CRYSTALLIZATION

CRYSTALLIZATION

Crystals are chemically homogeneous bodies, usually having
regular polyhedral forms, and whose molecules have arranged themselves regularly according to definite laws. The tendency to form crystals is common to almost all chemical compounds under certain conditions, the forms of the crystals being characteristic of the substance.
Crystals may form from a fusion, or by sublimation; but crystallization almost always takes place from solution.
In the majority of cases, the solubility of a substance increases as the temperature of the liquid rises, until a point is reached at which no more of the substance will dissolve, even though the solution is boiling. 'When a liquid has dissolved all of a solid that it can hold in solution at a certain temperature and pressure, it is said to be saturated for that temperature. Any decrease in the temperature results in the separation of a part of the substance, usually as crystals. There are a few instances where the maximum solubility is reached at temperatures much below the boiling point of the solution, the most notable of these salts being sodium carbonate and sodium sulphate, both reaching the maximum solubility below 35° C. During the formation of the crystal, there is a tendency to exclude from it all matter not homogeneous with it; hence this is an excellent method of purifying salts. But if a concentrated
solution, which is very impure, is allowed to crystallize, the impurities may become enclosed in or entangled among the crystals as they form, producing an impure product. This can often be prevented by stirring the solution while crystallizing, thus causing the formation of very fine crystals or "crystal meal," which may be more readily washed free from mother-liquor and impurities. The liquid from which the crystals have deposited, is called the 'mother-liquor' it contains the greater part of the soluble impurities present in the original solution, and also a considerable quantity of the salt, which has not deposited as crystals. The amount of the latter depends upon the temperature at which the crystallization took place. By fl1l'ther evaporation more crystals may be obtained, but they are less pl1l'e than those first separated. Thus the impurities
accumulate in the mother-liquor, and in many cases, being valuable salts themselves, are recovered, and add to the profits of the industry.
On the other hand, the mother-liquors from some processes
are the cause of much annoyance and expense to the manufacturer, since from their corrosive, poisonous, or offensive nature, they cannot be run into the streams or sewers, and their disposal in some other way becomes necessary.
If a concentrated solution is allowed to stand quietly while
crystallizing, especially if there is a considerable quantity of the liquid and the temperature falls very slowly, the crystals formed are usually large and well defined; on the other hand, if it be stirred, the crystals are small and imperfectly developed, constituting the crystal meal above mentioned. Since large crystals are very compact and offer a relatively small surface to the action of water, they dissolve very slowly, unless pulverized. Crystal meal dissolves more readily, and for this reason is becoming more and more popular with manufacturers.