“Mine cooling installations must supply either chilled water or cool air to the mining zones, and several mine cooling systems can be used, depending on the mine’s location, depth and configuration,” begins Theuns Wasserman, the General Manager for mine cooling and compressors at the Johannesburg-based Chart Industries company, Howden.
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A CAE model of the hard ice plant expansion project installed by Howden at the Mponeng mine in Carletonville.
Most deep-level mines, he continues, incorporate a combination of systems that are installed in stages as the mine develops. Key factors to consider when selecting a mine cooling system include:
- The mining depth.
- The underground heat loads and sources.
- The distance from the mining zone to the ventilation shaft.
- The available real estate and size constraints, on the surface and underground.
- The cost of power and the availability of water.
- The seasonal and daily ambient temperatures on the surface.
- The available air supply from the surface and in underground airways.
- The ease and cost of maintenance.
Where ice fits into mine cooling
Presenting an overview of the types of mine cooling systems, Wasserman lists the following: hard ice solutions, where ice is produced on the surface and sent to underground dams; surface bulk air cooling; spot cooling systems; underground refrigeration systems; and surface chilled water systems.
To get an idea of where ice fits in, he says the distribution of refrigeration must provide cooling to the mining areas as economically as possible. Subsequently, the thermal losses in transporting the cooling medium must be minimised. So the magnitude and sources of the heat loads will have a bearing on the type of refrigeration and distribution strategy employed, he says.
There are three commonly used fluids to cool underground mining zones. Chilled air, either generated on the surface or from underground Bulk Air Coolers; chilled water, which can be pumped to the mining zone and through air handling units to cool the air in the area being mined; and ice, which can be dropped deep into a mine dam to cool water before being pumped through air handling units.
Cooling air on the surface is usually relatively simple and generally the least expensive option. The air can be chilled down to 6.0 or 5.0 °C, with the amount of cooling stored in the air being limited by the available air flow and the ambient starting temperature. The dehumidification of the air, which is done on the surface, also helps to improve underground conditions.
“Generally, however, the efficiency of surface bulk air cooling is limited in deeper mines due to the effects of autocompression and strata headloads,” he explains.
Going deeper, chilled water has to be sent from the surface into the underground mine. “Water systems are expensive, though, because the water has to be pumped into and back out of the mine, with pumping costs often being far more expensive than the costs of running the refrigeration system itself,” says Wassermann.
Typically, about 9.0 MW of refrigeration can be provided for every 100 kg/s of water flow. For every 1 000 m change in depth, there is a 100 bar pressure head that has to be overcome to pump the water back to the surface. “The combination of pumping energy, due to the higher pressure, and size of water columns becomes a limiting factor for mines at extended depths,” he adds.
In addition, the deployment of underground refrigeration plants offers a viable option for deep-level cooling. However, their effectiveness is inherently constrained by heat rejection, which typically relies on discharge into the return airways. This introduces a critical limitation, where the total cooling capacity that can be installed underground is directly governed by the air volume of available return airflow.
The cooling energy in ice is stored and released because of the ice-to-water phase change. This is why ice is an effective medium for cooling ultra-deep mines. Typically, 100 kg of ice can store 39 MW of cooling, compared to 9.0 MW for the same mass of water. One kilogram of ice is equivalent to approximately four and a half kilograms of chilled water.
In a deep mine, where pumping is a significant energy consumer, using ice results in 23 kg/s of ice providing the same cooling as 100 kg/s of chilled water flow. Ice, therefore, reduces the pumping flow requirement by 77%, making a significant impact on the cost effectiveness of a hard ice plant.
Highlighting the results of a study that looked at how efficiently 10 MW of refrigeration could be delivered to a mine at different depths
Wassermann points out that, between depths of 3 000 and 4 000 m, a significant amount of cooling capacity on the surface must be added if using water as the transfer fluid (Figure 1). With ice, the capacity increment is relatively linear with increasing depth. “To get 10 MW of refrigeration to a mining zone that is 3 500 m below the surface, for example, almost 70% more surface capacity is needed if using a conventional water-based system – the top line of this graph – compared to adopting a hard ice solution – the bottom line,” he points out.
