Process Technology: An Introduction - Haan A.B. 2015

13 Crystallization and precipitation
13.6 Precipitation

Precipitation is an important industrial process, which is closely related to crystallization from solution. In precipitation a solute is made supersaturated so that dissolved solutes precipitate out. Precipitation differs from crystallization since the precipitate is usually amorphous, has a poorly defined size and shape, is often an aggregate, and is usually not pure. Precipitation can be carried out in one step to remove a large number of compounds in a single precipitate. This is the easiest application and is most common. Fractional precipitation requires a series of steps, with each step optimized to precipitate the desired component. It is not possible to do extremely sharp purifications by fractional precipitation. Precipitation and crystallization are often complementary processes, since precipitation is used for a “rough cut”, while crystallization is used for final purification. Precipitation is commonly used for the manufacture of pigments, pharmaceuticals, and photographic chemicals. In the production of ultrafine crystalline powders, precipitation is often considered as an attractive alternative to milling, particularly for heat sensitive substances.

Like all crystallization processes, precipitation consists of three basic steps: (1) the creation of supersaturation, followed by (2) the generation of nuclei and (3) the subsequent growth of these nuclei to visible size. The used equipment is often very similar to that used for crystallization. Precipitation operations usually have a mixing operation, where various reagents are added to make the solution supersaturated. This is followed by a holding period, which allows for an induction period before nuclei form, and a latent period before the supersaturation starts to decrease. Once the precipitate starts to grow, the desupersaturation often occurs at a constant rate. Towards the end of the desupersaturation period the rate decreases as the concentration approaches equilibrium. During this period, aging often occurs. During aging, small crystals redissolve, and the solute is redeposited onto the larger precipitates. The result is a uniform crop of fairly large precipitates, which is relatively easy to separate from the solution by centrifugation.

Precipitation processes are conveniently classified by the method used to produce the supersaturation: cooling, solvent evaporation, reaction, salting out, antisolvents, and pH. Supersaturation caused by cooling and or evaporation of solvent is very similar to solution crystallization. Many products are made by mixing reagents that react to form an insoluble precipitate. A representative example is the formation of gypsum by the addition of sulfuric acid to a calcium chloride solution:

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In aqueous solution, high inorganic salt concentrations also often cause the precipitation of solute. Antisolvents or nonsolvents can be added to achieve the same effect. The solubility of many compounds is effected by pH.