IMPACTS OF SMALL COLUMN ION EXCHANGE STREAMS ON DWPF GLASS FORMULATION: KT05- AND KT06-SERIES GLASS COMPOSITIONS

Abstract

This report is the second in a series of studies of the impacts of the addition of Crystalline Silicotitanate (CST) and Monosodium Titanate (MST) from the Small Column Ion Exchange (SCIX) process on the Defense Waste Processing Facility (DWPF) glass waste form and the applicability of the DWPF process control models. The KT05-series glasses were selected, fabricated, and characterized to further study glass compositions where iron titanate crystals had been previously found to form. The intent was to better understand the mechanisms and compositions that favored the formation of crystals containing titanium. Formation of these crystalline phases was confirmed. Increased Na{sub 2}O concentrations had little if any impact on reducing the propensity for the formation of the iron titanate crystalline phases. Other physical properties of these glasses were not measured since the intent was to focus on crystallization. Additional studies are suggested to investigate the potential impacts of Al{sub 2}O{sub 3} and K{sub 2}O on crystallization in glasses with high TiO{sub 2} concentrations. The KT06-series glasses were selected, fabricated, and characterized to further study glass compositions that, while broader than the current projections for DWPF feeds with SCIX material, are potential candidates for future processing (i.e., the compositions are acceptable for processing by the Product Composition Control System (PCCS) with the exception of the current TiO{sub 2} concentration constraint). The chemical compositions of these glasses matched well with the target values. The chemical durabilities of all the glasses were acceptable relative to the Environmental Assessment (EA) benchmark. Minor crystallization was identified in some of the slowly cooled glasses, although this crystallization did not impact chemical durability. Several of the KT06-series compositions had durability values that, while acceptable, were not accurately predicted by the current durability models. It was shown that for these high TiO{sub 2} concentration glasses, relatively high Fe{sub 2}O{sub 3} concentrations combined with relatively high Al{sub 2}O{sub 3} concentrations led to durabilities that were unpredictable. Several of the KT06-series glasses also had measured viscosity values that were not well predicted by the current model. A statistical partitioning routine showed that the measured viscosities became unpredictable by the current model when the Fe{sub 2}O{sub 3} concentration in the glasses was less than about 8.2 wt % at the elevated TiO{sub 2} concentrations. The current durability and viscosity models will have to be further evaluated should compositions in these regions become necessary for DWPF processing. Overall, the results presented for the KT06-series glasses show that TiO{sub 2} from the SCIX streams can be incorporated into DWPF-type glasses at concentrations of 6 wt % (in glass) without any detrimental impacts on crystallization or chemical durability that are of practical importance. The measured values for chemical durability and viscosity were acceptable for processing; however, not all of the values were predictable by the current PCCS models. Since the compositions selected for the KT06-series glasses were outside the current projections for DWPF processing with the SCIX streams (in terms of waste components other than TiO{sub 2}), these results help identify compositional regions that, if necessary for processing, would require modifications to the current models. Additional experiments are currently underway. Once completed, all of the measured data will be reviewed and compared to model predictions to better determine whether the validation range of the DWPF process control models can be confidently extended, or whether refitting of the models will be necessary.