Management of Thermal Hazards in Deep Mines in China: Applications and Prospects of Mine Cooling Technology

Abstract

With the continuous development of the mining industry and advancements in deep mining technology, mine environment optimization has become key to ensuring safety and improving the efficiency of mining. The high-temperature environment, particularly in deep mines, not only poses a serious threat to miners’ health but also significantly reduces operational efficiency. These issues have been determined based on the current application status and development trends of mine cooling technology, including traditional mechanical and non-mechanical cooling technologies, as well as emerging roadway insulation materials and mine cooling clothing applications. By comparing the advantages and disadvantages of each technology, the main challenges related to the use of current mine cooling technologies are pointed out, including the low energy efficiency ratio, high cost, and difficult implementation. Finally, this paper looks forward to the future development directions of mine cooling technologies, emphasizing the importance of intelligent, energy-saving, and environment-improving comprehensive system management and, in turn, promoting the progress and application of mine environment optimization technology and supporting safe and efficient deep mining.

1. Introduction

With the depletion of shallow surface resources and increasing mining depths, the challenges brought about by heat in deep mines cannot be ignored. Energy security, which is related to the overall solidity of a country’s economic and social development, is one of the most important contemporary issues [1,2,3]. In recent years, due to the impact of the COVID-19 epidemic and the international environment, the prices of major industrial energy sources, such as oil and natural gas, have skyrocketed. China is a country that is rich in coal, less so in oil, and poor in gas. In the future, coal will still dominate China’s energy consumption system [4,5,6]. According to statistics, coal resources within a 2000 m vertical depth in China amount to 5.57 trillion tons, and the resources buried above 1000 m account for 51% of the total [7,8]. As shown in Figure 1, the accident rate in China’s coal mines decreased significantly from 2016 to 2021 but showed an increasing trend thereafter, indicating that, with the development of mining work at deeper depths in recent years, the safety problems of some deep coal mines remain to be solved. In 2023, China’s total annual energy consumption rose to 5.72 billion tons of standard coal, with coal consumption accounting for 55.3% of the total energy consumption. It can be thus seen that China’s economic development still relies on coal as the main guarantor of sustainable energy power. Mine cooling technology is the core technology used to solve the problem of mine thermal damage. In the future, its technological development will be highly significant in the strategic application of deep resource mining in China [9,10,11].

With the continuous increase in mining depth, problems such as high temperature, pressure, and ground stress not only lead to a gradual increase in mining difficulties but also seriously affect the health and operational efficiency of staff [12,13]. In the high-temperature and -humidity environments of deep mines, the efficiency of traditional cooling technology has many shortcomings in terms of practical application, making meeting the actual cooling demand difficult. Due to the limited internal space in deep mines, it is very difficult to deploy effective cooling facilities in all the required areas. What is more, the technology consumes a lot of electricity and thus does not meet the requirements for developing green mining [14,15]. Therefore, optimizing the particular environments of deep mines and further developing mine cooling technology are major tasks that must be completed to ensure the sustainable development and supply of mineral resources in China. This study is based on the current application status and challenges of deep mine cooling technology, and predictions for future development trends are also explored. The causes of heat damage in deep mines are summarized, and the widely used mechanical and non-mechanical cooling technologies are introduced in detail. Their working principles, application effects, and existing problems are analyzed. In particular, we point out that although these traditional methods have somewhat alleviated the high-temperature problem of mines, the problems of, for example, high energy consumption, low efficiency, and environmental restrictions remain. In response to these challenges, this article outlines the future development directions of mine cooling technology, including the use of a variety of cooling methods in a combined mode to improve the thermal environments of deep mines, the use of renewable geothermal energy to regulate the temperature of roadways surrounding rock and coal deposits in order to optimize the underground thermal environment; the development of new high-efficiency cooling materials; and the use of intelligent ventilation systems to regulate the thermal environment. Through the comprehensive utilization of the above technologies and methods, we expect to achieve a more efficient and environmentally friendly mine cooling solution in the future and to guarantee the whole mining production process.

2. Main Heat Sources and Damage in Mines

2.1. Main Heat Source in Mines

Due to the different geographical environments and mining methods in different mines, the causes of the heat damage problems therein differ. However, we can summarize the reasons for these problems as follows [16]:

(1) Underground geothermal water heat release: The heat released from underground geothermal water is produced when groundwater is heated by the Earth’s internal heat source in the deep crust, and this heat is then carried up to the surface of the shallow crust. If this heat is released in the mine, it will cause the temperature inside the mine to rise [17]. Underground geothermal water not only increases the air temperature through heat exchange but also increases the air temperature and water content simultaneously through the evaporation of high-temperature water, resulting in a poor working environment within the mine [18,19,20].

