Coal preparation is the process whereby the raw run-of-mine coal is turned into a saleable clean coal product of consistent size and quality specified by the consumer. The end use of the coal falls into the following general categories:
- Electricity generation: The coal is burned to supply heat to drive turbines which generate electricity.
- Iron and steel making: The coal is heated in ovens, in the absence of air, to drive off gases (volatile matter) to produce coke. The coke is used in the blast furnace to make iron and steel. Coal can also be added directly to the blast furnace as in the pulverized coal injection (PCI) process.
- Industrial: Coal is used in the metallurgical industry as a reductant, whereby its carbon content is used to remove oxygen (reducing) in a metallurgical process.
- Heating: Coal can be used domestically and industrially as a fuel for space heating. It is also used as a fuel in dry kilns for the manufacture of cement.
Crushing and Breaking
Run-of-mine coal from the pit needs to be crushed to an acceptable top size for treatment in the preparation plant. Typical crushing and breaking devices are:
- Feeder breakers: A rotation drum fitted with picks that fracture the coal. The coal is delivered by a scraper conveyor and the drum rotates in the same direction as the coal flow. Feeder breakers are commonly used underground, however, there are some in use on surface in the coal preparation circuit.
- Rotary breakers: The breaker circuit of an outer fixed shell with an inner rotating drum fitted with perforated plates. Typical rotational speed of the drum is 12–18 rpm. Lifter plates pick up the run-of-mine coal which then falls across the diameter of the drum. The softer coal breaks and passes through the perforations while the harder rock is transported to the exit. The rotary breaker achieves two functions, size reduction and beneficiation by removal of rock.
- Roll crushers: Roll crushers can consist of either a single rotating roll and a stationary anvil (plate), or two rolls rotating at the same speed towards one another. The roll faces are usually toothed or corrugated. A common form of crusher is the two stage or quad roll crusher whereby the product from the first twin roll crusher falls into the second twin roll crusher set at a smaller aperture, with the result that a large-scale reduction can be achieved in one machine. A typical application would be crushing run-of-mine material down to 50 mm.
Crushing is sometimes used following the coal cleaning process, when large size coal is crushed to meet market requirements. Roll crushers or hammer mills are usually used. The hammer mill consists of a set of free swinging hammers rotating on a shaft that strike the coal and throw it against a fixed plate.
Coal is sized before and after the beneficiation (cleaning) process. Different cleaning processes are used on different sizes of coal, so that raw coal on entering the coal preparation plant will be screened (sieved) into three or four sizes which then go through to the appropriate cleaning process. The screening process is usually carried out by rectangular vibrating screens with a mesh or punched plate screen deck. At sizes below 6 mm wet screening is used to increase the efficiency of the sizing operation and at sizes below 0.5 mm a static curved screen (sieve bend) is placed before the vibrating screen to improve efficiency.
Following the beneficiation process, the clean coal is sometimes sized by screening into a variety of products for the industrial and domestic coal markets. Sizing of clean coal is rarely used for coal for electricity generating (thermal coal) or for steel making (metallurgical coal).
Storage and Stockpiling
Coal is typically stored and stockpiled at three points in the preparation and handling chain:
- raw coal storage and stockpiling between the mine and the preparation plant
- clean coal storage and stockpiling between the preparation plant and the rail or road loadout point
- clean coal storage at ports which may or may not be controlled by the mine.
Typically raw coal storage occurs after crushing and usually takes the form of open stockpiles (conical, elongated or circular), silos (cylindrical) or bunkers. It is common for seam blending to be carried out at this stage in order to supply a homogenous product to the preparation plant. Blending may be as simple as sequentially depositing different coals onto a conical stockpile to sophisticated operations using stacker conveyors and bucket wheel reclaimers.
Clean coal can be stored in a variety of ways, such as open stockpiles or silos. The clean coal storage system is designed to allow for rapid loading of rail cars or road trucks. Clean coal silos are usually constructed over a rail track allowing unit trains of up to 100 cars to be drawn slowly under the silo and filled to a known weight. In-motion weighing is usually used to maintain a continuous operation.
