Coal preparation plant
Encyclopedia
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
of soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
and rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
, preparing it for transport
Transport
Transport or transportation is the movement of people, cattle, animals and goods from one location to another. Modes of transport include air, rail, road, water, cable, pipeline, and space. The field can be divided into infrastructure, vehicles, and operations...
to market. A CPP may also be called a "coal handling and preparation plant" (CHPP), "prep plant," "tipple," or "wash plant".
The more of this waste
Waste
Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...
material that can be removed from coal, the greater its market value
Value (economics)
An economic value is the worth of a good or service as determined by the market.The economic value of a good or service has puzzled economists since the beginning of the discipline. First, economists tried to estimate the value of a good to an individual alone, and extend that definition to goods...
and the lower its transportation cost
Cost
In production, research, retail, and accounting, a cost is the value of money that has been used up to produce something, and hence is not available for use anymore. In business, the cost may be one of acquisition, in which case the amount of money expended to acquire it is counted as cost. In this...
s.
Run-of-mine (ROM) coal
The coal delivered from the mine that reports to the coal preparation plant is called run-of-mine, or ROM, coal. This is the raw material for the CPP, and consists of coal, rocks, middlings, minerals and contamination. Contamination is usually introduced by the mining process and may include machine parts, used consumables and parts of ground engaging tools. ROM coal can have a large variability of moisture and maximum particle size.Coal handling
Coal needs to be stored at various stages of the preparation process, and conveyed around the CPP facilities. Coal handling is part of the larger field of bulk material handlingBulk material handling
Bulk material handling is an engineering field that is centered around the design of equipment used for the handling of dry materials such as ores, coal, cereals, wood chips, sand, gravel and stone in loose bulk form...
, and is a complex and vital part of the CPP.
Stockpiles
Stockpiles provide surge capacity to various parts of the CPP. ROM coal is delivered with large variations in production rate of tonnes per hour (tph). A ROM stockpile is used to allow the washplant to be fed coal at lower, constant rate.A simple stockpile is formed by machinery dumping coal into a pile, either from dump truck
Dump truck
A dump truck is a truck used for transporting loose material for construction. A typical dump truck is equipped with a hydraulically operated open-box bed hinged at the rear, the front of which can be lifted up to allow the contents to be deposited on the ground behind the truck at the site of...
s, pushed into heaps with bulldozer
Bulldozer
A bulldozer is a crawler equipped with a substantial metal plate used to push large quantities of soil, sand, rubble, etc., during construction work and typically equipped at the rear with a claw-like device to loosen densely-compacted materials.Bulldozers can be found on a wide range of sites,...
s or from conveyor booms. More controlled stockpiles are formed using stacker
Stacker
A stacker is a large machine used in bulk material handling. Its function is to pile bulk material such as limestone, ores and cereals on to a stockpile. A reclaimer can be used to recover the material....
s to form piles along the length of a conveyor, and reclaimer
Reclaimer
A reclaimer is a large machine used in bulk material handling applications. A reclaimer's function is to recover bulk material such as ores and cereals from a stockpile...
s to retrieve the coal when required for product loading, etc.
Taller and wider stockpiles reduce the land area required to store a set tonnage of coal. Larger coal stockpiles have a reduced rate of heat lost, leading to a higher risk of spontaneous combustion.
Stacking
Travelling, lugging boom stackers that straddle a feed conveyor are commonly used to create coal stockpiles.Reclaiming
Tunnel conveyors can be fed by a continuous slot hopper or bunker beneath the stockpile to reclaim material. Front-end loaders and bulldozers can be used to push the coal into feeders. Sometimes front-end loaders are the only means of reclaiming coal from the stockpile. This has a low up-front capital cost, but much higher operating costs, measured in dollars per tonne handled.High-capacity stockpiles are commonly reclaimed using bucket-wheel reclaimer
Reclaimer
A reclaimer is a large machine used in bulk material handling applications. A reclaimer's function is to recover bulk material such as ores and cereals from a stockpile...
