2020年12月1日星期二

Zinc Tungstate / Bamboo Charcoal Composite Photocatalyst

In recent years, tungstate catalysts have attracted more attention for their narrower band gap and larger visible light range.

Zinc tungstate (ZnWO4) is a very representative tungstate material. As an efficient photocatalyst, ZnWO4 has excellent catalytic activity, good stability, low energy consumption and no secondary pollution, it has a wide range of applications in environmental protection, water treatment and many other fields. But inevitably, zinc tungstate, like bismuth tungstate, titanium dioxide and other photocatalysts, is easy to agglomerate and difficult to recover in practical applications, therefore, searching for an effective carrier is an effective way to improve photocatalytic performance and reduce the amount of zinc tungstate.

Bamboo charcoal is a three-year-old alpine bamboo as raw material, by nearly 1000 degrees high temperature firing from a charcoal. Bamboo charcoal has porous and porous structure, its molecule is fine and porous, the raw material is easy to regenerate, has strong adsorption ability, can purify the air, eliminate the peculiar smell, is the ideal material to serve as the carrier of zinc tungstate.



The molar ratio of ammonium paratungstate, zinc nitrate and citric acid was mixed with the molar ratio of 1: 1: 2, and the mixture was stirred in a constant temperature water bath at 85 °C. The dispersed bamboo charcoal was poured into the above solution and stirred for 10 min, and then with 7.28 mol / L ammonia water solution pH = 2.0 ~ 3.0 constant temperature stirring to the solution was gel-like, the sol transferred to the beaker, into the microwave oven, microwave oven power 640 W, microwave 10 min, and then in the box-type resistance furnace at 400 °C for 2 h, remove the grinding, the powder over 100 mesh sieve, that was ZnWO4 / bamboo charcoal composite materials.

After the composite of zinc tungstate / bamboo charcoal composite catalyst showed great photocatalytic activity, some enterprises use composite catalyst degradation rhodamine B, when the content of bamboo charcoal was 20%, the bamboo charcoal exhibited the maximum photocatalytic activity, and the degradation effect reached 92%.

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A New Shortcut for Preparing WC-CO Powder from Ammonium Metatungstate

 Many scholars have tried new methods to reduce production cost. The direct synthesis of tungsten cobalt alloy powder from ammonium metatungstate is an innovative attempt.

By ammonium metatungstate direct synthesis of WC-CO powder the process is roughly using ammonium metatungstate (APT), soluble cobalt salt and organic carbon source as raw materials, using spray conversion method, preparation of tungsten cobalt precursor by direct carbonization in situ synthesis method, the precursor for WC-CO carbide composite powder. The technology greatly simplifies the process of traditional technology, the cost is also reduced in theory.




Some scholars using the preparation of ammonium metatungstate by AMT (WO3>92.5%), soluble cobalt salts and ultra pure organic carbon as raw materials, according to the wc-6% (mass fraction) CO composition prepared tungsten cobalt mixed solution, and then the preparation of tungsten cobalt precursor powder in spray drying tower, calcined in N2 atmosphere, finally prepared by in situ synthesis method prepared by direct carbonization of WC-CO composite powder in H2 atmosphere. For the spray conversion, the calcination process uses fixed parameters: the solution concentration is set to 60% (mass fraction), feed rate 2000ml / min, inlet temperature 240 °C, centrifugal speed 12000r / min, low temperature calcination temperature is set to 200 °C; Set the direct carbonization temperature to 800, 850 and 900 °C. In situ synthesis of WC-CO powder by direct carbonization.

On the set of carbonation temperature test found that W at 800 °C began to slow carbonation, with the temperature rise, carbonization speed, to 900 °C, W has been carbonized more complete, under normal circumstances, W need to be carbonized at 1100 °C, but here the carbonization temperature is reduced by 200 °C, the main reason is that CO on the carbonization of W play a catalytic role.

The process of preparing WC-CO powder from ammonium metatungstate was carried out along the W / CO / C → WCO3 · CO6W6C → W2C-CO → WC-CO reaction. The WC-CO composite powder with hollow spherical structure can be successfully prepared, the average particle size is about 30 um, and the hollow sphere wall thickness is about 1.8 um; With the increase of carbonization temperature, the reaction system can be directly generated W2C-CO without further two steps, and then further carbonized into WC-CO composite powder.

