In the step of the leaching of wet ammonium paratungstate, the wet ammonium paratungstate is reacted directly with nitric acid to produce a dilute solution of ammonium metatungstate. In comparison with the prior art, the calcining process is prevented, so it is no longer necessary to use the rotary furnace equipments, thus the fixed investment is reduced, electric power needed is reduced by 120 kWh per ton of product, actual yield can be increased by 3%, and production cost is lowered. 

In the step of the spray-drying of the concentrated solution of ammonium metatungstate, the drying conditions (for example: inlet air temperature, outlet air temperature, and feeding speed) are controlled, such that the concentrated solution of ammonium metatungstate is rapidly dried into a powder with a high speed and a high efficiency, as a result, the ammonium metatungstate product has good flowability and good solubility as well as high purity. The ammonium metatungstate product produced according to the present invention has overcome the shortcoming of low solubility of the ammonium metatungstate produced by the prior art. 



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Below are the further descriptions of preparing steps:

A.The leaching of wet ammonium paratungstate: Pure water is added to a reactor and preheated to boiling; wet ammonium paratungstate [5(NH₄)₂O12WO₃₅H₂O] is added slowly to the pure water at a speed of 150 to 200 Kg/h while stirring; when adding the wet ammonium paratungstate, a 1:3 dilute nitric acid (HNO₃) is added slowly to control the solution at pH 3 to 5; the above process is stopped when the WO₃ content of the solution reaches 150 g/l; After boiling and being stirred for half an hour, the materials are allowed to pass through a suction filter for natural filtration, in order to produce a dilute solution of ammonium metatungstate. 

B.The conversion of dilute solution of ammonium metatungstate to concentrated solution of ammonium metatungstate: the dilute solution of ammonium metatungstate from step A is standing aged for 4 hours, followed by a primarily natural filtration to obtain a filtrate 1; the above filtrate 1 is pumped into a reactor in multiple times for heating to concentrate it, each time the filtrate 1 is pumped, a dilute ammonia water (NH₃H₂O) of 1:5 is used to adjust the solution at pH 3 to 4; when the density of the solution reaches 2, the pumping of the filtrate 1 is stopped, following 1 hour of cooling, the materials are allowed to pass through a suction filter for a secondarily natural filtration to obtain a filtrate 2; the above filtrate 2 is pumped into a settling tank, standing aged for 8 hours, then the materials are allowed to pass through a suction filter for a tertiarily natural filtration to obtain a concentrated solution of ammonium metatungstate. 

C.The spray-drying of the concentrated solution of ammonium metatungstate: The concentrated solution of ammonium metatungstate from step B is dried into a powdery product of ammonium metatungstate by the use of homemade advanced high-speed centrifugal spray-drying equipment or a spray atomizing dryer. The conditions for spray-drying are controlled at: inlet air temperature 220 to 280° C., outlet air temperature 110 to 160° C., and feeding speed 300 to 400 kg/h. 


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Ammonium metatungstate is an important tungsten-containing compound, mainly used as catalyst in petroleum cracking, organic synthesis, nitration, and so on. With the rapid development of petroleum
refining, petrochemical engineering and other industries, the ammonium metatungstate usage is increasing very fast. In the prior art, ammonium paratungstate is used as a raw material, and a crystallization process is utilized for the preparation of ammonium metatungstate; this process is expensive, costly and energy-consuming, and the obtained product is unstable in quality, particularly the solubility is poor.

A process for preparing ammonium metatungstate using ammonium paratungstate as raw material comprising: (A) leaching wet ammonium paratungstate with nitric acid to obtain a dilute solution of ammonium metatungstate; (B) concentrating the dilute solution of ammonium metatungstate to be a concentrated solution of ammonium metatungstate; and (C) spray-drying the concentrated solution of ammonium metatungstate to obtain powder of ammonium metatungstate.


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Spark plasma sintering method (referred SPS) using special pulsed DC current producing discharge plasma between tungsten oxide particle, so that the particle surface activation to give a good performance WC in the materials synthesis and rapid low-temperature sintering has a unique advantage. Besides it has a very significant technology condition advantages. For example: uniform heating, low sintering temperature, sintering time is short, high production efficiency, product organization even small and easy to operate. SPS is currently in thermoelectric materials, magnetic materials, functionally graded materials, composite functional materials and nano-materials and so achieved good results. Tungsten oxide using SPS to produce tungsten carbide the producing process as follows:
1. Producing the carbon content is 12% -17% tungsten oxides and carbon black mixture, while using thermal analyzer the reaction mixture of tungsten oxide and carbon black.
2. ball-milling in drum mill with ethanol for wet grinding 48 hours, drying in 100 ℃, sieving 149um sieve, charged in a graphite mold.
3. Sintered in SPS-3.20-MV type discharge plasma sintering apparatus.
4. Using optical microscopy and scanning electron microscopy analysis samples.

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Spark plasma sintering method (referred SPS), the technology is in between powder particles directly into the heating pulse current sintering. At the beginning the SPS technology has low productivity problem which has not been widely used. In recent years, with further research SPS as new technology for preparing material has attracted attention.
Tungsten carbide (WC) due to its high hardness, high flexural strength and excellent fracture toughness in many areas is widely using in many area. But the key to the producing high-performance WC is the grain size of the raw material powder. Traditional WC production process is: ammonium paratungstate →tungsten oxide → W → WC → ingredients → sintered block. But this process step is complicated and not easy to control the particle size. And using SPS producing WC is by pulsed DC current to produce discharge plasma between the tungsten oxide powder particles, so that the particle surface activation to give a good performance WC in the materials synthesis and rapid low-temperature sintering.

