Table of Contents
Transportation of Ore and Waste
For the purpose of this discussion, “transportation” signifies the operations involved in transporting ore and waste from the face or stope where it is broken to the surface but not the handling of the ore within the stopes, which will be discussed under Stoping. It therefore includes loading cars from chutes on the haulage and intermediate levels, hand-tramming, animal and locomotive haulage, scraper and conveyor-belt transportation, operation of main transfer raises between levels, dumping into shaft pockets, the operation of skip-loading devices, the caging of cars, and hoisting the ore in buckets, cages, and skips.
Transportation facilities of some sort must be provided when underground work is begun. These may or may not be retained as mine development expands and ore production begins, but they are likely to prove inadequate after the mine comes into part or full production. Thus, hand tramming and hoisting in buckets or in small cars on cages may serve when tramming distances are short and the amount of ore and rock to be handled is small, while large cars, locomotive haulage, and hoisting in skips may be required for economical transportation once the mine hits its production stride.
The distances material must be hauled and the amount to be handled are of first importance in deciding upon the method of transportation and the kind of equipment to be used. Facilities for handling ore and waste between the stopes and the haulage levels will vary with the stoping method employed as well as with the amount of ore and waste to be handled.
In an earlier Bureau of Mines publication it was pointed out that the cost of underground transportation constitutes a considerable percentage of the total mining cost in most mines, and in some mines may even exceed the cost of stoping. At 19 open-stope mines, transportation averaged 26.3 percent of the total underground cost; at 19 mines employing shrinkage stoping, 22.65 percent; at 3 mines employing block caving, 21.03 percent; at 8 mines employing square-set stoping, 12.77 percent; and at 4 mines employing cut-and-fill stoping, 12.9 percent. The average haulage and hoisting costs for these groups ranged from $0.1135 to $0.4025 per ton of ore mined, and the range for 47 individual mines was from $0,086 to $1,848. Elsing has tabulated transportation costs at 66 mines, which show averages by groups ranging from $0,120 per ton mined to $0,556. It is therefore apparent that underground transportation merits the careful attention of the operating staff.
Tramming and Hauling
Hand tramming is still widely used in small mines, in prospecting operations, and as an adjunct to mechanical haulage systems. For tramming small tonnages of rock and ore short distances on the main levels, for short transfers on sublevels, and in some types of stopes it is the obvious method to employ, although, if power is available, scrapers may be more economical for some of these purposes and small storage-battery locomotives for others, provided the tonnage to be handled warrants the capital outlay involved.
Hand tramming requires a minimum capital investment for equipment and for this reason, among others, is particularly adapted to prospects and small-scale operations such as those conducted by many “leasers” (lessees).
Small cars of 12-, 16-, or 20-cubic-foot capacity are usually employed, the 16- and 20-cubic-foot sizes being the most popular, although cars holding 2 to 2½ tons sometimes are used. In the Tri-State lead and zinc district, “cans” of 1,200 to 1,400 pounds capacity were used almost exclusively for many years, but more recently these have been superseded by cars in some of the mines. The cans are buckets that are run to the face on a flat truck and, after they have been loaded, are returned to the shaft and hoisted directly to the surface.
Cars are usually of either the end- or side-dumping type, with the end or one side, as the case may be, forming a swinging door suspended from a rod at the top of the car and latched at the bottom. In the end-dump type the car body or box often is mounted on a turn-table on top of the truck frame, so that the car may be turned for dumping into a chute at either side of the track.
Car bodies and truck frames may be of either wood or steel construction, though few wood cars have been employed in recent years. One advantage of the wood-box car is that if a board becomes broken it can readily be removed and replaced, whereas a damaged steel box requires considerable work in the shop.
For tramming ore and filling material in cut-and-fill stopes, a car with a scoop-shaped box open at the front end may be used to advantage.
Tracks for hand tramming usually are laid with light rail (weighing 8, 12, or 16 pounds per yard) on wood ties. The heavier rails are preferable if they are to be used for an appreciable time, since once they are well-laid they are less likely to become bent and out of alignment and thus will require less attention.
