October 28, 1922
Engineering and Mining Journal-Press
Gold-Mining Developments in the Black Hills
HOMESTAKE SOUTH MILL
NOTE: MOST PICS WILL BE AT END OF ARTICLE.
A Brief Description of Some of the Activities of the Homestake and Trojan Mining
Companies, the Only Properties Which Maintained Operations During the Recent Depression—The South Mill, at Lead, a Modern Reduction Plant
BY FELIX EDGAR WORMSER
Assistant Editor, Engineering and Mining Journal-Press
THE BLACK HILLS of South Dakota are neither black, nor are they hills, according to the commonly accepted meaning of that term. Their particular tint is chiefly a whim of the weather, and they are amply large enough to be designated full-grown mountains. Geologically speaking, they comprise a domical uplift rising above the Great Plains. Viewed from the plateau level, their height is unimpressive, and they seem to be a low, dark range, with a few summits towering here and there, but slightly above their neighbors. Investigation has shown that the central peaks are pre-Cambrian schists, and that they slope gently out to a rim of paleozoic sediments, which dip away from the hills. Intrusions of igneous rocks are common.
Set almost squarely in the center of the northern Black Hills are the important towns of Lead and Deadwood, both of which have been intimately associated with the mining history of the district. They are about four miles apart, although the actual distance depends upon whether the Burlington or the Chicago & Northwestern R. R. is chosen to do one’s transporting, as both communities are connected and excellently served by these railroads. The Northwestern takes a longer, but scenically, a more beautiful route, which gives the visitor superb views of the attractively wooded mountains. Lead has an elevation of about one mile above sea level; Deadwood is 1,000 ft. lower.
Prior to the war many small gold mines were active in the Black Hills. The Golden Reward, the Mogul, the Wasp, and numerous others, made names for themselves. The deposits were generally in the shallow Cambrian formation, which caps the Algonkian schists of the district. Many became profitable through working low-grade gold ore, and despite high freights, high labor, and high altitudes. In 1917, ten or twelve mines produced regularly. The decline in the purchasing power of gold affected them quickly, as it did every gold mine in the United States, and threatened their existence. Economies were instituted in the face of advancing labor and other costs, and one or two mines hung on tenaciously as long as they could, but finally succumbed to the great disadvantages under which they were operating.
A visit through the Black Hills will disclose many concentrators and mine plants in a sad state of decay, a grim reminder of the toll taken from South Dakota’s gold-mining industry during the last six years. As it will take a relatively heavy investment to reopen most of the properties, many have probably shut down for good. Others may resume again, although an exhaustion of ore deposits was imminent for a few.
GOLD MINES STRUGGLED TO KEEP OPEN
Throughout the entire depression, two mines succeeded in operating, where others failed—the Homestake Mining Co. and the Trojan Mining Co.—the former a gigantic, and aptly termed the greatest, gold mine in the world, and the other a much smaller venture. The worst is probably over, these companies can now view with great satisfaction the sterling fight they have made against economic odds.
Peculiarly enough, the two mines, although only five miles apart in a straight line, are different in a great many respects.
The Homestake is a deep low-grade gold mine, worked through vertical shafts piercing the Algonkian ore deposits of the company. It is a large-scale operation, and prior to the erection of its South mill normally treated about 4,500 tons daily. Homestake ore is of an average value of $3.50 to $4 per ton.
The Trojan is a shallow mine in the Cambrian, with a capacity of 400 tons per day: During the period of rising costs, it was necessary for both these companies to reduce expenses by decreasing development and prospecting, and increasing, so far as possible, the life of supplies.
The Homestake Mining Co. ordinarily has a program of 17,000 ft. of development work each year. During 1918 and 1919, this was reduced to 8,000 ft, but it is sufficiently advanced to obviate a heavy expenditure to bring it to normal proportions.
The Trojan did the best it could to raise the grade of ore mined, and endeavored to remove a minimum of $5 ore by careful selective mining. The struggles during the unpleasant last six years will add an interesting chapter to the rich history of the Black Hills. The population of Lead and Deadwood declined sharply. Whereas in 1900, these towns had a population of 12,045, in 1920 it was estimated at 7,416. Gold production of South Dakota also declined. Its peak was reached in 1912, when it amounted in value to $7,891,000, and the low point in the depression was in 1920, when it totaled $4,676,500, according to the U. S. Geological Survey statistics.
