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Encyclopedia Britannica - Main :: MIC-MOL |
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MINING , the general term for the working of deposits of valuable mineral. The term 1 is not limited to underground operations, but includes also surface excavations, as in placer mining and open-air workings of coal and ore deposits by methods similar to quarrying, and boring operations for oil, natural gas or brine. Mining may be subdivided into the operations of prospecting or search for minerals, exploration and development, work preparatory to active operations, and working. The latter includes not only the actual excavation of the mineral, but also haulage and hoisting by which it is brought to the surface, timbering and other means of supporting the excavations, and the drainage and ventilation of mines. Finally, under the heads of administration, mine valuation, mining education, accidents, hygiene and mining law, will be discussed matters having important bearing on mining operations. Special methods of mining are dealt with in the separate articles on COAL, GOLD, and other minerals and metals. QUARRYING and ORE-DRESSING, which may be considered as branches of mining, are also discussed in separate articles. Prospecting.-In the article on MINERAL DEPOSITS (q.v.) the distribution and mode of occurrence of the useful minerals and ores are fully discussed. The work of prospecting is usually left to adventurous men who are willing to undergo privation and hardship in the hope of large reward though the chances of success are small. The prospector is guided in his search by a knowledge of the geological conditions under which useful minerals occur. When the rocks are concealed by detrital material he looks for outcroppings on steep hillsides, on the crests of hills or ridges, in the beds of streams, in landslides, in the roots of overturned trees, and in wells, quarries, road-cuttings and other excavations. When the solid rock is not exposed the soil sometimes furnishes an indication of the character of the underlying rock. Sometimes the vegetation, shrubs, trees, &c., as characteristic of certain soils, may furnish evidence as to rock or minerals below. Search should be made in the beds of streams and on the hillsides for " float mineral " or " shoad stones," fragments of rocks and minerals known to be associated with and characteristic of the deposits. Fragments of coal, or soil stained black with coal, will be found near the outcrop of coal beds. Grains of gold or particles of ore may be detected by washing samples of gravel in a prospector's 1 Of doubtful origin. " Mine," both verb and substantive, come from the Fr., and is usually connected with Lat. minare, to drive or lead; but this would normally result in Fr. mener, not miner. Skeat, following Thurneysen, accepts a Celtic origin (cf. Irish mein, ore), but the New Eng. Dict, doubts this. MINING pan. By tracing such indications up the stream or up the hill-side the outcrop may sometimes be found, or at least approximately located. The outcrop of a metalliferous vein frequently manifests itself as a line of rocks stained with oxide of iron, often honeycombed and porous, the " gossan " or " eisen-hut," the iron oxide of which results from the decomposition of the pyrites, usually present as a constituent of such veins. Other metals, such as manganese, copper, nickel, may show their presence by characteristic colours. Finally, the surface topography will often throw much light on the underground structure. The shape of the hills and ridges is necessarily influenced by the inclination of the strata, by the relative hardness of different rock-beds, and by the presence of folds and fissures and other lines of weakness. A quartz vein or bed of hard rock may show itself as a sharp
ridge
Exploratory Work.Before opening and working a mine it is necessary to have as full and accurate information as possible as to the following: 1. The probable extent and area of the deposit, its average thickness, and the probable amount and value of the mineral; 2. The distribution of the workable areas of mineral in the deposit; 3. Conditions affecting the cost of opening, developing and working the mine or determining the methods to be adopted. Work undertaken to secure this information must be distinguished from prospecting, which is the search for mineral deposits and from development, work undertaken to prepare for actual mining operations. Exploratory work is associated intimately both with prospecting and with development, but the purpose is quite distinct from either prospecting, development or working, and it is of importance that this should be clearly recognized. It must be remembered' that the line between a workable deposit and one that cannot be profitably worked is often very narrow and that the majority of mineral deposits are not workable. The money that is spent in prospecting and in development is therefore liable to prove a loss. This is a recognized and legitimate business risk
risk
