Along with ventilation, mine cooling is an integral part of any mining operation, as it is central to creating a suitable climate for miners to work safely and efficiently underground, especially in deep-level mines where temperatures can be extremely high for the human body.
There are three main reasons why underground mines are often inhospitably hot, the first being that the deeper they are, the poorer natural airflow becomes. Secondly, certain geological conditions contribute to heat, such as the proximity of hot rock and geysers, with some mines recording virgin rock temperatures of up to 80 degrees Celsius. Additionally, significant heat is generated by equipment (such as electrical motors and mining fleet) used to support mining operations.
As a result, the science of mine cooling has developed steadily over decades, as mines became bigger and deeper, and effective mechanical cooling became essential. Today, mine cooling systems tend to be highly sophisticated, with customised and bespoke systems being designed for specific mines’ particular requirements.
With this country’s burgeoning mining sector having been the main driving force behind its economic growth for many years, it is little wonder that much of the knowledge and expertise that underpins today’s global mine refrigeration technology was pioneered in South Africa.
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While shallower mines can be cooled through ventilation, using fans to pull air through the underground workings, deeper operations require mechanical refrigeration systems that pre-cool the air before it is drawn through the mine.
Mine refrigeration has evolved significantly since the 1950s, when mines fitted with refrigeration systems were seen as cutting edge. However, advances in this technology have seen mine refrigeration become a standard feature in most South African deep-level mines in the 1970s and 1980s, and it has since been adopted by mining operations across the world, including in Canada, Australia, Europe, North and South America.
Other than just creating a suitable environment for working underground, today’s mine refrigeration contributes significantly to a mine’s health and safety compliance and practices, as a direct correlation has been found between mine temperatures and accidents.
Initial mine refrigeration solutions consisted of spot refrigeration machines placed in the hot areas of a mine so that work could be carried out. Once an area was mined out, these units would be moved to the next spot. However, as these small machines were designed for other applications, and sourced from the Heating, Ventilation and Air Conditioning (HVAC) industry, they proved to be poorly suited to the harsh environment, difficult to maintain and inefficient in a mining setting, with mines often deploying dozens of units to cool a single area.
The 1970s and 1980s saw a transition to bespoke centralised cooling systems, which were built underground and would distribute cool air throughout the mine. However, these custom designed refrigeration machines were often very big and proved difficult to build, especially in deep mines, where space was limited. These energy-hungry systems also produced hot air as a by-product of cooling (i.e. heat rejection from the condenser), meaning that the bigger they were, the more heat they would produce. Managing this heat in return airways was found to be a limiting factor constraining the capacity of the equipment and hence cooling potential of these systems.
The next step in the evolution of mine refrigeration was to place the centralised cooling system on the surface and distribute pre-cooled air to the underground workings. However, as air can only be cooled to a certain point, refrigeration systems that produced chilled water for distribution were developed. Water was chilled in the centralised cooling system and sent down the mine via large pipes. The addition of glycol to the water was used in some instances allowing colder water to be provided without the risk of freezing. But this system also has its inherent inefficiencies, from being expensive to build to incurring huge pumping and energy costs for the operator.
Since then, surface refrigeration machines were also designed to make ice, which is then sent to underground ice dams, from where cold water is pumped through the mine. The use of ice for mine cooling was also pioneered in South Africa, with two deep-level gold mines currently using this system.
Each of these technologies came with their own challenges and one of the biggest hurdles facing the industry today is the changing global legislation around refrigerants, which have to have a smaller impact on global warming. The challenge of providing low global warming potential (GWP) refrigerants is complex however as new refrigerants tend to be less stable and often come with flammability concerns. The latter would not be well suited to underground mining applications. Fortunately, the proposed legislation will allow manufacturers and owners alike to gradually phase out equipment, and phase down refrigerants with higher GWP. While we can expect to see changes in the future, the impact is not well defined in the short and medium terms because some refrigerants might disappear before their phase-down date due to commercial reasons (i.e. lack of demand), which means that, ultimately, new solutions for mine cooling will once again need to be developed. Also, it seems that new refrigeration equipment using HFC refrigerants will stop being manufactured in some developed countries from 1st January 2024.
Meanwhile, the industry is also striving for greater energy efficiency, with new innovations coming out of the HVAC sector, including materials, new compressor sets and components that monitor and control energy management, being leveraged by the mining industry. With energy being such a big consideration for most mining operations, the evolution of equipment remains a key focus point for mine cooling.