The Homestake is one of the oldest mines in the United States still operating. Its long life and enviable dividend record have combined to make it the greatest gold mine in the world. The total production from 1875 to the middle of 1922 is estimated to be $176,330,000, from which dividends of $42,693,500 have been paid. One of the striking things that impress the visitor to Lead, is the number of employees that have worked for the company ten, fifteen, twenty, or more years. I am informed that a Veteran’s League exists in Lead, whose membership is composed of men who have worked for the company at least twenty-one years, and the membership roster contains more than 150 names. When men stay in the employ of one company for long periods, an admirable community of interest must exist between employer and employee.
PIC Draw-chute leading to underground crusher at Homestake
There is evidence on all sides in Lead, that life in that gold-mining camp has been made as pleasant as it is possible to make it. The homes of the miners and employees look well kept, with attractive gardens or hillside settings. Lead itself is a clean city; in fact, the entire community does not typify the ordinary conception of a mining camp. The company has provided an excellent library, billiard rooms, a splendid swimming pool, and other recreational facilities that are available without discrimination to inhabitant and stranger alike.1 A moving-picture hall, large enough to more than fill the requirements of any city of equal size, supplies that popular entertainment to those who wish it. The Homestake Mining Co. may be proud of its welfare work, which is well worth study and emulation. At the present time, a new hospital is being provided, which will furnish additional medical conveniences.
MANY RAMIFICATIONS TO ENTERPRISE
It is noticeable that the Homestake Company has built a unit that is largely self-contained. It has its own timber tracts and yards, lime quarries, coal mines, hydro-electric power sources, and shops. In the foundries and machine shops, everything is built that can possibly be manufactured locally. Thus, it is not surprising in visiting the plant, to learn that here is a switch of distinctive Homestake design and manufacture, and there a sharpener made in the Homestake shops. Although discouraging at times to the salesman of mining machinery, this ability to fashion quickly repairs and machinery that may suddenly be demanded from some quarter, has not been without its advantages to the company, and many a “home-made” article has proved equal to the purchased kind on every comparable basis.
The Homestake works a deposit that is unique so far as gold mines in the Black Hills are concerned. It is commonly designated as a silicified slate, carrying free-milling gold in so fine a state as to be rarely visible, and profuse in the variety of minerals associated with the gold. Chief among these are pyrite and arsenopyrite, but they are not important gold-bearing minerals in the ore. A variety of hornblende, called cummingtonite, is a common mineral at depth, and its radiolarian or fibrous structure, interferes with crushing, but aids in leaching. Ferrous minerals are found in the lower levels of the mine and affect the metallurgical treatment.
Irving believes that the Algonkian slates, comprising the deposit, are of sedimentary origin. The lode is of varying dimensions at its widest point, being perhaps 1,000 ft. wide, although the mine-able width averages far less than this. It is about three miles long, and pitches southeast. Eruptives of acidic type cut the slates, and the entire mass was compressed into sharp anticlines and synclinal folds. The ore has been found to be associated with the folding. Fracturing and faulting took place owing to further compression, and porphyry dikes were subsequently intruded. The origin of the deposition is still somewhat obscure. It is only recently that the Homestake has systematically begun to learn the geological secrets of its great deposits, through careful study of the mine. Two company geologists are busy upon this task, and a description of their method has already appeared in these pages.’2
The mine itself is a huge operation in which both gloryhole, and underground shrinkage stopes are used. The glory hole, with its large fresh exposures, is a geologist’s delight. It gives a cheaply mined source of ore. The men working in the glory hole, which is now a half-mile wide, have built small precariously located trails in it for themselves, and like true mountaineers, use life-lines when necessary to drill and work in dangerous positions. The ore from the glory hole is drawn through mill holes in the mine and dropped on large underground grizzlies to be broken and trammed to the shaft.
Shrinkage stoping at Lead in the Homestake mine has been described in various papers.3 The walls stand remarkably well, and only a small amount of timber is required for support in the mine. The actual mining has undergone a gradual process of evolution. It is noticeable that the influence of several engineers on the staff, who have had experience in low-grade Alaskan gold mining, is being felt in underground operations. A recent modification of Homestake mining has been to carry some of the stope floors about 10 ft. higher than the haulage level, and to use chutes, thereby eliminating the use of shoveling collars, in transferring the ore from stopes to cars.
A development of great interest at the Homestake is the remodeling of the Ellison shaft, and the construction of the South mill, to replace several of the older units, which will be razed. The Homestake mine is worked through two shafts 1,730 ft. apart, the “Old Abe,” or B. & M. shaft, and the Ellison. The “Old Abe is situated in a part of the property that is subject to subsidence from the glory-hole operations, and it not so handy to the new South mill as the Ellison shaft. Furthermore, as the Ellison is in a section of the deposit closer to the place of future mining, it is natural that efforts should be made to enlarge its capacity. It formerly was without loading pockets and means for rapidly hoisting large tonnages of ore. Shaft dimensions had to be altered, skips substituted for cages, a new electric hoist installed, and numerous other changes made that practically completely remodeled shaft and equipment. Additional air compressors are being added.
