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June, 1955

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One of the most ambitious colliery modernisation schemes in Great Britain is that at Lynemouth in the No. 3 Area of the Northern (N. & C.) Division of the N.C.B. In 1951, it will be remembered,* we were able to give an outline of the scheme together with a report on the progress then made.

At that time most of the reorganisation work was concentrated underground and although the surface improvements had reached an advanced stage of planning and preparation very little evidence was available on site. That surface activities have not been allowed to lag behind, however, will be apparent from reference to the article we were able to publish** in December, 1953, and which dealt with the new winder installed at No. 2 shaft — the first automatic winding engine in the world to employ dynamic braking. More recently, *** in February of this year, an account was given of the dust-control devices employed on the very modern heapstead at Lynemouth Colliery.

From the foregoing it will be appreciated that the reorganisation scheme planned for Lynemouth is both sizeable and in keeping with the best of modern practice. One of the major steps in the scheme was the planning of a complete coal-preparation plant to treat the whole of the output which is planned to increase to 8,000 tons per day from the Lynemouth shafts plus nearly 4,000 tons per day from the nearby Ellington Colliery.

The new coal-preparation plant has been designed and constructed by Mitchell Engineering Group Limited to deal with this total output of nearly 12,000 tons per day of run-of-mine coal and, with a designed capacity of 800 tons per hour, is the largest plant in operation in Great Britain. Recently, through the kindness of Mr. D. Hindson, Area General Manager, we were able to visit the plant.

Several seams are treated and, because these are variable in character and since coal up to 8 in. in size was to be washed, it was felt that the dense-medium method of washing would be required as the basis of the design. The washing system thus incorporates the Ridley-Scholes float-and-sink system for treating the 8-in, to l½-in. coal, Acco-Baum washboxes for treating the 1½-in. to 0.5-mm. coal, and the Unifloc froth-flotation system for treating the slurry.

The plant has been designed so that the large coal and small coal-washing sections of the plant are substantially balanced, each dealing with about half the total load, thereby achieving the most economical rates of treatment. The layout is arranged so that the whole output is treated in two sections, of 400 tons per hour each, which can operate independently. The plant is arranged with the large-coal and small-coal sections at opposite ends of the plant with the flow towards the centre where the washed products meet and are conveyed to the screening and loading plant over the railways at the side of the main building.

These particular features of design have provided the flexibility of operation which is so necessary to the economics of such a large plant and have avoided the excessive heights of building which are so usual in very large plants.

Raw-coal handling

The plant is fed from a central 200-ton surge bunker which receives the raw coal from the Lynemouth shafts by belt conveyors and also coal from Ellington Colliery which is wagon-borne and delivered by track hopper to a belt conveyor.

Coal is delivered from the bunker by controllable electric feeders to two belt conveyors, each of 400 tons per hour capacity. These belts feed two slow-speed screens which scalp at 8 in. to remove the lump coal which is hand cleaned on the picking belt.

The scalping screens deliver the 8-in, to 0 coal to four high speed screens which separate at 1½ in. These four screens deliver the 8-in, to 1½-in. coal direct into four Ridley-Scholes primary baths while the 1½-in. to 0 coal passes into bunkers beneath the screens which provide 200 tons surge capacity between the large-coal and small-coal washing sections.

Large-coal washing: Ridley-Scholes system

The dense-medium process employed is the Ridley-Scholes system, for which Mitchell Engineering Group Limited hold the exclusive licence. This British system has been developed with the object of simplifying the gravity system for treating run-of mine coal, reducing the capital and operating costs and of extending the precision of heavy-liquid washing to smaller size coal. It comprises a method of float-and-sink separation by simple density displacement in a heavy liquid of predetermined density which can be controlled to give whatever quality of washed coal is required.

The operation will be understood by reference to the diagrammatic illustration of the system. Fig. 8

The washing unit comprises a shallow tank of tapering form with an inclined floor along which travels an ordinary rubber conveyor belt so that in effect, the tank has a moving floor on which the sinks settle and are frictionally conveyed along the tank to discharge at the surface of the medium at the shallow end. The liquid medium is pumped to the deep end of the tank and fed in by distribution inlets over the depth, so that the whole depth of liquid flows gently in a stream from the deep to the shallow end of the tank.

The coal is fed in at the deep end and the liquid medium, the belt conveyor, and the floating coal travel in the same direction along the tank at similar speeds. This is said to promote the best conditions for separation as relative motion between the materials and the liquid is reduced to a minimum and consequently no eddy currents are set up by changes of direction of one or more of the products or by counterflow of the liquid, such as may cause entanglement of middlings and refuse with the coal. Under these- smooth and undisturbed conditions the coal and impurities are able to stratify exactly according to their densities.