Track grades for most efficient hand tramming should be considerably steeper than for locomotive haulage. The ideal grade would probably be that on which the effort required to push a loaded car downgrade would equal that required to push the empty car up, although some prefer a somewhat steeper grade, so that the loaded car, when once started, will almost if not quite run by itself. The exact grade requiring a minimum of effort will vary with the size and weight of the car and the load carried, the type of bearings, lubrication, and the general condition of the equipment and track. Ordinarily, grades of 1 to 1½ percent in favor of the loaded cars will be found satisfactory for hand tramming.
Mules and horses continue to be employed for underground haulage, though much less extensively than formerly. The growing availability of electric power, the introduction of Diesel generating plants in sizes and of capacities to suit almost any condition, and the development of small storage-battery and trolley locomotives have favored the substitution of mechanical for animal haulage.
Animals are useful for gathering loaded cars from the loading faces to a siding on the main haulageway and returning the empties, particularly if large cars are employed or there are unavoidable grades to be negotiated that are too difficult for hand tramming but not steep enough for rope haulage. Cars may be handled one at a time or be made up into trains of two, three, or more, depending on conditions.
As a rule, mules are preferred to horses for underground work, and for heavy work animals weighing around 1,200 pounds are more satisfactory than smaller ones, provided there is enough head room. Smaller animals, of course, must be employed where head room is low or larger ones cannot be taken down the shaft. A cage compartment usually will accommodate a 900-pound mule.
According to van Barneveld, the economical limit of mule haulage is between 1,000 and 1,500 feet, and a mule should work at a speed not to exceed 5 miles per hour when taking a loaded train down a grade that needs only tractive effort to start, or 2 to 3 miles per hour when pulling a heavy load.
Van Barneveld cites data presented by Boericke regarding track grades and ton-miles per mule, as follows:
The following table from Boericke’s article indicates performance of mules under different conditions:
Ton-Miles Obtained Under Varying Conditions
A. Grades against the load. Maximum, 300 feet, 3 percent. Sharp curves. No brakes on empties. Return trip at 5-mile trot.
B. Long haul. Gathering mules hauled two cars to gathering station 1,000 feet from shaft over fair tracks; maximum grade against load, 1 percent. Eight-car train hauled from station to shaft. Too hard on mules; changed to gasoline haulage.
C. Good conditions. Level track, good curves. Large mules hauled six-car trains to shaft. Slow speed, 3½ miles an hour.
D. Good conditions. Track 1.5 percent in favor of load; straight track, electrically lighted. Cars run down by own weight after starting; mule trots at 5 miles. Train of four cars had to be spragged at end of run. Limit on return trip, four empty cars. Two mules haul 256 tons.
In considering mechanical vs. hand or animal haulage in a mine or section of a mine, a factor that always must be considered is the duty to be performed. If there are no other mitigating conditions, it is a good general rule that for a locomotive (or any type of machine) to pay for itself it must be kept busy; in other words, idle machines pay no dividends. When mechanical haulage units can be kept busy they will usually show a saving in operating costs as compared to hand or animal tramming.
Although a single example cannot reasonably be taken as a basis for broad generalization, the following, where the conditions were admittedly rather exceptional, is cited as being of interest in this connection:
The output was 400 tons per day on two shifts, 300 tons of which came from three or four loading chutes. The maximum haul was 750 feet and the average 500 feet. Although the track was good and the grade favorable, 8 men on each of two shifts, 16 in all, using 1-ton cars, were required to load, tram, and dump the ore, thus averaging 25 tons per man-shift. A storage-battery locomotive and 3-ton Granby-type cars were placed in service, the loading chutes were widened, and ore-storage capacity was provided, after which three men could handle 300 tons from the chutes together with some ore from development headings in 2½ hours, or an average of more than 100 tons per man in one-third of a shift. Hand-tramming costs were 22 cents and locomotive-haulage costs 5.2 cents per ton, including labor and power but not maintenance or depreciation.
Mechanical haulage in metal mines is of four general types: (1) Locomotive, (2) scraper or slusher, (3) rope, and (4) belt conveyor.
Locomotives for underground use are powered by electric, compressed-air, or, rarely, gasoline motors. Diesel locomotives have been used extensively in Europe, but have not been adopted in the United States. Studies are now in progress on how to adapt American Diesel locomotives to underground use and to determine conditions under which they may be employed with safety. Gasoline motors should never be employed for underground work because of the hazard from exhaust fumes.
