José Marcial

Effect of bubbles and silica dissolution on melter feed rheology during conversion to glass.

Nuclear-waste melter feeds are slurry mixtures of wastes with glass-forming and glass-modifying additives (unless prefabricated frits are used), which are converted to molten glass in a continuous electrical glass-melting furnace. The feeds gradually become continuous glass-forming melts. Initially, the melts contain dissolving refractory feed constituents that are suspended together with numerous gas bubbles. Eventually, the bubbles escape, and the melts homogenize and equilibrate. Knowledge of various physicochemical properties of the reacting melter feed is crucial for understanding the feed-to-glass conversion that occurs during melting. We studied the melter feed viscosity during heating and correlated it with the volume fractions of dissolving quartz (SiO2) particles and the gas phase. The measurements were performed with a rotating spindle rheometer on the melter feed heated at 5 K/min, starting at several different temperatures. The effects of undissolved quartz particles, gas bubbles, and compositional inhomogeneity on the melter feed viscosity were determined by fitting a linear relationship between the logarithm of viscosity and the volume fractions of suspended phases.

Nepheline structural and chemical dependence on melt composition

Abstract Nepheline crystallizes upon slow-cooling in some melts concentrated in Na2O and Al2O3, which can result in a residual glass phase of low chemical durability. Nepheline can incorporate many components often found in high-level waste radioactive borosilicate glass, including glass network ions (e.g., Si, Al, Fe), alkali metals (e.g., Cs, K, Na, and possibly Li), alkaline-earth metals (e.g., Ba, Sr,Ca, Mg), and transition metals (e.g., Mn, and possibly Cr, Zn, Ni). When crystallized from melts of different compositions, nepheline composition varies as a function of starting melt composition. Five simulated high-level nuclear waste borosilicate glasses shown to crystallize large fractions of nepheline on slow-cooling were selected for study. These starting melt compositions contained a range of Al2O3, B2O3, CaO, Na2O, K2O, Fe2O3, and SiO2 concentrations. Compositional analyses of nepheline crystals in glass by electron probe micro-analysis (EPMA) indicate that nepheline is generally rich in silica, whereas boron is unlikely to be present in any significant concentration, if at all, in nepheline. Also, several models are presented for calculating the fraction of vacancies in the nepheline structure.

Thermal Analysis of Waste Glass Batches: Effect of Batch Makeup on Gas-Evolving Reactions

Batches made with a variety of precursors were subjected to thermogravimetric analysis. The baseline modifications included an all-nitrate batch with sucrose addition, an all-carbonate batch, and batches with different sources of alumina. All batches were formulated for a single glass composition (a vitrified, simulated, high-alumina, high-level waste). Batch samples were heated from ambient temperature to 1,200°C at constant heating rates ranging from 1 to 50 K/min. Major gas-evolving reactions began at temperatures just above 100°C and were virtually complete by 650°C. Activation energies for major reactions were obtained with the Kissinger method. A rough model for the overall kinetics of the batch conversion was developed to be eventually applied to a mathematical model of the cold cap.

Crystallization of iron-containing sodium aluminosilicate glasses in the NaAlSiO4-NaFeSiO4 join

Although natural materials are the subject of most Earth science articles, fundamental studies on analogous synthetic materials, produced under laboratory-controlled conditions, can provide significant insight into expected behavior of natural systems. Iron, a common element in natural aluminosilicates as well as high-level nuclear wastes, plays a crucial role in crystallization behavior. In the present study, effects of Fe-Al substitution in nepheline-based aluminosilicate glasses (NaAl(1u2009−u2009x)FexSiO4, xu2009=u20090.0–1.0) were investigated to assess the role of iron in crystallization, employing semiquantitative X-ray diffraction (XRD), vibrating sample magnetometry (VSM), and electron probe microanalysis (EPMA). Fe promotes nepheline crystallization when substituted for Al in low additions (xu2009 u20090.5). Since effect of Fe is the subject of the present work and is the most common magnetic element, magnetic techniques were used to further analyze the phase assemblage. VSM measurements revealed that Fe oxides, i.e., hematite and magnetite, are present in cases even when their fractions are below the XRD detection limit, and backscattered electron micrographs confirm their presence. EPMA also shows that Fe incorporation in nepheline increases with increasing Fe-Al substitution, up to a maximum of xu2009=u20090.37 for the nepheline crystals in the sample with starting glass of Na(Al0.3Fe0.7)SiO4. The residual glass, on the other hand, contains approximately constant Fe concentration xu2009~u20090.54–0.59 for all samples with starting Fe addition 0.4u2009≤u2009xu2009≤u20090.8, and excess iron is expelled into Fe oxide phases. The significance of these results for geological processes and immobilization of high-level nuclear waste is discussed.