Non-isothermal kinetic (k, A and ΔE) and thermodynamic (ΔH, C_p and ΔS) parameters of ammonium paratungstate decomposition to the onset of formation of WO₃, were determined by analysing thermogravimetry (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) curves recorded at various heating rates (1–30°C min⁻¹). Thermal events encountered throughout the decomposition course were due to three endothermic (at 120 ° C (I), 195 ° C (II) and 359 ° C (IV)) and two exothermic (at 265 ° C (III) and 386 °C (V)) processes. The events I–IV were accompanied by weight losses resulting from the elimination of volatile components [(NH₄)₂O, NH₃ and H₂O] leading to the formation of intermediate solid products (metatungstate and polytungstate), whereas event V was due to the crystallization of WO₃ in the immediate vicinity of its formation (at 370 ° C).

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Tungsten metal products are made using a chemically refined intermediate ammonium paratungstate (APT) product. APT is produced by processing tungsten concentrates containing scheelite (calcium tungstate) or wolframite (iron/manganese tungstate). The processing steps usually include several impurity removal steps to make an APT product of the desired purity.

In the process for recovering tungsten from a tungsten concentrate wherein the tungsten content of said concentrate is separated therefrom, purified and converted into solid ammonium paratungstate (APT) by crystallization from ammoniacal solution, of which a portion is off-grade material, the improvement for recovering said off-grade APT with a minimum number of processing cycles which comprises mixing said off-grade APT with an ammonia solution containing about 2.5% to about 12%, by weight, of NH3 and autoclaving said mixture at a temperature above the boiling point thereof for a time sufficient to dissolve substantially all of said APT and recrystallizing solid APT from the resulting solution to yield a solid APT product.

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The thermal decomposition of ammonium paratungstate hydrate was investigated in dynamic air and inert gas (nitrogen and argon) atmospheres by simultaneous thermo-gravimetric analysis (TGA), differential thermal analysis (DTA) and mass spectroscopy (MS). The intermediate and end products obtained during thermal decomposition of ammonium paratungstate hydrate were characterized by X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR).

It was found that the kind of gas was not effective on the thermal decomposition mechanism of ammonium paratungstate hydrate below 665K. In the first decomposition step, anhydrous ammonium paratungstate was obtained by dehydration. In the second decomposition step, only deammoniation occurred and ammonium hydrogen tungstate was obtained. During the third decomposition step, dehydration, deammoniation and reduction of some tungsten atoms to lower valences were observed. At this stage, first, ammonium hydrogen tungsten oxide bronze and later ammonium tungsten oxide bronze were formed by two serial reactions. In the fourth step which occurs after 665 K, ammonium tungsten oxide bronze formed stoichiometric tungsten oxide in air atmosphere and tungsten suboxide in inert gas atmosphere.

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Ammonium metatungstate(APT) solutions are prepared from relatively insoluble APT by (a) forming an aqs. suspension containing up to 22% wt APT, calculated on the content of WO3; (b) maintaining the pH of the aqs. suspension at 1-75-3.5 by the addition of hydrogen ions, and (c) maintaining the temperature of the suspension at 50-120 degrees C until the APT has been dissolved and converted into metatungstate. The process enables ammonium metatungstate solutions to be more cheaply prepared, as ammonium paratungstate is cheaper than ammonium metatungstate. The process is convenient and efficient, and can be carried out using the commercially available refracting form of ammonium paratungstate. - Concentrated solutions of the metatungstate can be easily prepared.

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