The quickest and cheapest method is by surface explorations. The work of the prospector frequently furnishes much of the information required. By sinking additional pits or by ex-tending the costeaning trenches and uncovering the outcrop of the deposit more fully it is sometimes possible to obtain all the MINIM= information required for the most extensive and important mining operations. Even when the outcrop is oxidized, and surface the mineral character and richness of the deposit is Bxpiora- altered thereby, it is possible to determine variations doz. in thickness and the extent and distribution of the rich and barren areas by outcrop measurements. Information of this sort obtained by surface exploration is often as conclusive as similar information obtained from underground workings. If the deposit shows great variations in thickness in its outcrop along the surface it is probable that a drift or a slope would show the same thing in depth. If the workable areas are poor, and appear only at long intervals along the outcrop, the chances of discovering richer areas by a shaft are very small. In many cases underground exploration is necessary. For example, the deposit does not outcrop as in the case of blind Boring veins and fiat deposits below the general level of the country; or the outcrop lies beyond the limits of the property or under water or water-bearing formations, or is covered by quicksand, or is deeply buried. For such buried deposits boring is cheaper than sinking. In the case of coal, salt, iron ore, pyrite and other homogeneous minerals, boring may give all the information required. With a number of holes the average thickness and probable extent of the deposit may be determined, at least approximately. When the deposit is vertical or steeply inclined, horizontal or inclined bore-holes will be necessary. This will increase the cost of boring and will render the holes more likely to swerve from the true direction. In the case of metalliferous deposits of varying thickness or irregular distribution the information from bore-holes is less satisfactory. A large number of holes must be bored to obtain, even approximately, the average thickness and value of the ore and the shape and size of the ore bodies. In extreme cases the results from boring are likely to be untrustworthy and misleading unless the work is done on such a scale that the cost becomes prohibitory. While the information obtained by surface explorations is always valuable, and sometimes conclusive, as to the value under. of the deposit, it is usually necessary to supplement ground Ex- and confirm it by underground work. The outcrop ptoration. of a metalliferous vein is generally more or less altered by oxidation, and often a part of the valuable mineral has been converted into a soluble form and leached out. These conditions sometimes extend to a considerable depth. Below the oxidized outcrop the vein is often increased in value by secondary enrichment, sometimes to a depth of several hundred feet. In the case of such altered deposits surface exploration alone is likely to be misleading, and it is important to push the underground exploration far enough to reach the unaltered part of the deposit, or at least deep enough to make it certain that there is a sufficient quantity of altered or enriched ore to form the basis of profitable mining operations. As the sinking of shafts or the driving of narrow entries or drifts is expensive, and as the mineral extracted rarely pays more than a small fraction of the cost, it is usual to plan this exploratory work so that the openings made shall serve some useful purpose later. The mistake is often made of sinking large and expensive shafts, or driving costly tunnels, before it is fully proved that the deposit can be worked on a scale to warrant such developments, and, indeed, too often before it is known that the deposit can be worked at all; and in too many cases large amounts of money are thus unnecessarily lost by over-sanguine mine managers. It is, however, often advisable that the money spent in surface or underground exploration should at the beginning be spent for information alone. The information so gained not only determines the value of the deposit, but also serves to indicate the best methods of development and of working. The money so spent, if judiciously used, insures the undertaking against loss by diminishing the mining risk, and is thus analogous to premiums paid to insure against fire or other sources of loss. Development.As soon as it appears reasonably certain that the property is workable the mine will be opened by one or more shafts, drifts or tunnels, and the underground passagesfor active mining operations will be started. A drift or entry is a horizontal passageway starting from the outcrop and following the deposit. The former term is used in metal-mines and the latter in coal-mining. A tunnel differs from a drift in that it is driven across the strata to intersect the deposit. Either may be used for drainage of the mine workings, in which case it becomes an adit. A mine should always be opened by drift or entry if practicable, as thereby the expense of hoisting and pumping is avoided. Drifts, entries and tunnels find their chief application in mining regions cut by deep valleys. When, however, the deposit lies below the surface the mine must be opened by a shaft. If the outcrop of the vein or bed is accessible the shaft may be inclined and sunk to follow the deposit. This is in general a cheaper and quicker method of development for inclined deposits than by a vertical shaft, and it has the added advantage that much information as to the character of the deposit is obtained as the shaft is sunk. When the deposit lying below the surface is horizontal, or nearly so, or when the outcrop of an inclined deposit is not accessible, a vertical shaft will be necessary. Vertical shafts are better adapted to rapid hoisting, and have therefore somewhat greater capacity, than inclined shafts. They are to be preferred also for very deep shafts, or for sinking in difficult ground. Drifts and inclined shafts following the deposit may prove difficult of maintenance when the workings become large and settlement of the overlying strata begins. Large pillars of mineral should be left for the protection of the main openings, whether these be shafts or adits. In the case of very thick beds and mass deposits the main shaft or tunnel will preferably be located in the foot- wall
