Conveyors have long been touted as an alternative to earthmoving vehicles for the cartage of aggregates on-site, particularly over long distances. However, what really are the pros and cons of long distance conveyors – in terms of finance, safety and the environment – and is bigger necessarily better?
The landscape of the world continues to change in many different facets in the aggregate and mining industries. Aggregate reserves are becoming rare commodities in many areas, transportation issues are priorities because of rising costs and residents’ concerns, and industry requirements for specialised products are all factors in the discussion surrounding this transformation. These factors continue to drive the need for change in the way that aggregates are transported, not only from the source of the aggregate and/or the production area but also from the production site to the area of further processing or end use.
The decision to move product from the aggregate reserve to the crusher or to the area of stockpiling by truck or by conveyor is made because of a number of inputs that are analysed:
- Cost benefit.
- Product quality.
- Environmental issues.
Improvements in conveyor mobility, function, design, and flexibility, as well as escalating costs for operating haul trucks show the decision to use conveyors much more cost-effective.
The main benefits of using conveyors instead of haul trucks are as follows:
- Decreased operating expense.
- Limited inflationary effect.
- Improved product quality.
- Continuous flow.
- Environmental friendliness.
- The flexibility of design for conveyor systems.
Conveyors are beneficial not only in overland applications of short, medium, or longer length, but also in radial stacking or telescoping stockpiling applications where trucks have been traditionally used.
The maintenance costs of conveyors are considered less as more downtime is required to keep trucks and loaders running efficiently. In addition, most conveyor replacement parts are available locally. Conveyor systems are also less labour-intensive as trucks or loaders require one or sometimes two operators. Conveyors can also operate at maximum efficiency every hour of operation. This can decrease workforce and training requirements ( Table 1.).
While the big advantage of the haul truck continues to be its flexibility, even bigger disadvantages are increased fuel prices, increased labour costs to operate, maintenance costs, and the shortage of parts such as tyres, with lead times of more than six months being reported (see Figure 1)
Overland conveyors offer cost-effective material transport and a wide spectrum of capacities that can vary from a trickle of material all the way up to 30,000 tonnes per hour. Furthermore, with a lifespan of more than 20 years, the benefits of their high capacity and low cost operation are realised long after the initial truck fleet is retired.
As an example, a Texas-based mining operation has reported that its three kilometre-long conveyor has eliminated more than 140,000 truck trips and more than 917,000 vehicle kilometres on an annual basis. Added to the fuel savings, this operation is significantly reducing costs associated with labour, workers’ compensation costs, safety training, emissions, maintenance, and engine depreciation.
The above costs, and cycle time considerations when truck hauling in urban traffic congestion, were also factors in an Atlanta Airport project where an eight kilometre-long overland conveyor was chosen as the method to transport 30.5 million cubic metres of fill material from several nearby quarries to the construction site of a fifth runway. This overland conveyor also allowed the proper blending of crushed rock and overburden material in desired percentages as it was fed onto the belt.
A limited inflationary effect is achieved because rising fuel and energy prices have little effect on the operating costs of conveyors, and are not sensitive to fuel shortages. Electricity costs are fairly stable compared to diesel prices and conveyors can be run at off-peak energy times.
Improved product quality is seen because conveyors eliminate multiple handlings of material, which prevents compaction and contamination typically caused by trucks and/or loaders. Telescoping conveyors eliminate segregation and material degradation.
Continuous flow is another advantage realised because conveyors are not affected by weather and can be fully automated and operated at the touch of a button.
Aside from initial capital cost, operating expenses are the next major measured costs in aggregate operations. Initial comparative areas show that conveyor systems have high capacity and a low cost of operation; while depending on distance, the capital cost of a conveyor system can be higher than a haul truck. However, no matter what the distance, the cost of operating a conveyor will be less. Long range costs show that conveyors have limited inflationary effect and a long life span of at least 20 years.
TOPOGRAPHY, DESIGN EFFICIENCIES
Current conveyor technology can take on inclines up to 35 degrees while truck efficiency is compromised in any inclined application. There are also immovable issues such as the lay of the land. Haul trucks are not designed to tackle grades much above six per cent. It is clear that more and more operations are limiting truck transport to the area between the working face and the pit crusher. That means haul trucks are suitable mainly for level applications and truck efficiency is compromised in any inclined application.
