Since late in the last century, power shovels and draglines for digging and loading and locomotives and cars for hauling from the pit have been in general use at most large-scale open-pit operations. Starting with small steam shovels and with dump cars drawn by teams or “dinky” locomotives over narrow-gage tracks, the size and capacity of the equipment was increased, design and efficiency of equipment and methods and equipment for drilling and blasting were improved, and advances were made in other practices as operations were under-taken on a larger and larger scale and as the depths of stripping and ore pits became greater.
Typical of the evolution in equipment and methods are the developments on the Mesabi range, as described by Hunner. The following notes are abstracted from Hunner’s paper:
In 1903 one contractor at Chisholm, Minn., was using 60- and 70-ton railroad-type steam shovels with 2½- and 3-cubic-yard dippers for loading stripping into narrow-gage, 4-cubic-yard wooden dump cars hauled by saddleback, 8- to 18-ton steam locomotives; ore was loaded into railroad cars hauled by 45- to 70-ton, six-wheel, switching-type locomotives.
In 1914 a contract was let for removing 8,000,000 cubic yards of stripping at the rate of 2,000,000 cubic yards per year to uncover 6,000,000 tons of ore for open-pit mining. It was contemplated that later work would require the additional removal of 14,000,000 cubic yards of overburden to make available for mining an additional 19,000,000 tons of ore. In 1917 the contractors on this job were using the following equipment, typical of the best practice at that time:
9 six-wheel Baldwin steam switching locomotives, 20 by 26 inches.
8 Kilbourne and Jacobs 20-cubic-yard automatic air-dump cars.
49 Kilbourne and Jacobs 16-cubic-yard automatic air-dump cars.
1 eight-wheel, 25-ton locomotive crane.
1 model 100 Marion steam shovel, 5-cubic-yard dipper, weight 137 tons.
2 model 76 Marion steam shovels, 4-cubic-yard dipper, weight 107 tons.
1 model 36 Marion revolving shovel, 1½-cubic-yard dipper, weight 40 tons.
By 1919 one 300-ton revolving steam shovel with 6- and 8-yard dippers had been added to the equipment, which in one month of two-shift operation removed 250,000 cubic yards of sand and gravel overburden. Tracks were laid with 80- pound rails on 1.5 to 2.5 percent main-line grades.
Large revolving shovels were found to have decided advantages over 100- to 110-ton railroad-type shovels under certain conditions. In 1919 the M. A. Hanna Co. purchased two electric, full-revolving shovels (one Marion 300-ton and one Bucyrus 225-B) each equipped with 150-foot boom and complete dragline equipment. They used 8-cubic-yard buckets for stripping and 6-cubic-yard buckets for ore when equipped as shovels. As draglines they used 5-cubic-yard Page buckets. Power for shovel operation was supplied at 22,000 volts and stepped down to 2,200 volts.
In 1924 a Bucyrus 50-B revolving electric shovel mounted on caterpillar crawlers and equipped with a ¾-yard bucket was installed at the Wabigon mine and as Bucyrus 80-B revolving, electric, caterpillar-traction shovel at the Richmond mine, Palmer, Mich.
In 1926, two Bucyrus 120-B revolving, electric, caterpillar-traction shovels with 4-cubic-yard buckets were placed in operation at the Susquehanna mine, Hibbing, Minn. These three shovels were the first of their type and size using direct-current motors to be employed in the Lake Superior district. Since then shovels of this type have come into general use on the Mesabi range, most of them being equipped with 4- to 5-cubic-yard dippers.
In 1924 the first installation of standard-gage electric locomotives in open-pit ore mining in this country was made at the Wabigon mine, Buhl, Minn. The equipment comprised the following:
One Marion 300-E and one Bucyrus model 50-B, both equipped with direct-current motors operated by motor-generator sets; the model 300-E had full Ward-Leonard control, whereas the 50-B had partly rheostatic control.
Three 60-ton General Electric swivel-truck type designed to operate at 600 volts direct current with capacity to haul a train of 340 tons (including locomotive) on a tangent track against a 3-percent adverse grade at 7½ miles per hour. Each locomotive is equipped with an air-operated paragraph sliding-bow collector mounted on the cab for use on main tracks and with two air-operated side-arm collectors, one on each side, for use with side trolley wire on loading and dump tracks.
An overhead trolley-wire system was adopted on account of first cost, freedom from obstruction in the event of derailments and spill of stripping along the tracks, and greater flexibility and safety than a third-rail system.
In 1928-29 a similar installation was made at the Mesabi Chief mine, Keewatin, Minn., using one Bucyrus 225-B 90-foot-boom, 8-cubic-yard-dipper, full-revolving shovel, and two Bucyrus 120-B 32-foot-boom, 4-cubic-yard shovels, all full-revolving type with caterpillar traction and equipped with direct-current motors and Ward-Leonard control. General Electric 60-ton 8-wheel locomotives were used. The rolling stock comprises 16 air-dump side-pivot, drop-door cars of 30 cubic yards capacity. A Bucyrus heavy-type spreader is employed on the dumps for spreading the spoil and a Nordberg track shifter for throwing tracks and for general utility work.
Although the foregoing summarizes briefly the advances made up to about 1930 by only one company, the changes are typical and illustrate the evolution in modern excavation and haulage equipment for open-pit metal mines.
