OPERATING PRACTICE

MINING

 DEVELOPMENT 

     An effective method of sampling the native copper deposits has not been developed. The choice of ground to be mined is made by the mining captains on the basis of visual inspection, aided by feeling, for the small rough particles of metal may be felt where they are not easily seen. The standards for judgment are empirical, subject to such checks as are afforded by the weekly or monthly mill returns on the ore from all stopes of a given shaft or of the entire mine. In general, any rock in which copper is detected by sight or touch goes as ore, and that in which none is so detected is not intentionally mined.

STOPING

     Open stope. - Where the dip is between 35° and 45°, as it is in the mines near Calumet, the open stope is commonly used. A raise is put up to the level above, after which stoping proceeds, leaving a floor pillar under the level above and a sufficient number of round pillars in the stope to support the hanging wall. The commonest practice has been to advance these stopes outward from the shaft pillar to the limit of the ground tributary to the shaft, and before abandoning the level to recover as many of the pillars as can be taken out with safety; later still the floor pillars are removed provided they are rich enough to justify this independent operation. The more recent practice, however, carries the drifts to the limit farthest from the shaft and stopes on the retreat. In this way only enough pillars are left to give temporary protection to the breast of the stope, and as the stope retreats, all possible stope pillars and the floor pillars also are mined out. In either case no timber is used except in. the construction of loading chutes.

     In mining the deeper parts of the Calumet & Hecla conglomerate lode stope pillars are not used. Instead, timber stulls are used in sufficient number to afford a zone of protection near the breast of the stope, but as this zone moves along with the progress of the stope the timbers are removed, and caving of the hanging wall follows.

     Room and pillar. - In the deeper levels of the Quincy mine, where the dip is less than 40° and the lode narrow, a modification of the open-stope method has been adopted to meet the condition of great rock pressure. The interval between levels is 200 feet; stopes 100 feet long are supported temporarily by short stulls and later are partly or completely filled with waste rock from development operations. Between the stopes pillars 200 feet wide are left extending through from level to level, as round stope pillars have been found inadequate. On the advance, therefore, something over 50 per cent of the lode is mined, the remainder being left for support to be recovered so far as may be possible on the retreat.

     Shrinkage stope. - At the Isle Royale mine the dip ranges from 50° to 56° and is thus too steep to permit work in an open stope without timber. A timber bulkhead with numerous loading chutes is erected to separate the drift from the stope. Extraction is accomplished by inclined cuts; usually poor ore left unbroken affords sufficient support. The ore is allowed to accumulate to a convenient height, and the surplus is drawn off through the chutes. Upon completion of the stope all ore is drawn off and the stope is abandoned, beginning at the limits farthest from the shaft.

     Horizontal cut and fill. - At the Baltic, Champion, and Trimountain mines the dip approximates 70° and the lode is unusually wide. Two dry walls with openings for loading chutes are constructed of waste rock and covered with timber, forming a haulage way along the center of the lode on the level. As waste from mining operations accumulates behind and above these haulage ways circular dry-walled mill holes are extended upward from the loading chutes and kept even with the top of the fill. Stoping is carried on by horizontal cuts worked from the top of the fill. As each round is blasted down the broken rock is sorted by handling and visual inspection, the ore is thrown into the mill holes, and the reject is left to constitute the filling. Where the lode is rich and the amount of reject therefore inadequate to maintain the fill at a proper height additional filling material is obtained by exploratory stoping along the footwall and by blowing in wet mill tailings shot down from the surface. Floor pillars below the fill of the overlying level are supported by large timber cribs and are then stoped by inclined cuts from under the pillar.

     The size of shaft pillars varies with the depth. In some of the old inclined shafts no shaft pillars were left in the upper levels of the early days. As the shafts were deepened pillars were left, say, 50 feet wide or sometimes less, on each side. At still greater depths the shaft pillars were made 100 feet wide on each side, and now, in certain shafts at the deepest levels, 200 feet is left for each pillar, or a total block of 400 feet for the shaft. Where the shafts are carried in the footwall of the lode shaft pillars are not necessary, and by some it is regarded as better not to leave pillars in the lode over the shaft.

