The Zone-Refining of Selenium

The zone-refining process has been applied to selenium of 99.994% purity. The theory of zone-refining is based upon the phase diagram of binary alloys. But according to recent experiments, the segregation of impurities in zone-refining process of a metal which has relatively low melting point such as In or Se cannot be explained by the rule of phase diagram. Se contains impurities whose distribution coefficients k are less than unity. Besides, Se itself is transformed by a heat treatment. So it has not been confirmed whether the application of zone-refining to Se is effective or not. The specimens of Se were refined under various conditions. The travel rates of 7, 20, 40, 58 mm/hr were chosen. The effect of zone-refining was confirmed by the radio-activation and spectrographic analysis. The results obtained were as follows. The impurities Ga, Cu, Ca, Bi and Te segregated at the starting end. The impurities Al, Fe, Mg, Mn, Sb and Si segregated at the finishing end. Se crystallized, in the cases of the travel rates smaller than 20 mm/hr, and it resulted in vitreous Se at 40 mm/hr and 58 mm/hr. Zone-refining was most effective at the medium rate. In the case of purification of binary system alloys consisting of Se and an element whose k is larger than unity, the number of zone-travels required for the effective segregation of elements which have the tendency to move towards the starting end, must be at least larger than the ratio: bar length/width of molten zone.