# Underground Disposal of Coal Gauge Backfill in China
## 2 The Current Status of Coal Gangue Discharge on the Ground in China  
Coal gangue, as a solid waste generated during coal mining and washing, is a black-gray rock with low carbon content, low calorific value, and hard texture. Coal gangue is subdivided into the gangue generated during underground drifting and mining and the gangue generated during washing and dressing [47]. In addition to containing 20-30% carbon, coal gangue is generally dominated by oxides: SiO\({}_{2}\) at 30-60%, Al\({}_{2}\)O\({}_{3}\) at 15-40%, Fe\({}_{2}\)O\({}_{3}\) at 2-10%, CaO, MgO, and K\({}_{2}\)O at 1-4%. There are also other trace elements such as boron and nickel. More coal gangue is generated in North and Northwest China than in South and Northeast China. The amount of coal gangue discharged in North China is close to 3.5 billion tons, which far exceeds that in other areas and accounts for about 50% of the total discharge [48]. As shown in Figure 2, the total amount of coal gangue generated in 25 coal-producing provinces of China from 2017 to 2021 was estimated as 2.80 billion tons. This estimate was made by conservatively assuming that gangue accounted for 15% of coal production. Besides, the coal gangue produced in these five years accounted for 40.0% of the total amount of coal gangue. The top ten provinces in terms of coal production generated 2.59 billion tons of coal gangue in the five years. As shown in Figure 3, darker areas on the map indicate greater coal gangue production. The coal gangue produced by the top ten provinces accounted for 92.6% of the total in the past five years, while that produced by Shanxi, Shaanxi, and Inner Mongolia accounted for 70% [3], which shows the characteristics of intensive coal gangue production.  
As the intensification and the scale of China's coal production have been rising, coal gangue production is increasing, and the centralized tendency of gangue production has become more pronounced. At present, most coal mines adopt two types of gangue disposal. The first one is to discharge the coal gangue aboveground and allow it to be utilized by other enterprises. However, the industrial and mining enterprises only utilize the coal gangue to a minimal degree due to the following factors: the high coal gangue discharge cost, scattered distribution of aboveground gangue disposal facilities, environmental protection policies applicable to gangue transportation, high gauge utilization costs, the complex process and technology of aboveground gangue disposal, and the high requirements for recycling due to the gangue composition [49; 50]. The second one is to dump the gagne at a specified site in or near the mining area, thus forming waste heaps of varying sizes [51]. These waste heaps occupy valuable land resources, and the water leaching from the gangue pollutes the surrounding soil and groundwater [52]. Besides, the coal gangue contains combustible components, which may ignite spontaneously under proper conditions, leading to instability of the coal gangue [53; 54]. Noxious gases generated by the spontaneous combustion, including sulfur dioxide, nitrogen oxides, carbon oxides, and smoke, can cause air and ecological pollution, which threatens local residents' health [55].  
Figure 3: The percentages of coal gangue production in different provinces of China from 2017 to 2021.  
Figure 2: The annual coal gangue production in different provinces of China from 2017 to 2021.  
The recycling of coal gangue on the ground can hardly meet the gangue disposal requirements. In contrast, underground gangue disposal technology has the benefits of high-efficiency, in-situ disposal, and the centralized distribution of disposal facilities. There is an urgent need to develop underground gangue backfilling technology, which meets the demand for centralized, high-yield, and large-scale gangue disposal.