Mining. The art of mining comprehends all the processes whereby the useful minerals are obtained from their natural localities beneath the surface of the earth, and the subsequent operations by which many of them must be prepared for the purposes of the metallurgist. The art has been practised from the remotest times. It is referred to in the 28th chapter of the Book of Job; and an Egyptian papyrus, drawn in 1400 B.C., preserved in the museum at Turin, depicts the workings of a gold-mine. The first writer who treated mining systematically was Georgius Agricola. In 1556 he published in Latin an exhaustive treatise on the subject. The introduction of gunpowder as a blasting-agent in 1620 completely changed the conditions under which mining had up to that time been carried on, and the enlarged scale on which mining operations are now conducted has led to the invention of new methods of working, and to the introduction of machines of greater precision and power.
All mineral deposits are divided into two very broad divisions. The first includes the beds or seams of iron ore, coal, and salt. These are deposits laid out more or less horizontally and parallel to the stratification of the surrounding rocks. The second class includes mineral veins or lodes (see ORE-DEPOSITS). Various names have been given to these deposits. In the British colonies, for example, they are termed reefs (see GOLD), a somewhat misleading name. A lode may be defined as a repository of mineral matter which fills more or less completely a former fissure in the earth's surface.
The mining appliances employed are very different in the two classes of deposits. In the first class, it is desirable to make a hole of the shortest possible depth from the surface of the ground to the bed of mineral. A shaft is therefore sunk through valueless beds until the mineral is reached. Machinery of the best class is then used to extract the whole of the mineral, due precautions being taken to avoid danger from falls of roof and from noxious gases. In the second class of deposits, the inclination of the mineral vein has to be taken into account, as the deposit varies considerably in inclination and in size. The vein must therefore be studied foot by foot, downwards from the top. The miner does not look favourably on vertical veins. Certainly in most cases it would appear that the chance of vertical lodes being productive is much less than in inclined ones. In some cases a vertical shaft is sunk, and passages, known as cross-cuts, are driven from this to the vein at different levels. A vertical shaft presents the advantages of greater ease in sinking, hauling, and pumping. At the Comstock lode, in Nevada, thousands of pounds were wasted in sinking a perpendicular shaft, the advantages of which were urged with considerable plausibility. A deep shaft may cost from £10,000 to £50,000. In the case of an inclined shaft the ore obtained from the shaft itself enables some of the charges to be recouped. In a well-known Cornish copper-mine, Tresevean, after an inclined shaft had been used for many years, a new shaft, 1800 feet in depth, was sunk at a cost of £20,000; but success had already been assured before this great outlay was contemplated. The best arrangement for an extensive mine is to have a main vertical shaft and several secondary inclined ones. With inclined shafts it is out of the question to put in the highly-perfected engines used at collieries, the object being not the removal as quickly as possible of large quantities of material, but the exploration of the vein by slow and careful degrees at many points and with a moderate number of men.
In searching or prospecting for mineral deposits large sums of money are spent, sometimes in vain. The surface of the rock is usually covered by deposits of sand and gravel, vegetable matter, vegetation, and, in some cases, peat bogs. In consequence, many notable mines have been discovered by accident. Thus, the observation of the pellets picked up by birds led to the discovery of veins of gold ore in Lower Hungary. The famous silver-mines of Potosi are said to have been discovered by an Indian who, taking hold of a bush to prevent his falling, pulled it up by the roots and thereby disclosed glittering masses of native silver. Again, gold was discovered in California by James W. Marshall, in 1848, while cutting a small mill-race. In ancient times the search for mineral deposits was based on the indications given by the Divining-rod (q.v.); and there still exist intelligent miners who believe in this curious myth.
In the search for mineral deposits, the best evidence is obtained by putting down bore-holes. These are made by various methods, and are put down to a depth of a few feet when required for testing the character of the foundation subsoil, or, in other cases, to thousands of feet when required in seeking for or estimating the value of deposits of coal, salt, and ironstone. Ages ago bore-holes were put down by the Chinese to a depth of 3000 feet. Recently, in Europe and America, depths of 2000 feet have not unfrequently been attained. At Schladebach, near Merseburg, the deepest bore-hole in the world has been put down by the Prussian government in search of coal. The sinking occupied several years (1880-86), and the depth attained amounted to 5834 feet.
