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Colliery Engineering

December, 1935

The drying plant

Drying of the mixed ½ in. smalls, the flotation concentrates and slurry presented a ticklish problem, for not only did the colliery company specify a very low moisture content for these products, but the loads vary from 45 tons per hour of the ½ in. to zero material down to 15 tons per hour of flotation concentrates. To meet these conditions a thermal dryer was installed, this being of the Edgar Allen-Buell type (Büttner system). It consists of a drum 8½ ft. diameter by 32 ft. long, arranged for three speeds of rotation through a David Brown gearbox in order to accommodate the varying loads. Dust from the Waring filters is used for firing, and as 60 per cent, of this material is capable of passing through a 200-mesh no grinding plant is necessary. Two Buell pulverised-fuel burners are provided and also a small pilot jet burning a trifling quantity of fuel oil. The dust is fed to the burners from a 5-ton storage bunker by a feeder which delivers it to the two mixing valves. The wet coal is fed into the dryer by a water-cooled spiral conveyor, from which it is passed by lifting and conveying blades to the cross-shelving of the drum. This shelving (Fig. 12) is ingenious and comprises a number of cross-shaped members forming longitudinal shelves which have the effect of distributing the incoming wet material uniformly across the entire section of the drum as well as along its whole length. As the dryer rotates, the material is kept in constant motion, sliding from one shelf to another, thereby exposing fresh surfaces continuously to the action of the drying gases. One revolution of the drum is sufficient to effect the distribution of the material in the manner indicated, and the material gets turned over no less than four times during each revolution so that the evaporation capacity per unit of cylinder volume is exceptionally high — over 13 lb. of water per cu. ft. per hour in certain cases.

Apart from the thermal characteristics mentioned, there are a number of other important features of a mechanical nature which automatically follow as a consequence of this internal distribution arrangement. In the first place the load on the drum being balanced, the power absorbed is low. Secondly, owing to the short distance through which the material falls when sliding from one shelf to the next, the treatment to which it is subjected is of a gentle nature and very little dust is, therefore, generated due to the direct action of the drum itself. Thirdly, the material distributed along the shelves acts as a bed for the particles being showered through the drying gases to fall upon, so that wear and tear on the members is almost negligible. At the exhaust end a discharge gear is fitted which enables minor adjustments in the rate of the discharge to be effected. The exhaust gases, which leave the dryer at about 200° F. and 70 per cent. saturation, are drawn by the main exhauster fan through a centrifugal dust collector (Van Tongeren System), the dust being returned to the dried coal. This is effective in trapping the bulk of the dust, but to ensure that the finest particles are not discharged to atmosphere the exhaust gases are finally passed through a wet separator. This consists of a steel cylinder containing porcelain rings, supported on screens and subjected to high-pressure water sprays. The dust recovered, together with the spray and condensed water, is returned to the main washery sump.

Blending plant

Blending is effected in a two-shaft paddle mixer which is fed by three elevators. The first, situated at the feed end, handles the material from the dryer, which, when specially low-moisture contents are required, may include the whole of the washed smalls, the filtered flotation concentrates, and the slurry. Next, a small Redler elevator discharges into the mixer any filter dust not required for combustion in the dryer. The third elevator handles the whole or any proportion of the washed smalls taken direct from the dewatering screens and fed direct to the mixer; or if desired the dewatered smalls can be by-passed direct to the trucks from this elevator.

Usually the products from the mixer are discharged into a bunker and loaded into wagons, but, as an alternative when ½ in. washed smalls are not made, the mixer can discharge into the scraper conveyor, taking the mixed flotation concentrate and filter dust to the main clean-coal conveyor, i.e., for mixing with the dry-cleaned smalls and hand-picked large. The mixer is fitted with a water-jacket so as to assist in cooling down the dried products.

Electrical equipment

All the motors operating the various units are started up from a central contactor board and are arranged in groups for sequence actuation. Each group is self-contained, and the sequence is so ordered to prevent flooding at any point. Certain motors, not immediately affecting the coal circuits, can, of course, be started separately by their own hand switches. The fact that there are several alternative routes for the products made it necessary to have the contactor board arranged to cut out any group of motors not required whilst still retaining the automatic features. This was effected by arranging special selector switches on the contactor panel; the switches for the particular groups that are to run are then placed in the "on" position. The main starter push-button, situated on the separator floor, is then pressed and the motors in the various groups in the "on" position start up in sequence.

The push-button starting panel consists of one main start, one main stop, 21 individual stop push-buttons and four individual start buttons for the separators. Always the plant is started up by pressing the main start button and stopped by the main stop button. Should any individual stop button be pressed not only is that particular unit stopped but also all the motors depending on it. Any of the separators may, however, be started and stopped individually without affecting the running of the rest of the plant. A Klaxon horn is sounded before the motors connected with the contactor panel are started. All the electrical arrangements, together with the motors, were supplied by Metropolitan-Vickers Electrical Co. Ltd. The plant absorbs rather under 2 h.p. per ton of coal throughput, about half being due to the dust collecting. Where line shafting is employed to serve a number of units the transmission is effected by Whittle belts and Renold chains, and a number of David Brown reduction gears are employed for such applications as the rotary stirrer for the thickener tank, the Büttner dryer and other slow-moving machinery.

Summary and results

For convenience and simplicity it may be as well to summarise the throughput of the various units. The total input is 160 tons per hour of 4 in. to zero coal and this is handled as shown in the table opposite.

Table giving capacities of cleaning units
Dry cleaning	120	tons per hour	1 3/8 in. to zero
Washing	40	"	4 in. to 1¾ in.
Washing (alternative)	60	"	4 in. to 1 in.
Washing (alternative)	33	"	½ in to 1/32 in.
Elmore plant	10	"	1/32 in. to zero
Coal drying	45	"
Maximum loads
Dry cleaning plant	120
Nuts box	70
Fines box	50
Floatation plant	15

Table giving results from Rising Sun Plant
	Percentage Ash
Unscreened gas coal	6½
Washed cobbles/trebles/doubles	5 to 5½
Washed gas smalls	5 to 6
Dry cleaned smalls (½ to zero)	7
Dry cleaned singles	5
Dry cleaned pearls	6½
Dry cleaned peas	6
Flotation concentrate *	6½ to 7
* Actually ash contents down to 5 to 5½ per cent. can be obtained in this concentrate.

In addition, water clarification is effected at the rate of 650 gallons per minute for the wash water and at the rate of 200 gallons per minute for the flotation tailings. The washed coal is screened into cobbles, trebles, doubles and undersize, and the dry-cleaned product into 1 3/8 to 1 in., singles, pearls, peas and duff. Any of the following products can be delivered with the round coal on the picking belts :– washed cobbles, trebles, doubles and undersize, and dry-cleaned coal from 1¾ in. to zero. The table above gives some of the results obtained from the plant.

Drawings and Photographs accompanying the article