Electric locomotives are either of the trolley or storage-battery type, and each has its particular field. For long hauls and heavy trains the trolley type usually has a decided advantage, whereas for shorter hauls, such as in gathering service from a number of loading points to a main haulageway, the storage-battery type may be preferred. Small storage-battery locomotives of the “trammer” type that can be run on the mine cage and transferred quickly from one level to another have found wide favor where the mine output is small and comes from scattered areas and several horizons in the mine. For heavy duty, storage-battery locomotives have insufficient ampere-hour capacity for long service without frequent recharging, and spare batteries may have to be supplied for each locomotive. Where haulage is confined to one shift, the batteries may be charged on the opposite shift. When charging can be done between shifts or at times when the total mine load is light, storage-battery locomotives may have an advantage over trolley locomotives in keeping down power peaks.
Cable-reel trolley locomotives are sometimes used for gathering in short headings tributary to the main haulageways, where it does not pay to install trolley wire.
Compressed-air locomotives have not been employed widely in metal mines but have certain obvious advantages in gassy coal mines. Their over-all efficiency, including that of the compressor unit, is low, and the cost of power for their operation is high. In some instances they have been installed in preference to trolley locomotives because of the hazard of exposed trolley wires, particularly at loading chutes. They receive air at high pressures (600 to 850 pounds or more), which necessitates the installation of high-pressure compressors, receivers, air mains, and charging equipment at high initial cost and requires careful attention to prevent leakages and attendant loss of power.
Although the scraper as employed underground is ordinarily considered to be a mucking and loading machine, it is also a conveying machine and in some instances is used to transport ore and rock several hundred feet. It is then an auxiliary part of the transport system for moving ore from the face to a chute or mine cars or for transferring ore from a series of stope raises, through a sublevel drift, to an ore pass.
The term “rope haulage” is used here to mean the use of hoists for pulling cars on grades or flat slopes where the grade is too steep for locomotives or the method in general use in the mine.
Conveyor systems have come into common use in coal mines during the past decade. As this is written, belt conveyors are beginning to make their appearance in metal mines as a part of the underground transportation system, and an increasing number probably will be installed during the next few years.
It is not within the scope of this bulletin to discuss the mechanical design and construction of locomotives or other types of mine equipment; these are described in the catalogs and handbooks of the manufacturers. However, the broad question of application of different types of equipment to different mining conditions and uses, the results obtained, and costs of operation are within the province of the mining engineer.
In selecting a locomotive, its weight and horsepower must be proportioned to suit load, track grades and curvatures, condition of tracks, and type of bearings employed on the cars.
According to van Barneveld, the draw-bar pull of a locomotive is usually taken as 25 percent of its weight, and the trailing load it will pull is 15 or 16 times its weight when on level, straight track. When the train is running on an adverse grade, the draw-bar pull and consequently the load that can be pulled are reduced sharply, as shown in table 26. When roller-bearing cars or cars equipped with railroad-type journals are used, and with well laid and maintained tracks, a grade of about 0.25 percent in favor of the loads usually is advocated for motor haulage, although in many mines grades of 0.50 percent are standard. Under less ideal conditions, grades of 0.5 percent may be found desirable. Table 27 gives data on actual performance of mine locomotives.
The proper weight of rail to use will depend on a number of factors, including the weight of locomotives, cars, and loads, speed of operation, and length of time the haulage way is expected to remain in service. From the standpoint of operating cost and maintenance, the heavier rails are preferred, although the first cost obviously will be greater. Light locomotives weighing 1½ or 2 tons sometimes are run on 16-pound track. Thirty-pound rail was standard in many mines for haulage with 3½-ton locomotives; with larger equipment, 40-, 50-, or 60-pound rail is employed quite commonly. Table 28 presents track data taken from actual practice and covers hand-tramming, mule-haulage, and motor-haulage operations.
Tables 27 and 28 indicate the range in capacities of ore cars commonly employed in metal mines. For large output and long hauls the use of large-capacity cars usually is justified from the standpoint of economy in operation. Other considerations may be more important, however, and small cars may be employed for a number of reasons. Thus, the size of drifts it is feasible to drive and maintain often limits the size of car that can be used. In other instances ore may be gathered from many scattered points in the mine and trammed by hand to the main motor haulage way, and it may be cheaper to haul the small hand tram-cars in trains even over long distances than to dump them and reload into cars of larger capacity for locomotive haulage. Scrapping of existing small-capacity equipment and substitution of equipment of larger capacity often may be desirable after a mine has advanced from small- to large-scale operation, but in other instances, where the earlier equipment is still in good condition, the capital outlay required to replace it may not be justified by the anticipated savings in direct haulage costs.
