Model for the conversion of nuclear waste melter feed to glass
The rate of batch-to-glass conversion is a primary concern for the vitrification of nuclear waste, as it directly influences the life cycle of the cleanup process. This study describes the development of an advanced model of the cold cap, which augments the previous model by further developments on the structure and the dynamics of the foam layer. The foam layer on the bottom of the cold cap consists of the primary foam, cavities, and the secondary foam, and forms an interface through which the heat is transferred to the cold cap. Other model enhancements include the behavior of intermediate crystalline phases and the dissolution of quartz particles. The model relates the melting rate to feed properties and melter conditions, such as the molten glass temperature, foaminess of the melt, or the heat fraction supplied to the cold cap from the plenum space. The model correctly predicts a 25% increase in melting rate when changing the alumina source in the melter feed from Al(OH)3 to AlO(OH). It is expected that this model will be incorporated in the full glass melter model as its integral component.
The industrial glass melting process
The glass melting process starts from a granular mixture of natural and synthetic raw materials (the so-called batch) and yields a thermally and chemically homogeneous melt made available at a well defined temperature level. The consecutive steps of glass fabrication (forming, annealing, etc.) are not within the scope of interest of this paper. Figure 1 illustrates how a continuously working glass melting furnace functions in principle. Figure 2 presents in very general terms a heat balance of the glass melting process. It comprises amounts of heat related to the flow of matter led through the combustion space, amounts of heat related to the flow of matter led through the melting basin, and heat losses through the boundaries of the system. For simplicity, all quantities are referred to the standard temperature level T0 = 298 K of the environment and to isobaric conditions at p = 1 bar. The quantity of specific interest in this paper is the so-called exploited heat Hex. It comprises both the heat required to bring about the batch-to-melt conversion, and the heat stored in the homogeneous melt leaving the system boundary at a temperature Tex. For the quantification of the heat balance in figure 2, it is crucial to have an accurate account of Hex. It is true, the contributions Hin, Hoff, Hstack, and Hre related to the combustion space can be calculated in a most accurate way from measured process temperatures, and from the known amounts of fuel and air used. But for the wall losses Hwo and Hwu through the boundaries of the combustion space and the basin, respectively, rough estimates are available only. This is due to the complicated shape of a real glass furnace, to the presence of a considerable number of openings in the furnace walls, and – most of all, after some years of continuous furnace operation – to the unknown residual thickness of the refractory bricks of the furnace lining. Thus, if Hex is assessed at a high accuracy, then the balance in figure 2 can be completed, and even an accurate account of the wall losses may be obtained.
Thermal Analysis of Reactions in Soda–Lime Silicate Glass Batches Containing Melting Accelerants: I, One‐ and Two‐Component Systems
To identify each glass melting reaction in a multicomponent system, one-component and two-component reaction processes were studied using DTA, TGA, and XRD. Two-component mixtures were prepared by choosing pairs in the same ratio as in a commercial container glass batch composition (sand-soda ash-calcite-dolomite-feldspar). The presence of silica in the silicia-calcite system decreased the termination temperature of the decomposition of calcite, but did not alter the onset of decomposition. Similar behavior was found in the dolomite-silica system. A double carbonate (Na[sub 2]Ca(CO[sub 3])[sub 2]) formed via solid-state reaction in the calcite-soda ash system, and metasilicate/disilicate phases were detected during the fusion process in the soda ash-silica system. The effects of reaction accelerant additions in the soda ash-silica system were investigated using 1 wt% additions of sodium sulfate, sodium nitrate, and sodium chloride. Sodium chloride was the most effective melting accelerant, lowering the termination temperature of CO[sub 2] release by [approximately]80C compared with the soda ash-silica system with no additives. NaCl additions caused complete reaction and/or fusion of Na[sub 2]CO[sub 3] prior to its melting temperature. Sodium sulfate additions acted to suppress metasilicate/disilicate formation by coating quartz grains and shifted consequent CO[sub 2] release to higher temperature.