2020年8月11日星期二

Ultrafine Ammonium Metatungstate Acquisition Strategy

Ammonium metatungstate (AMT) is one of the most important raw materials and precursors for the preparation of WC.

spray drying method image

The structure and properties of WC are affected by the physical and chemical properties of precursor. The particle size and structure of precursor will greatly influence the process of reduction and carbonization in the late stage, and then affect the catalytic activity of WC catalyst.

In order to obtain precursor with good particle size and good structure, spray drying is usually used. Spray drying has many advantages, such as simple process, no pollution, homogeneous composition, and suitable for mass production.

The basic principle of spray drying method is ammonium metatungstate solution by spray drying, in a few seconds, water quickly evaporated into powder, physical and chemical properties of homogeneous granular, hollow spherical or spherical solid products.

Many scholars have found that in the spray drying process, the concentration of ammonium tungstate solution has an important influence on the morphology of ultrafine tungsten powder, and so does ammonium metatungstate.

Some scholars have investigated the effects of different feed concentration, feed rate and the addition of surfactants on the particle size of microspheres under aerobic spray drying conditions. The results show that the feed concentration is 5% at the inlet temperature of 200 °C, the outlet temperature is 90 ~ 100 °C, the air velocity is 550 L· min-1 and the feed rate is 7 m L · min-1 (d50) was 2.26 um. The AMT was prepared by the precursor solution, and the minimum median diameter (d50) was 2.26 um; AMT with solid solution concentration of 50% (WT) is the solid microsphere, and the minimum median diameter (D50) is 6.43 mu m. The greater the solubility, the greater the particle.

From an economic point of view, the concentration of the liquid should be as high as possible, because it can reduce the water content in the solution, thereby reducing the amount of hot air used for evaporating water, and reducing energy consumption. However, the ultrafine ammonium metatungstate precursor can be obtained by analyzing the technical requirements, and the ammonium metatungstate can be obtained with lower concentration.

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How to Extract Rare Element Rubidium with Phosphotungstic Acid

 Rubidium, as a rare and weak alkali metal element, has been widely used in organic catalysts, photomultiplier tubes, special glass and anticancer drugs.

extraction of rare element rubidium by phosphotungstic acid image

Rubidium does not have independent minerals, often and other alkali metals coexist in lithium mica, cesium garnet, lithium cesium garnet, natural carnallite, salt lake brine and underground brine. Because of the coexistence of rubidium and sodium and cesium, which are closely related to the physical and chemical properties, the extraction technology of rubidium is very difficult, which results in a large amount of brine rubidium resources not being rationally utilized.

The common methods for separation of rubidium and cesium are fractional crystallization, precipitation, solvent extraction and ion exchange. In industrial production, precipitation method is used to separate rubidium and cesium, which is mainly used to separate rubidium and cesium from high content solution. Solvent extraction is easy to realize continuous operation and has great potential for application.

Extractant is the most important extractant. Some scholars believe that the salts of heteropoly acids such as phosphotungstic acid and phosphorus molybdate have higher selectivity to rubidium and cesium. However, since the powder is too fine and unsuitable for the continuous operation of the exchange column, it is difficult to be used in the actual production process. Therefore, it is an ideal way to prepare the composite adsorbent by loading the heteropoly acid salt on other carriers.

Some scholars used calcium alginate as carrier and ammonium phosphate as active component to extract rubidium and cesium directly from mother liquor, the research shows that the composite adsorbent can extract Rb+ directly from the mother liquor containing about ten thousand times of impurities. The extraction rate of RbCl is above 92%, the desorption rate is nearly 100%, and it can be reused again and again.

At present, the total reserves of rubidium (excluding rubidium in seawater) are about 10 million 770 thousand tons, of which more than 92% and about 10 million tons exist in Saline Lake, in view of the role of rubidium and the reserves of rubidium, the rubidium resources in the brine of Saline Lake will be the focus of future development, and the solvent extraction method with ammonium phosphotungstate is the most promising method of industrial application.

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Microwave Ultrasonic Method for Customizing Ultrafine Ammonium Paratungstate

Ammonium paratungstate (APT) is the main raw material for the production of tungsten powder, tungsten carbide powder, tungsten wire and tungsten products.

microwave ultrasonic machine image

As everyone knows, the tungsten particle size, crystal morphology of its APT has a great "inherited" relationship, so, if want to produce nanometer tungsten powder and tungsten carbide powder, we first need to prepare ultrafine ammonium paratungstate (APT).

