Electricity is an important input for all forms of mining. It is a major energy source for the transport of personnel, material and ore, production machines and mineral processing.
In addition it is the exclusive power source for vital health and safety related applications such as the pumping of water, ventilation and refrigeration.
Most underground mines experience an inflow of groundwater. This water has to be pumped out to maintain safety in the workings. Some gold mines in South Africa are situated in an area where the ore body is overlain by water bearing dolomitic strata. One such mine pumps some 70 Ml of groundwater per day to surface.
The workings of mines are ventilated to remove dust and fumes and provide healthy atmospheres for workers. A survey of South African gold mines indicated that the average quantity of ventilating air circulated was some 6 cubic m per second per 1000 ton of rock mined per month. A mine producing 200 000 ton per month would therefore circulate 1 200 cubic m of air per second. That is 1,2 ton of air per second.
The virgin rock temperature increases with depth. The rate of temperature increase, or the geothermic gradient, varies according to the type of rock and therefore differs from place to place.
In mine workings heat flows from the exposed rock surfaces to heat up the air. In order to provide a safe environment for workers in deeper mines it is necessary to cool the ventilating air. This is achieved by refrigeration plants which supplies chilled water that is in turn used to cool the air. A survey conducted in 1993 indicated that the installed refrigeration capacity in South African gold mines exceeded 1 400 MW of cooling power.
In underground gold operations these health and safety applications may consume in excess of 55% of the total electricity used.
On the average gold mine the cost of electricity amounts to some 11,4 % of the total working cost. On collieries the average cost of electricity amounts to some 8 % of the total cost for underground mines and 6 % for opencast mines.
In 1999 the mining industry consumed 31 352 GWh or 18,4% of the electricity sold in South Africa.
National Electricity Regulator statistics for 1999 revealed that there were 1039 mining electricity consumers in South Africa of which 1002 obtained their electricity directly from Eskom. This situation came about for a number of reasons, e.g.:
Because of Municipal electricity prices were mostly higher than those of Eskom, mines were also reluctant to enter into electricity supply agreements with Municipalities.
- Many mines had been established in areas before the local Municipality was formed
- Local Municipalities were unable to meet the requirements of mines in respect of electricity supply and service
Through the years Eskom and the mining industry have developed a thorough understanding of each other’s needs and concerns that resulted in formal agreements, negotiated through the Chamber of Mines, which provide benefits to both parties. For example Eskom will, in the case of shortages of electricity, as far as is reasonably possible, allocate the available electricity to the pumping, winding and ventilation requirements of mines. In return mines agree to shed load when Eskom experiences under-frequency conditions in its system.
Because mines can control their electricity demand to some extent they are able to make use of tariff options that minimise their cost of electricity. These tariff options are Eskom’s Megaflex time-of use tariff and Nightsave, a tariff that does not impose a demand charge during off-peak times, i.e. at night.
For this reason the average price of electricity purchased by mines from Eskom during 2000 was US$0-01,86/kWh ( R0-12,91/kWh) compared to the average price for all customers of US$0-01,91 (R00-13,23/kWh).
(Note - the average exchange rate during 2000 was R6-94/US$1-00)
In further attempts to maximise tariff utilisation for cost containment mines have also participated in Eskom pilot studies on real time pricing.
The mining industry is, however, not only a major consumer of electricity it is also the major supplier to the electricity supply industry.
During 2000 Eskom burnt some 92,289 million ton of coal at an average cost ofUS$6-40 (R44-40/ton). This was 40,8% of South Africa’s total coal sales for that year making Eskom the largest single consumer of coal in the country. The revenue to the mining industry amounted to US$590,35 million (R4 097 million).
It is therefore clear that the mining industry, and especially the coal mining industry, has a special interest in the well being of the electricity supply industry.