The floating coal is assisted to travel along the surface by paddles arranged above the tank, and is discharged over a weir into a transverse trough situated at a suitable distance from the feed end. The coal is floated along the trough in the liquid with which it is discharged and reaches the adjacent drainage screen entirely by flotation, with corresponding avoidance of breakage. Any middling material with a specific gravity so near to that of the liquid that it remains suspended after some length of travel is eventually carried forward as the flow increases in the tapered section, and is discharged at a separate weir and may rejoin the washed coal or the sinks or may be kept separate as desired.

The belt conveyor plays an important part in the separation besides effecting a great mechanical simplification over the orthodox elevators and scrapers generally used for extracting the sinks and eliminating the heavy wear and tear of running such units in an abrasive liquid medium. Since the conveyance of the sinks depends on their frictional contact with the belt only material which is denser than the liquid medium can have the frictional adhesion to the belt necessary for its conveyance. Material lighter than the liquid cannot grip the belt and be transported with the sinks and loss of coal in the refuse is automatically eliminated. This is borne out by the results, the sinks product always being practically entirely free of coal. The belt also has the advantage of a large refuse capacity since it is the full width of the tank and is able to carry shale of any size. The use of the belt, coupled with the transport of the floats by overhung paddles, avoids the employment of wearing mechanical parts operating in the liquid medium.

Magnetite is the medium used in the bath and this is under substantially stable or uniform-density conditions, the stability being maintained partly by control of the coal and clay content of the medium and partly by longitudinal flow of the medium in the tank. The medium is recuperated by gravitational settlement and, in consequence, the medium-circulating system is exceeding simple.

The first plant on the Ridley-Scholes system was installed in 1940, at the Derwenthaugh Cokeworks of the Consett Iron Co., and in 1951, a modern Ridley-Scholes plant was inaugurated at Chislet Colliery, Kent. ****

Primary separating baths

The 8 in. to 1½ in. coal is fed by the four-bath feed screens to four Ridley-Scholes primary dense medium baths in which float-and-sink separation takes place at a specific gravity to suit the character of the coal treated, which is usually 1.4 sp. gr.

These baths comprise tanks of longitudinally tapering shape which contain magnetite liquid medium of the density required for separation. The floor of the tank is upwardly inclined and transversely troughed. The whole area of the floor is covered by an endless troughed rubber belt which enters through rubber seals in the end wall of the tank and slowly travels along the floor to deliver at the shallow end. The belt returns beneath the tank where the driving arrangements are located. Thus the carrying flight of the belt is the only moving element in the liquid and true float and sink separation can take place without any mechanical disturbance.

The medium flows steadily in a uniform stream from the deep to the shallow end of the tank and the coal to be separated is fed into the deep end and travels with the stream of medium. The floating coal is assisted to travel along the surface of the liquid by reciprocating paddles and is discharged, with flowing medium, over a weir into a transverse trough along which it floats to the drainage screen.

The dirt and middlings sink through the liquid medium on to the moving belt and are conveyed out of the medium and delivered over the discharge pulley on to the refuse drainage and spraying screen. The discharge pulley of the belt is vertically adjustable so that the amount of medium discharged with the dirt can be controlled and the components of flow in the bath thus adjusted.

Medium drainage and spraying screens

The floated coal discharged with flowing medium from the baths is floated along curved troughs to the drainage screens situated alongside the baths. These screens are upwardly inclined for good drainage and have 1.0 mm. wedge-wire decks through which the medium drains to the respective surge tanks for recirculation. The coal travels over these screens to the immediately following spraying screens which have 1.0 mm. wedge-wire decks and two batteries of sprays above them. The primary sprays are partially cleaned water overflowing from the thickeners and the secondary sprays are clean make-up fresh water. The coal is thus cleansed of adhering medium and is conveyed to the sizing and loading plant. The diluted medium which is drained through the spraying screens is collected in a pumping tank and delivered by rubber-lined pumps to the medium recuperation system.

The sinks delivered by the primary bath belts are delivered to two drainage screens, one for each pair of baths. Medium is drained from the sinks and returned to the surge tanks as in the case of the coal drainage screens and the sinks are delivered to the secondary baths.

* See Colliery Engineering, March, 1951, p. 93.
** See Colliery Engineering, December, 1953, p. 503.
*** See Colliery Engineering. February, 1955, p. 72.
**** See "New Coal-Preparation Plant at Chislet Colliery." F. Henley Price, Colliery Engineering, July, 1951, p 261.

Drawings and Photographs accompanying the article