« ForrigeFortsett »
The retort-silver is refined in a small reverberatory furnace, built of adobes and fed with wood, which can receive a charge of 600 pounds of crude bullion. This charge is worked off in four hours. A little litharge and lead are added to remove the impurities, sulphur, arsenic, lead, iron, and, possibly, a very little zinc which is present as zinc-blende, and car bonate of soda and borax are also used as fluxes. The loss is 7 per cent. on the crude bullion, and consists, to some extent, of silver and quicksilver; the amount has not been exactly ascertained, but is, doubtless, very small, as the silver is ladled out very rapidly when refined. It is cast into bars weighing about 70 pounds each, which assay .988 of silver on the average.
The slags from the refining-furnace, with the concentrated tailings or slimes from the tanks for washing amalgam, and sometimes other secondary products, are occasionally smelted in a small shaft-furnace. with addition of galena, and the resulting lead is used in refining the retort-silver.
An experienced amalgamator, who was working up some of the ordi nary tailings from the every-day run of the arrastras, furnished the writer with the following facts relative to their treatment on the patio. The Mexicans sometimes make the operation pay, especially when the ores contain other ores than native silver, because they employ their peons, whose labor costs very little.
The tailings were made into a heap (torta) containing 100 cargas, (about 16 tons,) and to this were added 720 pounds of salt, which was thoroughly mixed in, with water enough to keep the whole at a proper consistency. The next day 50 pounds of magistral (in this case sulphate of copper) were added and thoroughly incorporated. The third day 100 pounds of quicksilver were added and the whole left standing one day. Then every other day the mass was thoroughly mixed by driving mules about in it, in the usual way, and this operation, called the repaso, repeated until, by assays in the horn spoon, it was shown that the heap had been properly amalgamated, or was rendida. This requires more or less time, according to the temperature, the size of the heap, the nature of the ore, &c., and the whole operation requires great experience and care. When it was found that the heap was ready for washing, 25 pounds of quicksilver were added, with plenty of water, and the whole thoroughly mixed. In this operation of the patio there is a chemical action; the loss of quicksilver is necessarily large, and in this particular case amounted to 25 per cent. of the original amount charged. The resulting silver from the retort showed .990 fine. The amount obtained from the heap of 16 tons of tailings was only $145, leaving a loss of $5 on the expenses, not counting the amalgamator's time, and showing conclusively that simple amalgamation of native silver-ores, in the arrastra, is as effective and cheap a treatment as it is possible to employ, in the absence of an im portant proportion of combined ores, as sulphuret, chloride, or arsenical ores, and in a country so difficult of access as the interior and mountainous portion of Mexico.
THE REDUCTION OF SILVER-ORES IN CHILI.
This chapter was prepared at my request by Mr. James Douglas, jr., of Quebec, a gentleman whose high professional standing, and personal acquaintance with the subject, entitle his statements to respect.
The silver mines of Chili extend from its northern far toward its southern limits, the two last-discovered minerals happening to be those of Caracoles, in the debatable ground between Chili and Bolivia, and San Carlos in south latitude 360. But the most prominent mines heretofore worked have been in the Department of Copiopó, and there are to be found the only extensive and scientifically managed establishments for the reduction of silver-ores. All through the central provinces, however, but especially in the neighborhood of Arqueros, the landscape is often enlivened by a pretty little mill, consisting of a single light stone trapeze, turned by a rude turbine attached to the vertical shaft beneath the mortar, and a single Freiberg barrel or open tub amalgamator. They work up two or three cwt. a day of tractable ores, but never touch the more refractory. In Copiopó, on the other hand, the most difficult ores are treated; the machinery is very perfect; and the extraction of the silver as thorough as in any mills in the world.
There are seventeen establishments on or near the Copiopó River which work up the ores from the three minerals of Chañareillo, Lomas Bayas, and Tres Puntas; but the three mills owned by Messrs. Escobar & Ossa, in which the patent process of Herr Krähnke is used, do more proportionately than any of the others.
In these establishments calcination is not employed. The different ores are mixed accurately in given proportions—the chlorides and native silvers of Chañareillo, with the polybasite of Lomas Bayas and the base metals of Tres Puntas. The refractory ores, however, largely preponderate, as the pure silver-ores are yearly becoming scarcer. The broken ore is sampled by falling from a hopper, accurately placed above the apex of a pyramidal cone, from the angles of which protrude partitions. The stuff which collects in each compartment is resampled separately in like manner, till by repeating the act a perfectly accurate sample is obtained. As the hopper and the pyramid are carefully protected from wind by being incased in canvas, no dust escapes, and thus the error, which formerly resulted from the difficulty of always taking up proper portions of dust and coarse ore in sampling with the shovel, has been remedied. This error was found to be so great (for the ores there are always absolutely dry) that more silver has, at the end of the ycar, been obtained from the furnace than was supposed to have been put into the mill.
