Thermal analysis and X-ray diffraction of synthesis of powellite

Powellite (calcium molybdate) is an essential industrial product used as additive material to steel and for smelting of ferromolybdenum. Powellite often occurs as a secondary mineral and as pseudomorph after molybdenite in the oxidation zone of molybdenite deposits. The present work reports a thermal analysis study of synthesis of powellite by sintering of molybdite (molybdenium oxide) with calcite or calcium oxide using a derivatograph. The reaction products were identified microscopically and by using a Siemens Crystalloflex diffractometer. The DTA curve of sintering of molybdite with calcite shows the beginning of the reaction at 480°C with the formation of powellite. The intensive formation of powellite is represented by the medium and wide endothermic peak at 630°C. This is followed by a small endothermic peak at 790°C, representing the melting of unreacted molybdite. This is followed directly by large and sharp endothermic peak at 880°C, representing the dissociation of unreacted calcite. The wide and large endothermic peak at 1155°C represents the boiling of unreacted molybdite with appreciable vaporization. The DTA curve of sintering of molybdite with calcium oxide shows a medium and wide endothermic peak at 525°C representing the intensive formation of powellite and also the dehydration of calcium oxide. The small endothermic peak at 730°C represents the loss of carbon dioxide due to some carbonatization of calcium oxide with carbon dioxide from air. The medium endothermic at 790°C represents the melting of unreacted molybdite. The produced powellite is yellow in thin sections, has indistinct cleavage, crystallizes in the tetragonal system in the form of tabular crystals and is optically positive. This revised version was published online in July 2006 with corrections to the Cover Date.