An innovative technological approach to skip haulage could optimize costs and energy efficiency in hard rock mines

By Dr. Franz Wolpers

Open-pit metal mines are often shaped like an inverted cone, with the ore and overburden being drilled and blasted, then loaded and transported by shovel-and-truck systems. Mine trucks that weigh between 106 and 260 tons, and carry payloads from 136 to as much as 400 tons, transport these materials from the pit bottom along unpaved, slowly rising, winding haul roads to dumping areas outside the pit, or to a primary crusher station several hundred meters from the pit rim. Having run through the primary crusher, in larger operations the ore is then often transported via overland conveyors to the concentrator at rates of 10,000 mt/h and more.

The German company ThyssenKrupp Fördertechnik (TKF)—which supplies mining, crushing, processing and materials-handling systems to operations around the world—is now in the process of developing a system that allows hard rock ore and overburden to be transported more efficiently and with significantly lower environmental impact. Already being patented, it centers on an integrated conveying and processing system that does away with the need for heavy truck transport in this type of open-pit mine, which is where much of the world's copper, iron ore, gold and other commodities is produced today.

In overview, a rope-driven conveyor system is used to transport complete truckloads of material in track-mounted skips that run from a loading station in the pit bottom to the primary crusher on the pit rim. Inclined at up to 75°, it takes the shortest route possible up the pit wall. Like a freight elevator, as one loaded skip moves upward, an empty skip moves in parallel downward to the pit bottom. The two skips are connected via a rope system, pulleys and a traction-sheave drive system at the pit rim, such that the dead weight of the skips is fully balanced at all times and no unnecessary lifting power is needed.

This article describes the new system, and provides technical and cost comparisons with conventional heavy-truck transport. It highlights the system's major advantages with a concrete example, with the aim of giving open-pit operators food for thought when they are planning or redesigning mines in the future.

The Conventional Approach to Open-pit Transport
Freeport McMoRan Copper & Gold's Grasberg mine in Indonesia is typical in its layout with steep walls and winding haul roads. One of the world's largest copper-gold mines, Grasberg mines and processes around 220,000 mt/d of ore in a shovel-and-truck operation. In Figure 1, it can be seen that the heavily laden trucks move up and out of the mine in a train-like formation on ramped haul roads, traveling at average speeds of around 15–20 km/h.

Carrying ore from the pit bottom to the primary crusher, as well as overburden to dumping areas outside the mine, the line of trucks works its way upward on largely unpaved roads with gradients of up to 9%, until it reaches the top of the pit. After dumping at the crusher or in a waste area, the empty trucks drive back down into the pit by a separate route, with one truck cycle in an average pit typically taking between 20 and 40 minutes.

Grasberg runs six crusher lines in parallel, with TKF having supplied the operation with three of the world's largest (63 × 114 in.) gyratory crushers. The mine uses a fleet of up to 220 trucks to transport its ore and overburden, with the trucks' individual payload capacity ranging between 240 and 400 tons. For a maximum payload of, say, 240 tons, the dead weight of the truck will be around 160 tons, although the precise figure depends on the specific truck manufacturer. In essence, however, 160 tons of dead weight has to be moved in order to carry 240 tons of payload.

Across the world, open-pit mines are getting deeper and haul distances ever longer, so operators are using larger trucks with payloads of up to 400 tons to reduce the size of their truck fleets and the associated investment, labor and operating costs. Trucks of this capacity typically have a dead weight of some 260 tons, with an installed diesel-engine output of up to 3,000 kW (4,000 hp).

TKF's Alternative
With rising fuel prices and increasingly stringent environmental constraints likely to have a long-term impact on traditional open-pit mine operations, TKF has developed its new system for quasi-direct ore transport of ore and waste from the pit bottom or an intermediate level to a pit-rim crusher station. This system can then link in with overland conveyors for onward transport of both the ore and overburden.

To make a technical and financial comparison, the pit wall is assumed to have a 45°–55° slope, with a 200-m vertical rise for transporting the overburden or ore from the pit bottom (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 pit wall, two skips running in opposite directions on a ropeway system, each with an average payload of 136 tons of ore or overburden, and a track system for the two skips (See Figure 2). The crusher station, with a headframe and discharge equipment for the crushed material, is situated at the top of the pit wall. The electro-mechanical rope-drive system is arranged separately from the skip emptying and crushing stations.

