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June, 1935

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The cleaning problem at this colliery was governed by the necessity to obtain a product as low in ash as commercially possible without increasing the moisture content. Although "Silksworth" ranked amongst the best gas coals in the country even before the cleaning plant was installed, there is an increasing demand on the part of the gas companies for low-ash coal, and the colliery company was naturally anxious to spare no effort in meeting this requirement. To satisfy these conditions a Birtley plant was decided upon, this being designed to treat 180 tons of raw coal per hour from 2 in. to zero size. The general arrangement of the plant will be readily understood by reference to the flow sheet reproduced in Fig. 1. Briefly, the coal is screened on a battery of four Hummer units, the second pair of which is fitted with double decks. In the first pair the oversize above 7/16 in. is removed, while the second pair separate the material into 7/16 in. to ¼ in. and ¼ in. to 1/8 in. One wide Super Vee separator is arranged to treat the 2 in. to 7/16 in., and a second to treat the 7/16 in. to ¼ in., on one side and ¼ in. to 1/8 in. on the other. The undersize, i.e., below 1/8 in., after passing through an aspirator, is treated on two further wide Super Vee machines. From these last two machines the cleaned coal is passed over four vibrating screens where the 1/32 in. material is extracted, mixed with the relatively coarse dust from the aspirator settling chambers and finally treated on four additional machines specially designed for cleaning fine coal and dust.

The air-borne dust from the aspirators, together with the dust from the separator hoods, is delivered to three filters where it is deposited and ultimately delivered with the clean coal from the separators treating the large coal, the plus 1/32 in. coal from the two fines separators, and the cleaned dust from the smalls separators. The whole of this cleaned material is then returned to the main screening plant of the colliery where it is delivered to the picking belts for mixing with the round coal.

The shale from all the machines is collected and delivered to a shale bunker, whilst the middlings are returned to the raw coal elevator for recirculation.

Individual fans of the Gill propeller type supply air to each of the fan separators, whilst a separate centrifugal fan operates each aspirator. The whole of the air ranges are linked up to the three filters, and the cleaned air is exhausted to atmosphere by means of a single Gill type propeller fan.

A special feature of this plant is the care which has been taken to reduce breakage, and for this reason Redler conveyors have been incorporated. In this type of conveyor the coal is not carried bodily as with belt or bucket conveyors, but is induced to flow in a trough en masse by means of an endless chain — a condition rendered possible by the friction between the particles. Several types of chain are used and when necessary the equipment can be designed to convey the coal up very steep inclines and even vertically. In addition to the use of these conveyors breakage of the coal has been avoided on this plant by reducing the free fall at every conceivable point and the various bunkers are provided with spiral chutes.

Although the plant has been designed to treat 130 tons per hour continuously, the raw coal conveyor has been arranged to handle up to 200 tons per hour, in order to take care of fluctuations from the tipplers.

Referring now to the general arrangement drawing in Fig. 4, the raw coal is delivered from the main screening plant to a conveyor a which discharges into a concrete bunker b, of 150 tons capacity. A continuous bucket elevator with a capacity of 150 tons per hour raises the coal to the vibrating screens d which effect a separation at 7/16 in. The oversize is fed to the storage hopper 1 and the undersize to the double-decked vibrators, where it is divided into 7/16 in. to ¼ in., ¼ in. to 1/8 in. and 1/8 in to zero. The 7/16 in. to ¼ in. and ¼ in. to 1/8 in. sizes are then fed to the storage hopper 2 and the 1/8 in. to zero to the distributing conveyor n for delivery to hoppers 3 and 4. Sized coal from the hoppers 1 and 2 is delivered by gravity feeders to the separators g and f, while the in to zero sizes from hoppers 3 and 4 pass through the aspirators j and thence to the separators h.

Cleaned coal from separators f and g is discharged to the conveyor o, while that from the separators h is fed to the vibrating screens l to be screened at 1/32 in. The oversize passes to conveyor c and the undersize to the special fines separators m. The cleaned coal from these separators is delivered to the conveyor p and the shale to the conveyor u. Finally, the coal from the conveyors a and p, together with the dust from the aspirators j and the cleaned product from the separators h, is fed to the elevator q and conveyor r, and delivered on to the discharge ends of the picking belts of the main screening plant.

The shale is handled by the conveyor l to a further conveyor v and discharged in the hopper w; conveyor v also receives the material from conveyor u. The dust extraction plant consists of three filters x, suction being provided by a Gill-type propeller fan z. On being discharged from the filters the dust is taken by the conveyor y and discharged to the cleaned fines conveyor p.

Middlings are taken to the conveyor s and discharged to the raw coal elevator c for recirculation through the plant. Provision is also made for taking out middlings as a separate product when required.

Reference has already been made to the Redler conveyors and elevators in use on this plant; another departure is the system of grouping which has been adopted on the electrical control system. On previous plants control was exercised from a push-button board situated on the separator floor, it being possible from this point to start up or stop the whole of the motors on the plant in their proper sequence by pressing one or the other of two buttons. In addition, a "stop" button was provided for each motor, the contactors being so interlocked that in the event of a key motor being stopped, other predetermined motors in the sequence are stopped, thus preventing flooding at any point. Whilst all these safeguards are maintained in the present system, certain motors are grouped on a single contactor, and arrangements are provided so that certain units can be transferred from automatic to hand-operated control when required, the main start-and-stop system being maintained as in previous systems. Another point of difference lies in the fact that the earlier type of faceplate starter has been replaced by pendulum gear, which ensures the correct time interval between the starting up of the various units. Centralised control from the separator floor is still maintained.

All the motors and switchgear are of Metro-Vick manufacture; the 3 h.p. motors are of the totally enclosed, fan-cooled type, whilst the remainder are of the protected type or totally enclosed, according to whether they are situated in or out of doors. The large motors above 10 h.p. are of the pipe-ventilated type, with protective trunking. The total horsepower of the motors installed is 380 h.p., the largest unit being the 180 h.p. unit driving the main dust extractor fan. Actually, the horsepower absorbed is 265.

The aspirator fans are driven by 10 h.p. motors and are of the "Cyclone" type manufactured by Matthews & Yates Limited.

The plant is housed in a steel-framed, brick-filled structure and, as is usual with Birtley plants, the glazing is on the lavish side to ensure that no dark corners exist to foster the accumulation of scrap and other oddments or to cause accidents.

Drawings and Photographs accompanying the article