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What Factors Should be Paid Attention to in Producing High Purity APT

High purity quaternary ammonium paratungstate (APT) is a crystalline powder of ammonium paratungstate with a purity of 99.999% to 99.9999%.

At present, the methods of producing high purity APT by controlling impurity content in industrial production are: re dissolution method, ion exchange method, ammonium tungstate method, electrolysis method, recrystallization method, etc.. But because of the different chemical properties of impurity elements, using only one method to achieve the purpose of removing all impurities, so the preparation of high purity APT often need two times of ion exchange, activated carbon adsorption and other means of further purification.

It is found that the following three problems should be paid attention to during the purification of high purity ammonium paratungstate (APT).




(1). The higher the crystallization rate, the higher the impurity content in APT crystallization. When the crystallization rate is more than 65%, the precipitation of impurities is accelerated, and the precipitation rate of the four impurities is sorted as Fe >M o>P >S. Therefore, the ultra high purity APT should be controlled by controlled crystallization, and the crystallization rate should be controlled within 65%.

(2). Increasing the concentration of WO3 in ammonium paratungstate solution is beneficial to improve the purity of APT. The higher the initial concentration of WO3 is, the less impurity content in APT crystal. The decrease of Mo content in APT was faster than that of P, S and Fe in the ammonium tungstate solution with higher WO3 concentration. The precipitation rate of Fe decreased obviously with the increase of WO3 concentration.

(3) With the increase of crystallization temperature, the impurity content in APT decreased. The reason may be that the solubility of impurities increases at high temperature, and the content of APT crystals decreases. At the same time, homogeneous coarse grains are easily obtained at high temperature, and the impurities in the bulk crystals and homogeneous coarse grains are less.

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Tungsten Trioxide Gas Sensitive Performance Test

Tungsten trioxide is a transition metal oxide, with a semiconductor character, has great potential for gas-sensitive materials, sensitive to a variety of gases, especially toxic gases.

In recent years, with the emergence of nanotechnology, it was found that nano-tungsten trioxide has a higher sensitivity and response speed. There are three main methods for preparing nano tungsten trioxide, gas phase method, liquid phase method and solid phase method. In general, hydrothermal method has become the most widely used method for the preparation of nano WO3 by virtue of its economical, low cost and easy operation.



Commonly used to produce gas-sensing elements of tungsten trioxide mainly nanorods, nano-plate, nano-plate rod-like mixed structure and other forms, as will eventually become what form, are generally in the process of hydrothermal synthesis by adjusting the parameters formed by the results, therefore, even with the same hydrothermal method of nano-tungsten trioxide, often because of different regulators, the temperature is different, and makes the operation of a wide variety of the final nano-structure is also different.

Some scholars use water from tungsten trioxide thermal method, in order to verify the gas sensing properties of the above-mentioned three kinds of structure, using citric acid as acidifier, ammonium metatungstate will be mixed and sodium sulfate, were dissolved into distilled water, according to a certain percentage of the mixed solution by adding citric acid, by adding the amount of citric acid to adjust the reduction of rod, plate, plate-like mixed structure of three kinds of structure of nano-tungsten trioxide.

Through the test of the three structures, the following conclusions are obtained. The specific surface area of BET of nano plate, nano plate rod and nano rod WO3 is 17.55 m2/g, 11.01 m2/g, 10.57 m2/g, respectively, it can be seen that the specific surface area is nano plate > nano plate rod > nano rod. It can be said that the specific surface area is the key to determine the sensitivity of nano-tungsten trioxide, so the results will be clear at a glance.

In the atmosphere of three acetone, the same concentration of ammonia and formaldehyde, gas sensing properties of tungsten trioxide nano plate is the best, this is mainly due to the nano tungsten trioxide plate has a high specific surface area, can significantly change the effective resistance of the crystal and increase the active site of gas reaction. For gas sensitive reactions, the more active sites of metal oxides are, the more sensitive the gas sensitivity is, and the selectivity and sensitivity of gas sensors are higher.

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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.

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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.

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Zinc Tungstate / Bamboo Charcoal Composite Photocatalyst

In recent years, tungstate catalysts have attracted more attention for their narrower band gap and larger visible light range. Zinc tungstat...