Zone Refinement Effect

In order to calculate the effect of several passes an iterative calculation is needed using the initial profile at each pass to represent Cq. Clearly for the second pass, die concentration profile given in the right-hand side of tire above equation must be used. It is clear that tire partition constant of the impurity between the solid and liquid is the most significant parameter in tire success of zone refining. 

Because phenols are weak acids, they can be freed from neutral impurities by dissolution in aqueous N sodium hydroxide and extraction with a solvent such as diethyl ether, or by steam distillation to remove the non-acidic material. The phenol is recovered by acidification of the aqueous phase with 2N sulfuric acid, and either extracted with ether or steam distilled. In the second case the phenol is extracted from the steam distillate after saturating it with sodium chloride (salting out). A solvent is necessary when large quantities of liquid phenols are purified. The phenol is fractionated by distillation under reduced pressure, preferably in an atmosphere of nitrogen to minimise oxidation. Solid phenols can be crystallised from toluene, petroleum ether or a mixture of these solvents, and can be sublimed under vacuum. Purification can also be effected by fractional crystallisation or zone refining. For further purification of phenols via their acetyl or benzoyl derivatives (vide supra). 

The next stage in the zone-refining process is to move the furnace slowly and steadily to the right. The left-hand end of the bar will then cool and refreeze but with the equilibrium composition /cCq (Fig. 4.4c). As the furnace continues to move to the right the freezing solid, because it contains much less impurity than the liquid, rejects the surplus impurity into the liquid zone. This has the effect of inereasing the impurity concentration in the zone, which in turn then increases the impurity concentration in the next layer of freshly frozen solid, and so on (Fig. 4.4d). Eventually the concentrations ramp themselves up to the situation shown in Fig. 4.4(e). Flere, the solid ahead of the zone has exactly the same composition as the newly frozen solid behind the zone. This means that we have a steady state where as much impurity is removed from the... 

The zone-refining technique has been applied to Be purification. Typical analytical data for zone-refined samples are given in Table l . These data show that zone refining concentrates Fe, Al, Mn and Cr at the front of the sample and Ni and Cr at its base furthermore, it appears to have no effect at all on Si distribution The influenee of an electric fieldand the effect of H2 Nj and Al on zone-refining purification techniques have also been assessed. 

Percolation models differ from the zone-refining model essentially by the absence of mixing in the liquid, giving the liquid position-dependent properties. A simplified account of these models was described in Chapter 8. We will now provide a reasonably comprehensive account which may prove useful to the demanding reader, and then examine some properties of the chromatographic effect in a simple configuration. Let

open volume porosity of the medium, pso, and pliq the density of the solid matrix and melt, respectively, vliq the liquid velocity relative to the matrix, and Cso, and CHq the concentration of element i in the matrix and melt, respectively. Let us rewrite equation (8.3.14) as... 

This study has been conducted with the objectives 1) to analyze and determine the effect of the free convective mixing in the melted zone upon the over-all solute transfer process in zone refining under various conditions and 2) to take into account the solute transfer both in the melted zone and in the adjacent P.S.Z. (mushy region) in predicting the rate of over-all solute transfer. The temperature distribution, composition, zone travel speed, zone dimension and other important parameters are considered in this study. 

E. Kitazume, N. Sato, and Y. Ito, Effective concentration method for trace metals in solution by pH-zone-refining countercurrent chromatography, 1997 Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, 1997. 

Amongst the disadvantages of this purification technique is the occurrence of some secondary processes such as volatilization or oxidation of particular impurities, which may influence the course of the concentration process. Also, the fact that values of partition coefficient between the solid and liquid phases for various elements and compounds will differ, detracts from facility with which purification can be effected by zone refining. 

Additive and impurity rejection at the growing crystal front leads to uneven distribution in a crystalline polymer. This redistribution process has been studied by UV and fluorescence microscopy and by an electron microscope with energy dispersive x-ray analysis. In polymer samples which are quenched after rapid crystallization, the additive distribution is kinetically determined and may be modeled in a computer as a three-dimensional zone-refining process. In annealed polymer samples, low molecular weight additives are uniformly concentrated in the amorphous phase. The additive distribution reflects that of crystalline material within the polymer. Antioxidant and uv stabilzer redistribution probably does not have a major effect on polymer stability, but the redistribution of partially oxidized, impure polymer may be important... 

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