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Colliery Engineering

June, 1924

Coarse Crushing and Weighing.

A crusher of the swinging jaw type, manufactured by Messrs. Edgar Allen & Sons Ltd., of Sheffield, capable of dealing with a 12-in, cube, reduces the ganister to 3-in, cubes; the jaws are 24 in. broad and 14 in. apart at the top, its capacity being 12 tons per hour. The crushings are then elevated and passed to a similar machine, which further reduces them to 1-in, cubes. These are discharged by an automatic feeding device, into a gravity bucket conveyor which distributes them into the weighing pans. The discharging of the buckets and the weighing of the material is done entirely automatically. Placed consecutively in the track of the conveyor are the counterpoised weighing pans which receive the ganister crushings from the buckets. These are tipped by gear which, however, goes out of action when the pan has been loaded to an exact pre-arranged amount and comes into action again when the pan has been emptied. Some 60-h.p. is absorbed by this plant, which is electrically driven by a Crompton motor.

Mixing and Fine Crushing.

The mills for the fine reduction are two in number, and are of the underdriven, revolving-bottom type made by the Brightside Foundry & Engineering Company, of Sheffield, the total weight of each being about 26 tons. An accurately-measured quantity of milk of lime is mixed with the charge from the automatic weighing hopper and the whole is ground for a period of time depending upon the purpose for which the finished product is required. A further control of the grinding operation may be exercised by varying the crushing weights, which are adjustable. Each mill is driven by a 100-h.p., 500-volt, 50-cycle motor manufactured by the English Electric Company Ltd. The lime-mixing plant is separately operated through a worm reduction by a 15-h.p. motor by J. H. Holmes & Co., of Newcastle.

The amount of charge, like the grinding period, is subject to variation, but on an average, 18 cwts. of the crushings are reduced to mud at a time. An automatic device quickly empties the silica mud into tipping bogeys which convey it to hoppers delivering direct to the moulding tables. A general view of the mills and their housings is shown in Fig. 6.

Moulding the Bricks.

Direct supervision of the material is carried out by the laboratory staff at every stage of the brick's manufacture. A close check is kept upon the silica mud, representative samples being taken daily and subjected to a thorough chemical and physical investigation. It cannot be too much emphasised that the success of the whole operation is dependent upon this rigorous supervision. With regard to the silica mud, the product desired is one consisting of granular particles having sharp serrated edges, this form being best adapted to give the brick good binding properties.

The moulding is done entirely by hand on steel pallets; loose-sided steel moulds accurately machined and made at the Company's own works are used, some hundreds of shapes being kept in stock. A foreman moulder exercises constant supervision and keeps a strict watch on the operations. It must be remembered that the finished brick. differs considerably in dimensions to those it possesses in the mould, and as the size of the finished brick is guaranteed to within 1 per cent., it is evident that a very exact allowance must be made for expansion. Further, in special shapes, the action of "squatting" has to be considered and allowed for.

The shapes are thoroughly beaten to give a uniform density and the requisite mechanical strength. Finally, the moulder works up a fine skin on the material which is now ready for the drying machines.

The Drying Machines.

Pallets of the shapes are loaded on to rack cars and are taken to the dryers. These are of American design and were manufactured by Proctor & Schwartz, Inc., of Philadelphia, U.S.A. A view of the loaded cars inside one of the dryers is shown in Fig. 8. The Proctor dryer consists essentially of a closed heated chamber, the atmosphere of which is dry at one end and of high humidity at the other. It may be said to be divided into three zones, the atmosphere of each having a definite range of temperature and humidity. These zones are not separated from one another by screens, the division .being solely due to the relative positions of the dry air inlet, the outlet for the humidified air and the general disposition of the fans, baffle plate and heating coils. The air is kept in a continuous state of circulation about the shapes. In the first zone, which is filled with a more or less saturated atmosphere at 130° Fahr. the shapes stay long enough to be thoroughly heated to the centre. No air is admitted to this end of the machine except when the curtain is raised to admit fresh supplies of shapes. In the second and third zones average temperatures of about 145° Fahr. and 205&def; Fahr. are respectively maintained. The shapes are thus subjected to progressively higher temperatures and dryer atmospheres. In the third zone the moisture content of the brick surface has been practically reduced to zero and as quickly as further moisture diffuses from the interior it is swept away by the currents of hot dry air. On entering, the bricks have a moisture content of 14 per cent.; when running on bricks alone (9 x 4½ x 3 in.) the drying is completed, i.e., reduced to 0.09 per cent. moisture, in eight hours.

