Underground spray chamber platform, part of the cooling system at Agnico Eagle Mines’ LaRonde gold mine. CREDIT: AGNICO EAGLE MINES
Mining deeper has always been synonymous with challenges. In 1905, 150 metres was considered deep for Canada; by 1935, this classification had been pushed to 1,500 metres, and today 3,000 metres is a more common benchmark. Elsewhere, mines greater than 4,000 metres are considered ultra-deep.
The “depth” challenges are functions of technology and economics. The technological limitations are typically greater rock stresses, adverse working environments and the overall logistics of transporting personnel, material and ore through the mine efficiently.
Ventilation is the prime method of controlling the working environment for both personnel and machinery. Providing equitable conditions year-round underground can be very costly with ventilation being responsible for anywhere from 40-80% of a mine’s energy consumption. The fans driving air through the mines consume electricity; the winter heating of sub-zero air usually requires fossil fuels; and for deeper mines, there is the emerging need for summer cooling typically using electricity. However, there are various cooling options.
In Canada, the historic prime ventilation design driver came with diesel mechanization, this was most significant for the precious and base metal operations. The trends to employ larger mobile equipment working throughout a mine has led to some mines, by weight, handling 15 to 20 times more air than ore. Associated power costs increase disproportionately the further distance that air has to descend from surface. There can be structural and cost limits that define the maximum size of the conduits the air travels through.
Efforts to limit how much air is required are a continual goal. Diesel equipment is becoming cleaner with respect to harmful emissions, tele-remote mining can remove the human exposure concern, plus better temporal and spatial ventilation management (aka ventilation on demand) can greatly improve air utilization. Today, a major impetus is towards replacing diesel equipment with battery electric vehicles (BEVs), eliminating the recognized carcinogens in their exhaust emissions, and using more efficient motors, reducing the thermal energy transferred to the working environment.
All these efforts can result in significant savings that will help ensure a sustainable mining industry. However, for deeper mines such as Vale’s Creighton mine, Glencore’s Kidd Creek, and Agnico Eagle Mines’ LaRonde, all nearing or beyond 3 km depth, heat becomes a more dominant design criteria.
Hot environmental conditions, if not appropriately controlled, can lead to heat stress and heat strain on both personnel and equipment with respective fatal and catastrophic effects. For mines in the Canadian shield, surface climate and depth are the prime determinant in the temperature of the air heading to the workplace. Auto compression can add nearly 10°C/km of depth to surface conditions. This could result in mid-summer dry temperatures reaching 60°C at the bottom of a 3 km fresh air delivery system.
But there are modifying effects from water evaporation and heat transfer/thermal flywheel effects with the rock. Machinery heat and freshly broken or exposed rock can then add more heat en route to the workplace. Canada is also somewhat fortunate. Elsewhere in the world, mines can experience hot rock temperatures greater than 50°C at much shallower depths, or experience significant thermal water inflows at temperatures above 60°C.
Although dry temperatures are part of the concern for worker productivity, the air cooling potential through the evaporation of sweat is far more important. Most heat exposure management criteria are based on an evaporation measure which can be related back to humidity and humidex measures as given in weather forecasts.
The first line of defence against adverse hot thermal conditions is the removal of heat through ventilation. But this becomes increasingly ineffective as the air naturally heats up as it descends and passes through the mine. At a “critical depth,” it has no cooling capacity remaining and the air has to be rejuvenated through some method of cooling. In Canada, depending on specific conditions, this critical depth could be reached as early as 1,700 metres, is likely to occur by 2,500 metres and can be almost guaranteed at 3,000 metres.
With an increasing number of existing mines and new projects reaching these depths, cooling will become more common place in Canada.
Surface refrigeration plant in Brazil. CREDIT: BBE
Cooling is not new to Canada. Vale’s Creighton Mine has a natural heat exchange area at the bottom of its original open pit that dates back to the 1950s. It was originally designed to heat winter air, but has since been able to supply an estimated 17 MW of natural cooling that has accommodated mining to 3 km. During its 20 years of operating late last century, the Polaris mine in the Arctic Circle required mechanical cooling to prevent the thawing of the permafrost orebody it was mining. Glencore’s Kidd Creek mine, starting in the 1990s, progressively introduced a natural passive cooling system followed by a conventional surface vapour compression system to maintain suitable working conditions to 3 km. Maritime potash mines have also used cooling to dehumidify their air. Today, in Canada, Agnico Eagle’s LaRonde mine is leading the way in regard to refrigeration. It is reaching the third design horizon, whereby cooling will be required year-round to reach 3.25 km and deeper – this is even after allowing – 25°C winter air to enter the mine. Depending on the season, Agnico will operate a surface plant plus up to two underground plants at 2,600 metres and 3,100 metres below surface.
