than that of an equal blow (in foot-pounds) given by a lighter stamp with longer drop-the practical limits already referred to being observedbecause the longer drop gives greater final velocity to the stamp, and this tends to crush more and to pack less. The same principle underlies the effect of nitro-glycerine, as observed at the bottom of blastingholes, where the rock in the immediate neighborhood is shattered and pulverized by the suddenness of the explosive shock. 8. The superior effectiveness of frequent blows lies in the fact that there is a limit to the amount of crushing which can be practically performed by a single impact upon a given quantity of rock distributed over a given surface. Thus, a thousand foot-pounds, delivered instantaneously upon a surface eight inches in diameter, may be resolved into six hundred of minute motion or crushing, and four hundred of gross motion, or packing, and heat; while five hundred foot-pounds, under the same circumstances, may perform four hundred of crushing, and waste only one hundred. Two of the latter blows would then effect more with the same force than one of the former. There is another practical advantage of high speed. If stamps are left, as it were, standing in the pulp, between blows, the material settles around them and they "suck" when the lift commences. A great deal of power is frequently wasted in this way, by not picking up the stamps before they become partially buried. 9. But even if the efficiency of stamps were always exactly measured by the product of the three factors mentioned, that is, by the number of foot-pounds delivered per minute, (which is certainly not the case,) there would still be good reason for preferring rapid running. After the necessary stability and strength are secured, increased weight of machinery is an evil. If equal results can be achieved by substituting speed for weight, the change is advisable. 10. In the case of the Colorado mills, the argument is still stronger. Their (gross) average weight of stamp, 580 pounds, is not excessive; their average drop, 134 inches, is not too large to admit of high speed; but their average speed, say 30 drops per minute, is extremely low, and might be doubled with advantage. A bad arrangement for amalgamation is one excuse, which should be removed, not pleaded. Another serious objection, which Colorado experts are not so free in expressing, is a bad construction of battery foundations and frames. It is feared ' that high rates of speed would rack or upset the batteries. The difference in this respect between the mills of Colorado and those of other regions may be seen by comparing the drawings given in a previous chapter of this report with that on page 664 of my former report. The California mortar rests on a vertical block, and the blow of the stamp does not communicate vibrations to horizontal timbers. I believe the views I have expressed are coming more and more to be those of American millmen, even in Colorado. The true evidence of this tendency is to be found in the patterns of the new mills, rather than the practice of those persons who are frequently obliged to adapt themselves to the proportions or condition of antiquated machinery. Moreover, the manufacturers frequently adhere to the old patterns, or at least put higher prices upon machinery constructed after new ones; and few engineers have the opportunity of dictating from their own experience the details of their mills. Mine-owners think a stamp is a stamp, and a steam-engine a steam-engine; and desiring so many stamps with so much horse-power to run them, pick up what they want wherever they can get it most cheaply-at second-hand, if possible. But many causes, and particularly the keen competition among custom-mills, are bringing about a wholesome progress in this matter. CHAPTER XIII. THE WASHOE PAN AMALGAMATION. The third volume of the Report of the United States Geological Exploration of the Fortieth Parallel contains an admirable chapter, from the pen of Professor J. D. Hague, on the treatment of the Comstock ores. As the expensive character of that work, and the comparatively limited edition of it published by the Government, prevent its general circulation among the classes most interested in this part of its contents, a portion of the chapter referred to will be here abridged, with such notes and comments as may seem useful. The division of the Comstock ores into first, second, and third class is arbitrary and variable, having reference rather to the treatment chosen for each class than to the mineralogical constitution of the ore. The first class receives the most careful treatment, and usually possesses an assay value exceeding $150, or even $100, per ton. The second class, where it is distinguished at all, usually includes ores assaying from $90 to $150. The third class comprises all workable ore of still lower grades. The first-class ores form but a small proportion of the whole. For instance, the Savage mine produced, in the year ending July 1, 1868, 87,341 tons of ore, yielding an average of $40 84 per ton, of which only 277 tons were first class, having an average assay value of $449 40 per ton, and an average yield of $359 52; and 4,745 tons were second class, with an average assay value of $124 25 to $142 82, and yielding $78 16 per ton. The remaining 78,432 tons of third-class ore assayed $52 01 to $55 11, and yielded an average of $37 20. In the following year, out of a total of 69,287 tons, there were only 684 tons called first class, and having an average assay value of $275 47, while there was no second class distinguished, and 55,411 tons of the third class, assaying $50 78 to $60 29, yielded $34 64 per ton.