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The main washery at Pleasley was installed by the Simon Carves company of Stockport and commenced operation in May 1940. It was designed to handle the output from the new developments in the thinner seams where it was no longer possible to manually separate the dirt from the larger volumes of small coal being created. All run-of-mine (ROM) coal from the Deep Hard seam was broken to 3in and was processed by the washery as was coal smaller than 3in. from the Top Hard, Dunsil and Waterloo seams.
The washer itself was a Baum jig type and had a capacity of 130 ton/hour. About 80 tons of small coal per day were recovered by slurry dewatering and froth flotation whilst the washery water was clarified for re-use by a means of a flocculation plant.
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The mock-Tudor looking building was the washery. The cone-shaped tower received the washery discharge water and allowed the suspended fine coal to settle out the resultant slurry being run to de-watering screens. The building at the tower’s LH base held the main pump for recirculating the washery wate whilst the Art-Deco building on the RH side contained the flotation plant for extracting the finer coal particles still remaining in the slurry water. The circular structure between the buildings received the finer dirt particles or slimes which were treated to enable them to coagulate and settle out.
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When the washery was first installed, the run-of-mine coal arrived at the washery in railwayay wagons. They were unloaded by a tippler located further to the left in the photo above, and carried up to the top floor of the washery on the conveyor coming in at the top LH side.
Washed and graded coals were loaded into railway wagons underneath the washery as was the reject dirt. The dirt was then transported back to the North screens where it was unloaded and elevated into a bunker feeding the aerial ropeway which carried it the pit tip.
Around 1960 an overhead twin conveyor gantry was constructed between the South pit-top and a new surge bunker on the West side of the washery. Vibrating feeders at the base of the bunker discharged the contents onto a short conveyor running to the foot of the wagon tippler where it delivered directly onto the washery feed conveyor. The tippler itself was retained for use in case of breakdown of the conveyor system. One conveyor delivered ROM coal to the surge bunker whilst the second took away dirt delivered to it by a conveyor running from the washery in a another short overhead gantry.
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Separation of coal from dirt by this type of wet jig process depends on the fact that coal has a lower density than that of the dirt. Hence dirt will sink more quickly than coal when settling in water.
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The Baum wash box consisted of a large tank of water with a perforated horizontal screen about 1/3 of the way down. One side of the tank was partitioned off by a vertical plate extending part way down and capped at the top to form an air chamber. Pneumatic valves were positioned along the cap to admit and release compressed air above the water. Raw coal was fed into one end of the tank and the water was pulsed upwards from below the sieve by the air admitted behind the partition, thus causing the bed of coal and dirt to be lifted and become separated into different layers as it sank again. The valves operated in sequence and the pulse moved from one end of the tank to the other thus moving the bed along
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Large pieces of dirt sank immediately. They were removed via a window at the input end of the tank and taken away by a bucket elevator. The tank had a central weir and the coal was carried over into a second section where the same process was repeated to separate the remaining smaller pieces of dirt. At the discharge end the coal was carried over another weir into a chute whilst thedirt was removed through another window in the end of the box below it and taken away by second bucket elevator. Small dirt falling through the screen itself was removed by a worm-screw in the base of the box, discharging at each end onto the elevators.
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The coal discharged from the wash-box then passed to a pair of classifying screens where it was rinsed to remove the adhering coal particles and sorted into different sizes. There were four main sizes:
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These screens consisted of a series of inclined perforated steel plates, the size of the perforations increasing from plate to plate. The plate oscillated backwards and forwards, propelling the coal forwards. Coal which fell through the perforations fell onto a boom conveyor which loaded into railway wagons positioned in the respective siding under the washery.
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To improve economic efficiency and to minimise water consumption, the discharge water from the wash-box and the classifying screens was clarified in order to allow it to be reused. It was pumped to the top of the conical concrete tower where the suspended solids were allowed to settle to the bottom and form a slurry. Clean water was drawn off from near the top and piped back into the washery whilst the slurry was drawn off at the bottom and sent to de-watering screens where the coarser coal particles were removed and added to the washed smalls. The water from the slurry de-watering screens was further treated in the froth flotation and the flocculation plant to remove the residual solids.
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In the froth floation process, a foaming agent was added to the water containing mixed coal and dirt fines and, by agitation in a tank, a froth was formed with only the coal particles adhering to the surface of the bubbles. A series of tanks or cells were used and the froth was skimmed off from each and carried to a shallow bath at the base of a large rotating hollow drum with a finely perforated outer skin. A vacuum was applied to the inside of the drum and as it slowly rotated through the bath a coating of wet coal particles was sucked onto the surface.
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As the drum turned, water was extracted from the coating until, at about 3/4 of the way round, a blast of air from the inside detached a section of the coating which fell into a railway wagon located below, any remaining coating being removed by a scraper bar. The resultant “filter cake” as it was called, was then added to the coal used to fire the boilers.
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The reject tailings from the froth flotation cells contained fine silt and clay sized particles and in order to speed up their settlement, a flocculating agent was added to make them coagulate into larger clumps. They were then piped into the centre of a large circular concrete clarification tank where the solids soon settled to the bottom and the clean water ran off over the rim into a collecting channel ready to be returned to the main washery water. The thick slurry was drawn off from the sloping bottom of the tank to be mixed with the rest of the washery dirt and sent to the tip.
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