AT Rössing Uranium in Namibia, saving fuel and thereby reducing emissions has always been in fashion.
So they’re well placed for the cleaner and greener operating environment needed to help reduce the emissions that lead to climate change.
During past decades when the low price of uranium demanded all sorts of economies, one of them was to use haul trucks with diesel-electric units drawing power from overhead wires like a trolley bus. This reduces fuel consumption to a fraction of what it would otherwise be.
Electricity is substantially cheaper than diesel fuel in southern Africa so huge cash savings are made possible. Nowadays, as an added bonus for the times, the system substantially reduces exhaust emissions.
Werner Ewald, manager, Mining Operations at Rössing, explains that their Komatsu 730E trucks are powered by a Cummins K2000E (2000hp) engine.
Like similar trucks throughout the Group, the engine’s only purpose is to turn an electrical alternator that produces alternating current (AC).
The current goes through a bridge rectifier to produce direct current (DC) which in turn goes to the two wheel motors at the back of the truck.
The truck is thus driven by two DC electrical motors, not the diesel engine.
“Komatsu, as well as other truck suppliers, give us the option of trolley assist,” says Werner.
“By means of several more electronic and electrical components, the truck can hook up to a DC overhead line by means of a pantograph system and feed the two wheel motors directly from the overhead line.
“The engine and alternator are then no longer in the circuit and are simply idling along while the truck uses the overhead line to feed the two electrical motors. The overhead line system in use at Rössing is made by Siemens and is the same as the system used to drive electric trains. ”
Substations along the overhead line convert alternating current to direct current via a transformer/ rectifier substation.
The power on the overhead line is far more robust than the truck’s on board power source and as speed and voltage are in direct relation to each other, operators are able to raise the voltage closer to the electrical motor’s limit and therefore increase the speed of the truck while it is connected to the overhead line.
Typically, the truck speed increases by 5kph on a ten per cent gradient.
Fuel consumption on this gradient with a payload of 182 tonnes is about 350 litres an hour.
Trolley assist reduces this to only about 25 litres an hour.
The trolley lines are fitted to all permanent ramps leading out of the Rössing pit.
Trucks go “off line” at the top of the ramp and are then powered by the diesel/alternator again.