Application of Surfactant in Coal Industry

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. It will be very necessary and timely to introduce relevant information. Surfactants have been used in coal processing technology for a long time, but basic research on some basic phenomena has just begun. At present, due to the increasingly tense energy problem, extensive research on the application of surfactants in coal processing technology has been promoted. Many laboratories have made progress and developed a series of products. These applications will be discussed separately below.
Wetting of coal surface
Fuller et al. explained the heterogeneity of coal surface in the study of structural chemistry of coal.
It is confirmed by the research of immersion heat that low-grade coal absorbs more water than high-grade coal, and thus enters more extreme points. Minerals interact strongly with polar liquids such as water. As a result, higher heat of immersion is released. After the coal is corroded by alkali, the structure is significantly loosened, and the chance of immersion of the vulcanizing agent is greatly increased.
The heat of wetting of coal in the liquid can be considered as a measure of the affinity of the reagent during coal conversion. The rate at which the coal is wetted to establish an equilibrium state can be traced and measured with a miniature calorimeter. For example, Wightman et al. proved by studying the normal ethanol homologues of C1-C12 that the time for establishing an equilibrium state increases with the increase of the ethanol chain length. It is also recommended to reduce the wetting heat by increasing the ethanol chain length to limit the penetration rate of the wetting heat into the coal structure.
For different types of coal, it is of great significance to determine the interfacial tension and contact angle between the fine coal particles and water, 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 for measuring the contact angle of irregular fine coal particles. Recently, Neumann et al. developed a new technique for measuring the contact angle of sub-soot particles (with water). This method examines the dissolution characteristics of a certain amount of coagulation of coal particles with a particle size in the range of 25-300 microns before solidification, or in other words, matrix materials such as naphthalene, biphenyl, and o-phenylphenol. It also quantitatively explains the impregnation or inhibition effect of the easily solidified matrix material on coal particles in the early stage of solidification.
Neumann et al. used the infiltration technique to prove that the infiltration characteristics of coal depend on its carbon content, ash content and mineral impurity content. In addition, by adding a surfactant, the hydrophilic coal becomes a stronger hydrophobic coal. This is a necessary condition for flotation of coal particles.
Coal dust removal
The use of surfactant aqueous solutions to improve the wettability of coal particles has important practical significance for the control of coal dust. Non-ionic surfactants such as sodium o-xylene alkyl sulfonate have been used in this regard. Glanville et al. discussed various factors affecting the wetting rate of coal dust, and the results showed that the wetting rate determined by the Walket wetting rate experiment is mainly affected by the size composition of the temperature coal dust, and the concentration and concentration of special surfactants used. The influence of molecular structure. In the temperature range of 10℃--40℃. The wetting rate increases roughly linearly with the increase in temperature. Similarly, at a certain temperature, the wetting rate also has a linear relationship with the increase of the average particle size of coal dust.
Flotation
The coal obtained in the mining process is mixed with clays of various particle sizes. These clays are easily washed off the surface of coal particles with large particle size, but there is a certain amount of small particle coal in the coal slime. This part of the coal in the sludge is usually recovered by the froth flotation process. This process relies heavily on the different surface characteristics of the organic or inorganic compounds in the coal.
In the coal froth flotation process, the froth is formed by passing air into the coal slurry, so that the coal slurry contains impure and removed very fine coal particles and water. The flotation of coal from sludge or ore relies on the wettability of its surface and the contact angle, which is the angle formed by the contact between the solid and the inner surface of the bubble.
Three reagents are usually used in the flotation process. They are: (a) flotation promoter or accelerator, (b) modifier, and (c) foaming agent. The function of the flotation promoter or accelerator is to promote the contact between coal particles and air bubbles, that is, to form a film on the surface of the coal particles to be floated, making the coal particles hydrophobic. At the same time, it must be selective, without forming a thin film on the surface of coal particles that are not flotation. The flotation promoter or accelerator is usually kerosene and fuel oil.
The modifier may be a pH regulator, activator, flotation inhibitor, dispersant or flocculant. For example, a selective cationic polymer flocculant is the reactant of dimethylamine and epichlorohydrin.
Blowing agent is used to provide stable flotation foam, and the long-lasting stable foam can separate the coal. But it shouldn’t be so long that the bubble can’t burst, and post-treatment is needed.
Coal water slurry
The transportation problem has become one of the main difficulties in coal application. The slurry of fine coal with a solid content of more than 55% is difficult to transport with the commonly used slurry pump, because when the solid content of the system exceeds 5%, the water and solids tend to separate, thus in various areas of the pump transportation system Produce coal accumulation. In addition, the dehydration of the mud causes the pump transportation system to block and cause stagnation. Since water is the main contributor to the cost of transportation and processing operations, at the same time, during coal gasification, the evaporation device requires a lot of heat. Therefore, reducing the weight percentage of water in the cement slurry is an ideal solution envisaged.
People first investigate the influence of coal particle size on mud stability. Funk designed a coal-coal water slurry: the maximum content of coal is 75%, and the viscosity is less than 1000cps. This kind of mud can be transported in long distance pipelines and can be burnt 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. This kind of coal has a particle size distribution according to the formula given by Alred.
The coal/oil dispersion system is very useful for pipeline and tank truck transportation. It can be directly burned in oil burners without modifiers and is a basic step in the coal liquefaction process. In order to ensure that the more economical and practical coal/oil mixture, which is a partial substitute for fuel oil in boilers and heating furnaces, has satisfactory pumpability, storage and transportation and combustion characteristics, the stability is measured by periodic behavior. The proportion of the object to be evaluated.
Many literatures have reports on COM stabilizers. They all show that as a large class of cationic surfactants, they are the most effective COM dispersants; experiments have shown that coal particles are combined by physical adsorption. The surface of coal must first be nucleophilic. For this reason, cationic surfactants are required for COM stabilization. The coordination configuration in which the ion pair with polar terminal functional groups is oriented to the electron-donating site on the coal matrix and the hydrocarbons extending outward at the end is also in line with the aforementioned concept of stabilization of the network in the COM. In recent years, under the guidance of these experimental results, the coal-water slurry technology has been widely used in domestic improved oil-fired boilers.
Demineralization of coal
Surfactants are also used in the pretreatment of coal to remove inorganic components and modify the coal to make it more suitable for reprocessing, such as liquefaction at low temperatures. In coal/cement slurry, eliminating ash can reduce energy loss and loss. Various processes for removing minerals from coal are discussed in references. For example, Liotta has developed a method of crushing and removing minerals at the same time. It is first treated with a tetravalent alkali solution, and then physically separated by any conventional separation technique based on the difference in the specific gravity of the two substances. 
Flotation of coal with hydrocarbon emulsion can reduce the ash content of coal. This method uses a latex prepared from a hydrocarbon oil, a hydrophobic W/0 emulsifier and a hydrophilic surfactant. The process does not use a large amount of polymer and produces high-yield low-ash coal.