Principles of Metal Purification and Purity Evaluation
Techniques of metal refining and purification rely on the differences in physicochemical properties of the base metal and the impurities. In order to reach the desired ultra-high purity, a sequence of complementary refining steps is required. These steps
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Techniques of metal refining and purification rely on the differences in physicochemical properties of the base metal and the impurities. In order to reach the desired ultra-high purity, a sequence of complementary refining steps is required. These steps fundamentally apply chemical, electrochemical and physical methods, usually in this order. A substance is considered to be practically pure if its properties are determined by the atomic-crystalline structure and the intrinsic defects of the crystal lattice, while the relative effect of impurities is negligible [1]. Concentrations of impurities as low as one part in a million of the matrix metal can significantly influence the physical properties. The effect of a certain impurity also depends on the state of occurrence. Electrical properties are influenced practically by substitutional impurities, homogeneously dissolved in the material, while mechanical properties are strongly dependent on interstitial ones. The influence of minute quantities of imp urities on the physical properties of the base metal can be applied for the purpose of indirect purity evaluation, thus providing comparative methods to accompany direct elemental analysis.
2.1
Methods of Purification
Impurity elements can be removed from the base metal by assuring that they are eventually transferred - in elemental or compound form - into a separate phase. The methods of purification can be classified according to the main property utilized in separation (Table 2.1). The general scheme of purification is shown in Fig. 2.1. Aqueous precipitation techniques and fire refining are suitable for removing large quantities of different impurities, as applied in commercial hydrometallurgical and pyrometallurgical operations, and under appropriate conditions they can also be applied to fine purification. In order to achieve ultra-high purity, however, further ion-selective methods have to be applied, usually in aqueous media. The metal is extracted from the purified solution by electrolysis or hydrogen reduction of the crystallized and dehydrated salt. Remaining traces of impurities can be eliminated by refining electrolysis or by different physical methods, including vacuum melting, distillation, zone melting, hydrogen-plasma melting or, in special cases, electrotransport. The result of the procedure may equally depend on the primary (gross) and the secondary (fine) purification steps. The purified metal is annealed in ultra-high vacuum or hydrogen, and 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
Table 2.1. A classification of methods applied to metal purification No.
Nature of Property utilized separation
1 2
Chemical Chemical
3 4
Chemical Chemical
5 6
Electrochemical Physical
7
Physical
8
Physical
Method of separation
Selective reaction Selective precipitation Electrode potential Cementation Distribution of dissolved Ion exchange, solions between phases vent extraction E
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