APT Thermal Decomposition

Thermal decomposition. On heating APT to 300-800℃, ammonia and water are evolved. The decomposition becomes more complete as temperature and duration of heating are increased. The final decomposition product is determined by the decomposition the retention time in the furnace, and the reduction potential of the decomposition atmosphere.

If heating takes place with exclusion of air or in an inert gas atmosphere (e.g., nitrogen), part of the ammonia evolved is cracked and the hydrogen formed can cause a slight reduction of the hexavalent tungsten matrix(“autoreduction capability of APT”; partial formation of pentavalent tungsten). The degree of reduction and the formation of compounds is determined by the decomposition conditions. The product of this type of calcination is greenish blue to flashing dark blue in color and is called tungsten blue oxide (TBO).

If the decomposition is carried out under oxidizing conditions, a slight reduction can occur intermediately at low decomposition temperatures, but the final product is always tungsten trioxide (WO₃).

Besides temperature, time, and decomposition atmosphere, the amount of APT plays an important role since the mass of APT itself is responsible for producing a certain amount of ammonia and water as well as hydrogen and nitrogen. The powder layer, depending on its thickness and porosity (which increases during decomposition due to an increase in density), retains the gases released for some time. This fact explains the occasional contradictory literature on the decomposition of APT. For example, it is easy to understand that in a boat of comparable size 10g or 1000g will produce different atmospheres, especially inside the powder layers whose heights are also quite different.

 Under oxidizing conditions, the APT decomposition path is as follows:

 Between 10 and 100℃ only dehydration occurs and the product is crystallized, dehydrated APT:

(NH₄)₁₀[H₂W₁₂O₄₂] •4H₂O→(NH₄)₁₀[H₂W₁₂O₄₂]+4H₂O

In the temperature range 180-225℃, ammonia is released and the APT converts to amorphous ammonium metatungstate (AMT):
(NH₄)₁₀[H₂W₁₂O₄₂]→(NH₄)₆[H₂W₁₂O₄₀] •2H₂O+4NH₃

Between 230 and 325℃, ammonia as well as water vapor are evolved. The product is also amorphous:

(NH₄)₁₀[H₂W₁₂O₄₀] •2H₂O→(NH₄)₂[W₁₂O₃₇]+4NH₃+5H₂O

By increasing the temperature to 400-500℃, all residual ammonia and water is released, and the reaction product is tungsten trioxide:

(NH₄)₂[W₁₂O₃₇]→12WO₃+2NH₃+H₂O

Under a slightly reducing atmosphere between 220 and 325℃ amorphous, and above 325℃, crystallized, ammonium tungsten bronzes form: (NH₄)_χWO₃. Under stronger reducing conditions, conversion to lower tungsten oxides takes place.

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ammonium paratungstate characterization

Today, APT is the most important and almost exclusively used precursor for tungsten products. Only in tungsten melting metallurgy and for producing WC directly from ore concentrates are other starting materials used. Intermediates, such as tungsten trioxide, tungsten blue oxide, tungstic acid, and ammonium metatungstate can be derived from APT as shown in Fig. 5.15, either by partial or complete thermal decomposition or by chemical attack.
   Although a hexahydrate and a decahydrate exist, only the tetrahydrate (NH4)10[H2W12O42]•4H2O forms under industrial conditions, since the hexahydrate is unstable at temperatures exceeding 96℃, while the decahydrate crystallizes only from solutions at room temperature.
Characterization
PR: Its preparation will be treated in detail in chapter5.
PR: D: 4.61g•cm-1(X-ray); CR: monoclinic, p21/n; besides the technically important tetrahydrate, depending on drystallization conditions, a triclinic hexahydrate and an orthorhombic decahydrate can be formed.
A: It is today the most common, highly pure imtermediate for most tungsten products.
Ion associate complexes of isopolytungstates with secondary and tertiary alkyl amines play an important role in the technical solvent extraction process.
      APT is a white, crystallized powder. The average crystal size of commercial products ranges between 30 and 100 µm. The SEM image in Fig. 5.16 reveals mainly faceted crystals and only few intergrown or agglomerates. A typical grain size distribution of a crystallized APT is shown in Fig. 5.17. For special purposes, classified APT is also available. Specification of some physical properties and impurity concentrations as common today are given in Table 5.7. They reflect the high standard of the technical APT production.
Specially purified APT (by multiple-step liquid extraction of selected, very pure batches under clean-room conditions) for the production of 4N and 6N tungsten sputter targets shows a much lower impurity content.

