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to 80 per cent. of the value of the ore. When the chemicals were exhausted the yield fell below 40 per cent., only to recover on the receipt of a fresh supply of sulphate of copper. The following table will show the difference in percentage extracted while working with and without chemicals:
July and August, 1870-Percentage extracted, 54.40, working threequarters of the time without chemicals.
September-64.39, working one-half the time without chemicals.
Average assay value of the ore for the year, $143.21.
The ores operated upon were very rich. In such cases the policy of working ores with chemicals in preference to roasting may be questioned. The matter must be decided by a careful comparison of the expense and results of the respective methods. It must furthermore be taken into consideration that roasting involves dry crushing, and consequently a decrease in the working capacity of a mill of a given number of stamps. This again necessitates a larger outlay for the erection of more mills in order to crush a sufficient number of tons per diem to render possible the payment of any considerable amount in dividends. The expense of building furnaces, &c., must also be taken into consideration. Should, however, the ores prove of so rebellious a nature that the difference in percentage extracted by the roasting process be sufficient to overrule these considerations, then it is evident that the wet process must give way to dry-crushing and roasting.
At the Nevada Butte Mill, where this method of working with chemicals was introduced during the past summer, it was absolutely necessary to find some means of reducing the ore without roasting, as it was not sufficiently rich to admit of the latter method of treatment. Here also it was found that whereas only 35 per cent. of the value could be extracted by quicksilver alone, the use of sulphate of copper and salt increased the value to 75 per cent. and upwards. These ores contained still more lead than those of the Meadow Valley mine, and finally became so base as practically to belong to the class of smelting ores, causing an enormous loss of quicksilver, and rendering their beneficiation by the amalgamation process an impossibility. The enterprise had therefore to be abandoned.
The only chemicals thus far known to be applicable to the amalgamation process are salts of copper. A great benefit would be conferred upon the mining public by the discovery of other chemicals equally ef fective, but less expensive, and not so subject to decomposition by the iron of the pan or by the ore itself.
H. Ex. 211- -28
THE TREATMENT OF ORES OF NATIVE SILVER IN CHIHUAHUA.
The following account was written by H. B. Cornwall, E. M., of the School of Mines of Columbia College, New York, and is to be published in April in the columns of the Engineering and Mining Journal. It furnishes an interesting view of a remarkable industry.
Several districts in Mexico yield considerable quantities of native silver, but nowhere does this class of ore occur so abundantly as at Batopilas, in southern Chihuahua, and the neighboring country. Batopilas especially has become famous for its silver-ore, and the object of this article is to describe the mining and reduction of the native silver-ores of Batopilas, as exemplified by the actual workings of an American company, which has now been in eminently successful operation there for several years. In a future article, the treatment of the sulphureted and other combined ores of silver will be given, with some remarks on mining in Northern Mexico in general.
Batopilas is situated in a deep and very narrow valley, or barranca, among the western ranges of the Sierra Madre, in southeastern Chihuahua, about eight days' journey, by mules, from the nearest port on the Gulf of California. The neighboring mountain-ranges show different formations; sometimes the trails lead over trachytic rock, then over granite or diorite, and again over conglomerate and porphyritic formations. All of these may be met within a six hours' ride from Batopilas. The silver-bearing veins, however, are confined to the diorite, and their universal vein-rock or gangue is carbonate of lime, in the white, crys talline form of calcite. Accompanying the native silver, as will be more minutely described presently, are black sulphuret of silver, (plata negra,) ruby silver, (rosiclara,) arsenical iron, (fierro blanco,) galena, (plomo.) and zinc-blende, (copelilla ;) all of which occur, however, in very small quantity. Through the valley runs a river, always supplying a great deal more water than would be necessary to run as many mines as could be worked, and during the rainy season swelling to a torrent. On both sides of the river rise steep mountain ranges, and the silver-veins occur in considerable number in both ranges. The particular mine to be described lies on the east side of the river, and is worked by a main tunnel, entering the mountain some sixty feet above high water, and but a few yards from the river-bank.
