Sodium sulfate decomposition in dry atmospheres

Abstract

Na2SO4 in silicate glass batches is an environmental issue, since it releases SOx upon decomposition. Decomposition of Na2SO4 in different environments is studied by mass-loss measurements combined with evolved gas analysis, and thermochemical modeling. The decomposition experiments are undertaken in dry, pure O2(g), Ar(g), N2(g), and air(g) at 95 kPa total pressure. Thermochemical calculations using the code FAC*T predict SO2(g), Na(g), Na2SO4(g), NaO(g), and, in some cases, NO(g) as major emission species. The concentrations of these species increase with temperature. Na2SO4 decomposition initiates at ≈1373 K. Isothermal decomposition exhibits linear behavior with respect to time in 1473 to 1673 Κ range. At 1673 K, the decomposition rate is 24 · 10^-4 mg/(mm2 min) in UHP (ultra-high purity) O2 and 69 · 10^-4 mg/(mm2 min) in UHP N2. Evolved gas analyses identify SO(g)/SO2(g) as the emitted pollutants. In O2-rich atmospheres, SO(g) is a significant product as well as SO2(g). In inert atmospheres, SO2(g) is the sole decomposition product. At 1673 Κ in UHP O2, the concentration is 55 ppm (by volume) for SO(g) and 61 ppm for SO2. At 1673 Κ in UHP N2, SO2(g) concentration is 651 ppm. The decomposition is described by a surface reaction mechanism, in which SOx is generated by surface rearrangement of sulfur-oxygen complexes on the Na2So4 melt surface. Inert atmospheres increase SOx emission by facilitating this rearrangement process. O2-rich atmospheres passivate the melt surface, which favors the emission of lighter molecules such as SO(g).