Bore-holes may be made by a circular borer moved by a lever. The rods are of iron, with square heads, and are turned by a cross-head worked by a couple of men. In this way an auger-like cutting action is effected. With harder rock it is usual to advance by means of percussion. A chisel-headed tool is employed, which cuts holes of 3 to 4 inches in diameter. At each stroke the bore-master causes the tool to turn slightly. When sufficient debris has accumulated the rods are withdrawn, and an instrument put down to extract the powdered material and water. With a length of rods amounting to 400 to 500 feet the weight is enormous, and, in consequence of the concussion, difficulties arise. Men are not sufficient to raise the load. In some cases a lever is used to raise the rods a few inches or feet, and to let them fall suddenly. In other cases the rods are replaced by a rope. This, however, from being wet and dry alternately, is apt to snap suddenly, and the rods remaining in the hole are difficult to recover. The rods, too, may get twisted or the nature of the iron itself be altered by the vibration. In putting down a bore-hole, a tower or shears is erected over the hole. By making this 60 to 70 feet high, the rods may be extracted in lengths of 60 feet, and thus the 6 to 8 hours a day usually spent in unscrewing the rods are saved. In some cases it is necessary to tube or line the whole bore-hole. See BORING.
The Chinese method of boring with ropes has been imitated in Europe with great economy, but with great liability to fracture and consequent loss. This has been done by Messrs Mather & Platt of Salford, who employ a chisel-bit with circular sides so as to keep the bore-hole true. This is raised and allowed to fall a few inches or feet, according to the nature of the ground. It is attached to a weighty mass of iron with rings serving as guides. The whole mass is suspended by a flat hempen rope. This rope passes over a pulley to a drum on which a mass of rope can be accumulated. The pulley is attached to the piston-rod of a steam-engine. The action of the steam behind the piston lifts the pulley, and consequently the tool, the rope being clamped. The steam then causes the tool to fall, and on falling it automatically turns. A cylindrical tool can easily be inserted, and a core obtained that shows the nature of the rock and its inclination. At the Paris Exhibition of 1862 a proposal was made to drill with a tube in which diamonds were fixed. This was merely intended for use on a small scale; but it was soon applied to deep bore-holes. For this drill black diamond is employed, a substance with the full hardness of the ordinary diamond and a certain amount of toughness. Though very expensive, this method of boring is found advantageous when great speed is required. The fall of rock in bore-holes is apt to cause serious interruption on account of the jamming of the rods. The sudden strain given to release them is liable to cause fracture, besides which the full work is not done by the rods. This difficulty is obviated in several ways, notably by replacing the iron rods by wooden ones, 30 to 32 feet long, with iron connections. The free-falling cutter proposed by Kind and the hollow rods of Von Oeynhausen may be instanced as having rendered good service in the execution of great works.
