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Truckless Mining

Skip Conveying – An Approach to optimise Cost and Energy Efficiency in Hard Rock Mines

| Author / Editor: Franz M. Wolpers * / Marcel Dröttboom

Crushing at Freeport Mine, Indonesia

Thyssenkrupp has three crusher stations in operation in the Freeport mine in Indonesia (Fig. 11). Ore and overburden are crushed by the world’s biggest ever crushers of type 63"×114". A single crusher station has a capacity of 10,000 tonnes per hour. The overall height of the crusher station is 47 metres, almost comparable with the height of the crusher/skip system described here. Fig. 12 shows the three views of the crusher station at the top of the mine, roughly 22 metres below the skip emptying position.

To reduce the rope load and limit the drive moments, each skip is suspended in a hoist (Fig. 13). In the present case the rope has a diameter of 54 millimetres and runs over six rope sheaves per skip with a sheave diameter of 4320 millimetres. The two rope ends are firmly anchored in the headframe by means of an adjustable length compensation system.

The rope leading from the hoist in the headframe runs over a diverter sheave in the drive station and is led over two double-groove traction sheaves and a further diverter sheave to the second skip hoist. The drive moments of the two 1300 kilowatt rated motors are transmitted without virtually any slip through a total loop of 540 degrees by the two yellow traction sheaves (Fig. 14).

Figs. 15 to 17 show the top side and front view of the two identical drive units. Each drive train consists of a variable-frequency asynchronous motor, a disc service brake, a helical gear unit, a flexible coupling and a double-groove rope sheave fixed to the drive shaft by clamping elements. The diameter of the rope and traction sheaves is determined among other things by the German mining standard TAS (Technical Requirements for Shaft and Inclined Haulage Systems). This requires the ratio of sheave diameter to rope diameter to be greater than or equal to 80. For a 54 millimetres rope a sheave diameter of 4320 millimetres was chosen. The rope safety factor is greater than seven. In addition to the service brakes in the drive train each traction sheave is fitted with safety or holding brakes.

The two-sheave drive system is not new. It has proved successful in numerous heavy-duty elevators and ropeway systems. Fig. 18 shows a typical ropeway drive system in Switzerland. The drive power is 2 × 1150 kilowatts, the rope diameter 58 millimetres and the sheave diameter of 4.6 metres.

Skip Design Details

In this concrete case the skip for a truck load of 136 tonnes (75 cubic metres) of ore is 4 metres wide, roughly 13 metres long and has a fill height of 5 metres (Fig. 19). The skip has a design capacity of 90 cubic metres and requires a deadweight of around 90 tonnes to guarantee power transmission in the rope drive system. The quantity and size of the bogies are determined by the steepness of the track, the rail profile and the admissible wheel contact pressure. In this case a two-wheel bogie with 710 millimetre wheels was provided at each skip corner, matching the A100 rail. Side guide rollers are additionally fitted on each bogie.

Ore is fed into the skip through an opening below the rope sheave system. When the skip moves into the emptying station the discharge flap automatically opens to discharge the material. The skips have a locking mechanism on either side for unlocking and locking the discharge flap (Fig. 20). Once the skip reaches the topmost conveying position an external actuation mechanism releases an unlocking system and the pressure of the material immediately opens the discharge flap. Once the skip is emptied after around 25 seconds and the trip down has begun the flap is closed by two guide rollers and safely locked.

Fig. 21 shows a typical work cycle from skip loading to unloading. For a rise of 200 metres on an incline of 45 to 55 degrees the skips have to cover a total distance of 285 metres. With a maximum rope speed of around 11 metres per second and 6-part reeving, the skip speed is 1.9 metres per second (around 6.9 kilometres per hour). Skip loading takes 25 seconds and occurs while the second skip is emptied at the top station. Skip acceleration and deceleration require 5 seconds. A complete conveying cycle from loading to unloading therefore takes 180 seconds or 3 minutes; i.e. trucks with 136 tonnes of payload can drive into the tipping station at the bottom of the mine every 3 to 5 minutes.

Fig. 22 summarizes the design of the conveying and crushing system for the chosen example.

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