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Colliery Engineering

June, 1955

Secondary separating baths

The low-gravity separation in the primary baths which is necessary to produce the high quality demanded by markets for the larger sizes, results in sinks products which consist of middlings as well as refuse. The sinks from each pair of primary baths are fed from the respective drainage screen to a secondary Ridley-Scholes bath.

There are two secondary baths, one for each pair of primary baths, and these operate at a high density — normally 1.7 sp. gr. The floats, which consist of middlings between 1.4 and 1.7 sp. gr. pass from each bath to a drainage and spraying screen on which the middlings are first drained of medium and then sprayed in the same manner as the coal from the primary baths.

The middlings are crushed below 1½ in and mixed with the 1½ in.-0 raw coal for treatment in the small-coal washery.

The sinks from the secondary baths, which consist only of high-ash shale, are similarly drained and sprayed and delivered to the main refuse disposal conveyor.

Bath-medium circuits

Each of the separating baths, i.e. the four primary and two secondary, has its individual circuit with its own surge tank and pumping system.

In the case of each bath circuit the medium from the drainage screens passes to the respective surge tank from which it is pumped back to the bath feed inlets at the deep end of the bath.

The surge tanks take up the variations in the total quantity of medium in the respective bath circuits which result from the amount extracted with the products and subsequently returned from the thickeners. The surge tanks have capacity to hold the total contents of the respective baths, together with a working reserve; the baths may thus be emptied and refilled at any time within a few minutes.

The densities of the bath circuits are maintained by control of the medium returned from the thickeners which are automatically controlled to deliver high and low density media, respectively, to the secondary and primary baths. The bath densities may thus be increased by setting the automatic control to a higher value, while a quick increase may be made by introducing high-density new medium from the ball-mill storage tank. Reduction of density of any bath circuit may be made very quickly by the water dilution valves.

Dense-medium recuperation

The diluted medium resulting from the spraying of the washed coal, middlings and refuse, is pumped to two independent magnetite thickener systems. Each of the thickeners is preceded by three fine-mesh slurry screens which remove any coarser particles of coal or shale resulting from breakage, this oversize being delivered to the small coal washery. The underflow from the slurry screens is classified in the thickeners, the heavy magnetite settling quickly while the lighter coal and clay slimes pass into the overflow.

This overflow, carrying about 5 per cent. of slimes, is fed to the primary sprays on the spraying screens for the preliminary washing of the products. A portion of the thickener overflow is bled from the system to balance the build up of solids, this effluent passing to the small-coal washery.

Fresh-water sprays, equivalent in quantity to the amount of the effluent and the water loss in the products, follow the primary sprays on the screens. The products are thus finally washed free of magnetite and the coal is ready for sizing into market products.

The thickened magnetite is extracted from the thickeners by specially-designed oscillating ball-valve mechanisms which are operated by the automatic density controls.

Automatic density control

Each of the thickeners is equipped with an automatic control which comprises a weighing machine which continually weighs a constant volume of the discharged medium in a cone vessel which keeps the medium in motion to prevent settlement. The density of the weighed medium is continuously indicated and the settings of two pointers at two given densities determines whether the medium passes to the secondary-bath surge tanks or to the primary bath surge tanks. If the density of the discharged medium falls below the density indicated for the primary baths, the discharge from the thickener automatically stops for a given time to allow a quiescent settling period. After a predetermined time discharge automatically restarts and the discharged medium is again delivered according to the indicated density, or discharge is again stopped. The controls are all operated electrically by mercury switches actuating contactors with time relays, all of which are enclosed in the separate gravity control panels.

The medium extracted from the bath circuits on the products is thus recovered and returned at a density at least as high as the bath density. In the event of any rise above this value, which can only occur very slowly, special water valves may be operated to dilute the medium as required.

New-medium preparation

Two continuous wet ball mills, operating in closed circuit with classifiers, are provided for the preparation of newly ground magnetite to replace working losses. These mills grind magnetite finer than 200 mesh, the classifier overflows being delivered to a small thickener.

This thickener delivers a high-density medium of about 2.0 sp. gr. to a storage tank where it is held in reserve. This new medium is pumped to any of the surge tanks when required.

A storage house for 500 tons of magnetite is provided on the ground floor of the building and magnetite is delivered to the ball mills by belt conveyors and electric feeders.

