What Is Evaporative Crystallization?

Evaporative crystallization is a separation technique in which the solvent is evaporated by heating. As the solution passes from unsaturated to saturated and evaporation continues, the excess solute comes out of solution as crystals.

A simple example is a mixture of NaCl and KNO3 in which NaCl predominates: evaporation can be used to crystallise out the NaCl first and then recover the KNO3.

Choosing between evaporative and cooling crystallization

The choice depends on the shape of the solubility curve. When a steeply rising solute is mixed with a gently rising one and the steeply rising component is to be separated, cooling crystallization is used; when the gently rising component is the target, evaporative crystallization is used. In other words, evaporative crystallization suits substances whose solubility changes little with temperature, such as sodium chloride. Potassium nitrate is a steeply rising solute and sodium chloride a gently rising one, so evaporative crystallization separates out the NaCl while cooling crystallization recovers the KNO3. Plotting the solubility curve makes the classification clear: a solute whose solubility rises markedly with temperature is steeply rising, otherwise it is gently rising.

Filtration as a companion step

Crystallization is normally followed by filtration to recover the solid. The critical point in design and operation is speed. When a hot solution is filtered slowly, it cools inside the filter itself, so part of the product crystallises out prematurely and blocks the filter medium. This is particularly noticeable in potassium nitrate production. Keeping the filter and the receiving vessel hot and filtering quickly avoids premature crystallization, giving a better yield and a cleaner separation. Vacuum filtration is preferred where a faster separation is required.