(W132)Phase Equilibria at liquidus temperatures of the CaO?SiO_2–Na_2O?SiO_2-Na_2O?Al 2O_3?6SiO_2 Slag System

摘要

Direct utilization and land filling of the bottom ash from municipal solid waste incineration (MSWI) can cause leaching of heavy metals and weathering problems. Vitrification is an efficient treatment technology which can transform the ash into stabilized glassy slag. Total amount of CaO, SiO_2, Al_2O_3, Fe_2O_3, Na_2O and MgO accounts for more than 90 wt% of the vitrified bottom ash slag for most of the Dutch MSWI bottom ashes. In order to better use the vitrified slag, the knowledge of the thermodynamic properties and phase relations of the related slag system is required. In the present work, the phase relations of a part of the CaO─SiO_2─Al_2O_3─Na_2O system were investigated. The region of the tetrahedron CaO?SiO_2─Na_2O?SiO_2─Al_2O_3?SiO_2─SiO_2 which includes the bottom ash slag composition is on the focus in this research. Within this tetrahedron, the phase equilibria of the system CaO?SiO_2─Na_2O?SiO_2─Na_2O?Al_2O_3?6SiO_2 was experimentally studied at temperatures from 800 °C to 1200 °C. The liquidus temperature was determined with differential scanning calorimetry (DSC). The equilibrium experiments at liquidus temperatures were conducted under argon gas. The equilibrated samples were quenched with pressurized nitrogen, and examined with electron probe micro analysis (EPMA) and X-ray diffraction (XRD) for identification of microstructure and phase relations. Five primary phase fields, CaO?SiO_2, Na_2O?SiO_2, Na_2O?2CaO?3SiO_2, 2Na_2O?CaO?3SiO_2 and Na_2O?Al_2O_3?6SiO_2 were established. Three ternary invariant points were determined. Based on these experimental results, the projection of liquidus surface has been constructed from 800 °C to 1200 °C for the region focused in the present work.