2022年7月14日星期四

What is Halotungstate

Halotungstate is a kind of tungstate which is mainly composed of oxyhalide, it is a new type of visible light response photocatalyst.

halotungstate image

In alkaline earth tungstate, the luminescent intensity of barium tungstate is very weak. Therefore, people try to modify its structure. In 1982, domele and others synthesized barium tungstate with BaF2 and WO3 as raw materials. Ba2WO3F4 is a kind of halotungstate. Through the improvement of its structure. It has been found that ba2wo3f4 has strong fluorescence intensity and strong blue-green luminescence under UV excitation. It can be developed into practical UV and X-ray luminescent materials.

It has also been found that the rare earth tungsten composite halide oxide, which is made by the preparation of the halide tungstate, can glow stronger luminescence performance. For example, the study of lawo4cl shows that it has a strong absorption of X-ray, and its absorption wavelength is just within the range of medical usefulness. It is suitable for X-ray absorption materials. At the same time, it is also found that some rare earth ions show characteristic spectral properties in it. For example, the pure La3WO6Cl3 only has weak luminescence, and when Eu3+ and Tb3+ are doped, La3WO6Cl3 will be activated and emit extremely strong fluorescence.

To sum up, the main function of the halotungstate catalyst is to make X-ray machine. Although it has great application potential, it also has some disadvantages, such as the low efficiency of most of the light quantum, the low catalytic ability in the visible light area and the easy deactivation.

Therefore, the best way is to find a suitable carrier and immobilization method, using carrier to solve the shortcomings of suspension phase photocatalyst, such as easy agglomeration and difficult recovery, and using carrier synergy, can enhance the reaction activity of photocatalyst

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Scheelite Decomposition by Sodium Silicate

Sodium hydroxide decomposition is the main process for tungsten mineral raw materials. In the alkaline system, phosphorus, arsenic, silicon, molybdenum, tin and other elements contained in the mineral are leached to varying degrees and become the main impurities in the crude sodium tungstate solution. Some scholars have found that when sodium hydroxide decomposes tungsten ore, the calcium content of raw materials is beneficial to inhibit the leaching of impurities such as phosphorus, arsenic and silicon, and the leaching rate of these impurities decreases with the increase of calcium content.

scheelite decomposition image

The results show that the oxy acids of phosphorus, arsenic and silicon can form insoluble compounds with calcium, and these precipitates can still exist stably under the condition that scheelite has been decomposed. Because of this, in the purification process of crude sodium tungstate solution, adding appropriate amount of Ca(OH)2 can also remove the above three impurities. The precipitation rates of phosphorus, arsenic and silicon can reach 99.04%, 96.94% and 97.93% respectively.

Thermodynamic analysis shows that scheelite can be decomposed by sodium silicate. With the increase of pH value, the decomposition products were CaSiO3 and Ca3Si2O7. In the stable region of CaSiO3 with low pH value, the concentration of tungsten in the solution is relatively low; in the stable region of Ca3Si2O7 with high pH value, sodium hydroxide also participates in the decomposition reaction, and the concentration of tungsten increases rapidly with the increase of pH value. Therefore, appropriate sodium hydroxide should be added when sodium silicate decomposes scheelite.

The possibility of decomposition of scheelite by sodium silicate was verified. The results show that the amount of sodium silicate is an important factor affecting the overall level of leaching. Increasing the concentration of reagents and adding a certain amount of sodium hydroxide can further improve the leaching rate. Under the conditions of decomposition temperature 180 ℃, decomposition time 4 h, n (SI) / N (W) = 2.0, n (OH -) / N (W) = 0.5, n (H2O) / N (W) = 35.3, the leaching rate of tungsten is 96.9%.

The results of theoretical analysis are verified by decomposition experiments, and the trend of actual decomposition effect is consistent with that of thermodynamic analysis. The amount of sodium silicate is an important factor to affect the overall level of leaching. Increasing the concentration of reagents or adding sodium hydroxide can further improve the leaching rate of tungsten. When the mass fraction of WO3 is 55.0%, the decomposition temperature is 180 ℃, the decomposition time is 4 hours, and the molar ratio of silicon to tungsten, hydrogen peroxide to tungsten, water to tungsten is 2.0, 0.5 and 35.3, respectively, the leaching rate of tungsten is 96.9%.

