Prediction of chloride solubility in CaO–Al2O3–SiO2 glass systems

It is known that fly ash from municipal waste incinerators is particularly problematic because it contains a significant concentration of heavy metals and chloride. One of the methods for treating chloride is to entrap it in a glassy slag, which is produced by melting bottom ash with fly ash. The main components of the glassy slag are SiO2, Al2O3 and CaO. However, only a limited amount of chloride can be retained in the glass, and the relationship between glass structure and chloride solubility is still unclear. The present study was intended to examine the solubility of chloride in slag-like CaO–Al2O3–SiO2 glass systems in which CaCl2 was substituted for CaO. Glass compositions with varying non-bridging oxygen (NBO) content were investigated. It was found that NBO in glass has a significant influence on glass properties. The solubility of chloride was also found to strongly relate to the NBO content. It was proposed that chloride ions occupy the site around NBO and the chloride solubility should be determined by the NBO content. The maximum chloride content was found to agree with this glass structure model. This study could allow us to predict the maximum chloride content that can be dissolved in the slag.