Crystallization in solution - Processes and equipment types

Crystallization from solution is a basic process in many industrial sectors. Crystals can be produced in a wide range of sizes, from a few tens of nanometers to several millimeters, in the form of individual particles or structured agglomerates of particles. The best-known examples are crystals for fine chemicals and their intermediates, such as salts, sodium carbonate, zeolites, ceramics, detergents, fertilizers, active pharmaceutical ingredients or pigments. As a result, the tonnage and range of crystals produced is very broad. Production can reach several hundred tonnes per day for common products such as adipic acid or ammonium sulfate. Other products, such as aspirin or paracetamol, are produced in quantities of several dozen tonnes per day. Even smaller quantities are obtained for certain pharmaceutical compounds, for example. The economic value, social benefits and technical progress of crystalline products and associated processes are constantly increasing, particularly in sectors involving high value-added products. The central role of the crystallization process and the quality requirements placed on crystals, including safety and environmental criteria, are driving scientists and engineers to develop innovative products and processes. Crystallization is therefore a rapidly developing process.

Controlling the solid phase, particularly in particulate form, is an increasingly pressing industrial challenge. Divided solids are used as vehicles or carriers for active or consumable substances, as precursors for more elaborate materials, and as finished or semi-finished products.

Industry increasingly needs to "synthesize properties" for specific applications. This applies to many fields: basic chemistry, fine chemistry, pharmaceuticals, food, materials, pigments, catalysts, etc. Current needs are mainly to improve the repeatability of properties in the manufacture of split solids, and to develop new commercial-scale manufacturing processes.

As the crystal generation stage, solution crystallization plays a decisive role in the quality of the final product. The aim of these studies is to establish the interactions between the process and the use properties of the crystals. These interactions are difficult to establish, as usage quality may be a combination of several other, more elementary properties, such as crystal size and shape, aggregation state, crystal structure or purity.

Crystallization is the appearance of solid particles (crystals) in a solution that is supersaturated with respect to the product to be crystallized. This supersaturation can be created thermally (cooling of the solution or evaporation of the solvent) or chemically (addition of a crystallizing compound or chemical reaction).