2019年7月17日星期三

Bismuth Tungstate Preparation by Hydrothermal Method

Bismuth tungstate is a new type of photocatalyst which has attracted much attention in recent years.

It has not only good catalytic performance, stable photocatalytic activity, but also non-toxic environment. It is becoming a new hot spot in the field of photocatalyst.

The liquid phase methods for preparing bismuth tungstate include solvothermal method, precipitation method, microemulsion method, hydrothermal method, etc.. Among the many methods, hydrothermal method has become the most popular method because of its simple operation process, high utilization ratio of raw materials, less energy consumption, and conforms to the policy of energy saving and emission reduction.

bismuth tungstate preparation by hydrothermal method image

The working principle of the hydrothermal method is in a closed reaction vessel special (autoclave), using aqueous solution as reaction medium, heating the reaction vessel, creating a high temperature and high pressure reaction environment, the insoluble substances dissolved in normal temperature and recrystallization.

The main advantages of preparing bismuth tungstate by hydrothermal method: 1. The required reaction equipment is simple; 2. An optimum redox environment can be formed by controlling the reaction atmosphere through an autoclave; 3. The quality of bismuth tungstate is stable; 4. The hydrothermal method can effectively control the grain size and morphology of bismuth tungstate because of isothermal, isobaric and solution conditions; 5. Hydrothermal method does not require calcination and the addition of too many toxic agents, it is more environmentally friendly.

Some scholars used hydrothermal method to produce bismuth tungstate, and sixteen alkyl three ammonium bromide (CTAB) was used as surfactant. By adjusting the pH value in the solution, Bi2WO6 with different structure and morphology was prepared, the samples prepared by XRD, SEM and BET were characterized, the photocatalytic degradation of the prepared samples was also investigated, the optimum temperature of bismuth tungstate prepared by hydrothermal method is 180 °C, and the optimum pH value is 9.5.

bismuth tungstate preparation by hydrothermal method image

Hydrothermal method is the main method for preparing bismuth tungstate at present. Interestingly, the crystal morphology of bismuth tungstate obtained by hydrothermal method is different at different temperatures and different mineralizer, there are square, sheet, spiral, bird's nest shape, plum balls, various forms of bismuth tungstate is different, the range of use is also slightly different. 

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Bismuth Tungstate Preparation by Solid Phase Method

Bismuth tungstate is a new photocatalyst developed in recent years.

It has the characteristics of narrow band gap, good catalytic performance, stable photocatalytic and environmental protection, and has become a new hot spot in the field of photocatalyst.

bismuth tungstate preparation by solid phase method image

The preparation methods of bismuth tungstate mainly include solid phase method and liquid phase method. In general, they are fire process and wet process. Solid phase method is the preparation of bismuth tungstate by sintering at high temperature, compared with the liquid phase method, the solid phase method is more adopted by enterprises. Because most of the smelting enterprises adopt solid phase method to obtain tungsten powder, calcination equipment can be used in conjunction with each other, and the equipment foundation is the reason why solid phase method is more popular. The synthesis principle of bismuth tungstate solid phase method is mechanical processing containing Bi and W elements in the oxide material or its salt, then, the product obtained by mechanical processing and mixing directly through high temperature sintering can obtain bismuth tungstate products.

Some scholars believe that the preparation of solid phase preparation of the ambient temperature is the key to determine the preparation of bismuth tungstate, thus, he tested the 573K, 673K, 773K, 873K, 973K five temperature. Finally, the optimum calcination temperature of bismuth tungstate photocatalyst is 873 K, too low or too high calcination temperature will decrease the activity of bismuth tungstate photocatalyst.

Solid phase process is a relatively mature method for preparing bismuth tungstate, but its inadequacy is also accepted, but its deficiency is also obvious. The main reason is that the process needs to be heated by high temperature, and the energy consumption is greater, what is more serious is the size of the particles, the specific surface area and other factors, the performance of bismuth tungstate catalyst prepared by solid phase method is relative poor to liquid phase. Therefore, in recent years, the use of solid phase method of bismuth tungstate enterprises gradually reduced.

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Bismuth Tungstate--Dyeing Wastewater Treatment with Acrylic Composite Fiber

Dye pollution is one of the most important environmental pollution sources in the world.

Dye wastewater has the characteristics of complex composition, high chromaticity, large amount of emission, toxicity and biochemical degradation. It has been a difficult problem in wastewater treatment.

In recent years, with the development of photocatalytic technology, more and more photocatalysts have been developed. Bismuth tungstate has been widely used for its excellent photocatalytic degradation ability, there is no doubt that bismuth tungstate has better degradation effect on dye wastewater. However, in recent years, it has been found that bismuth tungstate is good, but it is easy to assemble, difficult to separate and difficult to recover. In order to solve this problem, some scholars have proposed a scheme for making composite materials of bismuth tungstate and acrylic fiber.

