Incorporation of Impurities

Impurities obviously reduce the purity of the crystalline product and are seldom beneficial to the crystallization processes or the product characteristics. As with the design of additives, it is important to understand how the impurities are incorporated in the crystalline product to reduce their uptake.

In general an impurity is included in the crystalline product through either direct incorporation in the lattice (solid solubility), the formation of inclusions, surface adsorption or poor washing of agglomerates. These incorporation mechanisms will be discussed in more detail.

Solid solutions can only be formed when the impurities mimic the main compound in form and charge.

Figure 7 An RDX (cyclotrimethylene trinitramine) crystal grown from the solvent cyclohexanone. A blocking type impurity in the system causes very large macrosteps on a particular surface and inclusions in the interior of the crystal.

They are especially likely to occur with organic crystals where closely related by-products are formed during synthesis of the product. Examples are 2,6-dimethylnaphthalene in naphthalene, cyclohexanone in caprolactam and bromide in sodium chloride. This phenomenon can only occur when the crystallization enthalpy of the impurity is very close to that of the main compound. If that is the case, not only will the impurity be easy to include in the crystal lattice, but the growth will not be retarded. In other words, compounds that form high concentration solid solutions have a very limited influence on the growth of the main compound. If the impurity resembles the main compound to a lesser extent the solid solubility will decrease and growth will be hindered. The free enthalpy involved in the incorporation of polymers or polyelectrolytes, which are the most effective additives in mineral scale prevention, is so large that they are not incorporated in the lattice by direct substitution for molecules or ions of the main component.

A second incorporation mechanism is inclusion formation. Inclusions can be formed in several ways. The most common mechanism in industrial crystallization involves imperfect regrowth of corners damaged by attrition. Pockets of mother liquor are then trapped. The concentration of impurity in the inclusion is then directly related to the concentration in the mother liquor. The overgrowth of poisoned areas is another type of inclusion formation, and occurs at higher driving forces for crystallization. This is essentially the only way to overcome the blocking effects of polymers and polyelec-trolytes. As discussed previously, the concentration of polymers or polyelectrolytes in solution is low because these additives are effective at low concentrations and most of the additive is adsorbed on the crystal surface. The amount of additive in the product will therefore not increase much, but of course the amount of other impurities present in the mother liquor, such as solvent, will increase.

A third incorporation mechanism is adherence to the surface. 'Normal' impurities, which cannot be incorporated in the lattice, can be washed off fairly easily. Tailor-made additives, however, are designed to adhere to the surface and are difficult to remove. Partial dissolution of the crystals is not an effective solution as these additives may also hamper the dissolution process. Washing is therefore very useful for the removal of 'normal' impurities, but less so for tailor-made additives.

The formation of voids in agglomerates on impurity uptake is a fourth mechanism of impurity uptake.

In conclusion, impurities that do not hinder crystal growth can be effectively removed from the product through a combination of careful crystallization and after treatments (washing, etc.). Compounds such as tailor-made additives are much harder to remove.

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