Investigation On Preparation Of Glass Ceramics Using MSWI Fly Ash Vitrified By Thermal Plasma

According to the regulation constituted by the Ministry of Environmental Protection (MEP) of the People's Republic of China, municipal solid waste incinerator (MSWI) fly ash is classified as hazardous material and must be stabilized or immobilized to make reasonably safe and stable.In this study, two typical types of MSWI fly ashes were used. Chemical composition and ash fusion point of fly ashes were analyzed. In order to study the effect of additives on ash fusion point, SiO2 and CaO with the different proportion were mixed into fly ash. DC thermal plasma was applied to dispose fly ash. The microstructures and crystalline components of fly ashes and produced slags were observed by scanning electron microscope (SEM) and X-ray diffraction (XRD). Results are shown that thermal plasma can vitrify fly ashes. The increase of SiO2 additive proportion makes the fly fusion point higher, while the addition of CaO could decrease the point under certain conditions. When the basicity of fly ash is about 1.5, it has the lowest fusion point. After vitrification, the slags are homogenous, vitreous and glazed in nature, opaque with dark color appearance, and amorphous and glassy with no crystalline peaks.Two type fly ashes were sintered for different times at different temperatures, respectively. The crystal transition, weight loss rate and basicity change of fly ash when sintering were studied, and the transfer characteristic of main chemical components were investigated too. Results show that there is sophisticated crystal transition when fly ash is vitrified. The sintering time has little effect on weight loss, while sintering temperature has significant effect on weight loss. The higher the content of Cl and SO3 in fly ash, the less the weight loss. The basicity of fly ash became lower and lower when sintering. The volatilization rate of main chemical components in slag presumably decreases in the following sequence: Cl>S>K>Na>Mg>P>Fe>Ti>Al≈Si>Ca.The transfer characteristic of heavy metal was investigated. The volatilization rate of heavy metals in slag presumably decreases as follow:Pb>Cd>Cu>Zn>Cr>Ni.The effect of additives SiO2 and CaO on heavy metal residual fraction, basicity, content of SiO2+CaO+Al2O3 and residual fraction of Na, K, Cl and S were studied. Results show that the additives SiO2 and CaO cause decrease of the weight loss lower. Additives SiO2 and CaO have different effects on residual fraction of Na, K, Cl and S. The average content of SiO2+CaO+Al2O3 of slags AG1-AG9 and BG1-BG9 is about 85%. The basicity of slags is in a broad range with 0.31-1.18. The effects of SiO2 and CaO on heavy metal residual fractions are different. The high content of SiO2 and CaO addition will exacerbate the volatilization of heavy metal.Base glasses vitrified from fly ash were used to make glass ceramics. The differential thermal analysis, XRD and SEM were used to investigate the heat treatment process. The mechanical properties and corrosion resistance of glass ceramics were analysed. It is shown that a mass of additive SiO2 will impede the nucleus formation and crystal growth. The effect of CaO is primarily on the crystal growth; it have minimal impact on nucleus formation. The particle size distribution of crystal particles ranges from 0.05μm to 0.4μm. The main crystalline components of glass ceramics are anorthite (CaAl2Si2O8) and diopside (CaMgSi2O6). The color of glass ceramics is various, such as dark black, black grey, soil-yellow, ochre, brown red and cyan. After crystallization, the mechanical properties and corrosion resistance of glass ceramics are significantly improved and better than granite and marble.According to the EPA TCLP 1311 method, the leaching behavior of heavy metals in the fly ash, slag and glass ceramics was conducted. It is indicated that the heavy metal leaching toxicity of slag and glass ceramic is much lower than the standards of TCLP and MEP, and slag and glass ceramic are both stable and harmless to surrounding. The addition of SiO2 can keep heavy metals leaching from slag, but the addition of CaO prick up heavy metals leaching. After crystallization, the heavy metal leaching toxicity of glass ceramic is higher than that of slag.