4. Horizontal transportation
4.7 Conveyor belts
4.7.1 Audit of conveyor belt system
4.7.1.12 Characterisation of Enclosed Belt Conveyors
Severe changes in direction can be achieved using a single belt. Angle stations discharge rock from the belt and re-load the rock after the belt has been re- directed. The cables are removed from the belts and, after passing through a series of sheaves and tensioning devices are re-directed. Although almost any change in direction can be achieved, a considerable sized station and excavation would be required to accomplish this.
Limits on length
To date, conveyors with lengths of less than 1000 m have been installed. There does not appear to be any major reasons to prevent the installation of considerably longer belts. Additional drive units can be installed at various points along the conveyor to reduce tension in the belt with a number of drive units required even for a relatively short belt.
Environmental compatibility
Except during discharge, the belt is completely closed at all times eliminating spillage and the release of dust into the environment. However, primary and secondary crushing systems will probably be required and would generate additional heat and dust. An advantage of the enclosed belt conveyor is the small cross sectional area of the belt that will have a much lower restriction to the airflow as compared to conventional conveyor systems.
Powering systems
Friction drive units driven by electric motors are used to induce tension in the profiles at the top of the folded belt. Additional drives can be located at various points along the conveyor. Where the conveyor turns at an angle of 90 degrees or more, an additional drive unit could be installed. As with any multiple drive conveyor additional cabling, switchgear and control mechanisms would have to be installed for each drive.
Feed and discharge arrangements
Lump sizes specified for the belts currently in use are small and some form of secondary crushing may be necessary. Loading can take place at any point along the length of the conveyor. The belt is opened up at the start of the loading station using rollers. The loading station covers the opening completely and the belt is closed immediately on leaving the station preventing spillage. Similar precautions such as those detailed in the section for conventional conveyors need to be taken for multiple loading points. On discharge, the belt is opened out completely over a pulley and is closed again after discharging. Discharge stations can be installed at any point along the conveyor.
Conveyor maintenance
The reduction of spillage, belt damage and belt cleaning requirements are a major feature of this type of design. Damage of the belt due to foreign objects is unlikely to cause tearing of the belt or damage to the conveyor structure, although all necessary precautions should still be taken to prevent them from entering the belt.
Installation considerations
Unlike other conveyors, this system is able to negotiate very sharp turns.
Changes in direction can be achieved with a bend radius of less than one metre.
The envelope of the conveyor cross section is small and although the support structure is lightweight it does need to be fixed firmly and rigidly. Because of its inherent flexibility, the system should be reasonably tolerant of misalignment and small amounts of ground movement.
Transportation of long lengths of belt underground may be problematic as the combination of belt and cable make the belt bulky. The length of each belt section will probably not exceed 300 m for the smaller belt and 150 m for the larger belt.
To join each belt length, the ropes can be spliced as with any wire rope and a rubber section added to bridge the gap.
Compatibility with other systems
This system is incapable of carrying personnel or equipment and therefore possibilities for an additional transporting system will be required in the return direction. It may, for example, be possible to transport backfill material back to the mining areas
4.7.1.13 Characterisation of Pipe Conveyors
As with enclosed conveyors, the pipe conveyor (Figure 4-52) is noticeably different in construction to a standard belt conveyor. The pipe conveyor has an open troughed section at the loading point. The belt is folded to form a pipe fully enclosing the rock being transported for the full length of the conveyor until it reaches the head of the conveyor where the tube is opened as it passes over the head pulley and discharges its load. Apart from the tubular cross section and idler arrangement, the conveyor is similar to a conventional conveyor with regard to the drive, take-up and tail arrangements.
rock---~-~~;+o
-4..---idlers
IJ-,t~""",,""-belt
Only top section shown
bracket~~~==========~
Figure 4-52: Tube conveyor (Du Plessis
et
a/., 1999)Capacity
Belts of this type have been rated for duties up to 2 200 tons per hour with pipe diameters between 100 and 500 mm specified.
Limitations on length
This type of conveyor has similar properties to that of a conventional conveyor.
The limitation on length will be primarily related to the belt strength. The required flexibility of the belt to allow the formation of the tube may not permit high strength belts to be used, however lengths of up to 5000 m have been specified by the manufactures. In addition, the nature of the design does not allow for intermediate drive units to be used hence the maximum length of a single conveyor will be limited by the tension produced at the drive unit.
Environmental compatibility
Except during loading and discharge the belt is completely closed at all times eliminating .spillage and the release of dust into the environment. Spillage associated with belt cleaning is also eliminated as the dirty side of the belt is always on the inside.
Powering systems
The drive units for this type of conveyor will be similar to those of a conventional conveyor and the same principles apply.
Feed and discharge arrangements
The feed and discharge arrangements for this type of conveyor will be similar to those of a conventional conveyor with a maximum permitted lump size of 125 mm.
This would require some primary crushing arrangement and in the case of the smaller diameter conveyor, secondary crushing may also be necessary.
Conveyor maintenance
The reduction of spillage, belt damage and belt cleaning requirements are major features of this type of design. Damage of the belt due to foreign objects could cause tearing of the belt or damage to the conveyor structure and thus all the necessary precautions would be required to prevent foreign objects from entering the belt.
Installation considerations
This type of conveyor system is able to negotiate reasonably sharp turns with radii of between 100 and 300 pipe diameters, depending upon the belt tension. Severe changes in direction could onlybeachieved using multiple conveyors.
. The conveyor cross section is smaller than for a conventional and although the support structure is lightweight it requires to be fixed firmly and rigidly. Thus, adjustments for ground movement may be difficult to achieve.
Compatibility with other systems
This system is not capable of carrying personnel or equipment.