The South African mining industry is currently faced with a number of challenges. The increasing depth of mines, particularly gold mines, combined with ever increasing energy charges, have presented problems when it comes to cooling the mine working area while maintaining energy efficiency.
Russell Hattingh, Engineering Manager at Johnson Controls Systems & Service Africa says: “Surface plants are commonly used in mines for bulk air cooling, which involves chilling the air above ground before sending it underground and distributing it in the ventilation network.
It is also common practice to produce chilled water on the surface and then send it underground for cooling purposes.
However, while surface coolers are effective for shallower mines, as mining operations become deeper it becomes more viable to provide cooling closer to the working areas underground, since the cost of pumping, combined with thermal loss, becomes prohibitively expensive.”
When it comes to underground refrigeration, the problem of space becomes a reality as it is expensive to mine out and equip haulages large enough to house the equipment.
Rejecting heat to the return air is also a challenge, as the upcast air has a limited capacity, since heat rejection cannot take place at too high a temperature because it becomes expensive and inefficient.
Hybrid systems are often used in an attempt to achieve economies, but these systems become less economical as mining operations go deeper.
Chilled water systems use vast quantities of water to provide cooling to the working areas, and this water must then also be pumped back out of the mine, contributing to astronomical energy consumption.
This is where ice cooling, in conjunction with existing cooling systems, becomes a viable option says Hattingh.
He explains that as mining reaches greater depths, the virgin rock temperature increases, and in South Africa where the rock temperature gradients can be quite steep, the temperature can easily be warmer than 50°C.
Since acceptable working conditions in mines are around 28/32°C (wetbulb/drybulb), this means that temperatures need to be dramatically decreased in order for the mine to function.
Additionally, the high heat loads place strain on cooling systems. Mining at increased depths also produces logistical issues, since a water cooling system requires a lot of pipes which take up valuable space in the shafts.
Ice cooling systems are available in different forms, but Johnson Controls believes that hard ice is more viable than slush ice, as it uses less water and does not require the addition of salt to the mine service water.
Producing solid tube ice on surface and then sending it underground to an ice dam can significantly save on the amount of water needed to cool the mine, which in turn means that less water needs to be pumped to the surface again. This can provide substantial energy savings while at the same time increase the efficiency of cooling systems at deep levels.
While ice cooling technology itself is not new, it is only in recent years that it has become a viable solution due to a number of factors says Hattingh.
The technology has evolved and matured, making it more competitive and efficient, and the needs of the industry have changed such that ice cooling is beginning to make financial sense.
While the initial capital investment for an ice cooling system is fairly high, in deep mines the solution becomes more economical, since it reduces logistical issues whilst easing system complexity, improving water management and reducing energy and pumping costs.
Says Karl van Eck, Regional General Manager Africa at Johnson Controls Global Energy Solutions, “However, in order to continue extracting minerals from the earth, mining companies have to dig to greater depths, particularly when mining for gold.
“This challenge facing the local mining industry, coupled with the increasing electricity costs are leading to shrinking margins that are putting marginal mines at risk.
“Greater efficiency and energy savings need to be achieved to keep these mines viable, and ice cooling systems can definitely help to achieve this to a larger extent than other cooling systems currently can.”