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After analysis using blue tungsten oxide to produce tungsten powder by cyclic oxidation found the tungsten powder particle size is small, and good performance. From the experimental data show that after the first reduction tungsten powder‘s specific surface area is 0.119m2 / g which is 1.95times to blue tungsten oxide’s. From this we can see that tungsten powder particle size is smaller than the blue tungsten oxide and distribution is more concentrated, but there is a certain amount of coarse particles of tungsten powder. In the second reduction which is using obtained tungsten powder in the air restore to tungsten trioxide then using hydrogen to make the second reduction. And the tungsten powder which obtained from the second reduction is finer. Comparing tungsten powder of the first reduction and the second reduction find in the second reduction there has more fine tungsten powder than first reduction, its particle size distribution is more uniform, but there are a small number of coarse particles of tungsten powder. The second reduction of tungsten powder specific surface area is 0.180m2 / g which is the first reduction of tungsten powder of 1.51 times and its diameter mainly in the 3 ~ 10um. It can be found using cyclic oxidation method obtained tungsten powder particles finer than directly by using hydrogen reduction, besides the distribution is more concentrated.

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Fine tungsten powder is an important raw material for producing tungsten carbide powder, to improve the performance of carbide tungsten is necessary to reduce or eliminate the coarse tungsten powder particles. Otherwise it will cause tungsten carbide cracks, fracture and other phenomena in the course. After studies the process of using blue tungsten oxide as raw material to produce fine tungsten powder research found that by using exceptionally pure ammonium paratungstate calcinate in ammonia to get blue tungsten and then go through the cyclic oxidation producing fine tungsten powder, and the tungsten powder properties is better which is less coarse. Producing process as follows:
1. Slow heating exceptionally pure ammonium paratungstate which is in ammonia to 600 ℃, when the temperature reach 600℃for 10min heat preservation to produce  blue tungsten oxide.
2. To obtain tungsten powder by restoring blue tungsten oxide in hydrogen.
3. The obtained tungsten powder slowly heating in air to 500 ℃, for the first time oxidation. When the temperature reaches 500 ℃ for 10min heating preservation to obtain tungsten trioxide.
4. The obtained tungsten trioxide in the hydrogen condition to process second reduction.

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Fine tungsten powder is widely used in various fields, but the manufacturing process conditions and material requirements it is very rigorous. Using blue tungsten oxide as raw materials to produce ultrafine tungsten powder also has certain material quality requirements which are including blue tungsten impurity content, particle size and volatiles requirements.
Firstly, blue tungsten impurity content requirement, purity and impurities of blue tungsten oxide will influence the tungsten powder grain size and quality directly. Selecting high-quality blue tungsten is the important factor for producing good performance and fine tungsten powder which is an important part in producing fine tungsten oxide process.
Since impurities can be a carrier of oxygen which will delay the residence time of oxygen in the powder layer, to promote the chemical vapor transport can lead to grow tungsten powder particles. The influence of particle size of blue tungsten oxide is when the fee's size from 7.90um increase to 12.94um, tungsten powder particle size reduced from 1.17um to 0.99um, which shows coarse particles of blue tungsten oxide is more advantageous for the preparation of fine tungsten powder. Finally, blue tungsten oxide volatiles requirements, blue tungsten oxide has different residual volatile matter such as NH3 and H2O which will make hydrogen humidity increases in the furnace and it is bad for the preparation of fine tungsten powder.

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The process of blue tungsten oxide restore to tungsten powder that the primary phase composition is β- tungsten oxide (WO2.9), γ- tungsten oxide (WO2.72), tungsten dioxide (WO2) and tungsten powder in this process. During reduction process which is from blue tungsten to γ-tungsten oxide, γ- tungsten oxide particle surface becomes rougher, rod-shaped and needle-like shape and irregular arrangement. This form is conduct to facilitate the reaction and to obtain fine tungsten powder. With the further reduction the γ- tungsten oxide transformed into tungsten oxide, and tungsten dioxide particles become denser, shaped like tungsten powder particle morphology. In this process, if works can control the process conditions well, may be can preparing fine or ultrafine tungsten powder by γ- tungsten oxide. In the final stage reduction process of tungsten powder, tungsten dioxide reduction to tungsten powders and the morphology of tungsten powder which is not very different with tungsten dioxide. Where is a tungsten dioxide grains turn into a tungsten powder grains. Therefore, the morphology changing is mainly prior to tungsten dioxide generated. In order to produce good performance and fine tungsten powder well control the technology conditions before generated tungsten dioxide is very important. 
 

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Weld strength is measured according to the regulations in JB / T 9864.2 ------ 1999.

Check and measurement of tungsten contacts is available by using universal measuring tool.

Tungsten contacts appearance and weld can be checked with a magnifying glass or by visual inspection.

Determination of the number of sections of crystal grains according to GB / T 4197 requirements.

The roughness of contact surface of tungsten contacts can be measured with surface roughness measuring instrument.

Purity of tungsten sheet is measuring according to GB / T 4324.1-4324.30 requirements.

Hardness measurements according to GB / T 230 rules: If the Vickers hardness measurement method is adopted, the load should be 4.904N, according to GB / T 4340 requirements. The conversion of measured values can refer to the relevant tables; the measurement results should be consistent with the provisions of the HRA.

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