Chapter 33

For the endless chain system, which is much used in the Wigan district, a double line of way is necessary, one line for full and the other for empty tubs. The chain passes over a pulley driven by the engine, placed at such a height as to allow it to rest upon the tops of the tubs, and round a similar pulley at the far end of the plane. The forward edge of the tub carries a projecting pin or horn, with a notch into which the chain falls which drags the tub forward. The road at the outer end is made of a less slope than the chain, so that on arrival the tub is lowered, clears the pin, and so becomes detached from the chain. The tubs are placed on at intervals of about 20 yds., the chain moving continuously at a speed of from 2½ to 4 m. per hour. This system presents the greatest advantages in point of economy of driving power, especially where the gradients are variable, but is expensive in first cost, and is not well suited for curves, and branch roads cannot be worked continuously, as a fresh set of pulleys worked by bevel gearing is required for each branch.

The endless rope system may be used with either a single or double line of way, but the latter is more generally advantageous. The rope, which is guided upon sheaves between the rails, is taken twice round the head pulley. It is also customary to use a stretching pulley to keep the rope strained when the pull of the load diminishes. This is done by passing a loop at the upper end round a pulley mounted in a travelling frame, to which is attached a weight of about 15 cwt. hanging by a chain. This weight pulls directly against the rope; so if the latter slacks, the weight pulls out the pulley frame and tightens it up again. The tubs are usually formed into sets of from 2 to 12, the front one being coupled up by a short length of chain to a clamping hook formed of two jaws moulded to the curve of the rope which are attached by the "run rider," as the driver accompanying the train is called. This system in many respects resembles the tail rope, but has the advantage of working with one-third less length of rope for the same length of way.

The endless rope system overhead is substantially similar to the endless chain. The wagons are attached at intervals by short lengths of chain lapped twice round the rope and hooked into one of the links, or in some cases the chains are hooked into hempen loops on the main rope. In mines that are worked from the outcrop by adits or day levels traction by locomotives driven by steam, compressed air or electricity is used to some extent. The most numerous applications are in America.

Ventilation.

One of the most important branches of colliery work is the management of the ventilation, involving as it does the supply of fresh air to the men working in the pit, as well as the removal of inflammable gases that may be given off by the coal. This is effected by carrying through the workings a large volume of air which is kept continually moving in the same direction, descending from the surface by one or more pits known as intake or downcast pits, and leaving the mine by a return or upcast pit. Such a circulation of air can only be effected by mechanical means when the workings are of any extent, the methods actually adopted being--(1) The rarefaction of the air in the upcast pit by a furnace placed at the bottom; and (2) Exhaustion by machinery at the surface. The former plan, being the older, has been most largely used, but is becoming replaced by some form of machine.

The usual form of ventilating furnace is a plain fire grate placed under an arch, and communicating with the upcast shaft by an inclined drift. It is separated from the coal by a narrow passage walled and arched in brickwork on both-sides. The size of the grate varies with the requirements of the ventilation, but from 6 to 10 ft. broad and from 6 to 8 ft. long are usual dimensions. The fire should be kept as thin and bright as possible, to reduce the amount of smoke in the upcast. When the mine is free from gas, the furnace may be worked by the return air, but it is better to take fresh air directly from the downcast by a scale, or split, from the main current. The return air from fiery workings is never allowed to approach the furnace, but is carried into the upcast by a special channel, called a dumb drift, some distance above the furnace drift, so as not to come in contact with the products of combustion until they have been cooled below the igniting point of fire-damp. Where the upcast pit is used for drawing coal, it is usual to discharge the smoke and gases through a short lateral drift near the surface into a tall chimney, so as to keep the pit-top as clear as possible for working. Otherwise the chimney is built directly over the mouth of the pit.

Mechanical ventilation may be effected either by direct exhaustion or centrifugal displacement of the air to be removed. In the first method reciprocating bells, or piston machines, or rotary machines of varying capacity like gas-works exhausters, are employed. They were formerly used on a very large scale in Belgium and South Wales, but the great weight of the moving parts makes it impossible to drive them at the high speed called for by modern requirements, so that centrifugal fans are now generally adopted instead. An early and very successful machine of this class, the Guibal fan, is represented in fig. 12. The fan has eight arms, framed together of wrought iron bars, with diagonal struts, so as to obtain rigidity with comparative lightness, carrying flat close-boarded blades at their extremities. It revolves with the smallest possible clearance in a chamber of masonry, one of the side walls being perforated by a large round hole, through which the air from the mine is admitted to the centre of the fan. The lower quadrant of the casing is enlarged spirally, so as to leave a narrow rectangular opening at the bottom, through which the air is discharged into a chimney of gradually increasing section carried to a height of about 25 ft. The size of the discharge aperture can be varied by means of a flexible wooden shutter sliding in a groove in a cast iron plate, curved to the slope of the casing. By the use of the spiral guide casing and the chimney the velocity of the effluent air is gradually reduced up to the point of final discharge into the atmosphere, whereby a greater useful effect is realized than is the case when the air streams freely from the circumference with a velocity equal to that of the rotating fan. The power is applied by steam acting directly on a crank at one end of the axle, and the diameter of the fan may be 40 ft. or more.