It is an innovative method to prepare superfine ammonium paratungstate (APT) by microwave ultrasonic combined crystallization. Many scholars have shown that compared with traditional heating crystallization, microwave ultrasonic combined crystallization has obvious advantages in speeding up the crystallization rate, refining the particle size and improving the purity of products. So what's the difference between microwave - ultrasonic combined crystallization and traditional evaporation crystallization?

Some scholars have used microwave-ultrasonic combined crystallization method for ultra-fine ammonium paratungstate (APT) preparation test, they used a certain volume of ammonium paratungstate solution placed in the use of XH-300A computer ultrasonic microwave combination synthesis extractor, set the ultrasonic frequency of 25 K Hz, adjust the microwave and ultrasonic power, in the specified value, using ultrasonic wave with microwave heating, at 45, 65, 80, 90, 95 degrees Celsius for evaporation crystallization, after a certain period of time, while the heat filter. After filtration, the filter material was washed with deionized water for 2 to 3 times, and the filtered material was dried in an electric constant temperature oven at 100 °C for 2 h to obtain ultrafine ammonium paramate powder.

The experiment concluded that in the absence of mechanical or magnetic stirring, adding surfactant, in the optimum preparation conditions of APT ultrasonic frequency of 25 K Hz, power 1000 W, microwave power 700 W, temperature 80 °C, ammonium tungstate concentration of WO3212 g/L, CO time 15 min, fine crystal APT-4 with average particle size of 7.6 m, uniform particle size distribution and crystal integrity can be obtained by microwave ultrasonic combined crystallization method.

Microwave ultrasonic assisted evaporation and crystallization can be prepared by micro APT in high temperature and without the addition of surfactant, can influence the crystallization rate and preparation of surfactants micro APT process APT purity to overcome the disadvantages of traditional heating. However, this method is only suitable for small scale production, more suitable for the special needs of the private custom, the main reason is the large microwave - ultrasonic equipment manufacturers almost did not come out, there are also expensive.

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Bismuth Tungstate Photocatalyst Modification

 In recent years, because bismuth tungstate (Bi2WO6) has narrower band gap and wider visible range than titanium dioxide (TiO2), it has a great tendency to replace TiO2.

bismuth tungstate photocatalysis image

However, in practical applications, it was found that bismuth tungate is good, but there are some shortcomings, mainly in its photogenerated electrons and holes easy to complex, quantum efficiency is not high. Therefore, scholars believe that bismuth tungstate should be added with some elements to excite the photocatalytic potential of bismuth tungstate.

Some scholars have made Au (Ag) / Bi2WO6 nanocomposites with bismuth tungstate doped with gold or silver. The results show that the degradation efficiency of RhB solution is 40% using Au (Ag) / Bi2WO6 nano photocatalyst, and the efficiency is 99.2%, higher than that of pure Bi2WO6.

Some scholars have suggested that the Pt/Bi2WO6 composite can be used for photocatalytic degradation of Mo methyl orange by doping bismuth with platinum. The results show that the photocatalytic activity of Mo solution is reduced to 100% by using 0.3Pt/ Bi2WO6 composite photocatalyst, and the photocatalytic activity of the photocatalyst is still active and recyclable.

It is said that the nano crystalline Bi2WO6 powder is mixed with a small amount of TiO2 powder and is hydrothermally mixed. The structure of the photocatalyst is still Bi2WO6 crystal structure, however, the properties of the two compounds show good visible light response range and catalytic activity.

Of course, if you think the doped TiO2 material is too low, then change to the king of new material graphene, it is found that graphene is an excellent conductor, and graphene, as an efficient electron promoter, can be combined with Bi2WO6 catalyst to increase its light capture capability and enhance its catalytic capability. The results show that rGO/Bi2WO6 has a catalytic effect on dye wastewater. The decolorization rate is 92%, which is about 30% higher than that of pure bismuth tungstate.

Recently, some scholars have proposed a more comprehensive doping scheme, which is based on bismuth tungstate as the main catalyst and doped graphene and titanium dioxide. TiO2 assisted Bi2WO6 to generate more holes, and graphene assisted Bi2WO6 to capture electrons. By combining, the potential of bismuth tungstate is fully activated. It not only has a photocatalytic rate of 99% of methyl waste water, but also has strong degradation ability to pesticide wastewater and organic wastewater.

With the development of bismuth tungstate photocatalyst, more bismuth tungstate photocatalytic activity will be improved in the future. In addition, another point to mention is that by chemical methods bismuth tungstate in polyester or acrylic fiber load, it is more favorable for the recovery and reuse of bismuth tungstate photocatalyst in practice.

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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 hydr...