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December, 1960

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A coal-preparation plant, which incorporates in its design some entirely new developments, is one of the interesting features of the reconstruction work being carried out at Dawdon Colliery. This colliery, situated on the cliff tops of the Durham coast near Seaham is some 60 years old and was originally owned by Londonderry Collieries Limited. The bulk of its output was shipped to the South of England from nearby Seaham Harbour.

Before Nationalisation, output had reached a figure of approximately 600,000 tons per annum, the coal being prepared for the market in a 100 T.P.H. Birtley dry cleaner and a Rheolaveur trough washer for small sizes.

After Nationalisation, it was decided to increase the productivity of the colliery and to raise its output by half, i.e. to approximately 900,000 tons per annum. Accordingly, a large reconstruction programme was put in hand.

Underground, a new level locomotive roadway was driven to connect three central loading points with a new pit-bottom, coal being hauled in 3-ton minecars by 100 h.p. diesel locomotives.

Complete electrification has been carried out on the surface including the installation of two tower-mounted winders (one with skip) and a new fan. A lamp room and administration block have also been completed.

A new coal-preparation plant was essential to handle the increased output, but it was decided to retain the existing dry cleaner to give maximum flexibility to cope with the stringent marketing requirements.

The site at Dawdon is congested and it was by no means a simple job to carry through the surface work whilst maintaining output. The fact that this was carried out to schedule speaks highly for the organisation and planning of the scheme and for the liaison between the N.C.B. and the various contractors involved.

Coal-preparation

Fig. 3 is a plan of the surface layout of the colliery, showing the position of the plant in relation to the shafts, and Fig. 6 is the flowsheet for the plant. The description of the method of coal preparation can be followed by referring to this drawing.

Raw coal handling

Run-of-mine coal from the new skip winding plant is delivered by an apron feeder to No. 2 belt conveyor, which conveys the product to the screening and inspection house. Foreign coal may also be delivered to No. 2 belt conveyor by means of a track hopper, vibro feeder and No. 1 conveyor. A circular electromagnet is situated at the head end of No. 2 conveyor for the purpose of removing tramp iron from the stream of coal, The coal then passes to a run-of-mine screen. This screen has two suspended bodies, the upper body is fitted with 6 in. diameter perforated plate, whilst the lower body is provided with perforated plate having 1 in. perforations. The purpose of this lower deck is to remove a certain quantity (in the region of 50 to 80 t.p.h.) of small coal which will be separately treated in the existing dry cleaning plant. The perforations in the lower body may be changed in the future to suit market requirements.

The plus 6 in. material passes to an inspection belt where any obvious foreign material is removed by hand, and delivered by chutes to minecars. The product remaining on the belt is chuted to a jaw crusher, which reduces the coal to minus 6 in.

The material which passes over the lower deck of the run-of-mine screen (predominately 6 in. to ½ in., but with some undersize material) is chuted to the raw coal bunker feed conveyor (No. 5). At this point it is joined by the crushed coal from the jaw crusher by means of No. 3 conveyor. The undersize material from the screen referred to above is collected, and fed to No. 4 conveyor. This conveyor delivers the coal via a conveyor system to the existing dry cleaning plant. It is also possible to transfer this small raw coal to rail wagons by means of a small loading hopper.

Provision is also made to pass the whole of this small coal to No. 5 conveyor so that the whole of the pit output may be sent to the coal washery.

No. 5 conveyor transfers the coal by means of a feed box to a reversible conveyor (No. 6) which in turn feeds either of the two 500 ton compartments of the raw coal storage bunkers, which are fitted with spiral anti-breakage chutes.

By-pass outlets are installed in each of the two compartments, whereby coal may be outloaded by means of conveyor No. 7 to rail wagons — this in the event of a breakdown in the coal-preparation plant. The coal may then be reintroduced by loading through the track hopper already mentioned.

Washing equipment

The main washing units comprise two identical ACCO SJL.80 washboxes having eight compartments and three elevators. Each box deals with a 6 in.-0 size raw coal feed at the rate of 200 tons per hour which it separates into three products — clean coal, middlings and shale.

From the base of each compartment of the raw coal bunkers, the coal is delivered direct to each washbox by means of vibro feeders, and Nos. 8 and 9 conveyors. Belt weighing machines are situated on each of these conveyors to indicate the tonnage being delivered to the washboxes.

