International comparison of the technical level of coal washing industry in China

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Abstract: A comprehensive comparison was made between the level of coal washing technology in China and the advanced international standards presented at the 14th International Coal Washing Conference. It was found that China’s coal washing technology is on par with international advanced levels, and it is already at the world-leading level in the field of heavy medium coal washing. However, the reliability of domestically produced coal washing equipment remains low, and China’s proportion of raw coal fed into processing plants as well as the type of plants used are below international advanced standards. The 8th issue of \"China Coal\" in 2002 published the author’s article \"An Overview of the 14th International Coal Washing Conference\", which provides an international comparison of the technical level of China’s coal washing industry. 1 Is the level of coal washing technology in our country comparable to international advanced levels? 1.1 The equipment developed independently in our country is already capable of meeting the needs for building coal washing plants with a capacity of 4 million tons per year. At present, our country has the capability to design and manufacture complete sets of equipment for such coal washing plants. In terms of specifications, the thickening machine has a capacity of 40 m²; the width of the heavy media separator tank is 4.5 m, the diameter of the heavy media cyclone is 700 mm, the volume of each tank in the flotation machine is 16 m³, the area of the screening machine is 27 m², the area of the filter press is 1050 m², and the area of the filter is 200 m². These devices form a series that meets the needs of large-scale coal processing equipment. In terms of varieties, the range includes gravity separation, jigging, flotation, slime treatment, drying, dewatering, etc., offering sufficient options to meet the equipment requirements of different processes. In terms of technical level, many devices have approached or reached international advanced standards. The equipment developed independently is now capable of meeting the requirements for building new plants with an annual processing capacity of 4 million tons, as well as for the technical upgrading of existing plants, regardless of their type, coal quality, or processing processes. ? 1.2 The coal washing process technologies can basically meet the requirements of coal washing production (1) Jig coal washing. Based on in-depth theoretical and laboratory research, the SKT series and X series jigs have been developed using a variety of new technologies. Their ability to handle different coal qualities as well as their level of automation are comparable to those of similar foreign equipment; currently, further development is being focused on achieving greater intelligence and full automation. The self-developed dynamic screen jigging machines have initially formed a series, with their technical specifications and theoretical research results being close to or exceeding international advanced levels. ? (2) Heavy media. The large-scale, pressure-free and pressurized three-product heavy media cyclone coal washing system developed through research and development boasts technical specifications that exceed international advanced levels, and it has been widely adopted. The large-scale heavy medium separators developed in our country offer better separation performance compared to the “TESKA” separators of the same type, and a series of such separators has been created. The 50–0 mm heavy medium cyclone separation process and equipment, which were developed first in the world, along with the subsequently developed -?0.5? mm grade slime heavy medium cyclone separation process, have simplified the separation process and achieved excellent technical results. The air-heavy medium fluidized bed dry coal washing system developed independently in our country was the first of its kind to be applied in industrial production on an international scale. (3) Flotation. On the basis of further improving traditional flotation processes and equipment, various series of mechanically stirred and jet cyclone flotators have been developed. At the same time, in-depth research has been conducted on various flotation columns and highly selective flotators designed to enhance the flotation efficiency of fine-grained materials, with significant progress achieved in this area. ? (4) Slurry water treatment. Based on the widespread use of filter presses, the concept of high-pressure air-piercing filter pressing for dewatering was proposed, and 100 m² coal refining filter presses were widely adopted. After more than 20 years of research and development, a series of disc pressure filter machines has been developed. Their technical specifications are comparable to those of similar international equipment, and they have been put into widespread use. New coal slime recovery processes and equipment, such as the large-scale horizontal sedimentation centrifuges, flat-belt filters, and large-scale disc vacuum filters developed, have also been put into use in industrial production. ? (5) Bituminous coal preparation, screening, and dewatering. Large multi-tooth roller crushers and high-power grading crushers have been developed and put into use, and large grading screens with an area of over 16 m² are also being employed in coal preparation plants ； A series of vibration dehydration screens with shifted centers of gravity were developed using modal analysis technology and have been widely deployed ； Energy-saving high-frequency vibration electromagnetic screens, as well as various high-frequency screens, have also been put into use in production. ? In addition, certain achievements have also been made in areas such as composite dry coal washing and the drying and crushing of slime filter cakes. Laboratory research has also been conducted on new technologies such as high-gradient magnetic separation, selective flocculation, frictional electrostatic coal sorting, as well as microwave and chemical desulfurization. ? 1.3 Automatic monitoring and control, as well as centralized control of the entire plant, have been achieved in the main stages of coal washing production. The automated instruments and sensors, computer software, and automatic control devices developed in China in recent years enable automatic monitoring and control of the key production stages in coal washing plants, as well as centralized control of the entire plant. “The projects completed during the 8th Five-Year Plan period, such as \"automatic control of key parameters of jig machines,\" \"online monitoring and control technology for product quality in flotation systems,\" \"online monitoring and control technology for product quality in heavy media systems used for tail coal processing,\" as well as \"standalone automation systems and industrial control network systems,\" together with the heavy media density control systems, computer-based centralized control systems for entire plants, and a large number of primary instruments and sensors that are already widely used in coal processing plants, have enabled the automation of these plants to evolve gradually from simple start/stop control to optimization and control of process parameters. ? 