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.