Understanding the influence of iron on crystallization kinetics of nepheline based glasses - 15472
- Department of Materials Science and Engineering, Rutgers, The State University of New Jersey (United States)
- School of Mechanical and Materials Engineering, Washington State University, Pullman, WA (United States)
Hanford site in Washington is home to 55 million US gallons of high level radioactive waste rich in sodium, alumina and iron. The strategy is to vitrify this waste in borosilicate glasses. Crystallization of nepheline/carnegeite (NaAlSiO{sub 4}) in glass melts during vitrification and canister cooling is a big challenge as it severely deteriorates the chemical durability of the glass. As the HLW borosilicate glasses are similar to basalt glass on a borate-free basis and as no borate phase crystallize on the liquidus, the crystallization chemistry of waste glasses can be described by the known phase relations of the geochemical basalt quaternary Na{sub 2}O-Al{sub 2}O{sub 3}-Fe{sub 2}O{sub 3}-SiO{sub 2} system. The objective of this study is to understand the influence of iron on the thermal stability and crystallization kinetics of model sodium- and alumina- rich simplified silicate glasses in the Na{sub 2}O-Al{sub 2}O{sub 3}-Fe{sub 2}O{sub 3}-SiO{sub 2} system designed in the primary crystallization field of nepheline. The results obtained from this study will form the baseline for understanding the nucleation and crystallization kinetics of multicomponent complex nuclear waste glasses in future. A series of glasses with compositions 25Na{sub 2}O-(25-z) Al{sub 2}O{sub 3}-zFe{sub 2}O{sub 3} -50SiO{sub 2} (mol.%) where z varies between 0 -5% has been prepared by melt-quench technique. Glasses have been labelled in reference to their Fe{sub 2}O{sub 3} content, i.e. Fe-z. We could not obtain amorphous glasses with Fe{sub 2}O{sub 3} > 5 mol.% . Glass batches comprising oxides and carbonates were melted at 1650 deg. C in Pt-Rh crucibles for 2 h. The melts were quenched in cold water to yield ∼90 g of glass. Thermal stability and crystallization kinetics of glasses was studied using differential thermal analysis (DTA). Thermal scans (30-1580 deg. C) were collected on glass particles (particle size: 0.5 mm-1 mm) at four different heating rates: 5, 10, 15 and 20 K min{sup -1} in air and inert (N{sub 2}) atmosphere. Non-isothermal crystallization kinetics of the glasses was studied using the Augis-Bennett method and Ozawa method. In accordance with the crystallization data obtained from DTA, glasses were heated in the crystallization temperature range and air quenched. Glasses were also heated from 30 deg. C to 800, 900 and 1000 deg. C for 1 h, respectively, in air and inert (N{sub 2}) atmospheres, at heating rate of 10 K min{sup -1}. Resulting glass-ceramics were characterized for their crystalline phase evolution by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Conversion of melt -to glass-ceramic during cooling DTA data was also collected for all the glasses during their conversion from melt -to -glass-ceramic during cooling at different cooling rates, in air and inert (N{sub 2}) atmosphere. In accordance with the DTA data, conversion of melt to glass-ceramic during cooling was studied by re-melting the glass frit in Pt-Rh crucibles at 1650 deg. C. The melt was allowed to cool at 10 K min{sup -1} and was air quenched at different temperatures (as per DTA data). The as obtained glass-ceramics were characterized by XRD and SEM Incorporation of Fe{sub 2}O{sub 3} in glasses decreases their crystallization temperature and increases their crystallization tendency. Parent glass (stoichiometric nepheline composition) is prone to surface crystallization. Incorporating 5 mol.% Fe{sub 2}O{sub 3} in these glasses transforms the crystallization kinetics from surface to volume nucleation. Crystallization kinetics of glass Fe-2.5 is affected by the environment (N{sub 2} vs. Air) while no significant impact of environment could be seen for glass Fe-5. Devitrification in all the glasses begins with the crystallization of carnegeite (NaAlSiO{sub 4}, orthorhombic) (∼1200 deg. C). Prolonged heat treatments leads to the crystallization of nepheline (NaAlSiO{sub 4}, hexagonal) (∼1200 deg. C) as a secondary phase. Fe{sub 2}O{sub 3} promotes the crystallization of nepheline over carnegeite via formation of magnetite as nucleation site. Iron incorporation pre-nucleates the glasses which upon heat treatment result in the formation of magnetite phase as nucleation sites, thus, shifting the mechanism of crystallization from surface to volume. Unlike carnegeite, the crystal structure of nepheline allows partial substitution of Fe{sup 3+}/Al{sup 3+}. This promotes the crystallization of nepheline over carnegeite in iron-containing glasses. Iron incorporation in nepheline-based sodium aluminosilicate glasses shifts crystallization mechanism from surface to bulk. Our results indicate that atmosphere does affect the crystallization kinetics of iron-containing aluminosilicate glasses but only when Fe{sub 2}O{sub 3} concentration is low. Since nepheline crystal structure can easily accommodate iron in comparison to carnegeite, iron seems to promote nepheline formation in these glasses at the expense of carnegeite through formation of Fe{sub 3}O{sub 4} phase as nucleation site. Future work will be focused on adding compositional complexity to these glasses and studying them further for their nucleation and crystallization behavior.
- Research Organization:
- WM Symposia, Inc., PO Box 27646, 85285-7646 Tempe, AZ (United States)
- OSTI ID:
- 22824369
- Report Number(s):
- INIS-US-19-WM-15472; TRN: US19V0941069415
- Resource Relation:
- Conference: WM2015: Annual Waste Management Symposium, Phoenix, AZ (United States), 15-19 Mar 2015; Other Information: Country of input: France; 2 refs.; available online at: http://archive.wmsym.org/2015/index.html
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
BASALT
BORATES
BOROSILICATE GLASS
CARBONATES
CRYSTALLIZATION
DIFFERENTIAL THERMAL ANALYSIS
HANFORD RESERVATION
HEATING RATE
HIGH-LEVEL RADIOACTIVE WASTES
IRON
IRON OXIDES
MAGNETITE
SCANNING ELECTRON MICROSCOPY
SERVICE LIFE
SILICON OXIDES
SODIUM
SODIUM OXIDES
STOICHIOMETRY
VITRIFICATION
X-RAY DIFFRACTION