TRANSPORTATION

     The ore is drawn from the stopes through chutes of varying design and construction into cars, which are commonly of several tons capacity. In the flatter stopes drag-line scrapers are used for moving the ore down the footwall. By the use of long platforms erected over the track scrapers are also used to some extent for loading cars where no chutes are provided, as in the deep levels of the Calumet & Hecla conglomerate, and for mucking in drifts. The so-called mucking machines and other forms of mechanical shovels have gained little foothold in this district.

     Power has largely displaced handwork for tramming. At most mines the cars are dumped directly into the skip, but at the Quincy and Seneca mines skip pockets are used. Hoisting is, of course, in balance, except in some of the old inclines. Each shaft is equipped with a rock house containing grizzlies and crusher, so that about 4 inches is the maximum size of material delivered to the railway cars for transportation to the stamp mills.

UNDERGROUND CONDITIONS

     The mines of the district are notably clean and comfortable. Natural ventilation commonly suffices, even in the deepest workings, to maintain the air clear, fresh, and cooler than the adjacent rocks. The rock temperature is low. A compilation of existing data on rock temperature by C. E. Van Orstrand, of the United States Geological Survey, gives the following gradients of increase with depth:

From 60 to 1,080 feet 1° F. to every 99.1 feet.
From 60 to 1,915 feet 1° F. to every 105.6 feet.
From 60 to 3,090 feet 1° F. to every 119.3 feet.
From 60 to 5,367 feet 1° F. to every 117.4 feet.

     The rock temperature on the eighty-first level of the Red Jacket shaft in the Kearsarge lode, at a depth of approximately 4,900 feet, as determined by Mr. Van Orstrand, is 86.2° F. It seems evident that, so far as temperature is concerned, mining may be extended much deeper without any such serious inconvenience as has been encountered, for example, in the deep St. John del Rey Irvine, in Brazil.

MILLING

     In all the mills the ore is first crushed by steam stamps of large capacity. Then follow successive concentrations and crushings, involving ball mills, rod mills, and jigs and tables. Flotation follows gravity concentration in the Calumet & Hecla and Quincy mills, and ammonia leaching is the final stage in the former. By the best practice a recovery of 95 per cent is obtained. The copper content of the mill concentrates or "mineral" ranges from 50 to75 per cent and averages about 60 per cent.

     A recent development of technical and economic interest is the reworking of the great tonnage of old conglomerate mill tailings accumulated in Torch Lake near the Calumet & Hecla and Tamarack mills. The mill records of tonnage and copper content of the tailings produced yearly for over half a century gave a basis for computing the copper contained in the entire mass. Both tonnage and metal content were checked by elaborate sampling, and the figures thus obtained were in striking agreement with those derived from the mill records. The winning of copper from these tailings will require many years at the present rate of production. The tailings are elevated by suction dredge and pumped to a plant on shore, where crushing concentration, flotation, and leaching art; employed, with a very satisfactory recovery. The cost per pound of producing copper from these tailings is the lowest in the district.

     As the copper is coarser and the rock softer in the amygdaloid than in the conglomerate lodes, the former have always yielded a larger proportion of their copper to initial treatment. Re-treatment of amygdaloid tailings is therefore not yet feasible.

SMELTING

     The copper in the product delivered to the smelters is already almost wholly in the metallic state and forms from 50 to nearly 100 per cent of the product, the proportion depending on whether it is mill concentrate or mass copper of the lower grades. The smelting process is therefore simple in principle, but, as most of the product is furnace-refined, and the standard of quality for Lake copper is high, much care is exercised in the smelting operations. Copper that carries a sufficient amount of silver to make the separation and recovery of this metal profitable is refined electrolytically. Electrolytic refining has been used also in order to purify some of the copper containing small amounts of arsenic, but in recent years nearly all of the arsenical copper is furnace-refined and sold for special uses. Lake copper has long been regarded by some consumers as excelling in certain qualities and has commanded a premium over electrolytic copper.

 NOTES