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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