Setting new standards for health, safety and energy-efficiency, Pierre Brisset of KTI-Plersch Kältetechnik, explains how ice cooling can enable sustainable productivity gains and profit consolidation. Faced with the challenge of increasing demand for more ore at a cheaper cost, several South African mines have resorted to ice technologies to manage heat at deep mining levels.
“Modern ice cooling systems enable to achieve acceptable work conditions underground and enhance safety with productivity gains and spectacular energy savings.
“Heat and humidity are a constant issue and limiting factor in these underground mines. Getting deep, the virgin rock temperature may reach 60°C. With the added heat of excavation, the working conditions become very critical for the miners, with detrimental effects to their health, safety and performance.
“As heat-loads increase with depth, the heat-rejection capacities underground become more limited. Meanwhile, surface water chillers and bulk air coolers also rapidly reach their limits. The mass flow of water to be introduced from surface to cool deep mines and the associated electricity costs to pump it back, are huge. Likewise, cold air circulation entails expensive airways. Furthermore, the efficiency of surface chillers is impaired by the heat exchange between the chilled water and the warm surrounding environment to the lower levels (chilled water) and/or by the auto-compression temperature rise (air chillers). In deep mines, Ice cooling then becomes a valid alternative or complement.
“The advent of a new generation of modern ice plants, makes it possible to overcome these limitations and reintroduce ice in the mines. After mixed experiences with liquid ice, encouraging results were obtained with solid ice (98 % Ice Mass Fraction).
“Ice is colder than water and/or conditioned air. It displays a far greater cooling potential. Furthermore, Ice falls down the pipe at higher speed (3 km in two minutes) and still reaches the low levels at 0°C, with minimal heat-loss. These unique properties make it a very attractive and energy-efficient cooling medium.
“To put it simply, the ice systems:
- Significantly lowered the temperatures at the heart of the mine
- Drastically reduced the mass flow of water to be pumped up to surface
- Overcame the limitations of heat rejection underground
- Provided megawatts of refrigeration
- Saved pumping energy
- Enabled thermal storage.
“These benefits stem from the unique physical property of the ice. Ice is pure cooling energy. Thanks to its unique latent heat of fusion, (capacity to absorb heat as it melts), a given mass of ice provides much more cooling than the same water quantity. Whilst one litre of water absorbs 4kj/kg, 1 kg of ice absorbs 333kj as it melts (from -0°C to +0°C). Introducing ice in the mine shaft, thus enables to reduce the liquid mass to be returned to surface.
“Another advantage is the reduced heat-loss while the cooling medium is travelling from the surface to the stope. Chilled water picks up 5oC for every 1 km travelled underground. Water is already lukewarm as it reaches the low levels. On the contrary, ice drops down in a few seconds, with only minimal warm-up and melt (The heat intake is in the range of 10 kJ/kg/1,000 m). After thawing, the melt water is still colder than any water introduced from the surface.
“As an example, at 2,000 m below surface, 25,000 t of 5°C water are required to fight a heat load of