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Dust Control Handbook for Minerals Processing
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In any minerals processing facility, dust is generated when ore is shattered or broken as in dumping, loading, transferring, or handling. Proper design, selection, and operation of equipment to minimize ore breakage can therefore reduce dust.
Two primary groups of equipment are used in minerals processing operations:
Processing equipment must be retrofitted with dust control measures; dust control measures can be implemented when bulk material handling equipment is designed. Although the choice of equipment for a specific operation is based on process needs, the use of alternate equipment, improved equipment design, or sometimes even a change in process can greatly reduce dust emissions. Simple measures such as providing shrouds, covers, or enclosures around a dust source can also help to contain dust emissions or allow the existing dust control system to operate more efficiently.
The following sections describe the most commonly used processing and bulk material handling equipment, their major dust emission points, and measures to prevent or reduce dust generation, emission, and dispersion. Several common minerals processing operations are also discussed.
The belt conveyor is one of the major pieces of equipment used to transfer ore between processing equipment and from one point to another within a minerals processing facility. It is also one of the most frequent sources of dust emissions.
Types of Belt Conveyors
There are two primary types of belt conveyors:
Troughed belt conveyors are the most common type of conveyor used in mining and minerals processing operations. Flat belt conveyors are used mostly in non-mining industries.
Pelt conveyors emit dust from the following four points:
Dust Prevention Measures
The amount of dust generated at belt conveyor transfer points depends on the way the material is loaded onto the belt. To reduce dust generation-
The material should be loaded onto the center of the belt.
The material and the belt should travel in the same direction and at the same speed, whenever possible.
Impact at Loading Point
A momentary deflection of the belt between two adjacent idlers may result when ore strikes the belt. As a result, a puff of dust may leak out under the skirting rubber seal. To prevent dust emissions at the loading point, adequately spaced impact idlers (1-ft centers) should be located at transfer points. These will absorb the force of impact and prevent deflection of the belt between the idlers, thus preventing dust leakage under the rubber seal.
Skirtboards are used to keep the material on the belt after it leaves the loading chute. They are equipped with flat rubber strips that provide a dust seal between the skirtboards and the moving belt.
The conventional skirtboard design uses vertical rubber strips. This design is not recommended for the following reasons:
Illustrated is an improved skirting design, which has the following important features:
Muckshelves can be installed in the belt conveyor's material impact zone to-
Dust curtains are used to contain dust within a conveyor enclosure. They should be installed at the head, tail, and exit ends. Dust curtains are made of rubber with 60-65 durometer hardness and can be hinged at the conveyor's head and exit ends to provide easy access during maintenance.
Belt Scrapers - A belt scraper should be installed at the head pulley to dislodge fine dust particles that may adhere to the belt surface and to reduce carryback of fine materials on the return belt. A scrapings chute should also be provided to redirect the material removed by the belt scraper into the process stream or container.
V-Plow - Product spillage or dust leakage may fall on the non carrying side of the belt and eventually build up on the surfaces of the tail pulley. This buildup may move the belt laterally and thus make the skirting rubber seals ineffective. A V-plow installed on the non-carrying side of the belt will clean the belt and prevent buildup of material and dust on the tail pulley, thus keeping the belt properly aligned.
The belt conveyor should be designed to operate at 7% of its full rated capacity. This reduces spillage, dust emission, and wear on skirting rubber seals.
The following measures are suggested to adjust the loading capacity of existing conveyors:
Transfer chutes transport ore from one piece of equipment to another. Significant dust generation can result if the transfer chute is not designed properly.
Dust Prevention Measures
The following points should be considered when designing a transfer chute:
Other types of chutes are used for controlling dust during bulk material handling:
Enclosures are used to contain dust emissions around a dust source. They can also lower the exhaust volume requirements of a dust collection system or help make the existing dust collection system work more efficiently.
