Microwave-assisted grinding
The use of rapid microwave heating to stress-fracture ore samples is discussed. Iron ores containing hematite, magnetite, and goethite were subjected to microwave energy in batch operations at 3 kW and heated to average maximum temperatures between 840 degrees C and 940 degrees C. Scanning electron microscope (SEM) photomicrographs verified fracturing along grain boundaries and throughout the gangue matrix. Standard Bond grindability tests showed that microwave heating reduced the work index of iron ores by 10 to 24%. Using a continuous-feed belt in a microwave applicator, samples heated more uniformly and quickly than in batch operations. In a microwave chamber designed to simulate a continuous-throughput operation at 3 kW, the grindability of a taconite ore was improved by 13% at a bulk temperature of 197 degrees C. Because stress cracking occurred at a lower temperature, less energy was consumed, thereby improving cost effectiveness. To further improve the economics, preliminary tests were conducted to increase heating rates by using higher microwave powers. The beneficial effects in the grinding circuit and in the extractive operation are discussed.<<ETX>>
Techno-Economic Considerations in the Commercial Microwave Processing of Mineral Ores
Microwave heating of mineral ores had previously been shown to result in process benefits such as reduced strength and improved mineral liberation, but the economics of the process were not attractive and no attention was given to feasible scale up. This paper provides an overview of the multi-disciplinary approach that has been required to address these failings and develop the technology to pilot scale. Thermal stress simulations show that the operation at high power densities and short residence times is the optimal operating strategy. Experiments using high power densities (>109 W/m3 absorbing phase) and short residence times (˜ 0.1 s) were used to confirm that the benefits can now be achieved at economically viable microwave energy inputs (˜ 1 kWh/t). In order to design applicators, reliable measurement of effective microwave properties of crushed ores is required. A new method has been developed to extract dielectric properties when the sample thickness is a multiple of half a guide wavelength at some point in the measurement band. Finite difference time domain modelling has been used to design and simulate applicators. A transverse E field applicator with a reflection compensating step has been developed, and a unit with a capacity of > 10 tons/h is being tested. Preliminary economic analysis shows that the overall cost of the process will be between US$ 0.16–0.85 per ton of ore.
Enhanced Magnetic Desulfurization of Coal
An oxidative alkaline pressure leach is shown to increase the magnetic susceptibility of coal-pyrite particles. The leach was successfully exploited as a pretreatment in the magnetic desulfurization of an eastern Canadian coal. Greater than 50% total sulfur (65% pyritic sulfur) rejection at 85% clean coal yield was achieved using a high-gradient magnetic separator (HGMS) at 1.3 T and 53 mm/s (2 in. per sec) flow rate, after pretreatment at 0.5 mol NaOH, 85°C (185°F), 930 kPa (135 psi) air pressure, 15 minutes retention time, and 20% solids by weight (w/w). Without pretreatment, sulfur rejection at these HGMS settings was less than 5%.