The vapour phase refining and separation of metals

The anticipated content of impurities in the refined metal may be calculated a priori by assuming thermodynamic equilibrium at both metal/gas interfaces, and using the relevant stabilities of the gaseous iodides. Adequate thermodynamic data could provide the activities of the impurities with that of zirconium close to unity, but the calculation of the impurity transport obviously requires a knowledge of activity coefficients in the original impure material, which are not sufficiently well known. 

The three impurities, iron, silicon and aluminium are present in the metal produced by the Kroll reduction of zirconium tetrachloride by magnesium to the extent of about 1100 ppm. After the iodide refining process the levels of these impurities are 350, 130 and 700 ppm respectively. The relative stabilities of the iodides of these metals compared to that of zirconium can be calculated from the exchange reactions 

This is an instructive example of the fact that thermodynamic data can only be used to calculate the extent to which a reaction can be carried out 

The separation of niobium from tantalum through the gaseous chlorides is carried out at higher temperature, about 900 K, and it is therefore to be expected, as is the case, that the thermodynamic data will provide a useful guide. These metals form a number of chlorides, mainly the tri- tetra- and pentachlorides. These latter are much more volatile than the tetrahalides, and the exchange reaction at 900 K 

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