Determination of phosphate hydrate transition temperatures

Margaret Ellen LaMothe

Tri-sodium phosphate chemistry was explored in this study due to its presence in radioactive waste streams and potential for precipitating and plugging underground transfer lines. Throughout the plutonium production era, pipeline plugging issues were documented and many resulted in abandoned pipelines due to the inability to sufficiently dissolve phosphate precipitates. Future Hanford activities depend on tank waste being freely transported from storage tanks through miles of underground pipelines to a vitrification facility that will transform liquid and slurry mixtures into a glass phase for long term waste storage. Evaluating phosphate plugging is a priority for both cost and time purposes. To prevent precipitation in future transfer processes, phosphate chemistry was evaluated at temperature transition regions where modeling results are uncertain. Transition temperature regions refer to a hydrate phase transition that takes place when sodium phosphate gains or loses hydration waters due to hydrogen bond interactions. At these regions, sodium phosphate can lose or gain water based on temperature and concentration of other ions in solution. Literature data on tri-sodium phosphate hydrate chemistry is scarce and, when available, lacks agreement. Hazardous and radiological tank waste components at elevated temperatures add to the complexity of these solutions. Reagent grade tri-sodium phosphate (Na3PO4-12H2O⋅0.25 NaOH) was studied at varied temperatures in common waste constituents to determine where hydrate transitions occurred. A Hach total suspended solids turbidity meter was used to denote these precipitation events. A hexahydrate, octahydrate and dodecahydrate were the three phases evaluated in the presence or absence of sodium hydroxide or sodium nitrate. Turbidity indicated that the transition of the hexahydrate to the octahydrate (6/8) was dependent on the increase in phosphate concentration present in solution. The transition from octahydrate to dodecahydrate (8/12) was dependent on the sodium hydroxide concentration. At the lower temperatures sodium hydroxide reduced the solubility of sodium phosphate (Ksp) and therefore resulted in decreasing sodium phosphate hydrate transition temperatures. Phosphate hydrate transition region testing is valuable for accurate computer modeling predictions to successfully transfer radiological and hazardous waste through miles of underground pipelines.

Determination of phosphate hydrate transition temperatures

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