Much attention has been given to details of design and construction of mine cars, but a discussion of this subject would be too voluminous for inclusion in this bulletin.
Today most mines employ steel cars equipped with roller bearings, though in a few large mines railroad-type journal bearings are used. The principal types of cars are distinguished chiefly by the methods of dumping, which, in turn, largely determine the several characteristic designs. End-dump and side-dump cars with end or side doors that are tipped endwise or sidewise to the dumping position have been mentioned under “Hand Tramming,” and these types, especially the side-dump cars, are used with motor haulage also.
Side-dump cars of the rocker type have been popular in recent years. The car body is V-shaped in cross section, the bottom of the V being rounded. Rounded trunnions at each end of the body below the center of gravity rock or roll on a frame attached rigidly to the truck frame; the body is rocked over on its side to discharge the contents of the car. This type of car can be emptied while it is moving slowly over the dumping point.
The Granby car is a favorite type for use in connection with locomotive haulage where rapid dumping of trains with minimum labor is desired. It is designed to be dumped while in motion. A roller on the under side of the car body rides up on a ramp constructed along one side of the track at the dumping point and lifts that side of the body. As the body tips, a side door on the opposite or low side is raised, and the contents is discharged. The locomotive pushes the train slowly along the ramp, and each car is dumped in turn without having to stop the train. The cars also can be dumped by means of an air jack or by a rope and winch at points where no dumping ramp is provided. If there is ample pocket capacity the train can be dumped and returned immediately for its next load in less time than ordinarily would be required to switch out the loaded train and couple up with a standing empty one. The train crew does the dumping, and a minimum number of cars is required as no cars are idle at any time. The Granby car is heavy, and some operators claim it is difficult to keep clean if the ore is at all sticky, but the advantage of quick dumping and low operating cost may often outweigh these objections.
The gable-bottom car has been employed widely, but in recent years the trend has been toward rocker-bottom or other types of cars. Gable-bottom cars have a swinging door on each side of the car, hinged at the top, which, when unlatched, permits the contents to discharge. The gable-bottom construction reduces the capacity of the body, and the doors usually require considerable repair work to keep them tight and prevent leakage of fine ore along the haulage roads. Bottom-discharge cars are used at some mines but are not as common as other types.
Solid-body cars with the bodies attached rigidly to the truck frame have a number of advantages but require a cradle or rotary dump for discharging and for this reason are not well-adapted for handling and distributing ore or waste to a number of dumping points. Where ore only is handled and there is but one dump point or large tonnages are dumped at each point, the cost of installing the dumping mechanism may be outweighed by the advantages of this type of car. They are low in first cost in relation to their capacity, they are sturdy and have no doors to maintain and keep in repair, no leakage of fines occurs as in the case of cars with poorly-fitting doors, and the height above the rail is low in proportion to their capacity.
Each type of car mentioned has its advantages under certain conditions determined by such factors as tonnage to be handled and the number of loading and dumping points; size of haulageways; physical character of the material handled, whether fine or coarse, wet, dry, or sticky, and the method of loading. Thus, a low car is desirable for loading by hand shoveling or by power scraper in drifts and crosscuts. Likewise, the height of loading chutes above the rail may limit the height of the car. If cars are to be hoisted on cages and dumped on the surface the size of the hoisting compartments and cages will limit the size of car that can be employed. For handling fine material such as sand filling as well as fine ore, a solid-body car without doors has obvious advantages.
Tramming and Haulage Costs
Costs of tramming and haulage are available from only a few mines, and it is difficult to make accurate comparisons because of differences in accounting practices at different mines. It is often the practice to combine costs of hand mucking and tramming, especially in development work. Most operators charge loading from chutes against tramming cost, some include also cost of chute maintenance and repairs, and some combine tramming and hoisting under the general head of transportation. In some instances dumping cars at the skip pocket may be charged to tramming if the dumping is performed by the train crew, whereas in others, where dumping is done by a separate station attendant or shaft crew, this operation may be charged to hoisting. Some mines segregate waste tramming from ore tramming, whereas many others do not.
