Purification of tin by zone refining with development of a new model
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INTRODUCTION
is an indispensable metal, the major uses of which are in tin plate, solder, and bearings. More recently, the application of tin to specific alloys vital to electrical, electronic, and various engineering achievements has been considerably increased. The high prices for tin metal, together with the increasing demand for it, have encouraged the purification of low-grade tin produced from various types of smelters, as well as the development of lower grade tin deposits, t~] Conventional dry refining techniques, including vacuum refining, electrolytic refining, and partial-solidification refining, have been generally used as the methods of purification and refining of tin.J21 Dry refining methods are still used for the removal of iron, copper, arsenic, and antimony from tin metal, but the removal of other irnpurifies such as lead and bismuth by dry refining is known not to be economical. Electrolytic refining of impure tin metal, in contrast to dry refining, is more efficient, both in terms of the purity of the product and the ratio of tin to impurities in by-products. It needs, however, a large stock of crude tin and, consequently, high initial investment for the process, apart from the higher capital cost for an electrolytic plant as compared with that for pyrometallurgical processes. On the other hand, partial-solidification refining, the well-known examples of which are zone melting and fractional crystallization techniques, has been considered a very efficient method applicable to the purification and refining of low-grade tin metal. The zone melting techniques, t3,4Jone of the partial-solidification refining methods, is useful for obtaining ultra-high purity in already nearly pure materials, but it is not applicable to the purification of large quantities of alloys containing appreHWA YOUNG LEE, Senior Research Scientist, JONG KEE OH, Principal Research Scientist and Head, and DONG HI.Yl LEE, Principal Research Scientist, are with the Extractive Metallurgy and Electrochemical Engineering Laboratory, Korea Institute of Science and Technology, Seoul, Korea. Manuscript submitted July 21, 1989. METALLURGICAL TRANSACTIONS B
ciable solute. Of course, this is attributed to the fact that zone refining is an extremely time-consuming process, which is a consequence of a diffusion layer of solutes existing at the solid-liquid interface. In most cases, even at somewhat lower zone travel rates, poor purification of alloys results, and there is a serious difference between the actual and theoretical distribution coefficient. Bollen et al. t5l and Hoshino and Utsinomiya [6] have proposed equipment capable of considerably reducing the refining time. In their works, the tube containing the substance to be treated was subjected to a high-speed rotation which was periodically reversed. Burton et al. tT] have examined the incorporation of solute elements into single crystals of germanium grown from a melt through the development of a theory which takes into account the contribution of solute transport in the melt t
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