* About 25 to 30 per cent. of the value of these ores is gold, and the remainder silver. In the bullion produced the relative proportion of the gold is a little higher, as it is more completely saved than the silver. The first-class ores are treated with dry crushing, roasting with salt, and subsequent amalgamation. The ores of the second and third classes are subjected to the "Washoe" process proper, as follows: Crushing. This is universally performed in stamp-mills, the larger pieces being "spalled" to a suitable size for feeding into the batteries. For this purpose Blake's rock-breaker is frequently used instead of the hand-sledge. The foundation of the battery is like that adopted in California,‡ consisting of heavy vertical timbers, firmly bolted together, and tightly packed with clay or earth. The mortars are usually placed directly upon these vertical mortar-blocks. The mortar in general use for wet * The earlier operations of the Comstock furnished a much larger proportion of rich ores, partly because the rich ores were eagerly extracted, and those of lower grade left standing. The greater part of the product of late years has been from material overlooked or discarded by the extravagant managers of the "flush times" of Washoe. It would be unfair to argue from the figures that the vein has to this extent "grown poorer;" they rather show that the operations of extraction and reduction have become cheaper, more skillful, and more rational.-R. W. R. + Generally assumed, roughly, at one-third the value.-R. W. R. Differing from the Colorado plan, as will be seen by reference to the chapter on that subject in this report.-R. W. R. crushing is an iron box or trough, 4 or 5 feet in length and depth, and 12 inches in inside width, cast solid. The feed-slit is 3 or 4 inches wide, and the discharge-opening is 12 to 18 inches high, the lower edge being 2 or 3 inches above the top of the die. The single discharge is generally used. The screens are of brass wire-cloth, 40 to 60 meshes to the inch, or (as is preferred for wet-crushing) of Russia sheet iron, perforated with holes to inch in diameter. The dies are cylindrical, 4 to 6 inches high, and usually cast on a square flat base, with truncated corners, so as to fill the bottom of the mortar, and yet be easily removed when necessary. The stamp stems are usually of turned wrought iron, about 3 inches in diameter, 10 to 12 feet long, and slightly tapered below to fit the sockets in the heads. The latter are cylinders of tough cast iron, about 8 inches in diameter and 15 inches high. The socket for the stem is about 7 inches deep. A similar, but larger, socket in the lower end of the head receives the shank of the shoe. Each end of the stamphead is encircled with a stout wrought-iron hoop, shrunk upon it like a tire. The shoes are usually about 8 inches in diameter and 6 inches high, with a tapering shank about 5 inches high and 4 to 5 inches thick where it joins the shoe proper. They are made of the hardest white iron,* and are replaced when worn down to about one inch in height. The collar or tappet, preferred in California and Nevada, is Wheeler's gib-tappet, which is cylindrical in form, (effecting the revolution of the stem during the lift,) and differs from others of that pattern in the manner of its attachment to the stem. This is effected, not by tapering the stem or cutting the screw-thread or key-seat upon it, but by means of a gib and two keys, which clamp the collar to the stem at any desired point. The rotary motion of the stamp, imparted by the friction of the cam against the tappet, is in very general use in Nevada. This is one of the advantages offered by the use of round shoes, stems, and tappets. The revolving cam, meeting the tappet and raising the stamp, causes it, while being lifted, to make a partial revolution about its vertical axis, which rotary motion being continued during the free fall of the stamp, produces a grinding effect between the shoe and die upon the substance to be crushed. Not only is the effective duty of the stamp at each blow increased in this way, but the shoe wears down much more evenly than when it falls without such rotary motion.† The guides, which are of wood, and supported by the cross-timbers of the battery-frame, are placed, one set below the tappet, about a foot above the top of the mortar, and the other set near the top of the stem, so that six inches or a foot of the latter may project above. *The manner in which shoes, heads, and stems are attached together in practice is described in the chapter on the Colorado process in this report.