2020年12月21日星期一

Cyclic Oxidation Method Helps Tungsten Powder Particle Size Rebalance

Tungsten powder is the basic raw material for making cemented carbide.

Most of the cemented carbides in the market do not have much demand for the size of tungsten powder, but they have strict requirements for the particle size uniformity of tungsten powder.

A new method of ammonium tungstate cyclic oxidation reduction method proposed in recent years for producing submicron tungsten powder, it is mainly through a number of ammonium tungstate - tungsten oxide circulation change, submicron tungsten powder and ultimately the average particle size. Therefore, we can also try to use circulating oxidation reduction method to help the common tungsten powder to achieve particle size rebalancing.


It is necessary to prepare blue tungsten for reduction of tungsten powder because of the preparation of ordinary particle grade tungsten powder. The general process of ammonium tungstate cycle oxidation reduction method is: the special pure ammonium paratungstate in argon to slowly raise the temperature rise to 600 °C, holding 10 min, to obtain blue tungsten → with the resulting purple tungsten in the first reduction of hydrogen, preparation of tungsten powder → tungsten powder in the air slowly heated to 500 °C for the first oxidation, when the temperature reached 500 °C for 10 min, to obtain tungsten trioxide → tungsten oxide in hydrogen for the second time to prepare Tungsten powder, end the process, get tungsten powder. 

The tungsten powder was tested with tungsten powder with a particle size of 3~10 and a specific particle size of 0.180 m2/g and a specific surface area of 83.86% m, the main reason is that the composition of blue tungsten is not a single phase, according to the oxidation - reduction deposition mechanism can be seen, blue tungsten in the reduction process is not synchronized to carry out, therefore, the tungsten particle size distribution is very uneven after the first reduction of blue tungsten. When the first reduction of blue tungsten tungsten powder is oxidized into WO3, and then restore, because the tungsten oxide particle size distribution is uniform, then the reduction process of tungsten trioxide is almost simultaneous, therefore, the particle size distribution of tungsten powder tends to be homogenized. In short, the first reduction process and can not draw tungsten powder with uniform particle size, this should not be too tangled, directly and then melted down a consolidation reduction effect on it.

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Ammonia Purification - Thermal Dissociation Method for Preparing High Purity Tungsten Trioxide

At present, the industrial production of pure tungsten trioxide or APT is mainly purify the crude sodium tungstate solution.

Ammonia purification of a thermal dissociation method is the South China University of Zhang Qixiao et al in the early 21st century proposed purification of ammonium arsenate solution method, it can be achieved by the crude ammonium tungstate solution directly prepared high purity tungsten trioxide.

The use of ammonia purification heat from the main base of high pure tungsten trioxide solution is directly preparing high purity tungstic acid (H2WO4), the quality of H2WO4 directly affects the quality of the final product and cycle times. The process of ammonia purification by thermal dissociation is:

1.The pulp industry H2WO4 slowly adding ammonia, WO4 will dissolve (NH42WO4 solution, and silica, crude H2WO4 of ferric hydroxide, manganese oxide and calcium tungstate are reserved for ammonia dissolution in slag.

2.Filtered (NH42WO4 solution, after heating and removing ammonia, they gradually precipitate the solubility of ammonium paratungstate crystals. 


3.The ammonium paratungstate dissolved in ammonia, is in the temperature range of 250-350 °C ammonium paratungstate for a sufficient period of time to make the properties of ammonium paratungstate crystals change, and then can be dissolved in ammonia to produce (NH42WO4 solution. With the progress of the thermal dissociation process, the impurities in the crystallization of ammonium paratungstate were dehydrated and oxidized. Most of the impurities were insoluble in ammonia, and only aluminum, arsenic and phosphorus still entered (NH42WO4 solution.

4.The obtained (NH42WO4 and then a first evaporation crystallization, to consolidate the effect, in general, the solution and then evaporate the crystallization of ammonium paratungstate will be much higher than the first purity.

The ammonia purification process has simple equipment, high pure tungsten trioxide thermal dissociation method, chemical reagent consumption, low cost, and the purity of WO3 reached 99.995%-99.999%, reached the requirements of high pure tungsten trioxide.