In order to open up a mine, tunnels or adit-levels are driven on the lode or to cut it whenever the contour of the country allows it. Shaft-sinking involves a larger outlay of capital and greater working costs. In the ordinary method of sinking shafts, the workmen standing upon the bottom of the pit blast out the rock, and send the excavated material to the surface by means of an engine, rope, and bucket. The sides of the shaft are supported by timbering or walling. In water-bearing strata many difficulties are encountered. Brunel, the father of the great engineer, proposed to obviate these by employing a circular frame with a cutting ring. On this, with hydraulic mortar, a wall was built and held firmly together by ties. In a second method, largely used in modern collieries, beams of cast-iron are employed, and 10,000 to 20,000 wooden wedges driven in, a succession of cast-iron segments or rings, known as tubbing, being built in. The shaft is thus sunk and the water pumped out. Tubbing a shaft is a very difficult operation, and the method has frequently been known to fail after £20,000 to £30,000 has been spent. In order to get over the difficulties and dangers, Kind, a German engineer, thought of sinking a bore-hole with sufficiently large tools consisting of solid masses of iron with sharp steel teeth. The shaft having been bored, rings of cast-iron could, he thought, be fixed in and the water pumped out. This was tried in 1840 in a very difficult case and was found impracticable, and not until 1860, when Chaudron, an eminent Belgian, took the matter in hand, was the method successful. A watertight bottom was made, half a dozen workmen at the surface doing all the work. The method has been employed in the United Kingdom in a few instances. One remarkable case may be mentioned. At the mouth of the Tyne are coal-measures of great value, and at South Shields attempts were made to work the coal under the sea. Difficulty, however, was caused by a band of magnesian limestone highly charged with water. The enterprise promised to be very costly. Tubbing was totally unsuccessful, notwithstanding the fact that enormous pumps were employed raising as much as 11,000 gallons of water per minute. The shaft was 14 feet in diameter, and if the pumping ceased the water rose in the shaft 12 feet in two minutes. Recourse was then had to the Kind-Chaudron method, which had previously been successfully tried on the Continent. The trépan or cutter of the boring tool was 3 or 4 feet in diameter, and the hole was bored to a certain depth. A larger cutter was then used. In this way the sides were formed into inclined planes, so that the fragments rolled into a suspended bucket in the smaller hole, the bucket being raised from time to time. When a place was reached where a watertight joint could be made, Chaudron's tubbing was applied and the shaft successfully completed. In this tubbing the bottom ring has a sliding case in which is placed a quantity of moss, which, when the whole length of tubbing comes to rest on the watertight bed cut for it under water by the borer, packs together and forms a tight joint. This method of sinking shafts is practically self-acting. It is economical and simple, and eliminates risk to human life. In ordinary shaft-sinking accidents are frequent, as a screw or a hammer falling down the yawning gulf is likely to produce a fatal injury.
An ingenious device for overcoming the difficulties of shaft-sinking was invented by a French engineer, Triger. This consists in damming back the water by employing a constant resisting force; that is to say, in pumping into the iron cylinder that forms the shaft such an amount of air that the pressure on the bottom from within should be equal to that from without. By means of a flooring in the cylinder, a lower air-tight compartment is formed, in which it is found that men can work under a pressure of atmospheres. In order that the men may enter or leave their working-place without disturbing the equilibrium of the forces, the principle of the canal-lock is applied, a second chamber being formed above the working one with trap-doors communicating with the shaft above and with the chamber below. One of these doors being always closed while the other is open, the excavated material can be drawn up without any appreciable loss of compressed air. This method has been successfully applied at a number of shafts on the Continent.
Another ingenious process of sinking through quicksand is that devised by H. Poetsch. This consists in freezing the water contained in that portion of the water-bearing ground which occupies the position of the intended shaft into a solid mass of ice, and then sinking through it by hand without having to pump any water. This method has proved successful at several Continental collieries and at the Chapin mine in Michigan.
The average depth of coal-mines before the introduction of the steam-engine did not exceed 100 yards, whilst a near approximation for the present time would be 400 yards. The deepest shaft in Great Britain is that of the Ashton Moss Colliery, near Manchester, which has attained a depth of 2850 feet. The seams dip at the rate of 9 inches per yard, so that parts of the workings are 3000 feet deep. The deepest shaft in the world was until recently that of a silver-lead mine in Bohemia, at Przibram, where the Adalbert shaft is 3432 feet in depth. This depth has, however, been exceeded in the Lake Superior copper-mining district, where in 1890 the Calumet shaft attained a depth of 3900 feet.
