2020年10月28日星期三

How to Prepare Ultrafine Tungsten Powder by Ultrasonic Spray Drying

 With the development of high and new technology, the requirement of pure tungsten and its alloy material is higher and higher.

There are many methods to prepare tungsten powder, and spray drying is one of the most important methods. The spray drying method or chemical synthesis method, it is a method of physical and chemical solution atomization obtain ultrafine particles through physical means of combining the process, including the original solution was prepared with solution blending, mixing, spray drying and subsequent processing. 

In recent years, the influence of nozzle structure, spray drying speed and temperature on particle size has attracted extensive attention. It is found that the concentration of precursor solution plays an important role in the morphology and particle size of ultrafine tungsten powder during ultrasonic spray drying. In order to prove this theory, some scholars have used 2 different concentrations of ammonium tungstate solution to spray drying respectively. The concentration of ammonium tungstate in solution 1 was 25% (mass ratio), and the concentration of solution 2 was 10%. The air pressure was 3 MPa and the drying temperature was 130 ~ 200 °C. Two kinds of different precursor powders were obtained, and the ultrafine tungsten powder was prepared by hydrogen reduction method.


After comparison test, it is concluded that when the concentration is 10%, the ammonium tungstate solution is low and the crystal grows slowly, so that the crystal powder has been dried when the crystal is not sufficiently crystallized, and the ultrafine tungsten powder precursor powder is obtained. When the concentration is 25%, the concentration of ammonium tungstate solution is high, the nucleation rate is high, so the crystal growth rate is very fast, and the crystallization process has been crystallized during drying, so the precursor powder of crystalline tungsten flake powder is obtained, and the particle size is coarser.

In conclusion, the concentration of ammonium acid solution is the main factor of the morphology and particle size of ultrafine tungsten powder, and the finer spherical tungsten powder precursor can be obtained in the range of 5% - 10%, which can be easily reduced to prepare ultrafine tungsten powder.

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Tungsten Powder Prepared by One Step Reduction Method

The preparation of tungsten powder from APT is usually calcined in the weak reducing atmosphere to produce yellow tungsten, blue tungsten or purple tungsten.

Then, the reduction of hydrogen to produce tungsten powder is a commonly used method in industry. The tungsten powder prepared by this process is a kind of very regular polycrystal, which is the main raw material for preparing tungsten alloy and tungsten carbide.

But some scholars believe that the traditional process is long, easy to bring in impurities, production costs are relatively high, so he proposed a step through tungsten oxide process, the direct preparation of tungsten powder from ammonium paratungstate one step reduction method.

The main process of one-step reduction method is as follows: 1. In the first step to control the formation of the intermediate of blue tungsten, a certain proportion of hydrogen and nitrogen mixed gas is replaced into ammonia in the reduction furnace. 2. Loading the amount of 50g, hydrogen flow 32 / h, heating rate of 10 °C / min, 500 °C insulation 200min, 900 °C insulation 1.5h, air-cooled to 100 °C out. In the hydrogen reduction reaction, it is found that the particle morphology of ammonium paratungstate to blue tungsten has no obvious change, but there is obvious crack on the surface of the particle. 




In the reduction process, there are a large number of water vapor generated, this part of water vapor mainly comes from two aspects: first, APT removal of crystalline water, this part of water accounted for most; In the reduction process, there are a large number of water vapor generated, this part of water vapor mainly comes from two aspects: first, APT removal of crystalline water, this part of water accounted for most; Second, due to the presence of weak reducing atmosphere, a small amount of tungsten trioxide in the reduction process was reduced to blue tungsten or purple tungsten produced when the water appears.

It is generally believed that this crack is due to the dehydration of APT and the cracks caused by ammonium removal, which is more conducive to the entry of hydrogen. In addition, the larger particle APT particles gap, will also be more conducive to hydrogen through and reduction of tungsten powder. Therefore, under the same reduction condition, the loading capacity of large particle APT can be more relatively.

The results show that the stepwise reduction of tungsten powder by ammonium paratungstate is feasible, and the yield of tungsten powder obtained under the optimum process conditions is more than 98.5%. It can also control the reduction temperature to control the particle size of tungsten powder. Prolonging the reduction time will cause the tungsten powder to grow up, and the grain of tungsten powder tends to be a complete polygon. The lower the initial hydrogen reduction temperature, the more conducive to the formation of fine tungsten powder; the larger the loading amount, the uniformity of tungsten powder will become worse.

Therefore, although the one-step reduction method has operability, but it is not perfect, first of all, the process requirements are very strict, and it is also required for the purity of ammonium paratungstate. Second, at the moment of nearly one hundred thousand yuan per ton of tungsten powder, the loss benefit of the law is almost equal to the cost of the traditional method.