The grinding is done altogether in the trapezes or Chilian mills; but these have been perfected in all their details. In Messrs. Escobar & Ossa's mill in the town Copiopó there are three double trapezes. Each wheel weighs 60 cwt. and is of metal. An automatic feed delivers the ore from a hopper, filled twice in the twenty-four hours. The three trapezes reduce to impalpable powder 12 tons in the twenty-four hours.
The pulp is received in slime-pits, whence it is carefully shoveled and allowed to dry by exposure to the air, every precaution being taken to prevent the lumps from breaking up. When perfectly dry, the cakes are ready for the barrels; of these there are five, arranged in a row, and
driven by the same line of shafting. They differ in size and in mode of gearing from the common Freiberg barrel. Each barrel is 8 feet long and 5 feet diameter, and revolves on spindles, which form the centers of heavy spiders covering the barrel-heads, and bolted to one another by strong iron bars. These form a cage, within which the barrel lies firmly secured. The barrel is made to revolve by a pinion playing into a toothed hoop, and it can be raised out of and lowered into gear with the greatest ease by means of an ingenious mechanism. The barrels are charged with 80 cwt. of ore, a 10 per cent. solution of dichloride of copper in brine, mercury, and metallic lead or zinc; but of course upon the accurate proportioning of the quantity and quality of the ore to the reducing reagents depends the whole success of the operation; and as the estab lishments of Messrs. Escobar & Ossa, which are under the immediate supervision of Mr. Krähnke alone, command the necessary skill, there alone this delicate process is worked with satisfactory results. In a notice appearing some months ago in Dingler's Journal, the active agent is said to be protochloride of copper. This is incorrect, as the equation afterward proposed in explanation of the reaction shows.*
The dichloride plays the same rôle in reducing the silver-sulphide directly to the metallic state as M. Laur, in his recent articles in the Annales des Mines, ascribes to it in the patio process:
2 Cu Cl+Na C1+Hg=Na Cl Cu2 Cl+Hg Cl. Na Cl Cu2 Cl+Ag S=Na Cl Cu Cl+Cu S+Ag.
Six hours suffice to effect the amalgamation; but one charge only is put into each barrel daily.
The separation of the amalgam from the sand is effected in a series of tanks provided with agitators.
There is a peculiarity in the subsequent treatment of the amalgam worthy, perhaps, of imitation. After being filtered in the usual way it is still further freed of mercury by being dried in a centrifugal machine, such as are employed in sugar-houses. The amalgam comes from the machine as fine sand, more uniformly deprived of free mercury than it can be in the filter. The plata piña is obtained as in Mexico, and then smelted in a small reverberatory to a fineness of .890 loss of silver by the draught being prevented by covering it with a very fusible slag.
The ores treated contain on an average 50 marcs to the cajon of 64 cwt. Ores with less than 20 mares to the cajon are smelted with copper and gold ores at the works of the same firm at Nantoko, whence a rich argentiferous and auriferous matte is shipped to England and Germany.
* I cannot agree with Mr. Douglas here. The reactions as given in Dingler's Journal and in the Berg und Hüttenmannische Zeitung (quoted September 26, 1871, in the Engineering and Mining Journal) are represented by the equation
3 Ag S+SbS3+3 Cu2 Cl+Na Cl=3 Ag S+Sb Cl3+3 Cu2 S+Na Cl.
If the argentic sulphide thus obtained is again treated in a hot solution with cuprie subchloride and sodium-chloride, and zinc is added, metallic silver is almost instantaneously formed. The reactions are
Ag S+Cu, C1+Na Cl+Zn-Ag+Cu2 S+Na Cl+Zn Cl.
The zinc probably acts as electro-positive metal, predisposing the atoms of argentic sulphide and cupric subchloride to a mutual exchange, so that the cupric subsulphide and argentic chloride are formed, which last is decomposed in a nascent state by the zinc, with the formation of zinc-chloride and silver. This may not be the correct theory; but the equations do not, in my opinion, bear evidence of its incorrectness.
R. W. R.
THE METALLURGICAL VALUE OF THE LIGNITES OF THE WEST.
This chapter was prepared by my deputy, Mr. A. Eilers, after thorough personal examination and inquiry.
No one who has visited our western mining districts and studied the economical relations of the beneficiation of their ores, can underrate the importance of the question of fuel.