The cutting of a path through the harder rocks, as carried on by the ancient miners, was particularly laborious. The work was executed in confined spaces, and a large amount of dust was produced. The miners' vocation was excessively unhealthy, inasmuch as they were obliged to inhale large quantities of dust; they thus became subject to disorders of the lungs to which they fell victims at an early age. Previous to the introduction of blasting the implements used were of the nature of wedges and hammers. Bit by bit pieces of rock were broken away, the operation being aided by natural fissures in the rock and by the brittleness of the hard material. In this way the ancient miners cut coffin-shaped galleries 5 feet in height. At the present time the galleries or levels are usually feet high and 5 feet wide, thus affording great facility for travelling and for ventilation. The invention of gunpowder is of much greater antiquity than its application to mining purposes. In the 14th century it was largely used for musketry and cannon, and even for blowing down defences; but, curiously enough, it was not applied to mining purposes until the beginning of the 17th century, and even then made its way so slowly that it was not largely employed until the 18th century. In the operation of blasting use is made of a borer or drill of iron, or, as is more usual, of cast-steel. This is struck with a hammer. A borer of larger diameter may be used, held by one man and struck by another. Of late years mechanical rock-drills driven by steam or by compressed air have come largely into use; see BORING, with illustration. The bore-hole, when finished, is then charged. The gunpowder is enclosed in a little bag of cloth dipped in pitch and provided with a fuse. Instead of using a cartridge of this kind, clay may be forced down the hole by a claying bar so as to shut off feeders of water, and the hole will be dry enough to receive a charge of powder. A needle of iron or steel is placed in the midst of the charge with the ring at its end protruding, and tamping is introduced. For this purpose it is best to put in clay piecemeal until the whole is filled up. Accidents may happen by pushing down the iron bar, which is apt to strike fire against hard rock. A second suspected cause is that, when a man takes a tamping bar and strikes it with a hammer, the air is compressed, and the temperature augmented sufficiently to explode the powder. These disadvantages are set aside by Messrs Bickford & Smith's safety-fuse (see BLASTING). The older fuse consisted of carefully selected straws filled with fine powder. The safety-fuse is of cotton with gunpowder in the middle. For use in wet ground it is covered with a waterproof composition. Considerable attention has been paid to the subject of consolidating the charge. Excellent work has been done with compressed powder; dynamite has become quite indispensable; and gun-cotton is also employed, the best form being the compressed variety invented by Abel. Nitrated gun-cotton or tonite has also given admirable results. The fullest benefit of these modern explosives can only be obtained by the use of strong detonators fired by electricity, by which it is possible to place a number of bore-holes in such a manner that when fired simultaneously they shall help each other.
For removing coal these high explosives are too quick in their action, and blasting-powder continues to be used. Millions of tons of coal are still obtained by its aid. In order to obviate the danger of explosions in fiery collieries, many ingenious substitutes for blasting have been proposed. For example, a hole is bored, and wedges inserted to force down the coal which has previously been under-cut with the pick. Another plan of great promise is that devised by Smith and Moore, in which cartridges of caustic lime are employed, water being forced into them by a force-pump. The pressure of steam generated by the usual charge of seven cartridges is 2850 lb., the cartridges themselves expanding to about five times their original size. The efficiency of these cartridges varies with the nature of the coal, the best results having been obtained in the Derbyshire collieries.
The work of the miner engaged in under-cutting the coal-seam is very arduous, and various coal-cutting machines have been invented with a view to lessen the labour and expense. They work with compressed air or electricity, and have the cutters arranged on the periphery of a rotating disc, or on a travelling pitch-chain. Though largely employed in America, they have not yet come into extensive use in Great Britain. The coal, when broken down, is placed in wagons, and drawn by horses or engine-power to the bottom of the shaft and raised to the surface.
The actual mode of working the coal, although varying greatly in every district, may be broadly divided into (1) the post-and-stall, or bord-and-pillar, or (in Scotland) stoop-and-room, method, where the first stage of excavation is accomplished with the roof sustained by coal; (2) the long-wall method, where the whole of the coal is allowed to settle behind the miners, no sustaining pillars of coal being left. The latter method, when well planned, is the safer both as regards facility of ventilation and less liability to accidents from falls. At a Durham colliery, working the Harvey seam, which is feet in thickness, 5185 tons of coal were obtained when working by the long-wall system, and 5052 tons when working by the post-and-stall system. In thick and highly-inclined beds it is usual to remove the coal by horizontal slices, and to fill the excavation with waste material. In some instances blast-furnace slag is used for the purpose.
