Optimising the demand of a mine water reticulation system to reduce electricity consumption

Abstract

South Africa has some of the largest and deepest mines in the world, reaching depths greater than 3 700 m below the surface. At these depths working conditions become intolerable, with virgin rock temperatures reaching up to 60 °C. Underground temperatures must be controlled to ensure safe and acceptable working conditions for the mining personnel. This is accomplished by means of a complex water reticulation system consisting of refrigeration plants and cascaded pumping stations. The water reticulation system is used to deliver cold water to the mining levels and to pump the used hot water back to surface. In deep-level mines the water reticulation system consumes up to 35% of the total electricity consumed by the mine. With electricity demand varying between 10 MW to 35 MW in typical deep-level mines, even a small reduction in electricity consumption will realise a significant cost saving. Investigations into water reticulation systems at different mines have shown that water usage varies between 1.25 kl and 4.15 kl per ton of rock mined. This large variation in the water consumed per ton of rock mined indicates that some mines may be using water inefficiently. Various energy efficiency methods have been implemented to reduce electricity consumption on mine water systems. Most of these are costly and time consuming. Very few of these methods addressed the problem of mine water wastage. Three techniques were identified which could reduce water wastage and consequently water consumption of deep-level mines. These techniques include leak management, stope isolation control and supply water pressure control. Initially the pressure control technique was tested at a typical deep level gold mine. A daily reduction of 1.4 Ml water was achieved which resulted in an estimated daily electricity reduction of 9.6 MWh. A total cost saving of R513 700 per annum is possible. The outcome of the test conducted on Mine 1 led to the implementation of all three water reduction techniques on a different mine. Leak management realised a total daily reduction of 7 MI with an additional reduction of 1.6 Ml per day possible from stope isolation and pressure control. An average daily electricity reduction of 92 MWh was achieved. This relates to an estimated cost saving of R5 617 000 per annum. Further investigations revealed that a combined daily electricity reduction of 170 MWh can be achieved by implementing water reduction techniques on five other mines. This relates to an estimated financial saving of R13 120 000 per annum.