If you have any inquiry of tungsten, please feel free to contact us:
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New Method for Reduction of Ultra Coarse Tungsten Powder by Ammonium Paratungstate

Ultra coarse tungsten powder is an important raw material for the preparation of ultra coarse tungsten carbide.

Compared with ultrafine tungsten powder, it has a series of advantages, such as less high-temperature defects, higher microhardness and smaller micro strain.

The common process of reduction of ultra coarse tungsten powder by ammonium paratungstate is usually the reduction of ammonium paratungstate into blue tungsten or yellow tungsten, and then by hydrogen reduction furnace. The tungsten powder prepared by this method is regular polycrystalline, and the particle size is mostly controlled between 2 microns and 5 microns, and the powder size is moderate.



In order to prepare ultra coarse tungsten powder, Li or Na was added to tungsten oxide, as far as the average degree of tungsten powder is concerned, the process of adding Li to ammonium paratungstate is the most effective, but there is a large amount of residue in the powder; At the same time, studies have shown that adding a small amount of Na element in ammonium paratungstate, can not only improve the Fisher particle size of tungsten powder, and can improve the uniformity of doping elements Po tungsten powder can greatly improve the tungsten powder to make the powder of FSSS, but the uniformity is poor.

Some scholars have suggested that Na and Li co permeation of ammonium paratungstate can improve the particle size of tungsten powder and improve uniformity. This is because the particle size of tungsten powder mainly depends on the particle size of ammonium paratungstate, the addition of crystal seeds before the crystallization of ammonium paratungstate solution can inhibit the formation of new nucleation. If the amount of seed "appropriate" grain ammonium paratungstate products obtained mainly by seed growth, product size without seed should be rough. The results show that the average particle size of the ammonium paratungstate powder increases from 4 m to 15 m after adding NaCL, and the average particle size of the secondary ammonium paratungstate powder after adding Li2CO3 grows from 4 um to 23 um, NaCL and Li2CO3 promote the growth of ammonium paratungstate powder particle size significantly, the combination of the two can grow up to 60 um.

In addition, the ammonium paratungstate powder added with the reagent is reduced to yellow tungsten or blue tungsten, and finally the coarse tungsten powder is formed under hydrogen reduction, in which the temperature control of hydrogen reduction also plays an important role. 

The use of low hydrogen flow rate, large loading volume, slow push speed, 1000 °C high temperature reduction conditions can further improve the alkali metal salt on the role of particle growth, the particle size of the tungsten powder was doubled to a maximum particle size of 150 um and an average particle size of 68 um.

If you have any inquiry of tungsten, please feel free to contact us:
Tel.: +86 592 5129696/+86 592 5129595
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EDTA Wastewater Photocatalytic Oxidation with Bismuth Tungstate

EDTA is a highly effective chelating agent. It has been widely used in chemical analysis, electroplating, medicine, industrial washing products, photographic products and other industries.

Unfortunately, EDTA can chelate with Mg2+, Ca2+, Mn2+, Cu2+, Fe2+ and other 50 kinds of metal ions to form water soluble chelates, especially the formation and some toxic metal ions is harmful and difficult from the material, excellent stability, will cause heavy metal diffusion, thus adding heavy environmental pollution.

Due to the stability of chelating agents, the degradation of EDTA is more difficult than traditional treatment methods, so bismuth tungstate photocatalyst has become a hot choice again.

The fact that bismuth tungstate is can play a big role, especially in the petal like Bi2WO6, visible light catalytic activity is high, can split water to produce oxygen and organic matter degradation, is a kind of photocatalytic agent ideal for environmental pollution control.
 
Some scholars used Bi2WO6 to do photocatalytic chemical test in the self-made photocatalytic reactor. 100m L containing a single heavy metal copper Cu-EDTA composite pollution solution, after the addition of Bi2WO6, the light intensity is 100m W/cm2. The photocatalytic reaction time was 4h.
 

The experimental results show that under visible light irradiation, Bi2WO6 is excited by visible light, producing photogenerated electrons on the conduction band and generating photogenerated holes on the valence band. The cavity can react with OH + in the solution or produce active oxide on the catalyst surface, which has strong oxidation capacity. The coordination bonds between Cu-EDTA complex ions were destroyed firstly, and EDTA was oxidized and TOC decreased with the oxidation of active oxide. On Cu-EDTA, Cu2+ dissolved in the solution and was adsorbed by Bi2WO6 again. Cu2+ capture photogenerated electrons are reduced to Cu0, deposited on the surface of Bi2WO6, formed CuO/, Bi2WO6 capture is reduced, EDTA capture h+ is oxidized, once again inhibit electron and hole recombination, improve photocatalytic efficiency.

Bismuth tungstate once again demonstrated its strong photocatalytic properties with the strength, not only is copper or EDTA, ethylenediaminetetraacetic acid manganese and magnesium and calcium, iron, EDTA almost all can be treated with bismuth tungstate.

If you have any inquiry of tungsten, please feel free to contact us:
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