(2) Heat release and airflow heat transfer from surrounding rocks and roadways: The increase in ground temperature with depth is called the geothermal gradient. With deeper mining, the global average temperature rises by about 2 °C for every 100 meters. The high-temperature surrounding rock releases heat into the roadways and increases the airflow temperature [21]. The temperature of the surrounding rock increases with the increased depth and its temperature rise rate is related to its thermal conductivity. When the mining depth exceeds kilometers, the temperature of the surrounding rock will be much higher than the surface temperature of the human body, and the heat released from the high-temperature surrounding rock becomes the main reason behind mine heat damage [22,23].

(3) Heat release from mechanical equipment: Underground workers are limited by their physical capacity, especially during coal mining activities such as tunneling, lifting, loading, and transportation. Mechanical equipment is used in these processes, converting useless work into heat energy. The heat energy generated from electric power is released into the air, contributing to the increased temperature in the mine [24].

(4) Oxidation heat dissipation of coal (coal gangue): The temperature of newly mined minerals is similar to that of the surrounding rock. As the temperature increases, the oxidation reaction rate also increases due to contact with oxygen, releasing heat energy into the air and raising the mine’s temperature [25,26,27].

(5) Other factors: Other factors such as underground workers, the climate, and the environment also contribute to heat damage in mines [28]. The heat generated by underground workers leads to increased temperatures in the roadways. The heat released by the human body is affected by working times and labor intensity. The greater the labor intensity, the more heat is dissipated [29,30]. When the mine is located in a high-temperature area, the initial airflow temperature into the mine will be higher, meaning that the roadways and surrounding rock have more difficulty in cooling [31]. Additionally, with increased mining depths, the underground geothermal water heat release significantly increases, as does heat transfer between high-temperature surrounding rock and airflow, underground electromechanical heat dissipation, and heat release from mineral oxidation. These factors will exacerbate mine heat damage, posing greater challenges in regulating underground thermal environments.

2.2. Influence of Mine Heat Disasters

In recent years, although there has been a general improvement in the safe production of coal, mine accidents due to high-temperature environments persist. On 27 October 2020, two ambulance workers tragically lost their lives due to heat stroke in the closed 7228 combined roadway of the Xutuan Coal Mine in Anhui Province, China. This unfortunate incident was caused by the influence of adjacent goaf and elevated temperatures resulting from coal oxidation and the surrounding rock’s temperature. The accident site recorded an alarmingly high temperature, as high as 44 °C, with humidity levels reaching 90%. Mine heat damage has emerged as the sixth largest type of natural disaster, following coal mine gas explosions, water inrushes, fires, dust hazards, and roof collapse [32,33]. Given its unique industrial context, the safety and stability of the mine environment are crucial for ensuring the well-being of miners and promoting sustainable development within the mining industry [34,35,36]. The high-temperature environment in mines poses a serious threat to the life, health, and production safety of miners. This not only decreases production efficiency significantly but also increases the likelihood of accidents and raises ventilation and safety costs [37]. High-temperature environments increase the body’s temperature, affect normal metabolism, disturb the circulatory system, and stimulate the nervous system. The survey results show that the labor production efficiency in high-temperature mines is only between 30 and 40 percent [38]. The high temperature in mines significantly affects workers’ blood pressure and heart and respiratory rates [39]. The incidence rate when working in harsh, high-temperature environments can reach 3.61 times that of normal conditions [40]. Fatigue is increased, leading to decreased attention, memory decline, and slightly slower responses in terms of work efficiency [41]. The heat and pressure generated during mining may cause changes in the formation’s structure, leading to ground collapse, cracks, and other phenomena. Additionally, high temperatures can trigger redox reactions in heavy metals in rocks, altering their chemical forms and producing harmful gasses that pose serious risks to the mine environment and worker safety [42,43,44]. The solubility of heavy metals also increases with increased ambient temperatures, resulting in easier dissolution from the ore and changes in the chemical composition of groundwater [45,46,47]. The presence of common mine wastewater, such as acid mine water containing toxic and radioactive elements, can degrade groundwater quality and, in severe cases, potentially lead to water pollution and pose a threat to human health [48,49,50]. Additionally, with high initial rock temperatures, the risk of disasters like gas outbursts and spontaneous ore combustion is increased [51,52].

3. Classification and Application of Mine Cooling Technology

Mine cooling technology can be roughly divided into two categories: mechanical cooling technology and other mine cooling technologies, each of which has its own unique application fields, advantages, and disadvantages.

