In ironmaking and steelmaking processes, dehydration of wet substances and minerals combined by H2O are often recognized to be a key to stable practices. This study aims at understanding dehydration behavior of goethite blended with graphite by microwave treatment simulating drying wet sludge and poor iron ore with combined water.Firstly blend specimens with various conditions, powdery or pelletized, were heated by microwave apparatus. Dehydration rate of goethite increased with increasing temperature. Denser specimen of pelletized was confirmed to be less effective to heat up the whole part. It was found that surplus addition of graphite decreased efficiency in dehydration rate. Thus, it was estimated that a specimen with smaller amount of graphite, whose blend ratio of FeO(OH):C was 1:1, had sufficiently larger distance between graphite particles, through which microwaves could adequately penetrate into the core of the specimen. To prove this hypothesis, Network Analyzer helped evaluate penetration depth of microwave by the measurement of permittivity. The measured results showed that penetration depth decreased with increasing apparent density consistently with the heating behavior by microwave treatment. At the blend ratio of 1:2 of FeO(OH):C, however, the specimen was estimated to behave as if it was under percolation.

Microwave penetration distance into powder mixtures of FeO(OH)/graphite (C) and tri-calciumphosphate/C was studied by changing the fraction of graphite powder and the degree of the compression. They are combined into a parameter of carbon volume fraction [Pct. Vc]. Experimentally, it was shown that the penetration distance decreased at high [Pct. Vc]. Measurement of permittivity became impossible at certain [Pct. Vc], which is related with the abrupt increase in DC conductivity (occurrence of percolation). In this study, dependence of DC conductivity of the mixture on [Pct. Vc] was expressed using generalized effective medium approximation. And then, average permittivity of the mixture below percolation threshold was measured and analyzed using a mixing rule based on effective medium approximation. In this procedure, permittivity of carbon was estimated to fit the data of the measured average permittivity. Transition from the dielectric to the conductive nature of the powder mixture influences the penetration distance; however, it was shown that the predicted penetration distances by equations using conductivity (σ) or complex permittivity (ε) of the mixtures are consistent with each other, namely, their extrapolations are continuous across the transition region of the percolation threshold.

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