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Conveyors vs trucks

01-Conveyors

[hr]Professor Peter Knights examines in-pit crushing and conveying and asks, are we learning from past performance?[hr]

01-ConveyorsTruck and shovel operations are currently the staple mining method employed by most surface mining operations worldwide. They can be purchased off the shelf, have comparatively low capital acquisition cost, and permit for flexibility in mine planning in the face of geological variance or volatile commodity price cycles. On the negative side, however, are their high labour requirements (each truck requires around seven operating and maintenance personnel for 24/7 operation), energy inefficiency relative to alternatives, and dependence on the logistics of diesel fuel and consumables such as tyres. Coupled with these are the safety implications of operating large fleets of mobile mining operations; roughly one third of all fatal accidents in surface mines are related to vehicle accidents.

Truck/shovel systems are “batch” systems in which the mining system is a sequence of unit operations each having stochastic (or variable) cycle times. Cycle time variance causes inherent inefficiencies at the boundaries of the unit operations, for example, shovels waiting on the arrival of trucks or trucks queueing to dump to the ROM crusher. Continuous (or semi-continuous) mining systems promise to eliminate many of these delays, improve labour productivity and reduce the energy requirements and emissions.

Continuous mining systems are not new: originating in Germany, bucket wheel excavators have a long history of operation in lignite mines around the world. However, whilst bucket wheels might be considered for excavating overburden in coal mines, they are not suited to hard rock applications. For hard rock mines a number of companies are evaluating the use of in-pit crushing and conveying systems, or IPCC systems as they are known. IPCC systems transport material by conveyor belt, inherently more energy efficient than trucks due to the large empty vehicle mass required to power truck movement. However, conveyor belts will only accept material within a fine particle size distribution. This is the function of the crushing component of the IPCC. For hard rock applications this will often be a gyratory crusher, but for soft rock it will be a lower profile sizer or roll crusher. The performance of hybrid roll crushers is fast approaching that of some gyratory crushers, which will further enhance mobile crusher solutions for hard rock applications. Crusher feed is via trucks in the case of semi-mobile IPCC system, or directly via a rope shovel or hydraulic excavator, as is the case with fully mobile IPCC systems. At the discharge end of the conveyor a stacker (for ore or coal) or spreader (for waste) is employed.

To date, seven IPCC systems have been installed in Australia, not all of which have performed as desired. Given that IPCC systems can potentially halve the workforce necessary to operate surface mines, why are there not more systems in Australia?  Industry professionals have advanced a multitude of explanations, not all of which are entirely accurate. Mining companies do not want to discuss the reasons for IPCC under-performance. However, failure is a learning opportunity. By keeping information in house, the industry is not being denied an opportunity to learn and improve.

[hr]“Given that IPCC systems can potentially halve the workforce necessary to operate surface mines, why are there not more systems in Australia?”[hr]

waste-movementFIGURE 1
WASTE MOVEMENT (MT PER YEAR) FOR AN INDICATIVE SURFACE MINE

As a result of discussion with a number of professionals, I believe that many Australian IPCC systems failed to live up to expectations due to the following factors:

HIGH INITIAL CAPITAL COST
IPCC systems have high upfront capital costs. Current systems necessitate EPCM contracts which extend the cost and lead times to implement IPCC systems compared to off the shelf truck/shovel solutions.  However, trucks have an economic life of around 60,000 hours, with shovels roughly double this. Over the life of a large surface mine the total capital cost of truck/shovel systems can easily exceed that of the IPCC system.

OVERESTIMATING SYSTEM PRODUCTION HOURS
An IPCC system is a series connected system. If one component is out of service, no production is possible. Studies have shown that utilisation losses (moving track shiftable conveyors, repositioning mobile crushers etc.) far exceed availability losses (downtime due to planned or breakdown maintenance). Such losses need to be well understood (and preferably simulated) prior to committing to an IPCC system. Mines should not expect 6000 operating hours per year from IPCC systems; somewhere in the range of 5200 to 5600 hours is the norm.

CAPACITY MISMATCH BETWEEN SYSTEM ELEMENTS
Matching throughputs from the loading equipment, crusher, belts and spreader is essential to optimise system performance. Unfortunately, changes in bench material characteristics can affect bulk densities and cause performance mismatches and non-optimal capacity utilisation of crushers and belts. The solution is to drill the orebody more extensively in order to better characterise material properties and equipment performance.

