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Dwindling fuel supplies and increased labour costs have changed the way mining equipment selections are made.

Recent performance trials, using the latest technology in mineral sizers, and the increasing maturity of large-scale materials handling systems, has seen the emergence of competitive and credible suppliers of continuous materials handling machines for all kinds of open-cut mining operations.

This article considers the possibility of a large open-cast mine being initiated, developed through its early operations, and expanded later in its life in a truck-free environment, substantially reducing resourcing.

This therefore leads to cheaper operating costs without substantial capital investment.

In 2006, the mining industry got a glimpse of a bleak future. Tyre suppliers suggested a crippling shortage, and insisted that mining companies suffice with the existing level of supply. That might not have been a significant issue if, at the same time, the miners weren’t uniformly seeking to expand their operations.

Since then, additional tyre production capacity has been developed, but mining companies still face a world where the scarcity of tyres may not be a once-off phenomenon. Combined with high fuel prices and a scarcity of skilled labour, the future is uncertain.

During the crisis, several mine operators considered using large-scale continuous mining and semi-continuous mining arrangements. These studies produced interesting results, some of which are being implemented while others are moving through the planning process.

However, for any mine handling a significant or competent rock mass, traditional styles of continuous extraction systems are not appropriate. Bucket-wheel excavators and continuous miners are able to supply a feed stream broken into small enough pieces to convey, but their use is limited to materials that can be readily cut or broken.

The majority of overburdens do not suit this style of operation and for more competent rock, reducing the feed stream to something small enough to be conveyed requires a crusher or sizer.

This article will therefore consider the more general example of a mobile crushing facility.

1. Equipment selection

1.1 Loading Units

Using continuous mining technology does not dictate the choice of excavation machine. Applicable units for feeding a mobile crushing system include:

• Hydraulic shovels
• Large front-end loaders
• Medium-sized rope shovels
• Large rope shovels

Any of these could load mobile in-pit crushing systems at varying rates.

1.2 Dipper selection

In selecting a dipper, you must pay attention to:

• the weight of the protective wear package;
• the weight of the dipper;
• the density of the material being excavated;
• voids left in the dipper due to the shape or fragmentation of the material; and
• the average skill level of the operators.

1.3 Face operation

Generally speaking, mining a wide face, while tramming the shovel in a circular pattern around the crusher hopper, offers the best overall productivity.

2. Crusher selection

Styles of crushers able to deliver material into the crusher (or sizer) at a peak rate approaching 11,000 tonnes per hour include:

• Large gyratory crushers
• Large sizers

A gyratory crusher is able to exert a considerable compressive force on any particle that fits into the chamber, significantly greater than a sizer can. For particularly hard materials, a rolls sizer is not able to compete with a gyratory crusher for performance or operating costs.

For most rock masses, a sizer might be a viable alternative to a crusher as they have much smaller head room requirements.

3. Mobile crushing rg

3.1 Machine Configuration

Typically, a mobile crusher rig has a hopper for receiving material from the shovel and an inclined apron plate feeder to transport material to the crusher, where it is delivered into a collection conveyor and a slewing / luffing boom conveyor.

Preferably, the upper carbody of the fully mobile crusher rig should be able to slew independently of the crawler set, to allow the unit to operate parallel to the face of the bench being excavated without constraining the position of the hopper in relation to the shovel.

This allows shorter swing cycles for the shovel, and therefore some improved productivity, offset by a slight increase in the capital cost.

4. Conveyor selection and specification

4.1 Crusher Discharge Boom Conveyors and other short conveyors

A number of short-centre conveyors are included in any IPCC system (crusher discharge boom, spreader boom, belt wagon, piggyback conveyors, etc). These are high-wearing items and some conservatism in their design is worthwhile.

4.2 Overland Conveyors (face, ramp, dump approach, dump pivot)

On the other hand, overland conveyors are generally much longer and can therefore operate at much higher velocities. Also worth considering is their need to be periodically relocated. They should therefore be made in a modular format with readily moveable components and with few means of anchoring them to the ground.

