1. Introduction
Although different types of conveyors find application in hard rock mines, this chapter discusses belt conveyors, which are by far the most common. Underground hard rock mines use belt conveyors for lateral and inclined ore transfer; however, belt conveyors are not as popular as they are for similar surface applications. Belt conveyors for underground service usually are designed more rugged and operate at slower speeds than a comparable overland conveyor.
Hard rock mine belt conveyors normally require the ore to be crushed before it is conveyed or at least broken enough to pass through a grizzly opening. Run-of-mine (ROM) material is not normally conveyed except for short level runs. The finer the ore is crushed, the longer the belt life, the more reliable the system, and the lower the operating cost. The reasons for this include less impact from lumps and the elimination of tramp material, such as rock bolts, rebar, drill steel, and scaling bars that can wreak havoc on a belt conveyor. The least cost and most reliable underground belt conveyor system ever installed is reputed to be the one at the Gaspé Copper Mine that employed both primary (jaw) and secondary (cone) crushers underground to size the conveyor feed.
Belt conveyors conserve energy because they are driven by electric motors with an efficiency of near 95% and their payload-to-dead load ratio is approximately 4:1. By comparison, the efficiency of the diesel engine in a haulage truck does not exceed 40% and a truck’s payload-to-dead load ratio is no better than 1½:1.
Belt conveyor systems are less flexible than truck haulage and require a high initial investment. Usually, this means that belt conveyors are the economical choice only when there is a relatively high production rate and the transport distance is significant. Belt conveyor systems are selected for other reasons in certain applications (for example, short conveyors are employed underground to optimize control of feed to a loading pocket and prevent a run of fines from reaching the shaft).
Special types of belt conveyors include extendable systems, cable belts, and high angle (HAC) belts. The special conveyor types have had few applications in hard rock mines and are not dealt with in this chapter. Many types of feeders exist for belt conveyors, but the most popular in hard rock mines is the vibrating feeder. The more expensive apron (caterpillar pan) feeder is still employed in certain applications, such as handling large lumps. Other types of feeders are rarely employed in underground mines. Even Ross chain feeders have been used, but their normal application is for crushers, not conveyors.
Due to the fact that many existing computer programs predate metric usage, conveyor calculations are most often completed in Imperial units of measure. This practice is followed in the text of this chapter.
2. Rules of Thumb
Costs
• An underground mine is more economically served by a belt conveyor than railcars or trucks when the daily mine production exceeds 5,000 tons. Source: Al Fernie
• As a rule, a belt conveyor operation is more economical than truck haulage if the conveying distance exceeds 1 kilometer (3,280 feet). Source: Heinz Altoff
• The ton-mile cost of transport by belt conveyor may be as low as one-tenth the cost by haul truck. Source: Robert Schmidt
• The installed capital cost of a long belt conveyor system to be put underground is approximately equal to the cost of driving the heading in which it is to be placed. Source: Jack de la Vergne
• Operating maintenance cost per year for a belt conveyor is 2% of the purchase cost of equipment plus 5% of the belt cost. To this should be added belt replacement every five to 15 years (five for underground hard rock mines). Source: Hans Nauman
Feed and Feeders
• In a hard rock mine, the product from a jaw crusher to feed a conveyor belt will have a size distribution such that the -80% fraction size is slightly less than the open side setting of the crusher. For example, if the open side setting of the underground jaw crusher is 6 inches, then the d80 product size = 5¾ inches. Source: Unknown
• For an apron feeder, the bed depth of material fed should be uniform and equal to one-half the width of the feeder. Source: Dave Assinck
• A vibratory feeder is best designed for a bed depth of about half its width. Source: Bill Potma
• The free fall of crushed ore to a belt must not exceed 4 feet. Chutes, baffles, or rock boxes should be employed to reduce impact and save belt life. Source: Heinz Schober
• The horsepower requirements for apron feeders listed by manufacturers are generally low. They should be increased by a factor of 30 to 50% to take into account considerations like starting torque, starting when cold, when the bearings are sticky, and when the bearings become worn. Source: Reisner and Rothe