There are inherent dangers in stockpiled coals. Stockpiles may be unstable. Walking on stockpiles should be forbidden because internal collapses can occur and because reclamation can start without warning. Physically cleaning blockages or hangups in bunkers or silos should be treated with the greatest care as seemingly stable coal can suddenly slip.
Coal Cleaning (Beneficiation)
Raw coal contains material from “pure” coal to rock with a variety of material in between, with relative densities ranging from 1.30 to 2.5. Coal is cleaned by separating the low density material (saleable product) from the high density material (refuse). The exact density of separation depends on the nature of the coal and the clean coal quality specification. It is impractical to separate fine coal on a density basis and as a result 0.5 mm raw coal is separated by processes using the difference in surface properties of coal and rock. The usual method employed is froth flotation.
There are two basic methods employed, one being a system using water, where the movement of the raw coal in water results in the lighter coal having a greater acceleration than the heavier rock. The second method is to immerse the raw coal in a liquid with a density between coal and rock with the result that the coal floats and the rock sinks (dense medium separation).
The systems using water are as follows:
- Jigs: In this application raw coal is introduced into a pulsating bath of water. The raw coal is moved across a perforated plate with water pulsating through it. A stratified bed of material is established with the heavier rock at the bottom and the lighter coal at the top. At the discharge end, the refuse is removed from the clean coal. Typical size ranges treated in a jig are 75 mm to 12 mm. There are special application fine coal jigs which use an artificial bed of feldspar rock.
- Concentrating tables: A concentrating table consists of a riffled rubber deck carried on a supporting mechanism, connected to a head mechanism that imparts a rapid reciprocating motion in a direction parallel to the riffles. The slide slope of the table can be adjusted. A cross flow of water is provided by means of a launder mounted along the upper side of the deck. The feed enters just ahead of the water supply and is fanned out over the table deck by differential motion and gravitational flow. The raw coal particles are stratified into horizontal zones (or layers). The clean coal overflows the lower side of the table, and the discard is removed at the far side. Tables operate over the size range 5 ´ 0.5 mm.
- Spirals: The treatment of coal fines with spirals utilizes a principle whereby raw fine coal is carried down a spiral path in a stream of water and centrifugal forces direct the lighter coal particles to the outside of the stream and the heavier particles to the inside. A splitter device at the discharge end separates the fine coal from the fine refuse. Spirals are used as a cleaning devise on 2 mm ´ 0.1 mm size fractions.
- Water-only cyclones: The water-borne raw coal is fed tangentially under pressure into a cyclone, resulting in a whirlpool effect and centrifugal forces move the heavier material to the cyclone wall and from there they are transported to the underflow at the apex (or spigot). The lighter particles (coal) remain in the centre of the whirlpool vortex and are removed upwards via a pipe (vortex finder) and report to the overflow. The exact density of separation can be adjusted by varying pressure, vortex finder length and diameter, and apex diameter. The water-only cyclone typically treats material in the 0.5 mm ´ 0.1 mm size range and is operated in two stages to improve separating efficiency.
The second type of density separation is dense medium. In a heavy liquid (dense medium), particles having a density lower than the liquid (coal) will float and those having a density higher (rock) will sink. The most practical industrial application of a dense medium is a finely ground suspension of magnetite in water. This has many advantages, namely:
- The mixture is benign, as compared to inorganic or organic fluids.
- The density can be rapidly adjusted by varying the magnetite/water ratio.
- The magnetite can be easily recycled by removing it from the product streams with magnetic separators.
There are two classes of dense medium separators, the bath- or vessel-type separator for coarse coal in the range 75 mm 12 mm and the cyclone-type separator cleaning coal in the range 5 mm ´ 0.5 mm.
The bath-type separators can be deep or shallow baths where the float material is carried over the lip of the bath and the sink material is extracted from the bottom of the bath by scraper chain or paddle wheel.
