Vitrified bottom ash slag from municipal solid waste incinerators - Phase relations of CaO-SiO2-Na20 oxide system

Abstract

Vitrification is considered to be an attractive technology for bottom ash treatment because it destroys the hazardous organics, contributes to immobilization of the heavy metals, and additionally it reduces drastically the volume. The main components of the vitrified bottom ash slag are SiO2 , CaO, Al2 O3 , Fe2 O3 , Na 2 O and MgO, and the compositions have direct effect on the glass formation during vitrification, and further on the physical and mechanical properties of the slag. To provide essential data for the utilization of bottom ash as vitrified slag, the phase relations and thermodynamic properties of the oxide system need to be systematically investigated. In the present study, the liquidus temperature of the typical vitrified bottom ash slag was determined by using Differential Scanning Calorimetry (DSC) measurement. High temperature equilibrium experiments were conducted to investigate the phase relations of the vitrified slag and sub-oxide system CaO-SiO2 - Na2 O by means of Scanning Electron Microscopy (SEM), Electron Microprobe Analysis (EMPA) and X-ray Diffraction (XRD). The results show that the melting temperature of the vitrified bottom ash is around 1120°C. In the CaO-SiO2 - Na 2 O system with less than 50 mol% SiO2 , the liquidus temperature increases with increasing amounts of Na2 O along the tie-line of Na 2 O- Ca 3 Si 2 O 7 . The ompound of Na 2 Ca 3 Si 2 O 8 is identified in the oxide system.