Environmental Impact of Sodium Sulfate Decomposition in Silicate Glass Manufacturing

Journal Title
Journal ISSN
Volume Title
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Sodium sulfate (Na2SO4) is commonly used in soda-lime-silicate glass manufacturing as a fining agent. The decomposition of Na2SO4 in glass tanks is an environmental linked to SOx and particulate matter emissions. This study focuses on the influence of atmosphere and reducing agents on Na2SO4 decomposition and the concurrent emissions release. Non-isothermal decomposition measurements identify 1373 K as the initial decomposition temperature for Na2SO4 alone regardless of atmosphere type. This implies that decomposition occurs in the molten phase. The isothermal decomposition occurs in a steady-state, with higher decomposition rates occurring in inert or reducing atmospheres. The decomposition mechanism involves rearrangements of Na-S-O complexes on the Na2SO4 melt surface followed by desorption of SOx molecules. This desorption step is rate-controlling. Decomposition atmosphere impacts the total SOx concentration and SO/SO2 ratio. Inert atmospheres facilitate decomposition via collisions with the Na-S-O surface complexes, resulting in higher total SOx concentration. Oxidizing atmospheres passivate the melt surface, thus favoring the desorption of smaller molecules such as SO. This phenomenon increases the SO/SO2 ratio in oxidizing atmospheres. Carbon is frequently used in combination with Na2SO4 to enhance the fining process. Higher carbon active surface area (ASA) decreases the initial decomposition temperature. In non-isothermal decomposition, carbon increases SOx emissions at temperatures above 1373 K. Isothermal decomposition is shifted to a non-steady state behavior. The decomposition rate scales with ASA. Carbon fractional conversion can be calculated by measuring CO/CO2 concentration by FT-IR spectrometry. Conversion vs. time data indicate three distinct regions: nucleation, desorption, and carbon depletion. Decomposition is initiated by chemisorption of O atoms from Na2SO4 on carbon active sites, forming surface complexes. These complexes subsequently dissociate, releasing SOx and leaving behind C(O) complexes. The C(O) complexes desorbs as CO/CO2, exposing new active sites.
Advisory committee members: Doreen Edwards, Arun Varshneya, Alexis Clare. Dissertation completed in partial fulfillment of the requirements for the degree of Doctorate of Philosophy in Ceramics at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University