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Coal washing is a processing technique that takes advantage of the differences in the physical and chemical properties between coal and impurities (gangue) to effectively separate them using physical, chemical, or microbial sorting methods, thereby producing coal products of uniform quality suitable for various applications. Depending on the coal washing method, it can be divided into physical coal washing, physicochemical coal washing, chemical coal washing, and microbial coal washing, etc. Physical coal sorting involves separating coal from impurities based on differences in their physical properties such as particle size, density, hardness, magnetism, and electrical properties. The main physical sorting methods include ① gravity-based coal sorting, which encompasses methods such as jigging, heavy media separation, incline chute separation, shaking table separation, and air separation. ②Electromagnetic separation utilizes the differences in electromagnetic properties between coal and impurities for sorting; this method is not used in actual coal processing operations. Physicochemical coal washing—flotation (abbreviated as flotation) is a sorting method based on the differences in the physicochemical properties of mineral surfaces. There are many types of flotation equipment in use today, mainly including mechanically stirred flotation and mechanically unstirred flotation. Chemical coal washing is a process that utilizes chemical reactions to concentrate the useful components in coal and remove impurities and harmful elements. Currently, chemical methods are commonly used in laboratories for desulfurization. Based on the types of commonly used chemical agents and their respective reaction principles, they can be classified into alkali treatment, oxidation methods, and solvent extraction, among others. Microbial coal washing involves the use of certain autotrophic and heterotrophic microorganisms, which directly or indirectly utilize their metabolic products to leach sulfur out of coal, thereby achieving desulfurization. Physical coal washing and physicochemical coal washing technologies are commonly used in actual coal washing operations; they can effectively remove inorganic sulfur in coal (pyrite sulfur). Chemical coal washing and microbial coal washing can also remove organic sulfur from coal. The commonly used coal washing methods in industrial production at present include jigging, dense medium separation, flotation, etc.; in addition, dry coal washing has also seen rapid development in recent years. Generally, a coal preparation plant consists of the following main processes; the basic flow is shown in Figure 1: (1) Raw coal preparation: including the reception, storage, crushing, and screening of raw coal. (2) Sorting of raw coal: The main sorting processes currently used in China include a combined jigging-flotation process ； Heavy medium-flocculation combined process ； Threshing-gravity separation-flotation combined process ； Lump coal heavy medium – fines coal heavy medium cyclone separation process ； There are also single jigging and single medium separation processes. (3) Product dewatering: includes the dewatering of lump coal and pulverized coal, the dewatering of flotation concentrate, and the dewatering of slime. (4) Product drying: Using thermal energy to dry coal, generally employed in very cold regions. (5) Treatment of coal slime water. Figure 1: Process flow of coal cleaning principles. The role of coal washing: (1) Improving coal quality and reducing pollutant emissions from coal combustion. Coal washing can remove 50%–80% of the ash content and 30%–40% of the total sulfur content in coal (or 60%–80% of the inorganic sulfur). Using washed coal helps to reduce emissions of dust, SO2, and NOx; processing 100 million tons of thermal coal can generally result in a reduction of 600,000–700,000 tons of SO2 emissions, as well as the removal of 16 million tons of gangue. (2) Improve coal utilization efficiency and save energy. An improvement in coal quality will significantly enhance its utilization efficiency. Some studies show that a 1% reduction in the ash content of coking coal leads to a 2.66% decrease in the amount of coke required for iron production, and an increase of 3.99% in the utilization efficiency of ironmaking blast furnaces ； Using washed anthracite in ammonia synthesis can save 20% of coal ； For coal used in power generation, every 1% increase in ash content results in a decrease in calorific value by 200–360 J/g, and an increase in the standard coal consumption per unit of electricity generated by 2–5 g ； Using washed coal in industrial boilers and furnaces can increase thermal efficiency by 3% to 8%. (3) Optimize the product structure to enhance competitiveness. The development of coal washing helps transform coal products from a single structure and low quality to a variety of high-quality options, thereby improving the quality of these products. In our country, there are many users of coal, and the requirements regarding coal quality and variety are continuously increasing. In some cities, the sulfur content in coal is required to be less than 0.5% and the ash content to be less than 10%; without developing coal washing, it is impossible to meet these market requirements. (4) Reducing transportation waste: Since China’s coal-producing areas are often located far away from the economically developed regions with high coal consumption, the volume of coal that needs to be transported is large and the distances involved are long; the average distance for coal transportation is around 600 kilometers. By washing the coal, a large amount of impurities can be removed, and for every 100 Mt of raw coal that is washed, 9,600 Mt·km of transportation capacity can be saved. This post was last edited by ryn on 2009-2-17 16:58]