The mode of working metalliferous veins differs greatly from that followed in the case of the more or less horizontal coal-beds. Horizontal galleries, termed levels, are driven upon the lode usually 10 fathoms (60 feet) apart. They are rarely perpendicularly above one another, as they follow the inclination of the vein. The levels are connected by means of small shafts, termed winzes. Represented on a vertical plane, the vein will thus be seen to be cut up into pillars which are worked by the method of stopping. Of this there are two varieties—underhand and overhand stopping. In the former the ore is gradually worked away downwards from the floor of one level, the ore and worthless mineral being taken out through the level next below. In most districts underhand stopping has been superseded by the more economical overhand method, in which the miners stand on timber platforms and break down the mineral above them.
The great depth and size of modern collieries necessitate the raising of greater quantities of coal through a single shaft than was ever contemplated in former times. The winding-engines of modern erection are consequently of extraordinary power. Thus, at Harris' Navigation Colliery the engines have cylinders with a diameter of 54 inches, and are capable of raising 6 tons of coal, or, with ropes and the cages containing the coal-trucks, a total load of 15½ tons, at a speed of 32 feet per second. The quantities which can thus be raised are enormous. It is by no means uncommon for 900 to 1400 tons to be raised from one pit in the day.
In collieries both coal and men are raised in the cages, but in the metalliferous mines the man-engine is largely used. This consists of a reciprocating rod or pair of rods fitted with steps, by which the miner is raised 8 to 14 feet at a stroke. Although this method obviates the tax on the energies of the men entailed by the climbing of ladders, it is by no means free from danger. Prussian statistics show that where man-engines are employed there are four times as many accidents as where cages and ropes or where ladders are used.
At the Épinac collieries in France a remarkable pneumatic system of raising coal and men is employed. An air-tight wrought-iron tube, 5 feet 3 inches in diameter, is placed in the shaft and fitted with a piston-cage carrying nine coal-wagons. The air being exhausted above the piston, a load of 3 tons of coal is raised at a rate of 19 inches per second. The great cost of the installation has prevented the method from being generally adopted.
In almost all mines the surrounding rock contains water which rapidly accumulates in the workings. Where the contour of the district is suitable, the best method of draining the mine is by means of an adit-level—i.e. a tunnel driven in the hillside. In some cases extensive areas are drained by adits. Thus, the great Gwennap adit in Cornwall, which is with its branches 40 miles in length, drains 30 sq. m. As further examples of long adits may be cited the Ernst-August adit in the Harz Mountains, which has a total length of 14 miles and cost £85,500, and the Rothschönberger adit at Freiberg in Saxony, which is 25 miles long. In cases where adits are unavailable, recourse must be had to pumps either of the lifting or forcing type. The principal type of engines is that known as the Cornish pumping-engine, which is a single-acting condensing beam-engine working expansively. Some of these engines are of enormous size, the cylinders in some cases being as much as 100 inches in diameter. Their great cost and ponderous character have led to the introduction of cheaper direct-acting engines which placed underground force columns of water to vertical heights of as much as 1000 feet. At a silver-mine at Klausthal, in the Harz Mountains, a pair of direct-acting rotary engines have been erected, driven by hydraulic power, with a head of 1959 feet. At twelve revolutions per minute these pumps force 330 gallons of water up 750 feet.