FAILURE TO ADJUST MAINTENANCE STRATEGIES
IPCC systems cannot be maintained in the same way as trucks or shovels. There is no redundancy in an IPCC system, so it is unacceptable to maintain equipment using reactive tactics. Instead, a “campaign maintenance” strategy should be adopted that is analogous to taking all components of the system out of service simultaneously and performing “mini shutdowns”. For example, following periods of 6 days of continuous operation, the system could be shut for 24 hours of maintenance, providing a theoretical system availability of 86%.  Extensive use should be made of sensors to detect performance or integrity deterioration order to accurately forecast and schedule maintenance workload during the shutdown intervals.

FAILURE TO PUT PLANNERS IN CHARGE
Germany has a history of successful operation of continuous mining systems. Why do Australian systems struggle to attain rated performance? I believe that the answer has to do with planning, in particular the level of detail and the degree of control planners have over operations. In Australian mines, Operations is king. This approach has to change if we are to successfully run IPCC systems.

Figure 1 shows a typical waste schedule for a mine operating two semi-mobile IPCC systems. Prestripping begins by using truck/ shovel systems. The vertical mining rate is initially high in order to accommodate early installation of the crusher station for the first IPCC system. Waste may also need to be dumped and levelled in order to accommodate spreader operations.

The exit strategy for the conveyor is via an inclined conveyor situated on the foot wall side of the pit.  Once the first IPCC system is commissioned, mining is conducted so as to widen the pit to the boundaries of the first few pushbacks. As the pit progresses, a second IPCC system is installed. Each IPCC reduces the number of trucks that would otherwise have been necessary. As a general indication, a semi mobile IPCC workforce consists of around 84 direct operators and maintainers, equivalent to the direct workforce needs of approximately 12 trucks. Thus if each IPCC can reduce the truck fleet by at least 12 trucks, then this will flow onto workforce savings which will also impact on overheads such as FIFO and camp expenses. This fleet reduction is of course dependent on truck capacity, truck cycle times and the mining throughput replaced by the IPCC system.  The operating cost of an IPCC system is around 1/3 of that for truck systems, but after taking into account drilling, blasting and loading costs, total mining costs will be around 20 to 25% cheaper.

The other important variable is the expected life of mine (LOM). If the life of mine is short, there is insufficient time in which to recover the capital costs of the IPCC system.  As a general rule, the LOM should exceed 8 to 10 years for an IPCC system to be economically attractive.

KNOW THE CONVEYOR
COMPARING THE SYSTEMS

BRIDGE CONVEYOR
Most bridge conveyor systems consist of mobile bridge sections: track or wheel mounted and carry chain or rubber belt conveying decks. Bridge sections are typically short (6 m on conveyor bridges and 16 m on chain type bridge systems) and are self-propelled. Depending upon seam (and hence mining) height, the discharge end of these systems can either run over or beside the main conveyor. This enables the bridge conveyor to discharge on the section conveyor as the bridge conveyor follows the continuous miner through the development sequence. Bridge continuous haulage systems provide a haulage system similar to the flexible conveyor train systems.

Bridge conveyors consist of several linked bridge segments using chain conveyors. At each intersection a crawler unit is required, where one operator for each unit might be required. An eighty metre pillar block would require a bridge conveyor with about eight segments and an overall length of 180 m.

Various flexible conveyor trains have been produced including both floor mounted and roof mounted continuous conveyor systems. Both systems offer some degree of operational flexibility. The discharge end of the flexible conveyor runs above the section conveyor. This enables the flexible conveyor to discharge onto the section conveyor as the flexible conveyor follows the continuous miner through the development sequence. The face end of the flexible conveyor is attached to the rear of the continuous miner or is self-propelled and kept at that position. Both roof and floor mounted flexible conveyor systems were trialled in Australian mines during the late 1980’s with limited success.

CHAIN CONVEYORS
Chain conveyors consist of four basic units: a breaker car module, conveyor bridge module, mobile bridge module and rigid haulage system. The system configuration and number of these units depends on individual mine application and production requirements. Systems can be up to 200 m of flexible chain conveyor with a feeder breaker behind a continuous miner. From the chain conveyor the coal is transferred via a belt interface onto the section belt. Chain conveyor systems often have a lower profile and thus are more suitable for lower seam workings.

