No abstract

Nepheline (Na 6 K 2 Al 8 Si 8 O 32 ) is a rock-forming tectosilicate mineral which is by far the most abundant of the feldspathoids. The crystallization in nepheline-based glass-ceramics proceeds through several polymorphic transformationsmainly orthorhombic, hexagonal, cubicdepending on their thermochemistry. However, the fundamental science governing these transformations is poorly understood. In this article, an attempt has been made to elucidate the structural drivers controlling these polymorphic transformations in nepheline-based glass-ceramics. Accordingly, two different sets of glasses (meta-aluminous and per-alkaline) have been designed in the system Na 2 O-CaO-Al 2 O 3 -SiO 2 in the crystallization field of nepheline and synthesized by the melt-quench technique. The detailed structural analysis of glasses has been performed by 29 Si, 27 Al, and 23 Na magic-angle spinningnuclear magnetic resonance (MAS NMR), and multiple-quantum MAS NMR spectroscopy, while the crystalline phase transformations in these glasses have been studied under isothermal and non-isothermal conditions using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and MQMAS NMR. Results indicate that the sequence of polymorphic phase transformations in these glass-ceramics is dictated by the compositional chemistry of the parent glasses and the local environments of different species in the glass structure; for example, the sodium environment in glasses became highly ordered with decreasing Na 2 O/CaO ratio, thus favoring the formation of hexagonal nepheline, while the cubic polymorph was the stable phase in SiO 2 -poor glass-ceramics with (Na 2 O+CaO)/Al 2 O 3 > 1. The structural origins of these crystalline phase transformations have been discussed in the paper.

No abstract

A spectroscopic study was conducted on six simulant nuclear waste glasses using multi-nuclear NMR, Raman, and Mössbauer spectroscopies exploring the role of Si, Al, B, Na, and Fe in the glass network with the goal of understanding melt structure precursors to deleterious nepheline crystal formation. NMR showed two sites each for Al, Si, and Na in the samples which crystallized significant amounts of nepheline, and B speciation changed, typically resulting in more B(IV) after crystallization. Raman spectroscopy suggested some of the glass structure is composed of metaborate chains or rings, thus significant numbers of non-bridging oxygen and a separation of the borate from the aluminosilicate network. Mössbauer, combined with Fe redox chemical measurements, showed Fe playing a minor role in these glasses, mostly as Fe 3+ , but iron oxide spinel forms with nepheline in all cases. A model of the glass network and allocation of non-bridging oxygens (NBOs) was computed using experimental B(IV) fractions which predicted a large amount of NBO consistent with Raman spectra of metaborate features. local charge balance on the oxygens of alkaline-earth boro-aluminate crystals, where results from 11 B and 27 Al magic angle spinning (MAS)-NMR stressed the importance of three-coordinated oxygen O(III), where parentheses here and elsewhere denote nearest neighbor coordination, and indicated the importance of Al(V) and Al(VI) in all composition ranges [19]. In another example, Du and Stebbins [12] presented a modified Dell and Bray-type model [20] to include aluminum (i.e., alkali-alumino-borosilicate) and to describe boron speciation. Various tetrahedra compete for sodium, the modifier generally preferring Al, then B, then Si [21]. Tetrahedra building, on the other hand, is believed to start with Si then proceed to Al(IV), Fe(IV), and B(IV) (if present), as long as charged tetrahedron nearneighbor avoidance is maintained [12,22].Both boron and aluminum coordination are affected by modifier-cation field strength. In aluminoboro-silicates containing both Na and Ca, increasing the amount of Ca increases Al(V) [13], but has been said to increase B(IV) [23] or decrease it [13]. It has been shown in aluminoborates that higher-fieldstrength cations (e.g., Mg), can shift the normally encountered "chemical ordering" (i.e., Al and B tetrahedra avoidance due to Coulomb repulsion) seen in Na and Ca aluminoborates to the "statistical random mixing" of tetrahedra seen in Mg alumino-borate [24], and to an extent in Ca alumino-borosilicates [13]. Higher coordinated alumina units, Al(V) and Al(IV), can be charge compensated by 2 + cations (Ca, Mg) or 3 + cations (B, Al, La), and at the same time tend to "stabilize" B(IV) tetrahedra, acting as weakly positive ions themselves [19,24,25].Effects of differing Al/B ratios are explained by the local charge balance model [19], as B(III) is needed to stabilize units containing the charged Al(IV), so as more Al is added, the fraction of B(III) increases relative to B(IV) [23]. It has also been shown by ...

No abstract

No abstract

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.

The selection of raw materials affects the rate of batch‐to‐glass conversion. In all‐electric melters, foam under the batch blanket limits the heat flux from the molten glass, thus slowing the rate of melting. Our study, in which we compare the melting behaviors of three batches formulated to vitrify high‐alumina high‐level waste, shows that a slowly dissolving refractory component can cause excessive foaming. Faster melting batches with gibbsite [Al(OH)3] or boehmite [AlO(OH)] as an alumina source produced substantially less foaming than a batch with corundum (Al2O3). Although gibbsite and boehmite dissolved below 500°C, corundum was still present in the batch up to 900°C; hence, the glass‐forming melt lacked alumina in the batch with corundum. The low viscosity of that batch caused the open pores to close prematurely at 660°C, trapping gases and expanding to foam. This would explain the literature‐reported slow melting rate of a batch with corundum, as compared to batches with gibbsite and boehmite.