Figs. r and 2 illustrate the development of a metal-vein by two adits, two inclined shafts in the lode, and by a deep vertical shaft connected with the lode by horizontal cross cuts. The stippled areas represent the ore shoots and the white areas the barren portions of the lode. The levels are supposed to be to fathoms (6o ft.) apart. As the mine is opened the deposit is subdivided into blocks of convenient size by parallel passages, which form later the main haulage roads, and by transverse openings i' i Ii% \ for ventilation. In metal- ^,.e X,y mines the main passages are t eo 40 no main passages is determined mainly by considerations of convenience and economy in excavating and handling the $007e. known as levels, and these FIG. I. are connected at intervals by winzes or small shafts. In coal mines, entries and headings, bords and walls serve similar purposes. The size of the blocks or the distance between the i 22 we, YI 0.. a .col Mananglar MIR* 1 so 60 70 BO 50 LL. mineral, and by the possibility of supporting the roof long enough to permit the excavation of the mineral without unnecessary risk or expense. In metal mining, when the workable portions of the deposit are small and separated by unworkable areas, the levels serve also the purpose of exploration, and in such cases must not be so far apart as to risk missing valuable mineral. In coal-mines main entries are often ro0 yds. apart, while in metal-mines the distance between levels rarely exceeds 50 yds. and sometimes is but 50 or 6o ft. In irregular and uncertain deposits this work of development should be kept at all times so far in advance of mining operations as to ensure a regular and uniform output. In some cases, where the barren areas are large, it may be necessary to have two or three years' supply of ore thus blocked out in advance. A mine, however, may be over-developed, which results in loss of interest
Working.When the, development of a mine has advanced sufficiently the operation of working or extracting the mineral begins. The method to be adopted will vary with the thickness and character of the deposit, with its inclination, and to some extent with the character of the enclosing rocks, the depth below the surface, and other conditions. The safety of the men must be one of the first considerations of the mine operator. In most civilized countries the safety of mine workers is guarded by stringent laws and enforced by the careful supervision of mine inspectors on behalf of the government. The method of mining adopted must secure the extraction of the mineral at a minimum cost. The principal item in mining cost is that of labour, which is expended chiefly in breaking down the mineral, either by the use of hand tools or with the aid of powder. Labour is also expended in handling the mineral in the working-places and in bringing it to the mine-cars in which it is brought to the surface. Narrow and contracted working-places are to be avoided, as in such places the cost of breaking ground is always large. Economy in handling makes it desirable to bring the mine-cars as near as may be to the point where the mineral is broken. This can be done in inclined deposits, it can often be done by the aid of mechanical appliances, though sometimes at an expense not warranted in the saving in the labour of loading. In steeply inclined beds the working-place can be so arranged that the mineral will fall or slide from the place where it is broken down to the main haulage road. The greatest difficulty is found where the inclination of the deposit is too great to permit the mine-cars to be brought into the working-place and yet not great enough to allow the mineral to fall or slide to a point where it can be loaded. While it is always desirable to provide large working-places, the size of .the working-place is limited by the thickness and size of strength of the overlying beds forming the roof Working- or hanging wall
Places. rocks the working-places may sometimes exceed loo or even 200 ft. in width. Indeed in metal-mines roo ft. is the usual distance from one level to the next. With weak and thin beds forming the roof the working-places are often not wider than 20 or 30 ft. as in most coal-mines. While the width of the working-place is thus limited by the strength of the roof, its length is determined by other considerationsnamely, the rapidity with which the mining work can be conducted and the length of time it is practicable to keep the working-place open, and also by the increased difficulty of handling the minerals sometimes experienced when the workings reach undue length. In long-wall and in the work of mining pillars the roof will be supported on one side only, the over-hanging beds acting as cantilevers. The working-place in such case is considerably narrower than in rooms or stopes, and there is also greater difficulty in supporting the roof because the projecting beds tend to break close to the point of support where the strain is greatest. This tendency is overcome by the use of timber supports so