s. These can achieve very high rates
Coal sampling
Sampling of coal is an important part of the process control in the CPP. A grab sample is a one-off sample of the coal at a point in the process stream, and tends not to be very representative. A routine sample is taken at a set frequency, either over a period of time or per shipment. Coal sampling consists of several types of sampling devices. A "cross cut" sampler to mimic the "stop belt" sample according to ASTM. ASTM is the standard in which coal must be sampled. A cross cut sampler mounts directly on top of the conveyor belt. The falling stream sampler is placed at the head section of the belt. There are several points in the wash plant that many coal operations choose to sample. The raw coal, before it enters the plant. The refuse, to see what the plant missed. Then the clean coal, to see exaclty what is being shipped. The sampler is set according to Tons per hour, Feet per minute and top size of the product on the actual belt. A sample is taken then crushed, then sub sampled and returned to the main belt. The sample is sent to an Independent lab for testing where the results will be shared with the buyer as well as the supplier. The buyer in many cases will also sample the coal again once it is received to "double check" the results. Continuous measurement of ash, moisture, kCal (BTU), sulfur Fe, Ca, Na, and other element constituents of the coal are reported by cross belt elemental analyzers. This information can be calibrated periodically to the lab data per ASTM methods.Washability
The washability characteristics of a coal reserve are provided by obtaining liberation data on the raw coal sample. Liberation refers to the amount of physical breakage required to separate material of different material densities. Low density material is clean coal whereas high density material is reject (rock). The intermediate density material is called middlings.Liberation data is commonly obtained by float and sink analysis. The procedures for this analysis are detailed in Australian Standard AS 4156.1 – 1994 “Coal preparation - Higher rank coal - Float and sink testing”.
Crushing
Crushing reduces the overall topsize of the ROM coal so that it can be more easily handled and processed within the CPP. Crushing requirements are an important part of CPP design and there are a number of different types.Screening
Screens are used to group process particles into ranges by size. These size ranges are also called grades. Dewatering screens are used to remove water from the product. Screens can be static, or mechanically vibrated. Screen decks can be made from different materials such as high tensile steel, stainless steel, or polyethelene.Gravity separation
Gravity separation methods make use of the different relative densities of different grades of coal, and the reject material.Dense medium process
Dense medium gravity separation methods use a material such as magnetiteMagnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
to form a medium denser than water to assist in separation.
A cyclone is a conical vessel in which coal along with finely ground magnetite (media) is pumped tangentially to a tapered inlet and short cylindrical section followed by a conical section where the separation takes place. The higher sp. gr. fractions being subject to greater centrifugal forces pull away from the central core and descend downwards towards the apex along the wall of cyclone body and pass out as rejects/middlings. The lighter particles are caught in an upward stream and pass out as clean coal through the cyclone overflow outlet via the vortex finder.
Fluid on entry commences in the outer regions of the cyclone body. This combined with rotational motion to which it is constrained creates an outer spiral. The existence of a top central outlet and inability for all the fluid to leave at the cone apex outlet, assist the inward migration of some of the fluid from the external moving mass. The amount of inward migration increases as the apex is neared, i.e. the radius decreased and the fluid which flows in this migratory stream, ultimately reverses its vertical velocity direction and flows upward to the cyclone over flow outlet, i.e. vortex finder. Since it is at the same time rotating, the result is an inner spiral.
The Heavy Media Cyclone is lined with very high quality ceramic tiles inside with a specially designed helical profile. A cyclone is the heart of the washing unit in a Heavy Media Washery. It is a non moving part and hence requires very low maintenance. However, the pressure at the inlet of the cyclone is a very important factor and it is suggested to maintain a minimum pressure of around D x 9 x 9.81 x density/100 (in bars), where D = the inner diameter of the cyclone in mm. It is important to note that the pressure at which pulp (mixture of coal and magnetite) is introduced in the cyclone is the principal means of controlling the forces within the cyclone. With the decrease in pressure more coal shall report to the discard/middlings, thus impairing the efficiency of separation.
