Thermal decomposition mechanism of ammonium phosphotungstate hydrate in air atmosphere

Production of W from scheelite mineral (CaWO4) by hydrometallurgical processes has grown up in the last decade. CaWO4 dissolves in HCl solutions containing H3PO4 as chelating agent and forms phosphotungstic acid (H-3(PW12O40), PhTA). Ammonium phoshpotungstate hydrate ((NH4)(3)[PW12O40NH2O, APhT) can be precipitated by addition of NH4Cl to solutions containing PhTA and WO3 can be produced by the thermal decomposition of APhT. W powder can be obtained by H-2 reduction of WO3. In W production, the thermal decomposition of APhT is one of the process steps. In this work, Thermogravimetric Analysis-Differential Thermal Analysis-Mass Spectrometry (TGA-DTA-MS), X-ray Powder Diffraction (XRD), Fourier Transform-Infrared Spectrometry (FT-IR), Scanning Electron Microscopy (SEM) and Thermo-Gas-Titration (TGT) analytical techniques were used to investigate the mechanism of thermal decomposition of APhT and the characterisation of its decomposition products. White coloured APhT losses its crystal water in the first decomposition step and turns its colour to light grey to form anhydrous ammonium phosphotungstate ((NH4)(3)(PW12O40), anhydrous APhT). Anhydrous APhT is stable up to 640 K temperature. Anhydrous APhT is transformed into tetragonal structured, light green coloured phosphotungstate (P(2)O(5)24WO(3), PhT) which is stable up to 1282 K temperature. PhT is then decomposed gradually into green coloured WO3 and P(2)O(5)18WO(3) mixture. The mixture formed does not vaporise until 1343 K temperature. After this temperature, sublimation takes place followed by melting at 1490 K temperature and loss of weight is observed due to evaporation. Thermal analysis up to 1573 K temperature showed that the samples are transformed into light yellow coloured, monoclinic structured WO3. (C) 2011 Elsevier Ltd. All rights reserved.