Main coal beneficiation equipments in use at global washeries are:
And in addition other equipments in use are double roll crusher, vibratory feeder, and coal ash monitor and thickener system.
The Pneumatic process of beneficiation is the best suited option for Coals which has very high percentage of Near Gravity Material. This method involves size reduction and screening of coal to segregate it into defined sizes followed by concentration i.e. separation of particles into fractions more homogeneous in nature.
This system consists of
The higher density raw coal is gathered at the bottom of the beneficiation bed and it moves to the waste side under the performance of the guiding block and it ultimately goes to the falling trough.
The heavy media plant consists of two major sections - H.M Bath section and fine coal section with other associated sections for Media handling, de watering, Stocking etc.
-50 mm to +6mm will be the feed to the Heavy Media Circuit from the present screening plant, for which provision has already been made.
The LOFLO bath consists of a static tank filled with heavy media and conveying arrangement for the removal of floated clean coal and sink rejects.
Both clean coal and Rejects from the bath are fed to the dedicated desliming & rinsing (D & R) screens from which they are fed to the clean coal and Reject bunkers via respective conveyors.
Under flow from the D & R screens are collected in Dilute Media Sump and fed to a Magnetic Separator for recovery of Magnetite, which is reused as media in the LOFLO Bath.
The media density in the Bath is maintained by an automatic Density control system, which regulates the amount of magnetite and water to provide the correct media density.
-6 mm coal will be diverted to the Fines feed tank. The fines tank is also fed with water & magnetic separator under flow to prepare the slurry, which will be pumped to the screw classifier in the H.M Bath Plant.
The screw classifier is used to classify fine coal in order to remove -500 micro fractions and hence deslime the fine coal fraction. Raked Coal from the screw classifier is further dewatered in the centrifuge to remove surface moisture from the classified product. This dewatered fine coal product from screw finally joins the coarse clean coal circuit.
Classifier over flow (-0.5 mm) is dewatered in thickener and further in vacuum Filter. This fine fraction available in form of cake is conveyed to the stocking area.
Barrel Washer is a combination of cylindrical and conical construction. In the barrel the beneficiation takes place based on the principles of hindered settling. The coal and water is fed in the cylindrical portion of the barrel which rotates at certain predetermined speed. The spiral inside the barrel creates waves in the water in which the pulsation of coal takes place. The lighter coal moves with the top layer of the wave and discharged at the conical end as low ash coal or clean coal. The high ash coal which settles on the bottom is carried by the spirals to upper cylindrical portion and discharged at the top end as rejects. Thus the rejects and clean coal move in the opposite direction inside the barrel.
Auto ash analyzer determines the ash by increasing the transmission of narrow beams of low and high energy gamma rays through coal. The absorption of low energy gamma rays depends on the ash content and on weight per unit area. The absorption of high energy gamma rays depends essentially on the weight per unit area of coal. The mitted energy signals are combined to give the amount of ash independent of coal build up on the conveyor.
The analog signals from the detector are amplified and converted with digital signals and fed to PC interface. The real time data acquisition is carried out and the results are displayed energy 30 seconds.
The display shows :
Vibratory feeder is a device that makes use of vibrations to feed material to a process. Vibratory feeders use vibrations and gravity to move materials. Gravity is used to determine the direction of flow of material and vibration is used to move the material. The material is almost always a dry material the unlike a liquid may not move down an incline without some assistance from the vibration effect.
A common vibratory feeder is cone shaped. Material is delivered in uncontrolled fashion into the top of the feeder and a controlled delivery of material flows from the bottom of the feeder
Sizing and liberation of coal from impurities is the main objective of crushing. Toothed Double Roll Crusher is being used for the purpose. It consists of two rotating toothed rolls in housing. The oversize lumps get crushed between the rolls. The gap between the rolls can be adjusted to meet the desired product size.
The aperture size of the screens is based on the washability studies. In this coal is demanded into different sizes and sent for washing. The fraction which does not require washing is by passed.
The Thickener consists of open tank with feed inlet at the top and a means of collecting sludge at the bottom. The contents are slowly stirred by the rake rotation and the solids settled are drawn off the bottom in continues underflow. Clear water is removed by the overflow weir.
Chemicals are added to the feed to increase settling rate of the solid.
The principle being flocculation of minute solid particles, its sedimentation leaving clear supernatant liquid for re-circulation in the system.