Skip Conveying – An Approach to optimise Cost and Energy Efficiency in Hard Rock Mines
In the Freeport opencast mine in Indonesia, six crusher lines operate in parallel. Thyssenkrupp Fördertechnik equipped the mine with the world’s biggest ever gyratory crushers, of type 63"×114". To transport ore and overburden the mine uses a fleet of up to 220 trucks. The loading capacity of the individual trucks is between 240 and 400 tonnes. Fig. 2 shows one of these trucks with a load of fine ore. For a maximum payload of e.g. 240 tonnes the dead weight of the truck is already around 160 tonnes depending on truck manufacturer. So to carry 240 tonnes of payload, 160 tonnes of deadweight has to be moved.
A fleet of fully laden trucks drives up to the crusher station at the top of the mine. After unloading into the crusher or onto the overburden dump, the empty trucks drive back down into the mine by a separate route. One truck cycle in an average pit can take between 20 and 40 minutes.
To reduce the truck fleet and the associated investment, labour and operating costs, larger trucks with payloads of up to 400 tonnes are being used as mines get ever deeper and distances ever longer. A truck like this has an average deadweight of 260 tonnes and an installed diesel engine output of almost 3000 kilowatts.
Thyssenkrupp Fördertechnik is developing a new mining method that allows ore and overburden in hard rock mines to be transported more efficiently and with significantly lower environmental impact than ever before (Figs. 3 and 4). Patents have been filed for the method and its details.
This article presents a technical solution for the quasi-direct transport of the ore from the bottom of the mine or an intermediate level to the crusher station and further transport of the crushed ore or overburden on overland conveyors to processing and handling points or overburden dumps.
Avoiding heavy-load Truck Traffic
To avoid heavy-load truck traffic in inverted cone-shaped opencast mines Thyssenkrupp Fördertechnik is developing an integrated conveying and processing method described in detail in the following. To allow technical/financial comparison, the mine is assumed to have an incline of 45 to 55 degrees. The vertical rise for transporting overburden or ore is assumed to be 200 metres from the bottom of the mine or an intermediate level to the crusher station at the top.
The conveying and processing system consists of an HLT (Heavy Load Truck) tipping station at the bottom of the mine (see Fig. 5), two skips running in opposite directions on a ropeway system with an average payload of 136 tonnes ore/overburden, and a track system for the two skips. At the top of the mine is a crusher station with headframe and discharge equipment for the crushed material. The electromechanical rope drive system is arranged separately from the skip emptying and crushing stations.
Trucks shuttle to and fro over short distances between the point of loading by the mine shovel and the feed station for the skip conveyor. Via an access ramp, loaded trucks reverse alternately into the tipping station. The skip is designed to take the full truck load plus a 10 percent weight tolerance. Dynamic loads caused by impacting material are absorbed among other things by the skip hanging in the rope system. Impacts on the skip discharge flap are cushioned safely by stationary pneumatic-tired buffer stations. During skip loading, rope sag on the transport section decreases and the ropes undergo additional extension. The resultant positional change of the skip, of up to 900 millimetres, is limited by a stop and is allowed for in the size of the opening of the feed chute. Fig. 6 to 8 show three views of the loading station in the bottom of the mine for illustration. Once the skip has been filled by a truck, it is pulled up on a track to the crusher station by a rope hoist, over a vertical rise of 200 metres.
Similar to a freight elevator, as one loaded skip moves upward, an emptied skip moves in parallel downward to the bottom of the mine (Fig. 9). The two skips are connected via a rope system, rope sheaves and a traction sheave drive system at the top of the mine such that the dead weight of the skips is fully balanced and no unnecessary drive power needs to be expended. Once it arrives at the top of the mine 200 metres above, the loaded skip is moved into the crusher station emptying position with a predefined time lag (Fig. 10). At the same time the second – empty – skip is positioned in the skip loading station at the bottom of the mine.
The skip headframe with rope sheaves and steel structure is an integral part of a semi-mobile or stationary gyratory crusher station with feed bin, crusher and discharge conveyor. When the skip moves into the highest conveying position above the crusher feed bin, the skip discharge flap opens automatically or under remote control. Over a period of roughly 25 seconds the material drops out of the skip into the crusher storage bin. While this is happening the second skip is filled at the bottom of the mine.
The crusher station is additionally equipped with an emergency or redundant truck charging system, a maintenance crane and a hydraulic breaker for breaking up oversize pieces of ore. Below the gyratory crusher is a discharge conveyor. This continuously feeds a conventional overland conveyor with crushed ore or overburden for transport away from the mine area. The rope drive system is anchored in a separate station roughly 30 metres away from the crusher and the edge of the mine. The overall height of the crusher station with headframe is roughly 50 metres.