3.1. Mechanical Cooling Technology

From the perspective of mining in China, mechanical refrigeration is the main method for preventing high-temperature mines. The application and implementation effects of mechanical cooling technology in some mines in China are shown in

Table 1. Application of some Chinese mining machinery refrigeration technology.

Serial Number	Mines in China	Elevation and Temperature of Mining Area	Cooling Method	Implementation Effect
1#	Pingdingshan coal Mine	−430 m; 33.2 °C~36.6 °C	Cold water cooling system	The temperature of the mining working face is reduced by about 5 °C
2#	Jinqu gold mine	280 m; 32 °C~33 °C	Underground centralized cold water cooling system with low-temperature mine water as the cold source	The temperature of the mining face is reduced to 27 °C
3#	Linglong gold mine	−710 m; 34 °C~36 °C	Ice-cooling system	The airflow temperature is reduced to 27.1 °C
4#	Shaxi copper mine	−770 m; 35.5 °C	Mine water local cooling system	The temperature of the mining face is reduced to 29.5 °C
5#	Xincheng gold mine	−1030 m; 33 °C~37 °C	Combined ice-cooling and air-conditioning cooling system	The airflow temperature is reduced by about 4 °C
6#	Sanshandao gold mine	−1140 m; 35°C~41°C	Cold watercooling combined with air-conditioning cooling system: air conditioning unit (840 kW)	The airflow temperature is reduced to 30 °C
7#	Xiadian gold mine	−662 m~−700 m; 37 °C~40 °C	Cold water cooling combined with air-conditioning cooling system: air conditioning unit	The mine was locally cooled to 26 °C

. Mechanical cooling methods are chosen according to the different mine conditions; for example, water cooling is suitable for mines with abundant water sources, and air cooling is more suitable for those with good ventilation. In high-temperature mines, the widely used mechanical refrigeration technology includes water cooling, ice-making systems, and air compression refrigeration [53].

3.1.1. Cold Water Cooling System

(1)

The mine cooling water cooling system works by using chilled water and airflow from the air cooler. It consists of a refrigeration center, cooling tower, cooling water pump, air cooler, and other components. Depending on the spatial layout positions, the system can be categorized into three types: ground-centralized, underground-centralized, and underground combined. The advantage of this cold water cooling system is its adjustable design for specific mines, allowing for future expansion or transformation. With an advanced control system in place, precise temperature control can be achieved to meet the strict temperature and humidity requirements. In a ground-centralized refrigeration cooling system, the refrigeration unit is set on the ground. Through the high- and low-pressure heat exchange unit, frozen water is transported to cool the mining face via airflow. Because the refrigeration station and cooling water circulation circuit are located on the ground, the ground-centralized refrigeration and cooling system has the advantages of taking up little underground space, conveniently condensing heat emission, requiring no explosion-proof treatment, and having high safety and low investment levels. However, with increased mining depths, the distance to the roadway is extended, and the distance required to lay down the cold water circuit increases, resulting in an increased loss of cold energy. In addition, high- and low-pressure heat exchange units also experience cold energy loss.

(2)

The downhole-centralized refrigeration cooling system consists of an underground refrigeration station, a cooling water circulation system, and fan coils. The refrigerator is located underground and has no heat exchanger; it only utilizes a cold water circuit with a short and easy-to-maintain water supply pipeline [54]. The underground centralized refrigeration and cooling system has the advantages of short water pipelines, minimal cold loss, and a small footprint, making it suitable for small mines. As the refrigeration station is located underground, a specialized electromechanical chamber needs to be excavated, and the equipment must be waterproof and explosion-proof. This efficient, energy-saving, safe, and stable system promotes green mining development.

(3)

Combined centralized cooling and cooling systems for upper and lower mines: Refrigeration units are set up both on the ground and underground, and the cold water prepared by the ground refrigeration unit is transported underground for heat exchange via a first-stage cold water circulation pipeline. The underground refrigeration unit adopts the fresh air heat removal method, introducing colder air into the air-cooled cooler and exchanging heat to reduce the cooling water temperature. After the two-stage cold water cycle, it enters the refrigeration unit again to cool down [55]. This system can reduce the cooling capacity and cold energy loss of the ground centralized refrigeration station, thus addressing the difficulties the underground refrigerator faces in discharging heat. However, the equipment is scattered and costly, meaning it is only suitable for large mines.