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Tungsten and Molybdenum Separation - Paratungstate B Guanidine Salt Precipitation Method

Paratungstate B guanidine salt precipitation method is a method for separating molybdenum and tungsten.

It is known that there are three kinds of orthopolyacids, namely, paratungstic acid A, paratungstic acid B, paratungstic acid Z, the structure of B is the most special, and its chemical formula is Na10[H2W12O42], containing two H atoms, so the selectivity is strongest.

The key point of B guanidine precipitation method is how to convert A into B. Scholars have found that SiO2 has a significant catalytic effect on the transition reaction, and can shorten the conversion process to a few hours, which makes the feasibility of B guanidine salt precipitation method.

After the conversion of sec-tungstic acid A to paratungstic acid B, the pH of the solution was increased from 6.7 to 8.1, ensuring that Mo42- could be polymerized in a large concentration range, in order to use guanidine salt precipitation agent can effectively separate and in suitable conditions of pH tungsten and molybdenum, tungsten and molybdenum separation under (pH>7.5), only the formation of paratungstate B can guarantee the high precipitation rate of tungsten.


Recently, some scholars believe that guanidine nitrate is an ideal selective precipitant for B. The concentration of guanidine nitrate was 1.063 times -1.068 times, pH7.70-7.79 was 45 minutes, , stirring speed of 120 minutes and other precipitation conditions, molybdenum containing paratungstate B and guanidine nitrate solution in the precipitation process can fully realize the separation of tungsten and molybdenum, and, in the process of transformation, and the recovery of ammonium paratungstate guanidine nitrate preparation at the same time.

After verification, paratungstate B guanidine salt precipitation method has been confirmed as an effective method for separation of tungsten and molybdenum, as to why there is no large-scale application to the laboratory, reason unknown. But good wine is not afraid of deep alley. In the near future, more scholars will further improve the method of B guanidine salt precipitation. Make it easier to popularize.

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Submicron Tungsten Powder Preparation by APT Cyclic Oxidation Reduction Method

Tungsten powder is mainly used to manufacture tungsten filament for incandescent lamp, inert gas shielded arc welding and atomic hydrogen welding electrode etc..

Submicron tungsten powder has the advantages of fine particle, well-developed surface, high activity, it is not as stringent as the preparation of nano tungsten powder, and the application range is better than ordinary tungsten powder, so the application scope is more and more widely. 

There are many main methods for preparing submicron tungsten powder at home and abroad: they are high-energy ball milling, dry atmosphere reduction, violet tungsten hydrogen reduction, molten salt electrolysis and plasma technology, etc. There are still many shortcomings in the preparation of submicron tungsten powder, such as time consuming, energy consumption, low production efficiency, high production cost, and difficult to realize industrial production. So the next to introduce an innovative submicron tungsten powder preparation method - ammonium paratungstate cycle oxidation reduction method.



The cyclic oxidation reduction method of ammonium tungstate was put forward by scholar Fu Xiaoming. Its main feature is the ammonium tungstate calcined in the air as tungsten trioxide, and then the use of cyclic oxidation reduction preparation of sub-micron tungsten powder, this feature is less pollution, simple and easy process route, easy to control process parameters, simple equipment and suitable for mass production of industrialization and so on, the submicron tungsten powder particle size distribution range is narrow, the structure is spherical, which is conducive to the homogenization of tungsten powder compacts in sintering process, and can achieve good size control.

Ammonium paratungstate oxidation reduction method is carried out under the reduction furnace, the main feature is the cycle of recycling, roughly as follows: the special pure ammonium paratungstate in argon to slow the temperature rise to 800 °C, holding 10 min, the tungsten powder was prepared by the first reduction of the tungsten powder and tungsten powder in the air to 500 °C for the first time. When the temperature reached 500 °C, the temperature was kept for 10 min, tungsten trioxide → tungsten oxide is prepared in hydrogen for the second time. Tungsten powder is also prepared. The second oxidation is carried out using the obtained tungsten powder. The second oxidation is carried out to obtain tungsten trioxide. To obtain tungsten trioxide → the third reduction was carried out again with the obtained tungsten trioxide to prepare submicron tungsten powder.

