Thorium Energy Conference 2015 (ThEC15) October 12-15, 2015, Mumbai, India FLUORINATION OF THORIUM OXIDE BY AMMONIUM BIFLUORIDE AND ITS REDUCTION TO METAL Abhishek Mukherjee a and Alok Awasthi b* a Fusion Reactor Materials Section, Bhabha Atomic Research Centre, Trombay, Mumbai–400085,India b Materials Processing Division, Bhabha Atomic Research Centre, Trombay, Mumbai–400085,India INTRODUCTION 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. Fluorination of thorium oxides; in general can be done using fluorine gas (F 2 ), hydrogen fluoride gas (HF), aqueous hydrofluoric acid (HF), ammonium fluoride (NH 4 F) or ammonium hydrogen fluoride (NH 4 HF 2 ). Of these, fluorine and hydrogen fluoride are corrosive and poisonous gases at the operating temperatures of 700°C and thus are difficult to handle. Aqueous hydrofluoric acid is again highly corrosive. NH 4 F is highly hygroscopic. Contact of water vapour and thorium fluoride has to be avoided, as there remains a possibility of oxygen contamination due to pyrohydrolysis of the fluoride. Hence among the commonly used fluorinating agent, the use of ammonium bifluoride is seen to be the most effective. There is thus no evidence to suggest solubility of oxygen in ThF 4 and the absence of thorium oxyfluorides in the product can be considered sufficient to term ThF 4 as oxygen-free. If oxygen purity of the starting fluoride could be ensured besides maintaining the inert atmosphere, then fluoride reduction should result in a purer thorium. After oxygen free ThF 4 is prepared, it is mixed with 20% excess calcium in a crucible in an argon environment and the charge is heated 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. Thus 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 CaF 2 and thorium metal would result. The reaction sequence of the process was studied by a visual observation [1], which is described in this work. As liquid thorium is 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. RESULTS AND DISCUSSIONS Fluorination of thoria begins right at room temperature with the formation of an unknown phase which could not be indexed using the standard JCPDS data. The unknown phase converts to (NH 4 ) 4 ThF 8 after few days and the ThO 2 peak completely disappears. (NH 4 ) 4 ThF 8 heated upto 500 °C eventually transforms to ThF 4 . The thermal curves on heating a freshly mixed ThO 2 with sufficiently excess NH 4 HF 2 showed at least six endothermic peaks and one exo peak. These peaks corresponded to the vaporization and decomposition of excess NH 4 HF 2 and the decomposition of ammonium thorium fluoride complexes. The XRD of the end products for charge mixture containing 100% excess NH 4 HF 2 was indexed completely with pure ThF 4 . Charge containing 10% excess NH 4 HF 2 formed ThOF 2 . A thorium oxy fluoride formation is probably caused due to the participation of unreacted ThO 2 . Therefore, for obtaining oxygen-free ThF 4 , it is best to ensure the full conversion of ThO 2 to (NH 4 ) 4 ThF 8 and complete disappearance of ThO 2 at either room temperature or slightly higher temperature when the evaporation of NH 4 HF 2 / NH 4 F is negligible [1]. Using this technique, oxygen free thorium fluoride could be made in a scale of 150 g and no problems of homogeneity in charges were encountered.