Introduction

Crystallization is the nucleation and growth of a solid phase in a mother liquor. It can be used as a separation technique, because the process results in the separation of the mother liquor and the solid phase. If the mother liquor and the solid phase have different compositions it can also be used as a purification technique. In such cases the mother liquor must contain impurities. The deliberate addition of impurities during the crystallization process, has been widely used to improve both the product quality and the process performance. However, most impurities present during crystallization processes are not added on purpose, but are by-products from previous reactions, dissolved salts and solvents that can have the same kinds of effects.

In industrial crystallization additives are generally used to improve the handling characteristics of the crystalline product or to prevent scaling. By employing correctly selected additives it is possible to reduce caking, improve the free flowing behaviour of a powder, increase the bulk density of a crystalline product, change the crystal size distribution, induce certain polymorphs and improve filtration behaviour.

Varied though this list may seem, all these effects can be achieved by using additives that have very specific interactions with the crystal surface. This fundamental physical effect causes a change in the system parameters when additives are introduced. Detailed knowledge of the molecular structure of the solid-liquid interface is very important for design of additives. Once this interface is understood, it is possible to design molecules that have specific interactions with one or more of the faces of a crystal. Often the same additive can be used in different application. For instance a growth retarder used to prevent scaling of a given compound can, when added after crystallization, also act as an anticaking agent. Blocking a single crystal face with a certain additive can induce morphology changes, but the same additive can also be used to act as a template for the nucleation of that same crystal face.

In many cases additives are not wanted in the product application. Ferrocyanide is a very effective anticaking agent for salt (NaCl), but the ferro-cyanide complex must be destroyed and the iron precipitated prior to electrolysis of the salt for the production of chlorine (Cl2). Nitrosyltriacetamide is also an effective anticaking agent for salt, but during electrolysis nitrogen trichloride (an explosive) is formed.

Apart from having a very specific influence on a crystal face, additives must also be effective at very low concentrations. Because one of the main uses of additives is in cheap bulk products (costing less than US $1 kg-1) the price of the additive also acts as an incentive to keep the additive concentrations as low as possible. Expensive additives, even if used in very low amounts, will adversely affect the cost of these cheap bulk products. High additive concentrations always give rise to significant additive incorporation in the crystal and thus decrease the product purity.

Additives that effectively control crystallization processes are also found in nature: the shells of crustaceans are formed as a result of bio-templated growth of calcium carbonate; the growth of the mineral components of our bones and teeth is carefully regulated; and studies of fish in polar regions have revealed some proteins that very effectively block the growth of ice crystals.

After describing the fundamental effects of the additive in the crystallization system, a general procedure for the molecular design of additives will be given. The review ends with some case studies.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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