A few examples of tramming and haulage costs are given in table 29, which, for the reasons above stated, cannot be compared precisely
Scraping has been discussed in detail in an earlier Bureau of Mines publication. The scraper was first employed in metal mines primarily as a mucking and loading machine but has come to be used widely for transporting ore and rock for limited distances. Thus, scrapers are used in stopes to drag ore from the face to the chutes, to place waste filling in the stopes, and, in scraper drifts and crosscuts, to drag ore from a line of raises to a common main ore pass or directly into cars on the haulage level. Direct loading into cars in headings and flat stopes by means of a scraper slide has been discussed briefly in the section on “Mine Development.”
In considering scraper transportation of ore, the digging function of the scraper cannot be disregarded, since it must first fill itself by digging before it can transport its load by dragging. A scraper designed for maximum digging efficiency may not work well as a means of transportation; it may catch on rough spots and spill most of its load along the way, may dig into the bottom if it is soft, or tear up flooring laid to protect the bottom. These difficulties can be overcome by proper design, though sometimes at the sacrifice of some digging efficiency.
Two general types of scrapers are employed underground—the box type and the hoe type. Both are bottomless, although some are constructed with a back plate curved forward to meet the floor at a flat angle, forming a partial sloping bottom. Figure 25 shows one form of hoe scraper at A. The box scraper differs from this principally in that it has sides. (Fig. 68.) Figure 25, B, shows a semihoe scraper that has short side plates extending upward and forward from the digging edge to the bale.
In general, the box scraper is applicable for handling finely broken or granular material, whereas the hoe type is better adapted to hard, chunky ore. The box scraper will dig fine material and, by reason of the sides, retain the load better; but in digging blocky ore the sides tend to ride up on the lumps and thus prevent penetration of the cutting edge or teeth into the muck pile, especially if the scraper is light in weight. By allowing spill to accumulate along the haulageway it will form banks on each side of the scraper rut, which will prevent further spillage even when a hoe scraper is used.
The proper size (capacity) of scraper for a given installation will depend on the quantity of ore to be handled and the rate of handling desired and will be limited in some instances by the dimensions of the heading in which it is to operate. The weight of the scraper for any particular condition is influenced largely by the size and weight of the broken ore. In soft, dry, loose muck a light scraper usually will dig satisfactorily, but in hard, chunky ore adequate weight is a primary requirement. Probably more initial scraper installations have failed because the operators started with a scraper too light to penetrate the muck pile or with a hoist too small to load and pull a heavy scraper than for any other reason.
As shown by graphic power-input charts taken from scrapers in operation, the maximum power during the scraping cycle is required when the scraper is digging into the pile when it is often two, three, or more times that required during the dragging period.
For transportation purposes the scraper should be designed so that its digging action will cease or be greatly reduced after the scraper is filled. In somewhat lumpy ore the sides of the box scraper usually will accomplish this. In soft, fine ore it may be necessary to curve the top part of the back plate forward, as shown in figure 68 at E. The force of the load against this top section then tends to lift the scraper. The same method may be employed with hoe-type scrapers, or, as an alternative, short, tapered side plates may be added (fig. 25, B).
Scrapers are best adapted to transportation underground where large quantities of ore must be moved comparatively short distances. Usually scrapers of large capacity are necessary for this, which in turn requires the use of hoists larger than are customarily employed for loading in drifts and crosscuts, the nature of the material handled being the same. The trend during the past 15 years has been toward the use of larger, more powerful scraper hoists capable of pulling heavier loads at higher speeds. Overpowered hoists are desirable for
scraper work, as they are subjected to rough use and frequent momentary overloading, and though the first cost is higher, the maintenance cost is considerably less.
A few years ago operators in the Lake Superior iron-ore districts were replacing all their smaller scraper hoists with hoists of 15 or more horsepower. More recently some companies are replacing 15-horsepower hoists with hoists rated at 25 horsepower. This is mentioned because scraping has been practiced to the virtual exclusion of hand shoveling and sublevel hand tramming in these districts for 15 years, and hence the experience there should indicate the most efficient practice.
Table 30 presents data on a few operations where scrapers are employed principally for transportation. With the one exception noted, all the hoists were electrically driven. The performance data include digging, as it is impossible, without making time studies, to separate the digging and transporting functions of the scrapers.
