1. Status Quo of Small- and Medium-Sized Mines in China
At present, many SM mines in China still use outdated mining methods such as the retention method, room and pillar method, and crumbling method for ore body mining 
. Similarly, inefficient mining equipment such as wind-driven rock drills, electric rakes, and rock loading machines are still in use in many SM mines 
. Moreover, the mining areas of many SM mines in China face significant safety concerns, environmental pollution 
, and solid waste discharge, which seriously impact their economic benefits.
1.1. Current Status of Mining Methods and Equipment in SM Mines of China
A significant number of SM mines in China still rely on the room-and-pillar method for ore mining. As illustrated in Figure 1
A, the room-and-pillar method involves leaving pillars to support the roof, and its simplicity and applicability to horizontal or gently inclined stratified ore bodies make it popular in SM mines 
. However, the method’s drawbacks are evident: it results in ore loss due to the placement of pillars to support the roof, significantly reducing the recovery efficiency of the ore and having a negative impact on the economic efficiency of SM mines 
Figure 1. (A) Schematic diagram of room-and-pillar method; (B) Schematic diagram of shrinkage stoping mining method; (C) Schematic diagram of sublevel caving mining method.
The shrinkage stoping mining method (as shown in Figure 1
B) is also a commonly used mining method in SM mines in China. It is a layered mining method that operates from bottom to top 
. However, the disadvantages of the shrinkage stoping mining method are also significant. Firstly, it has a high accident rate, making it difficult to guarantee the safety of workers. Secondly, large mining equipment cannot be used, resulting in very low mining efficiency. Furthermore, the shrinkage stoping mining method leads to a high loss depletion rate of ore, which has a negative impact on the economic efficiency of the mine 
Compared to the shrinkage stoping mining method, the sublevel caving mining method is more widely used in SM mines because it does not require large extraction production equipment, and the mining and ore extraction equipment and processes are simple, with high production capacity 
. The sublevel caving mining method is a mining method that manages ground pressure by caving the surrounding rock, as shown in Figure 1
C. However, it also has its disadvantages, such as the large amount of quasi-parallel cutting work, low mechanization of construction, and high rate of loss depletion 
As mentioned earlier, most of the mining equipment used in SM mines is relatively simple and inefficient. For instance, the pneumatic rock drill, which is a rock drilling machinery powered by compressed air (as shown in Figure 2
A), has been widely used in SM mines in China due to its low cost and equipment investment, especially in the retention method 
. However, the extraction efficiency of the wind-driven rock drill is too low, and upgrading the extraction machinery, such as the rock drill cart (as shown in Figure 2
B), under the premise of improving the mining method, can greatly improve mining and excavation efficiency. Similarly, electric rakes and rock loaders (as shown in Figure 2
C) are commonly used in SM mines in China for ore transportation, but they also have low transportation efficiency, necessitating the upgrading of these devices to more efficient transportation equipment, such as the load haul dump machine (LHD) (as shown in Figure 2
Figure 2. (A) Pneumatic rock drill; (B) Rock drill cart; (C) Electric rake and rock loader; (D) LHD.
2.2. Current Status of Mine Tailings Discharge in SM Mines of China
Here shows the production, integrated utilization, and combined utilization of solid emissions (including solid waste) in China in 2005, 2010, and 2015. In 2015, the combined utilization rate of mining-related solid emissions such as fly ash and coal gangue was between 65% and 75%, while the combined utilization rate of tailings was relatively low, with an average utilization rate of only 50%, as depicted in Figure 3
A. Therefore, the comprehensive utilization of tailings has become a critical issue that needs to be addressed urgently (as illustrated in Figure 3
Figure 3. (A) Utilization rate of mining waste in 2015; (B) Tailings discharge.
Most SM mines in China currently discharge the low-concentration tailings slurry generated from the processing plant directly into tailings ponds (as shown in Figure 3
B), which is also the main method of managing mining waste in China 
. As of the end of 2012, there were more than 12,000 tailings ponds in use in China, accounting for over 50% of the world’s total. Furthermore, there were nearly 9100 tailings ponds with lax management and prominent safety hazards, posing significant risks to the environment and people’s lives 
. Therefore, it is urgent to improve the management of tailings ponds in SM mines in China.
2. Problems in Small and Medium Sized Underground Mines
2.1. Hidden Danger of Goaf Safety
Goaf collapses can cause severe environmental problems, such as surface collapse, which not only impacts the ecological and geological environment but also greatly affects the development of the local mining economy 
. For SM mines, managing and maintaining the mining area is more challenging and can result in serious safety hazards, such as cave-ins 
. According to available statistics, there are more than 100 mines in China with a goaf area of 1 million cubic meters or more, and the accumulated mining goaf volume is up to 356 million cubic meters Figure 4
Figure 4. Mine quantity distribution (goafs) and goaf volume distribution.