The Waddle fan, represented in fig. 13, is an example of another class of centrifugal ventilator, in which a close casing is not used, the air exhausted being discharged from the circumference directly into the atmosphere. It consists of a hollow sheet iron drum formed by two conoidal tubes, united together by numerous guide blades, dividing it up into a series of rectangular tubes of diminishing section, attached to a horizontal axle by cast iron bosses and wrought iron arms. The tubes at their smallest part are connected to a cast iron ring, 10 ft. in diameter, but at their outer circumference they are only 2 ft. apart. The extreme diameter is 25 ft.

By the adoption of more refined methods of construction, especially in the shape of the intake and discharge passages for the air and the forms of the fan blades, the efficiency of the ventilating fan has been greatly increased so that the dimensions can be much reduced and a higher rate of speed adopted. Notable examples are found in the Rateau, Ser and Capell fans, and where an electric generating station is available electric motors can be advantageously used instead of steam.

Distribution of air underground.

The quantity of air required for a large colliery depends upon the number of men employed, as for actual respiration from 100 to 200 cub. ft. per minute should be allowed. In fiery mines, however, a very much larger amount must be provided in order to dilute the gas to the point of safety. Even with the best arrangements a dangerous increase in the amount of gas is not infrequent from the sudden release of stored-up masses in the coal, which, overpowering the ventilation, produce magazines of explosive material ready for ignition when brought in contact with the flame of a lamp or the blast of a shot. The management of such places, therefore, requires the most constant vigilance on the part of the workmen, especially in the examination of the working places that have been standing empty during the night, in which gas may have accumulated, to see that they are properly cleared before the new shift commences.

The actual conveyance or coursing of the air from the intake to the working faces is effected by splitting or dividing the current at different points in its course, so as to carry it as directly as possible to the places where it is required. In laying out the mine it is customary to drive the levels or roads in pairs, communication being made between them at intervals by cutting through the intermediate pillar; the air then passes along one and returns by the other. As the roads advance other pillars are driven through in the same manner, the passages first made being closed by stoppings of broken rock, or built up with brick and mortar walls, or both. When it is desired to preserve a way from one road or similar class of working to another, double doors placed at sufficient intervals apart to take in one or more trams between them when closed are used, forming a kind of lock or sluice. These are made to shut air-tight against their frames, so as to prevent the air from taking a short cut back to the upcast, while preserving free access between the different districts without following the whole round of the air-ways. The ventilation of ends is effected by means of brattices or temporary partitions of thin boards placed midway in the drift, and extending to within a few feet of the face. The air passes along one side of the brattice, courses round the free end, and returns on the other side. In many cases a light but air-proof cloth, specially made for the purpose, is used instead of wood for brattices, as being more handy and more easily removed. In large mines where the air-ways are numerous and complicated, it often happens that currents travelling in opposite directions are brought together at one point. In these cases it is necessary to cross them. The return air is usually made to pass over the intake by a curved drift carried some distance above in the solid measures, both ways being arched in brickwork, or even in some cases lined with sheet iron so as to ensure a separation not likely to be destroyed in case of an explosion (see figs. 5 and 8). The use of small auxiliary blowing ventilators underground, for carrying air into workings away from the main circuits, which was largely advocated at one time, has lost its popularity, but a useful substitute has been found in the induced draught produced by jets of compressed air or high-pressure water blowing into ejectors. With a jet of 1/200 in. area, a pipe discharging 1-2/3 gallon of water per minute at 165 lb pressure per sq. in., a circulation of 850 cub. ft. of air per minute was produced at the end of a level, or about five times that obtained from an equal volume of air at 60 lb pressure. The increased resistance, due to the large extension of workings from single pairs of shafts, the ventilating currents having often to travel several miles to the upcast, has led to great increase in the size and power of ventilating fans, and engines from 250 to 500 H.P. are not uncommonly used for such purposes.

Lighting.

