Microwave-absorbing characteristics of secondary copper smelting fly ash: High-temperature dielectric properties, thermal behavior, and microwave heating mechanism

Abstract

Investigating the microwave-absorbing characteristics of secondary copper smelting fly ash (SCFA), which is both a hazardous waste and valuable secondary resource, can be expected to expedite the efficient application of microwave heating technology in the comprehensive treatment of fly ash. Here, we have examined SCFA and its pure components, namely PbCl2, ZnO, and ZnS phases, with regard to thermal behavior and microwave-absorbing characteristics. Our results indicate that SCFA has excellent microwave-absorbing performance in the temperature range 30–800 °C, with a dielectric loss coefficient ε″ of 5.68 × 10−2 F m−1 at ambient temperature and a maximum value of 36.29 × 10−2 F m−1 at 550 °C. As a consequence, the SCFA could be heated to 700 °C within 12.6 min. The heating process of SCFA can be divided into three main stages: removal of adsorbed water and bound water (<260 °C), oxidation reactions of Pb, PbO, ZnS, and PbS (260–437 °C), and deoxygenation reactions of PbO2, Pb3O2, and Pb3O4 (437–1000 °C). The dielectric properties and thermal characteristics of SCFA are closely related, with PbCl2 exerting a significant influence. We propose herein a microwave-heating mechanism of SCFA based on analysis of its thermogravimetric characteristics, dielectric properties, and heating behavior in a microwave field.