By far the larger number of the districts which contain smelting-ores, i. e., argentiferous and auriferous lead or copper ores, are situated in the Great Basin, that great plateau between the Rocky Mountains on the east and the Sierra Nevada on the west, almost the whole of which is comprised at present in the boundaries of Nevada, Utah, and part of Arizona. This region is essentially a barren country. The extreme dryness of the atmosphere permits but a very scanty vegetation in the plains; and even in the detached mountain-chains running through itgenerally, from north to south, or from northwest to southeast-there are no trees found, except dwarf-pines and mahogany, at the head of sheltered ravines, and a few cottonwoods and willows, which fringe the insignificant streams, before the water sinks into the arid plains. Nearly all the mountain-chains in this region are rich in silver-ores. That class of these ores which is adapted to amalgamation, and rich in silver, has been worked with profit for more than ten years. But, before the advent of the transcontinental railways, mining was restricted to these ores alone, and the consumption of fuel could be met with the scanty supply of forest-trees in the immediate vicinity of the mining districts. Since, however, the Union and Central Pacific Railroads have brought the Great Basin nearer to the commercial centers of the East and the Pacific coast, thus reducing the expenses of freight and labor materially, other silver-deposits, containing poorer ores in greater abundance, have been rapidly taken up and worked. During the last year this industry has so expanded, that the State of Nevada alone has been able to show a production of over $22,000,000 in silver. But not alone are the poorer grades of amalgamating ores now worked profitably, aided, as the metallurgical process is, by such excellent inventions as that of the Stetefeldt and the Brückner roasting-furnaces, but the working of smelting ores has also been largely entered into. If I say "largely," I do not only mean to say that smelting-works are now scattered widely over the Great Basin, but that some of these conduct their operations on a really grand scale. In Eureka, Nevada, for instance, there are twelve furnaces in operation, which produced, during the last year, 5,665.5 tons of base bullion, worth $2,035,588, although only a small part of them ran regularly. Four of the Eureka furnaces have each a capacity of from 35 to 40 tons of ore per day. Three of these belong to the Eureka Consolidated Company, who have also two smaller furnaces. Nearly throughout the year this company have kept four furnaces running at a time, and one idle, and the daily consumption was 120 to 140 tons of ore and 4,000 bushels of charcoal. At this rate of smelting, the wood for ten miles around Eureka has been used up in a little over a year, which is not a strange statement, when we consider what I said
before, that there is very little wood in those regions, any way. Thus the question of fuel becomes, at once, a very important one, for the price of 33 cents per bushel of coal, which is now paid at the works, cannot rise much without threatening the very life of the industry.
In Utah, where over twenty furnaces were built, and had been partly in operation, in the fall of last year, some of the works have been compelled to pay as high as 30 cents per bushel for their charcoal, and very few are so favorably located as to get their coal for less than 18 cents per bushel. Many more smelting-works have been erected since the time spoken of, and the addition of every one of them must inevitably tend to raise the price of fuel. Even the most fortunate ones-those located high up in the mountains, where timber is comparatively plenty -cannot hope to escape in the next few years the danger of an enormous rise in the cost of wood and charcoal. And almost every smelting and amalgamating works in the Great Basin finds itself in precisely the same position. While the masses of poor ores are growing on their hands, fuel has a continual upward tendency.
Now there are two means by the combination of which this threatening danger can be averted. The first is the building up of a net-work of narrow-gauge railroads along the principal valleys, which will connect the mining districts with the Central Pacific Railroad; and the second is the employment of the vast stores of lignites occurring in the Rocky Mountain region, for metallurgical purposes. The utilization of this coal for the purpose named has not yet been attempted successfully, and I propose, therefore, to-night, to say a few words on this subject.
According to a late lecture of Professor Newberry, these lignites underlie not less than 50,000 square miles in the Great Basin and along both flanks of the Rocky Mountains. The principal beds now open and wrought I have had the good fortune to visit during the last summer. The mines are located at Carbon, Rock Springs, and Evanston, all three stations on the Union Pacific Railroad, and along the eastern slope of the Rocky Mountains, in Colorado. The coal in these localities, though from different beds, hardly varies in external appearance, but analysis has established a somewhat differing composition. It has a black
color, shining luster, a brown streak, and is very compact, the woodstructure, which is found intact in so many lignites, being almost totally obliterated.
The Carbon seam, one hundred and forty miles west of Cheyenne, is 8 to 10 feet thick, and had been extensively worked for over a year when the unfortunate fire broke out, in the latter part of 1870, which caused the whole mine to cave in. At the time of my visit, in the summer of 1871, work was progressing rapidly to re-open the mine, and regular operations have since been resumed. The coal in this bed is distinguished from that in the other beds by many small patches of resinous matter, very similar in appearance to amber. An analysis of this coal, furnished me by Mr. Wardell, the superintendent of the Wyoming Coal and Mining Company, gives-water, 6.80; ash, 8.00; volatile matter, 35.48; fixed carbon, 49.72.
The Rock Springs seam is opened in the midst of the Bitter Creek Desert. It is 10 to 12 feet thick, and a smaller seam lies close above it. This coal contains also some resinous matter, but not as much as the foregoing. The analysis shows-water, 7.00; ash, 1.73; volatile, 36.81; fixed carbon, 54.46.
The Evanston seam is by far the largest. It is from 22 to 26 feet thick, but the coal is not as good as that of the last locality. According to analysis it contains-water, 8.58; ash, 6.30; volatile matter, 35.22;