An entirely new development, patented by ACCO, and known as Automatic Feed Regulation, is incorporated in the raw coal feed circuit. Dependent upon the shale content in the feed, the tonnage of raw coal fed to the washbox can be automatically controlled, in order that the washbox will continue to operate at its optimum capacity. Although under certain conditions this will reduce the capacity to below the average, on other occasions the machine will operate at considerably more than average capacity, in view of the more favourable feed conditions.

The feed regulation can be applied either to a single variable in the feed or can be associated with a multiplicity of variables; in other words detectors are situated at various points in the washbox indicating which particular factor is becoming the bottle-neck, and automatically varying the feed accordingly. In addition, the feed regulation can be operated by integrating two variables so that the cumulative effect is the factor which determines the amount of feed regulation which will take place.

This form of control, which ensures continuous operation at optimum capacity, with a regular quality of washed products, enables other features to be incorporated in the plant, such as automatic control of air and water supply, an alarm system and recording devices, where these are considered advantageous.

Shale is discharged at the feed end of each of the ACCO automatically controlled washboxes, and delivered by elevators and chutes to the shale conveyor (No. 15) which in turn delivers the product to rail wagons through a small loading hopper.

"Heavy" middlings are discharged from the washboxes by means of the second elevators, the product being screened at ¾ in., the oversize being crushed, and recirculated to either one, or both washboxes, whilst the undersize is delivered by chute to the shale conveyor.

"Light" middlings are discharged by means of the third elevators, and are crushed to minus 2 in., and fed by scraper conveyor either into one or both primary washed coal classifying screens.

By this means, a separation is achieved in the washboxes at a suitable low gravity in the coals plus 2 in. to permit approximately half of that size of coal to be extracted as a domestic fuel, and blend the other half with clean coal and minus 2 in. middlings to make a gas coal.

The plant has been designed to permit the crushed middlings from the third elevators to be returned to the washboxes for rewashing, or delivered to a loading point outside the washery for use as a separate product should this be needed in the future.

Washed coal and circulating water from the washboxes passes over fixed wedgewire chutes fitted with ¾ mm. apertures before being classified into 6 in. by ½-in, and ½ in. by 0 by means of two resonance type screens.

The ½ in. by 0 product, together with the circulating water from the primary washed coal screens is laundered to two settling tanks, The coal is removed by means of drainage elevators and delivered to a scraper conveyor which in turn delivers the product to the three 100 tons per hour centrifuges (one of these a standby machine).

Circulating water and fines from the settling tanks are overflowed to the main pump sump, from where it is pumped to a Baum settling cone.

Settled solids from the base of the cone are fed to two slurry screens, whilst the clarified water is fed to the washboxes. The dewatered slurry flows to the two drainage elevators, and the underflow from the screens is returned to the main sump.

A special feature of all the washed coal and discard elevators is the inclusion of the patented ACCO "Bloklink" chain. This is a simple design of link assembly, composed of one long and two short links, which gives increased capacity and longer life than the normal chain in elevators of comparable width.

Froth flotation

A bleed from the rising main feeding the settling cone is delivered to a ¾ mm. scalping screen to catch any oversize and delivers this to one of the slurry screens. The underfiow from this scalping screen forms the feed to the froth-flotation plant. This comprises two 10 ft. 0 in. diameter by 10 ft. 0 in. high conditioners, each feeding a bank of eight frothflotation cells.

Froth from the cells is laundered to two vacuum disc filters, the filtrate being returned to the main sump, whilst the filtercake is delivered via No. 10 conveyor to the paddle mixer for mixing with the centrifuged washed smalls.

The two ACCO vacuum disc filters are each fitted with the patent ACCO Filter Level Control, designed and developed to overcome variations in the pulp level contained in the filter bath.

As is generally known on rotary vacuum disc filters variation in the pulp level prevents the formation of a uniform cake, the reason being that, when the level of the liquid in the filter bath is low, the more central region of the disc sector is not immersed in the pulp, thus freeing to atmosphere an area which should be under the effect of vacuum. This loss of vacuum reduces the pick-up of pulp on that portion of the disc sector which is submerged, and also results in less drying ability when the sector emerges from the bath.

The ACCO filter level control automatically maintains the liquid in the filter bath at the correct level by regulating the vacuum effect to suit the incoming feed rate. This ensures that when a reduced feed rate is entering the bath, the pick-up action is reduced over the submerged portion of the sector, thus balancing the filtrate removal with the liquor feed rate.

