Thermal analysis and X-ray diffraction of synthesis of scheelite

Scheelite (calcium tungstate) is the product of one of the processing methods of wolframite by its roasting with calcium oxide or limestone or its fusion with calcium chloride, followed by acid processing of calcium tungstate with the formation of tungstic acid. Scheelite occurs in contact metamorphic deposits, hydrothermal veins and pegmatites. The present work illustrates a thermal analysis study of synthesis of scheelite by sintering of wolframite with calcite and sintering of tungsten 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 wolframite with calcite shows the beginning of the reaction at 560 degreesC with the formation of scheelite. The intensive formation of scheelite is represented by the medium and wide endothermic peak at 740 degreesC. This is followed directly by a large and sharp endothermic peak at 860 degreesC, representing the dissociation of unreacted calcite. The DTA curve of tungsten trioxide shows three thermal effects. The sharp exothermic peak at 320 degreesC represents the oxidation of tungsten oxide content of lower valency. The endothermic peaks at 750 and 1090 degreesC are related to polymorphic changes of tungsten trioxide. The beginning of its sublimation is observed at temperature higher than 800 degreesC. The DTA curves of sintering of tungsten trioxide with calcite or calcium oxide indicate that the intensive formation of scheelite takes place by endothermic reactions at 660 and 545 degreesC respectively. The medium and small endothermic peaks at 520 and 730 degreesC on the DTA curve of tungsten trioxide with calcium oxide represent the dehydration of calcium oxide and the loss of carbon dioxide due to some carbonatization of calcium oxide with carbon dioxide from air, respectively. The produced scheelite is colorless in thin sections, has distinct cleavage (101), crystallizes in the tetragonal system in the form of tabular crystals and is optically positive.