AMPLE UNDERGROUND STORAGE FACILITIES
Three underground crusher stations and loading pockets were constructed just below the 800, 1,400, and 2,000 levels. A diagram, which has been taken from a sketch by K. D. Pyle, shows the arrangement of crushers, pockets, and skip-loading device. The Traylor jaw crusher used, is shown in an accompanying photograph, has an opening of 48 x 36 in., and is set to crush to 4 ½ in. It is cast in halves, and held together by two heavy steel bands. Special thought had to be given to the size of the various parts of the crusher so that they could be sectionalized and lowered through the Ellison shaft. The crusher is driven by a 125-hp., 2,200-volt, 720-r.p.m. motor, with a short belt (Lenix) drive, and has a capacity estimated at 200 tons per hour. In the figure, the ore coming from levels above 800, is trammed to chutes, and faIls upon a grizzly made of 6-inch steel shafting set 6 inches apart. The oversize passes into an apron feeder 5 ft. x 5 ft. 3 in., and undersize drops into the main ore pocket, which has a capacity of 1,500 tons. From the ore pocket, the ore passes to measuring pockets, which fill the seven-ton skips in use. A photograph of the measuring pocket is reproduced elsewhere.
The Ellison shaft, prior to its remodeling, was equipped for cages and the use of flat ropes. The new arrangement retains one cage with a counter-weight, both using 7 ½ x 5/8-inch flat rope as before, and handled by the old hoist. In addition, two skips using 1 1/2-inch round rope, are operated by the new electric hoist. To make a place for these skips, it was ‘necessary to make a simple alteration in the layout of the shaft. The Ellison originally had three compartments, two of them 5 x 10 ft. with cages, and the third, 6 x 10 ft., containing pipe and manway. By moving the divider between cage compartment and manway 6 inches toward the center of the shaft, three compartments were made. The unaltered 5-ft. compartment holds a cage as before, while the other two compartments contain the skips. As a smaller horizontal area is required for the skips, than for the cages, excess space is provided in the skip compartments, which is utilized partly by a cage counterweight in the skip compartment, adjacent to the cage, and partly by a pipe compartment.
The steel headframe formerly used at the Ellison shaft has been replaced by a newer, stronger, and larger one, with four sheaves, two of whose centers are 95 ft. 6 in. above the collar of the shaft and two (counterweight and cage) 65 ft. above. An idea of the size of the main sheaves may be gained from an accompanying illustration.
The rims are made of manganese steel, and, although manufactured elsewhere, were assembled with spokes, and trued at Lead. Owing to the splendid facilities of the Homestake company, for working iron and steel, this work was readily and satisfactorily handled in the local plants. The new seven-ton skips were designed and made in the Homestake shops, and weigh 10,750 lb. each. They travel with a speed of 2,000 ft. per minute.
A MODERN HOISTING INSTALLATION
The hoist, by Nordberg, is driven electrically, with Wagner-Ward-Leonard control, and has mechanical refinements that are as perfect as it is possible to make them. It has a rope pull of 36,300 lb. The two drums are 10 ft. in diameter, and have faces 7 ft. 2 inches in width, parallel-motion post-brakes, and complete safety devices. The installation is exceptionally compact. The hoist is direct connected to a 1,400-hp. 600-v. d-c. General Electric motor, having a full load speed of 63 ½ r.p.m. A flywheel motor-generator set supplies power for this motor, the alternating-current motor end, being rated at 1,208 hp.
The output of the d-c. generator end varies from 2,800 hp. during acceleration to 1,400 hp. during retardation. The flywheel weighs thirty-six tons, and in the event of an interruption to the supply of current, is sufficiently large to supply power for at least two hours -long enough to hoist all miners. The set runs at 600 r.p.m. The d-c. generator is capable of supplying 2,800 hp. during acceleration. It is estimated that the hoist will have a capacity of 2,000 tons in seven and one-half hours, from a depth of 2,200 ft. It has been designed to go to 3,230 ft. as its ultimate depth.