In this scenario, trucks shuttle to and fro over short distances between the loading points in the pit and the skip-conveyor feed station, where loaded trucks reverse alternately into the tip via an access ramp. The skips are designed to take a full truck load, plus a 10% weight tolerance. Dynamic loads caused by rock impacts and other factors are absorbed by the skip being suspended by the rope system, while impacts on the skip discharge flap are cushioned safely by stationary pneumatic-tired buffers. During skip loading, the rope sag over the transport distance decreases and the ropes undergo additional extension. The resultant positional change of the skip, of up to 900 mm, is limited by a stop and is accommodated by the size of the opening of the feed chute.

Figure 3 shows a schematic of the skip loading and transport system, as seen from above. Once a skip has been filled by a truck dumping directly into it, it is pulled up the track to the crusher station by a rope hoist, over the 200-m vertical rise. As one loaded skip moves upward, the empty skip moves in parallel, downward to the loading station. The two skips are connected though the rope system, the rope sheaves and a traction-sheave drive system, such that their dead weight is fully balanced all the time.

Once it arrives at the pit rim, the loaded skip moves into the crusher-station emptying position with a predefined time lag. At the same time, the empty skip is positioned in the loading station below. The skip headframe, together with the rope sheaves and steel structure, is an integral part of a semi-mobile or stationary gyratory crusher station, with a feed bin, crusher and discharge conveyor (See Figure 4).

When the skip moves into the highest conveying point above the crusher feed bin, the discharge flap opens automatically or under remote control, with the full load being discharged into the bin over a period of roughly 25 seconds. Loading of the empty skip takes place simultaneously.

The crusher station also has an emergency or redundant truck-loading system, a crane for maintenance work and a hydraulic breaker for breaking up oversize, with a discharge conveyor below the gyratory crusher continuously feeding a conventional overland conveyor with crushed ore or overburden for onward transport.

The Rope-drive System
The rope-drive system is anchored in a separate station roughly 30 m away from the crusher and the pit rim. The overall height of the crusher station, with the headframe, is roughly 50 m—which is fairly tall but not unusually so. For comparison, the three 10,000-mt/h TKF crusher units at Grasberg stand 47 m high, which is nearly the same as the height of the crusher/skip system described.

To reduce the rope load and limit the drive moments, each skip is suspended in a hoist. In this example, the rope has a diameter of 54 mm and runs over six, 4,320-mm-diameter sheaves on each skip. 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 (See Figure 5). The drive moments of the two 1,300-kW-rated motors are transmitted with virtually no slip through a total loop of 540° by the two yellow traction sheaves.

A top view of the two identical drive units is shown in Figure 6. 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 that is clamped to the drive shaft.

The diameter of the rope and traction sheaves is determined, among other things, by the German TAS mining standard, which covers technical requirements for shaft and inclined haulage systems. This requires the ratio of the sheave diameter to the rope diameter to be greater than or equal to 80, so TKF has chosen a sheave diameter of 4,320 mm for a 54-mm rope. 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 described here is not new, of course, and it has proved successful in numerous heavy-duty elevators and ropeway systems. A typical rope drive system from an installation in Switzerland, for which the drive power is 2 × 1,150 kW, the rope diameter is 58 mm and the sheave diameter is 4.6 m is shown in Figure 7.

The Skip Design
In this example, the skip for a 136-ton truck-load of ore—equivalent to 75 m3—is 4 m wide, roughly 13 m long and has a fill height of 5 m. The skip has a design capacity of 90 m3 and needs a dead weight of around 90 mt to guarantee power transmission in the rope drive system. The number and size of the bogies are determined by the steepness of the track, the rail profile and the allowable wheel-contact pressure. In this case, a two-wheel bogie with 710-mm wheels is mounted at each skip corner, matching the A100 rail. Side guide rollers are also fitted on each bogie.

Ore is fed into the skip through an opening below the rope sheave system. When the skip moves into the dump station, the discharge flap opens automatically. The skips have a locking mechanism on either side for unlocking and locking the discharge flap. Once a skip reaches the topmost conveying position, the flap is unlocked by an external mechanism and is then opened in a controlled manner by a hydraulically operated support carriage with thrust rollers. Once the skip is empty and the trip down has begun, the flap is closed by the support carriage and is safely locked.

In a typical work cycle from skip loading to unloading, for a rise of 200 m on an incline of 45°–55°, the skips have to cover a total distance of 285 m. With a maximum rope speed of around 11 m/s and six-part reeving, the skip speed is 1.9 m/s (around 6.9 km/h). Skip loading takes 25 seconds, as does emptying at the top station, while skip acceleration and deceleration each require 5 seconds.