Except on short drags, slow rope speeds greatly reduce the amount of ore that can be handled. A slow speed is desirable for digging, and this may be obtained in part by slipping the clutch during the digging part of the cycle. Better practice for transfer-level work or other long drags is to employ a two-speed hoist—low speed for digging and high speed for dragging. This can be done without any difficulty in selecting a motor of the proper horsepower, as the maximum pull occurs during digging, whereas higher speed is required during dragging when the load is lighter.
The data in table 30 were obtained in 1932, and since that time considerably higher speeds have been adopted for use, 350 feet per minute being common; speeds of 450 feet for pulling the load and 650 feet for returning the empty scraper have become standard in some mines.
A smooth bottom is required when dragging is done at the higher speeds, and it often becomes necessary to lay steel rails or plank flooring for the scraper to slide on.
Scraper hoists for underground use are powered by electric or compressed-air motors. From the standpoint of power consumption, the electric motor is to be preferred. With air hoists, theoretically about 4 horsepower is required on the air compressor to deliver 1 horsepower of useful work to the hoist. In actual practice the air-operated hoist requires six or eight times as much primary power as the electrically driven hoist. Actual meter tests of electric scraper hoists in scraping several thousand tons of ore have shown power consumption as low as 0.065 kw.-hr. per ton scraped with short drags and under favorable conditions. With electric drive the power requirement is usually less than 0.5 kw.-hr. per ton, except for very long drags or exceptionally difficult conditions. Matson has presented figures showing actual comparative costs of scraping with air hoists and with electric hoists. In the example given, 604,005 tons of ore were scraped with air hoists at a power cost of $0.0346 per ton and 91,124 with electric hoists at a power cost of $0.0042 per ton, a ratio of about 8 to 1.
Air hoists have certain advantages, however. Compressed air usually is available in all active sections of the mine, the exhaust
has some cooling and ventilating effect, the speed of air motors may be controlled by the throttle, and if overloaded they will stall without damaging the motor.
Electric-hoist motors may be either D. C. or A. C. If direct current is employed, power usually is taken from the trolley wires. Where numerous scrapers are in use and alternating current is available at 440 or 550 volts, it usually pays to run special transmission lines and employ A. C. motors, although D. C. motors have certain advantages for scraping. Thus, speed control is obtained more easily with D. C. motors; slow speed with high torque during digging and higher speeds for dragging can be provided for in the motor windings.
In straight transfer drifts or on other straight drags it is usually necessary to employ only one man for operating the scraper provided illumination is adequate. Sometimes a helper may be necessary at the loading point if the ore is very blocky or along the haulageway if the drift is crooked.
Next to labor, general repairs and rope renewal usually are the largest items in the cost of scraper operations, whereas cost of power is low, especially if hoists are electrically driven. Ropes receive rough treatment and have a short life because of overwinding on small drums and abrasion from rubbing on the back of the scraper and against rock. Scraping costs at one mine were distributed as follows:
In this example the ore was in large chunks, the scrapers weighed 1,760 pounds and had a capacity of 1 ton of ore, and the average scraping distance was 175 feet, the maximum being 400 feet. Five and one-third tons of ore was handled per foot of ½-inch wire rope.
At another mine the cost of loading in drift headings with scraper and slide and 25-horsepower electric hoists was divided as follows:
In top-slicing operations conducted by another company 455,000 tons of soft iron ore was scraped, 6 by 19 3/8-inch rope with independent wire center being used. With hoist drums of less than 9-inch diameter, rope consumption was 1 foot per 22.4 tons of ore. Using drums larger than 9 inches, the tonnage per foot of rope was 27.2.
Under average conditions 1 foot of wire rope usually will be worn out per 5 to 15 tons of ore scraped, if sizes of rope, scraper, hoist drums, sheave wheels, and hoist pull are suitably proportioned.
At El Potosi the cost of scraping 107,420 tons of ore in 1932, exclusive of operating labor, was as follows:
Hoist repairs probably would have been less had larger hoists been used. Those actually employed were of 6.5-, 7.5-, 10-, and 15-horse-power capacity.