3.1.2. Ice Cooling System

An ice-cooling system is shown in Figure 2. The ice-making cooling system exploits the fact that the melting point of ice is lower than that of water in order to lower the temperature by absorbing heat through ice phase change. The system consists of three stages: ice-making, conveying, and melting. In the ice-making stage, the ice machine uses a refrigerant to reduce the water temperature and freeze it. In the conveying stage, the ice block or ice–water mixture is sent to the area where cooling is needed. In the ice melting stage, the mixture of ice and ice water absorbs the surrounding heat and melts, reducing the ambient temperature. The shape of the ice is granular and muddy, and the appropriate transportation mode, such as wind or hydraulic transportation, should be selected according to the shape and position. Problems with this system include the fact that the transportation pipeline in the ice transportation link can be easily blocked and broken and that the melting rate of ice in the ice melting link is hard to control [56,57]. Because of its excellent cooling effect, mine ice cooling technology has been widely used across the world, especially in deep mines in China, South Africa, Canada, and other countries. The technology has flexibility and long-term stability, and it is able to adjust the amount of ice made and convey speed to meet the actual cooling demand. However, this system still has limitations that limit its wide application in high-temperature mines, such as the fact that equipment can easily be blocked and large losses can occur in the cooling capacity in deep mines.

3.1.3. Air Compression Refrigeration Cooling System

An air compression refrigeration cooling system is shown in Figure 3. Using air as a cooling medium, the air is compressed and then transported underground after cooling and drying. Underground, an expander is used to expand the air and reduce the temperature. The cold air is then sent to cool the working face of the mine [58]. Because of its simple system structure and flexible application, air compression refrigeration cooling technology is especially suitable for meeting cooling demands in small mines, so it has wide application prospects in the market. In the Barapukuria coal mine project, undertaken by China in Bangladesh, air compression refrigeration cooling technology successfully solved the serious heat damage problem in the heading face [59]. In order to ensure the effective application of cooling technology, it is necessary to ensure that the mine is adequately ventilated so as to provide a sufficient compressed air source for the air refrigerator. This shows that the system’s operation is closely related to the mine’s ventilation. In addition, due to the limited heat absorption of compressed air, this technology is only suitable for small mines with small cooling demands and not for larger ones.

3.2. Other Mine-Cooling Technologies

Other mine-cooling technologies do not require refrigeration equipment or refrigerants, so this method does not require a large amount of energy to provide a cold source for a mine [60]. The specific technical measures are as follows:

(1)

Numerical simulation technology (CFD) has been widely used in recent years to analyze the thermodynamic characteristics of mine airflows and the variation law of surrounding rock temperatures [61]. This technology is used to optimize mine development modes, design better ventilation routes, shorten ventilation distances, and reduce heat release from surrounding rock. Ventilation optimization also includes airflow organization inside the mine to reduce the impact of heat generated by hot magma and equipment on the operating environment.

(2)

Improve and optimize the existing ventilation system, increase the ventilation volume, avoid underground high-temperature geothermal areas, and use pre-cooled airflows to control heat damage. This method requires a lot of preparatory work and easily makes the ventilation system more complex.

(3)

In order to reduce heat release from surrounding rock, cooling can be achieved via the filling and spraying of thermal insulation materials. However, due to the particularity of the underground environment, this method has higher requirements for the toughness and anti-viral properties of thermal insulation materials.

(4)

For some mines with good hydrological conditions, spraying or water injection can be considered for cooling treatments. However, it should be noted that this method may increase downhole humidity and that its cooling effect may be relatively limited.

(5)

Heat pipe cooling is another cooling method that transfers downhole heat to the surface through a pipe, achieving downhole cooling and dehumidification. However, the arrangement of underground heat pipes can increase the ventilation resistance, meaning the cooling capacity and conditions for use are limited.

(6)

Underground workers wearing cooling clothes can effectively cool their bodies; this can greatly reduce the energy consumption of refrigeration and reduce cooling costs. However, their structure is more complicated, and wearing these clothes increases the burden on the human body, reduces work efficiency, and can easily cause frostbite.

(7)

Using the cooling characteristics of groundwater, the heat in the mine is brought out of the ground through the groundwater circulation system, which reduces heat transfer to the roadway and the temperature of the underground environment.

Other mine cooling technologies do not rely on traditional ventilation cooling devices, so their initial investment and operating costs are low. Although its cooling effects are limited compared with mechanical refrigeration technology, non-mechanical cooling technology is the most environmentally friendly and most aligned with the concept of green development.