The ore hoisted through the Ellison shaft is dumped into a relatively small ore bin in the headframe, from which it passes over a grizzly made of hard cast-iron bars, 6 in. deep, I ½ in. wide at the top, and 1 in. at the bottom, set to 3 inches, in cast-iron supports. The grizzly bars are made at the Homestake shops. The undersize goes to a 36-in, belt conveyor, part of Stephens-Adamson equipment, and is carried over a series of Dings magnetic pulleys to a trommel with 3-inch openings.
The oversize from the grizzly is fed to four No. 6 gyratories set to 3 inches, and the product passing through them joins the 36-in, conveyor belt stream. The trommel feeds its undersize to a 36-inch shuttle conveyor 49 ft. long. The oversize is passed through two No. 6 gyratory crushers set to 2 in. before falling on the shuttle conveyor used to distribute the ore to a large ore bin constructed in the country rock, 100 x 20 ft. and 40 ft. deep. This bin supplies the South mill and furnishes ample storage capacity.
NEW REDUCTION PLANT IMPOSING
Ground was broken for the new plant in March, 1921, and one unit of the mill was placed in operation in September of this year. The main dimensions of the building are 150 x 160 ft., and it has a slime settling and classification annex, 100 x 40 and 70 x 70 ft. respectively.
The South mill is a steel structure with corrugated iron sides that occupies a conspicuous hillside position facing north, and to the east of the main mine buildings. The capacity of the new mill is estimated at 1,800 tons daily, and additional units may be added as required. An interesting feature of its construction is the elimination of any slope to the roof of the mill. The Black Hills are subject to heavy snowfall, and the flat roof will doubtless be—in fact has already been—called upon to withstand a heavy load of snow. The designers of the mill believe that by constructing its roof strong enough to withstand severe snow loads, the greater illumination gained by a flat roof, will more than offset the additional cost involved. Steel work was provided by the Moravia Steel Co., whose engineers designed the building in cooperation with Homestake engineers and officials.
The South mill is built to replace the old “Homestake” 220-stamp mill, whose operation has been seriously affected by subsidence caused by open-pit and other minIng in its immediate vicinity. Although the Homestake mill is still operating, it is only because each battery of ten stamps has its individual motor drive. The entire building has settled noticeably.
Homestake metallurgical practice has been thoroughly described in several exhaustive and excellent papers4 and has been prominently associated with world progress in stamp milling, amalgamating, and cyanidation of gold ores. The milling plant is not confined in one compact unit, but is distributed in several buildings, which are sometimes located thousands of feet apart.
The Amicus, Golden Star, Homestake, Pocahontas, Mineral Point, and Monroe are individual older stamp mills which have in the aggregate 1,000 stamps. Clarifying houses and sand plants also are built separately near the stamp mills. Inside amalgamation is practiced, quicksilver being added to the mortar, the crushed ore is passed over plates, classified by means of cones into sands and slimes, and the sands are leached and the slimes treated in a large filter plant in Deadwood. The South mill will partly replace the older units, and will not tax the capacity of the present sand and slime plants, which are large enough to handle its product.
The capacity of the slime plant has been increased by the use of “booster pumps” and auxiliary tanks. The filter presses in the plant are run at 33 lb. pressure, compared with 16 lb. formerly used, as it has been found that tons of solution, rather than time of contact is the more important factor. The only additional cost involved in passing the slimes from the South mill, through the plant, is that of chemicals. It would not be surprising to find that the operation of the South mill will set a new record for low-cost gold milling.
The flow sheet of the South mill was designed by Allan J. Clark, chief metallurgist, who has been identified with Homestake metallurgy for more than twenty-five years. It represents the careful study of metallurgical practice as applied to gold ores for a long period, and the selection of machinery, and a flow of pulp that would give the most economical recovery of gold from Homestake ore. Older local practice is chiefly followed in that a large area of amalgamating plates and classifying cones, and the use of stamps for coarse crushing, have been retained. Stamps have been built heavier; whereas the stamp duty of the smaller size was 4.2 tons per day, the new ones will more than triple this.
Four-ton cars, hauled by compressed-air locomotives, dump the ore into a steel bin of inverted V-shape design, having a capacity of 7,200 tons, from which the ore is fed by means of suspended Hendy Challenge feeders, to 120 stamps. The water feed for the stamps is supplied by an iron water pipe placed parallel with, and back of the mortar, to which five short pieces of smaller pipe have been welded. These are connected with cocks and short lengths of discarded rubber air hose leading to the mortar. This construction lessens vibration of the pipe. One-third of the mill has been equipped with short trammels, placed between feeders and stamps, and if their performance warrants, they will be used throughout. The stamps are arranged in two rows of sixty each, back to back, with ten of them in two mortars placed on each block. They are driven, however, in units of five by 25-hp. motors—a departure from the ordinary Homestake practice of having an individual drive for every ten stamps. The weight of the stamps has been increased from 900 lb. to 1,550 lb., and their rate of drop increased from 88 to 100 per minute, whereas the length of drop has been decreased from 10 to 8 inches. The camshaft is 6 inches in diameter, and the cams have a 32 3/4-inch horn. A blanton key, 4-in, stem, three-key tappet, and two mortars on one block are used. The concrete blocks’ batter is 1 1/2 inches per foot.