The lighting of underground workings in collieries is closely connected with the subject of ventilation. In many of the smaller pits in the Midland districts of England, and generally in South Staffordshire, the coals are sufficiently free from gas, or rather the gases are not liable to become explosive when mixed with air, to allow the use of naked lights, candles being generally used. Oil lamps are employed in many of the Scotch collieries, and are almost universally used in Belgium and other European countries. The buildings near the pit bottom, such as the stables and lamp cabin, and even the main roads for some distance, are often in large collieries lighted with gas brought from the surface, or in some cases the gas given off by the coal is used for the same purpose. Where the gases are fiery, the use of protected lights or safety lamps (q.v.) becomes a necessity.

Composition of gas evolved by coal.

The nature of the gases evolved by coal when freshly exposed to the atmosphere has been investigated by several chemists, more particularly by Lyon Playfair and Ernst von Meyer. The latter observer found the gases given off by coal from the district of Newcastle and Durham to contain carbonic acid, marsh gas or light carburetted hydrogen (the fire-damp of the miner), oxygen and nitrogen. A later investigation, by J. W. Thomas, of the gases dissolved or occluded in coals from South Wales basin shows them to vary considerably with the class of coal. The results given below, which are selected from a much larger series published in the _Journal of the Chemical Society_, were obtained by heating samples of the different coals in vacuo for several hours at the temperature of boiling water:--

+------------+------------------+---------+----------------------------------+ | | | Volume | Composition in Volumes per cent. | | Quality. | Colliery. | per ton +---------+-------+------+---------| | | | in cub. | Carbonic| | Marsh|Nitrogen.| | | | ft. | Acid. |Oxygen.| gas.| | +------------+------------------+---------+---------+-------+------+---------+ | Bituminous | Cwm Clydach | 19.72 | 5.44 | 1.05 | 63.76| 29.75 | | " | Lantwit | 14.34 | 9.43 | 2.25 | 31.95| 56.34 | | Steam | Navigation | 89.62 | 13.21 | 0.49 | 81.64| 4.66 | | Anthracite | Bonville's Court | 198.95 | 2.62 | .. | 93.13| 4.25 | +------------+------------------+---------+---------+-------+------+---------+

In one instance about 1% of hydride of ethyl was found in the gas from a blower in a pit in the Rhondda district, which was collected in a tube and brought to the surface to be used in lighting the engine-room and pit-bank. The gases from the bituminous house coals of South Wales are comparatively free from marsh gas, as compared with those from the steam coal and anthracite pits. The latter class of coal contains the largest proportion of this dangerous gas, but holds it more tenaciously than do the steam coals, thus rendering the workings comparatively safer. It was found that, of the entire volume of occluded gas in an anthracite, only one-third could be expelled at the temperature of boiling water, and that the whole quantity, amounting to 650 cub. ft. per ton, was only to be driven out by a heat of 300° C. Steam coals being softer and more porous give off enormous volumes of gas from the working face in most of the deep pits, many of which have been the scene of disastrous explosions.

The gases evolved from the sudden outbursts or blowers in coal, which are often given off at a considerable tension, are the most dangerous enemy that the collier has to contend with. They consist almost entirely of marsh gas, with only a small quantity of carbonic acid, usually under 1%, and from 1 to 4% of nitrogen.

Fire-damp when mixed with from four to twelve times its volume of atmospheric air is explosive; but when the proportion is above or below these limits it burns quietly with a pale blue flame.

Coal dust.

The danger arising from the presence of coal dust in the air of dry mines, with or without the addition of fire-damp, has, since it was first pointed out by Professor W. Galloway, been made the subject of special inquiries in the principal European countries interested in coal mining; and although certain points are still debatable, the fact is generally admitted as one calling for special precautions. The conclusions arrived at by the royal commission of 1891, which may be taken as generally representative of the views of British colliery engineers, are as follows:--

1. The danger of explosion when gas exists in very small quantities is greatly increased by the presence of coal dust.

2. A gas explosion in a fiery mine may be intensified or indefinitely propagated by the dust raised by the explosion itself.

3. Coal dust alone, without any gas, may cause a dangerous explosion if ignited by a blown-out shot; but such cases are likely to be exceptional.

4. The inflammability of coal dust varies with different coals, but none can be said to be entirely free from risk.

5. There is no probability of a dangerous explosion being produced by the ignition of coal dust by a naked light or ordinary flame.

Danger arising from coal dust is best guarded against by systematically sprinkling or watering the main roads leading from the working faces to the shaft, where the dust falling from the trams in transit is liable to accumulate. This may be done by water-carts or hose and jet, but preferably by finely divided water and compressed air distributed from a network of pipes carried through the workings. This is now generally done, and in some countries is compulsory, when the rocks are deficient in natural moisture. In one instance the quantity of water required to keep down the dust in a mine raising 850 tons of coal in a single shift was 28.8 tons, apart from that required by the jets and motors. The distributing network extended to more than 30 m. of pipes, varying from 3½ in. to 1 in. in diameter.

