The Purification of Base Transition Metals
Possible methods of purification, outlined in Sect. 2.1, are applied in practice to purify the technically important 3d-type (first series) transition metals used in modern industry. This group includes the elements in the d-block of the fourth row of the
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The Purification of Base Transition Metals
T. Kekesi and M. Isshiki
Possible methods of purification, outlined in Sect. 2.1, are applied in practice to purify the technically important 3d-type (first series) transition metals used in modern industry. This group includes the elements in the d-block of the fourth row of the periodic table, from titanium to zinc. Although zinc is not considered a transition metal by virtue of its chemical characteristics, it is also included in the discussion due to the similar practical significance and purification techniques. These metals are produced in large quantities at a commercial level of purity and are used as pure materials (Fe, Cu, Zn and Ti), or as components in various grades of alloys (V, Cr, Mn, Co, Ni and Zn). Having discussed the principles underlying the methods of metal purification in the previous chapters, the attention is now focused on: • • • •
the major industrial applications of high-purity grades starting materials for purification practically proven methods of separation achieved levels of purity
for each important transition metal, approaching the subject primarily from the angle of overall purification by chemical methods, supplemented by physical steps of specific effects. In general, similarities in the properties of the base metal and the impurities usually make the elimination of certain elements especially difficult. Therefore, a combination of different chemical and physical purification methods may have the desired effect of overall purification. It is especially difficult to remove impurities from the most reactive members in the first transition series, such as Ti, V and Cr. The attainable levels of global purity, indicated by the measured RRR values, are relatively low in these cases.
3.1
The Purification of Titanium
The application of ultra-high-purity titanium in VLSI chips requires virtually complete elimination of: (i) alkali metals, which migrate into the gate insulation film and cause deterioration of MOS interface characteristics; (ii) radioactive elements, which cause microprocessor operational errors by (Yparticle emission; and (iii) heavy metals, especially Fe, Cr and Ni, which impair interface junctions [1]. Titanium is also used as a minor addition to low-activation structural alloys, to improve the metallurgical properties. Y. Waseda et al. (eds.), Purification Process and Characterization of Ultra High Purity Metals © Springer-Verlag Berlin Heidelberg 2002
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T. Kekesi and M. Isshiki
Commercial titanium sponge is usually obtained by the (Kroll) magnesium reduction process after removing relatively large residual amounts of the reductant metal and salt impurities. Eventually, no significant impurities are introduced by the reducing metal [2]. Minute amounts of chlorides may remain sequestered in pores, unaffected by the washing treatment. These residues are fully removed by vacuum melting. The iron impurity of the Kroll sponge can hardly be avoided because of the contamination from the reactor wall. The sodium reduction
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