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Colliery Engineering

August, 1933

Fig. 3 illustrates the actual table deck of one of the Newbiggin tables and shows clearly the skimmers for coal and middlings and (nearest the camera) the discharge chute for shale.

To secure a steady flow of coal it is necessary that the oscillation of the table should be smooth and without jerk. The purpose of this motion is to assist in producing a uniform thickness and resistance of bed and to permit the coal, under the pneumatic influence, to flow in the direction of the discharge by gravity. It is therefore important that there shall be no lost motion or slackness in the oscillating mechanism. A roller bearing of particular design to take both axial and radial loads is employed. This is mounted on an eccentric sheave keyed to a portion of the main shaft, which is turned eccentrically relative to the shaft itself. By the provision of a number of keyways on this part of the shaft, the sheave can be locked in several positions relative to the eccentric portion of the shaft. A number of different eccentric throws are thus available. The roller-bearing housing is bolted to a heavy forged eccentric rod, the opposite end of which connects to the table through a gudgeon pin mounted in a machined cast-steel bracket, also bolted to the table. The provision of the latter ensures perfect alignment between the table and the mechanism, thus eliminating vibration and undue stresses on the supports. The gudgeon pin is of large diameter, and is made of specially heat-treated nickel-chrome steel, giving an extreme degree of hardness. The pin works in a pair of similarly-treated hard steel steps. The pin and steps are manufactured to a very fine clearance, so that there is absolutely no play in the whole mechanism during operation. The pin is centrally drilled for grease-gun lubrication.

This arrangement has proved most satisfactory in practice, lost motion due to wear having been practically eliminated, and a very smooth oscillatory motion is obtained.

Many detail improvements have been made in the general design of the cleaning unit for ease of adjustment and maintenance. The skimmers are each carried from bridges, which extend across the table above the point of the V of the skimmer. A sleeve, mounted at the point of the vertical plough walls of the skimmer, moves on a square threaded vertical spindle carried from the centre of the bridge, locking nuts being provided. The bases of the skimmers are of perforated steel sheeting reinforced with edge beading and longitudinal ribs. The discharge lips are screwed down to the sides of the table, steel distance pieces being provided to allow for the vertical adjustment of the skimmers.

The perforated deck of the table, which is of steel sheets carried on a strong lattice grid made of flat steel strip, has a very robust construction. The decking of each cell is independent of the next. This enables the perforated sheets to be easily changed, and one cell can be renewed without the others being interfered with. The sheets have a considerable life, usually six to twelve months, according to the hardness of the shale in the coal being treated.

Fig. 4 illustrates the manner in which the clean coal is progressively skimmed from the surface of the layer on the table, the middlings being skimmed off immediately before the final elimination of the refuse.

In the Newbiggin plant, improvements have also been made in the general design, with the result that the whole process is entirely dustless. The dust nuisance was one of the bugbears of the earlier dry-cleaning plants, and probably did more to militate against the advancement of this method than any other factor. The dust hooding of the Newbiggin plant is of completely new design, and an operating-chamber is formed between the two tables, which is sealed from the rest of the building. A very slight air pressure is introduced into this chamber, which, when inspection doors are opened, causes air to flow in and thus prevent any escape of dust into the surrounding plant. All the operator's controls and inspection doors are in this chamber so that the entire process can be observed and controlled under absolutely clean conditions. This arrangement has been extraordinarily successful, and no dust escapes from the tables.

The system of "mist-cloud" dust settlement, which has been developed with these plants, is incorporated, but the disposition of the atomisers has been re-arranged, and a remarkably complete settlement of dust is now achieved, so that only clean air escapes from the building through the ventilators at the top. One of the great advantages of this system, apart from the cleanliness of the plant, is that there is no dust at the loading points.

Fig. 5 shows the interior of the expansion chamber over the tables, and illustrates the tiers of primary and secondary sprays for precipitating the extremely fine dust which rises into this upper zone. The purpose of these sprays is to create a humidity in the chamber akin to a mist or fog. In operation, a high- speed jet of air is discharged across the mouth of the water nozzle and draws up a fine thread of water, which is immediately split up and dispersed in tiny particles by the air stream. The mist particles, in their very fine state, combine with the dust particles, and settle together in small masses, which fall into the chutes and pass away with the coal. After traversing the mist, the cleaned air is allowed to pass to atmosphere through roof ventilators, which are designed to prevent back-draught, due to wind effects. The spray from each jet is very small, but the jets are numerous, thus giving a better effect than fewer and larger sprays. Much experimental work has been done to determine the best type of jet, size of nozzles, air pressure, water level, &c., and also the disposition of the batteries in the chamber. The object is to produce a continuous mist in the chamber without any large drops of water, which are useless for combining with the dust particles. A most satisfactory combination of these factors, as typified in the Newbiggin arrangement, has now been reached.

Owing to the very small amount of dust which rises and is actually treated in this way, the effect on the moisture content of the coal is practically undetectable. Even the dust which is actually treated only becomes moistened after settlement. The condition of the treated dust in the coal confers further benefits in the elimination of the dust nuisance when loading and discharging the coal at the colliery and docks.

The clean coal is collected from the sides of the table by chutes, which are constructed integrally with the air boxes and feed into a totally enclosed Redler conveyor. This delivers the coal to wagons beneath the plant or, alternatively, to a vibratory screen to make sized products. The clean coal can also be delivered to the picking belts to mix with the large to make unscreened coal.

Refuse is delivered by chute to wagons, and the middlings are collected from the table by a totally enclosed Redler conveyor and returned to the raw-coal scraper conveyor for re-treatment by means of a belt conveyor and vertical elevator.

Fig. 6 shows the clean-coal chutes from the tables passing into the Redler conveyor mentioned above, while Fig. 7 shows the neat layout of the driving end of the tables with the protected chain drives and the control rods of the variable-speed gear for the pulsators and star feeders. This latter view shows how the whole of the mechanical part of the dry-cleaning unit is driven from one motor. Switchgear for all the motors, for both wet and dry plants, is housed in a spacious switch-house, which is fireproof and isolated from other parts of the building; all the motors are started from here. The title block gives an exterior view of the plant, with the main raw-coal conveyor delivering to the cleaning-plant bunkers.

Drawings and Photographs accompanying the article