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The Application of Surfactants in the Coal Industry

With increasing attention to low-carbon economies and global climate issues, the rational utilization of coal energy and energy conservation measures have become increasingly important. Therefore, an introduction to the application of surfactants in coal processing technology is necessary and timely. Surfactants have been used in coal processing technology for a long time, but basic research on some fundamental phenomena has only just begun. Currently, due to the growing energy crisis, there has been extensive research on the application of surfactants in coal processing technology, and many laboratories have made progress and developed a series of products. The following discusses these applications separately.

Wettability of Coal Surfaces

Fuller et al. elucidated the multiphase nature of coal surfaces in their study of coal structural chemistry.

Studies on coal immersion heat have confirmed that low-grade coal has greater water absorption than high-grade coal, resulting in more polar points entering. The interaction between minerals and polar liquids such as water is very strong, thus releasing higher immersion heat. After coal is corroded by alkali, the structure becomes significantly loose, and the chance of sulfurizing agents soaking in is greatly increased.

The wetting heat of coal in a liquid can be considered as a measure of the affinity of the reagent for the coal during the coal conversion process. The rate of establishing equilibrium after coal wetting can be traced by microcalorimetry. For example, Wightman et al. demonstrated through studies of C1-C12 normal ethanol homologues that the time to establish equilibrium increases with the lengthening of the ethanol chain. They also suggested reducing the wetting heat by increasing the ethanol chain length to limit the penetration of wetting heat into the coal structure.

For different types of coal, determining the interfacial tension and contact angle between fine coal particles and water is meaningful for coal directional flotation processes and the production of more valuable oil/coal-type concentrated fuels.

There was no suitable method for measuring the contact angle of irregular fine coal particles in the past. However, recently, Neumann et al. developed a new technique for measuring the contact angle of secondary coal smoke particles (with water). This method examined the dissolution characteristics of a certain amount of easily coagulated substances before the solidification action or, in other words, the matrix substance such as naphthalene, biphenyl, and ortho-phenyl phenol, among others, and quantitatively explained the coagulation or inhibitory effects of pre-coagulated matrix substances on coal particles.

Neumann et al. confirmed through wetting experiments that the wetting characteristics of coal depend on its carbon content, ash content, and mineral impurity content. In addition, adding surfactants can transform hydrophilic coal into more hydrophobic coal, which is a necessary condition for coal flotation.

Coal Dust Removal

Improving the wettability of coal particles using surfactant aqueous solutions is of great practical significance for controlling coal dust. Nonionic surfactants, such as sodium n-dodecylbenzenesulfonate, 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 Walker wetting rate experiment is mainly affected by the temperature, particle size composition of coal dust, and the concentration and molecular structure of the special surfactants

used. Within the temperature range of 10°C to 40°C, the wetting rate increases approximately linearly with increasing temperature. Similarly, at a specific temperature, the wetting rate increases linearly with the increase in the average particle size of coal dust.


Coal obtained during the mining process is often mixed with various clay particles of different sizes. While these clays are easily washed off the surfaces of larger coal particles, a certain amount of fine coal particles remains in the coal slurry. This portion of coal in the slurry is typically recovered through froth flotation processes. This technique heavily relies on the different surface properties of organic or inorganic compounds present in coal.

In the froth flotation process for coal, foam is generated by introducing air into the coal slurry, thereby incorporating impurities, removed ultrafine coal particles, and water into the foam. The flotation of coal from slimes or ores relies on the wettability and contact angle of the coal surface, which represents the angle between the solid and the inner surface of the bubble.

Three types of reagents are commonly used in flotation processes: (a) flotation collectors or promoters, (b) modifiers, and (c) frothers. The function of flotation collectors or promoters is to facilitate the contact between coal particles and bubbles, thereby forming a thin film on the surface of the coal particle to render it hydrophobic. Simultaneously, it must exhibit selectivity, ensuring that no film forms on the surface of coal particles not intended for flotation. Flotation collectors or promoters typically consist of coal oil and fuel oil.

Modifiers may include pH regulators, activators, flotation depressants, dispersants, or flocculants. For instance, a selective cationic polymer flocculant is produced from the reaction of dimethylamine with epichlorohydrin.

Frothers are employed to generate stable flotation foam, facilitating the separation of coal. However, the foam should not be too persistent to the extent that it cannot rupture, necessitating post-processing.

Coal-Water Slurry

Transportation has become one of the primary challenges in coal applications. Transporting slurries containing fine coal with a solid content greater than 55% is particularly difficult using conventional dewatering pump systems. When the solid content of the system exceeds 5%, water and solids tend to separate, leading to coal accumulation in various regions of the pump transport system. Additionally, the dewatering nature of slurries leads to clogging and retention in pump transport systems. Since water constitutes a major contribution to the cost of transportation and processing operations, and significant heat is required during coal gasification, reducing the weight percentage of water in coal slurries is an ideal solution.

Initially, the influence of coal particle size on slurry stability is examined. Funk designed a coal-water slurry with a coal content of up to 75% and a viscosity below 1000 cps. This slurry possesses the advantage of long-distance pipeline transport and can be directly burned without dewatering. It is prepared using a method that controls the particle size and its distribution according to a specific particle size distribution formula. Such a particle size distribution formula is highly beneficial for providing novel coal briquettes. These briquettes exhibit a particle size distribution as per the formula proposed by Alred.

Coal/oil dispersion systems are highly beneficial for pipeline and tanker transportation, enabling direct combustion in fuel furnaces without the need for modifiers. Moreover, they represent a fundamental step in coal liquefaction processes. To ensure that coal/oil blends, which serve as more economical substitutes for fuel oil in boilers and furnaces, possess satisfactory pumpability, storage, transportation, and combustion characteristics, their stability is evaluated by periodically determining the specific gravity of effluents.

Numerous studies have reported on COM stabilizers. They all indicate that cationic surfactants, as a major category, are the most effective COM dispersants. Experimental evidence suggests that coal particles are bound by physical adsorption. However, it is essential for the coal surface to exhibit nucleophilic properties first. Therefore, COM stabilization requires cationic surfactants. The coordination configuration, where ions with polar end functional groups are oriented towards the coal matrix and outwardly extended hydrocarbons, also conforms to the concept of stabilizing reticulated bodies in COM. In recent years, guided by these experimental findings, coal-water slurry technology has been extensively applied in improved fuel boilers domestically.

Coal Desalination

Surfactants are also utilized 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/water slurries, eliminating ash can reduce energy losses and consumption. Various processes for removing minerals from coal have been discussed in the literature. For instance, Liotta developed a method for simultaneously crushing and removing minerals. It initially treats the mixture with a tetravalent alkali solution, followed by physical separation using any conventional separation technique based on the different specific gravities of the two substances.

Using a latex of hydrocarbon compounds for coal flotation can reduce the ash content of coal. This method employs an emulsion prepared from a hydrophobic W/O emulsifier and a hydrophilic surfactant. The process yields high-volume low-ash coal without the need for significant amounts of polymers.

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