The following guidelines are suggested when designing an enclosure for a dust source:
Crushers reduce coarse material to a desired size. The crushing process uses mechanical energy and rubbing to fracture the rock. The forces applied to rock fragments during crushing processes are-
All crushers generate dust. Crushers that primarily use impact forces produce large amounts of fines and dust. Those that primarily use compression forces produce dust in proportion to the stage of reduction: dust production increases progressively from first- to third-stage crushing.
Types of Crushers
There are six main categories of crushers used in minerals processing operations:
Crushers emit dust primarily from two points:
Dust control measures are not usually considered in the design of a crusher. However, the use of shrouds or enclosures for crushers can contain the dust so that a dust control system can operate more efficiently. The following measures are recommended:
Screens are used to sort material according to size. The material fed into a screen is separated into a least two sizes:
Dust is generated in all dry screening processes. However, the amount of dust depends on the particle size contained in the ore, the moisture content, and the type of screening equipment used. Generally, a screen processing finer material produces more dust. Also, screens agitated harder and faster produce more dust than those vibrated more gently and slowly.
Types of Screens
The four most common types of screening equipment are-
In general, screens emit dust from the following points:
The top one-third of the screen surface where incoming material hits
The openings between moving parts (the screen) and stationary parts (the discharge chutes)
Dust Prevention Measures
The rate of dust generated by screens cannot be altered. However, properly enclosing the screen can reduce dust emissions. A complete enclosure that can be easily removed for maintenance and inspection should be used. One commercially available enclosure system consists of a special rubber cloth, rubber molding, and simple metal hardware. The rubber cloth can be attached to the screen to provide an almost perfect dust seal between the screen and the discharge chutes. The top of the screen can also be enclosed by the rubber cloth to prevent dust escape. These lightweight rubber covers not only provide a dust-tight enclosure but also allow easy maintenance, inspection, and replacement of screening surfaces.
A tight sealing system reduces dust emissions and also minimizes air flow, which reduces the exhaust volume for the dust collection system installed downstream.
Some screen manufacturers provide sheet-metal covers to enclose the top of the screen. These covers are effective when properly maintained. However, they do not provide a dust seal between the moving screen surfaces and the stationary chutes.
Storage Bins and Hoppers
Bins and hoppers are used to store ore temporarily. They act as buffers to absorb the surge between unloading and consumption.
The ore is fed into the bin by various equipment, such as conveyors, elevators, and screens. The material is normally discharged from bins and hoppers through gravity or vibrating feeders.
Bins and hoppers primarily emit dust from-
Dust emissions during feeding operations can be minimized by-
Such spouts are sophisticated versions of the telescopic chute and are used to load and stack ore into bags, trucks, and railroad cars. The spouts apply three basic principles of dust control:
2. Dust capture close to the source
3. Preventing air flow caused by the falling material
The falling material is enclosed by a flexible duct, acting as a chute, which retracts as the height of the material pile increases. The duct also prevents air flow during free fall of material between the chute and stockpile. The generated dust is captured by the same flexible duct and is conveyed, countercurrent tot he material flow, to a dust collector.
Another method commonly used to reduce dust emission is to transfer dusty air through a vent into an adjoining bin or silo. Of course, this assumes that multiple bins are used, that the bins are totally enclosed, and that the adjoining bin has room for the displaced air to expand.
A typical bucket elevator consists of a series of buckets mounted on a chain or belt that operates over head and foot wheels. A steel casing usually encloses the entire assembly. The buckets are loaded by scooping up material from the boot (bottom) or by feeding material into the buckets. Material is discharged as the bucket passes over the head wheel.
Types of Bucket Elevators
Pucket elevators emit dust from two points:
The steel casing that encloses the buckets and chain assembly contains dust effectively unless there are holes or openings in the casing.
Emissions at the boot of the bucket elevator can be reduced by proper design of a transfer chute between the feeding equipment and the elevator. Dust production can be reduced significantly by keeping the height of material fall to a minimum and by gently loading material into the boot of the elevator.
