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September, 1948

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The case for a central coal cleaning plant serving several collieries is readily established, provided that the advantages are not outweighed by the cost or difficulty involved in transporting coal from the various pits. In the present instance, the centralised preparation of these coals has an additional advantage in that it enables a uniform blend of washed coal to be sent to the coke ovens, as the whole of the washed coal output from this plant is carbonised. Centralisation also facilitates the technical control that is called for in the preparation of washed coal for making metallurgical coke. The plant at present deals with 10,000 tons of run-of-mine coal per week, and in order to meet the demands of an additional battery of ovens which is just going into commission, the throughput will have to be increased to 18,000 tons per week working two shifts.

The incoming coal is brought by rail to the washery from four collieries, with an average haul of about three miles, and is marshalled on four separate roads, each of which is provided with a track hopper, into which the coal is dumped from hopper-bottom wagons. Each hopper is equipped with a Sherwen vibrating feeder, the first having a capacity of 150 tons per hour, and the other three each having a capacity of 60 tons per hour. The quantity of coal delivered by each feeder can be adjusted by means of rheostat controls. All feeders discharge on to a 36-in. troughed blending conveyor, which in turn delivers the blended raw coal on to a similar inclined conveyor 300 ft. long, this lifting it from the reinforced concrete blending tunnel beneath the tracks up to the head of the preparation plant. Here it is screened at 4 in. on a Ty-rock vibrating screen, and the oversize is crushed in an Edgar Allen single-roll crusher. All the coal fed to the plant is friable, and it is doubtful whether it ever contains more than 10 per cent, of 4-in. oversize.

The 4 in. to zero passing the scalping screen, and the broken oversize, are mixed and are dry-screened at 4 in. on Gyrex units, the oversize being fed to number one Tromp bath, whilst the undersize is first dedusted in two jet type aspirators, and is then wet screened at ¼ in. ; the 5/8 in. x ¼ in. oversize goes to the second Tromp bath, whilst the undersize is laundered to a reinforced concrete sump outside the washery. From this raw fines basin, the coal is elevated to an open discharge Rheolaveur system consisting of four fines troughs and three slurry washing troughs. From this system, the washed coal is laundered to a washed coal sump, from which it is elevated to a battery of three Reineveld centrifuges, one of which acts as a standby, the moisture content being reduced from about 30 per cent. down to 7½ per cent. After this operation the washed coal is loaded out to wagons by means of a troughed belt conveyor.

The shale from both fines and slurry systems is laundered to a shale basin in the basement of the building, and is elevated to a common refuse hopper outside the washery, the re-wash from both systems being laundered back to the raw fines basin.

The Rheolaveur system was installed for a nominal capacity of 90 tons per hour out of a specified throughput of 175 tons per hour of mn-of-mine coal. Normally, more than half of the feed goes to this section of the plant.

The fine coal washing plant dealing with the ¼ inch to 0 inch raw coal consists of four superimposed washing troughs, each fitted with the necessary upward current and free discharge Rheo extraction boxes for the removal of the rewash middlings and the fine shale. Separation in the fine coal washing troughs is at 1.55 specific gravity giving a yield of approximately 90 per cent. at a theoretical float ash of 4.30 per cent. (1941) and 4.38 per cent. (1946).

The yearly average washing results over the above years are given below :–

	1941	1946
Ash in washed fines from end of trough, percent.	5.44	5.60
1.40 specific gravity floats in shale, percent.	1.85	2.00

The above indicates a remarkable consistency of washing results over a long period of time, and the following schedules will show the consistency on a week to week basis :–

Washed coal ash content
Ash per cent.		Ash per cent.
October	5.5	January	5.8
,,	5.7	,,	5.1
,,	5.5	,,	5.6
,,	5.3	,,	5.5
November	5.3	February	5.6
,,	5.6	,,	5.3
,,	5.6	,,	5.9
,,	5.5	,,	5.0
	----		----
Average	5.50	Average	5.48
	----		----

The overflows from the raw fines, washed fines and shale basins are laundered to a common pump sump which is equipped with two main pumps, one of which acts as a standby. The main pump discharges on to a fixed sieve clad with 2 mm. wedge wire above the slurry tower, which is of reinforced concrete and 35 ft. diameter. This slurry tower acts as a constant head tank, and provides the service water for the Rheo system; the settled solids from the bottom of the cone are discharged into the head of the first Rheo slurry washing trough.

A tapping is taken from the rising main, classified to remove any oversize and diluted if necessary down to about 6 per cent, solids, after which it is dosed with lime to bring the pH value up to 10 or 11. The solids are then flocculated by the addition of caustic starch, after which the water is laundered to a 50-ft. diameter thickener which gives a clear water overflow, for return to the washery, and a thickened underflow which is taken to a battery of three rotary vacuum filters, two Unifloc units and one Rovac, having a total area of 650 sq. ft. of filtering surface.

