Structural Evolution of Ammonium Paratungstates during Thermal Decomposition

Mixed metal oxide systems (e.g. Mox(V,W)yO3-z) are employed for the partial oxidation of light alkenes. Ammonium paratungstate (APT) and ammonium heptamolybdate (AHM) are used as precursors for the production of WO3 and MoO3, respectively. The catalytic activity of these materials may depend on the treatment of the precursors. Therefore, studies of the decomposition process in order to identify and quantify tungsten oxide phases and their formation under various atmospheres, reveal correlation between catalytic activity and structural evolution of APT is very important. Here, we present results obtained from bulk structural studies on the thermal decomposition of APT in various reducing and oxidizing atmospheres. In this work the decomposition of APT is studied in situ by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Using these two complementary methods allows us to follow the evolution of the short-range and long-range structure of the phases formed during the decomposition of APT, to elucidate the evolution of the primary and secondary structure under different conditions. 

Experimental 

Ammonium paratungstate (APT), (NH4)10H2W12O42*4H2O (OSRAM) was used as purchased. Transmission X-ray absorption spectra were measured in situ with the sample pellet in a flow reactor (4 ml total volume) under a controlled reactant atmosphere. In situ XAS experiments were performed at the W LIII edge (10.204 keV) (Hamburger Synchrotron Radiation Laboratory, HASYLAB, beamline X1), using a Si (311) double crystal monochromator. Temperature programmed decomposition was carried out at temperatures between 300 and 773 K in atmospheres of pure helium, 5 % hydrogen in helium, 20 % oxygen in helium, 10 % propene in helium, and 10 % propene/10% oxygen in helium. For the in situ XAS measurements APT was mixed with boron nitride and pressed into 5 mm in diameter pellets. Analysis of the gas phase was carried out with a quadrupol mass spectrometer, QMS 200 (Pfeifer), with a time resolution of ~ 2 s. Further details about the experimental XAS set-up used can be found in .

Metallo-Organic Deposition of Tungsten Oxide Films from Alkylammonium Tungstate Solutions

Metallo-organic deposition of tungsten oxide films from alkylammonium tungstate solutions

This paper describes a simple and inexpensive metallo-organic deposition (MOD) process for forming electrochromic tungsten oxide (WO₃) films on glass. The thin films of WO₃ were made by air firing (500–700°C) films from xylene/2-propanol solutions of bis-(di-n-octylammonium) tetratungstate, [(n-C₈H₁₇)₂NH₂]₂[W₄O₁₃]. The process coats glass with undoped films ranging in colour from faint yellow to dark brown, and can be used to make gradients of these colours. The colour is determined by the firing parameters and results from residual carbon and tungsten suboxides in the film due to incomplete firing. Increased firing temperatures or longer firing times removes the carbon and produces films with higher crystallinity. Electrochemical doping with acid (H⁺) switches the colour gradient films to a uniformly blue colour.

Processing of Ammonium Paratungstate from Tungsten Ores

To obtain highly purified, lamp grade ammonium paratungstate crystals from any of several different tungsten ores, the ore is reduced to finely divided status and slurried in heated HCl solution to convert tungsten values to WO.HO. Recovered tungstic oxide is washed and dissolved in heated aqueous solution of sodium carbonate or sodium hydroxide with the pH maintained at about 8 to 8.5 to form soluble sodium tungstate. Sodium hydroxide is added to raise the pH to about 10.5 to 11.5, and magnesium chloride is added in amount sufficient to somewhat neutralize the solution. Sodium hydroxide is added to raise the pH to about 10.5 to 11.5 to precipitate as hydroxide the magnesium and additional metallic impurities. At least one of ammonium sulfide and thioacetamide is and the heated solution is acidified to a pH in the range from about 2 to 3 to precipitate any molybdenum as MoS. The tungstate solution is then contacted with an organic, water-immiscible ion exchange liquid in which the active ingredient is an amine salt to extract the tungsten values. Tungsten values are then stripped from the ion exchange liquid with ammonium hydroxide to form ammonium tungstate solution, which in turn is separated. From the ammonium tungstate solution is crystallized highly purified ammonium paratungstate. The process is adaptable to continuous type operation.

