# Underground Disposal of Coal Gauge Backfill in China
## 1 Introduction  
According to the statistics released by the International Energy Agency (IEA), the world's total coal production increased continuously from 2017 to 2019. However, due to the impact of COVID-19, the world's coal production in 2020 dropped to 7.721 billion tons, a reduction of 4.8%. Except for China, the coal production in many countries, including Indonesia, Russia, Australia, Mongolia, and the United States declined. The top ten countries in terms of coal production were China, India, Indonesia, the United States of America, Australia, Russia, South Africa, Kazakhstan, Germany, and Poland [1]. The world's total coal production rebounded in 2021, but is still not at pre-COVID-19 levels. China's coal production in 2021 amounted to 4.13 billion tons [2], 52.4% of the world's total. Offering robust support to the national economic and social stability of China, coal will remain its main energy source in the foreseeable future [3]. A large amount of coal gangue is generated during coal mining, accounting for 15-20% of the total coal production. The solid wastes discharged on the ground surface then form waste heaps. According to incomplete statistics, the accumulated coal gangue in China has exceeded six billion tons, with waste heaps numbering between 1500 and 1700 and occupying over 13,300 hectares. The annual increase of the coal gangue in China is 500 to 800 million tons (e.g., 795 million tons were produced in 2020) [4; 5; 6]. Coal gangue accumulated on the ground occupies large areas of land, causing severe air pollution and presenting other environmental hazards, including vegetation damage, water and soil loss, groundwater pollution, or even spontaneous combustion of waste heaps and landslides [7]. Given the environmental impact of coal gangue in the mining areas, many countries have elaborated their national gangue management regulations and strategies. Since the 1960s, developed countries, such as Germany and U.K., have issued plans to encourage the use of coal gangue to build materials in land reclamation, roads, and dams. Somewestern countries have established waste exchange systems, which makes waste exchange a resource [8]. In France, Germany, and the U.K., coal gangue was used as filling material for road embarkments or the road base of arterial highways [9]. The use of coal gangue for power generation started early in developed countries. For example, Germany and Holland began to build coal power plants alongside coal preparation plants in the 1970s to recycle coal gangue for power generation. As some developed countries become less dependent on coal over time, the projects utilizing coal gangue for power generation have stalled in recent years [10]. Coal gangue combined with sulfate-reducing bacteria (SRB) could be used as a microbial organic fertilizer [11]. Land reclamation using coal gangue has been proven successful in Hungary, the former Soviet Union, the U.K., and the U.S. Coal gangue can also be used as cover material [12; 13]. Although aboveground utilization of coal gangue in China has started only recently, an explosive development has occurred. Currently, about 140 million tons of coal gangue in China are annually used for power generation, which is equivalent to conserving approximately 38 million tons of standard coal [14]. Since the 1980s, China has consolidated the subgrade use of coal gangue as filling materials during road engineering [15]. Extensive theoretical and practical explorations of coal gangue recycling to prepare building materials and chemical raw materials have been conducted worldwide. Because of the high content of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\)[16], coal gangue has been used for manufacturing building materials, such as cement [17; 18], bricks [19; 20], lightweight aggregate concrete [21; 22], and glass-ceramics [23]. Coal gangue, rich in Si, C, and Al, is also used to develop aluminum salt chemicals [24; 25], silicon salt chemicals [26; 27], carbon chemicals [28], and reinforcing materials [29].