• Power requirements for apron feeders are about twice as high as for comparable belt feeders. Source: Reisner and Rothe
• 75-90% of belt wear occurs at the loading points. Source: Lawrence Adler
Belt Conveyor Design
• On well-engineered systems, using appropriate controls to limit acceleration, the (static) factor of safety for belt tension can be reduced from 10:1 to 8:1 for fabric belts and from 7:1 to 6:1 for steel cord belts. Source: D. T. Price
• The standard troughing angles in North America are 20, 35, and 45 degrees. In Europe, they are 20, 30, and 40 degrees. A 20-degree troughing angle permits the use of the thickest belts, so the heaviest material and maximum lump size can be carried. A troughing angle of 35 degrees is typically employed for conveying crushed ore. Source: Unknown
• For conveying crushed ore, the cross-section of the material load on the belt can usually be accurately calculated using a 20-degree surcharge angle. It should be considered that when conveying over a long distance, the dynamic settling of the load could reduce the surcharge angle to 15 degrees. Source: Al Firnie
• The availability of a belt conveyor is 90%; if coupled with a crusher, the availability of the system is 85%. Source: Wolfgang Guderley
• Stacker conveyors (portable or radial) should be inclined at 18 degrees (32%) from the horizontal. Source: Dave Assinck
• In-pit conveyors should not be inclined more than 16½ degrees (29%) from the horizontal. Source: John Marek
• A downhill conveyor should not be designed steeper than 20%. This is the maximum declination for containing material on the belt under braking conditions. Source: Al Firnie
• The pulley face should be at least 1 inch wider than the belt for belts up to 24 inches wide and 3 inches wider for belts greater than 24 inches. Source: Alex Vallance
• The length of skirt boards should be at least three times the width of the belt. Source: Jack de la Vergne
3. Tricks of the Trade
• The easy way to have your conveyor system designed for a particular application is to call your friendly local belt or idler sales agent and ask him to do it for you on his computer program. This service is usually performed promptly and without charge. The procedure will help insure that you have put together all the information required and identify possible problem areas or items of potential controversy. You may talk the agent into giving you a copy of his program so that you can readily make adjustments in-house to accommodate changes resulting from subsequent detailed design. Source: Jack de la Vergne
• One way to have a computer program designed for overland conveyors work for an underground mine installation is to change the output as follows: raise the drive HP by 5%, increase the belt width by 6 inches, shorten the carrying idler spacing by six inches, and kick up the idlers by one class and one size. Source: Jack de la Vergne
• A continuous ore handling system (conveyor) is more easily automated than a batch system (rail cars or trucks). Source: Fred Edwards
• To increase the capacity of an operating belt conveyor, it is more practical and less expensive to install a linear drive conveyor booster than to replace the drive. Source: Dowty Meco
• When installing a long horizontal belt conveyor, it is normally specified to vulcanize all the splices. If one of them is made a mechanical splice, it will facilitate the belt cut required as a result of initial belt stretch in operation. Source: Gus Suchar
• Supporting columns for a belt conveyor system can be protected from wayward travel of mobile equipment by enclosing the bottom portion with a culvert section and filling it with concrete. Source: Dennis Sundborg
• Rollback on a belt conveyor may be prevented by chevron patterns on the belt, but avalanching of fine material can only be overcome by reducing the bed height of the material on the belt and, if necessary, increasing the flight speed to restore capacity. Source: Warren Holmes and others
• When the travel of a screw take up for an underground belt conveyor is too short, consider installing two in tandem and/or using a steel cord belt before resorting to a gravity take-up. Source: Ian McKelvie
• A wing snub pulley can make a good belt cleaner. Source: Dave Assinck
• A wing tail pulley helps prevent particles from being trapped between the belt and the pulley face that would otherwise damage the belt. Source: Dave Assinck
• Half-trough pulleys will shorten the transition distance at the tail end, but they can cause the belt to lift off the idlers when empty. As belt loading fluctuates, the belt line will change dramatically, so the feed zone cannot be sealed effectively. Source: Martin Engineering
