Open-pit mining is always a massive undertaking. And, as with all things which we tend to simplify, it is not just a big hole in the ground. There’s quite a science involved, as Duncan A Bullivant explains.

Ore bodies come in all shapes and sizes – pipe-shaped, vein-type, steeply dipping, stratified or irregular. Open-pit mining, which is quite different from strip mining, is often the most economical method of removing ore for further processing. It is a massive earthmoving exercise that usually takes the form of a cone-shaped excavation but, depending on the size and shape of the ore body, it could be any shape. Palabora copper, Mamatwan and Sishen iron-ore, and Venetia, Koffiefontein and Finsch diamond mines are prime examples of the diversity of open-pit mines in South Africa.
Excavation usually involves preparation by first loosening up the earth using the standard drill and blast method. Then, using a dragline excavator, a bucket wheel excavator or a hydraulic excavator to dig, scoop and dump ore onto a conveyor belt or into tipper trucks, the mined material is transported to the next processing stage. This makes the process cyclic, therefore enabling higher levels of efficiency. Since no room is available to dump waste within the pit, it is dumped outside the mined-out area but as close to the edge of the pit as possible so as to minimise transport costs, and for use in any later mine rehabilitation project.

Pit design
Benches are normally excavated between 2 and 15 m in height, in stacks of three or four, between which a crest is placed for the haul road. The more benches in a stack, the greater he road gradient. Benches have a steeper face angle, approximately 35o, while the stack and overall slope angle is approximately 45o. This is to prevent slope failures.

Figure 1: The typical open-pit design

From an analysis of the overall slope geometry, as a rule, as steep a slope as possible should be mined in order to reduce the overall stripping ratio. But, this rule is limited by the haul road’s maximum gradient, which is typically between eight and ten degrees. This requires more frequent and wider crests, and the need to have flatter slope angles in places to provide slope stability. Slope failure can be disastrous, even catastrophic, particularly in ultra deep open-pit mines a kilometre or more deep. In designing the slope, slope angle, slope height and the horizontal to vertical in situ stress ratio are some of the factors taken into account, as is the characteristics of the earth being mined and ground water.

Transport costs
Mineral, and especially waste transport, costs comprise the greatest amount of an open-pit mine’s working costs. To reduce this cost aspect, especially when the pit gets deeper, the following options are possible:
• In-pit crushers, together with a conveyor belt, instead of truck transport. (a continuous transport system (conveyor belt) is usually much cheaper in terms of R/t hauled), can be installed at a steeper angle, thereby saving stripping costs by virtue of reduced stripping ratio.
• Trolley-assist on the main haul road. (electrical power supply to trucks) – faster trucks, steeper roads for cheaper R/t costs .
• Computerised truck dispatch – more efficient use of trucks .
• Steeper bench slope angles (in other words, a reduced stripping ratio) where stability allows them – especially at the bottom of the pit when the life of the mine approaches end.

Given the high cost of rock transport, up to 50% of an open-pit’s total operating costs, and the prevailing cost of fuel per litre, many large pits are being forced to rethink strategies for reducing fuel consumption. Continuous transport systems (and the associated in-pit crusher if drill and blast is used) begin to out-perform truck based systems – since they are run on electricity, not diesel fuel. While the electricity is still cheaper than the fuel option, electricity costs, given Eskom’s huge increases of late, need to be carefully considered. Nonetheless, electricity can provide a considerable ore and waste transport cost saving. These savings become all the more important as the depth of the pit increases as the cost of transport increases exponentially with increasing vertical transport distance.

As a result of their application flexibility, truck and shovel systems are always popular and widely applied in mining, but in terms of energy efficiency, trucks use only 40% of the energy input to move the load, the remaining 60% is used to move the tare mass of the truck itself. Conveyors use 80% of the energy input to move the load. Energy costs of trucks are generally still 50% higher than that of conveyors in some parts of the world.
Production rates and strategic reserves, together with the R/$ exchange rate, should drive the regular trade-off analysis between the two systems, which, given the need to minimise costs, should be regularly reviewed.

Reference
Duncan A Bullivant, Current Surface Mining Techniques, Journal for the Transportation of Materials in Bulk: Bulk, Solids Handling, vol 7, n6, December 1987, pp827-833.