With the concern for 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 the introduction of the relevant will be very necessary and timely. Surfactants have been used in coal processing technology for a long time, but basic research on some fundamental phenomena has just begun. At present, the increasing tension of energy problems has promoted the extensive research on the application of surfactants in coal processing technology.
- Wetting of the coal surface: low-grade coals are more water-absorbent than high-grade coals, and thus enter more polar nature points. Minerals interact strongly with polar liquids such as water. As a result, a higher heat of impregnation is released. When coal is corroded with alkali, the structure is significantly loosened and the opportunity for leaching of sulphide reagents is greatly increased.
The heat of wetting of coal in a liquid can be thought of as a measure of the degree of affinity of the coal for the reagent during conversion. The rate at which equilibrium is established after coal wetting can be determined by tracing with a microcalorimeter.
- Coal Dust Removal: The use of surfactant aqueous solutions to improve the wettability of coal particles is of great practical significance for coal dust control. Nonionic surfactants, such as sodium o-xylene alkyl sulfonate, have been used in this regard. The results proved that the wetting rate determined by Walket’s wetting rate experiment is mainly affected by the size composition of the coal dust at temperature, as well as the concentration and molecular structure of the particular surfactant used. In the temperature range of 10 ℃ – 40 ℃. The increase in wetting rate with increasing temperature is approximately linear. Similarly, the wetting rate increases linearly with the average particle size of the coal dust at a given temperature.
- Flotation: Coal obtained in the mining process is mixed with clays of various particle sizes. These clays are easily washed off the surface of the larger particle size coal grains, but a certain amount of small coal particles are present in the sludge. This portion of the coal in the sludge is usually recovered by a froth flotation process. This process depends to a large extent on the different surface properties of the organic or inorganic compounds in the coal. In froth flotation of coal, the froth is formed by passing air through a slurry of coal, which contains impure, removed very fine coal particles and water. The flotation of coal from sludges or ores is based on the wettability of the surface and the contact angle, which is the angle created by the contact between the solid and the inner surface of the bubble. Three types of reagents are commonly used in the flotation process, which are (a) flotation promoters or accelerators, (b) modifiers, and (c) blowing agents. The function of the flotation promoter or accelerator is to promote the contact between the coal particles and the air bubbles, i.e., to form a thin film on the surface of the coal particles to be flown, making the coal particles hydrophobic. At the same time, it must be selective so that it does not form a film on the surface of coal particles that are not being flown. Flotation promoters or accelerators are usually paraffin and fuel oil.
- Coal water slurries: Transport problems have become one of the major difficulties in coal applications. Slurries containing fine coals with a solids content greater than 55 per cent are difficult to transport by commonly used slurry pumps because water and solids tend to separate when the solids content of the system exceeds 5 per cent, thus creating coal aggregates in various areas of the pump delivery system. Also, the dehydrating nature of the slurry clogs and stalls the pump delivery system. As water is a major contributor to the cost of transport and processing operations, and in coal gasification, the evaporation unit requires a large amount of heat. Therefore, reducing the weight percentage of water in the cement slurry is envisaged as the ideal solution.
- Demineralisation of coal: Surfactants are also used in the pre-treatment of coal to remove inorganic components and to modify the coal to make it more suitable for reprocessing, e.g. liquefaction at low temperatures, etc. In coal/cement slurries, the elimination of ash reduces energy loss and losses. Flotation of coal with a hydrocarbon emulsion reduces the ash content of the coal. This method uses an emulsion prepared from hydrocarbon oil with a hydrophobic W/0 emulsifier and a hydrophilic surfactant. The process produces high yields of low ash coals without the use of large amounts of polymers.
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