The cyclone-type separator enhances the gravitational forces with centrifugal forces. The centrifugal acceleration is about 20 times greater than the gravity acceleration acting upon the particles in the bath separator (this acceleration approaches 200 times greater than the gravity acceleration at the cyclone apex). These large forces account for the high throughput of the cyclone and its ability to treat small coal.
The products from the dense medium separators, namely clean coal and refuse, both pass over drain and rinse screens where the magnetite medium is removed and returned to the separators. The diluted magnetite from the rinsing screens is passed through magnetic separators to recover the magnetite for re-use. The magnetic separators consist of rotating stainless steel cylinders containing fixed ceramic magnets mounted on the stationary drum shaft. The drum is immersed in a stainless steel tank containing the dilute magnetite suspension. As the drum rotates, magnetite adheres to the area near the fixed internal magnets. The magnetite is carried out of the bath and out of the magnetic field and falls from the drum surface via a scraper to a stock tank.
Both nuclear density gauges and nuclear on-stream analysers are used in coal preparation plants. Safety precautions relating to radiation source instruments must be observed.
Froth flotation is a physio-chemical process that depends upon the selective attachment of air bubbles to coal particle surfaces and the non-attachment of refuse particles. This process involves the use of suitable reagents to establish a hydrophobic (water-repellent) surface on the solids to be floated. Air bubbles are generated within a tank (or cell) and as they rise to the surface the reagent-coated fine coal particles adhere to the bubble, the non-coal refuse remains at the bottom of the cell. The coal bearing froth is removed from the surface by paddles and is then dewatered by filtration or centrifuge. The refuse (or tailings) pass to a discharge box and are usually thickened before being pumped to a tailings impoundment pond.
The reagents used in the froth flotation of coal are generally frothers and collectors. Frothers are used to facilitate the production of a stable froth (i.e., froths that do not break up). They are chemicals that reduce the surface tension of water. The most commonly used frother in coal flotation is methyl isobutyl carbinol (MIBC). The function of a collector is to promote contact between coal particles and air bubbles by forming a thin coating over the particles to be floated, which renders the particle water-repellent. At the same time the collector must be selective, that is, it must not coat the particles that are not to be floated (i.e., the tailings). The most commonly used collector in coal flotation is fuel oil.
The briquetting of coal has a long history. In the late 1800s relatively worthless fine coal or slack was compressed to form a “patent fuel” or briquette. This product was acceptable to both the domestic and industrial markets. In order to form a stable briquette, a binder was necessary. Usually coal tars and pitches were used. The coal briquetting industry for the domestic market has been in decline for some years. However, there have been some advances in technology and applications.
High-moisture low-rank coals may be upgraded by thermal drying and subsequent removal of a portion of the inherent or “locked in” moisture. However, the product from this process is friable and prone to the re-absorbtion of moisture and spontaneous combustion. Briquetting of low-rank coal allows for a stable, transportable product to be made. Briquetting is also used in the anthracite industry, where large-sized products have a significantly higher selling price.
Coal briquetting has also been used in emerging economies where briquettes are used as cooking fuel in rural areas. The process of manufacture usually involves a devolatilizing step whereby excess gas or volatile matter is driven off prior to briquetting in order to produce a “smokeless” domestic fuel.
The briquetting process, therefore, usually has the following steps:
- Coal drying: Moisture content is critical because it has an impact on the strength of the briquette. Methods used are direct drying (a flash dryer using hot gas) and indirect drying (a disc dryer using steam heat).
- Devolatilizing: This is only applicable to low-rank high-volatile coals. The equipment used is a retort or a beehive type coke oven.
- Crushing: The coal is often crushed because a smaller particle size results in a stronger briquette.
- Binders: Binders are required to ensure that the briquette has adequate strength to withstand normal handling. The types of binders that have been used are coke oven pitch, petroleum asphalt, ammonium lignosulphorate and starch. The typical addition rate is 5 to 15% by weight. The fine coal and binder are mixed in a pug mill or paddle mixer at an elevated temperature.