TEMPORARY BELT SUPPORT
A temporary belt support system is comprised of a telescopic conveyor utilising a belt bending section and collapsible A-frame belt supports mounted on skids. Temporary belt support systems are available that can facilitate belt extensions during belt operation.

These systems allow the inserting of new belt structure and idlers without interfering with production. These systems are used predominantly in low seam mining.

PIPE CONVEYOR
Pipe conveyors are self-advancing and retreating via a monorail system and a hydraulic winch system. Maximum effective haulage length is approximately 200 metres. Due to the closed conveyor concept spillage is nonexistent. The design relies on a stretchable rubber belt driven by multiple friction rollers acting on a vertically vulcanised drive strip. Pipe conveyors include tear drop conveyors; both systems use a closed loop of conveyor belt.

Instead of running over rollers as a traditional conveyor system would, tear drop conveyors are suspended from a number of idlers on “j” sections that pull the belt into a tear-drop shape for much of its travel. This brings the benefit of enclosing whatever is being transported, removing the need for external structures to be built around the belt to stop dust escaping. Bringing the contents of the belt together helps generate bridging, which stops the material being transported falling back down the belt.

Tear drop conveyors have the ability to handle curves with a radius of about 5 metres, much tighter than conventional conveyors. The tear drop conveyor belt only takes the weight of the load but not the tensile load; the “j” sections take the tensile load, and also help the conveyor to reach an 80-degree elevation.

This continuous haulage system is supported by a monorail system from a track driven hopper car, which will also act as the loading device for the conveyor system. The hopper car can be equipped with a roof bolter and enough storage space for 100 metres of monorail and an inboard lump breaker.

PNEUMATIC CONVEYOR
Negative pressure (vacuum) conveying systems are ideal for coal recovery because coal can be loaded and conveyed from several faces to a common storage hopper. Coal is loaded directly into the conveying system at the face by the vacuum action of the system. The vacuum system has proved itself in removing slurry and waste from sumps.

The vacuum coal loading system involves use of air injector pumps to generate the vacuum, a separator/surge hopper to remove the coal from the air stream, plastic PVC pipe for haulage and flexible loading tubes for loading the coal at the face.

ALLISON GOLSBY, CONSULTMINE

PROFILE
Professor Peter Knights
BMA Chair and Head of division of mining school of mechanical and mining engineering, university of Queensland

Peter Knights is BMA chair and professor and Head of the division of mining engineering within the school of mechanical and mining engineering. at the university of Queensland.

From 1996 to 2004 he was employed as an assistant professor with the faculty of engineering of the catholic university of chile, based in Santiago, chile. He was subsequently named as associate professor and Canadian chair in mining. peter holds a Bachelor’s degree in mechanical engineering for the university of Melbourne, Australia, a masters degree in systems engineering from the Royal Melbourne institute of technology and a ph.d. in mining engineering from McGill university, Canada.

For the past twenty-five years peter’s research work has focussed on maintenance and reliability engineering. He is best known for: promoting the now widespread use of logarithmic scatter plots (also known as jack-knife diagrams) to characterise and prioritise downtime events; for his industry training courses on root cause failure analysis and for developing pragmatic approaches to rcm.

His recent work has focussed on mine data analytics and the reliability and planning issues associated with novel mining systems. He has a number of maintenance publications in prestigious international journals such as the Journal for Quality in maintenance engineering and the Journal of reliability engineering and system safety.

REFERENCES

Foley, m. 2012. “in-pit crushing: Wave of the future?”, Australian Journal of mining, pp.4653, may/June.

Harcus, m. 2011 “Back to the future”, mining magazine, pp45-59, June.

Londono J G, Knights p, Kizil m, 2012. a review of in-pit crusher conveyor (ipcc) application, in proceedings 2012 Australian mining technology conference, 8-10 oct, perth pp 63-81 (crc mining : Brisbane)

Turnbull d. and cooper a. 2009. in-pit crushing and conveying (ipcc) – a tried and tested alternative to trucks. the ausimm new Leaders’ conference Brisbane, QLd, 29 – 30 april.

Golsby allison. 2013 “What stacks up?Benchmarking continuous Haulage”, Australasian mining review, issue 7, p 34

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