Solubility.
The solubility in water is low: 20g wo3/l at 20℃; 60g wo3/l at 90℃.

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process steps of tungsten carbide powder

 A process for producing a tungsten carbide powder, comprising the steps of:

(a) mixing an aqueous ammonium tungstate solution with a carbon powder in a proportion to reduce and carburize ammonium tungstate to form a slurry,

(b) drying the slurry to prepare a precursor,

(c) subjecting the precursor to a reduction and carburization by heating to a temperature, at which a reduction and carburization proceeds, in a non-oxidizing gas atmosphere to form a reduced and carburized product,

(d) mixing the reduced and carburized product with a carbon powder in a proportion required to carburize a W2 C component and/or a W component in the reduced and carburized product into WC, and

(e) subjecting the reduced and carburized product mixed with the carbon powder to a carburization by heating to a temperature, at which a carburization proceeds, in a hydrogen atmosphere,

wherein an amount of the carbon (C) powder in step (a) with respect to the tungsten (W) component in ammonium tungstate by atomic ratio C/W is within a range of 3-4.

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Nickel-Silver tubular tungsten tarbide powder Introduction

1.Characters Nickel-Silver tubular tungsten tarbide powders are patent products, the deposited wear resistant layers mainly have the following characters: 1) Better Wear Resistance Copper and copper alloy is used to replace iron or nickel alloy of wear resistant composite material in the patent Nickel-Silver tubular tungsten carbide powders. Due to the lower hardfacing temperature, the products are able to avoid decomposition of WC-W2C in hardfacing, thus keeps excellent wear resistance. 2) Higher Bonding Strength with Base Metal Comparing to traditional iron tubular rods, copper alloy is more easy to wet WC-W2C powders than iron, thus improves bonding strength of WC-W2C powders and base metal, reduces fall of WC-W2C layer in use, and avoids failure of wear resistant parts. 3) Larger Particle Size of Tungsten Carbide Powders are more Wear Resistant Comparing to tungsten carbide rope, larger particle size of tungsten carbide powders can be used in patent Nickel-Silver tubular tungsten carbide powders, so as to have higher wear resistance. 2.Applications Patent Nickel-Silver tubular tungsten carbide powders are widely used for hardfacing especially for field repair.

Brief Introduction of Tungsten Carbide Powder

Tungsten carbide powder is the intermediate in the line from W powder to cemented carbides. It can be produced from different raw materials and by different processes. By far the biggest percentage is manufactured by the conventional method -carburization of tungsten powder- and covers the widest range of powder qualities in regard to average grain size (0.15 – 12 µm). All other methods in use yield in very fine or very coarse powder grades.

Spherical Cast Tungsten Carbide

Extremely hard and dense, CTOMS's Spherical cast tungsten carbide  powder, an eutectic mixture of WC and W2C phases, is one of the most wear resistant products on the market. Commonly blended with nickel based alloys, it is perfect for hardfacing applications by PTA, laser cladding and other processes for use in applications where superior wear protection is needed, such as mining, oil drilling and agriculture.

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High Density Tungsten Powder Application

  High Density Tungsten PowderApplications:

• Thin film surface applications of its polymer mixture is ideally suited for radiation protection.
   
• The large particles of  makes  the ideal heavy metal filler for injection-molding applications.
   
• The unique shape and resulting high flowability of these particles make  Ultra suitable for metal spray industry applications.