disposed as to ensure the breaking of the overhanging roof at a safe distance from the working-face and prevent the interruption of the work that might otherwise result. While it is always desirable to work the deposit so as to. extract the mineral completely, it frequently happens that this can only be done at greatly increased cost. In complete the case of cheap and abundant minerals and low- Extraction grade ore deposits it is sometimes necessary to of mineral. sacrifice a considerable proportion of the mineral, which is left for the support of the overlying strata. A similar sacrifice in the shape of pillars is often necessary to support the surface, either to avoid injury to valuable structures or to prevent a flooding of the mine. As already noted large pillars must always be left to protect shafts, adits and the more important mine-passages necessary for drainage, ventilation and the haulage of mineral. In the early history of mining there was but little attempt at systematic development and working, and the mines were often irregular and tortuous. Fig. 3 is A an old Mexican silver-mine of this type. In such mines the mineral was carried out on the backs of men, and the water was laboriously raised by a long line of suction-pumps, operated by hand, each lifting the water a few feet only. With but slight modifications permitting the use of pumps and hoisting-machinery equally simple methods of mining may be seen to-day when the deposit is of small extent. Fig. 4 is a portion of a mine which consists of a series of irregular chambers with the roof sup-ported on small pillars left at intervals for the purpose. In the systematic mining of larger deposits, the simplest plan consists -in mining large areas by means of numerous working-places under the protection of pillars of mineral left for the purpose, and later mining these pillars systematically, allowing the FIG. 4. overlying rock beds to fall and fill the abandoned workings. In shallow mines the pillars are small and the saving of the mineral of minor importance. In deep mines the pillars may furnish the bulk of the product, and the control of the fall of the roof, so as to permit the successful extraction of the mineral, demands a well-schemed plan of operation. In the robbing of pillars, timber is necessary for the support of 4 ZA- the roof in the working-places, and later to control the fall of the roof while the pillars are mined. More effective support and control of the roof may be secured by the use of rock-filling alone or with timber. By the use of rock-filling it is even possible to dispense with pillars of mineral; or, if pillars are left, the use of rock-filling greatly facilitates subsequent robbing operations. Rock-filling will be used whenever a large proportion of barren material must be mined with the ore. If rock-filling must be brought from the surface its use will generally be confined to mines in which it is difficult to support the roof in any other way. Rock-filling yields and becomes consolidated under heavy pressure, and therefore does not furnish a rigid support of the overlying strata, but rather a cushion to control and equalize the subsidence. With soft material, pillars must be large, even at moderate depths below the surface, and it involves less labour to leave Room- and long rectangular pillars than to form numerous Pillar- square ones. This leads to the adoption of the Mining. room and pillar system so common in coal-mining. Fig. 5 is a mine in u bed of soft iron ore worked by a series 0 ~~ - //'; ~ 4//.00Ord/l ,,? 0E2 =2 ia/ w of inclined shafts, from which long horizontal rooms branch off right and left. The usual method of working metal-mines is by overhand and underhand stoping, using rock-filling' or pillars of mineral stoping. to support the roof. Fig. 6 represents a portion of one of the Lake Superior copper-mines worked by overhand stoping. A stope is that portion of the working assigned to a party of miners, and the block
old slope fa/se shell divided into three or four stopes at varying heights above the main level, the lowest being known as the cutting-out stope, the others as the first and second back stopes in ascending order. In steep pitching beds sufficient excavated material is allowed to remain in the stope for the support of the machines and men, the excess being drawn out from time to time and loaded into cars. The rest of the mineral is allowed to remain until the stope has so far advanced that its support is no longer needed. This method of mining requires but little timbering, only a single line of timber and lagging over the level, called the stull. When the roof is weak, or when it is undesirable to leave so much ore in the stopes, false stulls are sometimes erected in the upper part of the stope. The ore below the false stulls can then be drawn out without waiting for the completion of the top stope. When the mineral does not stand well in the pillar it will be necessary to erect a line of timbers with lagging so as to sheathe the under-side of the pillar and prevent L) Li /eve/ /eve/ /A W V ~~~ it is.i, drfl sky, /s ,ape v4 its falling. It is not desirable to leave large areas standing upon pillars in the mine, and as soon as the work on any level is completed the pillar below should be mined out as far as is safe, and the abandoned portion of the mine allowed to cave in and lessen the weight on the pillars elsewhere. The block
