As an alternative to traditional truck and shovel mining operations, “truckless mining” (In-Pit Crushing and Conveying systems or IPCC) is attracting global interest for its significant sustainability benefits and potential to reduce operating costs in mining operations.
With a sometimes confusing array of IPCC systems coming onto the market and an abundance of claims regarding their potential advantages, working installations and projects in active development beyond the study phase don’t necessarily match the hype.
Despite the trend towards “off-the-shelf” offerings from the market, one system does not suit all operations.
The selection of the most appropriate materials handling system for a given operation must consider economic, social, environmental and technical issues. Attempting to apply an existing design of sizer or transfer conveyor to a new operation without considering all relevant issues is unlikely to provide the best solution.
Of paramount importance is understanding and acceptance of the system by the operating team that will eventually have to make it work. The best design and delivery of an IPCC system will count for little if the operating and maintenance teams, for whatever reason, do not adequately understand the issues that influence production.
Selection criteria
An appropriate IPCC system for a specific operation cannot be selected without consideration of a wide range of site-specific factors and must satisfy two competing criteria to warrant selection for a given operation:
- Be physically able to excavate and deliver material (whether ore or overburden) to some form of out-of-pit system at the required capacity
- Be cost-effective during both the capital and operating phases of the operation
Trucks are well suited to short hauls and selective mining and dumping. As haulage distances increase or required system capacities increase, conveyors and mobile mining/dumping machines start to offer cost efficiencies. Skilled labour shortages and increasing concerns over greenhouse gas emissions also favour the latter techniques, provided that suitable power is available.
In presenting a model IPCC system, it is important for mining owners to have confirmed the following:
- That the layout of the reserve and material type is suitable for strip-mining techniques favoured by in-pit conveying systems, where benches are predominantly regular and long
- That a downstream system is available and suited to receiving material from a conveying system where dumps won’t require excessively difficult or expensive conveyor movements in order to be built
- That a reliable and affordable power supply is available
If the project cannot pass these gates with confidence, then the suitability of an IPCC system is under question from the start.
Having satisfied these initial criteria, there are two main options for the basic IPCC layout.
Mobile face operation
An excavation unit delivers material directly onto an in-pit conveying system for delivery out of pit. This type of system is used at Loy Yang in Victoria, Australia, for coal and overburden removal, and is in development for Vale’s S11D iron ore operation in Brazil.
In-pit dump to conveyor
Material is delivered to an in-pit dump hopper then onto the conveying system. Material delivery from the mining face can be by short-haul truck operation such as used for overburden at Clermont Coal mine in Queensland, Australia, dozer push or even dragline, such as Drummond’s Pribbenow mine in Colombia.
In-pit dump hopper arrangements tend to rely on either mature technology such as the short-haul truck operations or novel site specific techniques, such as those employed by Drummond. While exceptions exist, the majority of vendor offerings for IPCC systems are focused on the mobile face operation approach and discussed here in further detail.
IPPC components
The Operating Sequence must not only span regular operation, from drilling through to delivery of the material out of the pit, but also encompass how the system travels between benches and relocates once a face is completed. The following sections step through this sequence and highlight issues for consideration.
The sequence must also take into account:
- Power supply during both operation and relocation
- How horizon relocations are managed when the pit extents are reached
- How flexible the system is for operation on irregular benches or faces
Drill and blast
An IPCC system introduces additional equipment to the mining face, including sizing units, face conveyors and transfer conveyors.
The requirement for material to be conveyed after a minimum of processing also applies. While limits on run of- mine (ROM) size also apply to truck operations, a high proportion of large lump sizes will directly impact crusher or sizer selection and its related capital cost.
Excavate
Excavator selection is directly linked to system capacity. The capacity of a given excavator for a truck operation will not necessarily align directly to that for an IPCC system.
A truck and shovel system is limited by the capacity of the trucks. The excavator is sized to minimise truck waiting time so has unutilised capacity if trucks are unavailable or manoeuvring into position.
An IPCC excavator is feeding a continuously running conveying system. By sizing the conveying system capacity above that of the excavator, the full capacity of the excavator can be utilised. It becomes the bottleneck and the ‘lost’ capacity of the same machine if a truck operation is recovered.
Size
The focus of the sizing system is to reduce the ROM to the largest practical size for conveying. The sizer or crusher must be selected so as to ensure a new bottleneck isn’t created so soon after the excavator.
There is a noticeable trend in designs on the market to remove the superstructure slew bearing in sizing rigs due to concerns about operation vibrations. This in turn has impacts on achievable bench width.
Transfer off face
Once excavated and sized, the material must be transferred away from the operating face and out of the pit. This area potentially creates the most confusion when attempting to select equipment. An IPCC system is often described as a process stream. This is correct, but with the added complication that the environment the head of the process operates in, is continuously changing as benches get created, extended or completed. All components of the ‘process stream’ must be suited to this change – not simply inserted from an existing design and expected to be compatible.
Completion of all benches
The bench operation must take into account starting each bench and transferring systems between benches. A sizing rig and/or transfer conveyor that can complete a given bench may not be capable of creating the box cut into the next bench further down or above without the assistance of further equipment (and its associated cost).
Bench widths and the operating sequence must include the creation and possible removal of ramps to allow interbench movements. Some bridge conveyor designs, for example, require particularly long ramps due to the large turning radius of the machines. This may be acceptable but needs to have been factored into cost estimates for ancillary fleet to create the ramps and possible impacts on production while the ramp is created.
Bench conveyor relocation
Relocation of the face conveyor is required once the bench sequence has been completed. The frequency of relocation will depend on the number of benches in the sequence (eg one or three) but usually occurs frequently enough to noticeably influence operation cost. This is one of the most visible operations with regards to acceptance of or resistance to a new system from operations groups.
Other factors
Power supply
An IPCC system demands more electrical power to be supplied to the mining face than an equivalent truck and shovel operation. Conveyor drives, mobile equipment and ancillary equipment such as driven hopper cars require a reliable power source that is also suited to being relocated with the system.
Horizon relocation
Once a bench system has progressed to the pit extents, the IPCC system will generally require removal to a new horizon to continue operation.
While system components should already be suited to relocation due to regular conveyor track-shifts, both machines and conveyor drive heads/restraints may need to be trammed over long distances and potentially steeper grades than encountered in regular track shifting. As pit design needs to cater for this, it helps to have mine planning and materials handling design teams working closely during design phases.
Flexibility
Continuous mining systems such as IPCCs have a long and successful history in European and La Trobe Valley Australian coal mines due in no small part to the homogenous nature of these deposits. These systems are suited to long, regular benches and strip-mining techniques.
As attempts are made to apply IPCC systems to more varied deposits, their flexibility in managing irregular bench heights, shapes and distributions of material must be considered.
This is a condensed version of Timothy Atchison and David Morrison’s paper In-Pit Crushing and Conveying Bench Operations,delivered at the AusIMM Iron Ore Conference 2011 in Perth. For a full copy please contact Lucy Griffin.
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