Proper venting to a dust collector, as well as proper enclosures and chutes between the elevator discharge and the receiving equipment, will control dust emission at the discharge end of the bucket elevator.
Feeders are relatively short conveyors used to deliver a controlled rate of ore to the processing equipment.
Although dust is emitted from all types of feeders, the amount of dust depends on-
Dust emission points from a feeder are-
Reduce dust production during feeding operations by-
Screw conveyors are one of the oldest and simplest types of equipment used to move ore. They consist of a conveyor screw rotating in a stationary trough. Material placed in the trough is moved along its length by rotation of the screw.
Screw conveyors emit dust primarily from-
Normally, screw conveyors are totally enclosed except at the ends, where emissions can be controlled by proper transfer chute design.
The trough cover is usually fastened by nuts and bolts. However, to maintain a proper dust seal, a self-adhesive neoprene rubber gasket should be installed. Many manufacturers provide two-bar flanges and formed-channel cross members that make a continuous pocket around the trough. The flange-cover sections are set in this channel. Once the channel section is filled with dust, an effective dust seal is created.
Abrasive materials can wear out screw conveyor troughs quickly unless a special coating or abrasion-resistant material is used for the trough.
Pneumatic conveyors are tubes or ducts through which material is moved by pressure or vacuum (suction) systems.
Pressure systems can be either high or low pressure. Low-pressure systems operate at pressures obtainable from a fan; high-pressure systems use a compressed-air source. When material is fed into a pressure system, the airstream immediately suspends is and conveys it to a cyclone- or filter-type collector. The conveying air then escapes through the cyclone vent or a filter.
Vacuum systems offer clean, efficient pickup of material from rail cars, trucks, or bins and hoppers for unloading into other types of equipment. Cyclone receivers or filters are used at the end of such systems to separate the material.
Since pneumatic systems are totally enclosed, dust emissions do not usually occur unless the system has work-out areas.
Dust Prevention Measures
Because maximum wear in the conveying ductwork occurs at elbows, long radius elbows made of heavy gauge material should be used. The elbows can also be lined with refractory or ceramic material to further reduce the war and abrasion.
In low-pressure pneumatic systems, dust may leak through joints. Self-adhesive neoprene gaskets should be used at all joints to provide a dust-tight seal.
Grinding and pulverizing reduce ore to a desired fineness for further treatment.
In its basic form, a grinding mill consists of a horizontal, slow-speed, rotating cylindrical drum. Rod, pebble, and ball mills are the most common types of grinding mills used in minerals processing operations. Steel rods, balls, or pebbles roll freely inside the drum during rotation to provide the grinding action.
Dust is emitted from a grinding mill-
Most grinding mills are fed by a belt conveyor or a feeder. The ground ore is discharged to a screen, conveyor, or elevator.
Properly designed enclosures and chutes, as well as rubber dust seals between moving and stationary components, should be installed at the feed and discharge ends to minimize dust emissions.
Dryers remove water or other volatile material from solid substances primarily by introducing hot gases into a drying chamber. The hot air readily absorbs moisture from the material.
Rotary dryers, flash dryers, spray dryers, and tray and compartment dryers are a few of the many types of dryers. However, in minerals processing operations, rotary dryers are the most commonly used.
Rotary dryers consist of a horizontally inclined rotating cylinder. Material is fed at one end and discharged at the other end. In direct-type rotary dryers, hot gases move through the cylinder in direct contact with the material, either with or against the direction of its flow. The cylinder is equipped with flights, which lift the material and shower it down through the hot gas stream. This type of dryer has a potential for high levels of dust emission. In an indirect-type rotary dryer, heat is applied by combustion gases on the outside of the cylinder or through steam tubes inside the cylinder. This type of dryer has much less tendency to emit dust. It is usually used when continuous drying of powdery or fine material is needed.
Dust emission can be a problem in any dryer in which material is agitate or stirred. Dust is emitted only from the discharge end.