The chamber of the machine is 50 ft. long by 20 ft. wide, and is 6½ ft. high. It holds 24 cars, eight on each of the three tracks. Each of the cars carry 104 pallets with six bricks on each pallet, making the total capacity of the dryer 14,976 bricks. The framework consists of light angle iron, and this is covered with panels of thin iron sheets interleaved with heat-insulating material. At the feed end is a flexible metallic curtain; the delivery end, a view of which is seen in Fig. 8, is fitted with sliding doors. Each track is provided with a creeper the length of one car only, so that, as a car of most bricks is fed in, the whole row of eight cars is moved forward one car length, causing a car of the dried bricks to be discharged at the delivery end.

Along each side of the chamber a system of fans and steam coils is arranged, a view of one of these units being shown in Fig. 12. There are four coils on either side, each consisting of 616 ft. of 1 1¼-in. diameter wrought-iron tubing, giving a total heating surface of 4,928 sq. ft. The fans, 72 in. in diameter and running at 237 r.p.m., are fixed in front of the coils and placed so as not to be opposite each other. A smaller fan 32 in. in diameter suffices to draw off the moisture-laden air through a tube exhausting outside the building. In all, 25 h.p. is absorbed. The whole of the moving parts are driven from a countershaft running in a tunnel beneath the machine. The temperature control unit consists of a small air compressor and Brice regulating instruments, one for each zone. Each instrument can be set at any desired temperature over a range between 80° Fahr. and 220° Fahr., the temperature then being maintained automatically within very narrow limits.

This system of drying would appear to possess great advantages over the ordinary hot floor method. The operation, especially with difficult shapes, can be carried out more quickly and with less risk of cracking and the setting up of internal strains. This is mainly due to the fact that the heat is applied evenly to all surfaces, and that the preliminary drying takes place in a humid but rapidly circulating atmosphere. On the hot floor the shapes are dried in a stationary atmosphere; if the operation is in the slightest degree forced the outer portion dries before the interior and stresses are set up. Very large shapes, however, which would retard the sequence of operations in the machine and upset the time schedule of the whole process, are necessarily dried on a hot floor, which simply consists of a reinforced concrete floor heated by steam pipes passing underneath. Great care is, of course, taken to ensure that the drying of large shapes is performed with sufficient slowness to prevent cracks developing. They are not handled in the green state after moulding but are left until quite dry and hard.

Burning the Bricks.

The eight circular down-draught kilns in which the bricks are burnt are the largest of their type in England. Four are built entirely of Consett refractory material reinforced by stout bands of steel, the whole construction being exceedingly massive and substantial. Each has a capacity of 100,000 standard sized bricks or 330 tons of material. In stacking the bricks care is taken to allow sufficient room at the top for expansion. The firing period extends over twelve days, a finishing temperature being ensured of at least 1,500° C. The kilns are then sealed up and allowed to cool by radiation alone, no other assistance whatever being given. The waste gases from the flues are led to a Babcock & Wilcox boiler which has an evaporating capacity of 6,000 lb. of steam per hour at 160 lb. pressure and 100 degrees of superheat ; this boiler is equipped with two Weir feed pumps controlled by a Cope automatic regulator. The kiln draught is provided by a Sirocco propeller fan driven by a 40 h.p. Robey engine, a water gauge pressure of two and a half inches being maintained. Finally, the gases are delivered into a chimney 120 ft. high. By means of a system of by-passes the boiler may be cut out of the flue circuit and the gases passed to the chimney direct. Sufficient steam is generated adequately to supply the Proctor drying machine, the drying floor for large silica shapes and two large drying floors in the adjoining fire brick works.