and each party of contractors has one or more mills or timbered chutes through which the rich ore is conveyed to the level below and loaded in cars. The ore as mined is hand-picked and the barren material allowed to remain in the stope where it r/z///,ff-Of 7 #4 Wg / //A r A / rr ~i~.i :Jiioi%,.../ Year falls. In this method of mining no pillars need be left under the levels, as the rock-filling gives sufficient support to the roof. This method of mining affords the maximum of safety to the miners. In the working of thick deposits the block of ground between two levels is divided into horizontal sections or floors which Working are worked either from above downward or from of Thick the bottom upward; in the first case the separate Deposits. floors are worked by one of the caving systems; in the second, generally with the aid of filling. Fig. 9 illustrates the working of a block of ground by the top-slice caving system. Above, the ground has been completely worked out from the surface, and the space formerly occupied by ore is now filled with the debris of the overlying strata which has caved in above the block of ore now being worked. There is considerable thickness of old timber left from the working of the upper levels. This mat of timber forms a roof under the protection of which the mining of the ore proceeds downward floor by floor. The working-floors are connected by winzes with the main haulage roads below. These winzes serve for ventilation, for the passage of the workmen, and for chutes through which the ore is dumped to the level below. The working out of each floor is conducted much as if it were a bed of corresponding thickness. Haulage roads are driven in the ore so as to divide the floor into areas of convenient size. These separate areas are then mined in small rooms, each room being timbered as in mining under a weak roof rock. The room is driven in this way from one haulage road to another or to the boundary of the ore body. On completion of any room the timbers are withdrawn and the overlying mass of timber and rock is allowed to fall and a new room is started immediately alongside of the one just completed. In this way the whole floor is worked out and the mat of timber and overlying rock is gradually lowered and rests upon the top of the ore forming the floor below. Before abandoning a room it is usual to cover the bottom of the working-place with lagging-poles, which facilitate the mining of the floor below. In this manner one floor after another is worked until the floor containing the main haulage roads of the level below is reached. In the meantime a new level and a system of haulage roads have been driven a hundred feet below, and winzes have been driven upward to connect with the old level which is to be abandoned. The floor containing these old haulage roads now becomes the top slice of the one hundred-foot block of ground below and is mined out as described. Several floors may be mined simultaneously, i4Q {`ice ~ql! !t-S~T~'C~`%,Ot,.2$~i,']1r.9/~1^X+~'iS`r'1'~v~< ff. the workings in the upper floor being kept in advance of those below, so as to allow the broken mass above to become consolidated before it is again disturbed by the working places of the next floor. This system permits the complete extraction of the ore at moderate cost and without danger to the men. The subdrift caving system, fig. ro, differs from the top-slice system mainly in the greater thickness given to the working floors, which may be from 12 to 40 ft. in thickness, whereas in the top-slice system the height of the floor is limited by the length of the timbers used in the working-rooms, rarely over fl or ro ft. The subdrift system requires a smaller amount1 of narrow work in excavating the necessary haulage roads, and is therefore better adapted to hard ores in which such narrow work is expensive. The mining of each floor is carried on in sections with small working-places which are first driven of moderate height to their full length and width, leaving a back of ore above and pillars of ore between to support the upper portion of the upper layer or floor. These pillars and the back of ore above are then mined in retreating back towards the haulage road. The subdrift system is somewhat cheaper than the top-slice system, the output per man being greater. The bottom-slice caving system of mining begins at the bottom of a hundred-foot block of ground, a floor being excavated under the whole area, leaving pillars of sufficient size to support the ground above. These pillars are then filled with blast holes which are fired simultaneously, permitting the whole block of ground to the level above to drop. A floor is then reopened in this fallen ore, leaving pillars for temporary support which are blasted out as before. This is the cheapest of the three caving systems, but is applicable only when the deposit lies between walls of very solid rock, as otherwise wall rock is liable to cave with and become mixed with ore, which adds greatly to the expense of handling. When rock filling is available, as when the ore contains much barren material to be left behind in mining, the ore body is divided into blocks of convenient height as above, and these blocks are divided into floors, the bottom floor of each block however being attacked. Each floor is opened up by subsidiary haulage roads and worked out in small rooms which are timbered and filled with broken rock when completed. An adjoining room is next excavated and filled, and thus the whole floor is worked out and replaced with rock-filling. Work is then, started on the floor above, the upper floors being connected