Fundamentals and Equilibrium

A phase, e g. a solution has to be supersaturated so that new crystals can arise or existing crystals can grow. Supersaturation can be achieved by cooling a solution or by evaporation of the solvent. This is called cooling and evaporative crystallization. For vacuiun crystallization flash evaporation is used to create supersaturation. In this case cooling and evaporation superimpose. 

Sometimes a drowning-out medium is added to a solution. It reduces the solubility of the solute and hence leads to supersaturation. This is called drowning-out crystallization. The solubility of many aqueous solutions of inorganic salts can be reduced by the addition of organic solvents (e.g., acetone, methanol). In reactive crystallization two or more reactants form a product which is less soluble and therefore crystallizes. For example, reactions between an acid and a base lead to the precipitation of a solid salt. This is called precipitation crystallization. However, it should be mentioned that this term is neither clearly defined nor uniformly used. 

Although there is no strict and universally valid distinction between a solution and a melt, it is purposive to differentiate between crystallization from solution and from melts. The process is called crystallization from the melt, if the crystallization temperature is close to the melting temperature of the crystalline phase. In solution crystallization this is not the case. While the kinetics of crystallization from solution is often limited by mass transport, the limiting factor of crystallization from melt is often heat transport. 

Sometimes solutes in a solution have to be concentrated by freezing the solvent. This is called freeze crystallization or freeze concentration. 

Mersmann et al., Thermal Separation Technology Principles, Methods, Process Design, VDI-Buoh, DOI 10.1007/978-3-642-12525-6 8, 

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