Screening Theory of Moist Fine Coal

　　With the rapid development of China's national economy, the requirements for coal preparation technology have undergone fundamental changes, and the demand for obtaining (secondary) energy from raw coal as raw material is continuously increasing. Most of the thermal coal in China's mining areas is characterized by high moisture content, large amounts of fine coal, and lower ash content with smaller particle size, making it suitable for a combined screening and washing process. That is, the low-ash fine coal screened out is directly supplied to power plants and other users, while the coarse coal is processed by washing to improve productivity and reduce coal preparation costs. However, during coal mining, due to coal seam water seepage, underground dust suppression spraying, and poor management, the external moisture of the mined raw coal often exceeds 7%. Moist fine coal, under the influence of external moisture and clay content, tends to stick together or adhere to the screen surface, resulting in low screening efficiency and deteriorated screening process. When the external moisture reaches 7%–14%, screening with 6mm or 3mm mesh is generally difficult to accomplish with vibrating screens. To solve the problem of dry deep screening of moist fine coal, extensive and in-depth research has been conducted domestically and internationally on the theory of dry screening of moist fine materials.

　　Regarding the evaluation of the difficulty of screening moist fine coal, the UK developed a method to assess the difficulty of dry screening of moist fine raw coal (Shi Deming, 1986), where the difficulty is measured by a processability index. They defined the processability index as the time (in seconds) taken to discharge 12 kg of 25–0 mm raw coal from a vibrating cone. The longer the time, the greater the processability index and the more difficult the dry screening. When the index exceeds 5 seconds, screening with an ordinary vibrating screen becomes very difficult. It was also pointed out that the processability index is related to coal particle size distribution, moisture content (especially surface moisture), content of argillaceous shale, and the clayification characteristic curve.

　　In the study of particle screening theory, as early as the 1940s, Gaudin proposed the single-particle screening probability theory in his book "Principles of Mineral Dressing," and at the same time, Targgart proposed the single-particle probability screening theory in his "Mineral Dressing Handbook." With the advent of vibrating screens, to describe particle motion laws, Klockhaus proposed the formula for average material velocity in the 1950s, which was further developed by Kluge, who proposed the upper limit theory of projection intensity Kv=3.3. In the 1960s, Swedish scholar Mogensen, after detailed research on the basis of single-particle screening probability theory, proposed the probability screening principle, laying the theoretical foundation for the later development of probability screens (Zhang Enguang, 1990). In the 1970s, French scholar Burstein proposed the equal-thickness screening principle, providing a theoretical basis for the later development of equal-thickness screens (Zhang Enguang, 1990). This principle adjusts the projection acceleration or screen surface inclination at different sections of the screen so that the material has greater projection intensity and movement speed at the feed end, promoting rapid stratification, while the projection acceleration decreases and discharge speed slows at the discharge end, increasing the screening opportunity for fine particles. This treatment keeps the material thickness basically consistent or decreasing along the entire length of the screen surface, i.e., equal thickness. In the 1980s, Zhao Yuemin and Chen Qingru broke through the single-particle screening probability theory proposed by domestic and foreign scholars and proposed a screening probability distribution model for particle groups along the screen length direction. They discovered an abnormal upward phenomenon in the screening distribution curve in the fine particle range during dry screening of moist fine raw coal caused by particle adhesion forces. In the 1990s, Beenken divided the material screening process into three zones: single particle, thin layer, and thick layer, proposing kinetic models for each zone; Vaisberg et al. (1990) regarded the material segregation on vibrating screens as an instantaneous "explosion" process and established a screening probability model. In 1994, Brereton et al. (1994) proposed the influence of screening machine process parameters on screening effects and put forward evaluation criteria.

　　①Brereton, T. et al., 1994, The influence of screen feed rate and length on performance characteristics.

　　Since the 1960s, the German company Hein Lehmann has proposed the principle of tension-relaxation screening for moist fine coal after decades of exploration. This principle uses the screen surface to alternately tighten and relax, causing the screen surface to bend and generate large acceleration and deformation of the screen openings. The material is vertically thrown up and moves forward in a jumping manner, which allows good loosening and stratification of moist fine materials, enabling smooth dry deep screening (Shi Deming, 1986; Schmidt, 1980; Walenzik, 1985).

　　In the 1990s, domestic and foreign scholars began exploring the microscopic adhesion mechanism of moist fine materials and its impact on the screening process. German scholars Norgate and Weller (1993) proposed an adhesion model for screening moist fine materials during their research on the screening process. Using the relationship between model parameters and feed moisture content, they successfully predicted the impact of feed moisture changes on product quality and dry screening equipment for iron ore. German scholars Hollinder and Hoberg (1994) studied the adhesion model of moist fine materials adhering to each other under the influence of surface moisture from the perspective of physical and chemical properties of material surfaces and proposed several methods to reduce the adhesion and bonding forces between moist fine materials. Chinese coal preparation workers summarized the empirical method of low-frequency large-amplitude dry screening of moist fine coal from extensive practical experience (Hao Fengyin et al., 1993), and Zhao Yuemin and Liu Chusheng conducted further in-depth theoretical research.

　　The above content is shared by Xinxiang Kunlun Screening Machinery Design Co., LTD. People from all walks of life are welcome to learn from each other.