Electric pit haulage has not been adopted generally in the Lake Superior district chiefly because there is a large amount of serviceable and efficient steam equipment on hand, and the remaining life of the pits is too short to justify the capital outlay for electrification. The revolving caterpillar-traction electric shovel, however, has become standard because the savings in operating cost through its use are so great that it has paid to scrap steam-operated shovels. Among its advantages are its mobility without use of tracks, eliminating the pit crew required with the old railroad-type shovel (the shovel crew consists usually of only 2 men); elimination of delays to loading when coaling; ability to dig and load in any direction; superior efficiency, control, and power characteristics of electric shovels.
Track shifting and grading, spreading of spoil on the stripping dumps, and clean-up of stripping on top of the ore were formerly done by hand; this work is now done principally by locomotive cranes, bulldozers, and dirt spreaders.
Bank drilling, formerly done by hand with “jump drills,” long- handled shovels, or post-hole diggers in soft ore, or by piston drills, is now done principally with mobile, caterpillar-mounted, electric churn drills from the top of the bank along the berms or benches of the pit.
More recently, motorized pit haulage (gasoline- or Diesel-powered trucks or trucks and trailers) has been adopted for the smaller pits and for clean-up work in larger pits where adverse grades are too severe for locomotive haulage. In other districts large operations have been equipped at the outset for motorized haulage, large trucks of 30 to 35 tons capacity being used. Truck haulage has the advantage of flexibility, elimination of tracks and track laying, and ability to negotiate steep grades and to load in restricted space in corners and local dips in the pit. For long, main-line hauls in the pit and from the pit to beneficiation plant, it is probable that locomotive haulage is generally more economical and combinations of truck and rail haulage (dual haulage system) may prove most satisfactory under some conditions, using truck haulage to bins on the main line or to the crest of the pit, followed by rail haulage to the plant. Based upon a recent investigation of truck vs. rail haulage in bituminous-coal mines, Toenges and Jones conclude that with equipment in its present stage of development and within the range of conditions studied, for a new mine a dual haulage system would effect a saving in transportation cost if the round-trip haul exceeds 2.7 miles.
The first use of trucks for large-scale open-pit haulage in metal mines in the United States was, so far as the authors are aware, at the United Verde mine. According to Alenius, the original plan was to mine the material below the 160-foot level by glory-holing, and in 1924 several glory holes were started. They did not prove practical for the following reasons:
- Inability to drive raises in the desired locations on account of the underground fire and existing stopes.
- Dilution of ore and inability to sort material properly.
- Indefinite production.
- Operating hazards.
Early in 1925 a plan was devised by W. V. De Camp, then assistant general manager, by which the lower pit would be mined by small shovels working on low benches. The material was loaded into trucks, which dumped into several transfer raises extending from the 1,000- foot mine level. Figure 136 shows a plan of the pit as of July 1, 1929, and figure 137 is a typical section through the No. 3 raise
showing dumps on three levels. The major equipment employed was as follows:
4 electric, full-revolving, 1¾-cubic-yard shovels.
10 six-wheel, four-wheel rear drive, 4-cylinder, 10-ton trucks and one 6-cylinder, 10-ton truck.
6 tractor trucks with 2 front wheels and caterpillars on rear, 6 cylinder, 10-ton capacity.
2 four-wheel, 4-cylinder, 5- to 6-ton trucks.
1 four-wheel, 6-cylinder, 3½-ton truck.
1 four-wheel, 6-cylinder, 2-ton truck.
1 four-wheel, 6-cylinder, 1-ton truck.
1 gasoline-powered, full-revolving crane.
1 tractor-powered road scraper.
During 1928, 167 cubic yards or 390 tons of material was trammed per truck-shift, the average one-way haul being 500 feet. Additional data on this and other tractor operations are given later.
Recently several open-pit mines have installed conveyor-belt systems for transporting the ore from gathering points in the pit to bins on surface outside the pit limits. Excavation is by power shovel or by tower drag scrapers. When power shovels are used they load into trucks that haul the ore to raises leading to the conveyors, and when drag scrapers are used, the ore is dragged to the raises by hoists. Both systems are employed at the Spruce mine, Eveleth, Minn., where the installation is believed to be the largest of this kind in an American metal mine, although others were planned when it began operation in 1937. The system was described
in Skillings Mining Review, October 16, 1937, pp. 1-2, 10-11, et seq. Figure 138 is a longitudinal section of the system in which the lower part is the continuation to the left of the upper part. The general arrangement is shown in plan in figure 139, figure 140 is a section of a truck-dumping station, and figure 141 is a section of a tower- or drag-scraper station. The ore in each case is delivered to jaw crushers, whence it passes through transfer raises to pan feeders that deliver it to the conveyor belts. There are nine belts having a combined length of 4,481 feet, of which 3,170.5 feet are in underground drifts. All belts are 33 inches wide, and when traveling at 500 feet per minute the rated capacity is 500 tons per hour. General specifications of the equipment are given in figures 139, 140, and 141. Although operating costs are not available to the authors at this time (August 1938), careful estimates by engineers of the company prior to beginning
operations indicated a saving of 9 cents per ton as compared to locomotive haulage.
Hoisting of ore in skips running on inclined tracks on the side of the pit or in a shaft connected to the bottom of the pit has been practiced for many years. This practice is adapted particularly to small pits, where the relative volume of stripping or where track or truck-road grades would be prohibitive for haulage purposes, and to the final stage of power-shovel pit operation when mining out the benches and bottom ore.