Eutectic freeze crystallisation (EFC) is a special form of melt crystallisation. In an EFC process, an aqueous stream with dissolved salts is cooled to the eutectic temperature and both salt and ice crystals are formed. Their yield is controlled via the cold withdrawn from the crystallizer. The salt and water crystals can be separated easily in a settler due to the difference in density. A TNO HWC® is used to separate the ice crystals from the mother liquor. This is important for the EFC process, because minimising the amount of mother liquor in the washed ice will maximise the overall yield of the process. In addition, an efficient ice-mother liquor separation improves the purity of the water, which increases the possibility of re-using the water in the process or reducing the treatment costs in case the water needs to be disposed of.
A third application for the TNO HWC® is to transfer solids from one solvent to another in a very efficient way. The operating principle of this ‘solvent switch’ differs from melt crystallisation or EFC as the product is a suspension instead of a melt. This application is interesting when high product purity and/or low wash liquid consumption is required. Examples include the purification of carnallite (KMgCl3.6H2O), KCl and NaCl crystals using counter-current washing in an TNO HWC®. Other potential applications in this field are the separation of polymer particles from organic solvents (for instance, in emulsion polymerisation), continuous adsorption/ion exchange and the recovery of heterogeneous catalysts.