At Nkana scraping costs in open stopes for a 6-month period in 1935 were as follows:
In recent years there has been a notable increase in the use of scrapers for transferring broken ore from a series of stope raises into an ore pass leading to the haulage level. This practice often greatly reduces the number of long raises that otherwise would have to be driven in the wall rock and eliminates loading and tramming on sub-levels. The stope raises are driven from one or both sides of the transfer drifts rather than directly above, so that as the ore piles up it will not block the drift in assuming its natural angle of repose but will leave room over the tops of the sloping piles for passage of the scraper. The raises may be timbered or untimbered but are left open at the bottom. If the ore is sticky and tends to pack, this system is particularly advantageous, since the ore can run freely through the raise and out into the transfer drift, thus eliminating troubles caused by “hang-ups,” which are common in chutes equipped with gates.
As indicated by the figures in table 30, large scrapers often can handle 50 to 75 tons per hour or more in this kind of work, only one or two men being required for the operation; 100 to 200 tons per hour have been scraped through transfer drifts under favorable conditions. Scraping distances are usually under 150 to 200 feet but doubtless with suitably designed equipment considerably greater distances are economical.
At present (October 1938) very few data on underground conveyor systems in metal mines are available.
At the La Rue mine on the Mesabi range, Minnesota, belts were installed some time ago on an old haulage level 15 feet below the bottom of the ore. Scrapers deliver the ore from the top-slice stopes to chutes, from which it drops through gates onto the 30-inch belt. The conveyor system is in seven sections and delivers the ore from the 125-foot level to the washing plant on the surface. Although no data are available, it was stated that costs were considerably below those for tramming and hoisting.
At the Spruce mine on the Mesabi range, a conveyor system was installed during the summer of 1937 to handle open-pit ore. Part of the ore is mined by electric shovels that load into trucks, which deliver it to a raise driven from an underground level to the bottom of the pit. The rest is mined by two tower excavators using 3-yard drag buckets, which will operate within a maximum clean-up radius of 700 feet, the average digging radius being 170 feet. Each tower surmounts a raise to which the scraper delivers the ore.
At the top of each of the three raises the ore passes through a crusher before it enters the raise proper and thence falls to a pan-feeder, which feeds the belt. The belt system, fed from the three raises, is 4,991 feet long and in nine sections, each driven by a 75-horsepower, 440- volt, A. C. motor. The belt is 7-ply and 30 inches wide. Belt speed is 500 feet per minute, at which rate the system has a rated capacity of over 500 tons per hour. (It is reported that actual capacity exceeds this considerably.) The maximum adverse belt slope is 30 percent, or 17°. By means of this sytem ore is delivered directly to shipping bins on surface about 1,000 feet from the top of the incline that connects with the mine level. Although this system is part of an open-pit operation, about 70 percent of the belt is underground, and it is mentioned as an example of the suitability and capacity of conveyor belts and to call attention to their probable future wide application to underground transportation, where large tonnages of ore must be handled.
Another underground conveyor belt that was also used in connection with open-pit mining operations was installed at the New Idria mine. This is probably one of the earliest installations in an underground metal mine. It was employed principally to convey waste from the hanging wall of the ore deposit to a waste dump. The waste is blasted or caved into a raise leading to an underground grizzly chamber 27 feet above the 300-foot adit level. From the grizzly it drops onto an apron feeder 42 inches wide by 11 feet long, which feeds it onto the conveyor belt at the rate of about 350 tons per hour, although 1,000 tons per hour can be handled without difficulty. To prevent large boulders from cutting the belt, a steel plate with round holes 1 inch in diameter is placed at the end of the feeder at an angle of 38°. The fine material falls through the plate onto the belt in advance of the large chunks and forms a cushion for the latter to fall upon. The conveyor belt is 42 inches wide, 8-ply, rubber-surfaced 3/16-inch on the top side and 1/32-inch on the bottom. It is 1,250 feet long, center to center of head and tail pulleys; 670 feet is in the adit and the rest extends over the dump on surface. Speed of the belt is 310 feet per minute when waste is being handled. It is driven through a tandem drive, situated midway between the ends, by a 40-horsepower, 850- r. p. m., 440-volt, A. C. motor with spur-gear reduction unit; variable-speed drive is provided so that the belt may be run at 310 feet per minute when waste is handled and 45 feet per minute when ore is handled. At the slow speed, hand-sorting is practiced, the ore being low in grade. At the end of 1930 the belt had been in service 3 years and appeared good for several years more; maintenance expense had been negligible.