Acrylic fiber is also called polyacrylonitrile fiber, also known as synthetic wool, and bismuth tungstate does not seem to be related, how are they made of composite materials, and what effect will it have?

wastewater treatment image

The researchers used wet spinning technology to prepare bismuth tungstate-acrylic composite fiber, the general process is as follows:

At room temperature, 20 mL DMSO was added to the conical flask, and five water bismuth nitrate and two sodium tungstate were added into the flask according to the amount of substance 2 to 1, and each ultrasonic was dispersed for a period of time. Adding acrylic fiber, stirring with magnetic force for14 h, prepared into a mass fraction of 10% of the spinning solution, prepared into a mass fraction of 10% of the spinning solution. In 40% dimethyl sulfoxide aqueous solution for wet spinning, after that, the obtained composite fiber is placed in a hot air drying box and dried at 35 °C.

The product began to rise from 40 degrees of temperature, with a certain temperature gradient, heating up to the specified temperature, The bismuth tungstate acrylic fiber with photocatalytic property was obtained by constant temperature reaction for10 h.

What are the advantages of bismuth tungstate acrylic fiber prepared by such a great deal of setbacks? The experimental results show that the bismuth tungstate acrylic fiber not only continues the photodegradation characteristics of bismuth tungstate, the defects of bismuth tungstate for difficult separation and difficult recovery of dye wastewater are also improved, especially for methylene blue wastewater, the photo degradation ability of bismuth tungstate acrylic fiber reaches a small height.

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How to Achieve Bubbles Adsorption by Tungsten Ore Flotation

In the process of tungsten ore flotation, under the action of flotation reagent, bubbles appear on the surface of flotation machine, the target ore particles begin to float, and gradually enriched and adsorbed in the bubbles.

People get the needed ore particles by collecting and processing the bubbles, in this process, how does the flotation bubble adsorb ore particles?

Generally speaking, tungsten ore particles and bubbles are close to each other, the water between the cracks should be removed first, because the water molecules are disordered freely, so it is easy to be removed, when the bubbles and the ore particles are further approaching, the hydration film on the ore surface is thinned by the extrusion of the bubbles, the process of sticking ore particles to bubbles can be divided into three stages.

tungsten ore flotation image

The first stage is the approaching and contacting stage of the ore particles. In the process of flotation, due to the mechanical and aerating effects of flotation, the ore particles and bubbles are constantly colliding, the particle bubble between ordinary aquifers are crowding out, until the hydrated film on the surface of the ore particles contacts the hydrate film on the surface of the bubble.

The second stage is the thinning and fracture stage of the hydration film between the ore particles and the bubbles, because there is a stable hydration film on the surface of the ore particle, a similar hydration film is also present on the bubble surface, when the particles and bubbles close to each other and the hydration film of each other is thinned, at last, the hydration film becomes very unstable and finally causes a large range of rupture. 

The third stage is the attachment stage of the ore particles on the bubble. When the ore particles contact the bubbles, most of the hydration film are squeezing out from the ore surface, and the surrounding ore particles are gradually expanded, and the ore particles begin to adhere to the bubbles. 

At the ore dressing plant, technical personnel will be arranged in the flotation machine on the edge of the regular observation bubble reaction process, because the degree of adsorption of the bubble often determines the amount of the final harvest of tungsten ore, if abnormal situation is found, it is also convenient and timely to follow-up. Of course, how to achieve the bubble adsorption process perfectly is hard to say, all the details determine success or failure. Usually in the use of flotation agent, temperature, mineral paste adhesion firmness, mineral water hydrophobic size, mineral particle geometry is also an important factor in determining the bubble adsorption effect.

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2019年6月24日星期一

Zirconium Tungstate Thin Film

A thin film material is a film material with a thickness less than 1 microns. It is mainly used for semiconductor functional devices and optical coatings.

At present, the field has been involved in computer storage equipment, pharmaceuticals, high temperature insulation products, thin-film batteries, solar cells etc..

Compared with traditional block materials, thin film materials have incomparable advantages, such as small grains, large specific surface area and special surface structure. Therefore, zirconium tungstate is a kind of excellent negative thermal expansion material. The thin films prepared with zirconium tungstate can be conveniently used in the compensation layer of thermal stable structures or into large capacity heat driven devices, This is also the other general material is difficult to take advantage of it, so these years the zirconium tungstate thin film or zirconium tungstate composite thin film has also been concerned and research hot spots. 

It is well known that zirconium tungstate is mainly produced by the synthesis of ammonium paratungstate and zirconium oxychloride, and the difficulty of making the powder is a major obstacle to the popularization of zirconium tungstate thin film.

zirconium tungstate thin film solar cell image

The research shows that the methods of preparing zirconium tungstate thin film are mainly current beam evaporation powder method, wet chemical method, radio frequency magnetron sputtering method, alternating radio frequency magnetron sputtering method. Among them, the evaporation process of the current beam evaporation method is not well controlled, and the WO3 is easy to volatilize at high temperature, the stoichiometry of the film element is inconsistent with the Zr2W2O8 target; The purity of zirconium tungstate thin film made by wet chemical method and radio frequency magnetron sputtering method is not very high; The only thing worth mentioning is the alternating magnetron sputtering method based on RF magnetron sputtering, the method of preparation of zirconium tungstate thin films on quartz substrate deposition and annealing process, the sample obtained is cubic phase zirconium tungstate thin film, the purity and quality are improved, but the corresponding increase in procedure leads to higher cost.

