Temperature and frequency dependencies of the permittivities of selected pyrometallurgical reaction mixtures

Abstract

High temperatures are required in pyrometallurgical processes and consequently they are extremely energy intensive. Microwaves can offer several potential thermal energy efficiency advantages such as selective and/or rapid internal heating. The fundamental parameters which determine the interaction of microwaves with any given non-magnetic material are the real and the imaginary permittivities. In the current research, the permittivities of various mixtures of several pyrometallurgical raw materials plus carbonaceous reducing agents were determined as a function of both temperature and frequency, using the cavity perturbation technique. The raw materials investigated were an oolitic iron ore, an electric furnace dust and a chromite ore. As the temperature of a mixture increased, the permittivities began to increase rapidly, at temperatures lower than for the raw material alone. Subsequently, as the temperatures increased even further, the permittivities began to decrease and consequently peaks were typically observed in both the real and the imaginary permittivities. The changes in the permittivities of the mixtures were interpreted in terms of reduction reaction information available in the literature and also the amounts of the various species present as calculated by the Equilibrium Module of HSC Chemistry® 7.1. The initial rapid rise in the permittivities of the mixtures was attributed to the presence of the carbonaceous reducing agent, while the decrease was ascribed to the consumption of the carbonaceous reducing agent and also the reduction of metal oxides to metal. The permittivity data can be utilized to better understand the heating behaviour of the mixtures and also can be employed in numerical modelling.