Graduate Thesis Or Dissertation
 

Preparation of yttrium and rare-earth metals by metallothermic reduction

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  • An investigation was made of various methods for the preparation of yttrium or rare-earth metals by metallothermic reduction. Particular emphasis was placed on the use of high-purity chloride salts as intermediates. Methods were stressed which appeared adaptable to production of high-purity, ductile metal on a plant scale. Reactions were chosen for study on the basis of thermodynamic considerations and also from a standpoint of chemical and physical properties of reactants and byproducts. Consideration was given to construction materials and their contribution of impurities to the metal products. Yttrium or rare-earth chlorides, fluorides, bromides or iodides were investigated as intermediates for metal production. Emphasis was placed on the development of methods for preparing these halides and for refining them to a high state of purity. Rare-earth chlorides were prepared either by slow dehydration of hydrated salts while in contact with ammonium chloride, or by a more rapid technique involving the direct chlorination of mixtures of rare-earth oxides and carbon. In either case, chloride salts were purified by vacuum distillation or by filtration of molten salts. Lithium, sodium, potassium and calcium were tested as reductants for anhydrous halides. These metals were compared as reductants on the basis of operating characteristics, purity of metal product, and yield of product. Yttrium and rare-earth metals prepared in this study were consolidated, usually by inert-atmosphere arc melting, and the consolidated metals were evaluated on the basis of chemical analyses and mechanical workability. Of the many systems studied, best results were obtained by the lithium reduction of rare-earth chlorides in Inconel or molybdenum apparatus. Metal made by this procedure was freed of byproduct lithium chloride by vacuum distillation, and the rare-earth metal, which appeared as a sponge, was consolidated by arc-melting. Yttrium was prepared by this technique on a scale of 15 pounds of metal per batch. Metal recoveries were greater than 95 percent and the major impurity, oxygen, was usually present in amounts less than 500 ppm. Samarium and europium cannot be prepared by reduction of their halide salts. These metals were produced by reduction of their oxides in a high vacuum employing lanthanum, cerium, or zirconium as reductants. Yields were usually over 90 percent during tests conducted at 900 to 1450°C.
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