Thermal evolution of mercury from waste rare earth phosphor: Effect of thermal treatment conditions and kinetic analysis

Abstract

Waste phosphor (WP) from fluorescent lamp is a hazardous waste and an important secondary resource for recycling mercury and rare earths. To avoid mercury pollution during rare earth recovery, it is necessary to conduct deep mercury removal before rare earth recovery. However, the mechanism of mercury removal in WP is not clear, which restricts the progress of efficient mercury removal methods. Effects of treatment temperature and time on thermal evolution of WP were analyzed by different characterization methods. Model-free, model-fitting and z(α) master plots method are used in combination to explain the complex mechanism of mercury removal in WP. The mechanism of mercury removal will change gradually with the process of mercury removal. The average Eα for mercury removal is 100.76 kJ/mol and 190.36 kJ/mol at conversion α (i.e. mercury removal efficiency) of 0.1–0.3 and 0.3–0.9, respectively. Simple mercury compounds are decomposed at α less than 0.3 (∼600 °C), according with the model of random nucleation and subsequent growth. A large amount of tightly combined mercury is released rapidly at α of 0.3–0.75 (600–700 °C), ascribing to the decomposition of carbonate and the structure transformation of silicon oxide. The process mechanism gradually change from 1D/2D diffusion to 3D diffusion, increasing the mass transfer resistance of mercury diffusion. Thermal desorption of residual bound mercury at α larger than 0.75 (∼700 °C) is controlled by phase boundary reaction. More efficient mercury removal effects can be obtained by adding additives to roasting. The results are beneficial to the research of deep mercury removal methods of WP.