Fractional solidification

Zone refining is one of a class of techniques known as fractional solidification in which a separation is brought about by crystallization of a melt without solvent being added (see also Crystallization) (1 8). SoHd—Hquid phase equiUbria are utilized, but the phenomena are much more complex than in separation processes utilizing vapor—Hquid equiHbria. In most of the fractional-solidification techniques described in the article on crystallization, small separate crystals are formed rapidly in a relatively isothermal melt. In zone refining, on the other hand, a massive soHd is formed slowly and a sizable temperature gradient is imposed at the soHd—Hquid interface. 

A variety of other phenomena influence fractional solidification of organic compounds (68). 

M. Zief and W. R. Wilcox, Fractional Solidification Marcel Dekker, Inc., New York, 1967. 

C, is the concentration of impurity or minor component in the solid phase, and Cf is the impurity concentration in the hquid phase. The distribution coefficient generally varies with composition. The value of k is greater than I when the solute raises the melting point and less than I when the melting point is depressed. In the regions near pure A or B the hquidus and solidus hues become linear i.e., the distribution coefficient becomes constant. This is the basis for the common assumption of constant k in many mathematical treatments of fractional solidification in which ultrapure materials are obtained. 

FIG. 22-1 phase diagram for components exhibiting complete solid solution. (Zief and Wilcox, Fractional Solidification, -ool. 1, Marcel Dehher, New York, 1967, p. 31. )... 

The distribution-coefficient concept is commonly applied to fractional solidification of eutectic systems in the ultrapure portion of the phase diagram. If the quantity of impurity entrapped in the solid phase for whatever reason is proportional to that contained in the melt, then assumption of a constant k is valid. It should be noted that the theoretical yield of a component exhibiting binary eutectic behavior is fixed by the feed composition and position of the eutectic. Also, in contrast to the case of a solid solution, only one component can be obtained in a pure form. 

General Reeerences Van t Land, Industrkd CrystaUizaUon ofMdts, Taylor Francis, New York, 2004. Mullin, CrystaiUzathn, 4th ed., Butterworth-Heinemann, 2001. Myerson, Handbook of Industrkd CrystaUizaUon, 2d ed., Butterworth-Heinemann, 2001. Pfann, Zone Melting, 2d ed., Wiley, New York, 1966. U.S. Patents 3,621,664 and 3,796,060. Zief and Wilcox, Fractional Solidification, Marcel Dekker, New York, 1967. 

Newton Chambers A process for purifying benzene by fractional solidification cooling is accomphshed by mixing it with refrigerated brine. The process does not remove thiophene. 

Fractional solidification and its applications to obtaining ultrapure chemical substances, has been treated in detail in Fractional Solidification by M.Zief and W.R.Wilcox eds, Edward Arnold Inc, London 1967, and Purification of Inorganic and Organic Materials by M.Zief, Marcel Dekker Inc, New York 1969. These monographs should be consulted for discussion of the basic principles of solid-liquid processes such as zone melting, progressive freezing and column crystallisation, laboratory apparatus and industrial scale equipment, and examples of applications. These include the removal of cyclohexane from benzene, and the purification of aromatic amines, dienes and naphthalene. 

ZONE REFINING. One of a class of techniques known as fractional solidification, in which a separation is brought about by crystallization of a melt without solvent being added. See also Crystallization. A massive solid is formed slowly and a sizable temperature gradient is imposed at the solid—liquid interface. 

Zief, M. Wilcox, W. R. Fractional Solidification Dekker New York, 1967. 

FIO. 22-10 Steady-state separation of azobenzene and stilbene in a center-fed column crystallizer with total-reflux operation. To convert centimeters to inches, multiply by 0.3937. Zief and Wilcox, Fractional Solidification, ix)l. 1, Marcel... 

---