with the main haulage roads by winzes which are maintained through the filled ground. Several floors can be mined simultaneously, the work in the lower floors being kept well in advance. Instead of mining in horizontal floors the filling method permits the ore to be mined in vertical chambers or slices which extend from one level to the next above and from one wall of the deposit to the other. When a chamber has been excavated and completely filled the slice adjoining is mined out, or at times a block of ground may be left untouched between two filled chambers and then mined out. In the latter case the top-slice caving method will usually be employed for the working of such intervening pillars. In order to lessen the cost of handling the rock-filling, the excavation sometimes takes the form of inclined working-places, parallel to the slope naturally taken by the rock when dumped from above into the working - FIG. IO. ~//7//4 "/V/ place. This method of mining and filling can be used when the work is done in horizontal floors or in transverse chambers. In the United States the Nevada square set system of timbering is used in connexion with rock filling (fig. 11). The use of the heavy timbers and continuous framing which - characterize this system facilitates greatly the work of mining and maintaining the haulage roads on the different floors, and gives more rigid support to the unmined portions of the block of ground above. These advantages compensate for the greater first cost. Where each floor is timbered by itself with light timbers, as is the practice on the continent of Europe, the consolidation of the rock-filling under pressure gives rise to considerable subsidence of the unmined ore, which has frequently settled 20 ft. or more before the upper part of the block is reached. This occasions much added expense in the maintenance and retimbering of the haulage roads on the upper floors. The shrinkage of the rock-filling and the settlement of the workings Intl" I1II- 111fh1\~ . II 1\ ~i~~l~lI~II~Ii~` ^_ ~1I 111111/11 I12111141111111'I\\\ 1111 !!ii'ilil''111111'111~IIIIIIIII~IIIINIIIiiI _ 'I II'!IiNNItiIIill!IIIIIIIi&\ ~I-I 11-11-1II lIIII L %/////i FIG. 11, can be greatly lessened by the use of hard rock with a minimum of fine stuff; but even so the advantage lies with the American system of timbering. The cost of filling has been greatly reduced by the system of flushing culm, sand, gravel and similar material, through pipes leading from the surface into mine workings. Material as coarse as I in. in diameter may be carried long distances underground with the use of little more than an equal volume of water. This method originated in the Pennsylvania anthracite mines in 1887, but has been employed in recent years on a large scale in Silesia, Westphalia and other European coalfields. In some cases it has been found advantageous to quarry and crush rock for the purpose of using it in this way. Examples of other mining methods will be found under COAL. Where mineral deposits lie near the surface underground mining may be replaced by open excavations, and the reduced Open cost of mining makes it possible to remove the Workings. overlying soil and rock to considerable depths. The depth to which open working can be pushed depends upon the size and value of the mineral deposit and upon the expense of removing the over-burden. Open excavations several hundred feet in depth are not uncommon. Where practicable steam shovels are employed, even when it is necessary to break up the material beforehand by blasting. Steam shovels are not well adapted to deep excavation unless provision is. made for the rapid handling of the cars when filled. For deep workings the miffing method is usually employed, inwhich the ore is excavated in funnel-shaped pits, each of'Which connects with underground haulage roads by a shaft. The ore is mined in the ordinary way, by pick and shovel if soft, or by the aid of powder if necessary, and the funnel-shaped bottom of the pit is maintained at such an angle that little or no shovelling is required to bring the excavated material to the shaft. Before the bottom of these pits reaches the level of the haulage roads below, a new set of roads will have been driven at a lower level and connected with the excavations above by the shafts. The cost of mining by the milling method does not greatly exceed the cost of steam-shovel work. For the special methods by which placer deposits. are mined see Gor.D. Underground Haulage.The excavated material is brought to the hoisting shaft, or sometimes directly to the surface, in small mine cars, moved by men or by animals, or by locomotives or wire-rope haulage. The size, shape and design of the cars depend on the size of the mine passage and of the hoisting compartments of the shafts; on whether the cars are to be trammed by hand or hauled in trains; whether they are loaded by shovel or by gravity from a chute; and whether they are to be hoisted to the surface or used only for underground trans-port. The cost of underground haulage is lessened, by the use of cars of large capacity. In the United States cars in the coal and iron mines hold from 2 to 4 tons. In Europe the capacity ranges from i000 to 15001b, though the tendency is to increase the size of the cars used. In mines of copper, lead and the precious metals, in which the cars are moved by hand, the usual load is from 1200 to 3000 lb. These small cars are constructed so that the load may be dumped by pivoting the car bodies on the trucks. . Larger cars are usually dumped by means of rotating or swinging cradles, the car bodies being rigidly attached to the. axles or trucks. When loaded by shovel the car is made low to economize labour. Wooden rails, protected by iron straps, are sometimes used on underground roads for temporary traffic; but. steel rails, similar to, though lighter than, those employed for. railways are the rule. For hand tramming, animal and rope haulage, the rails weigh from 8 to 24 lb per yard, for locomotive haulage 30 to 40 lb. Grades are made, whenever possible, in favour of the load, and of such degree that the power required to haul out the loaded cars shall be approximately equal to that for hauling back the empties, viz. about i of 1%. Sharp
