By Himanshu Nagpal
Extensive amount of water is used in deep underground mines for various purposes like mineral processing, drilling, dust suppression, washing equipment, ore transport in slurries and refrigeration (for deep mines). Australia’s mining industry water consumption in 2008-2009 was 858 M m3 where as Canada’s mining industry water consumption in 2009 was 675 M m3 . The Boliden Tara Mines site in Ireland is the Europe’s largest zinc mine, the world 9th largest. Underground mining in Tara mines requires around 4.5 million m3/yr of water.
Due to high elevation change, the pressure of used water becomes very high at deep levels of mine. Pressure reducing valves (PRVs) are used at regular intervals to maintain the allowed pressure in pipes which can range from 6 – 12 bar. The used water must be pumped out (dewatering of the mine) using pump stations at various locations. This dewatering of mine is an energy intensive process. For Tara mines, the pumping contributes 10% to its total energy consumption (185 GWh/yr). This potential energy of water going down the mines shaft can be recovered using pump as turbine (PATs) in place of PRVs.
Example Case Study –
Kopanang Gold mine
Free state, South Africa
Single shaft mining at depth of 1350 m and 2240 m.
Figure 1 shows the simplified mine water reticulation network for this mine
Figure 1 Simplified mine water reticulation network
The average water consumption of the mine is around 19 Ml/day or 220 l/s. The available elevation head is 2286 m. The required pressure in pipes is 100 m or 10 bar. The theoretical energy consumption of dewatering the mine is
Figure 2 shows the mine reticulation network with energy recovery devices. PATs are installed in place of PRVs and a turbine pump is installed on a single shaft.
Figure 2 Mine water reticulation network with energy recovery devices
The PAT design conditions for specific production level:
Average water flow on the production level – 110 l/s
Average Upstream pressure – 52 bar
Average downstream pressure – 10.5 bar
Efficiency – 77.2 % (based upon pump-turbine efficiency curve)
353 kW of power at peak flow of 110 l/s and efficiency 77.2%
Investment cost – 230,000 Euros
Payback period – 3.8 years
Requirements for TARA mines
Our objective is to do a feasibility study to estimate the potential of hydropower recovery in Tara mines based upon measured data. The steps involved in the feasibility study are as follows
Measurements of flow data though PRVs
Measurements of pressure drop data across PRVs
Based upon the measurement of flow and pressure data, define a design-criteria for PAT
Cost estimation base upon designed system
Estimate cost savings and payback period