Another valuable feature is that the moisture in the cake will not become greater under light load conditions, but will, in fact, tend to lessen, because the excess vacuum is diverted to the drying area of the sector.

An alarm coupled to the automatic level control attracts the attention of the operator, if the control device is being called into use for excessive periods thus ensuring attention when necessary.

Tailings from the cells are fed to a thickener, together with flocculating reagents. The thickened product is then pumped by diaphragm pump to waste, whilst the clean water is returned to the settling cone.

Washed coal handling

Dried coal from the centrifuges is fed to the ½ in. by 0 conveyor, and delivered to a paddle mixer to be mixed with the filter cake. The paddle mixer in turn feeds the main 6 in. by 0 or ½ in. by 0 washed coal conveyor. This conveyor feeds a shuttle conveyor which distributes the product to the 1,000 tons capacity washed coal bunker.

The 6 in. by ½ in. washed coal product from the two classifying/dewatering screens is conveyed for final classifying to two further resonance type screens in series situated in the washed coal screening and loading house. Provision has been made to by-pass all the 6 in. by ½ in. product from No. 1 washbox and screen direct to the 6 in. by 0 or ½ in, by 0 washed coal conveyor (No. 13) feeding the 1,000 ton capacity washed coal bunker.

The 6 in. by 4 in., 4 in. by 2 in., 2 in. by 1 in. and 1 in. by ½ in. products are fed direct from the screens to boom loaders which load the wagons.

A ½ in. screen plate is provided at the feed end of these screens to take out any undersize made in passing through the plant. This undersize is directed to the conveyor feeding the washed coal bunker. Part of the coal can be diverted to the same route.

Main Contractors

AUTOMATIC COAL CLEANING Co. LTD.

Other plant suppliers
Item	Manufacturers
Vibro feeders	Fraser & Chalmers Engineering works
Troughed belt conveyors	ACCO/Hugh Wood & Co. Ltd.
Separator magnet	Rapid Magnetic Co. Ltd.
Run-of-mine coal screen	Nortons-Tividale Limited
Inspection belt conveyor	ACCO/Hugh Wood & Co. Ltd.
Raw coal crusher	Armstrong Whitworth (Metal Industries) Limited (Kue Ken)
Middlings crushers	British Jeffrey-Diamond Limited
Scraper conveyors	Jenkins De Brouwer
Classifying screens	Robert Cort & Son Limited
Slurry screens	Robert Cort & Son Limited
Centrifuges	Rheolaveur General Construction Limited
Slurry scalping screen	W. H. Barker & Son (Engineers) Limited
Oil feeders	Denver Equipment Co. Ltd.
Conditioners	Denver Equipment Co. Ltd.
Coal flotation machine	Denver Equipment Co. Ltd.
Disc filters	ACCO/Davey, Paxman & Co. Ltd.
Thickener dosing equipment	R. O. Stokes & Co. Ltd.
Thickener mechanism	R. O. Stokes & Co. Ltd.
Paddle mixer	New conveyor
Bunker doors	James W. Ellis & Co. Ltd.
Main circulating pump	E. N. Mackley & Company
Air blower	Keith Blackman Limited
Clarified water pump	E. N. Mackley & Company
Drainage pumps	E. N. Mackley & Company
Oil pump	E. N. Mackley & Company
Vacuum pump	Alley & Maclellan Limited
Filter blower	Whittaker Hall & Co. (1929) Ltd.
Filtrate pump	Sultzer
Diaphragm pump	Johnson
Air compressor and receiver	Broom & Wade Limited
Valves	Newman Milliken Glenfield & Kennedy Limited
Electric motors	Associated Electrical Industries Limited. Bruce Peebles Limited in special instances
Switchgear	Belmos Co. Ltd.
Lighting and wiring	A. Anderson & Son
Heating	Northern Gas Board
Building structural steelwork	J. W. Ellis & Co. Ltd., or equal
Hoist blocks	Felco Hoists Limited
Remainder	ACCO

Acknowledgments

For permission to publish this article and photographs we are indebted to Mr. S. W. Potts, General Manager, No. 2 (Mid-East Durham) Area, Durham Division, National Coal Board, and we would very much like to express our thanks to the Automatic Coal Cleaning Co. Ltd., for supplying information and photographs of the plant.

Drawings and Photographs accompanying the article