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.
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.
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).
Applications for the TNO HWC®
The first and most commonly used application for the TNO HWC® is suspension-based melt crystallisation. This is a powerful technique to obtain very high purity organic chemicals and metals. Pure crystals will be formed when an impure melt is cooled in a controlled way, because most impurities with a deviating shape and/or size will not fit into the very regular crystal lattice. A TNO HWC® can be installed downstream of the crystalliser to carry out the separation of the pure crystals from the impure mother liquor. Melt crystallisation usually has a higher selectivity than distillation and extraction. The high selectivity, the low energy consumption and the absence of solvents make melt crystallisation a perfect example of a ‘green’ production process, while it also fulfills the increasing demand for high-purity products in industry. The TNO HWC® has been proven to work successfully at lab scale and pilot scale for over 100 compounds. It has been adopted at semi industrial scale in two applications.
15cm HWC column operating with Para-xylene.
30 cm semi-industrial application of HWC operating with high purity Electronic grade chemicals.
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.