Process for Producing Ammonium Paratungstate

Ammonium paratungstate (or APT) is a white crystalline salt of ammonium and tungsten, with the chemical formula (NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O.

Ammonium paratungstate is produced by separating tungsten from its ore. Once the ammonium paratungstate is prepared, it is heated to its decomposition temperature, 600 °C. Left over is WO₃, tungsten(VI) oxide. From there, the oxide is heated in an atmosphere of hydrogen, reducing the tungsten to elemental powder, leaving behind water vapor. From there, the tungsten powder can be fused into any number of things, from wire to bars to other shapes.

A process is disclosed for producing ammonium paratungstate which involves adding hexamethylenetetramine to a first solution of ammonium tungstate, adjusting the pH to about 2 with an acid to form a precipitate which contains the major portion of the tungsten and the hexamethylenetetramine and separating the precipitate from the resulting mother liquor. The tungsten hexamethylenetetramine precipitate is then dissolved in aqueous ammonia to form a second ammonium tungstate solution which is then heated at from about 90℃ to about 100℃ to form a precipitate essentially all of which is ammonium paratungstate and a mother liquor which contains essentially all of the hexamethylenetetramine. The ammonium paratungstate precipitate is then separated from the mother liquor.

Calcium Tungstate Crystals Aim to Light up Dark Matter

German scientists hunting dark matter are set to produce half a tonne of high-purity calcium tungstate for their detectors, one 1kg crystal at a time. The CRESST-II experiment based in Gran Sasso, Italy is currently seeking this enigmatic substance, thought to explain the universe’s structure, with 10kg of calcium tungstate (CaWO4). Now Andreas Erb and Jean-Côme Lanfranchi are preparing crystals for its larger successor EURECA, which will begin operation in the French Alps in 5–10 years.

Gravitational effects suggest as-yet-unobserved dark matter in the universe outnumbers more familiar atomic matter four to one. Erb, Lanfranchi and their colleagues are hunting leading theoretical candidates, Weakly Interacting Massive Particles (WIMPs). That name reflects their size – up to a lead atom’s mass – and the limited interaction with atomic matter that makes them hard to find, or ‘dark’. ‘They have to interact weakly to agree with the matter needed,’ says Richard Gaitskell from Brown University in the US, who isn’t involved in the calcium tungstate experiments.

Erb explains that the detectors should be able to pick up dark matter particles when they hit atomic nuclei in the crystals. ‘A higher sample mass gives a higher probability of such events.’ But distinguishing the miniscule amount of heat WIMP–nucleus collisions would produce requires detectors cooled to 10mK and shielded from ambient radioactivity.

CRESST/EURECA is the only team hunting dark matter with calcium tungstate, which has two advantages. Firstly, its different atoms cover a range of possible WIMP masses. ‘No matter the mass, you always have a nucleus with a high probability of interacting,’ Erb says. Second, it would also emit light when a WIMP hits it and monitoring the different signals will help the scientists eliminate background noise. 

Having initially purchased crystals, this need for extreme sensitivity drove Erb and Lanfranchi to produce their own. ‘They weren’t pure enough for the background we want,’ Erb recalls. To avoid oxidation at calcium tungstate’s 1600°C melting temperature, the crucibles are made from rhodium, with their 12cm diameter vessel costing €120,000 (£97,300). Erb says that if they can grow two or three 1kg crystals per week then they will have the required amount for EURECA in about five years.

New Report Sheds Light on the Sodium Tungstate Sales Global Market

The report provides a basic overview of the Sodium Tungstate industry including definitions, classifications, applications and industry chain structure.
Development policies and plans are discussed as well as manufacturing processes and cost structures are also analyzed. This report also states import/export consumption, supply and demand figures, cost, price, revenue and gross margins.
The report then analyzes the global Sodium Tungstate market size (volume and value), and the sales segment market is also discussed by product type, application and region.
The major Sodium Tungstate market (including USA, Europe, China, Japan,.) is analyzed, data including: market size, import and export, sale segment market by product type and application. Then we forecast the 2016-2021 market size of Sodium Tungstate.
Finally the marketing, feasibility of new investment projects are assessed and overall research conclusions offered.

With 183 tables and figures the report provides key statistics on the state of the industry and is a valuable source of guidance and direction for companies and individuals interested in the market.