Crystallisation from a melt or solution has the potential to yield a product with a very high purity in a single equilibrium step. After crystallisation the pure crystals need to be separated from the impure mother liquor. This is often done by means of standard solid-liquid separation techniques like filtration or centrifugation. For high purity products, additional washing is required to remove residual mother liquor from the crystal cake. An attractive alternative is to use a wash column, which combines continuous solid-liquid separation with efficient counter-current washing using very little or no wash liquid. The TNO Hydraulic Wash Column combines a high washing efficiency with a very high specific production capacity.

The industrial purification of chemicals through e.g. distillation requires large amounts of energy and/or solvents. An alternative method, developed by TNO, consists of melt crystallisation in combination with the TNO Hydraulic Wash Column (TNO HWC®) and delivers products with purity up to 99,9% in one step and a reduction in energy consumption of 20% up to 90% compared to distillation.

Process boundaries

To take full advantage of the TNO HWC®, the crystals should have reasonable filtration properties, the viscosity of the mother liquor should not be too high and, in a melt crystallisation application, the product should be stable at its melting temperature. A rule of thumb for all HWC applications is that a mean particle size of 100 μm will usually be sufficient when the liquid has a viscosity comparable to that of water (1 mPas).