3.2.1. High-Performance Mine Cooling Clothes

High-performance cooling clothes belong to the category of individual protection in non-mechanical cooling technology. They have the advantages of simple operation, low production costs, and efficacy [62,63]. Because clothing micro-spaces are closely related to human skin and clothing, the thickness of these spaces and their internal microclimate parameters have a direct impact on human thermal comfort and the thermal protection performance of the clothing. Therefore, more and more scholars in China have begun to study clothing micro-spaces. Liu Heqing et al. (2018) discussed in detail the changes in climatic parameters in the micro-spaces inside air-cooled clothing and how human skin temperature is affected by factors such as ambient temperature, labor intensity, and ventilation [64]. Jiang et al. (2018) summarized three research methods used to explore the influence of air layers under clothing on the heat-transfer performance of clothing systems, analyzing their advantages and disadvantages and the influence of thickness, volume, position, direction, non-uniform shapes, and motion states on individual clothing items. By analyzing and summarizing the relevant research at home and abroad [65], Li Wanyue et al. (2023) discussed the influence of different amounts of underwear space on clothing contact, pressure, and thermal–wet comfort. The results show that the ambient temperature has the most significant effect on the temperature and humidity of the airflow in the clothing micro-space and against the skin [66].

According to the different materials, heat transfer media, and technologies, common mine cooling clothing can be roughly divided into air-cooled, liquid-cooled, phase change material, semiconductor material, and chemical ice bag clothing [67].

(1)

Air-cooled clothes: Air-cooled clothing uses air as the cooling medium in the heat transfer process between the airflow and the skin’s surface; this promotes sweat evaporation and thus achieves cooling. The most common air-cooled clothes are miniature fan-cooled and vortex-cooled. In a micro-fan cooling suit, the fan is woven into the clothing, generating airflow through its rotation to the skin’s surface. An eddy current cooling suit adopts eddy current tube-cooling technology to provide effective cooling protection for underground workers. A cooling suit uses compressed air as the cooling medium, and the cold end is tightly attached to the skin’s surface via a cold and hot separation mechanism [68]. An air-cooled suit is suitable for complex mining environments due to its simple structure, convenient operation, and taking up little space. The application of air-cooled clothing integrates the expertise of heat transfer and fluid mechanics. By regulating the gas flow and heat exchange process in the tiny space between the skin’s surface and clothing, body temperature is regulated, and thermal comfort is improved. A system diagram of the heat transfer and cooling process is shown in Figure 4.

(2)

Liquid-cooled clothes: These types of clothes rely on micro-pumps to cool the liquid medium. This is then transmitted to various parts of the human body through a network of in-built pipes, removing heat from the skin’s surface by convection and heat conduction. After absorbing heat, the cooling medium returns to the refrigeration device for re-cooling so that the cooling effect can be recirculated. Liquid-cooled clothing is widely used in high-temperature operations such as fire protection, mining, electricity, and transportation due to its strong cooling capacity and reliability. The disadvantage is that the refrigeration source needs to consume electric energy, and the battery needs to be replaced over time, thus proving inconvenient for underground operations [69].

(3)

Phase change cooling clothes: When the skin surface temperature exceeds the phase change temperature, the material will absorb heat and achieve cooling. A phase change material cooling suit uses a phase change material to absorb heat when the ambient temperature is higher than the phase change temperature [70]. Phase change cooling clothing is widely used because of its simple structure, strong cooling capacity, and low pollution levels. Although phase change materials have an excellent refrigeration effect, their temperature cannot be controlled, which means that the skin’s surface temperature can easily be supercooled. Phase change materials need to be stored cold repeatedly, and their continuous working time is short.

(4)

Semiconductor cooling clothes: Semiconductor cooling clothes are based on the principle of thermoelectric refrigeration, whereby semiconductor refrigeration sheets are applied to clothes. Multiple semiconductor refrigeration sheets are connected in a series and then connected in parallel with the water pump; this can effectively realize body cooling. Wen Hu et al. (2017) proposed a new design, skillfully stitching semiconductor materials in parallel onto basic clothing [71]. When the power supply is turned on, the cold end of the material can effectively absorb the heat of the surrounding environment, thereby achieving a cooling effect. An example of semiconductor refrigeration protective clothing is shown in Figure 5.

(5)

Chemical ice bag cooling clothes: Chemical ice bag cooling clothes absorb heat through the chemical reactions of an internal coolant to achieve the purpose of refrigeration. The coolant is based on ammonium nitrate, ammonium chloride, urea, and other substances in granular form. The advantage of chemical ice bag cooling clothes is that they can be used on demand without additional refrigeration equipment and are easy to operate. However, their disadvantage is that the reaction process of the chemical ice bag is irreversible, so it can only be used once and cannot be recycled. In addition, its cooling effect cannot be accurately regulated, so it is not suitable for use in extreme environments such as high-temperature mines. There are some problems in the application of existing human cooling clothing in high-temperature mine environments, such as insufficient technical maturity, short cooling efficiency, complex cooling system structures, and poor wearing comfort.