The filter cake, which amounts to about 15 tons per hour, is normally loaded out with the washed coal, but as an alternative can be loaded out separately.

Returning now to the washing of the larger sizes, the 4 in.-5/8 in. and the 5/8 in.-¼ in. coal are fed to their respective Tromp baths in which the medium consists of finely ground rolling mill scale and water, the fineness of grinding being such that the heavy medium settles out slowly, giving a progressive increase in density from the top of each bath to the bottom. The cleaned coal is removed from the bath by means of a push-plate scraper approximately 6 ft. wide and running at 30 ft. per minute, the shale sinking to the bottom of the bath being removed by a chain and bucket elevator. The middlings, which sink below the surface and are not heavy enough to get to the bottom of the bath, are carried from the feed side to the discharge side by means of four horizontal currents of heavy medium, which finally pass out of the bath through a wedge-wire screen on which the middlings are retained until removed by means of an inclined push-plate conveyor. The separate products are discharged on to their respective screens, where they are first sprayed with swill water from the process, and are finally sprayed with clean water.

The underflow from these screens is fed to a pair of vibrating screens clad with 50-mesh bronze wire cloth, to remove coarse particles of coal and shale and discharge them to the Rheo system. The underflow from these screens goes to the recovery tank where the medium settles to the bottom and is re-circulated in the Tromp system, a tapping being taken from the overflow to keep the system reasonably free from very fine coal and clay slimes.

The electrical equipment throughout is of Messrs. Metropolitan-Vickers' manufacture, the bulk of the motors being of the squirrel cage, drip-proof type, but to meet special cases as regards starting load and exposure, slip-ring machines and totally enclosed, fan-cooled frames are included. Originally, practically the whole of the motors were grouped for sequence starting, and although this feature has in the main been retained, it has been found desirable to take certain motors out of the sequence. In general, however, the electrical equipment is controlled by group push-button panels operating through air-break contactor gear, which is housed in a separate room at ground level.

Having described briefly the preparation plant and its equipment, it seems desirable to go into a few points that are of special interest.

In the first place, it is unusual, and indeed it must remain unusual, to find such an extensive plant in this country treating run-of-mine coal where no sized products are made, as normally such a comprehensive layout would be expected to produce cobbles, doubles, trebles, singles, to be loaded out separately. The West Durham coals that are treated in this plant are so friable and so strongly caking, however, that in the first place they could never reach the consumer in anything approximating to their proper grading, and again they are so strongly caking that they would find little, if any, application outside their proper field, which is the production of metallurgical coke.

Turning back for a moment from this question of coking properties, it is of interest to note that the middlings, either with or without a certain amount of filter cake, are burnt on chain-grate stokers under water-tube boilers, and that they have proved to be a better steam raising fuel than the raw slacks normally sent to the washery. Whilst there is a good case to be made for the burning of middlings at or near to the pit, this is a rather special instance, and the reason why it is possible to maintain a better head of steam with a fuel containing rather more ash and having a rather lower calorific value, is due to the fact that these middlings tend to burn "dry" and give a uniform bed over the grates, whereas the raw coals are so strongly caking that they are quite unsuitable for steam raising. Although it has been emphasised that this is a special case, there is no suggestion that substantial economies could not be effected throughout the country by burning middlings, but it is unlikely that there will be many more cases where the middlings prove to be a better fuel than the raw coal itself.

The blend of coals sent to the washery is not particularly dirty, and in common with the West Durham coals generally, the larger sizes contain the bulk of the dirt and the middlings, simply because the coal itself is so friable and breaks down so easily, that the only large pieces able to withstand mining and transport are either refuse, or pieces of coal held together by mineral matter of one sort or another.

The coal coming from the Tromp baths runs about 4½ per cent. of ash, that from the Rheolaveur about 5½ per cent. of ash, whilst the filter cake averages about 15 per cent.; the final blend sent to the coke ovens contains 6 ot 7 per cent. of ash and 7 to 8 per cent. of moisture. This latter figure is important, for although this plant is not linked up to a normal gas grid, it is in essence a part of the steelworks, and here, all the surplus gas from the ovens can be usefully consumed. In consequence, it has been well worth while to provide machinery for turning out a coking coal with low moisture content, so that coke-oven gas can be put to a much more useful purpose than merely evaporating moisture in the ovens.

The overall results (dry basis) for the fourth quarter of last year were :–

Washed coal	82.2 per cent.
Middlings	6.7 per cent.
Refuse	11.1 per cent.

The plant was designed and erected by the Birtley Company, Ltd., the Rheo troughs and centrifuges being supplied by Rheolaveur General Construction Co. Ltd.

Drawings and Photographs accompanying the article