Effects of impurities on an industrial crystallization process of ammonium sulfate

In general, crystallization is a process including a liquid-solid phase transition. This process was used for a very long time by mankind and one famous example is the production of the so-called “white gold”, the sodium chloride. The Martin Luther University is situated in Halle at the river Saale which is famous for its history in the production of food grade salt. Archaeological findings of tools for the salt production at the “Hallmarkt” close to river Saale from the year 2000 before Christ are evidence for a considerable salt production of Celtic, German and Slavic tribes. In 13 th century saltwater was taken from springs and was brought to so-called boiling houses, where the water was evaporated and therefore salt was produced. Afterwards the wet salt was dried in big baskets and the final product table salt could be sold. From this example for early and simple evaporation crystallizations there was a steep development and today crystallization processes are highly efficient and generate very pure products with high yields. These industrial crystallization processes for the production of inorganic salts like ammonium sulfate are large scale processes. By taking a look at the crystallization unit of such a process and fading out all upstream or downstream process operations like centrifugation, filtration, drying or storage, the dimension become clear. The most important criterion for the success of the process is the product stream which contains the desired crystalline product. This product should have specified properties after leaving the crystallizer. On the one hand the quantity (yield) of the crystallization should be as high as possible to make the process profitable. On the other hand there is the product quality which includes parameters like crystal size and crystal size distribution, the shape of the product crystals and their purity. All of these parameters depend on the design of the crystallizer (and process conditions) and on the properties of the feed stream. Common types of crystallizers are the FC (Forced Circulation) crystallizer, the DTB (Draft Tube Baffle) crystallizer or the Oslo-type crystallizer [Hof04]. All of these types are continuous crystallizers which are often used for mass products like inorganic salts. Already by choosing the one or the other type of crystallizer the size of the product crystals is fixed into a defined range.