In light of these problems, the future development of cooling clothing for use in mines will mainly focus on the following issues: (1) miniaturizing parts, (2) intelligently controlling body temperature, (3) finding better phase change materials, and (4) optimizing the design of composite cooling clothing.

3.2.2. Roadway Insulation Materials

In terms of composition, thermal insulation materials for mining mainly include two categories. One type is inorganic materials such as fly ash cement-based composite materials, which are low-cost, have low thermal conductivity, and have good compressive strength. The other type is organic polymer-based thermal insulation materials. Although they have exhibited excellent thermal insulation performance, they are costly and poorly flame-retardant. These materials are, therefore, not applicable in complex underground high-temperature environments. Roadway insulation materials achieve cooling by controlling the heat source and reducing heat dissipation to surrounding rock [72,73]. The insulation heat source cooling method is based on the active prevention and control of heat sources. The idea of prevention and control is that thermal insulation material is sprayed onto the surface of the surrounding rock to form a thermal insulation layer, thus hindering the heat conduction from deep underground to the roadway and reducing the heat of the airflow [74,75]. In deep mine environments, in view of the high-stress state of the surrounding rock, the thermal insulation material for the high-ground-temperature roadway must have the dual functions of heat insulation and load bearing. As a material sprayed on the surface of the surrounding rock of the roadway, its thermal conductivity must be low enough to ensure effective insulation. At the same time, its compressive strength must be high enough to meet the bearing capacity demands of deep mine thermal insulation materials. The thermophysical properties of rock are the basis of research on the cooling of thermal insulation materials. Through studying rock mechanics, thermal physical parameters such as the conductivities and specific heat capacities of roadway rock can be understood. In this way, the heat transfer and storage capacities of rock can then be analyzed [76,77,78]. The heat transfer process of a porous material is shown in Figure 6.

As early as the twentieth century, the former Soviet Union tried to use materials such as polycarbamates and boiler slag to mix into concrete spray layers in order to isolate heat sources in high-temperature roadways. Taking the Dahongshan Copper Mine as the research object, Zhang Xi et al. (2022) discussed the cooling effect of spraying thermal insulation materials on the surrounding rock of high-temperature roadways, verifying their effectiveness. Xue Hanling et al. used and tested silicone rubber hollow glass beads as a thermal insulation material [79]. Wang Feifan et al. (2017) used fluent numerical software to simulate the thermal insulation effect of sprayed fly ash concrete. In their study, they found that fly ash thermal insulation concrete has a good thermal insulation effect in that the temperature in the roadway was significantly reduced [80]. The above studies mainly focus on the research and development of roadway thermal insulation materials and the study of thermal insulation effects, but there is a lack of research on the influence of roadway thermal insulation layers and raw rock temperatures on the cooling characteristics and heat regulation of airflows. Furthermore, no further explorations of the change characteristics of surrounding rock temperatures have been conducted in the field. In view of this, when studying thermal insulation materials in mine roadways, researchers should consider the influence of time on the temperature of surrounding rock so as to provide a theoretical basis for the actual design of thermal insulation layers in mines.

4. Traditional Ideas for Solving the Problem of Mine Thermal Damage and the Shortcomings of Mine Cooling Technologies in Application

4.1. Traditional Ideas for Solving the Problem of Mine Thermal Damage

The traditional way of solving the problem of mine heat damage is shown in Figure 7. Firstly, a conventional analysis of mine ventilation heat exchange and cooling and underground heat sources, as well as a geological survey, are carried out. On this basis, the mine thermal environment can be predicted, and the cooling ventilation network can be optimized. Then, the underground thermal environment is evaluated. When the underground thermal environment does not meet the thermal comfort standard requirements, the appropriate cooling method is selected according to the mine’s own situation, and the cooling effects and costs are analyzed. Finally, after taking cooling measures, the mine ventilation network is optimized, and the underground thermal environment is re-evaluated. When the underground thermal environment meets the thermal comfort standard, the mine’s thermal energy utilization is studied [81].