To sum up, the zirconium tungstate thin film will take a place in the future high and new technology field with its excellent performance, but immature, inexpensive schemes also limit its use. However, as a new thing, the study of zirconium tungstate thin film is still in the initial stage, it is worth looking forward to the future.

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Bismuth Tungstate Photocatalytic Degradation of Tetracycline Wastewater

China is a major producer of antibiotics and one of the most serious countries that abuse antibiotics.

Livestock farming is the hardest hit by the use of antibiotics, with the rapid development of aquaculture, the residues of antibiotics in water, sediment and soil become more and more serious.

Tetracycline is the most common antibiotic used in livestock farming. Its main characteristics are broad-spectrum antibacterial and cheap, unlike other organic wastewater contamination, tetracycline and other antibiotic compounds usually have a shorter degradation half-life in the environment, however, the use of tetracyclines in livestock and aquaculture has never ceased to allow them to enter the environment, manifested as "persistent". Such a consequence is that antibiotics selectively inhibit some environmental microorganisms in the environment and induce the emergence of some antibiotic resistance groups, thus triggering a particular ecotoxicological effect, this is why some large pharmaceutical factories and farms become the main pollution factors and ecological extinction of the surrounding rivers.

In recent years, photocatalytic degradation technology has been paid more and more attention to environmental pollution control, and photocatalytic treatment of antibiotic pollution seems to be a good solution, thus, some areas began to use photocatalytic technology to control tetracycline pollution.

bismuth tungstate photocatalytic degradation of tetracycline wastewater image

The photocatalysts include tungstate series, titanium series and bismuth series. Tungstate is one of the best known photocatalysts, and bismuth tungstate is one of the best. In recent years, bismuth tungstate photocatalyst has attracted much attention in the field of environmental protection, mainly due to its following advantages: (1) Good UV and visible light response; (2) Thermal stability; (3) Photocatalytic stability; (4) The material sources are easy and the cost is relatively low; (5) Environmental friendly.

Some scholars used bismuth tungstate catalyst and doped with small amounts of ZnFe2O4 to degrade tetracycline wastewater, in the process of photocatalytic reaction, the oxidation of hydroxyl radical promotes the deamination, demethylation and other oxidation reactions of tetracycline, in the long photocatalytic reaction time, the intermediates of the macromolecules will eventually be mineralized into CO2, H2O and NH4+, and the photocatalytic degradation efficiency will reach 98%.

Studies have shown that bismuth tungstate is the bane of antibiotic wastewater such as tetracycline, and its importance to human survival is self-evident. As we all know, avian influenza and super drug resistant bacteria occur frequently in recent years, which is associated with the abuse of antibiotics in the aquaculture industry. We do not expect the pursuit of profit businessmen can find their conscience, but only hope that the rivers and lakes, the sea can be more clean, the development of light degradation technology also allows people to have more expectations of the environment.

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Bismuth Tungstate Preparation by Microemulsion Method

Bismuth tungstate (Bi2WO6) is an oxide with perovskite like structure, and is a typical n type semiconductor. Bismuth tungstate has become a new hot spot of photocatalysis because of its unique crystal structure.

The preparation methods of bismuth tungstate include solvothermal method, precipitation method, microemulsion method, hydrothermal method, etc.. Among them, microemulsion method is a new method for preparing ultrafine powders, which can be used to prepare nanometer bismuth tungstate, which is relatively simple and the powder morphology is easy to control.

Microemulsion refers to two mutually insoluble reaction solutions which form isotropic and thermodynamically stable emulsions under the influence of surfactants, the reactants are nucleated, agglomerated, agglomerated, crystallized and grown in tiny vacuoles and then treated at high temperatures (usually less than 240 degrees) to form nanoparticles.

Microemulsion can be divided into three types: water-in-oil and oil-in-water type, intermediate state, also known as positive phase microemulsion (O / W), reverse microemulsion (W / O) and double continuous phase microemulsion.

bismuth tungstate preparation by microemulsion method image

Some scholars have prepared Bi2Wo6 photocatalyst by microemulsion mediated hydrothermal method: The oil phase of n-butanol and cyclohexane is mixed with an aqueous phase consisting of dilute nitric acid solutions and Na2WO4 solutions, and the magnetic stirring for 15 minutes, forming precursor, adjust pH value, pour into the high pressure reactor, the hydrothermal reaction was carried out at a temperature of 180 °C for 24 hours, the product is naturally cooled to room temperature, and the product is dried at 80 °C by centrifugal separation and washing, and then Bi2WO6 photocatalyst is obtained.

Microemulsion is characterized by the formation of tiny reactors under the action of amphiphilic molecules of surfactant, which prevents the formation of agglomerated nanocrystals. Compared with other liquid phase methods, microemulsion method has the advantages of simple process, good product dispersion, and the surface active agent can be used to modify the surface of nanomaterials, the prepared bismuth tungstate has controllable particle size and uniform morphology, so the photocatalytic property of bismuth tungstate produced by microemulsion method is highly recognized.

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