In General
• The large quantities of material handled for these operations leads to the use of oversize equipment–from bucketwheel excavators to haul trucks, to wide, fast belt conveying systems.
Material
• Materials can range from lignite and low rank coal to ores for base and precious metals. In addition, large quantities of overburden must be moved. This overburden can change greatly in material characteristics as different layers of strata are removed on the way down to the ore level.
Considerations for Specific Industries Conveying
• Wide, high-speed, high-capacity conveyors are the rule rather than the exception. For example, German lignite operations use conveyors with belt widths up to 124 inches, (3200 mm) operating at speeds up to 2100 fpm (10.5 m/sec). Typically these operations are pushing equipment suppliers for bigger, faster, higher-tonnage systems.
• There are often extreme levels of impact in conveyor loading zones from uncrushed, run- of-mine materials. Loading zones should be designed for these forces with impact idlers and impact cradles. To handle this impact, many operations incorporate catenary idlers, creating difficulties in the sealing of load zones.
• Changing material characteristics–from different layers of overburden for
example–may allow accumulations that can choke and clog chutes. The installation of air cannons on transfer chutes may be useful.
Belt Cleaning
• High belt speed and material velocity lead to high frictional temperature and high vibration levels. Belt cleaners must be engineered to withstand these conditions. Pre-cleaners with a high volume of urethane are often used to extend service life and dissipate heat.
• High conveyor operating speeds may not allow higher pressure secondary cleaners to be installed. However, larger head pulleys may have enough room for two lower-pressure pre- cleaners below the material trajectory.
• The continuous production schedules and high tonnage conveyed will require systems
engineered for long life and low maintenance. In some operations, this may allow the use of “maintain while in service” systems; i.e., belt cleaners installed on mandrel-style or
telescoping mounts that allow the blades to be changed while the conveyor runs. The
adoption of this maintenance practice will require approval of the appropriate regulatory agency and internal safety department. • Return belt cleaning is important because of
the potentially large size or sticky nature of material. These materials become trapped between the belt and bend pulleys and can damage the belt either by puncturing it or increasing the tension. The buildup of materials can also quickly cause mistracking. Return-side belt cleaning devices must be designed for high impact and to prevent entrapment of materials in the suspension systems. Pulley cleaners are often applied in addition to belt plows.
• Cleaning devices for the inside of the belt must be designed for high impact and to prevent entrapment of materials in the suspension systems. Pulley cleaners are often applied in addition to belt plows.
Dust Management
• Spray-applied water is the typical dust suppression method. However, high rates of water application will increase problems with carryback or screen blinding. Surfactant or foam suppression can be considered as an alternative.
• The truck dump leading to a crusher will generally require dust control systems.
See also Coal Mining, Metal Mining & Production
chapter
17
Safety
Belt conveyors and their transfer points can be dangerous. By their very nature, they feature many “pinch” points and speeding objects. They apply large amounts of mechanical energy to what is basically a loaded rubber band. Operations and maintenance personnel must always be aware of the power of a conveyor, and they must maintain a healthy respect for its potential to injure or kill an untrained or unwary individual.
However, it should be noted that conveyors can be one of the safest ways–if not the safest–to move the large quantities of material required for large-scale industrial processes. Other forms of bulk haulage–from trucks to trains to ships–all carry their own risks and safety concerns, and may in fact be more unsafe than belt conveyors. As CEMA noted in Belt Conveyors
for Bulk Materials, few personnel are required for operation of conveyors
and they are exposed to fewer hazards than other means for transportation.
While it must be recognized that accidents can happen, it must also be observed that they can be prevented. Conveyor safety begins with a design that avoids foreseeable hazards. Plant management must require
equipment that is safe and easy to maintain. Installations must be designed with free entrance and exit from conveyor transfer areas. Vendors should provide information and training on the safe use of their products. Management must insist upon good housekeeping practices in terms of the cleanup of fugitive material, but also in terms of the disposal of discarded machine elements and packaging materials,
The establishment and maintenance of safe practices in the design, construction, operation, and maintenance of conveyors and conveyor transfer points will aid greatly in the prevention of accidents. Guidelines for safety in conveyor design and operation have been spelled out by CEMA in Safety Standards for Conveyors and Related Equipment and adopted by the American National Standards Institute in ANSI B20.1-1976. These works and/or their international equivalents should be consulted as a guide for the design and construction of any belt conveyor system.