4.2. The Shortcomings of Mine Cooling Technologies in Application

Through a comprehensive study of various cooling methods, we have found that, although mine cooling technologies have developed rapidly, there remain corresponding deficiencies across all kinds of cooling technologies. On the whole, the shortcomings of the existing mine cooling technologies in application are as follows:

(1)

Large energy consumption does not conform to the concept of green mining. Mechanical cooling technology has gradually become the main energy consumption method for deep mineral resource mining. The refrigeration equipment used in mechanical cooling technology is expensive, and the power consumption for air conditioning is high. Most mechanical cooling technologies are suitable for local cooling areas. With increased mining depths, the transportation route lengthens. Therefore, it is necessary to increase the power of the compression and pressurization components in order to improve the refrigeration capacity and achieve an appropriate working temperature. At this time, the required electric energy also increases, which greatly improves the energy consumption.

(2)

High technical and maintenance costs lead to poor operability. In practical applications, some mine cooling technologies encounter problems such as complicated operation, difficult maintenance, and complicated system laying. As such, they require the guidance and operation of professional and technical personnel, which limits the possibility of its wide application.

(3)

Only paying attention to mine temperature control leads to ignoring other issues. When only temperature control requirements are considered, high humidity can easily occur in the mine. Various cooling technologies often focus on mine cooling without, or rarely, considering the problems associated with high humidity. In some mechanical refrigeration cooling technologies, temperature control occurs at the expense of humidity control; this is thus contrary to the original intention of improving a mine’s working environment.

(4)

The value of geothermal mines is not fully utilized. Their resources not only provide a heat source for mine cooling but, more importantly, can be used as a renewable energy source for development and utilization. Compared with traditional fossil energy, geothermal energy has the advantages of sustainability, environmental protection, and low carbon emissions. Through reasonable technical means, mine geothermal energy can be converted into electric, heat, and other forms of energy, providing clean energy for industrial production, residential life, and other fields.

5. Development Directions for Thermal Environment Improvement in Deep Mines

Deep mining is the most urgent task for the development of metal mineral resources across the world. Improving the cooling efficiency of deep mines and reducing the cost of heat damage control are important factors for the sustainable development of deep mineral resources. In order to improve the thermal environment of deep high-temperature mines, various methods should be adopted for simultaneous cooling; in this way, the cooling system can achieve the best effect, and active cooling measures can be implemented during the process of mine development. The key to efficient and low-cost cooling in deep high-temperature mines is to combine mine ventilation temperature regulation with geothermal mining. The idea of mine heat damage control is transformed into the mine geothermal utilization mode. Based on information acquisition, information analysis, automatic control, and display systems, an intelligent ventilation system able to adjust underground thermal environments can be established [82,83]. Improving the design and operation mode of the ventilation system can also make the airflow smoother and reduce the time that dust particles spend in the system, thus effectively reducing the accumulation of dust in the mine environment [84,85,86].

5.1. Intelligent Ventilation Systems Regulate Thermal Environments

Mine ventilation is the most basic way of adjusting the underground thermal environment; it is also an important part of other cooling methods. As an important part of smart mine construction, intelligent ventilation in mines is gradually becoming a hot topic in the industry. An intelligent ventilation system with a thermal environment adjustment function can be constructed, an example of which is shown in Figure 8. The system integrates information collection, analysis, automatic control, and display functions [81,87]. On the one hand, by optimizing the ventilation network, the system takes the air temperature as an important index for ventilation route selection, aiming to reduce the temperature rise in the process of airflow transportation. The system can then dynamically adjust the air volume distribution of the ventilation network according to information on underground personnel distribution, equipment status, mining progress, and climatic conditions so as to improve the air volume in specific areas and improve the working environment. On the other hand, the intelligent ventilation system combines human body-sensing technology, intelligent algorithms, and automatic control methods to achieve accurate temperature control for specific personnel or equipment. The system automatically selects the best cooling area by identifying and locating the temperature control target; it uses air curtains and other methods to isolate the area and accurately cool it down, thus reducing cold energy loss and improving cooling efficiency. During the cooling process, the system dynamically adjusts parameters such as the fan frequency, refrigerator power, airflow direction, and size according to the target temperature in order to ensure that the cooling target is always within a comfortable temperature range and to avoid overheating. By constructing an intelligent ventilation system with a thermal environment regulation function, accurate temperature control in local areas of the mine can be achieved. This is the key to effectively reducing cooling and mechanical refrigeration energy consumption.

