Min′ing, the process by which mineral matters of commercial value are taken from their natural position and made available for shipment. The substances mined consist not only of metals and the ores of metals, but of various nonmetalliferous substances, as coal. In the broadest sense of the term mining may also include the min
ning of such substances as natural gas, mineral oils, clay, building-stone, natural fertilizers and salt, though other terms are commonly applied to the processes by which these substances are made available.
Occurrence. The materials mined occur in various forms, chief among which are (a) bedded deposits chiefly beneath the surface, (b) vein deposits and (c) surface deposits.
(a) Bedded deposits include (1) such bodies of valuable mineral matter as lie in beds essentially parallel to the associated layers of rock and (2) deposits which are disseminated through stratified rocks. Examples of (1) are many iron and all coal beds. An example of (2) is the copper in the conglomerate beds of the Lake Superior region. The former (1) are really layers of rock, formed later than the rock which lies below them, but before that which lies above. The second group (2) usually are of secondary origin; that is, the valuable mineral matter was concentrated where it now is, after the rock was formed.
(b) Veins are the fillings of cracks or fissures in rock. The filling is generally accomplished through the agency of underground water, which dissolved the mineral matter from the rock, brought it to the fissures and there deposited it. Veins are called mineral veins or, often, lodes, if the material is valuable for commercial purposes. The walls of veins are often impregnated with useful mineral matter like that of the vein proper, and the term is then made to cover those parts of the walls which contain the valuable matter as well as the vein itself. Veins differ from bedded deposits in that they usually are more irregular, less continuous, and have no definite relation to the bedding planes of the rock. Both bedded and vein deposits may be horizontal, vertical or inclined at any angle. Bedded deposits usually were horizontal to begin with, but the beds of ore, coal etc. may have been tilted the same as the beds of rock above and below. Veins likewise occur in any position, depending on the direction of the fractures in which the vein stuff was deposited.
(c) The third class of ore deposits is represented by gravel, sand or earthy material, in which there is some useful mineral which can be extracted with profit. Thus certain deposits of sand and gravel, like those of Cape Nome and various mountain valleys of the western part of North America, contain gold. Tin ores sometimes occur in the same way. In such deposits mines are opened. The specific gravity both of gold and tin is much higher than that of the associated sand and gravel, and this fact is taken advantage of in mining. Running water is caused to flow over the gravel and sand with a current which can be regulated. It washes away the lighter material, leaving the desired metals or ores behind. This is the principle of the method known as placer mining. Mines in these loose surface materials are placer mines. Certain other surface deposits are mined, but in a very different way. Thus in certain marshes and bogs bog-iron ore accumulates. This, in reality, is a bedded deposit, but is of recent origin, and not buried beneath later beds of rock or sediment. If worked at all, the ore is taken out bodily. Bog-iron ore is at present but little used.
The Problems of Mining. After the existence of valuable deposits of mineral matter has been determined, many conditions affect the method of mining it. These conditions are so complex that they must be considered individually in the case of each mine. In each case the methods must be adapted to the local situation. The aim always is to extract the maximum of ore with the minimum expenditure of time and money and with the least danger to life and property. The first things to be determined are (1) the shape and position of the ore body, whether it is in distinct beds or veins or disseminated through a large mass of rock; whether the veins or beds are horizontal, vertical or oblique etc.; whether it is near the surface, far from the surface or both; (2) the extent of the ore body; (3) the character of the ore itself as regards hardness, tenacity etc.; (4) the nature of the rock in which the ore occurs, so far as concerns its hardness, texture, structure etc.; (5) the topography of the immediate locality where the mine is located. Questions of another sort, as facilities for transportation to and from the mine, water supply, fuel supply etc., have to be considered in connection with every mine.
Various means are employed for the determination of the exact position of the ore body. This is the work of exploration. Exploration is carried on partly by sinking small test pits; partly by stripping the loose material from the surface, exposing the rock which contains the substance to be mined; partly by shafts sunk into the rock; partly by the help of the compass, as in the case of magnetic iron ores; and partly by means of drill holes. In each case the methods of exploration best fitted to the situation should be adopted. The test pits are very much like open, shallow wells. The stripping of the surface is the method often adopted where the mineral vein comes to the surface of the rock, but is covered by soil and other loose debris. Vertical shafts are often sunk alongside the vein, and tunnels or cross-cuts are then run from the shaft across the vein. Veins are often cross-cut in this way in many places by way of exploration. The drills which are used in preliminary tests are hollow iron tubes having an inner diameter of one or two inches. In the “bit” at the lower end of the tube diamonds are set. With the bit resting on the surface of the rock to be drilled, the tube is made to rotate at high speed and cuts its way down into the rock. A cylindrical core of rock appears in the tube or “core barrel” as the drill descends. Water is constantly pumped down the inside of the tube, and rises between the drill rods and the wall rock, washing up the fine grindings of the drill. With the drill it is possible to obtain specimens of the rock at various depths, and this is often of value in the further work of the mine. Drill holes have been made several thousand feet deep. This method of exploration is expensive, but effective.
Another problem with which the miner has to deal is the approach to the ore body. Roughly speaking, mining methods may be divided into two general classes: (1) surface mining or open work and (2) underground mining.
Open work is done where a small amount only of worthless material must be removed in order to uncover the deposit. This method is employed, for example, in the iron mines in the Mesabi range in Minnesota, where the ore is covered by glacial drift 10 to 100 feet thick. Such covering may often be stripped off with steam shovels. Where the ore is soft and friable, as in the case of some of the iron ore of the Mesabi range, it too can be removed by steam shovels; but if the material to be mined is hard, it is blasted and taken out in masses.