Small-coal washing — Acco-Baum washboxes

The 1½ in.-0 raw coal is delivered from the storage bunkers by electric feeders, by which a uniform load is maintained, and is fed by belt conveyors to the small-coal washing section. This comprises four Acco-Baum washboxes which are equipped with the well-known Acco automatic-controlled shale discharge. *

Each washbox is equipped with two refuse elevators and the floats automatically control the pulsations in the respective compartments, whereby the rate of shale discharged to the elevators is controlled according to the amount of dirt in the raw coal.

The washed coal delivered from the washboxes is delivered to four sizing screens which extract the ½ in. to 0 coal and wash water and deliver the 1½ in. to ½ in. washed coal to a collecting conveyor for delivery to the main washed-coal conveyor which collects the large coal from the Ridley-Scholes section for delivery to the sizing and loading section.

The 1½ in. to 0 coal, with the wash water, is drained on four wedge-wire screens which deliver the dewatered ½ 2 in. to 0.5 mm. coal to the washed slack conveyor. The wash water drained by these screens is delivered to a spitzkasten on the ground floor of the building from which the overflowing clarified water is delivered to the pump sump.

The washboxes are serviced by four main pumps, three of which are in normal service and one standby, the connections being made so that any one pump may be out of commission.

The refuse discharged by the elevators is collected by conveyors and delivered to the main refuse belt to join the refuse from the large coal washery.

Slurry washing — Unifloc froth-flotation system

The thickened slurry from the spitzkasten is pumped to particle-size control headboxes which deliver a feed of wash water containing 0.5 mm. to 0 coal to two conditioners where oiling and mixing take place.

The slurry is first cleaned in two banks of primary froth-flotation cells from which the clean froth is delivered to two banks of secondary cleaning cells which deliver clean frothed coal. This coal is dewatered on four Unifloc string-discharge vacuum filters. The filter cake is collected on a belt conveyor which delivers it to the washed slack conveyor.

The tailings from the primary and secondary cells are delivered to a Unifloc clarification plant which comprises two thickeners in which the tailings are flocculated and settled. The clarified water from the thickeners is returned to the small coal-washing circuit, thereby maintaining a clean condition of the wash water.

The thickened tailings are extracted by high pressure diaphragm pumps which pump the sludge to the sea by pipeline — a distance of about one-third of a mile.

Washed-coal sizing and loading

The washed coal from the dense-medium section and the 1½ in. to ½ in. from the washboxes, and also lump coal from the picking belt, comprising all sizes down to ½ in., is delivered by a main belt conveyor to the sizing plant which is located above the railroads adjacent to the main building.

The screening equipment comprises two separate banks of screens which are able to run independently. A series of belt boom loaders pass beneath the screens for loading to wagons. The sizes of coal produced are: plus 4 in., 4 in. to 2 in. 2 in. to 1½ in. and 1½ in. to ½ in.

The washed slack and filter cake are delivered by belt conveyor to a paddle mixer which intimately mixes the filtered slurry with the washed coal and delivers the ½ in. to 0 product to two roadways by chute.

Refuse disposal

The pit stone from the Lynemouth shafts is delivered by a main refuse conveyor to a central refuse-loading point. This conveyor is also arranged to bypass raw coal to wagons in case of emergency. The main refuse conveyor from the washery delivers to this conveyor which thus centralises the whole of the colliery refuse disposal.

At the loading point there are arrangements for loading refuse to railway wagons, or, alternatively, to dumper cars for disposal to a coastal pit heap.

Buildings

The whole plant, gantries and bunker, are housed in modern buildings which have been architecturally designed to harmonise with the rest of the colliery extensions. The washery buildings fabricated by Hills (West Bromwich) Limited are steel-framed and sheathed with a specially designed and manufactured form of asbestos sheeting. This, which is of a new type of troughed section with reinforcing lining to give adequate strength with heating insulation, was supplied by Boddy Roofing Co., Ltd.

Appropriate areas of glazing are arranged in accordance with the architectural features of the design and the plant is provided with extensive artificial lighting of the fluorescent strip type by A. Andersons & Sons Limited.

The plant is heated and air-conditioned by steam unit heaters, which are disposed throughout the plant to create the necessary warm air circulation.

An automatic lift is provided for easy access to all parts of the plant and for easy delivery of spare parts to the different sections of the plant.

The electrical control equipment was supplied by Allen West & Co. Ltd.

* See "A New Washbox at Eppleton," Colliery Engineering, February, 1954, p. 47.

Drawings and Photographs accompanying the article