Rules Of Thumb : Crystallization From Solution

1. The feed to a crystallizer should be slightly unsaturated.
2. Complete recovery of dissolved solids is obtainable by evaporation, but only to the eutectic composition by chilling. Recovery by melt crystallization also is limited by the eutectic composition.
3. Growth rates and ultimate sizes of crystals are controlled by limiting the extent of supersaturation at any time.
4. Crystal growth rates are higher at higher temperatures.
5. The ratio S 1⁄4 C=Csat of prevailing concentration to saturation concentration is kept near the range of 1.02–1.05.
6. In crystallization by chilling, the temperature of the solution is kept at most 1–28F below the saturation temperature at the prevailing concentration.
Growth rates of crystals under satisfactory conditions are in the range of 0.1–0.8 mm/hr. The growth rates are approximately the same in all directions.
Growth rates are influenced greatly by the presence of impurities and of certain specific additives that vary from case to case.
Batch crystallizers tend to have a broader crystal size distribution than continuous crystallizers.
To narrow the crystal size distribution, cool slowly through the initial crystallization temperature or seed at the initial crystallization temperature