Skip Conveying – An Approach to optimise Cost and Energy Efficiency in Hard Rock Mines
Comparison of Transport Methods
A system was analysed to convey ore with an average density of 1.8 tonnes per cubic metre. Trucks with an average payload of 136 tonnes are used to load the skips. The vertical rise is 200 metres over an incline of 45 to 55 degrees. The rope system and drive system with 2 × 1300 kilowatt drive power is designed for 20 cycles per hour and a handling capacity of 2720 tonnes per hour. It can be assumed that this design will be capable on average of conveying over 2000 tonnes per hour of ore or overburden.
The table in Fig. 23 compares typical parameters of the two systems “skip conveying” and “truck traffic”.
For an average handling rate of 2000 tonnes per hour of ore, seven trucks with a payload of 136 tonnes must each travel 2 × 2500 metres on a road with an incline of 8 percent or 4.6 degrees to overcome a vertical rise of 200 metres.
If the moved payload is set in relation to the deadweight of a truck, which must be multiplied by two due to the empty trip back into the mine, the transport efficiency for the trucks is just 37 percent. The ratio of payload to truck weight of 1.2 is very unfavourable. With the skip system the deadweight of the skips is completely balanced. The drive system does not have to expend additional energy to transport the empty skip.
The comparison of installed power shows 2600 kilowatts for the skip system and 7 × 1082 kilowatts = 7574 kilowatts for truck traffic, giving a ratio of installed power of 1:2 to 3.
If we compare the use of manpower – excluding crusher and maintenance personnel – 20 to 25 truck drivers per day will be needed for multi-shift mine operation, plus one driver each for a water spray truck and a grader for road upkeep.
One significant factor with the two systems is their CO2 footprint. If a skip conveyor system is used instead of the truck system in the example discussed with a handling rate of 2000 tonnes per hour of ore, CO2 emissions can be reduced by up to 29 tonnes per day.
The combination of tested technologies for the use of a skip conveyor offers:
- The shortest conveying distance through use of a steep incline conveyor.
- Up to 50 percent lower transport costs within a mine.
- Energy saving: Energy needs to be expended only for transporting the payload. The deadweight in the system is completely balanced by the second, empty skip.
- The crusher station can be positioned at the top of the mine or at an intermediate level.
- Significant reduction of CO2 footprint due to reduced truck traffic.
- Environment-friendly technology with minimum noise and dust emissions.
- Higher availability as the system remains fully available in bad weather conditions (snow, fog, rain).
- Reduced costs for mine road upkeep and maintenance of remaining trucks to supply ore to the skip system.
- Lower operating and personnel costs.
- Lower investment costs as the truck fleet can be significantly reduced.
As mentioned at the beginning, Thyssenkrupp Fördertechnik is currently developing this innovative skip conveying system with integrated ore/overburden crushing. The company is studying the design of drive and conveying components as well as investigating technical implementation for a suitable mine. Obviously such a system would have to be adapted to actual mine conditions and the technical/financial aspects of using the system would have to be clarified in advance – together with the mine owner.