Three of the veins cut by this tunnel had been worked in old times, and from one of them it is reported that some ten millions of dollars were taken. Probably the report is little if at all exaggerated, for the immense waste-heaps and the size of the workings under ground show how extensively the vein was worked, and if the present richness of the ore extracted from another vein can be taken as a criterion, certainly an immense amount of silver must have been obtained. The veins have the general trend of the mountain-range, although they converge at different points, and some of them cross the valley. All are proved by the present tunnel to be true veins, as they are cut from 300 to 600 feet below the surface-level of the old workings, and show the same character below as above. As is always the case where the silver occurs pure, the
ore is not uniformly distributed, but occurs in pockets, and sometimes a vein which has yielded a large bonanza at one time may be worked for several hundred feet without yielding more than a very moderate amount of silver. Still, although the silver is not continuous throughout, yet,. in the vein above alluded to, the old workings show a continuous body of ore, varying from 3 to 10 or 12 feet in width, and to a depth (these statements are made from memory, and not from notes) of over 200 feet. The present tunnel has cut eight large veins, all bearing silver, and by judicious working of these veins, following the indications of rich ore, new deposits may be constantly opened, so that once such a district is developed it may be as successfully worked as if there were but one vein with a continuous body of ore. The Mexicans were not prudent miners. If a large and rich deposit was opened they worked it out as rapidly as possible, not providing by dead work on other veins for the time when their bonanza should be exhausted.
It has been said that galena and zinc-blende accompanied the silver. These minerals, taken in connection with a lively appearance of the calcite, and the presence of arsenical iron, are the miner's guides. By their occurrence he judges where to look for the silver, and an experienced man can follow them up, until from a merely promising rock he proceeds to the silver itself. The country-rock is diorite, very hard and tough near the mouth of the tunnel, but becoming more tractable further in, and always changing decidedly when near a vein. Occasionally the diorite, in a somewhat altered state, and mixed with calcite, forms the vein-rock, as is the case in the largest and richest vein now worked in the tunnel; but invariably, wherever pay rock occurs, there the silver is found with the above accompanying minerals.
As regards the method and expense of mining and reducing the ore, the following facts are given, taken from notes furnished the writer, during a twelvemonths' stay in Batopilas, by the superintendent of the company, who is also the vice-president.
The cost of mining per ton, including all dead work, such as running the front of the tunnel, prospecting, &c., is $33; the actual cost of extracting the silver-ore, including necessary drifts, and the work on all the veins yielding silver in paying quantities, is $8 per ton. Hauling to the works, on donkeys, about half a mile, 62 cents per ton. In another article other details of mining expenses and methods will be given.
The ore is sorted into three classes: first class, value $2,500 and upwards per ton; second class, value $1,000 to $2,500; third class, all under $1,000, averaging perhaps $250.
The third-class ore is dumped at the stamps, the better ore is kept in a store-room and weighed out. All the ore is crushed in a battery of three small stamps, weighing about 300 pounds each, with a fall of 9 inches, and a capacity of 8 tons per twenty-four hours. The ore falls through a screen with five-eighth inch slits, and is then charged in the arrastra. The lumps of silver are separated by the screen, cleaned by hand, and, with the larger lumps of pure silver from the mine, refined with the retort silver. The stamps are run by a horizontal water-wheel, which will be described under the arrastra. This latter apparatus it is unnecessary to describe. Suffice it to say that it is a large Mexican arrastra, 9 feet in diameter, with two stone mullers or runners, weighing 600 to 800 pounds each. The wheel that runs it is, however, peculiar. The arrastra is built on the top of a pile of masonry in a deep pit. In the center of the arrastra rises a shaft, revolving on a pivot which rests in a plate raised a little above the bed of the arrastra, and from this shaft horizontal arms project beyond the rim of the arrastra. From
these arms descend rods which support a horizontal wheel, that thus revolves around the arrastra a few inches above the bottom of the pit. In the periphery of this wheel, at intervals of 6 inches, are inserted rectangular floats, slightly concave, and set up edgewise, as if to receive the water from a tangential, horizontal chute. These floats are called cucharas, (spoons,) and hence these arrastras are called arrastras de cuchara as distinguished from the arrastra de mula. The water acts on this wheel solely by its momentum acquired while descending very rapidly through a tapering chute, having a fall of 8 feet, with a length of 12 to 15 feet. It is very evident that there is a great loss of power here, but as the works are supplied with a superabundance of water by a ditch, and the three arrastras are capable of reducing all the ore required, this makes little difference. These arrastras are universally employed in Mexico when water-power is at hand. Such a wheel, with a diameter of 20 feet, will carry the two runners of the arrastra as fast as four stout mules, which could not work more than eight hours per day, and it runs the battery of stamps as above stated.
From the stamps the ore is taken to the arrastras, into each of which a ton, more or less, according to the size, is charged at once. A few buckets of water are thrown in, just enough to give the mass a certain consistency, which is very essential to the proper conduct of the grind ing process. If there is too little water, the ore is raised up and pushed forward by the mullers, without being ground; if there is too much water, it packs beneath the mullers. Water is from time to time added to preserve the proper consistency of the ore, and after the operation has been carried on about eight hours sufficient quicksilver is added to amalgamate all the silver in the ore. Generally the arrastra is charged with one ton per day of the third-class ore, requiring some 25 pounds of quicksilver, and after three days' run, or whenever the amalgamator thinks proper, rich ore is added, requiring proportionally more quicksilver, for the purpose of getting a suitable amount of amalgam collected in the arrastra, preparatory to cleaning up. Some hours after adding the quicksilver the amalgamator takes a portion of the charge out in a horn spoon, washes it, and thus judges whether there is the proper amount of quicksilver present. These assays are regularly made, but, after a little experience with any ore, he soon learns to gauge the amount of quicksilver very closely.