However, bigger isn’t necessarily better. A common misconception is that oversized equipment lasts longer and requires less maintenance. Conveyors should be sized according to the expected load yet allow for a small capacity increase. For example, going with a 90cm belt when you only need a 76cm belt will ultimately result in additional belt and idler rolling resistance, and less energy efficiency.
Also, overland conveyors do not need to be eight kilometres long over rugged, mountainous terrain to be considered as a legitimate option. Overland conveyor lengths as short as 150 metres have been shown to have significant cost reductions over haul trucks. Payback of a system from 300m to 900m has been shown to be as short as 88 days to 182 days.
Electric motors consume 64 per cent of the electricity produced in the United States, according to a northeast utilities provider whose report cites the example of a 75kW AC induction motor that costs approximately $USD5000, yet will use as much as $USD35,000 worth of electricity over its lifetime.
With that said, it is clear that small improvements in efficiency can generate notable energy savings. As such, today’s conveyor manufacturers normally install new premium efficiency motors on each unit they build. Since these motors perform most efficiently near their designed power rating, it is recommended to run at between 75 per cent and near 100 per cent of full load rating. If you’re running at 50 per cent of capacity or lower, the efficiency of the motor drops dramatically. Motor size should be matched to the wattage requirements of the load.
Again, some producers will oversize a motor, thinking that it will require less maintenance. However, the energy savings realised from a properly sized motor will outperform any maintenance savings derived from a larger model.
Monitoring and minimising rolling resistance requires paying close attention to the quality of your belt and idlers. Manufacturers are now designing belts with special covers that help to counter rolling resistance. Idlers may look similar but they vary in performance. Speak to your conveyor manufacturer for a recommendation about the right bearing and seal type for your application. Proper idler spacing is also important. Again, consult with your conveyor manufacturer for idler spacing and rolling resistance formulas.
Systems must be designed and operated relative to the maximum material lump size. This is particularly applicable to material conveyed from the primary crusher. As the lump size increases, the potential capacity for a given belt width decreases. Some producers may skimp on belt width while running at higher speeds or capacities. While they may size their belt according to lump size, they may be running at a capacity that is not suitable to that lump size.
A basic rule of thumb dictates that you should never run at an excess of 80 per cent of volumetric capacity. Plus, when you’re running material that is at maximum lump size, it is best to remain under 80 per cent volumetric capacity to maintain standard edge distance and safe operation.
SAFETY, MAINTENANCE, ENVIRONMENT
Obviously safe conveyor maintenance does not allow for crawling under the unit to get to the other side. A ground line or overland conveyor must be designed with crossover points (for pedestrians and/or vehicles) at convenient intervals, so that producers can safely, easily and efficiently complete maintenance tasks on both sides of the system.
However, beyond costs per tonne, there are other key drivers in an emerging overland conveying trend.
Consider the ever-tightening environmental regulations. Individual mobile haulage units emit and stir pollution along the entire transfer path. As production sites expand and encroach upon suburban development, concerns arise over dust, noise and traffic. As such, quarry owners are often involved in decade-long negotiations over leasing and permits.
For instance, when faced with government and community opposition, a California-based aggregate operation proposed the use of overland conveyors over scrapers and trucks. It was armed with an environmental impact statement issued by the US Bureau of Land Management. It reported that the use of overland conveyors over haul trucks would reduce estimated PM-10 emissions (ie particulate matter larger than 10 microns) by mobile haulage from 95kg per day to as little as 2.3kg per day. In addition, community concerns could be eased by the fact that an overland conveyor would offer quiet material transport, and
when designed properly, could blend in with the environment.
A long distance conveying system needs
to be based in reality, and according to specific site parameters. The following questions should be considered during the purchasing process:
- What is the lay of the land?
- What is the percentage of incline?
- Will you need to cross roads or
- What is the best way to route the conveyor?
- What is your power availability?
- What are your power requirements?
Answering these questions will provide a clearer picture of what type of conveyor system will not only work for you but will make your operation safer for personnel, environmentally friendlier and more economical.
Source: Superior Industries