Kinetics modeling of the volatilization of mercury compounds involved in spent mercury-containing catalyst under microwave irradiation

The industrial design and development of microwave-induced volatilization of hazardous substances in waste greatly depend on their dielectric properties. The dielectric properties of different mercury compounds in spent mercury-containing catalyst (SAC) were investigated by the cavity perturbation method at different temperatures. The decomposition absorption mechanisms for HgS and HgCl₂ show the same trend according to thermogravimetric analysis results over the temperature range of interest. The SAC composite exhibits sufficient microwave absorption from 20 °C to approximately 600 °C owing to the superior dielectric properties of non-volatile mercury-containing compounds (HgCl₂ and HgS). At temperatures above 600 °C, the dielectric properties of the SAC are determined by the residual mercury and carbon matrix. Phase analyses indicate that the entire process of microwave heating could be divided into three stages: (1) The volatilization temperature of unbound water and some impurities are between 20 °C and 100 °C; (2) The main thermal disintegration region of mercury species is between 100 °C and 600 °C; (3) HgCl₂ and HgS volatilize completely to gas at 600 °C. Meanwhile, HgCl₂ decomposing more readily than HgS.