Characterization of Tungsten Blue Oxide

Characterization. Industrially produced TBO is characterized chemically by the residual ammonia content and oxygen index, leaving open the composition in regard to the different compounds and the amount of amorphous species. These determinations would be much too tedious and expensive for routine purposes. As long as APT quality and calcinaiton conditions are kept constant, the composition will be reproducible. Consequently, in many companies it is preferred not to buy TBO but APT, and to perform the blueing in-house.

Physical characterization of TBO includes particle size and distribution measure-ment by laser diffraction (macroporsity) as well as specific surface area measurement (microporsity). Particle size measurement by FSSS (Fisher Sub-sieve Sizer), as also sometimes used, is misleading, because of the porosity of the TBO particles. An empirical relationship between FSSS and particle size measured by laser scattering can, however, be detected if the microporosity of the samples is uniform (constant blueing conditions).

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Tungsten Blue Oxide Physical properties

Physical properties. The physical properties of the TBO, such as particle size, size distribution, and internal porosity, are of importance for the subsequent hydrogen reduction process. Size, size distribution, and shape of the particles are related to the starting APT as a consequence of the crystallization process and do not alter significantly during the decomposition process, while the internal porosity is a result of the blueing process. The degree of porosity is influenced by the calcinations parameters (time, temperature). The specific surface area in commercial TBOs can vary between 1 and 15m²/g.

Both macromorphology and microporosity of the TBO particles determine the permeability and thus the diffusion resistance of a TBO powder layer, which is a determining factor for the material exchange (H₂→H₂O) during reduction.

Homogeneity of Blue Tungsten Oxide

Homogeneity. The product of a rotary furnace is more homogeneous in regard to differences between individual particles, since the powder is constantly being mixed by the turning motion. The TBO particles are composed of different compounds but are similar in composition, while pusher-furnace-derived TBO particles can also differ in composition, depending on their position in the powder layer.

Tungsten Blue Oxide--Chemical Composition

Chemical Composition. TBO is not a defined chemical compound but is a mixture of different constituents, such as ammonium, hydrogen and hydronium tungsten bronze phases, tungsten trioxide, tungsten-β-oxide(WO_2.9 or W₂₀O₅₈), and tungsten-γ-oxide(W_2.72 or W₁₈O₄₉). Under more reductions, even traces of WO2 andβ-tungsten can be present. The relative amounts of the various compounds in the TBO depend on the calcinations parameters.

•temperature,

•heating time,

•composition and pressure of atmosphere,

•mass of APT flow with time,

•gas flow,

•layer height in the boat (pusher furnace),

•slope and rotation rate (rotary furnace).

The oxygen index (molar ratio O/W) is commonly used to characterize the degree of reduction of TBO. However, since most TBOs also contain ammonia and water in addition to W and O, a more complete description is given by x(NH₃)•y(H₂O)•Wo_n.

A series of analyzed industrial samples gave the following ranges for the coefficients x and y and the index n:x=0.02-0.09, y=0.02-0.14, and n=2.82-2.99.

Qualitative and quantitative X-ray analyses of the same samples revealed quite a large scatter in composition: tungsten bronzes, 0-45%; WO₃, 0-45%; WO_2.9, 5-20%; WO_2.72, 0-25%, and amorphous, 30-55%.

Amorphous species form by dehydration which, on further heating, convert into crystalline binary tungsten oxide as well as tungsten bronzes. The conversion from amorphous to crystalline is a slow process. Therefore, if the heating period is short, as in rotary furnaces, the time available for overall crystallization is insufficient. This is why rotary-furnaces-derived TBO can be high in amorphous oxide when compared to TBO from pushers.

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Tungsten Blue Oxide Industrial Production

 TBO is formed by calcinations of APT under slightly reducing conditions. The conversion can be performed either in multitube push-type furnaces or in rotary kilns. Various atmospheres are used. Generally, in push-type furnaces a flow of hydrogen or hydrogen-nitrogen mixtures is applied. In rotary furnaces one usually takes advantage of the reducing capacity of the gases evolved during the decomposition (H, H₂, NH₃), leading to the desired formation of reduced tungsten species.

Temperature may vary between 400 and 900℃. Literature values can be misleading, because some are related to the real temperature of the powder layer while others are furnace temperature measured at the wall of the heating compartment or tube. These temperatures can differ considerably, due to the overall endothermic behavior of the APT→TBO decomposition reaction. The exposure time in a rotary kiln is usually much shorter than in the pusher, and the decomposition temperature is therefore higher for obtaining a similar degree of thermal decomposition.

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Tungsten Trioxide

Tungsten Trioxide

In a technical scale, WO3 is almost exclusively produced by calcination of APT under oxidizing conditions (in air). Usual equipment consists of rotary furnaces operating at 500-700℃. Sufficient air supply must be provided to suppress any reducing reaction by the partly cracked ammonia. The ammonia evolved can be recovered by absorption in cold water and concentrated by subsequent distillation.

The WO3 particles are pseudomorphous to APT. This means that the shape and size of the particles are the same as the APT crystals, but they are built of very small WO3 grains (Fig. 5.18) forming a large oxide sponge with a high degree of microporosity (specific surface area). Their grain size and agglomerate structure depend on the calcinations condition (heating rate, temperature, and time). Higher temperature and low heating rates result in coarser grains. Above 700℃, coarse, faceted WO3 single crystals form due to enhanced chemical vapor transport of the oxide.

Low-temperature calcined WO3 (approximately 500-550℃) is highly reactive an dissolves easily in water, which is not the case for higher-temperature calcined WO3.

For special purposed, especially in the case where a high specific surface area is necessary and APT pseudomorphology is undesirable, WO3 can be produced also by calcinations of tungstic acid.

As precursor for the W and WC powder production, WO3 lost its importance mainly to tungsten blue oxide. WO3 is also used as a yellow pigment.

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Tungstic Acid

Tungstic Acid

In order to make use of the high APT purity in modern processing, tungstic acid is produced today by treating an aqueous APT crystal slurry with hydrochloric acid. In this way APT is decomposed and H2WO4 is precipitated. After filtration, it must be thoroughly washed to remove ammonium chloride. The earlier process of precipitation from sodium tungstate solutions by addition of acids no longer has industrial importance.

Tungstic acid has a high active surface. The former, most important intermediate today is only used in smaller quantities for special purposes:

1.       production of ultrafine tungsten and tungsten carbide powders in order to circumvent the sometimes disturbing pseudomorphology of APT-derived products, and

2.       for tungsten chemicals.

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