of a stirred vertical mill was greatly enhanced.