Usually, crystallization processes are carried out slowly and the mother liquor is in contact with sufficiently large crystals so that at the end of the process, the mother liquor is saturated at the final temperature.Â
In such cases, the yield of the crystallization process is calculated from the initial solution composition and the solubility of the solute material at the final temperature.Â
If appreciable evaporation of the solvent occurs during the crystallization process, the evaporated must be taken into account in determining the yield. Whenever crystals are anhydrous, the yield is obtained by taking the difference between the initial composition of the solution and the solubility of the solute corresponding to the final temperature of the process.
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In cases where material precipitates as a hydrated salt, we have to take into account the water of crystallization as this water is not available for retaining solute in solution. Under these circumstances, the key to calculations of yield is to express all compositions in terms of hydrated salt and excess water as excess water remains constant during crystallization operation and compositions or amounts thus expressed based on excess water can be deducted to obtain the correct results.Â
Crystals Made From
These constituent particles of a crystal are arranged in an orderly and repetitive manner. The constituent particles may be atoms molecules or ions. They are arranged in orderly three-dimensional arrays called space lattices. Crystals are classified according to the angle between the faces and this is the area of the science of crystallography. Different forms of crystals based on the angle between faces and lengths of axes are, cubic, tetragonal, orthorhombic, hexagonal, monoclinic, triclinic and trigonal.Â
Caking of Crystals
Caking of crystalline materials is caused by to a small amount of dissolution occurring at the surface of crystals and subsequent re-evaporation of the solvent. Due to caking the crystals can get very tightly bonded together.Â
Since the vapour pressure of a saturated solution of a crystalline solid is less than that of pure water at a given temperature, condensation can take place on the surface of the crystals even though the atmospheric relative humidity is less than 100%. The solution thus formed enters into/penetrates into the pack of crystals as a result of capillary action of the small gaps between the crystals and caking can result due to subsequent evaporation of moisture when the atmospheric humidity falls.Â
Crystalline materials can also cake at a constant relative humidity as the vapour pressure of a solution is less in a small capillary as compared to it in a large capillary as a result of the temperature effect. As condensation occurs the small particles get first dissolved and therefore the average size of capillaries increases and the vapour pressure of the solution may increase sufficiently for evaporation to take place.
When the particle size is non-uniform, a crystalline material will cake more rapidly since the porosity of a bed of particles of mixed sizes is less and fine particles are more readily soluble.
Thus the tendency of crystalline materials to cake can be reduced by forming crystals of relatively large and uniform sizes or by adding a water-repellent agent, such as stearic acid.Â
Effect of Impurities on Crystal Formations
(i) Soluble impurities may get adsorbed on the surface(chemistry) of the nuclei or crystals' nucleation sites and retard the rate of nucleation and crystal growth.Â
(ii) The shape of the crystal may get modified as the adsorption of impurities may occur preferentially on a particular face.Â
The impurities may decrease the rate of crystal growth. In some cases, it is desirable e. g. addition of a small quantity of glue or tannin to boiler feed water prevents nucleation and growth of calcium carbonate crystals and thus reduces scaling.Â
Take these Notes is, Orginal Sources:Â Unit Operations-II, KA Gavhane