• Designers, fabricators, and installers of inclined conveyors pay much attention to alignment at the head end where the drive is usually located. Accurate alignment of the tail end is just as important because this is where tension is least and the belt most likely to start wandering off track. Source: Goodyear
• Training idlers can be responsible for more problems than they correct. Their prolonged use typically results in separating the belt plies due to the sidewall tears they inflict. A good operator can keep the belt trained without training idlers. Source: Dave Assinck
• Belt training can be improved at a problem area by shimming the idlers to cant very slightly forward (direction of travel) and, if necessary, slightly off square to the direction of travel. Source: Goodyear
• A staggered idler spacing may be employed on long inclined conveyors to obtain a cost savings. Close spacing is not normally required at the top end where belt tension is highest. Staggered spacing should be avoided in all other cases. Source: Jack de la Vergne
• The calculation of tail pulley tension (manual or computer) is not accurate because the value is obtained by difference. The design of tail pulley, attachments, and take-up should make allowance for a higher design tension than is calculated. Source: Jack de la Vergne
• Cement or very dry concentrate should be transported by screw conveyor and not on a belt conveyor. Source: Mular and Bhappu
• A load-out conveyor is required to properly automate a skip hoist hoisting system. Source: Largo Albert
• The head pulley should always be lagged for wet service. In practice, they are invariably employed for underground mine installations and inclined conveyors on surface. Source: Khoa Mai
• For underground conveyors, the use of impact cassettes (slider beds) that employ low friction bar sections have gained design preference for use at load and transfer points and are replacing the one-time traditional impact idlers for this duty. Source: Heinz Schober
• The metal portion of skirtboards should have a minimum one-inch gap to the belt and the gap should normally be tapered wider in the direction of movement. The exception is the skirtboards on a through conveyor. In all cases, the rubber skirts on the board should just touch the belt. Source: Jack de la Vergne
• Skirtboards on a vibrating feeder do not touch the deck. You should taper the gap from rear to front to prevent jamming of lumps under the skirt. Source: Bill Potma
• A slight increase in width between feeder skirtboards from back to front will reduce friction significantly. Source: Bill Potma
• A slight decrease in width between feeder skirtboards from back to front will help release trapped material. Source: Heinz Schober
• A skirt board taper should be very slight, otherwise the width of the feeder throat may be reduced enough to invite hang-ups. A taper may not be necessary for alluvial material, such as bank sand for a backfill plant. Source: Jack de la Vergne
• In a hard rock mine, the product from a jaw crusher may tend to be slabby, while the product from a gyratory crusher may tend to be blocky, the latter being easier to pass through a feeder or transfer point on a conveyor system. Source: Heinz Schober
• It is better to feed a belt a foot or so past a constriction of the feed to accommodate any bounce-back of material caused by turbulence. Source: Martin Engineering
• Installing a high-side vibratory feeder eliminates the skirtboards and the inherent problems. The only price paid is a slight increase in drive motor capacity. Source: Heinz Schober
• A vibratory feeder that is seated on a frame or pedestal needs approximately 6 inches of lift from rest to release the mounting springs. The design should allow this clearance between the feeder skirt or any other obstruction and the bottom of the feeder pan. Source: Ed Cayouette
• A vibratory feeder that is suspended must be lifted to unhook the suspension and then lowered on a temporary support. A clearance of 6 inches will allow for this and other problems that may be encountered, such as adjusting the feed slope. Source: Ed Cayouette
• To calculate the practical capacity of an apron feeder, assume 75% of the bed is full and select a flight travel that does not exceed 50 fpm. Source: Dave Assinck
4. Belt Conveyor Design
The first step in designing a belt conveyor is to determine the following design criteria.
• Capacity [normally expressed in tons per hour (tph)].
• Layout dimensions (length, lift, and azimuth) for each leg of the conveyor.
• Material origin (ROM, grizzly, jaw crusher, cone crusher, etc.).
• Material description (specific gravity, bulk density, angle of repose, abrasion, foliation, moisture content, pH, and contaminants).
• Material size (crusher setting, grizzly opening, screen analysis).
• Ambient conditions (temperature range, humidity, etc.).
• Applicable statutory mine regulations.
