(57f) Semibatch Evaporative Crystallization of Multiple Solutes

Fractional crystallization has been examined as a potential means of accelerating the remediation of mixed liquid waste at the Hanford Site. In the proposed process, a feed stream containing waste is partitioned by evaporating water and crystallizing a number of sodium salts; the salts are separated and ultimately treated in a bulk vitrification facility, while Cs and other radioactive species remain in solution and are sent to a waste-treatment facility. The primary criterion by which performance was judged included the Cs content of the recovered crystals, the fraction of sodium recovered as crystals, and the fraction of feed sulfate that is diverted to the recovered crystals.

The technical challenges associated with crystallization of the sodium salts, which are comprised primarily of sodium nitrate, sodium carbonate, sodium sulfate and other species, were explored in a two-stage, semibatch, evaporative unit, with each stage operated at constant volume and temperature. Feed solutions were prepared to simulate those expected from the waste tanks, but no radioactive materials were included. During a run, the level in the crystallizer was maintained constant by adding feed to compensate for the reduction in volume associated with evaporation. The temperature of the system was maintained within ±1 °C by adjusting the system pressure. The slurry produced in each stage contained multiple crystalline products, each of which contributed to the overall crystal size distribution and presented unique challenges in the solid-liquid separation requirements. The slurry generated in Stage 1 was filtered and the resulting filtrate was used to prepare the feed to Stage 2. The crystals recovered from each stage were sized by sieving and the distribution of species determined by polarized light microscopy. The Cs content of the recovered crystals was determined by chemical analyses and mass balances on the system and its components were used to certify the validity of a run.