If due to some reason the cyclone feed pump is not being able to deliver the required pressure at the inlet of the cyclone, feed should immediately be stopped and the pipelines, tank and pump should be properly checked for any jamming and any jamming should be properly cleaned before starting the circuit or the feed.
Dense medium baths (DMBs)
Different types of DMB include:- Teska bath
- Daniels bath
- Leebar bath
- Tromp shallow bath
- Drewboy bath
- Barvoys bath
- Chance cone
- Wemco drum
Dense medium cyclones
A mixture of raw coal and dense medium enters the cyclone at a predetermined flowrate and pressure. A vortex is formed, and the high density reject forced to the outside to the underflow orifice discharge, also called the spigot. The cleaned coal goes longitudinally down the axis of the cylone and leaves the cyclone through the overflow orifice, also called the vortex finder. From the overflow orifice, the coal goes into the overflow chamber, and is discharged to the next stage of the process.Fine coal methods
Fine coal is cleaned using froth flotation methods. Denver cells and Jameson cells are two flotation methods used.Spirals perform a simple, low cost and reasonably efficient separation of finer sized material, based on particle density and size.
Dewatering product coal
Water is removed from the product to reduce the mass, and runoff on the stockpile. Different methods of dewatering exist, including:- Coarse coal centrifuges
- Screen bowl centrifuges
- Slurry screens
- Dewatering cyclones
- Horizontal belt filterBelt filterThe belt filter is an industrial machine, used for solid/liquid separation processes, particularly the dewatering of sludges in the chemical industry, mining and water treatment...
s
Dewatering tailings (reject)
Water is removed from tailings to recycle water. Filters, centrifuges and thickeners are used in this part of the process. The blackwaterBlackwater (coal)
Blackwater is the pollution produced from coal preparation.It contains copious amounts of hazardous substances, including carcinogenic compounds and heavy metals. Blackwater is permanently impounded in toxic waste facilities, as it cannot be processed into a form that can be returned to the...
which is produced as a by-product
By-product
A by-product is a secondary product derived from a manufacturing process or chemical reaction. It is not the primary product or service being produced.A by-product can be useful and marketable or it can be considered waste....
is typically placed in a coal slurry impoundment.
Thickeners
Thickeners are used for dewatering slurries of either tailings or product. A thickener is a large circular tank that is used to settle out the solid material from the water in the feed slurry. The separated water is clarified and reused as process water in the CPP.Thickeners are sized according to the volume of feed slurry to be processed. Typical size ranges are from 13 to 40m in diameter and 3-4m high. The floor of the thickener is conical, sloping gently down toward the centre.
The feed is pumped into the feedwell, at the centre of the thickener, near the top. The feed is normally dosed with flocculant
Flocculation
Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...
before delivery to the thickener.
The thickened mass is moved toward the bottom centre of the thickener by large rakes that rotate around the tank. Rotation speed is very slow, and drive torques can be high, especially for larger diameter thickeners. Drive torque is usually monitored continuously, as high densities could cause failure of the rakes and drive equipment. Rakes may have the capacity to be raised to reduce drive torque.
The thickened slurry, also called thickener underflow, is pumped out of the bottom of the thickener. In the case of product coal, further dewatering is usually required before shipment. Thickened tailings can be pumped to a tailings dam, combined with larger sized rejects for disposal (co-disposal), or dewatered further before disposal.
Control and instrumentation
Control and instrumentation is a very important part of a CPP. Measurement of flow, density, levels, ash and moisture are inputs to the control system. PLCs are used extensively in plant design. SCADASCADA
SCADA generally refers to industrial control systems : computer systems that monitor and control industrial, infrastructure, or facility-based processes, as described below:...
systems are typically used to control the process. Other instrumentation found in plants include density gauges and online elemental coal analyzers
Coal analyzer
Coal analyzers are used by coal producers, coal preparation plants and coal-fired power plants to determine coal quality in real time. Coal quality parameters of greatest interest include ash, moisture, sulfur, and energy value...
.