Zone refining surface

Before this treatment, the cassiterite content of the ore is increased by removing impurities such as clay, by washing and by roasting which drives off oxides of arsenic and sulphur. The crude tin obtained is often contaminated with iron and other metals. It is, therefore, remelted on an inclined hearth the easily fusible tin melts away, leaving behind the less fusible impurities. The molten tin is finally stirred to bring it into intimate contact with air. Any remaining metal impurities are thereby oxidised to form a scum tin dross ) on the surface and this can be skimmed off Very pure tin can be obtained by zone refining. 

Tellurium [13494-80-9] M 127.6, m 450 . Purified by zone refining and repeated sublimation to an impurity of less than 1 part in 10 (except for surface contamination by Te02). [Machol and Westrum J Am Chem Soc 80 2950 1958.] Tellurium is volatile at 500°/0.2mm. Also purified by electrode deposition [Mathers and Turner Trans Amer Electrochem Soc 54 293 1928]. 

Dislocations. Dislocation densities (dislocation lines/cm ) in electron-beam-zone refined tungsten have been measured to be between 5 x 10 and 10. A diminution from the center of a rod to its surface was determined. In cold-woriced foils, filings, rods, and thin wires (as worked or aimealed) the densities vary between 10 and 10. ... 

Several elementary aspects of mass diffusion, heat transfer and fluid flow are considered in the context of the separation and control of mixtures of liquid metals and semiconductors by crystallization and float-zone refining. First, the effect of convection on mass transfer in several configurations is considered from the viewpoint of film theory. Then a nonlinear, simplified, model of a low Prandtl number floating zone in microgravity is discussed. It is shown that the nonlinear inertia terms of the momentum equations play an important role in determining surface deflection in thermocapillary flow, and that the deflection is small in the case considered, but it is intimately related to the pressure distribution which may exist in the zone. However, thermocapillary flows may be vigorous and can affect temperature and solute distributions profoundly in zone refining, and thus they affect the quality of the crystals produced. 

A solute distribution exists in the melt because the solidification is carried out at a finite rate. For example, if k0 < 1, then solute is rejected and accumulates at the surface which is solidifying, and this creates solute gradients in the melt which tend to be relaxed by molecular diffusion and any convection which may exist. The interfacial distribution coefficient, k, refers to the solid to liquid solute concentration ratio at the interface. It is k which is used in transport calculations when one is trying to understand the dynamic behavior of zone refining systems. It usually is found that equilibrium exists locally at the solid-liquid interface, in which case k ko. 

The flow phenomena involved in zone refining will be discussed briefly. In particular we shall consider surface tension driven flow in a cavity containing a low Prandtl number, Pr, fluid (a low Pr number is typical of liquid metals and semiconductors). It will be shown that simplified models of such flow, which simulate the melt configuration in zone refining, predict multiple steady-state solutions to the Navier-Stokes equations exist over a certain range of the characteristic parameter. 

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