Microwave heating behaviour of nickeliferous limonitic laterite ores

Abstract

The nickeliferous laterite ores, in which the nickel occurs in oxide form, represent a significant potential resource of metallic nickel. However, in comparison to the nickel-containing sulfide ores, the extraction costs are relatively high and thus it will be necessary to develop new processing techniques, which are both technically and economically viable. In the present research, the potential application of microwaves for the heating of a nickeliferous limonitic laterite ore ((Fe,Ni)O(OH) · nH2O) was investigated. Firstly, since the nickeliferous limonitic laterite ore contains considerable moisture, both free and combined, then thermogravimetric analysis (TGA) was performed in order to characterize the changes, which result from the dehydration processes. Derivative thermogravimetric analysis (DTGA) curves were calculated from the TGA data. Secondly, the real (ε′) and imaginary (ε″) permittivities of the ore were measured at frequencies of 912 and 2460 MHz at temperatures up to about 1000 °C using the cavity perturbation technique and these results were related to the DTGA curves. Also, the loss tangent (tanδ=ε″/ε′) was calculated from the permittivity data. Finally, the microwave heating behaviour of the nickeliferous limonitic laterite ore was determined at 2460 MHz. The results show that the both the real (ε′) and imaginary permittivities (ε″) and the loss tangent (tanδ) increase with temperature and change as both the free and the combined moisture are removed. The permittivities (ε′ and ε″) increased with increasing slope of the TGA curve and vice versa during the goethite to hematite dehydroxylation reaction, where there was a maximum in the permittivities (ε′ and ε″). It is proposed that these changes, which occur during the dehydroxylation reaction, are a result of the liberation of hydroxyl units from the goethite structure. Furthermore, some hydroxyl units were retained in the hematite structure (i.e. hydrohematite (α-Fe2−x/3O3−xOHx)) even after the goethite to hematite transition and thus the permittivities of the dehydroxylated ore were higher than that of normal hematite. With regards, to their microwave heating behaviour, it is shown that despite the relatively low permittivities (ε′ and ε″) of these materials at low temperatures, they can be heated using microwaves. The microwave heating process can be improved by conventional preheating of the sample or by the addition of charcoal as a susceptor or by selecting a crucible material, which acts as a susceptor. There is a rapid increase in the permittivities over the temperature range of about 600–800 °C and this combined with the low thermal conductivities of these oxides, can result in rapid internal heating of the sample and thermal runaway.