The Laboratory.

A handsome laboratory plays an important part in almost every stage of manufacture. It is fitted with gas and electric furnaces and every conceivable appliance for testing refractories, normally and under load conditions. The equipment also includes apparatus for conducting expansion tests, microscopical examinations and for taking microphotographs. The routine tests for hardness, resistance to impact and crushing stresses mentioned in connection with the raw material are conducted here, as also are a similar series of tests conducted on the finished bricks.

Two experimental kilns capable of containing 400 standard bricks are a further interesting feature of this department. They are used for the purpose of expediting experimental results, since the cycle of operation in the large kilns extends over five weeks. The chemical side of the laboratory practically forms a department in itself, though, of course, all results are correlated with the data provided from the other work which is carried out.

Quite apart from the representative tests conducted by the laboratory staff, each individual brick undergoes rigorous inspection in the loading sheds for size, soundness and shape, immediately before being despatched by rail.

The Fell Coke Works.

The high standard of the silica produced has solved all difficulty in using one of the latest types of American ovens. A study of high temperature carbonising practice in that country decided the Company to install the Wilputte design as being the most suited to meet the particular conditions at Consett. To facilitate transport a site was chosen in close proximity to the blast furnaces and steel works that absorb the bulk of the output. This site is on high, sloping ground and a considerable amount of excavating was required to accommodate the plant and extensive sidings. It is interesting to note in this connection that a seam of coal — the Three Quarter — was found to come within 20 ft. of the foundations of the ovens, and it was considered advisable that this should be removed before the building of the structure was commenced. The contract for the design and erection of this installation was placed with the Coppée Company (Great Britain) Ltd., who own the patents throughout the British Empire. The new plant, which was completed in March this year, comprises a battery of 60 Wilputte ovens and the usual type of Coppée by-product and benzol installations.

Transport and Crushing Arrangements.

Further evidence of the resourcefulness of the makers of "Consett" is exemplified in the fact that in order to facilitate the handling of the coal for the ovens they have recently sunk a new pit — the Victory — quite near by. The other main source of supply is the Eden Colliery, another of the company's eleven pits. All the coal is brought to the ovens on a private line. It is delivered direct by bottom discharge wagons into reinforced concrete hoppers situated below rail level, advantage having been taken of the natural slope of the ground to avoid the use of a preliminary elevator to deliver it to the crushers. From the hoppers the coal is distributed to either of the two crushing units by an adjustable automatic feed. A general view giving a very good idea of these arrangements and the plant is shown in Fig. 30. The crushers are of the hammer-mill pattern supplied by the Pennsylvania Crusher Company and are driven by separately housed, 200-h.p. direct-coupled totally enclosed motors. Each crusher has a capacity of 100 tons per hour, one only being required to supply the present needs of the battery. When the latter is duplicated, however, a third unit will be installed in the space left for the purpose and this will form a stand-by in case of a breakdown. The coal is reduced to about one-eighth inch cubes and less and is passed to the boot of a bucket conveyor. The reinforced concrete storage bunker, into which the coal is delivered direct from here, has a capacity of 2,000 tons; standing some 120 ft. above ground level and very finely proportioned it forms an imposing structure.

To distribute the coal to the ovens an electrically driven charging lorry is used. A view of this, showing it being filled under the bunker, will be seen in Fig. 24. It will be seen to have four conical hoppers which are spaced to correspond with the loading hoppers of the bunker and with the four charging holes in the top of each oven. An adjustable double-cone baffle device which may be set at a definite point will give any charge desired between limits of about 115 and 140 cub. ft.

Drawings and Photographs accompanying the article