Production of Large Crystals

PRODUCTION OF LARGE CRYSTALS

Batch production of large, uniform crystals may be carried out in agitated reaction vessels by slow, controlled or natural cooling. Spontaneous nucleation is improbable until solution A is cooled to X. Crystallization then follows the path XB. Better control is gained if the solution is artificially seeded. Seeding is shown at X^’ . Crystallization then follows the broken line X^’ B, the aim being to maintain the solution in the metastable region where growth rate is high and natural nucleation is low. The course of the crystallization is shown in Figure 9.5. Initially, spontaneous nucleation may be allowed by cooling from A to X. As crystallization takes place, the degree of supersaturation and the concentration of the solute fall, ultimately reaching saturation at B when growth will cease. Closer control is secured by artificially seeding the supersaturated solution in conditions of no natural nucleation. Seeding is indicated by the point X0 . The course of the crystallization is then indicated by the broken line, X’ B.

FIGURE 9.5 The production of large crystals. The conditions of supersaturation.

An important principle for the continuous production of large even crystals is used in Oslo or Krystal crystallizers. A metastable, supersaturated solution is released into the bottom of a mass of growing crystals on which the solute is deposited. The crystals are fluidized by the circulation of the solution, and classification in this zone allows the withdrawal of sufficiently large crystals from the bottom.