In underground work the cheapest and easiest method, if the topography and the position of the material to be mined permit, is to approach the ore by tunnels. This may be done where the ore body lies in the side of a hill or mountain, so that it may be approached from the side. If the ore can be taken out through a tunnel, it saves the expense of hoisting it tip through a shaft. The tunnel often also saves the expense of pumping out the water, which often is a troublesome factor in mining. In many cases, however, it is necessary to approach the ore by a shaft. The shaft may be either vertical or inclined, according to local conditions, especially according to the position of the ore body. The inclined shaft is sometimes known as an incline. A shaft is usually rectangular in section. In very small mines it may be no more than four feet square, but in large mines the shafts are much larger, large enough to permit the working of at least two hoists, called cages or skips. There must also be room for pipes, both for carrying down compressed air and for the carrying out of water. Large mines frequently have shafts as much as 12x18 feet in diameter.
If the material to be mined lies in a horizontal bed, the shaft is sunk down to the level of the material to be mined or a little below. Tunnels are then made, leading sidewise from the shaft into the substance to be mined. The material to be mined is worked out along these tunnels, carried to the shaft and hoisted to the surface. When the material which is being mined is worked out, it often is necessary to put in timbers to keep the mine open. If, for example, a horizontal bed of coal is being mined, the taking out of the coal leaves the rock above unsupported. Timbers may be put in to hold up the roof, or columns of coal may be left here and there to serve the same purpose. Where practicable, the tunnels driven from a shaft are made to rise slightly from the horizontal, so that the water encountered in the tunnel may drain into the sump or pool at the bottom of the shaft. Prom the sump it is pumped out. The slight inclination of the tunnel also facilitates the transfer of the substance mined to the shaft. If the tunnels are parallel with a mineral vein, they are called drifts. When they cut the vein, they are called cross-cuts. Tunnel is a rather indefinite name which may be applied either to drifts or cross-cuts. The process of drifting and cross-cutting is called “driving a level.” Tunnels are usually as much as 6x6 feet in section. Tunnels are often run at various levels; for example, tunnels may be run at the 50-foot level (that is, 50 feet below the surface), at the 100-foot level, at the 150-foot level and so on. When the drifts and cross-cuts have been made, and the different levels put in communication by vertical shafts, the ore between the various openings is said to be “blocked out.”
Some ore has been extracted in the process of blocking out the ore. When a body of ore has been blocked out, extraction of ore begins in earnest. The processes employed for loosening the substance to be mined are various. Sometimes it is loosened by the pick or some other sort of hand-tool. Sometimes it already is soft or incoherent. Often it is so hard that it must be blasted. Ore is often worked from below rather than from above. That is, ore between the 50-foot level and the 100-foot level is often worked from the latter and carried along it to the main hoisting shaft. The working out of the ore between levels is “stoping.”
Another problem which the miners have to encounter is the drainage of the mine. In most deep mines the amount of water seeping in is great, and it must be pumped out about as fast as it enters. For this purpose force-pumps are used. In deep mines several or many force-pumps may be needed. Pumps which are able to elevate the water 300 or 400 feet each are in common use. In mines near deep valleys it sometimes is possible to secure drainage by driving a drainage tunnel or “adit” from the shaft to the valley.
Still another problem which has to be faced in mining is that of ventilation. In coal-mines where noxious gases are sometimes plentiful, in mines where many men are at work, in mines where there is much blasting and in deep mines where the temperature is high at the bottom ventilation is of the utmost importance. Ventilation may be either natural or artificial In shallow mines the hoisting shaft is usually large enough for ventilation. If there are several shafts connected with one another, the circulation of the air in the mine is facilitated. If there is an adit, it helps still further. Artificial circulation is secured by fans placed at the entrance of mines, or by the use of compressed air which is carried into the mine. Compressed air is often introduced for power, and as it escapes it serves for ventilation as well.
Transportation. When the material to be mined has been loosened from its natural position, it is carried to the main gangways in barrows, chutes or cars. In the main gangways are tramways. The cars are of iron or wood, with wheels so close together that the cars can be run on short curves. The motive power is furnished by men, mules, steam engines or electricity. In vertical shafts the material is hoisted in buckets or cages. In inclined shafts, the hoisting is done with skips. The skips are on wheels, the rear wheels being much larger than the front ones. When the skip reaches the horizontal dumping platform at the top, the forward pitch dumps the ore automatically. The hoisting power in large mines is steam or electricity.
Ore Dressing. In most instances the ore must be subjected to a preliminary treatment before it is fit for metallurgical processes. The method of treatment varies with the ore. In many cases the masses and lumps of ore are crushed, or even ground to powder, often by the pounding of huge hammers in a stamp mill. The valuable part of the ore is then separated from that which is without value, the process of separation being different in different cases. Where the ore is much heavier than the waste, the separation is brought about through differences in specific gravity. The final extraction of the metal, like the preliminary treatment of the ore, differs with the nature of the ore. Gold, for example, is passed over a copper plate coated with mercury. The mercury forms an amalgam with the gold, and is afterward separated from it by heating until the mercury is volatilized. At this stage the product passes from the hands of the miner to the hands of the metallurgist. Ores of other metals are treated by other processes. The concentration of the crude ore is milling. Under this term the extraction of the metal from the ore is also sometimes included. Properly speaking, the process of mining ceases when the ore is ready for the mill or for shipment.