PIC Settling cones in classification section
The screens used with the stamps have ½ inch mesh, and the pulp passing through them joins the undersize from the trammels, and flows to six dewatering cones with 65-deg. slopes to their sides. The spigot discharge of these cones is reground in six 5-ft. x 10-ft. ½ inch Allis Chalmers rod mills, operating at 22 r.p.m., with 100-hp. Motors, and a rod charge of 29,000 lb. Each mill has a rated capacity of 200 tons per day and works in closed circuit with a Dorr duplex classifier, 4 ft. 6 in. x 21 ft. 4 inches, having a slope of 2 ¼ inch per foot, and making twenty-six strokes per minute.
The classifier overflow, an 80-mesh product, unites with that from the dewatering cones and passes over twenty-four 9 x 12-ft. amalgamating copper plates, which are silver plated with 2 oz. of silver to each square foot of plate. This gives an area of 21.6 sq.ft. of plate per stamp.
The tables are so arranged that each pulp stream flows over two plates. Four 7-ft. dewatering cones receive the pulp in the next stage in its treatment, the underflow of these being reground in one 5 x 14-ft. Allis Chalmers tube mill after passing through one Dorr classifier. The product of this mill is passed to an amalgamating plate and then to eight 7-ft. cones. The underflow of these cones is transferred to Cyanide Plant No. 1 for further separation into sand and slime. The overflow of the four tube-mill cones is passed to eight 10-ft. cones, whose spigot discharge passes to the eight 7-ft. cones. The overflow of the eight 10-ft. and eight 7-ft. cones passes to three 32-ft. Dorr double-tray thickeners, the overflow going back to the mill as wash water, and the underflow to the slime plant in Deadwood, for further treatment.
At this point I wish to make grateful acknowledgment of the many courtesies shown to me by the members of the Homestake company’s staff in affording opportunities for me to familiarize myself with the company’s activities.
The Trojan Mining Co.’s chief workings are situated close to the right of way of the Spearfish branch of the Burlington railroad, at an elevation of about 6,000 ft., or 1,000 ft. higher than the elevation of Lead, S. D., and on the north side of Green Mountain. The mine is in the Cambrian formation, and its geology is similar to that of many of the mines of equal size in the Black Hills. A series of Cambrian strata lies unconformably upon the upturned metamorphic schists of the Algonkian. These strata have been fractured and fissured in zones of fairly uniform trend, with a strike of 20 deg. NE-SW. The local name for the fractures and fissures is “verticals,” owing to their vertical position. They have been mineralized, and the shoots, or valuable portion of the verticals, vary in height from a few inches, to 25 ft., with an average of perhaps 6 ½ ft. The verticals pass through shales, dolomite, conglomerates, and igneous rocks, all of which affect the richness of the deposit. The limestone seems to be particularly favorable to the deposition, and where the vertical passes through a limestone stratum, mineralization generally extends further to either side of the vertical than when it passes through some other rock. The miners term the limestone “sand-rock” because of its usual sandy and oxidized nature, and consider it a valuable ore indication.
The verticals may be extremely narrow in width— a fraction of an inch—or they may widen out to several inches. They often occur bunched together, and where they intersect, the ore is generally richest. The roof is most often a shale which has proved impervious to further deposition in the vertical, and the floor a conglomerate or quartzite.
The ore is discolored, and seldom shows free gold. It contains pyrite, which is gold-bearing, and which gives trouble in cyaniding. The ore also contains gold in an extremely fine state of division, and is often bluish gray in color—characteristic of the local limestone—and it, also, gives trouble in cyaniding. Although it has been thought that the presence of tellurium is responsible for the difficulty in cyaniding, the explanation has also been made that the hard blue rock contains troublesome sulphides.