4.3 System Productivity

4.4 Determining operating time

The series nature of the system (shovel crusher, conveyors and spreader) suggests the system will be inoperative whenever any of the machines stop. Conveyor movements and interruptions due to unforeseen maintenance issues may also occur from time to time.

It is worth noting that the shovel can still operate loading a truck fleet in those times when the IPCC system is out of service for a protracted period. An example of this type of opportunity might occur during the relocation of the system from one cut-back to the next, when the conveyors could be out of service for a week or so.

4.5 Mine Configurations

Geotechnically, the walls of any deep mine are more stable if the geometry of the pit is close to circular and sharp corners are avoided. That sort of constraint will therefore dictate the configuration of the IPCC materials handling equipment.

Conversely, strip mines allow long straight benches.

Configuring IPCC systems can therefore be broken into systems that either involve:

• straight benches with trackshifted face conveyors; or
• more randomly shaped benches with piggyback conveyors.

5. IPCC ancillary equipment requirements

To connect the face operations to either an ore processing plant or to the mine waste dumps, several pieces of ancillary equipment may be required.

5.1 Belt wagon

A belt wagon allows the mobile sizer station to operate on benches above or below the elevation of the face conveyor. It also allows the conveyor system to be located further from the blast zone than it could without a belt wagon.

5.2 Bridge Conveyor

The bridge conveyor is similar to the belt wagon, with the difference primarily in the support mechanism beneath the much longer boom. The bridge conveyor links benches that are more remote from one another than might be employed via a belt wagon.

5.3 Hopper Car

Beneath the discharge of any boom conveyor sits a mobile hopper car. These rail-mounted steel structures include chute-work to receive the material from either the mobile sizer station or a belt wagon discharge conveyor.

5.4 Cable Reel Car

The cable reel car is a rail-mounted device coupled to the hopper car. The cable reel is selected with adequate capacity to carry a cable half the length of the conveyor. The belt wagon, mobile crusher and shovel can all be powered from this cable reeler.

5.5 Relocatable Conveyor Systems

Each of the trunk conveyors are made up of modular components. Given the nature of this continuously relocating application, changes in conveyor alignment, length and lift can be accommodated relatively easily.

For particularly tight mine geometries, piggyback conveyor systems can be employed in place of the track-shifting style. They offer flexibility and can therefore readily adapt to changes in mine operations.

Another variation on the relocatable conveyors are self-propelled crawler-mounted bridge conveyors. They can be employed in a heap leaching operation, waste dump handling or for face operations beneath the mobile sizer.

5.6 Tripper

A tripper delivers material from a conveyor belt at any point along its length.

5.7 Spreader

Mounted on crawler tracks, the spreader unit travels up and down the dump while distributing the waste. Its two boom-mounted conveyors carry material from the tripper to the centre of the spreader and transfer it to the discharge conveyor. The conveyors slew independently of one another to allow flexibility in the placement of material onto the dump.

5.8 Transport Crawler

A transport crawler is necessary for moving conveyor drive stations.

Powered by diesel, the crawler has a track unit fixed to a slewing frame with hydraulic lift gear and clamp levers. It can fix itself to an item such as a conveyor drive head, lift, relocate and then position the item into its new location.

6. Handling waste and product simultaneously

To complete the ‘truck-free’ mine, some means of simultaneously handling waste and ore / product must be provided. In many mining operations, waste, low-grade ore and high-grade ore can be encountered on the same bench. As it makes little sense to relocate all of the conveyors and loading units when variations in the material are encountered, a dispatch centre could be employed.

Shuttles, similar in concept to (but significantly smaller than) the tripper, diverting the stream onto a spreader, are employed at the head of each incoming conveyor to enable the dispatch operator to divert the incoming stream to either an ore-handling stream, a waste-handling stream or a low-grade stockpiling system as required.

6.1 Other ancillary equipment

Using a mobile in-pit crushing and conveying system could save about 50% on dozer operating hours for waste handling. Furthermore, little support from graders is required for road maintenance and water cart usage is minimal.

