## 2 Status Quo of Small- and Medium-Sized Mines in China
### 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 2A, 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 [12]. 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 [13].  
The shrinkage stoping mining method (as shown in Figure 2B) is also a commonly used mining method in SM mines in China. It is a layered mining method that operates from bottom to top [14, 15]. 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 [16, 17].  
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 [18]. The sublevel caving mining method is a mining method that manages ground pressure by caving the surrounding rock, as shown in  
Figure 2: **(A)** Schematic diagram of room-and-pillar method; **(B)** Schematic diagram of shrinkage stopping mining method; **(C)** Schematic diagram of sublevel caving mining method.  
Figure 2C. 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 [19].  
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 3A), has been widely used in SM mines in China due to its low cost and equipment investment, especially in the retention method [20]. 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 3B), 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 3C) 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 3D) [21].  
Table 1 shows the comparison of benefits between continuous and intermittent loading for a certain set underground mining site. The comparison results show that Scheme 4 is the most economical scheme, requiring only one pass, and also the scheme with the highest concentration ratio of production operations and the highest stope and labor productivity, therefore replacing outdated mining machinery with more effective equipment not only reduces mining costs, but also improves production efficiency and guarantees.  
Figure 3: (**A**) Pneumatic rock drill; (**B**) Rock drill cart; (**C**) Electric rake and rock loader; (**D**) LHD.