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January, 1937

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Despite the increasing adoption of mechanical methods for the preparation of coal, the larger sizes have hitherto defied all attempts at mechanical cleaning. Hand picking is still practised for cobbles and large coal because, although far from perfect, it has so far proved the only practicable method. It is, in fact, inaccurate and inconsistent due to fatigue, oversight and lack of concentration on the part of the pickers; the labour cost is high, and the picking belts occupy the greater part of the space in the average screening plant.

After about three years' experiment the Birtley Co. Ltd. have developed an automatic picking machine which, if its performance under demonstration is borne out under actual colliery conditions, may well revolutionise the treatment of the larger sizes of coal. We were invited to witness a demonstration of the machine at the maker's works, and found that it dealt accurately with coal, shale and middlings. We understand that it is capable also of separating hards from softs.

The machine depends for its operation on the difference in electrical conductivity between coal and shale. Each lump is passed separately beneath a set of brushes, and if its conductivity is that of coal it is allowed to pass unimpeded to the delivery chute, but above a certain conductivity, denoting the presence of shale, a gate rises to deflect the lump to a shale chute. Each lump, it will be noted, is "examined" individually, and to enable this to be done, the incoming coal is passed through a marshalling device which arranges the lumps in a track with an interval of about 18 in. between lumps. In the machine there are a number of tracks arranged side by side, depending upon the required capacity, but each row is treated independently.

The machine illustrated in Fig. 1 has five tracks and is arranged to throw out shale and middlings into separate chutes, clean coal passing out at the front. Incoming coal is delivered to the chute at the far end of the machine. The bottom of this chute is formed into five troughs, and the chute is given a combined circular and reciprocating motion which causes the material to slide into the troughs and form five rows of lumps. These lumps are at the moment packed closely together, and it is necessary to space them out to enable them to pass singly under the contact brushes. This is achieved by delivering the lumps to a spacing conveyor running sufficiently fast to draw away each lump, as it falls off the chute, a distance of about 18 in. from its successor. The action can be seen in Fig. 2, which shows the chute discharging to the spacing conveyor in a three-track machine.

Each lump in each track now passes under a brush contact consisting of a spring-loaded plate, the circuit being completed by the belt, which is of steel mesh. These plates are visible in Fig. 2, the left-hand plate being shown engaging a lump. The currents involved are naturally minute — of the order of 5 to 50 micro-amps. — and the feasibility of the machine is due to the commercial development of the industrial type of thermionic valve. The brush is connected to the grid circuit of a thermionic valve, and if the conductivity of the lump is above a certain value, the voltage applied to the grid is sufficient to cause the valve to flash over and operate a relay. This in turn energises a solenoid which raises a gate in front of the lump, causing it to fall into a chute; if the lump is passed as coal, the gate is not raised and the lump jumps over. In Fig. 3 are shown the solenoids and gates for a five-track machine, the belt being seen behind the gates and two of the gates being raised to deflect rejected material.

After the shale picker the coal passes to the middlings picker, which is of somewhat different construction. The flat-plate type of brush is suitable for eliminating shale, for the material is homogeneous, and a contact anywhere is sufficient to ensure operation. Middlings present a different problem, for here it is necessary to detect dirt bands and intergrown pieces. The exact arrangement depends upon the type of coal; in general it consists of a series of wire combs such as are shown in Fig. 4. In the arrangement illustrated the lumps first pass under a vertical comb, making a circuit between the comb and the belt, and thence between two horizontal combs forming the poles of a circuit. In order to detect a layer of dirt which has formed a parting on the bottom face of a lump it is necessary to carry the horizontal brushes close down to the belt. In the course of experiments it was found that lumps were apt to force the lower wires of these combs into contact with the belt and operate the gate direct, but the difficulty has been overcome in an ingenious manner. The belt for the middlings picker is not of steel mesh but is of rubber, and its surface is covered by a large number of stainless-steel pins which pierce right through the belt. On the underside of that portion of the belt beneath the vertical comb is placed a contact brush, which enables the circuit to be completed through the comb, lump, pins and brush. This brush does not extend to the portion of the belt below the horizontal combs, so that the pins in this region are dead.

The electrical equipment and valves for the machine we inspected were of B.T.H. manufacture, the valves and relays being enclosed in dust-tight iron cases. Adjusting knobs enable the sensitivity of any valve, or of all the valves in gang, to be adjusted to suit the coal. It is possible, in fact, to set the valves to operate on atmospheric moisture, and they normally respond to the contact of one's finger on one of the brushes. The size of lump evidently does not affect the accuracy of sorting, the reason being that the difference in conductivity between coal and shale is much greater than differences due to size. We also enquired as to the effect of moisture. Experimental work in this direction is still in process, but it is found that wet material can still be sorted, by suitably adjusting the sensitivity of the valves, because shale contains salts which dissolve to form a good electrolyte.

Having regard to the space occupied by picking belts we must confess to some amazement at the small dimensions of the machine; in fact, next to accuracy of sorting, the saving in space and in labour are two of its strongest recommendations.

As evidence of the small space occupied, a three-track demonstration machine has been mounted on a lorry, and is available for transit to any colliery interested.

A machine is already in service at a colliery, and we understand that comparative tests with hand picking, conducted by dividing a consignment between the machine and the belts and analysing bulk samples, showed the machine to be superior.

Drawings and Photographs accompanying the article