Every morning, after the silver appears to be thoroughly amalgamated, a large excess of water is added and the arrastra kept in motion for four to six hours; the heavier particles then settle, the amalgam separates from the fine ore, and after the machine has been at rest for a short time, the water is run off, carrying with it all of the finely ground and desilverized ore. The coarser grains of ore, not yet sufficiently reduced, remain and are ground with the next charge. The tailings thus obtained are very poor-so poor that the most experienced men in the place are unwilling to pay $3 per ton for them, with the object of extracting the silver on the patio. They contain nearly all of the galena. zinc-blende, and arsenical 'iron of the ore, a very little quicksilver and amalgam, and any arsenical silver (ruby silver) that may occur, with the exception of a trace that stays in the amalgam, either owing to its density or to native silver adhering to it. Most of the sulphuret of silver, being less brittle, and therefore not so easily reduced to powder, settles to the bottom of the arrastra and is taken out with the amalgam, in which it is plainly visible after washing. The rich tailings, removed after the rich silver-ore has been added, and just before a clean-up, are more valuable, and are saved for concentration or treatment on the patio.
As regards the quicksilver required in this part of the process, it is found that ore containing coarse silver needs less than ore with fine silver, in proportion to the amount of silver.
When the rich tailings have been run off, the top layer of coarsely ground ore is removed with iron scrapers and reserved for the next charge. Then the amalgam is scraped up and carried in wooden bowls (bateas) to the washing-tank. This amalgam seems, to the superficial observer, scarcely anything more than coarse sand and slime, but, on adding to a suitable amount of it, in a shallow wooden bowl, the proper quantity of quicksilver, and washing it, stirring and rubbing it constantly with the hand, the clean amalgam is obtained. The dirt thus removed from it is rich and is reserved for concentration by washing on the plane-table. About 10 per cent. of the quantity of quicksilver already employed in the arrastra is added to the amalgam in this process of cleaning. A small portion of amalgam from the finer-grained silverores is sometimes very carefully washed, the black silver (sulphuret) being removed by grinding the amalgam on a stone and washing it thoroughly, and the resulting very pure amalgam, after straining, is retorted carefully, and furnishes the bullion used in paying expenses. It is even purer than the refined silver cast into bars.
The clean amalgam is now strained in canvas cloths, and this is the most tedious part of the process. As the amalgam is at present retorted a very firm amalgam is required, so that it is not found sufficient to strain the amalgam in large bags by merely twisting them with a stick, but the quicksilver must be thoroughly pressed out from small balls, not over 2 to 2 inches in diameter, by squeezing and rubbing them in the canvas with the hands. The coarseness of the silver, which is frequently present in nails, renders the separation of the quicksilver impracticable by any other means yet tried; from the very finegrained amalgam obtained on the patio the quicksilver is much more easily expressed. Probably if some other system of retorting were introduced this part of the process might be made less laborious. The strained amalgam is charged into quicksilver-flasks, from which the bottom has been removed. About 65 to 70 pounds of amalgam are introduced, and then the flasks are set aside to allow as much of the quicksilver to drain off as possible, and also to harden the amalgam. In a day or so, or as soon as four of these flasks are ready, they are removed to the retorting-furnace, where they are set on end over holes in a slab which forms the bottom of the furnace. There is nothing to prevent the amalgam from falling out of the flasks except four narrow strips of iron set into the mouth of the flask; the amalgam never runs out when the fire is properly managed. Other quicksilver-flasks, open at each end, are placed below the holes in the bottom of the furnace, their lower ends being beneath the surface of water in a tank which lies under the furnace. After luting the lower ends of the amalgam-flasks with clay and ashes there is thus no outlet for the quicksilver except into the water, where it condenses. A charcoal fire is slowly kindled around the flasks and they are thus retorted. This simple furnace, universal in that part of Mexico, has supplanted the old copper-bell apparatus, but is itself susceptible of great improvement.
We have now followed the silver as far as the refining process. It is evident that whatever loss of quicksilver may have resulted is purely mechanical. There is no chemical action in the arrastra, nor is any needed. Consequently the loss of quicksilver is small. A careful account is kept of it, and the result shows only three-quarters of an ounce lost per marc (eight pounds) of silver produced.