The use of carbonation and fractional evaporative crystallization in the pretreatment of Hanford nuclear wastes
Dumont, George Pierre, Jr.
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The purpose of this work was to explore the use of fractional evaporative 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. Experimental semi-batch crystallizations of sodium salts from simulant solutions of double-shell tank (DST) feed demonstrated that the recovered crystalline product met the purity requirement for exclusion of cesium and nearly met the requirement on sodium recovery. Batch fractional evaporative crystallization involves the removal of multiple solutes from a feed solution by the progressive achievement of supersaturation (through evaporation) and concomitant nucleation and growth of each species. The slurry collected from each of these crystallization stages was collected and introduced to filtration and washing steps. The product crystals obtained after washing were sampled for analysis by polarized light microscopy (PLM), dried, and sieved. The PLM results aided in identification of species crystallized in each stage. Carbonation was used as a supplemental method to evaporative crystallization in order to increase the sodium recovery in DST experiments. Carbonation was necessary due to the high aluminum ion concentration in the solution, which leads to formation of a viscous gel during evaporation. This gel was avoided by reacting carbon dioxide with hydroxyl ions, which modified the system behavior. Through two stages of carbonation, each followed by evaporation, the effect of carbonation on sodium recovery was demonstrated.
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