Traditional mine ventilation uses ventilation facilities to form airflow, bring fresh air into the mine, and expel high-temperature air, harmful gasses, and dust from the well. On this basis, an intelligent mine ventilation system can improve ventilation and cooling efficiency and reduce energy consumption. In addition, an intelligent ventilation system also has a remote linkage control function, which can supply a regional air volume on demand and as needed. By combining human sensing, intelligent algorithms, and other technologies, the system can accurately track the target (person or device) and cool it, thus effectively reducing cooling energy consumption in the target-free area. This personalized cooling method not only improves the efficiency of energy utilization but also helps to improve worker comfort and further ensures safe production in mines. However, although the advantages of mine intelligent ventilation systems are obvious, most systems are still in the demonstration or construction stage. Research on underground thermal environment control technology based on intelligent ventilation is relatively weak, and mine intelligent ventilation systems with the characteristics of underground thermal environment evaluation and optimization still require further development and optimization.

5.2. Multiple Cooling Methods: Combined Cooling

As shown in Figure 9, one way of improving thermal environments in deep mines is to swap from a single cooling method to a combined cooling mode with multiple cooling methods. This mode combines mechanical cooling technology, an optimized ventilation system (high-temperature roadways should be avoided when designing ventilation routes), the full utilization and transformation of geothermal resources, workers wearing personal cooling equipment, underground hot water dredging, and other methods to ensure the optimum safety and environment for the whole mine system. When implementing the combined cooling method, it is necessary to implement a targeted design according to the specific environmental conditions of each mine to ensure that the various cooling methods used can coordinate and cooperate. However, this also increases the complexity and design of the mine cooling system, leading to higher technical and financial requirements. In general, the combined cooling method is a strategy that examines the mine system as a whole, achieving a more efficient cooling effect by integrating and optimizing various measures. This helps to improve the working environment and safety of deep mines, laying the foundation for the sustainable development of the mining industry.

5.3. Geothermal Utilization and Mine Thermal Environment Improvement

As one of the key factors of thermal damage in deep mines, geothermal energy is essentially an environmentally friendly thermal energy resource. At present, researchers all over the world are actively studying the development and utilization of geothermal resources. Geothermal energy can be mined cooperatively by means of heat storage and mining in mining layers, heat mining in rock layers, and waste heat recovery. The heat storage and mining of mining layers and the heat mining of rock layers can not only obtain geothermal energy but also combine with the mine ventilation system to adjust the temperature of the surrounding rock and roadway, optimizing the underground thermal environment and effectively controlling heat damage [88,89].

The specific working principle is to set up an injection well under the main air intake roadway of the mine and inject the water into the rock layer by collecting low-temperature geological gushing water in the mine. Together, the flow of injected water and the thermal conductivity of the rock strata reduce the temperature of the surrounding rock and ore in the main roadway of the mine. With the cooling of the surrounding rock of the main roadway, the airflow temperature flowing through the main roadway will also decrease and then flow in the ventilation network to comprehensively improve the underground thermal environment [90]. At the same time, because of the convective heat transfer between the cold airflow and the surrounding rock, the high-temperature roadway in the ventilation system can also be quickly cooled. In addition, the thermal production wells arranged in the deep rock strata extract geological hot water in the high-temperature rock strata through negative pressure extraction technology. This hot water is transported to the heat pump and converted into hot water, which is then transported to the ground for reuse. The low-temperature water generated during the operation of the heat pump is transported to the working face for cooling. The cold water that completes the heat exchange is then transported back to the reservoir and continues to be used for rock layer cooling and heat extraction, as shown in Figure 10.

6. Conclusions

(1)

Mine ventilation and air conditioning are increasingly becoming the dominant means of energy consumption in the mining of deep mineral resources. Therefore, the advancement and economic efficiency of mine thermal environment control technology have a direct, decisive role in the maximum mining depth of the mine. In the future, the mine’s intelligent ventilation system will become a new trend.

(2)

Mine cooling can be carried out by improving mine ventilation conditions, increasing the air volume, protecting personnel, reducing heat transfer, reducing heat sources, etc. The methods presented in this study belong to other mine cooling techniques. Other mine cooling techniques are generally economical and applicable, but their cooling effects are limited. The most direct and effective method is mechanical cooling technology.

(3)

In the future, the trend for cooling deep mine thermal environments will be a combined cooling mode using multiple cooling methods. The combined cooling method starts from the overall system of the mine and integrates and optimizes various cooling measures to achieve more efficient cooling effects.

(4)

The synergistic mining of deep mineral resources and geothermal energy in rock strata is essentially a combination of mining and stratum heat extraction. This method not only allows us to obtain renewable, clean energy from geothermal energy, providing power for economic development but also reduces environmental pressure through conserving energy and reducing emissions. In addition, it can also play a role in controlling mine heat damage, improving the working environment of miners, and ensuring their health and safety.