These three 3-km-deep mines start to indicate the diverse options and ingenuity that can be used to cool mine air, ranging from passive seasonal heat exchange through to mechanical cooling. Other options that the industry has considered but have yet to be proven as either guaranteed, or scalable, or economic and socially acceptable, or a combination thereof are seasonal “cold” energy storage in ice or another medium, and district cooling use lake water. Waste heat conversion is another opportunity, be it from the mining and milling process, including power cogeneration, or heat exchange from hot water and air streams.
Underground vs. surface options
Ideally, one would like to cool the air as close as practicable to the point of need. This would be underground, where the cooling wouldn’t be wasted on air that is not going to hot working areas. However, underground solutions are invariably more costly and complex to put in place.
Typically, mines tend to start with a surface cooling system, but as with Agnico Eagle’s LaRonde, this may not be appropriate in winter. Surface systems are usually much larger than an underground plant providing the same cooling to the underground workplace. They cool a larger volume of air, but are considerably cheaper to build and are less disruptive to production. It is these surface applications that should see the greatest potential to use alternate technologies to the long used mechanical vapour compression systems.
A hybrid system that BBE has used at many mines is a split air conditioning system, it’s similar to residential applications but much larger. These systems use underground coolers to obtain the greatest positional efficiency but the cooling transfer medium (water) is supplied from surface. Here the heat rejection side of the mechanical process can be readily achieved to atmosphere. Such systems have been applied to many South African platinum mines, Hecla Mining’s Lucky Friday mine and on the shaft development of Rio Tinto’s Resolution project. The main limitation to such systems is the depth to which chilled water can be circulated and the associated pipe size. To date, BBE’s split systems have delivered cooling to a depth of 1,000 metres with plans to go to 2,000 metres.
There are two significant considerations for a refrigeration plant to be installed underground: a sufficient amount of air needs to be available to deliver the required cooling, and similarly, there has to be a appropriate volume to remove the heat from the cooling plant. For deep mines using BEVs, these considerations will play a major role in dictating the overall ventilation to be supplied from surface because more air may be required than for a shallower depth BEV mine. Another consideration for deep BEV mines is maintaining a perceptible airflow sufficient for the sweat-based evaporative cooling of workers. Hence mine ventilation infrastructure needs to be designed with future demand considerations from the start.
BBE has been designing and constructing mine cooling systems for over 25 years, they are continually improving our products to meet changing priorities. For example: moving toward more greenfield solutions, shifting away from permanent structures to prefabricated buildings that can be easily decommissioned; and using pre-assembled solutions that can be quickly assembled on-site plus moved to other locations. We also continue to explore alternate solutions. At a remote Australian mine, absorption waste heat based cooling has been used in association with the mine’s power generation. In the future, this could be linked to small nuclear reactors. In South Africa, our systems have used off-peak power to generate ice, storing cooling to manage electrical peak usage and time of day energy unit costs.
We continue to seek and evaluate ways to optimize how cooling is supplied to mines. However, it can be challenge to replace or match conventional technology. A well designed mechanical plant will have a guaranteed performance and can generate 5 MW of cooling from 1 MW of electricity.
At some point, there will be a paradigm shift to include new technologies, however, with a risk averse industry it is hard to predict when this will occur. Also some solutions may not be suited to many sites. Every deep mine would welcome the free, natural, green 17-MW cooling facility that Vale’s Creighton mine enjoys, but the lead-time planning dating back 70 years would be hard to replicate.
The other important factor is the worker. Unfortunately, our current workforce may not be as young, fit, and healthy to be best suited for working in the heat. This is starting to be increasingly recognized in heat exposure management, and the result could be lower temperature exposure limits. This could necessitate the introduction of cooling earlier at shallower mine depths, but it also underlines the importance of developing individual based strategies. These could include local cooling (AC vehicle cabs), personnel cooling, plus environmental and biometric predictive monitoring to help define potentially hazardous locations and whether the worker is at risk. Furthermore, even if the human was removed from the mine, equipment can have thermal operation limits.
Currently in Canada, there are several mines introducing refrigeration, or planning for an impending need. New deep mines are being also designed from the inception with cooling considerations. Cooling is coming to Canada in a big way. BBE Consulting Canada is here to help.
– Stephen Hardcastle is the managing director of BBE Consulting Canada. He can be reached at firstname.lastname@example.org.