A study on rotary dryers by the Barber Greene Company concluded that the dust carryout increased proportionately to the square of the exhaust gas volume.
Dust Prevention Measures
The hot, dust-laden gases from the dryer are carried to a dust collector, and dust normally does not escape unless the ventilation system is work out or improperly maintained.
Large volumes of processed material are stored in open or enclosed stockpiles. Open stockpiles are normally used when the material size is large. Enclosed stockpiles are used when material is either very fine or must be stored dry. Stockpiles are considered active when material is continuously removed or added. They are considered inactive when material is not added or withdrawn for long periods. All types of stockpiles can be significant dust source.
Generation of dust emissions from stockpiles is due to-
Dust Prevention Measures
Dust from stockpiles can be reduced through the following measures:
- Stone ladders, which consist of a section of vertical pipe into which stone is discharged from the conveyor. At different levels, the pipe has square or rectangular openings through which the material flows to form the stockpile. In addition to reducing the height of free fall of material, stone ladders also provide protection against wind.
- Telescopic chutes, in which the material is discharged to a retractable chute. As the height of the stockpile increases or decreases, the chute is raised or lowered accordingly. Although some free fall of material from the end of the chute to the top of the stockpile occurs, proper design of the chute can keep the drop to a minimum.
- Stacker conveyors, which operate on the same principle as telescopic chutes. The conveyor has an adjustable hinged boom that raises or lowers it according to the height of the stockpile.
- Locating stockpiles behind natural or manufactured windbreaks
- Locating the working area on the leeward side of the active piles
- Covering inactive piles with tarps or other inexpensive materials
Haul roads are used in minerals processing operations to transport ore from the quarry to the processing plant, as well as within the facility. Large amounts of dust can be generated during this process.
Dust emissions from hauling operations vary, depending on-
Dust emissions from haul roads can be minimized by-
Truck and Railroad Car Dumping
Ore is dumped or unloaded from trucks or railroad cars in most minerals processing facilities.
Dust generation and emission during the dumping of ore are caused by-
Enclosures should be used to contain dust during dumping. A sufficiently large enclosure will contain most of the dust, as well as aid internal recirculation of the dust-laden air.
Enclosures for railroad- car dumping operations must have openings at each end to allow cars to enter and exit. However, these openings can crate a wind-tunnel effect. To minimize this effect, the size of these openings should be kept as small as possible with shrouds, rubber curtains, etc. Shrouds or rubber curtains should also be used for other types of enclosures to reduce the area through which air can escape or enter.
In addition to the above design guidelines, the following operational measures are suggested:
Power Handling and Packing
Pulverized material, such as sand, silica flour, hydrated lime, or other powdery material, is normally shipped either in bulk quantities through truck or railroad cars or in small volumes using paper bags, drums, or barrels. Although the process of filling a bag, drum, or barrel is simple, packing fine material can be an extremely dusty operation.
Two types of mini-bulk-packing processes commonly used in minerals processing are-
This process uses bagging machines commonly known as packers. Several different types of packers, such as fluidized auger or screw type, belt or sling type, drop type, and impeller type, are available. However, the two most commonly used packers in the minerals processing industry are-
The open-mouth type uses gravity to fill sewn or adhesive-sealed bags. The material is fed into a weigh hopper, and when the correct weight is reached, the feed gate closes and the contents drop into the open-mouth bag.
The spout fluidizing packer uses compressed air to force the material through a nozzle into a paper bag equipped with a seal valve. When the desired weight is reached, the compressed air supply is cut off and the feed stopped. The internal pressure of the contents of the bag then forces the valve to close.
Barrel or Drum Filling
Sometimes the final product is shipped in barrels, drums, or containers. The material is fed into the receptacle by gravity.
Considerable amounts of dust may be emitted during bagging operations. The following are common occurrences:
Dust emissions caused by bagging machines can be minimized in the following ways: