Zone-melting process

Reiss, H. Mathematical Methods for Zone-Melting Processes. Trans. AIME... 

Refined tellurium contains traces of lead, copper, iron, selenium, and other impurities. Highly pure tellurium can be obtained either by distilling refined tellurium in vacuum or by the zone melting process. The last traces of selenium can be removed as hydride by treating molten tellurium with hydrogen. 

What has been done to speed up the zone melting process ... 

In the zone melting processes, a more likely achieved condition is illustrated via the impurity concentration proffle in the molten zone in Figure 6.3.15(a) the impurity mass fraction Un changes from the value m,/ at the freezing interface (where it is assumed to be in equilibrium with the solid mass fraction Uis o in the newly frozen section ABB A via Mis = K sMi/ o) to Uiib, the impurity mass fraction in the bulk of the melt at a distance Si (usually << /), the boundary layer thickness. Ideally, if the complete molten zone is well mixed, then... 

Problems 6.3.14 and 6.3.15 illustrate that the purification achieved in a single-pass zone melting process is less than that achieved by a normal freezing process. 

This makes it possible to conclude that the intensity of fiber filler destruction depends on the zone of processing installation where the filler is introduced (Fig. 7). As the Figure shows, with glass fibers introduced into the loading zone, their destruction occurs rapidly (curve 1). When fibers are introduced into the melt zone of an extruder the fiber destruction is less intensive. 

The preceding sections have established that the U-series information from arc lavas is a complex end-product of several different processes and this is especially true for U-Th isotopes. The two observations that seem to be most robust are (1) the need to transfer the Ra-excess signal from the subducting plate into the melting zone and to the surface in less than a few 1000 years, and (2) the need for a partial melting process which allows for Pa in-growth. There are a number of different models which can take account of these two requirements but which have different implications for the U-Th isotope arrays. 

As for fractional crystallization and fractional melting, element-element plots with a logarithmic scale should show straight lines for the solid as well as for the liquid, since both differ by a constant coefficient. Contrary to fractional crystallization but similar to fractional melting, discussed above, and to percolation, to be presented below, zone-melting is a very powerful process to separate incompatible elements. 

Electrotransport In this process too a super-clean atmosphere is necessary. In electrotransport (a kind of solid-state electrolysis) a large dc current (typically 200 A cm-2) is passed through a rod of the metal at a temperature 100-200°C below its melting point. In the rare earth metals the interstitial impurities slowly move towards the anode, while several metallic impurities move towards the cathode. In this case too, as in zone melting, the purest portion of the bar is its central part. 

Secondary refining processes such as zone melting and solid-state electrotransport (Section III,D) should yield ultrahigh-purity Pa metal. 

Terbium metal is obtained from its anhydrous trifluoride, TbFs, or trichloride, TbCls, by thermal reduction with calcium, carried out in a tantalum crucible. Terbium produced by such methods may contain traces of calcium and tantalum. High purity metal can be prepared by various methods such as vacuum remelting, distillation, amalgam formation, floating zone melting, and various chemical processes. 

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. 

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