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August, 1933

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Newbiggin Colliery is situated at Newbiggin-by-the-Sea, in Northumberland, about 20 miles from Newcastle. The seams worked are the New Main, Main, Yard and Plessey, the output being of the order of 400,000 tons per annum.

Last year it was decided to install coal-cleaning plant, and as the cleaning of the 3-in, to 1-in, coal presented little difficulty, a very simple type of jig-washing installation was built for this purpose. The capacity of this particular washing unit is 40 tons per hour, the coal being treated in two jigging-type wash boxes, each of 20 tons per hour capacity. The coal is sized after washing into 3 in. to 2 in. and 2 in. to 1 in. by jigging screens. Labour on this section of the coal-cleaning plant amounts to only one man per shift, while the power consumption is 30 H.P., the approximate operating costs being 2d. per ton. The raw coal contains about 11-15 per cent. of ash. which is reduced in the clean coal to 4.5-5 per cent.

The cleaning of the small coal below 1 in. in size presented a rather more difficult problem, and after thorough investigation it was decided to install a static dry-cleaning plant, with a capacity of 60 tons per hour, the coal being cleaned on two tables. The power used on the dry-cleaning plant amounts to 80 H.P., while a further 40 H.P. is used for the conveyors and coal-handling plant. The whole plant is run by one man and a boy, and the cost of coal-cleaning, including labour, power and maintenance amounts to only 1½d. to 2d. per ton.

The seams cleaned vary considerably in character and ash content. On an average the raw coal above 1/16 in. contains 14 to 19 per cent. ash, this being reduced in the cleaning process to 7 per cent., the free coal in the refuse being 1½ per cent. Fixed ash in the different seams varies from 4 to 6 per cent. The small coal, 1 in. to zero, is collected from the colliery shaker screen by means of a cross-belt conveyor, which delivers it to a 30-in, trough-belt conveyor forming the main feed to the dry-cleaning plant. This latter conveyor is also fed by another conveyor, which collects from an underground track hopper for the treatment of stored and other foreign coal.

Over each dry-cleaning table, a feed hopper of 15-tons capacity is provided, to which the coal is distributed through a number of outlets by means of a totally-enclosed scraper conveyor. Fig. 1 shows how the coal is fed from the storage bunker through a star-feeder on to the cleaning table, and it is now proposed to deal with the principles underlying the operation of the process, which has been developed and patented in this country by Huntington, Heberlein & Co. Ltd.

Briefly stated, the underlying principle consists in the establishment of a condition of equilibrium between the force exerted by a static-air pressure (in contra-distinction to that exerted by an air stream, which is uni-directional) and the weight of a densely-packed bed of material of high resistance, which is supported by the pressure as a whole and not by interaction between the air force and the individual particles. When this condition is established, the packed bed becomes distended, due to internal pressure and occupies a greater volume. Freedom of internal movement, as represented by a condition of "fluidity" in the bed, then exists and permits of ordinary displacement of the material by any denser material which may be introduced into the bed.

In practice, these conditions are realised by forming a bed of particles of widely varying sizes, to obtain close packing and allowing the bed to flow at uniform thickness over a pervious support which forms the upper surface, or roof, of a large pneumatic chamber of suitable design for the maintenance of a steady mean air pressure, without the generation of directional currents and consequent variations of pressure at different parts of the air chamber.

A considerable amount of research has been carried out on the process since the first plants were installed about 1928. This has been principally in the direction of investigating the static-pressure pneumatic system to determine its optimum conditions, and also experimental work to improve the separating tables. It has now been established that a fluctuating pressure pneumatic system in which the fluctuations are clearly defined, of high amplitude, and of considerable periodicity, in combination with a rapid and smooth oscillation of the table, is most effective in maintaining the coal bed in its distended and fluid condition. In practice, an air chest of all-steel construction and of very considerable volume is now used. This is of rectangular shape, its width being the same as that of the first cell of the separating table. The most effective form of pulsator, which now comprises standard equipment, consists of a series of rotating vanes arranged in a vertical plane across the end of the air chest connected to the fan. The opening area of the pulsator is the same as the cross-sectional area of the air chest, and forms the entrance thereto, being disposed at right angles to the axis of the chest. This area is proportioned in relation to the area of the separating table to permit of the required mean pressure being maintained in the air chest.

A fan capable of building up pressure against the shutting of the pulsator is connected by an air duct to the pulsator. A pressure substantially greater than that required in the air box is generated in the duct and, by the operation of the pulsator, the required character of pressure fluctuations is established in the air chest. The arrangement of the pulsator in a plane at right angles to the axis of the chest enables a plane pressure wave to be generated over the whole cross-section of the air chest, and in combination with the shape of the latter, the system can be resonantly tuned to give the best conditions of fluctuating pressure with the minimum damping of amplitude

The pulsator vanes, which are relatively narrow (usually 6 in. to 9 in. wide) and which extend across the width of the air chest, are carried on forged spindles running in ball bearings mounted in a rectangular cast-iron frame forming the connection between the air duct and the air chest. Each spindle is separately driven through mitre gears from a single vertical shaft mounted in a totally enclosed casing at the side of the main frame. This shaft is driven through a variable-speed gearbox by a horizontal main shaft at the base of the pulsator. Each of the vanes rotates in a machined port and gives a definite period of closure in the cycle. This creates the clearly-defined pressure waves required.

It is now the usual practice to operate two separating machines in parallel, the respective pulsators being coupled through a breeches duct to a common fan. The pulsators are synchronised to open alternately, as this assists the fan in maintaining the pressure by steadying the load, and also reduces the power required as compared with two separate tables under independent operation. Fig. 2 illustrates the actual pair of pulsators on the Newbiggin plant and shows one pulsator in the open position (nearest the camera) whilst the far pulsator is in the closed position.

The table is of all-steel construction, the deck being constructed upon a built-up steel chassis of great strength. The original arrangement of a series of cells or troughs of successively decreasing widths has been definitely proved, by practical experience and after considerable experimental work in other directions, to be the best means of carrying on the concurrent operations of stratification and separation of strata. The number and proportions of the cells have been modified in many cases as a result of experience in the treatment of different kinds of coal. There may be two, three or four cells, according to the characteristics of the coal being treated. The most usual number, however, is four, and the introduction of the extra cell has been found to be beneficial in giving a sharper line of demarcation between the coal and the shale, and also increases the capacity of the unit.

The cells are proportioned in regard to the required capacity and the necessary stratifying area. The latter figure is about 1.0 to 1.3 sq. ft. per ton of coal treated per hour, according to the cleaning characteristics of the coal and the capacity of the table. The required capacity determines the width of the first cell, which is now made from 6 to 10 ft. in length and is usually about half the total table area. This is because it has been found efficient in practice to discharge a large quantity (usually about 60 per cent.) of the coal at the first skimmer. The other cells are shorter (2 ft. 6 in. to 6 ft.), their proportions being determined largely by the middlings and shale content of the coal, as sufficient margin must be left at the last skimming of the clean coal to ensure that no contamination shall occur. The perforated floors of the cells are at slightly different levels and are connected as before by forwardly-declined impervious steps. These have been reduced in height, while their arrangement and design, in relation to the positions of the skimmers and the contractions in the table sides, have been improved to enhance the even and regular flow of coal from one cell to the next, which is an essential of the process.

Drawings and Photographs accompanying the article