An additional advantage inherent in the use of an IPCC system is the ability to install dust management controls at both loading and dumping points. This could substantially reduce potential airborne dust issues with substantially less need for attention to pit retention of dust during excavation of the upper benches.

7. IPCC system operation

7.1 Drilling and Blasting Safety Considerations

The majority of rock excavated and moved by the IPCC system requires blasting. The risks associated with blasting and the potential for physical damage to equipment has occasionally been sufficient to preclude consideration of the IPCC concept. However, there are ways in which equipment can be protected from damage while allowing the drill and blast operation to deliver sufficient fragmentation.

In some respects, the care required in blasting the overburden works in favour of the mobile in-pit crushing and conveying systems, which by necessity, have to operate near drilling and blasting operations. While the crusher, belt wagon, bridge conveyor and the shovel can be moved away from any blast area, the face conveyor locations are not so flexible and some damage from fly rock should be expected over the life of the mine.

7.2 Mining Option 1 – Trackshifting pre-stripping operation

Trackshifting is the process of moving a conveyor laterally by dozers.

Trackshifting is particularly suitable for operations where the IPCC system is being used to pre-strip ahead of a dragline operation. In that case, a long, straight face can be provided. From the face, just three or four conveyors are required to carry material away from the pre-strip face and out to the waste dumps.

7.3 Mining Option 2 – bench-by-bench development

The bench-by-bench arrangement for an IPCC system involves piggyback conveyors. While using piggyback conveyors increases the system’s complexity, it greatly improves the system’s flexibility, particularly with regard to wall shapes.

7.4 Conveyor Operation

Excavated material is conveyed either around or out of the pit on conveyors. Ore handling systems deliver directly to the process plant feed stockpile. Waste-handling systems deliver either to an out-of-pit dump or – if pit room is available – to backfilling operations.

For combined waste and ore handling systems a despatch centre may be required.

8. Operating cost estimate development

8.1 Staff Requirements

A team of about 26 personnel are required to operate and maintain a continuous materials handling system of this type, including supervisors, engineers, equipment operators and operations crews.

8.2 Electrical Power

Not all of the installed electrical power is used during operation. In developing an estimate of electrical energy consumption, it should be recognised that the shovel and crusher move along the full length of the in-pit conveyor. This means that, on average, the total energy consumption for this aspect of the mining operation would consume about half the possible total demand.

8.3 Maintenance of the system

The total maintenance labour hours expended per year depends on the machine type and operating regime.

An estimate of labour on site over a year could be calculated based on the machine working 20 hours a day, plus 135 man-hours of maintenance input (11 x 12 hours shifts). Although this will vary slightly with different plant arrangements, in general, this would be a typical requirement for a mobile sizer, belt wagon, 20MW conveyor network and spreader.

9. Infrastructure Implications

Given that few trucks and other mobile equipment are required in a continuous material handling system for the excavation, this significantly reduces the infrastructure associated with operating and maintaining this equipment. Additionally, this reduces:

• operational labour and therefore housing requirements
• maintenance labour and therefore housing requirements
• workshop space, diesel fuel storage and tyre handling systems

As the predominant technology involved in the IPCC system is the same as that incorporated in the crushing and ore handling systems, no significant new skills are required.

10. Project implementation implications

Typical lead times for the style of equipment employed in a continuous materials handling system is about 24 to 30 months between placing orders and first mining. This time can vary as demand for mining equipment fluctuates, but an allowance for at least that time is recommended.

Since the timing of equipment delivery is similar to that of stockyard machines that operate ore stockpiles (for much the same reasons), the decision to implement an IPCC system should be made when the project is approved.
Examining the economic benefits of an IPCC system (and reaching a conclusion) is best considered at project feasibility stage.

If you would like to receive the full copy of David's conference paper please contact:  Maria Whaley, Capability Marketing Manager, SKM Mining & Metals on +61 7 3244 7353 or mwhaley@skm.com.au.

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