The normal way to purify organic compounds without using a solvent is by distillation. However, it is suggested in a review article from Covalence (van Overbeke J et al, Chimica Oggi, 2000 (Nov/Dec) p18) that melt static crystallisation may be a energy efficient process, which leads to less degradation and higher purity products for solids which have low to moderate melting points.

The usual process consists of crystallisation from the melt, draining the mother liquors, partial melting again to remove impurities from the crystal surface (sweating), then melting the product to discharge. The cycle may be repeated to improve the purity. A typical cycle may take from 10-30 hours, the longest time being the “sweating” step.

It is possible to obtain purities of 99.5% and above with this method. Typical applications include purification of p-xylene, (50,000 t/a) O-phenylphenol (Bayer), m-dichlorobenzene (Metachem, USA). It is claimed that melt crystallisation, even when run in batch mode, is more cost effective than solution crystallisation, presumably because of the high space-time yield, and ease of recycling liquors.