• Applicable insurance stipulations (FM).
• Access dimension and weight restrictions to reach the workplace.
When the criteria are established, the calculations may proceed. Excellent handbooks exist (i.e. Belt Conveyors for Bulk Materials, CEMA) that provide step-by-step procedures; however, it is more convenient to use a standard commercial or custom in-house computer program. Once an initial run is made, the design can be polished to reflect special conditions and to optimize standardization of components.
The CEMA handbook is considered the standard reference. CEMA procedures are typically incorporated into commercial computer programs used to design belt conveyors. The design capacity for a hard rock conveyor system is less than determined by the CEMA procedure and may be expressed as a percentage of the CEMA determination. For example, a hard rock conveyor system capacity might be expressed as being designed on the basis of 70% CEMA for the flow sheet capacity and 85% CEMA for short-term peaks.
While handbooks are valuable to understand the design process, and computer programs are mathematically perfect, they may not adequately address all requirements for a practical design. One common problem area in mines is determining the belt width that will reliably handle lumps in the ore stream. This dilemma crops up underground because capacity is not usually the final determinant for belt width. A similar problem may be found in the concentrator when narrow belts are installed at a high inclination to feed a FAG or SAG mill that wants lumps in the ore to operate efficiently.
5. Conveyor Belt Width
Belt width is typically first dimensioned on the basis of capacity, which is a purely mathematical exercise. Final determination often depends on the characteristics of the material on the belt, particularly particle size distribution.
Handbook Formulas
• The CEMA Handbook states that the belt width ought to be between three and five times the maximum lump size. Unfortunately, this lump size is often taken to be equal to the crusher setting. The CEMA definition of maximum lump size is later found where it states that lump size means “the largest lump that occasionally may be carried.”
• The 1992 SME Handbook states on page 2,171 that “an important factor in selecting minimum normal belt width is to select a figure that is at least three times the maximum lump size.”
• The 1973 SME Handbook shows on page 18-35 that the belt width should be equal to 3.5 times the longest dimension of the occasional lump.
• Edition III of the CE Handbook on page 1,357, Table 16, recommends a 36-inch belt for d80 = 8 inches and 42 inches for d80 = 10 inches.
• Edition VII of Mark’s Handbook, on page 10-75, Table 14, recommends a 36-inch belt for 8- inch lumps and a 42-inch belt for 10-inch lumps, when “sized” (crushed) material is conveyed.
The belt width can be less for a single flight conveyor than for a conveyor involving transfer points that handles the same material. A 30-inch wide belt may be satisfactory to handle ore from a jaw crusher set at an open side of 6 inches on a straight run, but this belt will not be wide enough for a long conveyor system with transfer points, especially when the angle turned is acute.
As a general rule, a belt width of 36 inches is satisfactory for an open-side crusher setting up to 8½ inches, while a 42-inch belt is satisfactory for an open-side setting up to 10½ inches. A 54-inch belt is desired for muck passing through a 12-inch by 15-inch grizzly.
6. Case Histories
Table 17-1 shows belt conveyors at various Canadian Mines.
Table 17-1 Belt Conveyors at Canadian Mines
7. Power Requirements
Calculating the power requirements for a conveyor system is a tedious chore when done manually. The following procedure provides a good approximation of the drive HP required. The procedure is based on the assumption that conveyor power requirements may be determined by adding the power required to actually lift the material to the power required to lift the material a distance equivalent to the friction losses sustained by a level installation.
Total Lift, H = Hg + Hf
Table 17-2 provides the equivalent lift, Hf in feet for various conveyor belt lengths and speeds.
Table 17-2 Equivalent Lift for Belt Conveyor Lengths
Example
Determine the drive power requirements for the following conditions.
• A flat (level) conveyor
• An uphill gradient of 20%
Determine the power generation potential for the following condition.
