Crystallization, hydrates sodium chloride

The reactor effluent, containing 1—2% hydrazine, ammonia, sodium chloride, and water, is preheated and sent to the ammonia recovery system, which consists of two columns. In the first column, ammonia goes overhead under pressure and recycles to the anhydrous ammonia storage tank. In the second column, some water and final traces of ammonia are removed overhead. The bottoms from this column, consisting of water, sodium chloride, and hydrazine, are sent to an evaporating crystallizer where sodium chloride (and the slight excess of sodium hydroxide) is removed from the system as a soHd. Vapors from the crystallizer flow to the hydrate column where water is removed overhead. The bottom stream from this column is close to the hydrazine—water azeotrope composition. Standard materials of constmction may be used for handling chlorine, caustic, and sodium hypochlorite. For all surfaces in contact with hydrazine, however, the preferred material of constmction is 304 L stainless steel. 

Water molecules can reorientate in order to neutralize the ionic forces of crystalline materials, such as sodium chloride (Na+Cl ), allowing hydration of the crystal to occur. Thus water is a good solvent. 

In general substances which consist of positive as well as negative ions have high boiling points because a lot of energy is required to loosen the particles from the crystal lattice. These substances mostly dissolve well in a solvent whose molecules possess a dipole. These dipoles surround the ions, due to which the latter are no longer able to unit to form a crystal. This surrounding by molecules of the solvent is called solvation, or, in the case of water, hydration. In Fig. 3.11 you can see what happens to the ions when sodium chloride (NaCl) is dissolved in water. 

Some salts, such as sodium chloride, copper carbonate and sodium nitrate, crystallise in their anhydrous forms (without water). However, many salts produce hydrates when they crystallise from solution. A hydrate is a salt which incorporates water into its crystal structure. This water is referred to as water of crystallisation. The shape of the crystal hydrate is very much dependent on the presence of water of crystallisation. Some examples of crystal hydrates are given in Table 8.6 and shown in Figure 8.20. 

Cadmium Tungstates.—The anhydrous normal salt, CdWO, remains as colourless crystals when a mixture containing sodium tungstate (4 parts), sodium chloride (16 parts), and cadmium cliloride (11 parts) is heated to complete fusion and allowed to cool. The hydrate, CdW04.2H20, is formed by double decomposition. ... 

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