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January, 1932

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The increasing quantities of coal being treated at washing and cleaning plants, in spite of the continued depression, is one of the most notable phases of the coalmining industry at the present time. This development is due in general to the demand for a cleaner product on the part of consumers, but the movement has also been stimulated by the growth of machine-mining, which involves a higher proportion of dirt in the coal, and by the fact that dirtier and thinner seams are now being worked following on the exhaustion of the higher-quality beds. It is not without significance that the greater part of this increase in the last few years has been due to dry-cleaning plants, the number of which in operation in this country now approaches one hundred. This affords remarkable evidence of the way in which dry-cleaning plants are finding favour, and of the fact that substantial advantages are being secured as a result of their use.

Description of Plant.

The equipment at the Louisa pit has a capacity of 100 tons per hour, and was supplied by the Birtley Co. Ltd., Co. Durham. The plant has been erected within an existing coal shed, a wall of which, flanking the L. & N.E. Railway lines, has actually been incorporated in the new structure. Referring to Fig. 6, raw slack is taken from an existing cross-conveyor in the screen house, this conveyor having been lengthened so as to feed on the new conveyor A. This latter unit is of the belt type, 30 in. wide by 117 ft. between the centres, and elevates the coal to a 150-ton-capacity concrete storage bunker. From this bunker the slack is fed into the boot of the raw coal elevator B and is raised to Hum-mer vibrating screen C, which separates it into two sizes — 2 in. to 7/16 in. and 7/16 in. to zero. Oversize coal is fed to the hopper F1, and the undersize is discharged to a second Hum-mer screen D. This unit divides the coal into approximately 7/16 in. to 3/16 in., and 3/16 in. to zero, the oversize passing into the hopper F, and the undersize first to the scraper conveyor E and then into the hoppers F2 and F3.

The hoppers F2 and F3 feed into the standard-type aspirators G and G1, in which the fine particles are extracted before the coal is delivered to the "Vee"-type separators H2 and H3, the aspirated fines passing to the cleaned-coal conveyor K. Hoppers F and F1, are fitted with gravity feeders to control the flow of coal to the separators H and H1, which are also of the standard Vee-type. The necessary air to the separators is supplied by the fans G, G1, G2 and G3.

Cleaned coal from all the separators is delivered by means of mild-steel chutes to the conveyor K, and is discharged from thence on to the cross-conveyor L. This latter conveyor, in turn, delivers the cleaned product on to the scraper conveyor L1, which is provided with chutes to feed it in the correct proportions to the larger coal from the screen picking-belts.

Middlings are carried by the mild-steel chutes to the conveyor M and then discharged into the elevator B for recirculation in the plant. The shale extracted is delivered direct to wagons which travel on a new shale road which runs alongside the storage shed.

Handling the Dust.

Particularly effective measures have been taken at this plant to eliminate the dust problem and this drawback to dry cleaning can now be stated to have been successfully solved. At the time of our visit, with the plant in full operation, the atmosphere inside the building was remarkably free from dust, and no dust whatsoever could be detected at the discharge from the filter. These results have been achieved by the perfecting of the Birtley's Company's established practice and no new departures have been introduced apart from the construction of the cyclones, which are now built of concrete, and the improved form of relief valve which has been fitted.

The Waring dust extractors in use are a combination of a cyclone and bag-filter. A steel air-duct, Fig. 6, connects the hoods of the separators and a second duct connects the top outlets of the casings to the fan P. To clean the bags the fan pressure is cut off and the automatic valve is opened to atmosphere about once an hour; this causes the bags to collapse and the dust falls to the bottom, whence it is removed by the spiral conveyor R and delivered to the cleaned coal conveyor K.

High-torque, squirrel-cage type motors of "Metrovick" manufacture provide power to the various units and are controlled from a main contactor board. This board is operated by push buttons on the separator floor, the contactors being so arranged that the motors automatically start, in their proper sequence, on the pressing of a single push-button. A single "stop" button also closes down the whole plant and further switches are provided for each motor, the contactors being so interlocked that in the event of a key motor being stopped, other predetermined motors in the sequence are likewise stopped, thus preventing the system from being flooded at any point.

Apart from the old storage yard wall which has been utilised, the building consists of a steel framework with brick panels and reinforced-concrete floors.

The accompanying table gives the result of a recent analysis of the cleaned products, the samples having been taken over a period of 5 hours, from 11 a.m. to 4 p.m. In this case the dirt contained 2.3 per cent. of coal lighter than 1.5 specific gravity, and, as a matter of interest, it may be mentioned that the ash in the clean coal is more than 1 per cent. below the figure guaranteed.

Table Showing Analysis of Cleaned Coal Samples
Birtley Dry Cleaning Plant, Holmside and South Moor Collieries Ltd.
Size	Percentage Weight	Percentage Ash	Bulk Ash (per cent.)
+ ½ in.	34.7	5.0	5.7
½ in. to 1/8 in.	41.4	4.9	" "
1/8 in. to 1/32 in.	13.8	6.5	" "
- 1/32 in.	10.1	10.4	" "

Drawings and Photographs accompanying the article