Basics of Industrial Crystallization from Solution

Crystallization may be carried out from a vapor, from a melt, or from a solution. Most of the industrial applications of the operation involve crystallization from solutions. Nevertheless, crystal solidification of metals is basically a crystallization process, and much theory has been developed in relation to metal crystallization. This topic is so specialized, however, that it is outside the scope of this subsection, which is limited to crystallization from solution. 

Ciystallization from solution is an important separation and purification process in a wide variety of industries. These range from basic materials such as sucrose, sodium chloride and fertilizer chemicals to pharmaceuticals, catalysts and specialty chemicals. The major purpose of crystallization processes is the production of a pure product. In practice however, a number of additional product specifications are often made. They may include such properties as the ciystd size distribution (or average size), bulk density, filterability, slurry viscosity, and dry solids flow properties. These properties depend on the crystal size distribution and crystal shape. The goal of crystallization research therefore, is to develop theories and techniques to allow control of purity, size distribution and shape of crystals. 

Suspension crystallization processes are the most common in industry. After crystallization, the crystalline product is usually separated from the solution by filtration. The separation of crystals from the mother liquor has an important influence on the whole process. The reader is referred to some basic handbooks on filtration, such as Solid-Liquid Separation as edited by Svarovsky [48] and those by Wakeman and Tarleton [49], and Rushton et al. [50]. 

Solvent extraction of metals embodies all aspects of coordination chemistry rates, equilibria, stereochemistry, crystal field theory, covalent bonding, hard-soft acid-base theory, hydrogen bonding, steric hindrance, enthalpy and entropy. All of these basic principles can link together to produce pure metals on an industrial scale from dilute aqueous solutions — a remarkable achievement of elegant coordination chemistry. To achieve this result it is only necessary to form within the aqueous medium a neutral species containing the metal to be extracted. 

Dissolve or suspend 0.5 g of the acid in 5 ml of water in a small conical flask, add a drop or two of phenolphthalein indicator, and then 4-5 per cent sodium hydroxide solution until the acid is just neutralised. Add a few drops of very dilute hydrochloric acid so that the final solution is faintly acid (litmus).t Introduce 0.5 g of p-bromophenacyl bromide (m.p. 109 °C) dissolved in 5 ml of rectified (or industrial) spirit, and heat the mixture under reflux for 1 hour if the mixture is not homogeneous at the boiling point or a solid separates out, add just sufficient ethanol to produce homogeneity. (Di- and tri-basic acids require proportionately larger amounts of the reagent and longer refluxing periods.) Allow the solution to cool, filter the separated crystals at the pump, wash with a little alcohol and then with water. Recrystallise from dilute ethanol dissolve the solid in hot ethanol, add hot water until a turbidity just results, clear the latter with a few drops of ethanol and allow to cool. Acetone may sometimes be employed for recrystallisation. 

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