A research team at the University of Queensland has found that utilising low-temperature rocks in underground mines can be an efficient way to cool deeper tunnels, reducing energy consumption and risks to workers.
Due to their high ambient temperatures, deep rock formations, and complex ventilation networks, underground mines present challenging working conditions. Without proper thermal design, these factors can compromise worker safety and equipment performance.
Australian underground mines, along with those worldwide, are extracting resources from increasingly deeper orebodies. This trend is driven by advanced exploration tools that can locate orebodies at greater depths, as well as the depletion of many near-surface deposits. To meet the rising global demand, particularly for critical minerals, this shift towards deeper mining is essential.
As mines extend deeper, increased geothermal energy, operating machinery, and restricted airflow can create thermal hazards that compromise worker safety and equipment performance.
Published in August in the journal Process Safety and Environmental Protection, a study from the University of Queensland highlighted that heat stress poses a significant risk to underground mine workers. Traditional cooling methods aimed at ensuring employee safety, such as enhancing mine airflow with mechanical ventilation, consume substantial amounts of energy.
To address this, lead author and Sustainable Minerals Institute (SMI) visiting student Kaiqi Zhong and her colleagues modelled a system that sent hot air through naturally cooler tunnels closer to the surface, before recirculating it.
The results showed that the air from the active mine area was cooled within the heat exchange zone by 11 per cent and was kept cooler for hours afterwards.
Zhong explained that simply increasing airflow did not fix heat hazards and could stir up more dust, adding that the technology was suitable for underground mines with disused roadways.
She said: “It may be particularly advantageous for deep mines with high heat hazards and access to natural low-temperature sources.”
Zhong’s supervisor at SMI, Professor David Cliff from SMI’s Minerals Industry Safety and Health Centre, noted that some mining companies spent vast amounts on cooling mines, especially in already hot countries like Australia.
Prof Cliff said: “Given the high capital costs involved in the current methods, it merits a lot of further investigation to see if the costs and practicalities work out.”
Relevant studies published recently in Nature have focused on optimising cooling strategies by investigating water injection parameters and air cooler configurations.
Studies on the synergistic mining of geothermal energy have shown that adjusting the duration and rate of water injection can significantly reduce the temperature in roadway environments and enhance heat recovery.
Other research utilised simulation and numerical analysis to redesign air cooler configurations, resulting in a significant reduction of ambient temperatures at both working faces and return airways.
An emerging innovation holding promise is the application of hybrid predictive methodologies, which combine numerical simulations with adaptive neuro fuzzy inference systems to provide highly accurate wet bulb globe temperature forecasts, enabling real-time auxiliary fan power optimisation and reduced energy consumption.
A paper published earlier this year in Scientific Reports investigated the changes in the airflow temperature field during the construction of an ultra-deep mining shaft, analysing it through the lens of fluid dynamics. The study was grounded in Newton’s law of cooling and the first law of thermodynamics.
The authors proposed a new concept known as the theory of equilibrium enthalpy interface, which qualitatively and quantitatively determines the critical conditions for the three changes in wind temperature within ultra-deep shafts.
The findings indicated that when airflow moved from the bottom to the top of an ultra-deep shaft, the change in airflow temperature did not follow a simple linear relationship. Instead, it exhibited a curvilinear pattern, resembling a cubic function that initially decreases, then increases, before ultimately decreasing again.
The authors noted that there were 66 metal mines in China and more than 100 mines globally with a depth exceeding one kilometre, with some reaching depths of three to five kilometres.
Using empirical data collected at various mines worldwide, the study showed that both the virgin rock temperature and the airflow temperature in many mines at depth significantly rose as the depth of mining increased.
This severely deteriorates the underground working environment, posing risks to the health and safety of workers.
Some examples are the President Steyn gold mine in South Africa at depths exceeding three kilometres, with virgin rock temperatures reaching 63 degrees Celsius; Robinson gold mine also in South Africa, at a depth of 2,700 metres, recording 41.1 degrees; and Mount Isa copper mine in Australia reaching 60 degrees at a depth of two kilometres.
Other examples include the Ibbenbüren coal mine in Germany, which reaches a depth of one-and-a-half kilometres with bottom rock temperatures soaring to 60 degrees Celsius, and the Creighton mine in Canada, where virgin rock temperatures of 48 degrees Celsius are recorded at a depth of nearly two-and-a-half kilometres.
The results revealed that the high-temperature zone corresponding to the shaft airflow was located at the depth change between 1,375 metres and 1,622 metres. In this range, the intake air temperature reached between 293.1 and 313.15 Kelvin (approximately 20 to 40 degrees Celsius).
The authors said it was evident that an increase in the depth of mining operations led to a significant rise in the temperature of deep mine airflow, which not only posed a threat to the health of miners but also affected the normal progression of mining and excavation activities, decreasing labour productivity and increasing the rate of accidents.
