Application of surfactants in coal industry

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通用场景

With people's attention to low-carbon economy and global climate, the rational use of coal energy and energy saving and emission reduction measures are becoming more and more important, and it will be very necessary and timely to introduce relevant measures. Surfactants have been used in coal processing technology for a long time, but the basic research on some basic phenomena is just beginning. At present, due to the increasing tension of energy problems, it has promoted the extensive research on the application of surfactants in coal processing technology, and many laboratories have made progress and developed a series of products. These applications are discussed separately below.

The wetting of the coal surface

Fuller et al. described the heterogeneous nature of coal surfaces in their structural chemistry studies.

It is confirmed by immersion heat study that the water absorption of low grade coal is higher than that of high grade coal, so more polar particles enter. Minerals interact strongly with polar liquids such as water. Thus a higher impregnation heat is released. After alkali corrosion, the structure of coal is loosened significantly, and the chance of sulfide agent immersion is greatly increased.

The wetting heat of coal in liquid can be considered as a measure of the affinity of the reagent in the coal conversion process. The rate of equilibrium state of coal after wetting can be traced by microcalorimeter. For example, Wightman et al. demonstrated that the time to establish the equilibrium state increases with the increase of the ethanol chain length by studying the normal-structure ethanol homologues of C1-C12. It is suggested to reduce the wetting heat by increasing the ethanol chain length to limit the penetration rate of wetting heat into the coal structure.

For different kinds of coal, the measurement of the interfacial tension and contact Angle between fine coal particles and water is of great significance for the directional flotation process of coal and the production technology of more valuable oil/coal type concentrated fuel.

In the past, there was no suitable method to determine the contact Angle of irregular fine coal particles. More recently, Neumann et al. have developed a new technique for determining the contact Angle of secondary soot particles. This method examines the dissolution properties of a certain amount of solidifiable material, or in other words, base materials such as naphthalene, biphenyls and o-phenylphenol, of coal particles with particle size in the range of 25-300 microns before solidification, and quantitatively explains the impregnation or inhibition effect of the matrix material on the coal particles during the early solidification period.

Neumann et al. proved by infiltration technique that the wetting properties of coal depend on its carbon content, ash content and mineral impurity content. In addition, hydrophilic coal can be changed into stronger hydrophobic coal by adding surfactants. This is a necessary condition for the flotation of coal particles.

Coal dust removal

It is of great practical significance to improve the wettability of coal particles by using surfactant aqueous solution for the control of coal dust. Nonionic surfactants, such as sodium o-xylene alkyl sulfonate, have been used in this regard. Glanville et al. discussed various factors affecting the coal dust wetting rate, and the results showed that the wetting rate measured by Walket wetting rate experiment was mainly affected by the size composition of the coal dust at temperature, and the concentration and molecular structure of the special surfactants used. In the temperature range of 10℃- 40℃. The wetting rate increases roughly linearly with the increase of temperature. Similarly, the wetting rate is linear with the increase of the average particle size of coal dust at a specific temperature.

flotation

Coal obtained in the mining process is mixed with clays of varying granularity. These clays are easy to wash away from the surface of large granular coal, but there is a certain amount of small granular coal in the coal slime. This part of the coal in the sludge is usually recovered by the foam flotation process. This process relies heavily on the different surface properties of organic or inorganic compounds in the coal.

In the foam flotation process of coal, the foam is formed by passing air into the coal slurry, so that the coal slurry contains impure, removed very fine coal particles and water. The flotation of coal from mud or ore depends on its surface wettability and the contact Angle, which is the Angle between the solid and the inner surface of the bubble.

Three reagents are usually used in the flotation process, they are: (a) flotation collector or accelerator, (b) modifier, (c) blowing agent. The function of flotation promoter or accelerator is to promote the contact between coal particles and bubbles, that is, to form a thin film on the surface of the coal particles to make the coal particles hydrophobic. At the same time, it must be selective, and no film is formed on the surface of coal particles that are not being floatated. The flotation promoters or accelerators are usually kerosene and fuel oil.

The modifiers may be ph regulators, activators, flotation inhibitors, dispersants or flocculants. For example, a selective cationic polymer flocculant is a reactant of dimethylamine and epichlorohydrin.

Foaming agent is used to provide stable flotation foam, long lasting stable foam can be used to separate coal. But not so long that the bubble cannot burst, and post-treatment is needed.

Coal-water slurry

Transportation has become one of the main difficulties in coal application. Slurry with a solid content of fine coal greater than 55% can be difficult to transport with a slurry pump normally used, because when the solid content of the system exceeds 5%, the water and solids tend to separate, resulting in the accumulation of coal in various areas of the pump transport system. In addition, the dewatering of the mud causes the pump transport system to be blocked and retained. Since water is a major contributor to costs during transportation and processing operations, and at the same time during coal gasification, evaporation units require a large amount of heat. Therefore, reducing the weight percentage of water in the cement slurry is an ideal solution.

Firstly, the effect of coal particle size on mud stability was investigated. Funk designed a coal - coal water slurry: coal with a maximum content of 75% and a viscosity of less than 1000cps. The mud has the advantage of being able to be transported over long distances by pipeline and can be burned directly without dehydration. It is prepared by a method in which the particle size and its distribution are controlled according to a certain particle size distribution formula. The particle size distribution formula is very useful for providing new coal billets. The coal billet has a particle size distribution according to the formula given by Alred.

The coal/oil dispersion system is very useful for pipeline and tanker transportation, it can be burned directly in the oil furnace without the modifier, and is the basic step in the coal liquefaction process. In order to ensure satisfactory pumpability, storage and combustion characteristics of the more economical coal/oil mixture as a partial substitute for fuel oil in boilers and heating furnaces, stability is assessed by periodically acting to determine the specific gravity of the effluent.

COM stabilizers have been reported in many literatures. They all show that cationic surfactants are the most effective COM dispersants. The experiment shows that the coal particles are bonded by physical adsorption. The surface of coal must first be nucleophilic. For this purpose, COM stabilization requires cationic surfactants. This coordination configuration in which the ions with polar end-group functional groups are oriented towards the electron-donating site of the coal matrix and the end-extending hydrocarbon also conforms to the concept of network stabilization in the aforementioned COM. In recent years, under the guidance of these experimental results, coal water slurry technology has been widely used in improved oil-fired boilers in China.

Demineralization of coal

Surfactants are also used in coal pretreatment to remove inorganic components and to modify the coal to make it more suitable for reprocessing, such as liquefaction at low temperatures. In coal/cement slurries, the removal of ash reduces energy loss and loss. Various processes for removing minerals from coal are discussed in the references. Liotta, for example, has developed a way to crush and remove minerals at the same time. It is first treated with a tetravalent alkali sink, and then physically separated by any conventional separation technology depending on the specific gravity of the two substances.

The use of hydrocarbon emulsion for coal flotation can reduce the ash content of coal. This method uses a emulsion prepared from a hydrocarbon oil with a hydrophobic W/0 emulsifier and a hydrophilic surfactant. This process does not use large amounts of polymers and produces high yields of low-ash coal.