The general procedure in mining is to follow the verticals persistently. To do this, it is necessary to run a series of parallel drifts or rooms, with pillars of barren country rock between. A glance at a map of the Trojan mine shows a large number of scattered but parallel rooms that greatly resemble the appearance of a coal mine. During the war, a higher grade of ore was sought than would normally have been required. Five-dollar ore was the standard chosen and maintained, although the plant was designed to treat three-dollar ore. As a consequence, sampling was carefully and frequently done. The size of the rooms was reduced, and only ore close to the verticals and satisfying grade requirements was removed. The ground stands well. Round stulls are used when necessary, and the ore breaks easily.
The mine is generally flat, and mining is now confined to one level. However, in following the verticals, it is frequently necessary to vary the grade. This gives a certain irregularity to the level of the rooms. Horse haulage is used to haul the ore car, or cars, to a gathering station, where a string of four cars is hoisted by a small electric hoist up a 12 percent grade. At the top of this grade, the cars are dumped into a storage pocket, and chutes leading to the main haulage level and slope, 150 ft. below.
The main haulage slope is 1,500 ft. long on a 6 percent grade. Eight 1 1/2 -ton capacity cars are hauled up this slope to the surface, by a single-drum electric hoist. There the rope is detached from the forward end of the ore train, and fastened to its rear, so that the cars may now be lowered on the surface down a 15 percent grade, about 2,000 ft., to a surface gathering station. Gasoline locomotives tram the ore cars from the surface gathering station to the mill, another 2,000 ft. Gasoline locomotives have proved entirely satisfactory at the Trojan mine. It costs about 6c. per ton, to tram the ore with these locomotives.
The Trojan mill is a cyanide plant with a capacity of 400 tons, using leaching of sands and agitation, and counter-current decantation of slimes. The ore delivered by the gasoline locomotives falls into one of three ore bins, having a total capacity of 985 tons. The ore from the bins is passed over grizzlies, set to 1 1/2 in., the undersize dropping into a hopper, and the oversize passing through a No. 5 Gates gyratory crusher set to 1 ¾ in. The gyratory product falls into a hopper over a shaking feeder, supplying an 18-in, rubber-belt conveyor 110 ft. long, leading to a mill bin with a storage capacity of 365 tons.
Secondary crushing is done by a Monadnock 7-ft. Chilean mill, and a more recent installation of a 6 x 6-ft. Allis-Chalmers ball mill, with herringbone gears. The management echoes a sentiment I find expressed elsewhere, that the added cost of herringbone gears is well worthwhile in increased smoothness of operation, although dirt must be carefully excluded. Five-inch balls are used in this mill. Both grinding machines are served by Dorr duplex classifiers, the ball-mill classifier operating in closed, and the Chilean, in open circuit.
A Dewco 5 x 8-ft. ball mill, using 2 ½ in. balls, receives the oversize sand from both the Chilean and Allis-Chalmers mills, and operates in closed circuit with the Chilean mill classifier. The Allis-Chalmers mill grinds to a product running 3 to 4 percent plus 10 mesh; the Dewco mill to 8 percent on 60 mesh. Bucket elevators lift the pulp from the Chilean mill classifier, to one Wiegand and two Dorr duplex classifiers, where the separation of sand and slime is made. Ordinarily this runs 59 percent slime, and 41 percent sand. The sand is distributed in six sand vats, 28 x 8 ft., where it receives a leaching treatment lasting from seven to eight days. The slime, containing 46 to 50 percent moisture, passes to two 40 x 14-ft. Dorr thickeners part of whose overflow is returned to mill storage and part passes through the zinc boxes after clarification.
PIC Interior of Trojan mill showing ball-mill installation
The underflow from the thickeners is passed to five 17 x 22-ft. Dorr agitators operating in series, where the pulp is diluted to a 21/2 solution to 1, of solids. The pulp is then transferred successively to three Dorr thickeners operating on the counter-current principle. The overflow from the first thickener is clarified in a Butters filter and passed through zinc boxes, which are dressed twice a month to remove the precipitated gold.
Three Portland filters dewater the tailing from the secondary thickener, No. 3, which then passes to a large tailing pond. Extraction varies from 75 to 85 percent on five-dollar ore. This result might possibly be improved by the introduction of additional machinery, and a modification of the flow sheet, but economic conditions in the last five years have not permitted gold mines to indulge in much experimentation, and purchases of new equipment. In 1919, however, the Allis-Chalmers ball mill was installed to enlarge the capacity of the mill. The Trojan has made the best of a difficult situation, and has succeeded in maintaining operations, where other mines have shut down, is ample evidence that its course has been properly laid out. The Bureau of Mines has experimented with the blue rock of the Black Hills, as it is a common constituent of many other ores in the district, but no recommendations have been made.
