Stabilization of a chlorine-rich fly ash by colloidal silica solution

Fly ash from municipal solid waste incinerators (MSWI) consists of various substances, including a lot of heavy metals. In cases where fly ash contains a lot of chlorides, it is very difficult to apply general treatment methods because chlorides could hinder hydration in cementation and cause great loss in vitrification. In this study, we report a promising method for the treatment of fly ash containing a high concentration of chlorides. A colloidal silica solution was induced to stabilize the fly ash. The fly ash used in this research has a chlorine level over 35 wt.% as well as containing heavy metals, such as Pb (1120 ppm) and Zn (5430 ppm). The samples were prepared at a temperature of 600-800 °C for 2 h after mixing with the fly ash and a 4 wt.% colloidal silica solution. The effect of the colloidal silica was evaluated by the leaching test, Toxicity Characteristic Leaching Procedure (TCLP). The solidified products with fly ash and colloidal silica at temperatures of more than 700 °C had an excellent resistance, with Pb2+ <0.02 ppm, Zn2+ <0.52 ppm, and Cd2+, Cr2+, Cu2+, Mn2+ <0.01 ppm, and it could be influenced by the phases containing Ca and Si formed above 600 °C and reduction of a chemically weak phase.