The process and equipment used to prepare submicron tungsten powder by cyclic oxidation of ammonium paratungstate are similar to those of traditional industrial production of tungsten powder. Therefore, the process route is simple and easy, the process parameters are easy to control, the equipment is simple, suitable for industrial production in large quantities.

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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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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.

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2020年9月15日星期二

Green Catalytic Activity of Phosphotungstic Acid for Four Hydrogen Sulfide

Four hydrogen sulfide is a chemical intermediate of nitro spray paint, rubber and other solvents, so is often used in the protection of hydroxyl groups.

The catalysts, Lewis acid and ion exchange resin used in the preparation of the four - hydrogen catalyst, all of which have good catalytic effect. However, there are some shortcomings in these methods, such as longer reaction time, reflux, solvent, complex catalyst preparation procedure and unsatisfactory yield.

Phosphotungstates have copper phosphotungstic acid, zinc phosphotungstate and aluminum phosphotungstate. As a kind of heteropoly acid salts, phosphotungstates are widely used as catalysts. It has been proved by experiments that the catalytic activity of phosphotungstic acid for the catalytic hydrogenation of four hydrogen peroxide is better than that of the four hydrogenation of alcohol, thus, a green environmental reaction route is provided for the four hydrogen alkylation reaction better than other methods.

green catalytic activity of phosphotungstic acid for four hydrogen sulfide image

The results show that there are three kinds of phosphotungstates such as copper phosphotungstate, zinc phosphotungstate and aluminum phosphotungstate, which have good catalytic effect on tetrahydropyran. In the reaction catalyzed by copper tungstate, the reaction time of secondary alcohol and tertiary alcohol is longer, and the activity of Bibb alcohol is relatively low; the reaction of aluminium tungstate catalyzed alcohol (except n-butanol, SEC butyl alcohol and tert butyl alcohol) is in progress, the catalytic activity of alcohol is very high, and the structure of alcohol has little effect on the catalytic effect; And zinc phosphotungstate as a catalyst, regardless of which structure of the alcohol structure of its activity are very high.

It can be seen that phosphotungstic acid can provide a simple, new and efficient and environmentally friendly reaction pathway for the protection of hydroxyl groups, which is the effective catalyst for tetrahydropyran.

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Pesticide Pollution Treatment by Potassium Phosphotungstate

Chlorothalonil is an efficient fungicide absorbed in organic pesticide, organic compounds but it is also easy to cause pollution of soil.

In the process of agricultural operations, the land often contains chlorothalonil which is difficult to degrade, not only the soil ecological structure is affected, but it is often washed by rainwater to enter the river or infiltrate the ground water, causing serious pollution.

In recent years, with the tungsten based photocatalyst for environmental, economic, effective and has become a new method for the treatment of organic wastewater, some scholars have found that most of the organic pesticides is easily degraded and mineralized into H2O and CO2 and other small molecules, so the photocatalytic degradation of chlorothalonil is feasible.

.degradation of chlorothalonil by phosphotungstic acid image

Tungsten photocatalysts mainly consist of tungstate and tungsten heteropoly acids, the commonly used method is to use a combination of loading and photocatalyst to form a new composite preparation for photocatalytic work, a common load has cloth, holes, sieves, or other chemicals. Some scholars believe that phosphotungstic acid has good degradation effect on chlorothalonil, and in order to verify his opinion, potassium chloride is used to degrade phosphotungstic acid.

During the experiment, he added potassium chloride and phosphotungstic acid to 2.5:1 in proportion to H3PW12O40 water solution and stirred it to produce a white precipitate, continue to stir 10h until the precipitation is complete, centrifugal separation, washed with deionized water, white precipitate to no Cl, and then dried, baking at 300 hours, made into potassium tungstate.

Through the contrast test of two chlorothalonil solution showed that potassium phosphotungstate catalyst of chlorothalonil solution has good photocatalytic effect, not only this, have the effect of all similar pesticides, and very small amount of it. It is estimated that 1 grams of potassium phosphotungstate degradable 10L chlorothalonil solution (5mg/L), light degradation time is 4 hours, the degradation rate is 86.22%, the effect is ideal. And the use is also very convenient, that is, before the allocation of pesticides, according to the proportion of phosphorus potassium tungstate stirring can be used.

