Mechanism of Sulfate Segregation during Glass Melting
- PDF / 661,552 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 31 Downloads / 217 Views
II5.5.1
Mechanism of Sulfate Segregation during Glass Melting Pavel Hrma, John D. Vienna, and Joel S. Ricklefs Pacific Northwest National Laboratory Richland, WA 99352-999, U.S.A. ABSTRACT Sulfate retention in glass during the vitrification process can be as low as 1/3 of the solubility limit, or can exceed the solubility limit if suspended in the glass in the form of droplets. This study is focused on the mechanism of incorporating and segregating sodium sulfate during the melting of an alkali-alumino-borosilicate glass batch. Batches were ramp heated at 4°C/min to temperatures ranging from 600°C to 1050°C and fractured for examination. Observation of the melts showed that as the batch temperature increases and the primary oxo-anionic, predominantly nitrate melt decomposes, the sulfate residue accumulates inside gas bubbles and is transported in them to the melt surface, where it remains segregated. The degree of sulfate incorporation into the final glass depends on the relative rates of sulfate dissolution in the borosilicate melt and sulfate lifting inside bubbles.
INTRODUCTION Sulfate has received considerable attention in commercial glass technology, where it is used as melting accelerator, foam breaker, and fining agent [1]. Molten sulfate wets the grains of silica sand before it is fully dissolved in the glass. Because of this coating by sulfate, silica sand is prevented from dissolving early in the glass-forming melt, and the low-silica glass-forming melt retains high alkali content until sufficiently high temperatures are reached. As a result of a retarded increase in silica content while the temperature is rising, the melt maintains a low viscosity. This behavior has several beneficial effects on glass melting and fining: other refractory components are readily incorporated into the glass-forming melt; the low viscosity of glass-forming melt allows an easy escape of glass bubbles; reduced primary foam leads to an improved heat transfer and faster melting; residual grains of silica sand that survive to the maximum temperature provide nucleation sites for bubbles from decomposing sulfate and thus facilitate fining. However, an excessive sulfate stabilizes secondary foam in glass and can form a segregated salt layer on the molten glass surface. Salt-layer formation is a major concern in vitrifying sulfate-rich low-activity wastes (LAWs). The LAW fraction of Hanford tank wastes is an aqueous solution with Na+, K+, OH-, NO3-, NO2-, SO42-, PO42- and various aluminum-containing ions. The slurry (LAW solution mixed with glass-forming chemicals and minerals) is sprayed through nozzles onto the melt surface, where it becomes a dry crusty material called a cold-cap that is converted to glass in a thin layer between the glass melt and the cold-cap. The lowviscosity, highly corrosive primary melt (alkali-nitrate, -sulfate, -nitrite, -hydroxide, phosphate, -chromate, etc.) forms at roughly 350°C, reacts with refractory batch components, producing a glass-forming borosilicate melt, liberating a large volume of
II5.5.
Data Loading...
