A modified fluoride reduction route for the preparation of thorium metal

Thorium oxide is one of the most stable oxides known in nature. Commercially, thorium metal is produced by calciothermic reduction of its oxide. The process leads to the production of thorium powders, which if not handled carefully, can often lead to oxygen contamination in the final metal. In this paper, we report an alternative possibility of producing bulk thorium metal by calciothermic reduction of its fluorides. Fluoride being less hygroscopic and having higher boiling and melting point is preferred over the chloride intermediate. The process involves mixing of as prepared ThF₄ and 20% excess calcium in a crucible in an argon environment and heating the charge to beyond the melting point of thorium. The heat of reaction during calcium reduction of thorium fluoride is not sufficient to increase the temperature of the reacted mass to above the melting points of the products. This is unlike the common metallothermic reduction process, viz. thermit process, in which both the products, the metal and the slag, melt due to the intense heat of the reaction and separate from each other due to density difference. The slag-metal separation in this process has to be ensured by heating the charge to above the melting point of the higher melting substance, otherwise a mixture of CaF₂ and thorium metal would result. The reaction sequence of the process was studied by a visual observation. Besides, liquid thorium being very reactive, a proper choice of crucible material becomes important. In this paper, we describe our experiences on the interaction of liquid thorium with the crucible materials. In this work, we used two kind of crucibles: standard molybdenum crucibles and custom made fully dense yttria crucibles. Yttria crucibles of 25 mm dia and 85 mm length were specially made by Glass and Advanced Materials Division, Materials Group. An alternative route for production of metallic thorium is reported. The advantage in the process is that it can make thorium in bulk form which greatly reduces the issue of handling powders. However, as the process involves the melting of thorium, selection of suitable crucible becomes a key to its success. Yttria crucible appears to be a good choice