• A downhill gradient of 20%
Facts:
1. The belt length is 1,000 feet
2. The belt speed is 300 fpm
3. The conveyor output is 300 tph
4. The drive train efficiency, E is 88.5%
Solution:
Output, Q = 300 tph = 10,000 Lbs./minute
Flat
H = Hg + Hf = 0 + 60.2 = 60.2 feet
Belt Horsepower = QH/33,000 = 10,000 x 60.2/33,000 = 18.2 BHP
Drive HP = BHP/E = 18.2/0.885 = 20.6 HP
Select 25 HP motor +20%
H = Hg + Hf = 200 + 60.2 = 260.2 feet
Belt Horsepower = QH/33,000 = 10,000 x 260.2/33,000 = 78.8 BHP
Drive HP = BHP/E = 78.8/0.885 = 89.1 HP
Select 100 HP motor -20%
H = Hg + Hf = -200 + 60.2 = -139.8 feet
Belt Horsepower = QH/33,000 = 10,000 x -199.8/33,000 = -60.5 BHP
Power generation = -BHP x E = 60.5 x 0.885 = 53.5 HP
Equivalent generator output = 53.5 X 0.746 = 40 kW
8. Production Capacity
A belt conveyor production capacity is normally measured by installing a belt scale (“weightometer”). The material is weighed as it passes over a “weigh bridge,” the belt speed is measured, and the results are electronically integrated to provide a rate output. These scales are obtainable off the shelf and are adaptable to both existing and new conveyor belt installations. An accuracy of over 99% may be obtained with proper calibration.
A simple system has been developed to verify the weightometer results or to determine the output when no weightometer is installed. In this method, the loaded conveyor is stopped and a measured length of material on the belt is removed and weighed. The length of material removed is compared with a specified length that will result in short tons from a measurement in pounds. The specified length may be obtained from the following formula.
L= 3V/100
In which L= specified length (feet) and V =belt speed (fpm)
Example
Determine the conveyor output for the following case.
Facts:
1. The belt speed is 200 fpm
2. The material on a 5’-0 length of belt weighs 250 Lbs.
Solution:
The specified belt length = 3 x 200/100 = 6.00 feet
The weight on the specified length = 250 x 6/5 = 300 Lbs.
The conveyor output = 300 tph
9. Friction Around a Circular Drive
The following relationship can be used to relate the tensions to friction around the drum of a conveyor drive pulley.
μ= Coefficient of friction
θ = Contact angle (in radians)
For rubber belts on a bare steel drum, μ= 0.30 (dry), μ=0.18 (wet)
For rubber belts on a ribbed, rubber-lined drum, μ= 0.42
10. Feeder Selection and Design
A feeder is employed to maintain and control bulk material (crushed ore) being transferred from a static source (bin) to another device (moving belt, stationary crusher) to optimize capacity and reliability. A feeder is not normally required at a transfer point from one conveyor to another. (A “belt bender” that employs the same belt after the change in direction is not normally used in hard rock mines.) The transfer point, if well designed, will alter the travel of material to the new direction while maintaining the forward flight velocity at belt speed.
For open pit operations, apron feeders are commonly employed. One reason for employment is their flexibility with respect to flight angle. For underground mines, the popular choice in North America is the vibratory feeder because it is less expensive to purchase, usually reliable, provides ready access for maintenance, and normally is less costly to maintain. Other types, such as a belt feeder, hydro-stoke feeder, or apron feeder may better handle some ores that are particularly abrasive, lumpy, or sticky. Of these, the apron feeder is most often employed.
The apron feeder is relied upon in mines of the Former Soviet Union and a few large mining companies elsewhere. In the past, sometimes a “picking” belt was employed after the feeder that was wider and slower than the main belt. It was also practice to feed a belt with a finger or scalping grizzly that allowed fine particles to fall first, providing a bed to lessen impact from the coarse feed. Neither of these procedures is as popular today, but there is still occasional good application for the second procedure.
Feeder capacity should at least match belt capacity. The feeder model required for a particular application can be selected from manufacturers’ catalogues that provide tables and guidelines for this purpose.
Feeders employed underground today are not really efficient. They use more energy shearing off the material from its static consolidation and overcoming friction than they do accelerating the material to belt speed. New feeder designs (used elsewhere) that employ gravity alone to overcome friction and shear and provide acceleration may soon be available for practical application in hard rock mines.