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Photocatalytic Activity of Bismuth Tungstate Upgraded by Polyester Fiber

Bismuth tungstate (Bi2WO6) is the most common Aurivillius type oxide and is a hot spot in the field of photocatalysis.

Most of the traditional photocatalytic reactions are carried out in the suspensions of bismuth tungstate catalyst powders. Although they have high photocatalytic activity, bismuth tungstate powders are easy to agglomerate and difficult to separate and recover. In practice, it was recognized that some measures had enabled bismuth tungstate to work, and some scholars had thought of loading bismuth tungstate onto the cloth.

bismuth tungstate photocatalysis image

Of course, loading bismuth tungstate on cloth is not a simple mix, and it also requires a series of chemical procedures that combine bismuth tungstate with cloth to function. Acrylic, polyester fiber and so on, have the same effect at last, and their main difference is synthetic method.

Some scholars used polyester as the carrier of bismuth tungstate and prepared them by hydrothermal method, the polyester fabric was treated with acetone and ethanol at 50 °C for 30 min and 80 °C by ultrasonic cleaning. Then soak 40 min in butyl acrylate and dry. Twelve sodium sulfate and Bi2WO6 powder were dissolved in 50 mL deionized water, and then 2 h at room temperature and then removed and dried to obtain the load polyester fabric.

The results show that Bi2WO6 loaded polyester fabric has better UV resistance and water repellency than Bi2WO6 powder alone. The photocatalytic results show that the Bi2WO6 load of polyester fabric on the degradation of methylene blue was significantly higher than that of Bi2WO6 powder, the degradation efficiency of methylene blue reached 92%, this is because the load after Bi2WO6 got a stable platform, can play a balanced effect.

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Graphene, Bismuth Tungstate, Titanium Oxide

Bismuth tungstate is a popular new visible light catalyst, especially petal shaped bismuth tungstate.

It has good light absorption and high stability, and has broad application prospects in the field of photocatalysis.

The scholars in our country believe that the proper doping of bismuth tungstate, the preparation of semiconductor compound and assistant modification are effective ways to improve the photocatalytic property of bismuth tungstate.

treatment of methyl wastewater image

In recent years, graphene has brought tremendous changes to the material industry because of its unique two-dimensional structure, large specific surface area and high electron transmission rate. It has been recognized as the "king of new materials", Chinese scholars believe that graphene as an efficient electronic additive, and Bi2WO6 composite catalyst, will greatly improve its light capture electronic capabilities, thereby enhancing catalytic capacity. However, scholars believe that only graphene is not enough, and the number of photocatalytic holes will also be increased to improve the cavity manufacturing capacity of bismuth tungstate.

In the photodegradation process, the main active species, HO, is obtained by the reaction of holes and H2O. In recent years, adding cavity additives into photocatalyst is a popular method. Amorphous oxide amorphous TiOis a common assistant, and TiO2 is also a photocatalyst. It is found that the amorphous TiO2 film can be applied to the surface of BiVO4 and GaAs electrodes to transfer the photogenerated cavity rapidly.

With bismuth tungstate (Bi2WO6) as the main catalyst, graphene (rGO) is an electron assistant, and TiO2 (TIO2) is a hole assistant, three Musketeers combined to degrade methyl wastewater. Ti (IV) -rGO/Bi2WO6 has been prepared by hydrothermal impregnation deposition method to degrade methylene orange. The test concluded that the catalytic rate of composite catalyst are pure Bi2WO6, Ti (IV) Bi2WO6 / rGO/Bi2WO6, 88 times, 67 times and 17 times, enhance the photocatalytic performance is mainly due to the Ti (IV) and rGO double additive synergistic effect, rapidly capture and transfer of photogenerated holes and electrons promote separation between them, thus effectively reducing the recombination of photogenerated carriers.

From the cost point of view, bismuth tungstate is the most expensive, followed by graphene, titanium dioxide is the cheapest, they reduce the use of bismuth tungstate, cost a great deal of medicine, and the effect is greatly increased. Therefore, the use of new additives to modify the photocatalytic materials will be most likely to become mainstream methods in the future.

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Cyclic Oxidation Method Helps Tungsten Powder Particle Size Rebalance

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