Perhaps the single feature which distinguishes the Trojan flow sheet from others, is the use of the Wiegand classifier, invented by William Wiegand, chemist of the company. It consists essentially of a bowl, with a shallow sloping interior, in which four arms with rakes, travel at the rate of 1 1/4 r.p.m. Pulp is fed to the classifier by means of an annular ring, with perforated holes, and is distributed evenly into the bowl. The slimes overflow into a column, with a large mouth placed in the center of the bowl. By altering the height of this column, the quality of the slimes discharged, can be regulated. The sand is raked with arms up the sides, and is discharged over the edge of the bowl, to fall into an annular launder. It is claimed for the machine, that it gives a better separation of sand and slime, than other rake classifiers. It has proved entirely satisfactory at the Trojan mill.
Both the Homestake and Trojan companies now use the dark flaky Aëro brand low-grade cyanide. Considerable difficulty was experienced at first in properly adding this chemical to solutions. It generates acetylene and other gases on being added to water, but the experiments of the Homestake company have not disclosed anything harmful in them. Aëro cyanide has been found highly satisfactory in cyaniding Black Hills ore. The low-grade chemical has been found to be just as efficient as the purer white material, but care must be taken in its addition to the circuit. It may be that the trouble, which users of this product have had in Ontario, is due to the method of adding the chemical.
Preliminary dissolving in barrels or vats is necessary. The flaky cyanide may be fed by hand, or, as in Homestake practice, by a small screw feeder into a suitable sprayed receptacle and screen, to any undissolved residue. An ingenious arrangement devised by Mr. Whitney, of the slime plant, uses a 10-inch paddle wheel, placed in a launder, and actuated by the flow of solution, as it passes from one tank to another, to drive the screw feed. The cyanide drops into the center of an annular spray of water. Rapidity of feeding can be regulated by step pulley. The absence of any strong odor with this arrangement is striking.
Cyanide consumption in the Trojan mill is 0.4 lb. per ton. A protective alkali content of 1.8 lb. is used. The bullion produced varies from 700 to 500 parts silver and 280 to 500. parts gold.
I wish to thank Mr. C. E. Dawson, manager, Mr. William Wiegand, chemist, and Mr. Goodrich, mill superintendent, of the Trojan Mining Co., for the many kindnesses extended to me on the occasion of my visit to their plant. My thanks are also especially due to Mr. A. A. Lease, of the Homestake engineering staff, for supplying the photographs in this article, illustrating Homestake operations, and for those in the plates of the half-tone section which follows.
1 A paper by B. C. Yates, Superintendent of the Homestake Mining Co., entitled “Welfare Work at the Homestake Mine,” describes the recreational facilities. It was published in Engineering and Mining Journal of July 31, 1920.
2 Geological Methods of the Honiestake Mining Co.,” by L. B. Wright and J. O. Hosted,
3 ”Mining at the Homestake Mine,” by B. C. Yates, Black Hill’s Mining Men’s Association, 1904; abstracted in Mining Engineer’s “Handbook”; Peele. “Stoping at the Homestake Mine,” by J. Tyssowski, Engineering and Mining Journal, July 9. 1920.
4 The Metallurgy of the Homestake Ore,” by Allan J. Clark, and W. J. Sharwood; Transactions of the Institution of Mining and Metallurgy, Vol. 22 (1912-1913). An exceptionally valuable contribution to metallurgical literature.
Notes on Homestake Metallurgy,’ By Allan J. Clark; Tranactions of the American Institute of Mining Engineers (1915), Vol. 52, p. 3.
Engineering and Mining Journal, Dec. 21, 1921.
BLACK HILLS HOMESTAKE STAMP MILL
LOADING POCKET ASSEMBLED ABOVE-GROUND FOR INSTALLATION UNDERGOUND- HOMESTAKE MINE
LOCOMOTIVE THAT USES COMPRESSED AIR FOR POWER
DOOR IN MINE TO CONTROL AIR FLOW/DRAFTS (PART OF VENTILATION AND SAFETY SYSTEM)
MANGANESE STEEL SHIV, INSTALLED AT TOP OF HEADFRAME, WHICH SUPPORTS ORE AND HOIST WEIGHT FROM DEEP IN MINE TO THE SURFACE. ALSO PROTECTS CABLE/WIRE ROPE FROM DAMAGE. OLD STYLE WIRE ROPE WAS ACTUALLY FLAT, SAY 7 INCHES, AND ABOUT 3/4" THICK
NEW 7 TON ORE SKIP FABRICATED AT HOMESTAKE MINE SHOPS
ANOTHER VIEW OF NEW 7 TON ORE SKIP FABRICATED AT HOMESTAKE MINE SHOPS
OLD STYLE MAN AND ORE SKIP. THIS ONE HAS PROTECTED SIDES LINED WITH SHEET METAL AND WIRE LATH, ALONG WITH A GABLE ROOF. MANY DID NOT HAVE THIS FEATURE, AND MINERS WERE PRONE TO LOSS OF LIMB IF HANDS OR ARMS WERE CAUGHT ON SHAFT SIDES DURING ASCENT OR DESCENT. IF ANY ROCKS FELL FROM ABOVE, MINERS ON SUCH SKIPS WERE STANDING DUCKS.