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Cadmium Tungstate Single Crystal Growth by Crucible Descent Method

Cadmium tungstate (CdWO4, CWO) single crystal is a scintillation crystal material with excellent comprehensive properties, which can be widely used in nuclear medicine imaging, safety inspection, industrial CT, oil logging, high energy physics and other technical fields, especially in medical X-CT, container inspection system. Up to now, it has been reported that the single crystal of cadmium tungstate has been grown by Czochralski method at home and abroad. It is very difficult to grow high-quality large-scale cadmium tungstate single crystal. The main problem is that the optical transmittance of the grown single crystals is not uniform at the beginning and the end, the repeatability of the single crystals grown in different batches is poor, and there are many crystal defects such as cracking, inclusions, color centers, etc. 

cadmium tungstate single crystal image

In view of the technical problems existing in the current Czochralski growth of cadmium tungstate single crystal, a transparent and complete CdWO4 single crystal with a size of φ 40mm x 70mm was successfully grown by the crucible descent method for the first time. The properties of the single crystal were characterized by X-ray diffraction, transmission spectrum and X-ray excitation emission spectrum.

In the experiment, CdWO4 polycrystal with theoretical stoichiometric composition was synthesized by using a special platinum crucible to grow cadmium tungstate single crystal. After the polycrystal was filled into the platinum crucible, the top of the crucible was completely welded and sealed to realize the growth of cadmium tungstate single crystal by the crucible drop method under the closed crucible. The results show that the platinum crucible can seal the vapor in the upper part of the melt reliably, and the component of the melt will not be volatilized. During the whole process of crystal growth, the toxic cadmium oxide vapor can be avoided to escape, the composition of the melt can keep constant, and almost all the melts grow into relatively uniform transparent single crystals.

The stoichiometric polycrystal was synthesized by high temperature solid-state reaction. The crystal growth was guided by oriented seed crystal. The temperature of the growth furnace was controlled at 1350-1400 ℃, the temperature gradient of the solid-liquid interface was adjusted to 30-40 ℃ / cm, and the large-scale transparent and complete cadmium tungstate single crystal was successfully grown by the crucible drop method under closed condition. The process avoids the volatilization of the melt components in the process of crystal growth, can maintain the uniformity of the melt components, and stable temperature field is conducive to reduce the crystal cracking. Therefore, the crucible drop method for the growth of cadmium tungstate single crystal is a valuable process for application and development.

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Supported Tungstophosphoric Acid Catalyst Preparation

Theoretically, many of the carriers found at present can be used as the carriers of tungstophosphoric acid, but most of them use oxidants as the carriers of oxidants or are easy to decompose when alkali is azeotropic. At present, SiO2, HZSM-5 molecular sieve, Y-type molecular sieve, HP, HMS molecular sieve, worm like molecular sieve, activated carbon, silica gel and so on are the main carriers of phosphotungstic acid catalyst.

supported tungstophosphoric acid catalyst image

At present, the main preparation methods of phosphotungstic acid include impregnation method, adsorption method and sol gel method.

1. Impregnation method
The impregnation method is to dissolve tungstophosphoric acid in water, then impregnate the carrier with this solution and stir it. After a certain period of time, take it out for drying, and then obtain the supported tungstophosphoric acid catalyst. Or the active component is directly loaded on the carrier by physical mixing, hydrothermal dispersion, CO impregnation or ion exchange. However, there are some problems in the stability of the products.

2. Adsorption method
The reflux adsorption method is to add the carrier into the phosphotungstic acid solution, after heating and reflux for a certain time, filter, wash, dry for several hours, and then activate at a certain temperature. The method has good uniformity of the load, but the load is not strong enough and easy to fall off.

3. Sol gel method
Using ethyl orthosilicate as precursor, using alcohol (such as n-butanol, ethanol, etc.) as solvent, inorganic acid (nitric acid, hydrochloric acid, etc.) was used as catalyst for sol gel solution, then added phosphotungstic acid to form gel, drying and standby.

At present, the research on supported tungstophosphoric acid catalyst is still in the primary stage. In order to realize the industrial production of supported tungstophosphoric acid as soon as possible, further research is needed. The main research directions are as follows: further study on the loading mechanism between tungstophosphoric acid and support, especially the mechanism of in-situ synthesis; To solve the problem of dissolution of heteropoly acid in liquid-phase reaction, improve the activity and service life of the supported tungstophosphoric acid catalyst; to study the thermal stability of the supported tungstophosphoric acid catalyst; to study the influence of the modification of the physical and chemical properties of the support on the performance of the supported tungstophosphoric acid catalyst.

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Tungsten Hexachloride Price on OCT.25, 2022

Basic Trading Condition Product Name Tungsten Hexachloride Details Purity:99.90% Quantity:100.00KG Price:120.00 USD/KG EXW, XIAMEN, CHINA Pa...