Coal preparation plant
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A Coal Preparation Plant (CPP) is a facility that washes coal of soil and rock, preparing it for transport to market. A CPP may also be called a Coal Handling and Preparation Plant (CHPP), "tipple" or "wash plant".
The more of this waste material that can be removed from coal, the greater its market value and the lower its transportation costs.
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[edit] 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.
[edit] 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 handling, and is a complex and vital part of the CPP.
[edit] 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 trucks, pushed into heaps with bulldozers or from conveyor booms. More controlled stockpiles are formed using stackers to form piles along the length of a conveyor, and reclaimers to retrieve the coal when required for product loading, etc.
Taller and wider stockpiles redce the 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.
[edit] Stacking
Travelling, lugging boom stackers that straddle a feed conveyor are commonly used to create coal stockpiles.
[edit] 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 reclaimers. These can achieve very high rates.
[edit] 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.
[edit] Washability
The washability characteristics a coal reserve is provided by obtaining liberation data on the raw coal. 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”.
[edit] Crushing
Crushing reduces the overall topsize of the ROM coal so that it can be more easily handling and process within the CPP. Crushing requirements are an important part of CPP design, and a number of different types are available to choose from.
[edit] 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.
[edit] Gravity Separation
Gravity separation methods make use of the different relative densities of different grades of coal, and the reject material.
[edit] Jigs
Jigs are a gravity separation method for coarse coal. Different types of jig include:
- Baum Jig
- Under-air jig
[edit] Dense medium process
Dense medium gravity separation methods use a material such as magnetite to form a medium denser than water to assist in separation.
[edit] 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
[edit] 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.
[edit] Fine coal methods
Fine coal is cleaned using froth flotation methods. Denver cells and Jameson cells are two flotation methods used.
[edit] Dewatering
[edit] 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 filters
[edit] Dewatering tailings (reject)
Water is removed from tailings to recycle water. Filters, centrifuges and thickeners are used in this part of the process. The blackwater which is produced as a by-product is typically placed in a coal slurry impoundment.
[edit] Thickeners
A thickener is a large circular tank that is used to settle out the solid material from the tailing material. This allows a greater proportion of the tailings water to be separated, clarified and recycled in the CPP. Thickeners are sized according to the volume of tailings to be processed. Typical size ranges are from 13 to 40m in diameter and 3-4m high.[1] The floor of the thickener is conical, sloping gently down toward the centre.
The tailings is pumped into the feedwell, at the centre of the thickener, near the top. The tailings is normally dosed with flocculant 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.
The thickened slurry, also called thickener underflow, is pumped out of the bottom of the thickener. This can be combined with larger sized rejects for disposal.
[edit] 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. SCADA systems are typically used to control the process.
[edit] Notes
- ^ Stuart Nicol, The Principles of Coal Preparation, 1997, p.275
[edit] See also
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