A complete conveying cycle, from loading to unloading, therefore takes 180 seconds or 3 minutes, so trucks with 136 tons of payload can drive into the tipping station at the bottom of the mine every 3 to 5 minutes.

Skip Conveying System Truck Fleet
System 2 skips, 136 tons each 7 trucks, 136 tons each
Transport Distance 0.29 km, 55° slope 2×2.5 km, 4.6° (8%) slope
Transport Efficiency (moved payload/moved power-effective total load) 100% 37% (136/(2×113.5+136)
Ratio of Payload to Truck Weight - 1.2 (136/113.5)
Total Installed Power 2×1,300 = 2,600 kW 7×1,082 = 7,574 kW
Installed Power Ratio (truck traffic : skip system) 1 2-3
Manpower (without crusher operations and maintenance staff) 0 20–25 truck drivers per day, plus 1 water truck driver and 1 motor grader driver for haul road maintenance
CO2 Reduction (compared with truck system) 29,000 kg CO2 per day
Table 1—Comparison between conventional truck traffic and the new skip-conveying system in an open-pit mine.

Economic Comparison
As a way of illustrating the advantages of TKF's system over conventional truck haulage, a number of assumptions have been made to provide the basic parameters for the comparison. In this case, the system handles ore with an average density of 1.8 mt/m3, trucks with an average payload of 136 mt are used to load the skips, and the vertical rise is 200 m over a 45°–55° incline.

Using 2 × 1,300-kW drive power, the rope and drive systems are designed for 20 cycles per hour and a handling capacity of 2,720 mt/h. It can be assumed that this design will be capable of handling more than 2,000 mt/h of ore or overburden on average. Table 1 compares typical parameters of the skip-conveying and truck haulage systems.

For an average handling rate of 2,000 mt/h of ore, seven trucks, each with a payload of 136 mt, must travel 2 × 2,500 m on a haul road with an 8% (4.6°) incline to overcome a vertical rise of 200 m. If the payload being moved is set in relation to the dead weight of a truck, which must be multiplied by two because of the empty trip back into the pit, the transport efficiency for the trucks is just 37%. The ratio of payload to truck weight is a very unfavorable 1.2:1.

With the skip system, by contrast, the dead weight of the skips is balanced completely, so the drive system does not have to expend additional energy to transport the empty skip. A comparison of the installed power shows 7 × 1,082 kW (7,574 kW) for the truck fleet but just 2,600 kW for the skip system, giving an installed power ratio, in this case, of nearly 3:1. Ratios are typically between 2:1 and 3:1, according to TKF's calculations.

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. Being fully automatic, the skip system has no labor requirement, thereby saving the cost of up to 27 operators.

Another significant advantage for the skip conveyor is its lower CO2 footprint. If a skip conveyor is used instead of trucks to handle 2,000 mt/h of ore, TKF has calculated that CO2 emissions can be reduced by up to 29 mt/d.

The Financial Advantage
With its combination of tested technologies, a skip conveyor system offers significant advantages compared with a truck haulage system, including:

  • In-mine transport cost savings of up to 50%;
  • The shortest transport distance by using a steeply inclined system;
  • Energy savings, since energy only needs to be expended for transporting the payload. The dead weight in the system is completely counterbalanced;
  • The crusher station can be positioned at the pit rim or at an intermediate level; and
  • The mine's CO2 footprint can be cut significantly through reduced truck traffic.

TKF's skip-conveyor system represents environmentally friendly technology with minimal noise and dust emissions. It also has the potential to provide higher transport availability, since it remains fully operational in conditions such as snow, fog or rain.

In addition, mines can cut their costs for haul-road upkeep, and for the maintenance of the fewer trucks that are needed to move the ore from the pit shovels to the skip-loading system. Having fewer trucks also means lower operating and personnel costs. Last but not least, mines will be looking at lower investment costs because truck fleets can be cut significantly. TKF is now developing this innovative skip-conveying system with integrated ore and overburden crushing. The company is studying the design of the drive and conveying components, as well as investigating technical implementation at a suitable mine. A system like this will, of course, have to be adapted to actual mine conditions, and the technical and financial aspects of using the system will have to be clarified in advance, working together with the mine-operating company involved.

TKF believes that many open-pit mine operators who are faced with rapidly rising production costs while using conventional transport concepts now have a genuine, cost-effective alternative.

Dr. Franz Wolpers is the executive vice president for TKF's materials-handling business unit, and head of the company's central R&D division. He can be reached at +49 6894 599 434 or at [email protected]

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