- Briquette manufacture: The coal-binder mixture is fed to a double roll press with indented surfaces. A variety of briquette shapes can be made depending on the type of roller indentation. The most common form of briquette is the pillow shape. The pressure increases the apparent density of the coal-binder mix by 1.5 to 3 times.
- Coating and baking: With some binders (ammonium lignosulphorate and petroleum asphalt) a heat treatment in the range of 300°C is necessary to harden the briquettes. The heat treatment oven is an enclosed conveyor and heated with hot gases.
- Cooling/quenching: The cooling oven is an enclosed conveyor with recirculating air passing to reduce the briquette temperature to an ambient condition. Off-gases are collected, scrubbed and discharged to the atmosphere. Quenching with water is sometimes used to cool the briquettes.
Briquetting of soft brown coal with a high moisture content of 60 to 70% is a somewhat different process than that described above. The brown coals are frequently upgraded by briquetting, which involves crushing, screening and drying the coal to approximately 15% moisture, and extrusion pressing without binder into compacts. Large quantities of coal are treated in this way in Germany, India, Poland and Australia. The dryer used is a steam-heated rotary tube dryer. Following extrusion pressing, the compacted coal is cut and cooled before being transferred to belt conveyors to railcars, road trucks or storage.
Briquetting plants handle large quantities of highly combustible material associated with potentially explosive mixtures of coal dust and air. Dust control, collection and handling as well as good housekeeping are all of considerable importance to safe operation.
Refuse and Tailings Disposal
Waste disposal is an integral part of a modern coal preparation plant. Both coarse refuse and fine tailings in the form of slurry must be transported and disposed of in an environmentally responsible way.
Coarse refuse is transported by truck, conveyor belt or aerial ropeway to the solids disposal area, which usually forms the walls of the tailings impoundment. The refuse can also be returned to the open pit.
Innovative cost-effective forms of transporting of coarse waste are now being used, namely, crushing and transportation by pumping in slurry form to an impoundment pond and also by a pneumatic system to underground storage.
It is necessary to select a disposal site which has a minimal amount of exposed surface while at the same time provides for good stability. A structure that is exposed on all sides permits more surface drainage, with a greater tendency for silt formation in nearby water courses, and also a greater probability of spontaneous combustion. To minimize both these effects, greater quantities of cover material, compacting and sealing, are required. The ideal disposal construction is the valley-fill type of operation.
Preparation-plant waste embankments may fail for several reasons:
- weak foundations
- excessively steep slopes of excessive heights
- poor control of water and fine material seepage through the dump
- inadequate water control during extreme rainfall events.
The principal categories of design and construction techniques which can greatly reduce environmental hazards associated with coal-refuse disposal are:
- drainage from within the refuse pile
- diversion of surface drainage
- waste compaction to minimize spontaneous combustion
- waste pile stability.
Tailings (fine solid waste in water) are usually transported by pipe line to an impoundment area. However, in some instances tailings impoundment is not environmentally acceptable and alternative treatment is necessary, namely, dewatering of tailings by belt press or high speed centrifuge and then disposal of the dewatered product by belt or truck in the coarse refuse area.
Tailings impoundments (ponds) operate on the principle that the tailings settle out to the bottom and the resulting clarified water is pumped back to the plant for reuse. The pool elevation in the pond is maintained such that storm in-flows are stored and then drawn off by pumping or small decant systems. It may be necessary periodically to remove sediment from smaller impoundments to extend their life. The retaining embankment of the impoundment is usually constructed of coarse refuse. Poor design of the retaining wall and liquefaction of the tailings due to poor drainage can lead to dangerous situations. Stabilizing agents, usually calcium-based chemicals, have been used to produce a cementation effect.
Tailings impoundments normally develop over an extended period of the mine’s life, with continually changing conditions. Therefore the stability of the impoundment structure should be carefully and continuously monitored.