Purification - zone melting

The primary application for floating-zone melting is crystal growth rather than purification. Semiconductor-grade siUcon is not purified by zone refining siUcon chlorides are distilled and then reduced with hydrogen. 

Either low- or high-porosity-solids-phase concentrations can he formed in the purification and melting zones. End-fed units are characterized by low-porosity-solids packing in the purification and melting zones. 

In addition to the above, purification of /V-methylacetamide can be achieved by fractional freezing, including zone melting, repeated many times, or by chemical treatment with vacuum distn under reduced pressures. For details of zone melting techniques, see Knecht in Recommended Methods for Purification of Solvents and Tests for Impurities, Coetzee Ed. Pergamon Press 1982. 

Naphthol [135-19-3] M 144.2, m 122.5-123.5°, pK 9.57. Crystd from aqueous 25% EtOH (charcoal), water, benzene, toluene or CCI4, e.g. by repeated extraction with small amounts of EtOH, followed by dissolution in a minimum amount of EtOH and pptn with distilled water, then drying over P2O5 under vacuum. Has also been dissolved in aqueous NaOH, and ppted by adding acid (repeated several times), then ppted from benzene by addition of heptane. Final purification can be by zone melting or sublimation in vacuo. [Bardez et al. J Phys Chem 89 5031 7955 Kikuchi et al. J Phys Chem 91 574 1987.]... 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

Zone melting is a possibly generic approach to IL purification. The solidification of ILs often resulfs in the formation of glass. However, it is possible to determine/choose conditions under which single crystals of ILs with a melting point down to -25°C (but not all) can be grown [39]. Where crystallization is seen, then separation of impurities can be demonstrated. 

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. 

As shown by Fig. 22.16, the concentrations of solute atoms are significantly reduced in the material that is solidified early in the solidification process when k < 1. One-dimensional plane-front solidification can therefore be used as a method of purification. However, purification is carried more effectively out by modifying the process and adopting a zone-melting technique. 

Considerable purification is achieved during zone melting. The final transient at the end begins when the leading end of the zone reaches the end of the specimen. At that point, the solidification becomes very similar to plane-front solidification. Additional passes produce further purification and very small solute concentrations in the first part of the specimen. An asymptotic limit exists, however, as taken up in Exercise 22.2. 

Suppose that multiple zone-melting passes are made along a bar of length L in an effort to purify it as much as possible. As the number of passes increases, a limiting situation will be reached in which no further purification is obtained by increasing the number of passes. Let uSL(x) be the ultimate solute distribution along the bar after an infinite number of passes. 

Metal Purification. Depending on the relative boiling points, purification may be carried out by RE distillation, alloying element distillation, or zone melting. 

From equation 7, it may be seen that the tendency toward constitutional supercooling increases as the freezing rate increases, the temperate gradient G decreases, the impurity content w increases, the separation (w i) between Hquidus and soHdus in the phase diagram increases, and the stirring decreases (5 increases). This explains why zone melting is limited to purification of materials with low impurity contents, and why substantial temperature gradients and low zone-travel rates are necessary. 

---