1. Field of the Invention
The invention relates to a relatively non-reactive container for metal production, and more particularly to a container for samarium production which has a replaceable inner liner of resistant metal.
2. Description of the Art
Although rare earth metals, defined for purposes herein as elements having the atomic numbers 39 and 57 through 71 (namely, yttrium and the lanthanide elements), have been known and studied for many years, commercial uses for these materials have generally been slow to arise. Samarium in particular, due possibly to its relative scarcity when compared to some other rare earths, has not found a wide range of industrial applications. Fairly recently, however, the development of samarium-containing high energy permanent magnet materials, such as the intermetallic samarium-cobalt compositions, has substantially increased the demand for samarium metal of high purity.
Rare earth metals, including samarium, have been prepared for many years by fused salt electrolysis techniques, in which a molten flux containing rare earth compounds (e.g., rare earth halides) is electrolyzed using appropriate inert electrodes. Disadvantages of the technique include the product contamination resulting from corrosion and degradation of electrodes and cell containers, and the difficulty of separating the product metal from molten flux. In addition, the process is not well adapted to high production rates for certain metals, including samarium.
An even older preparative method is that involving metallothermic reduction of rare earth halides, using an active metal. The method has been developed to the point that small quantites of very high purity rare earth metals can be prepared, if some fairly elaborate precautions are taken. Various methods utilize lithium, sodium, potassium and calcium as reducing agent, but results are generally unsatisfactory for those rare earth metals which have the lowest boiling points, including samarium, europium, ytterbium and thulium.
A. H. Daane, D. H. Dennison and F. H. Spedding, in their paper "The Preparation of Samarium and Ytterbium Metals", Journal of the American Chemical Society, Vol. 75, pages 2272-2273 (1953), report a method for volatile rare earth metal preparation which involves heating a mixture of rare earth oxide and lanthanum metal in a tantalum crucible, and condensing the vapors of rare earth metal onto a perforated tantalum crucible lid. This method was conducted under a vacuum and at a temperature of 1450.degree. C.
The noted method of Daane et al. can be advantageously utilized for the commercial production of samarium, since reasonably large quantities of product can be obtained if the crucible size is appropriately increased, and the requirements for equipment and other facilities are not stringent. Also, metal of satisfactory purity for most uses, e.g., the preparation of magnet alloys, is obtainable without elaborate precautions, due, at least in part, to the limited number of reactants and the limited number of materials which are allowed to contact the metal product.
Typically, a crucible for such preparation of samarium is fabricated from pure tantalum metal having sufficient thickness to impart the desired mechanical stability to the finished crucible. Although tantalum is one of the more resistant materials against attack by molten rare earth metals, some adhesion to the interior of the crucible is always observed at completion of a samarium preparation. It is not feasible to discard a crucible prior to its perforation or other mechanical breakdown, due to the very high cost of fabricated tantalum, so each crucible must be manually cleaned by chipping or grinding to remove the adhering material. This operation not only utilizes considerable expensive labor, but also tends to weaken the crucible, further shortening its useful life.
A need clearly exists for a means to extend the life-time of tantalum crucibles for samarium production and to reduce or eliminate the necessity for time-consuming cleaning procedures on used crucibles.
Accordingly, it is an object of the present invention to prevent significant attack on a tantalum crucible which is used for samarium metal production. It is a further object of the invention to provide a liner for a tantalum crucible which will protect the crucible from attack and which can be discarded after use.
Another object is to improve the economics of samarium metal production by eliminating or greatly reducing cleaning procedures on used tantalum crucibles.
A still further object is to permit the use of crucibles which are less resistant than tantalum toward attack by molten rare earth metals.
These and other objects will appear more clearly from consideration of the following disclosure.