Dry solidification of chloride salts and heavy metals in waste incineration fly ash by mayenite.

Abstract

There are hazardous substances such as chloride salts and heavy metals in the municipal solid waste incineration fly ash (WIFA). During thermal treatment, the concentrated chlorides promote the volatilization of heavy metals, increasing the ecological risk. The water washing method is also employed as a pre-treatment for WIFA, but a substantial volume of wastewater with high chloride content is produced that poses challenges for effective treatment. This study integrates chemical stabilization with heat treatment method and suggests the utilization of a calcium aluminum oxide-mayenite (CA) for the solidification of chloride salts and heavy metals in WIFA. The experimental results indicate that adding CA for heat treatment has a significant solidification effect on chlorides. Under the conditions of WIFA: CA mass ratio of 1: 1 and temperature of 1200 °C, the chloride ions were solidified by forming Ca₁₂Al₁₄O₃₂Cl₂, with a fixation efficiency of up to 85 %, and most of the chlorides in WIFA became insoluble instead of soluble. Most of the heavy metals in WIFA were immobilized and doped into the crystal structure of CA, forming the catalytic metal-rich Ca₁₂Al₁₄O₃₂Cl₂ phase, which was subsequently applied to the degradation of chlorobenzene. Under an initial concentration of 512 ppm, the degradation efficiency of chlorobenzene reached 50.4 %. Through the introduction of CA, not only the solidification of chloride and heavy metals is achieved, but the high-value resource utilization of the final heat treatment product is also realized, providing a new method for the disposal of fly ash.