Permittivities of Electric Arc Furnace Dust/Lignin Mixtures as a Function of Temperature, Frequency and Gas Composition

Abstract

Increased electrification of pyrometallurgical processes and reduced greenhouse gas (GHG) emissions are important environmental targets both in Europe and around the world. Microwave heating is considered a potential new energy source that can use electricity from renewables and is consequently a more sustainable pyrometallurgical method. Studying the permittivities of the materials of interest under different conditions is a crucial step in simulating understanding the behavior of these materials in microwave fields and designing microwave processes. The present work aims to investigate the permittivities of electric arc furnace (EAF) dust/lignin mixtures under various conditions. The effect of the reducing gas composition on the permittivity changes as a function of temperature and frequency was studied.Reduction did not occur within the EAF dust/lignin mixture in an air atmosphere. The changes in the permittivities of the mixture were associated with reactions in the range of 500°C–600°C and were related to the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide. In an argon (Ar) atmosphere, the EAF dust/lignin mixture permittivities exhibited two main peaks, one at approximately 600°C and another at approximately 840°C and these were due to the pyrolysis of lignin (biochar formation) and the reduction of both zinc oxide and zinc ferrite to metallic zinc and iron, respectively. In a hydrogen–argon (4% H2–Ar) atmosphere, the EAF dust/lignin mixture permittivities displayed two peaks at approximately 600°C and 715°C that were attributed to the pyrolysis of the biochar and the reduction of both zinc oxide and zinc ferrite, respectively. In the hydrogen-containing atmosphere, the second peak occurred at a lower temperature. After the second peak in both the Ar and H2–Ar atmospheres, when the reduction reactions were completed, rapid decreases in the permittivities were observed. At this stage, the biochar was consumed, and the metals were produced, which resulted in a decrease in the permittivities.

Keywords: Permittivity, microwaves, heating behaviour, EAF dust, Lignin, biochar, cavity perturbation