Safety explosives.

In all British coal-mines, when gas in dangerous quantities has appeared within three months, and in all places that are dry and dusty, blasting is prohibited, except with "permitted" explosives, whose composition and properties have been examined at the testing station at the Royal Arsenal, Woolwich. A list of those sanctioned is published by the Home Office. They are mostly distinguished by special trade names, and are mainly of two classes--those containing ammonium nitrate and nitrobenzene or nitronaphthalene, and those containing nitroglycerin and nitrocellulose, which are essentially weak dynamites. The safety property attributed to them is due to the depression of the temperature of the flame or products of explosion to a point below that necessary to ignite fire-damp or coal dust in air from a blown-out shot. New explosives that are found to be satisfactory when tested are added to the list from time to time, the composition being stated in all cases.

Aerophores.

Methods for enabling miners to penetrate into workings where the atmosphere is totally irrespirable have come into use for saving life after explosions and for repairing shafts and pit-work under water. The aerophore of A. Galibert was in its earlier form a bag of about 12 cub. ft. capacity containing air at a little above atmospheric pressure; it was carried on the back like a knapsack and supplied the means of respiration. The air was continually returned and circulated until it was too much contaminated with carbonic acid to be further used, a condition which limited the use of the apparatus to a very short period. A more extended application of the same principle was made in the apparatus of L. Denayrouze by which the air, contained in cylinders at a pressure of 300 to 350 lb per sq. in., was supplied for respiration through a reducing valve which brought it down nearly to atmospheric pressure. This apparatus was, however, very heavy and became unmanageable when more than an hour's supply was required. The newer forms are based upon the principle, first enunciated by Professor Theodor Schwann in 1854, of carrying compressed oxygen instead of air, and returning the products of respiration through a regenerator containing absorptive media for carbonic acid and water, the purified current being returned to the mouth with an addition of fresh oxygen. The best-known apparatus of this class is that developed by G. A. Meyer at the Shamrock colliery in Westphalia, where a body of men are kept in systematic training for its use at a special rescue station. This corps rendered invaluable service at the exploring and rescue operations after the explosion at Courrières in March 1906, the most disastrous mining accident on record, when 1100 miners were killed. A somewhat similar apparatus called the "weg," after the initials of the inventor, is due to W. E. Garforth of Wakefield. In another form of apparatus advantage is taken of the property possessed by sodium-potassium peroxide of giving off oxygen when damped; the residue of caustic soda and potash yielded by the reaction is used to absorb the carbonic acid of the expired air. Experiments have also been made with a device in which the air-supply is obtained by the evaporation of liquid air absorbed in asbestos.

Underground fires are not uncommon accidents in coal-mines. In the thick coal workings in South Staffordshire the slack left behind in the sides of work is especially liable to fire from so-called spontaneous combustion, due to the rapid oxidization that is set up when finely divided coal is brought in contact with air. The best remedy in such cases is to prevent the air from gaining access to the coal by building a wall round the burning portion, which can in this way be isolated from the remainder of the working, and the fire prevented from spreading, even if it cannot be extinguished. When the coal is fired by the blast of an explosion it is often necessary to isolate the mine completely by stopping up the mouths of the pits with earth, or in extreme cases it must be flooded with water or carbonic acid before the fire can be brought under. There have been several instances of this being done in the fiery pits in the Barnsley district, notably at the great explosion at the Oaks colliery in 1866, when 360 lives were lost.

Methods of winding.

The drawing or winding of the coal from the pit bottom to the surface is one of the most important operations in coal mining, and probably the department in which mechanical appliances have been brought to the highest state of development.

Cage.

The different elements making up the drawing arrangements of a colliery are--(1) the cage, (2) the shaft or pit fittings, (3) the drawing-rope, (4) the engine and (5) the surface arrangements. The cage, as its name implies, consists of one or more platforms connected by an open framework of vertical bars of wrought iron or steel, with a top bar to which the drawing-rope is attached. It is customary to have a curved sheet iron roof or bonnet when the cage is used for raising or lowering the miners, to protect them from injury by falling materials. The number of platforms or decks varies considerably; in small mines only a single one may be used, but in the larger modern pits two-, three- or even four-decked cages are used. The use of several decks is necessary in old pits of small section, where only a single tram can be carried on each. In the large shafts of the Northern and Wigan districts the cages are made about 8 ft. long and 3½ ft. broad, being sufficient to carry two large trams on one deck. These are received upon a railway made of two strips of angle iron of the proper gauge for the wheels, and are locked fast by a latch falling over their ends. At Cadeby Main with four-decked cages the capacity is eight 10-cwt. tubs or 4 tons of coal.