Accelerating treatment of radioactive waste by evaporative fractional crystallization
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The purpose of the work described in this thesis was to explore the use of fractional crystallization as a technology that can be used to separate medium-curie waste from the Hanford Site tank farms into a high-curie waste stream, which can be sent to a Waste Treatment and Immobilization Plant (WTP), and a low-curie waste stream, which can be sent to Bulk Vitrification. The successful semi-batch crystallization of sodium salts from two single shell tank simulant solutions (SST Early Feed, SST Late Feed) demonstrated that the recovered crystalline product met the purity requirement for exclusion of cesium, sodium recovery in the crystalline product and the requirement on the sulfate-to-sodium molar ratio in the stream to be diverted to the WTP. The experimental apparatus, procedures and results obtained in this thesis on scaled-down experiments of SST Early and Late Feed simulated solutions were adapted and reproduced under hot-cell with actual wastes by our partners at Hanford. To prepare the application of the pretreatment process to pilot scale process, several varation to the feed solutions were investigated including the presence of carboxylates and amines organics compounds and solids particles. Results of the study showed that 4 organics species presented complications to the process (NTA, HEDTA, EDTA and sodium citrate) while the other species (Formate, acetate, glycolate and IDA) and solids particles did not in the conditions of the stored wastes. In this thesis, the kinetics of the crystalline species formed at the condition of the early feed certification run (66 °C and 25 g/h evaporation) were determined along with the effect of the operating temperature and evaporation rate on these kinetics. On one hand, the study of evaporation rate values ranging from 25g/h to 75g/h showed that an increase in evaporation rate increased the specific nucleation while decreasing the specific growth rate. On the other hand, experiments on operating temperature ranging from 35 °C to 75 °C displayed that the nucleation rate of all species increased with temperature at the exception of sodium carbonate monohydrate and burkeite crystals, and that the growth rate of all species increased with temperature at the exception of sodium nitrate. Furthermore, sulfate based crystals such as trisodium fluoride sulfate were only roduced at 45 °C and 75 °C. A simple steady state MSMPR population balance model was developed expressing the total population density function as the sum of the specific population density functions. The specific semi-batch crystallization kinetics were implemented in this model.