Screening the coal

The in situ ash contents of the seams vary from a minimum of 35 per cent, in the High Main to about 25 per cent. in parts of the Bottom Grey seam.

The aim is to produce a uniform product having an ash content not exceeding 10 per cent, and to achieve this, crushing and blending of the different qualities of coal will be necessary. It has not been considered necessary to resort to washing the coal, and this has meant paying increased attention to digging cleanly from the seams.

Cleaning of the seams before loading the coal is done mainly by hand labour, assisted by graders and bulldozers. Experiments are being made with a tractor-mounted power brush which should reduce labour. The screening plant is situated about two miles from the site, and is approached by road.

Coal is transported by ten-ton rear-dump lorries, along the public highway to the screens and is discharged into one of three 12-ton-capacity hoppers feeding on to the three 36-in, main coal belts, through Sherwen electro-magnetic variable feeders. The capacity of each belt is around 150 tons per hour.

These belts discharge over Niagaras vibrating screens with 2-in, square mesh, the larger coal passing on to the picking belts, where low-grade material is removed by hand, being passed via a cross belt into a storage hopper of 20-ton capacity which is cleared periodically and dumped back in the cut.

If the coal is of "high quality," it then passes direct from the picking belts on to the boom loaders which load it directly into 20-ton rail wagons.

In the case of inferior coal, this is diverted through British Jeffrey-Diamond crushers on leaving the picking belt and, after crushing to under two-inch size is returned on a separate belt for blending with the under-two-inch material which passed through the Niagara screens on the high-quality belts. High quality coal can also be crushed if required to satisfy blending requirements.

A cross-feed at the ends of the boom loaders can be made to fill a hopper for removal of coal by road transport instead of by rail, if this is required.

There are four outgoing tracks and one for incoming empty wagons. Two steam locomotives are in use for shunting, and all outgoing wagons pass over a rail weighbridge on leaving the plant.

New coal-haul road

At the time the contract was let, the intention was to use normal highway transport to carry the coal to the screens, but several factors have since led to a change of plan.

First among these is the statutory regulation which would have meant limiting the payload to around 14 tons if a normal three-axle vehicle was used. The need for good off-the-highway performance was also realised and it was impossible to reconcile this with the public-highway limitations. Added to this, the proposed route was by no means ideal, as it incorporated several sharp bends and a steep gradient.

Costain Mining Limited therefore decided to construct a private haul road, 1½ miles long with a 30-ft. carriageway, between the site and the screening plant at Bebside.

The road involved in its construction the building of three fly-over bridges. Bailey bridging-triple / single extra widened-was chosen as being the most rapid form of construction to meet the requirement. Two of these, each 120 ft. long, cross main roads and are supported on 18-ft. high reinforced-concrete columns four feet in diameter. The third, which crosses the River Blyth and is 190 ft. long is supported on timber trestles. Foundations are of mass concrete, and decking to the bridges is of sawn greenheart planking. Guard frames have been fitted to the bridges as a precaution against falling coal.

The assembly was carried out on the embankment, and the spans lifted into position by a Loraine crane with 70 ft. jib.

More than 50,000 cubic yards of drift and clay had to be excavated in cutting a way for the road. And for building the road itself about 100,000 cubic yards of rock fill-yellow sandstone and hard white post stone-were taken from the Acorn Bank site and laid, graded and rolled. The fill varies in depth between 3 ft. and 60 ft., and was transported by a fleet of 22ton Euclids. At the screening plant at Bebside the hoppers have been built up to increase their capacity by 40 tons to receive the larger loads. The new runway over the hoppers is formed of prestressed concrete beams, mainly 18 to 22 ft. in length.

The round trip from site to screening plant and back takes 30 minutes including the time for the trailers to discharge, so that when the full complement of haulers is operating 40 tons of coal will be delivered to the screens every 7½ minutes.

Approval to the road scheme was obtained from two local authorities, two Ministries and the River Board; the negotiations for wayleaves with live individual landowners were carried through quickly. From the angle of road safety, much is to be gained by keeping such heavy traffic off the public highway; and the scheme received enthusiastic approval from the local authorities concerned and the County police.

The building of this road is not part of the contract. It is a free-enterprise venture by Costain Mining Limited, who built the road themselves in 3½ months at a cost of Ј100,000.

Electrical layout

All power for operating electrically-driven plant on the site is taken from the public supply system at 20 kV and 50 cycles per sec. Two 3,000 kVA transformers reduce the voltage to 3-3 kV before it is transmitted to the site by overhead line. A portion of the supply passes through a pair of frequency boosters which convert it to 4.16 kV., 60 cycles per sec., for use by the two draglines. All other plant operates at the supply frequency of 50 cycles per sec.

Overhead lines convey the current from the substation to the two ends of the cut at either 4-16 kV or 3-3 kV., whence it is fed to the machines via flexible cable. Tied switchgear is skid-mounted and of the all-weather type. Additional field transformers are necessary to reduce the voltage still further for use by the coal shovels, hoisting derricks and for site lighting.

Organisation

The Acorn Bank operation is being run in the manner of a separate company with headquarters on the site, and the accommodation is therefore necessarily larger than in normal for a contract of this size. The central administrative block is a single-storey brick-built building and houses the following staff -

Additional accommodation is provided for the National Coal Board's resident engineer and his staff.

There are also provided canteens for use of office staff and the men working in the adjacent plant yard. A small dining room is available for the use of visitors when the occasion arises. One of the larger canteens can be turned into a cinema-the site owns and operates its own cine-camera and projector-and film shows on safety precautions, plant operations and other subjects are regularly given to both supervisors and operatives.

The plant depot, consisting of repair shops and stores buildings is situated adjacent to the administrative block. Five large Romney huts of corrugated sheeting on tubular steel ribs house the repair shops, each being devoted to a different class of work, such as machine shop, tractor repairs, Euclids, riggers, etc. The stores building which extends along the back of these shops is provided with separate service counters to supply each shop with the necessary spares, stored in conveniently-located racks.

A canteen and mess-room are also provided near the working site, and it is planned to move these as the work progresses.

Some of the problems to be overcome on a job of this size and complexity can be attributed to the sense of urgency which at present attaches to all work of this nature, and which has had its effect in limiting the time available for detailed planning on the job. Experience in the operation of large draglines has also been very limited in this country, and the technical difficulties in working a multi-seam site of this depth are many. The comparatively short life of the site, by normal standards elsewhere and the need to comply with the requirements of agriculture and local industry have also added to the problems.

In spite of these difficulties, or perhaps as a result of them, there is at all levels a spirit of challenge, coupled with the determination to succeed in a project which is of importance to the nation.

Drawings and Photographs accompanying the article