2 The heavy medium coal sorting technology is at the international leading level. A comparison of the technical performance of heavy medium cyclones for secondary products worldwide is shown in Table 1. As can be seen from this table, except for a slightly larger diameter, the other technical specifications of China’s heavy medium cyclones for secondary products are not significantly different from those of foreign ones. ? Table 1 Comparison of technical performance of two-product heavy media cyclones internationally ? (Omitted) Currently, only China, Russia, and the UK have developed three-product heavy media cyclones; 11 different models are actually in use in the coal processing industry, of which 3 are from Russia, 7 from China, and 1 from the UK. The main performance parameters of domestic and foreign three-product heavy-media cyclones are listed in Table 2. As can be seen from Table 2, the 3NWX1200/850 type three-product heavy medium cyclone developed in China is currently the largest in size and most advanced in technical specifications among pressure-free feed three-product heavy medium cyclones to be used in industrial production on an international scale. In terms of three-product heavy medium cyclones, China is at the international leading level in terms of both diameter specifications and other parameters. Table 2 Comparison of the main technical parameters of three-product heavy-media cyclones at home and abroad? (Omitted) In terms of process design, China has developed large-scale, pressure-free feed three-product heavy-media cyclones as the primary equipment, along with smaller-diameter heavy-media cyclones. By using a suspension of one density and a single medium system, it is possible to implement a simplified heavy-media coal sorting process that enables effective separation of raw coal into three fractions ranging from 85 to 0.1 mm without any grading process. This approach addresses the challenge of maintaining the advantages of heavy-media coal sorting while reducing investment and processing costs; it also introduces a process in which a single medium system is used for the heavy-media separation of coarse slime. Compared with typical international heavy media coal washing processes, this technology eliminates the need for sludge removal, classification, high-density suspension preparation systems, and high-density separation systems. It reduces the amount of flotation required, thereby saving more than 50% in equipment and space in the main processing plant; investment costs are reduced by over 50%, and processing costs are cut by 20%. As a result, the economic efficiency of heavy media coal washing is **improved**. In terms of the automatic monitoring and control systems for the heavy medium coal washing process, China has utilized the dynamic equilibrium relationships among various parameters in this process to derive a mathematical model for multi-parameter dynamic intelligent control of heavy medium suspension density, a approach that has not yet been reported abroad. At present, the relatively advanced automatic monitoring and control system for heavy media coal washing on the international stage is the “53SU1000” type system developed by the American company PTI; however, the control algorithm used remains PID control. The self-correction control methods and techniques for the density of heavy medium suspensions in our country are more precise than the traditional PID methods used domestically and internationally. They maintain the control accuracy of the heavy medium suspension density at ±0.006 kg/l, which represents a 40% improvement in precision compared to the ±0.01 kg/l achieved abroad. Adopting an artificial intelligence-based control method for refined coal ash content enables online feedback control of the refined coal ash content. No reports have been found indicating that automatic measurement and control systems for heavy medium coal washing abroad possess this function. ? 3 There is still a significant gap in the reliability of domestic coal sorting equipment. Through inspections and visits, it was found that the reliability of such equipment lags far behind international advanced levels. Due to the relatively low overall industrial level in our country, the raw materials and manufacturing processes used for large-scale coal washing equipment and automated components are backward, resulting in poor reliability of such equipment; the average reliability of coal washing equipment in our country is only around 70%. Especially after joining the WTO, how the domestic coal washing equipment manufacturing industry can cope with the competition from foreign manufacturers and rapidly improve the reliability and technical performance of its products is a major challenge we face. ? 4 The proportion of raw coal processed in our country, as well as the type of plants used, are below international advanced levels. (1) The proportion of raw coal processed is low, resulting in a high ash content in the refined coal. Currently, the average coal selection rate across the country is 32%, with state-owned key coal mines accounting for 46.7%, local state-owned coal mines at 27.6%, and township coal mines at 9.1% – a figure that is far below the 50%–95% level seen in other countries. The average ash contents of coking coal and commercial thermal coal in our country are 9.88% and 22.43%, respectively, while in the United States they are only 7% and 17% during the same period. ? (2) The coal preparation plants are small in scale, resulting in low production efficiency. The average annual production capacity of coal washing plants across the country is 310,000 tons; among these, state-owned key coal mines have a capacity of 1.5 million tons, while rural coal mines have only 70,000 tons. In the United States, the average production capacity is 3.4 million tons. Due to the low level of automation and poor equipment reliability, coal preparation plants in China rely on a large number of workers, resulting in low productivity – 17 tons per worker – which is only 10% to 15% of that in foreign countries. In summary, through the on-site visits and discussions at the 14th International Coal Washing Conference, it can be concluded that the relevant technologies in China’s coal washing industry are not far behind international advanced levels, and the country is at the international forefront in terms of heavy-media coal washing technology. However, the reliability of domestically produced coal washing equipment still lags significantly behind international standards, and the proportion of raw coal subjected to washing processes needs to be increased further.