3.2. Hidden Danger of Goaf Safety Tailings Ponds
According to statistics, as of 2016, tailings ponds in China covered an area of more than 3 × 104
, including a significant amount of agricultural and forest land, causing immense pressure and a serious burden on society 
. Due to the strong acidity of tailings after flotation, tailings drainage water can cause immeasurable losses to occupied land, crops, surface water, underground water sources, and aquatic organisms. Moreover, long-term exposure of sulfide and other harmful components in acid tailings can produce a large number of harmful gases, which are more likely to induce dust pollution in strong winds 
Tailings ponds are not only a significant source of pollution but also a potential source of danger, which can lead to accidents or environmental safety hazards, as shown in Figure 5
A. When a tailings pond breaches, it can cause destruction of nearby vegetation, buildings, and pose a risk to the safety of residents. For instance, in 2019, the Córrego do Feijão Mine suffered a serious tailings pond failure (as shown in Figure 6
B), leading to severe pollution of rivers and soil along the route 
. This event resulted in the death of a large number of flora, fauna, microorganisms, and fish, and created drinking water difficulties for 250,000 people due to the presence of high levels of pollutants and heavy metal ions in the tailings 
Figure 5. (A) Tailing Pond; (B) Brazilian iron ore tailings dam break accident in 2019.
2.3. Depletion Losses of Ore
Most SM mines in China still use mining technologies from the last century, resulting in a significant amount of high-quality resources remaining in goafs. However, the safe and efficient recovery of residual ore resources has been a significant challenge in mining technology today 
. There are no typical cases of success at home and abroad to facilitate large-scale promotion, mainly due to the following reasons:
Firstly, goafs have a complex form and prominent safety risks. The interior of goafs is often crisscrossed and connected from top to bottom, making it prone to caving and collapse, leading to large-scale ground pressure activities 
. Ensuring the safety of residual ore recovery is, therefore, extremely challenging, which is a significant constraint.
Secondly, the endowment conditions of residual ore resources are complex, and the spatial morphology changes significantly. In some mines, the ore body occurrence changes from thin to thick, and the dip angle changes from horizontal to steep. The hanging wall of most of these mines has unstable rock masses, making mining technical conditions extremely complicated 
. In addition, years of disordered mining have resulted in numerous goaf groups, leaving a large number of high-grade residual mineral resources in the goaf groups and their edges, with different forms, uneven thickness, and great difficulty in safe recovery technology.
Finally, different types of residual ore resource mining technology require different approaches. The types of residual ore resources in most goaf groups mainly include top and bottom pillar, interpillar, and corner ore. Due to the different characteristics of residual ore resource endowment and distribution of mined-out areas in each panel, the corresponding residual ore recovery process is also different. Therefore, the selected technical scheme must be targeted, safe, reliable, feasible, and economical 
3. Suggestions for Small- and Medium-Sized Underground Mines
Prospects for the Application of Backfill Mining Methods
SM mines within China are plagued with poor levels of mining technologies and equipment, safety hazards and environmental damage caused by research, therefore, to address these problems suggest the following points to start with.
(1) Using backfill mining methods can not only increase the recovery rate of ore but also reduce the discharge of waste materials (such as tailings and waste rock) generated during the mining process. This approach can also increase the productivity and capacity of the mine, which has significant overall benefits. For example, the Shishudi Gold Mine in Henan Province, China, is currently conducting experimental backfill studies to explore the potential for practical applications of this mining method.
Therefore, SM mines need to choose the appropriate backfill mining method according to the actual situation of their mine, which can greatly improve mining efficiency while ensuring the safety of mining production. For instance, at the Shishudi Gold Mine in Henan, China, the upward horizontal cut and backfill stoping method is used, which can significantly increase ore recovery while ensuring safety.
(2) Therefore, the construction of a backfill system is essential for the successful implementation of the backfill mining method. The system is designed to ensure safe and efficient mining, manage mine site safety and address the issue of mining waste disposal 
. It is crucial for the future development of SM mines in China to actively build backfill systems, optimize the backfill mining process solutions and reduce backfill costs. Through the backfill system, the goaf (or stope) is backfilled using the backfill mining method, which can prevent subsidence and surface collapse caused by mining operations to a certain extent. If ecological cement is used in the filling system, it can reduce the emission of harmful gases such as carbon dioxide, have less impact on the environment, and greatly improve the compressive strength of the filling body compared to ordinary cement 
. Additionally, while building the backfill system, it is important to further strengthen the research on the comprehensive utilization technology of remaining tailings and waste rock to achieve the secondary utilization of mine waste. This can help solve the problem of high investment costs and large occupied areas of tailings storage 
(3) Depending on the resource endowment of each mine, it is important to develop highly efficient and cost-effective mining technology that is tailored to the existing mining conditions, while also introducing mechanized mining equipment that can maximize resource recovery efficiency and mining intensity 
. By doing so, the entire process of mining, excavation, loading, transportation, hoisting, and beneficiation can be mechanized, reducing labor costs and safety risks. The Shishudi Gold Mine is a typical example for other small- and medium-sized mines in China, as it actively seeks to improve mining methods while vigorously introducing advanced mining equipment.
(4) SM mines should actively manage tailings ponds and prevent potential safety hazards and environmental pollution from occurring 
. At the same time, they should also explore ways to reuse mine solid waste in an economically and environmentally friendly manner. Mine solid waste can be used as aggregate for backfill material in the backfill mining method (as mentioned earlier) 
and also as a valuable secondary resource. For example, it can be used as raw material in the production of building materials 
, glass ceramics 
, fertilizers 
, and other products. Proper management of tailings ponds and the reuse of mine solid waste can significantly reduce the environmental impact of mining activities and contribute to sustainable mining practices