In metal mines, where, as a rule, mechanical haulage is inapplicable, the cars are moved by men (trammers). This is expensive, but is made necessary by the small man and amount of material to be handled at any given Animal point. The average speed is about 20o ft. per H8alage. minute, and the distances preferably but a few hundred feet. Animal haulage is employed chiefly in collieries and large metal mines; sometimes for main haulage lines, but oftener for distributing empty cars and making up trains for mechanical haulage. In mines operated through shafts the animals are stabled underground, and when well fed and cared for, thrive notwithstanding their rather abnormal conditions of life. Mine cars are sometimes run long distances, singly or in trains, over roads which are given sufficient grade to impart consider-able speed by gravity, say from I to 21%. The grades must not be too great for brake control nor for the hauling back of the empty cars. Cars may thus be run through long adits or through branch gangways to some central point for making up into trains. Near the top and bottom of hoisting shafts the tracks are usually graded to permit the cars to be run to and from the shaft by gravity. Locomotive haulage is applicable to large mines, where trains of cars are hauled long distances on flat or undulating roads of moderate gradients. Steam locomotives have been largely superseded by compressed air or electric locomotives. Compressed air locomotives are provided with cylindrical Sand Flushing. steel tanks charged from a special compressor with air at a pressure of 500 to 700 lb per sq. in. The capacity of the tank depends on the power required and the dis-L etance to be traversed by a single charge of air. Haulage. ulage. 3 The air passes through a reducing valve from the main to an auxiliary
For heavy gradients rope haulage has no rival, though for moderate grades it is often advantageously replaced by electric and compressed air haulage. Gravity or self-acting 8 aHge. planes are for lowering loaded cars, one or more at a time, from a higher to a lower level. The minimum grade is that which will enable the loaded cars in travelling down the plane to pull up the empty cars. At the head of the plane is mounted a drum or sheave, and around it passes a rope, one end of which is attached to the loaded cars at the top, the other to the empty cars at the foot. The speed due to the excess of weight on the loaded side is controlled by a brake on the drum. The rope is carried on rollers between the rails. There may be two complete lines of track or three lines of rails, one being common to both tracks, and the cars passing on a middle turnout or " parting "; or a single track with a parting. An engine
loaded cars are hauled by a stationary engine
empty cars running down by gravity, dragging the rope after them. This is similar to shaft hoisting, except that the grades are often quite flat. In the tail-rope system of haulage, best adapted for single track roads, there are two ropesa main and a " tail " ropewinding on a pair of drums operated by an engine. The loaded train is coupled to the main rope, and to the rear end is attached the tail-rope, which reaches to the end of the line, passing there around a large grooved sheave and thence back to the engine. By winding in the main rope the loaded cars are hauled towards the engine, dragging behind them the tail-rope, which unwinds from its drum. The trip being completed, the empty train is hauled back by reversing the engine. The ropes are supported between the rails and guided on curves by rollers and sheaves. High speeds are often attained. Branches, operated from the main line, are readily installed. In the endless rope system the rope runs from a grip wheel on the driving engine to the end of the line, round a return sheave, and thence back to, the engine. Chains are occasionally used. The line is double track and the rope constantly in motion, the cars being attached at intervals through its length by clips or clutches; the loaded cars move in one direction, the empties in the other. There are two modes of installing the system: either the rope passes above the cars and is carried by them, resting in the clips, or it is carried under the cars on rollers, the cars being attached by clips or a grip- carriage. (For details see Hughes, Text-book of Coal Mining, pp. 236272; Hildenbrand, Underground Haulage by Wire Rope.) Rope haulage is widely used in collieries, and sometimes in other mines having large lateral extent and heavy traffic. With the tail-rope system, cars are run in long trains at high speed, curves and branches are easily worked, and gradients may be steep, though undulating gradients are somewhat disadvantageous. In the endless-rope systems cars run singly or in