Crystallization from Solutions

CRYSTALLIZATION FROM SOLUTIONS

When a material crystallizes from a solution, nucleation and crystal growth occur simultaneously over a wide intermediate temperature range so that a study of these processes is more difficult. In general, however, they are thought to be similar to nucleation and crystal growth in melts. The three basic steps, induction of supersaturation, formation of nuclei, and growth of crystals, are explained with reference to the solubility curve shown in Figure 9.2.

A solution with temperature and concentration indicated by point A may be saturated by either cooling to point B or removing solvent (point C). With further cooling or concentration, the supersaturated metastable region is entered. 

FIGURE 9.2 The solubility-supersolubility diagram.

FIGURE 9.3 The effect of agitation on the rate of growth of a crystal of sodium thiosulfate.

If the degree of supersaturation is small, the spontaneous formation of crystal nuclei is highly improbable. Crystal growth, however, can occur if seeds are added. With greater supersaturation, spontaneous nucleation becomes more probable and the metastable region will be limited approximately by the line B^’ C^’ . If the solution is cooled to B^’ or concentrated by solvent removal to C^’ , spontaneous nucleation is virtually certain. Crystal growth will also occur in these conditions. The rate of growth, however, is depressed at low temperatures.

During crystal growth, deposition on the faces of the crystal causes depletion of molecules in the immediate vicinity. The driving force is provided by the concentration gradient setup, from supersaturation in the solution to lower concentrations at the crystal face. A large degree of supersaturation, therefore, promotes a high growth rate. A reaction at the surface, in which solute molecules become correctly orientated in the crystal lattice, provides a second resistance to the growth of the crystal. Simultaneously, the heat of crystallization must be conducted away.

Agitation modifies the rate of crystal growth for given conditions of temperature and saturation. Initially, agitation quickly increases the rate of growth by decreasing the thickness of the boundary layer and the diffusional resistance. However, as agitation is intensified, a limiting value is reached, which is determined by the kinetics of the surface reaction. In Figure 9.3, the effect of agitation on the rate of crystal growth in solutions of sodium thiosulfate of differing degrees of supersaturation is described.

As with melts, soluble impurities may increase or retard the rate of nucleation. Insoluble materials may act as nuclei and promote crystallization. Impurities may also affect crystal form and, in some cases, are deliberately added to secure a product with good appearance, absence of caking, or suitable flow properties.

The temperature at which crystallization is performed may be determined by the crystal form or degree of hydration required of the products. Reference to the solubility curves given in Figure 9.4 shows that crystallization at 50^○C yields FeSO₄ · 7H₂O, at 60^○C yields FeSO_4N · 4H₂O, and at 70^○C, FeSO₄. The majority of materials, however, have one or possibly two forms. The degree of super-saturation of solution 1 is 5 g/L, of solution 2 is 10 g/L, and of solution 3 is 15 g/L.

FIGURE 9.4 Solubility curves.