Crystallization is a powerful and versatile technique to separate components from a liquid mixture. This liquid mixture may be an impure melt or a solution, which refers to the case that one or more components are dissolved in a liquid (solvent). Crystallization is a separation process that makes use of differences in solubility of the components present in the melt or the solution. Crystallization can occur when the solubility of one or more components present in the liquid is exceeded. An excellent and well-known example of a separation based on crystallization is the formation of ice in sea water. On average 1 kg sea water contains 965 g water and 35 g dissolved salts. The dissolved salts consist of 19.25 g Cl–, 10.7 g Na+, 2.7 g SO42-, 1.3 g Mg2+0.42 g Ca2+, 0.39 g K+ and 0.25 g of minor components. So, seawater in molten state can be considered as an impure melt of water with dissolved ionic impurities.The consequence of the presence of the ionic impurities is that they depress the freezing point of the water and elevate the boiling point. These effects do not only hold for salt in water but for almost all impure solutions or melts. The freezing point depression for sea water amounts about 2°C, which means that the sea will start to freeze when the temperature of the water drops below -2°C. When the seawater is freezing, the ice contains only very little salt as the salt ions do not fit in the very regular crystal structure of the water molecules in ice. Again, this is a rather typical characteristic of crystallization: impurities typically do not fit in the regular crystal lattice of the crystallizing component, which explains why crystallization is a powerful separation process with a relatively high selectivity. This can for instance be illustrated by the fact that icebergs formed in the sea could be used as drinking water, because almost all salt has been expelled from the ice. In most crystallization processes the crystals are the targeted product, but this is not necessarily true. A well-known process in food processing is freeze concentration, which again makes use of the crystallization of ice to concentrate beverages like coffee, fruit juices or beer. In such a process the concentrate is the targeted product and not the ice crystals! The advantage of freezing above evaporation is that volatile components like fragrances will remain in the freeze concentrate, whereas they would co-evaporate with the water during evaporative concentration. In addition, the low operating temperatures during crystallization will prevent or significantly slow down thermal degradation processes.
The figure at the left displays the solubility curves of sugar and salt (kitchen salt, i.e. NaCl) in water. It shows that the solubility of sugar increases significantly with increasing temperature, whereas the solubility of salt is not strongly dependent on temperature. From the solubility curves it can be derived that evaporative crystallization is the most logical option for salt crystallization, because cooling a saturated salt solution will not result in the formation of lot of salt. For sugar, the solubility is much more dependent on temperature. So, it would be possible to crystallize sugar from a hot, concentrated solution via cooling crystallization. In industrial practice, beet sugar is made in a batch crystallizer where a concentrated solution is boiled and cooled under vacuum.
Sugar and salt are examples of products where crystallization does not only serve as separation/purification technique, but where it is also responsible for getting crystals with the right size (and shape) for further application of the products.
It is important to note that the fact that crystals are often very pure does not automatically imply that the product is also very8 pure. The reason is that the pure crystals are still suspended in impure mother liquor at the end of a crystallization process. The final purity of the product does therefore also depend strongly on the efficiency of the solid-liquid separation used to separate the crystals from the mother liquor. Filter presses and centrifuges are well-known conventional solid-liquid separators. Scholz and Ruemekorf have reported that filter presses typically produce a filter cake with about 20% residual mother liquor, whereas the crystal cake in a centrifuge will contain 3-10% residual mother liquor, depending on the size and shape of the crystals. For applications where a high product purity is demanded, it is difficult to impossible to attain this with conventional solid-liquid separators. For instance, the product purity is limited to 99.0 wt% for a centrifuge with only 5% of residual moisture if the mother liquor contains 20% impurities. Sometimes washing steps are added after the centrifugation/filtering, but in that case 10-20% of the product is needed as washing liquid in order to reduce the impurity content in the cake by roughly 2/38. In the given example the product purity would increase to 99.6 wt%, which is often insufficient, but the 10-20% of contaminated washing liquid contains so much product that this will typically be recycled in the process. The practical consequence is that the overall yield of the process/crystallizer drops accordingly. Wash columns have been developed to overcome the described disadvantages associated with the use of conventional solid-liquid separators in order to fulfill the industrial need to make high purity products in a cost and resource efficient way. The strong points of the Hydraulic Wash Column is that it combines solid-liquid separation by means of filtration with a highly efficient counter-current washing in which the washing liquid is used but not consumed. Furthermore , the Hydraulic Wash Column is a truly continuous apparatus in which high specific production capacities can be reached in the range of 5-20 tonnes of washed product per hour in a 1 m2 column. The countercurrent washing is much more effective than in centrifuges and the amount of remaining impurities/mother liquor is often as low as 0.1-1%. More information on the use of wash columns is given in section 3.5, while section 3.6 gives the specific benefits of the SoliQz Hydraulic Wash Column.
In summary, crystallization is one of the most effective and selective separation processes/principles available in industry. Crystals will contain almost no or only a very limited amount of impurities inside the crystals provide that they are made in a well-designed and operated crystallizer/process. However, in order to benefit from the intrinsically high purity of the crystals it is crucial to separate them almost completely from the impure mother liquor. (Hydraulic) Wash Columns are particularly suited for this task. The combination of cooling/melt crystallization with the Hydraulic Wash Column is an excellent example of a green, cost and resource efficient separation process applicable to a large variety of organic and inorganic, especially when they are required in high purity.