Steve Davis, Senior Bulk Handling Expert at Advisian, explores the ins and outs of different conveyance types and how they can be deployed for different applications.
Conventional troughing belt conveyors are the most common conveyances for bulk materials with good reason. They are cost effective with multiple component suppliers and well understood by our design, operations and maintenance teams. These conveyors have high carry capacity and reasonable terrain capabilities. They have horizontal curve capability. Speeds in excess of 10 metres per second are proven as are high tension belts. Overland conveyors exceed 20 kilometres for a single flight.
There are alternative conveyance types that are suitable when conventional troughing conveying is either unsuitable or the layout and cost is compromised through designing around the limitations. Typical reasons for considering alternate system include steep angle conveying, enclosed conveying, cross country rough terrain, routes where multiple transfer stations can be avoided, high temperatures, long distances, low footprint, poor ground conditions and others.
The temptation is to use conventional and conservative conveying to define a solution rather than finding a different conveyance that provides a better or alternate solution. Here are some of the larger options, and there are many more in use and in development.
Metso ‘Cable Belt’, the longest of which single flight installed is in WA and is a 31-kilometre single flight overland, has better incline capacity than troughing belts and can manage horizontal curves. This potentially results in single flights across country that is too rough for troughing conveyors. Most components are specific to the conveyor. Carry capacity appears to be limited by available belt width and lower trough angle than conventional belts.
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Doppelmayr’s RopeCon system uses a wire rope structural support which carries a flat belt with convoluted sidewalls. The conveyance is strung between steel towers that can be more than one kilometre apart. The footprint is low. Access for maintenance is via a specialised trolley that also runs along the ropes. These conveyances do not have curve capability but are often able to run a straight line between points that is not possible for other systems. In between elevated rough terrain sections, the belt can be carried on rails at ground level. Capacities are high and conveying distance relatively high. Most components are specific to this type of conveyor. There is a long span across the Nile in Sudan, for which other conveyances would have required an expensive bridge and would have interfered with local agriculture. Every year sees more of these conveyances being installed.
Agudio’s FlyingBelt system also uses a wire rope structural support and support towers. This system supports a conventional belt on catenary idlers and uses a maintenance trolley. The conventional belt gives an opportunity to run elevated in rough terrain and then as a convention troughing conveyor when practical, and this does give horizontal curve capability. This conveyance is relatively new but there are already a few installations. This system has capacity up to 10,000 tonnes per hour and spans to one kilometre.
Chevron belts and cleated conveyors use all conventional components except for the belt and belt cleaners. These are used where an increase in conveyor angle by 10° or more can reduce plant footprint or avoid using a two-stage conventional conveyor and transfer system. These are generally used for lower capacities and in plant. Specific types of belt cleaner now allow conveying of sticky materials.
Sandwich belts also use many conventional components. Two belts are used to sandwich the bulk materials. These machines can convey vertically and even beyond vertical, as in self unloading ship loop belts, to feed above and back toward the load point. Conveying capacity so far is 3500 tonnes per hour.
Flexwall uses a flat belt with a convoluted sidewall arrangement and cleats. Many different arrangements for these machines are possible from steep to vertical incline through ‘S’ shapes, ‘L’ shapes and others. Idlers and pulleys are like conventional components but there are some special parts. These are best for dry materials and are mostly smaller capacity units.
Pipe conveyors load and discharge bulk in a conventional manner in a troughing section of a special flat belt. Between loading and unloading the belt is rolled into a circular shape and resembles a pipe. Pipe conveyors have excellent curve and incline capability and can wind through complex multi-curved routes. Many components are like conventional conveyors. There are many examples in use. Capacity is limited by the diameter of the pipe and is about 4000 tonnes per hour depending on density and other factors. The footprint is generally narrower than a troughing conveyor. The longest single flight is 15 kilometres. Pipe conveyors are also excellent for containment and dust and do not need a gallery enclosure. Pipe conveyors can be installed in tunnels where there is a requirement to conceal, such as through a suburban area to a port.
Thyssenkrupp and Contitech have developed the Megapipe. This is a larger diameter version of the pipe conveyor with a special belt having moulded cleats. The belt can travel multiple curves and steep or vertical slopes. Capacity is high and particle size greater than found in most pipe conveyors, leading to potential use for haul truck replacement in the mine pit environment.
Beumer have a ‘U’ conveyor. This is like a pipe conveyor except that the belt is rolled into a vertical walled U section rather than a full pipe. This promises many of the benefits of a pipe conveyor for multi-curves and a higher capacity.
Pouch conveyors also fully enclose the carried material, and are more route flexible than any other conveyance, being able to wind through multiple tight curves and inclines in almost any path. Capacity is relatively small at perhaps 1000 tonnes per hour maximum and depends on density. Multiple distributed drives keep the belt tension low. The small cross sections, ability to run carry and return in separate paths allows these machines to be installed in small spaces and follow existing walkways making them useful in brown fields installations.
Bucket elevators are mainly used for vertical conveying and allow loading conveying and discharge in a small footprint. Buckets are attached to chains or to belts (steel cord or fabric) and can haul up to approximately 3000 tonnes per hour depending on material and height. The tallest single flights are 175 metres. Bucket elevators are generally enclosed and are good for controlling dust.
Air support conveyors are similar to conventional conveyors, however the carry idlers and sometimes return idlers are replaced by a perforated trough which provides a film of air to support the belt. Loading and discharge is the same as conventional. These conveyors are relatively short and cannot have horizontal or vertical curves. They are useful for dusty and friable materials where the lack of idlers does not disturb material. There is a potential energy saving in the reduced friction carry, but this must be offset against the air supply energy.
Air slides are generally rectangular trough sections where air is introduced through a membrane. The air reduces friction in the carried material between material and the trough and the material flows down a slight gradient under gravity. Direction changes can be above 90° angle and need little space. They are useful for powdery materials such as alumina. Length is limited by the required slope from load to discharge.
There are many other conveyances in the materials handling arsenal, and new possibilities are being developed at all times.
Light rail systems offer possibilities for low cost contained high-capacity transport over unrestricted length. Systems such as Railveyor, MRL BOSS, ARC and Cont-e-bahn are all in various stages of development. The last three systems are Australian developments.
A study example assessed transfer of 5000 tonnes per hour primary crushed ore between points that were 14 kilometres apart. Because the terrain on the direct route was not conducive to conventional conveyors, a route was selected to allow their use. This resulted in some 32 kilometres of conveyors with three interim transfers and drives. The alternative options included Metso’s cable belt, Dopplemayr RopeCon and Agudio FlyingBelt. All three conveyances have capacity and can cross rough terrain. RopeCon and FlyingBelt both had the capacity to transfer almost in a straight line in a single flight, and when assessed on a total installed cost basis appear attractive propositions. However, as is often the case with studies, conveyances were compared solely on the mechanical cost and ranked on incorrect perceptions of risk.
I have recently been involved in a project where the design parameters were different in the extreme to my normal materials handling problems. Many conveyance types were considered in two stages. The first stage excluded any conveyance that could not carry the high temperature being handled and would not be able to fit on the limited footprint. This left roughly twenty different potential conveyances, and only one which eventually met all requirements. Of course, the first pass was how can we convey using troughing conveyors!