The ventilation of subterranean workings is a problem of the greatest importance. The air is contaminated by the respiration of men and horses, by the combustion of lights, by the smoke of explosives, and by deleterious dust. Added to which, in the case of collieries, the insidious fire-damp or carburetted hydrogen exudes from the coal. Mingled with air this gas forms the explosive mixture to which so many miners owe their death (see FIREDAMP, CHOKE-DAMP, SAFETY-LAMP). It is obvious that the ventilating current must be sufficient to dilute this mixture below the firing-point and to sweep it away. The general mode of ventilating a colliery is to have two shafts, a downcast and an upcast. The pure air entering by the downcast shaft traverses the roadways of the colliery. By means of doors and stoppings, the current is caused to travel in the required direction so as to reach the innermost workings of the mine. It then passes to the upcast shaft and returns to the surface. The motion of the air-current is caused by furnaces or by mechanical ventilators. In the former case, a large furnace is kept burning at the bottom of the upcast shaft, the air in which it heats and causes to expand. In this way a volume of air is obtained suitable for very extensive workings, as much as 120,000 to 250,000 cubic feet of air being passed through the shafts per minute. In the case of mechanical ventilators, the vitiated air is withdrawn from the colliery by the exhausting action of centrifugal fans, which may be made either of large diameter to run at low velocity, or of small diameter to run at high velocity. At several important collieries these fans attain enormous dimensions, in some cases as much as 45 feet in diameter and 14 feet in width.
Almost as important as ventilation in relation to the safety of human life is the accurate construction and the preservation of mine-plans. In many cases the plans are laid down without any reference to the phenomenon of the variation of the magnetic needle. Trusting to old plans constructed in this way, the miner may drive straight into old workings filled with water, the tapping of which would be death to all employed in the colliery.
The progressive legislation in connection with mines (e.g. the Coal-mines Regulation Act of 1872, amended 1886, which prescribes for the inspection of mines by duly appointed inspectors, &c.) has proved beneficial in diminishing the proportion borne by the accidents to the number of miners employed; for whereas in 1850, when the output of coal in the United Kingdom did not exceed 50,000,000 tons, the number of miners employed being about 200,000, the deaths slightly exceeded 1000 in the year, in 1877, when the output of coal was 134,000,000 tons and the number of miners double that in 1850, the deaths were only 1200 in number. The deaths from explosions of firedamp during the eleven years 1875–1885 formed but 23.57 per cent. of the total deaths, the remainder being due to falls of roof and other causes. For statistics of mineral production, see GREAT BRITAIN, UNITED STATES, &c.
In England and Ireland the crown has the right to all mines of gold and silver; but where these metals are found in mines of tin, copper, iron, or other baser metal, then the crown has only the right to take the ore at a price fixed by statute. In Scotland gold-mines belong to the crown without limitation, and silver-mines when three-halfpence of silver can be extracted from the pound of lead. As a general rule, in the United States as well as in Britain, whoever is the owner of freehold land has a right to all the mines underneath the surface, for his absolute ownership extends to the centre of the earth; but under special grants and contracts it is not uncommon for one person to be owner of the surface of the land and another to be owner of the mines beneath; or several persons may be owners of different kinds of mines lying one above the other in the different strata. On the public lands of the United States, a title or license may be obtained by any citizen from the general land office at Washington, at the rate of $5 per acre of surface pre-empted; no royalty is paid, but the claim must be worked in accordance both with local regulations and with the general mining laws, which prescribe as one condition the performance of a certain amount of work annually. If this condition is not fulfilled, the mine may be 'denounced,' and any other person secure the claim.
BIBLIOGRAPHY.—The literature of mining is very extensive, but the following may be cited as useful works of reference: Callon's Lectures on Mining, translated by Dr C. Le Neve Foster and W. Galloway; Sir Warington Smyth, Coal-mining (7th ed. Lond. 1890); J. J. Atkinson, Practical Treatise on the Gases met with in Coal-mines (Lond. 1879); R. Hunt, British Mining (1884); H. M. Chance, Report on Coal-mining (Philadelphia, 1885); B. H. Brough, Mine Surveying (2d ed. Lond. 1889); Arundel Rogers, Mining Law (Lond. 1876); Report of the Accidents in Mines Commission (1886). The principal sources of information on mining matters are the Transactions of the Mining Institutes, the technical journals, and the annual reports of H.M. Inspectors of Mines. See also articles COAL, COPPER, GOLD, IRON, LEAD, DIAMOND, &c.