short trains, curves are disadvantageous, unless of long radius, speed is relatively slow:and branch roads not so easily operated as with tail-rope. The tail-rope plant is the more expensive, but for similar conditions the cost of working the two systems is nearly the same. An advantage of the endless system is that the cars may be delivered at regular intervals. Hoisting.When the mine is worked through shafts, hoisting plant must be installed for raising the ore and handling men and supplies. On a smaller scale hoisting is also necessary for sinking shafts and winzes and for various underground services. As ordinarily constructed, a pair of horizontal cylinders is coupled to a shaft on which are mounted either one or two drums (fig. 12). The diameter of the cylinders wladlag is such that each alone is capable of starting the Eagme load. As the cranks are set 9o apart, there is no dead centre, and the engine is able to start under full load from any point of the stroke. This is important in mine hoisting, which is intermittent in character and variable as to power and speed required. The cylinders are generally single-expansion, though compound engines are occasionally used for heavy work. The engine is direct-acting, the drums making one revolution for each double stroke. In geared hoists the drums are,on a separate shaft, driven from the crank-shaft by tooth or friction gearing, and make one revolution for, say, 4 or 5 double strokes. The hoisting speed is therefore slower, and as less engine power is required for a given load the cylinders are smaller, though making more strokes per minute. Large and powerful geared hoists are not uncommon. The dimensions of the drum depend on the hoisting speed desired and the depth of shaft or length of rope to be wound. Drums are either cylindrical or conical. Conical drums (fig. 12) tend to equalize the varying load on the engine due to the winding and unwinding of the rope. On starting to hoist, the rope winds from the small towards the large end of the drum, the lever arm, or radius of the coils, increasing as the weight of rope decreases. A similar equalizing effect is obtained by the use of flat rope and reel, the rope winding on itself like a ribbon. Tapering ropes, tail-ropes suspended from the cages, and other means of equalization, are also employed. If, for a two-compartment shaft, a pair of drums (or a single wide drum) be keyed to the engine shaft, with the ropes wound in opposite directions, the hoisting is " in balance," that is, the cages and cars counterbalance each other, so that the engine has to raise only the useful load of mineral, plus the rope. This arrangement allows no independence of movement: when the loaded cage is being. hoisted the empty must be lowered. Independent drums, on the contrary, are loose upon their shaft, and are thrown on or off by tooth or friction clutches. The maximum load on the engine is thus greater and more power is required than for fixed drums.' Steam consumption is economized, when-ever possible, by throwing in the clutches of both drums and hoisting in balance. Fixed drums are best for mines in which the hoisting is done chiefly from one level; independent drums when there are a number of different levels. Hoisting engines are provided with powerful brakes and frequently with reversing gear. In deep shafts hoisting speeds of 3000 or 3500 ft. per minute are often attained, occasionally as much as 5000 ft. Formerly hemp and also fibre ropes were commonly used. Except in a few instances Hoisting these were long ago superseded by Ropes, iron-wire ropes, which in turn have been replaced by steel because of its greater strength. For hoisting in deep shafts, and to reduce the weight of rope, tempered-steel wire of very high tensile strength (up to 250,000 or 295,000 lb ultimate strength per sq. in.) is advantageously employed. A 1-in. ordinary steel rope has a breaking strength of about 32 tons, which, with a factor of safety of six gives a safe working load of 54 tons. A 1-in. plow-steel rope has breaking and working strengths respectively of at least 48 and 8 tons. Standard round rope (fig. 13) has six strands of 19 wires each and a hemp 'core. Flat rope is in favour in some districts. It is composed of several four-stranded ropes, without hemp centres, laid side by side, and sewed together by wire (fig. 14). It is not as durable as round rope and is heavier for the same working strength. As the sewing wires soon begin to break, a flat rope must usually be ripped apart and resewed every six or eight months. Numerous patent ropes, some having wires and strands of special shapes, have been introduced with the idea of improving the wearing properties. Such, for example, are the Lang-lay, locked-coil and flattened strand rope. Hoisting ropes are weakened by deterioration and breakage of the wires, due to corrosion and repeated bending, and should be kept under careful inspection. To prevent excessive bending stresses the diameter of drum and sheave must bear a proper ratio to that of the rope. A ratio of 48 to r is the minimum allowable; better 6o to 95 to I, and for highly- tempered steel ropes ratios of 15o to r or more are desirable. To prevent corrosion the rope should be treated at intervals with hot lubricant. With proper care a steel rope should last from two to three years. A frame of wood or steel, erected at the shaft mouth, and End of Article: MINING If you wish, you can link directly to this article.
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