CONE MILLS WERE USED AFTER THE CYANIDE PROCESS BEGAN. wHEN ONE CAN CONTROL THE SIZE OF THE ORE, A CHEMIST CAN DETERMINE THE OPTIMUM CONTROLS TO LEACH OUT THE VALUES FROM ROCK ORE
HOMESTAKE MINES PLATED THEIR AMALGAMATION PLATES WITH 2 OZS. OF SILVER PER SQUARE FOOT, AS A MEANS TO INCREASE EFFICIENCY, BOTH IN ALLOWING HIGHER ADHESION OF MERCURY, AND BETTER RECOVERY OF FINE GOLD (Debatable perhaps). MERCURY WAS ATTACHED TO THE COPPER PLATES, AND FINE GOLD PASSING OVER WOULD STICK TO THE MERCURY
HUGE ELECTRIC MOTORS AND LARGE PULLEYS USED TO LIFT OR LOWER ORE AND MACHINERY INTO THE MINE VIA A VERTICAL SHAFT
COMPRESSED AIR CHARGING STATION FOR AIR POWERED MINE LOCOMOTIVES. FANS HELPED THE VENTILATION FACTORS UNDERGROUND
HOMESTAKE SOUTH MILL
A SLUSHER, SORT OF LIKE AN OPEN END WHEELBARROW TROUGH, THAT AATTACHED TO A CABLE, WAS USED TO MOVE BROKEN ORE AND ROCK UP OR DOWN AN INCLINED TUNNEL OR CHUTE. DRAMATICALLY LESSENED THE NEED TO SHOVEL ALL THIS DIRT INTO A MINE CART, AS THE SLUSHER WOULD ALLOW CHUTE FILLING OF CARS
SLUSHER- DIFFERENT STYLE
MINE SLUSHER- DIFFERENT STYLE
MINE SLUSHER- DIFFERENT STYLE
CONTRAPTION MADE WITH A CAMSHAFT TO CONTROL HEAVY RODS THAT HAD A MODERATELY HEAVY WEIGHT ON THE BOTTOM, USED TO CRUSH ORE. NOISY AND A LOT OF DUST. HARD ON THE LUNGS AND HEARING
ONCE ORE WAS CRUSHED TO A SMALL ENOUGH SIZE, IT WAS INITIALLY PASSED OVER THE AMALGAMATION PLATES WHICH ALLOWED FREE GOLD AND SILVER TO ADHERE TO THE MERCURY ON SUCH PLATES. ALL MATERIAL AT THE BOTTOM WOULD THEN GO TO A GRINDING MILL (BALL OR TUBE), THAT WOULD CRUSH THE ORE REALLY SMALL, AND THAT WOULD GO ON TO A CYANIDE OR FLOTATION TREATMENT TO GET OUT MORE GOLD AND SILVER VALUES.
TROMMEL. A REVOLVING DRUM FULL OF SPECIFIC SIZED HOLES. ORE OF CERTAIN SIZE WOULD FALL OUT, AND ORE TOO BIG TO FALL OUT WOULD BE SENT TO ANOTHER GRINDING OR BREAKING CIRCUIT.
UNDERGROUND CRUSHER. USED TO MAKE A UNIFORM PRODUCT THAT ELIMINATED WORK AND EFFORT OF SURFACE MILLS AND TRANSPORT. MORE EFFICIENT FOR THE MILLERS;NOT ANY BETTER FOR THE GUYS BELOW THAT HAD TO DEAL WITH THE DUST AND NOISE.
any undissolved residue. An ingenious arrangement devised by Mr. Whitney, of the slime plant, uses a 10-inch paddle wheel, placed in a launder, and actuated by the flow of solution, as it passes from one tank to another, to drive the screw feed. The cyanide drops into the center of an annular spray of water. Rapidity of feeding can be regulated by step pulley. The absence of any strong odor with this arrangement is striking.
