Microwave heating has been extensively explored in various fields of materials processing. This technology exhibits unique characteristics including volumetric and selective heating, which eventually lead to many exceptional advantages over conventional processing methods including both energy and cost savings, improved product quality, faster processing and greater ecofriendliness, making microwave heating appropriate for applications in metallurgy. This paper presents a critical review on the use of microwave energy in metallurgy, with emphasis on both fundamentals of microwave heating and recent experimental efforts on extractive metallurgy via pyrometallurgical and/or hydrometallurgical routes. Applications to metallurgical processes for extraction of various metals, including heavy metals (Fe, Ni, Co, Cu, Pb and Zn), light metals (Al and Mg), rare metals (Ti, Mo, W and Re) and precious metals (Au, Ag and Pt), are reviewed and discussed.
The microwave absorption capability of an eastern high
volatile
bituminous coal from West Virginia was investigated by measuring the
dielectric properties from room temperature to ∼900 °C
in ultrahigh purity (UHP) argon, at both 915 and 2450 MHz. The dielectric
properties remain relatively constant below 500 °C. As the temperature
continues to increase, however, the relative dielectric constant and
loss factor increase dramatically. This is due to the release of volatiles,
resulting in the increased electrical conductivity and higher microwave
loss. The calculation
of microwave penetration depth shows that the pyrolysis process significantly
improves the microwave absorption capability of the coal at high temperatures.
The calculated reflection losses of the coal sample suggest that the
maximum microwave absorption with the reflection losses of −41.25
dB and −32.54 dB can be obtained for the coal having thicknesses
of 0.14 and 0.20 m at 915 and 2450 MHz, respectively. The sample dimension
has a significant effect on the overall performance of microwave absorption
of coal during pyrolysis and thus on the efficiency of microwave coal
pyrolysis.
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