Temperature, frequency and compositional dependencies of the permittivities of hydroxide minerals

•Permittivities of hydroxides as a function of temperature and frequency.•Permittivities exhibited an inverse dependency on frequency, particularly the imaginary permittivities.•Dehydroxylation processes characterized by an increase, a maximum and then a decrease in the permitivities.•Minerals with a higher iron content typically had higher permitivities.•Hydroxylated mineral had higher permitivities than the dehydroxylated mineral. In the last few decades there has been increasing interest in the application of microwaves as an energy source in extractive metallurgy for the treatment of ores and concentrates. The fundamental parameters required for the design of industrial systems are the real and the imaginary permittivities, which are both frequency and temperature dependent. However, there is a dearth of data on the permittivities, of not only the ores and the concentrates, but also the minerals that comprise the ores. In the present research, the permittivities of selected hydroxide minerals were determined using the cavity perturbation technique as a function of both temperature and frequency. The behaviours of the permittivities were interpreted using both thermogravimetric analysis (TGA) and thermodynamic modelling. The water in these minerals, in its various forms, makes a significant contribution to the permittivities. Also, generally, the higher iron-bearing hydroxides have higher permittivities than the magnesium silicates or aluminosilicates. It is shown that the permittivities of the hydroxide minerals are low, but the hydroxyl ion makes a significant contribution. The effect of frequency is more pronounced both during dehydroxylation and at high temperatures and, in both cases, is attributed to increases in the ac conductivity.