It is essential to note, he says, that a combination of fluid configurations and equipment technologies can be employed in a mine’s refrigeration strategy, which typically evolves in different stages over the life of a mine.
A typical hard ice plant:
Pointing to a system diagram, he says that Howden’s hard ice solution starts on the surface, where large quantities of ice are produced.
The ice is conveyed to the mine shaft through vertical pipe chutes. It then falls into an ice dam, where it is stored as a combination of ice and chilled water at between 2.0 and 5.0°C. From the dam, the chilled water is sent to various air coolers at mining zones.
The chilled water from the ice dam is utilised in the air handling units, and the majority of the hot water is returned to the ice dam; only the water equivalent to the ice flow is pumped back up to the surface to be refrozen. “Consequently, the total chilled flow to the mining zones far exceeds the ice flow, which results in a significant reduction in the total water being pumped back to the surface.
Howden installations and ice technology
Howden has been involved in pioneering mine cooling systems since the 1960s, when surface chillers operating on R11 and R12 refrigerants (also known as Freon) were used for medium to deep mines for bulk air cooling (BAC). Chilled water systems soon followed, enabling chilled service water to be sent underground. “But as the underground workings went deeper, water flow rates became excessive, resulting in increasing pumping costs and maintenance issues.”
Following research in the late 1970s and early 1980s, Howden-engineered ice plants were installed at the ERPM Mines in 1986 and at Mponeng in 2014. These hard ice systems use mechanical refrigeration with an ammonia refrigerant and plate ice technology: The ice is formed on vertical plates. When the required ice thickness is reached, the refrigeration cycle is reversed, causing hot gas to be passed through the plate, which defrosts the ice in contact with the surface. The sheet then slides off the plate and is broken up, ready for conveying.
Another ice technology that has been employed, although not by Howden, is soft or slurry ice, which is produced under a vacuum, where the pressure in the vessel is reduced to the triple point of water, where all three phases of water, i.e. vapour, liquid and solid, exist in an equilibrium. Large Mechanical Vapour Recompression (MVR) compressors are used, and a saline solution is required to form ice crystals in a 15% ice slurry. An ice concentrator is then used to separate the ice slurry from the brine mixture.
The primary difference between hard ice and soft ice is their ice mass fraction (IMF), which is the ratio of solid ice to water. “Ultimately, we aim for an ice system with the least amount of liquid. Hard ice has an ice mass fraction of between 93 and 98, which means there is between 7.0% and 2.0% liquid in that ice. Soft ice slurries typically produce ice with a 70% IMF, so for the same cooling effect, 30% more water must be pumped back,” says Wasserman.
The Mponeng Ice Plant expansion
Howden has recently completed a hard ice plant expansion project at the Mponeng mine in Carletonville, which, at 4.0 km underground, is the deepest mine in the world. This expansion will increase the nameplate ice production capacity to 200 t/h.
“Mponeng employs all types of large refrigeration systems, including ice, hard ice and soft ice. In 2014, we completed the first hard ice plant there, with an initial production capacity of 100 t/h. Then in 2023, we were contracted to expand the plant to double its capacity, which has just been completed,” says Wasserman.
At the heart of the refrigeration system is Howden’s range of WRV Screw Compressors. The expanded plant will incorporate four of these compressor packages, potentially to provide a total installed refrigeration capacity of 24 MW. Heat rejection is being achieved via custom-engineered evaporative condensers, with four banks of 12 coils each.
Howden has supplied ventilation equipment to every major mining company in the world, from frozen sites in the Arctic to the hottest nations in Africa. A wide and quality portfolio of cooling systems is available, from surface bulk air coolers to hard ice plants.
“For the deep mines we have here in South Africa, hard ice solutions are becoming increasingly important, and we have the experience and the expertise to effectively deliver cost-effective plant cooling to enable safe mining at these ultradeep levels,” concludes Theuns Wasserman.
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