3. Literature Review
3.2 Horizontal transportation
3.2.3 Conveyors
Conveyors are becoming popular in South African mines and offer the flexibility of combining personnel and rock transportation. A diagram showing a personnel/rock conveyor is shown in Figure 3-2. Presently, such belts are in operation at Target Gold Mine, East Rand Proprietary Mines Gold Mine, Tshikondeni Coal Mine, Lavino Chrome Mine, and the Bafokeng-Rasimone Platinum Mine.
Belt conveying is considered an efficient means to transport rock over short to medium distances and can operate at steeper gradients than trackbound transport. Conveyors are flexible in their capabilities for receiving rock from one or more locations and are generally considered safe and reliable with a wide range of capacities.
MacNulty (1999) reported that, in normal operation, belts could run at grades of up to 22°, and at maximum speeds of 2,5 m/s (9 km/hr) for personnel transportation and 5,0 m/s for rock transportation. In comparison, the angle of limitation for tracked and trackless vehicles is approximately 2° and 12° respectively with greater transporting speeds. It was commented that conveyors have the disadvantage of not being able to transport material and required additional systems such as a monorail or trackless transport to move material.
MacNulty listed support methods for conveyors as:
• Attached to the sidewall with knee brackets.
• Supported on the footwall using stands.
• Supported from the hangingwall using roofbolts and chains.
• Combinations of the above.
It was concluded that generally, roof supported conveyors suffered less damage from mining vehicles, but the conveyors were more difficult to maintain.
Conveyors generate considerable quantities of heat and this heat is dissipated into the atmosphere. Baker (1981) estimated the heat output as 30% higher than that from a locomotive system. Associated with the heat is the risk of fire, in 1981, 15% of all underground coal mine fires in the USA were a result of ignition of belt conveyors. For underground conveyors, their fire resistance characteristics are given preference to their cut, gorge and abrasive resistance.
Advantages quoted by Baker (1981) of conveyor systems over conventional locomotive/rope haulage systems included:
• Little additional equipment was needed where conveyors are used for rock transport.
• Conveyor personnel riding could be used where the installation of other means of personnel riding was prohibited.
• Personnel were transported individually and the hazards associated with crowding workers at riding stations were avoided.
MacNulty (1999) listed some advantages of conveyor belt transport as being:
• Specific energy consumption less than other haulage systems.
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Operating cost and skilled labour costs reduced.
System could be automated.
Disadvantages included:
High initial capital investment.
Lack of ability to negotiate curves and inclinations greater than the angle of repose of the material being conveyed.
Poor flexibility (generally applicable to permanent installations).
Sensitivity to large particle size and abrasive material.
Breakdown results in a 'standstill' of the conveyor system.
Directional changes require tipping and loading facilities.
Fire risk
Hughes (1997) reported on the design considerations for the installation of the decline personnel/rock conveyors installed at Target Gold Mine. Due to the nature of the orebody mining is concentrated far from the shaft. This led to the decision to use a conveyor for the transportation of ore; later it was decided to use the conveyor for personnel transportation as well. The initial design was for an
·1800 m conveyor at an angle of 8,5°, running at a speed of 2,5 m/so Adjacent to the conveyor, it was decided to install a monorail system for bulk material transportation. Track and trackless transport was considered, but the monorail system was chosen due to the steep inclination of the haulage and the probable damage to the conveyor installation from trackless vehicles. Provisions implemented to facilitate personnel riding on the conveyor belt included:
• Boarding and alighting platforms.
• Additional clearance between top and bottom belts.
• Brakes to prevent runaway belts.
• A personnel barrier above the tail pulley.
• Additional slipping of the hangingwall and sidewall to provide adequate clearance for riders.
• Belt slip, rip and misalignment detectors.
• Chute blockage trips.
• Additional lights, alarms, signals and notices at boarding and alighting platforms.
Particular attention was also paid to:
• The belts acceptance by the workforce.
• Training facilities.
• Misuse of the system.
• Riding on the bottom belt.
• Maintenance and supervision.
The implementation of the "man riding" belts has contributed to the quick access of personnel to the work face and to the productivity of the mine. It is believed
that a significant cost saving was realised compared to the use of a dedicated personnel carrier or chairlifts.
Figure 3-2: Typical boarding and alighting arrangement (Hughes, 1999).
The specifications of Target's personnel and rock belt included:
Table 3-1: Belt conveyor specifications (Hughes, 1999).
DESCRIPTION DATA
Length; Lift; Gradient; Speed 1 800 m; 266 m; 8.5°; 2.5 m/s Belt width; Type 900 mm; ST1250 F (fire retardant)
Material duty Waste rock and gold ore at 225 tons/hr (3.75 tons/min)
Men duty (top and bottom 30 persons/min (2.25 tons/min based on 75
belts). kg/person at 5 m intervals)
Motor and gearbox Two 160 kW, 525 V AC induction motors, through 26: 1 reducers
Drive configuration Tandem drives,shaft mounted on return pulley.
Brakes Two electro-hydraulic brakes with 800 mm
diameter disc
Pulleys Lagged, crowned at edges
Electrical control Twin 525 V variable speed thyristor convertor drives with vector controlled IGBT's
Take-up and tensioning Sinking: Winch take-up at top end
Permanent: Gravity tower take-up at tail end Idler configuration, top 3 roll, 35° trough, series 25, 127 mm diameter,
spaced at 2 m
Idler configuration, bottom 2 roll, 10° 'V' return, 127 mm diameter, spaced at 2m
Leonard (1997) commented on the problems experienced with a conveyor belt installed for East Rand Proprietary Mines. The mine had installed a maximum tensile strength fabric belt on the rock-carrying conveyor instead of a steel core belt. It was believed that conditions in the decline during development would not be suitable for steel core splicing. However, the splices made in the fabric belt broke frequently, which resulted in major production delays. The proper splicing techniques only became evident after numerous laboratory and on-site tests. The lesson learned was: "When considering a class of belting off the standard for an application in a critical area --beware!"