Studies on the interactions of metal oxides and Na2 SO4 at 1100 and 1200 K in oxygen

The interaction of different metal oxides such as Co3O4, NiO, Al2O3, Cr2O3, Fe2O3 and SiO2 with Na2SO4 at a temperature of 1100 and 1200 K in flowing oxygen has been studied. The thermogravimetric studies for each system were carried out as a function of Na2SO4 in the mixture. The presence of different constituents in the reaction products were identified by X-ray diffraction analysis and the morphologies of the reaction products were characterized using metallography and scanning electron microscopy (SEM). The formation of products was also investigated by thermodynamic computation of free energies of the reactions and the study of relevant equilibrium phase diagrams. The soluble species in the aqueous solutions of the reaction products were determined quantitatively using atomic absorption spectrophotometry. The high temperature interaction products usually contain a 3-phase structure namely, Na2O·M2O x , M2O x and metal sulphide and/or metal sulphate. The formation of Na2O·M2O x depends upon the solid state solubility of metal oxide in the molten salt at high temperatures. Under limited solubility conditions Na2O·M2O x is invariably formed, but as soon as this condition is relaxed the oxide. M2O x , precipitates and forms a separate phase.