-R. W. R. I have copied this paragraph verbatim from Professor Hague's chapter; but I must take leave to doubt the existence of an effective grinding action, such as he describes, at least from stamps run at ordinary speed, say 30 to 70 drops per minute. The circular revolving stamps have their advantages, no doubt; the chief ones being convenience and regularity of wear. But their dynamic advantage, if it exists at all, is much overrated, as the statistics of the best square stamps will show. If I remember correctly, some comparative tests, made under the superintendence of Mr. S. S. Robinson, in one of the largest stamp-mills of the Lake Superior copper region, did not indicate a greater crushing capacity for the revolving stamps. And it may well be questioned whether the most recent German batteries (which still retain the square stamp) are not as effective as our own. -R. W. R. The cams are of tough cast iron, and usually double-armed.* The proper curve of the face is the involute of a circle, the radius of which is equal to the distance between the center of the cam-shaft and the center of the stamp-stem. This form keeps the bottom of the tappet constantly perpendicular to the radius of the cam-curve, and thus lifts the stamp vertically and uniformly. The involute is described in practice by cutting from a thin board a circular piece, the radius of which is equal to the horizontal distance between the centers of shaft and stem, as above. At a given point on the periphery is fixed one end of a thread, having the length of the greatest desired lift of the stamp, and to the other end of the thread is attached a pencil-point. The circular piece, with the attached thread wound on its periphery, is laid on a smooth board, on which the involute is to be traced, and the thread, being constantly stretched "taut," is unwound until it forms a tangent to the circle at the point where the other end is attached. The line described by the pencil-point is the desired curve. This is frequently modified somewhat, receiving a greater curvature at each end, to diminish the shock of catching the stamp and the wear upon the tip of the cam in letting it fall again. The face of the cam is 2 to 2 inches wide. It is placed as near the stamp-stem as is possible without contact. The cams are keyed or wedged to the iron cam-shaft, which varies in diameter from 4 to 6 or 7 inches, according to its work. In some mills a single cam-shaft drives all the batteries; but short shafts, one for each battery or pair of batteries, are preferred, as permitting stoppage of part of the mill without interfering with the rest.t A common order of fall in the usual five-stamp battery is 3, 5, 2, 4, 1.‡ The weight of stamps in most general use is between 600 and 700 pounds. They are usually run at about 70 or 80, sometimes 90 or even 100, blows per minute. They drop from 7 to 10 inches, according to their speed, the greater number of blows per minute requiring shorter lift. In wetcrushing on Comstock quartz, and discharging through No. 5 or No. 6 screen, the average duty is about two tons in twenty-four hours.§ In some mills it is said to reach three tons. Feeding is usually performed by hand, but in some mills automatic feeders are employed, which give satisfaction. The arrangement comprises a hopper filled with ore,|| and a chute, leading to the feed-slit of the battery, so inclined that when agitated it will cause the ore to slide down. The chute is hung on a pivot, and a rod is attached in such a manner that the tappet will strike upon it when the stamp falls so far as to require a fresh supply of rock. The shock agitates the chute and causes the ore to move down and fall into the battery. The consumption of water is usually between 250 and 300 cubic feet per ton of rock treated, or from one-third to one-half of a cubic foot per stamp per minute. This, includes, however, the water used in the pans, which does not pass through the batteries, and which amounts, perhaps, to one-twelfth or one-eighth of a cubic foot per stamp per minute, leav *See remarks on this subject at page 734 of my last report.-R. W. R. This arrangement also permits the regulation of speed for each battery, according to the nature of ore crushed, etc. In a mill so arranged, experiments to determine the best rate of speed could be easily instituted.—R. W. R. See my last report, page 736.—R. W. R. Two tons daily for a 650-pound stamp, falling 8 inches and giving 75 blows per minute, represent 1.91 tons per horse-power developed at the stamp, a high efficiency, due to speed and the use of Blake's crusher.-R. W. R. See pages 663, 664, 736 of my last report.-R. W. R. ing one-fourth of a cubic foot and upward of battery-water per stamp per minute.* The mills of Virginia City and Gold Hill, that have no springs or other sources of water of their own, are supplied by the Virginia and Gold Hill Water Company. This company obtains water by means of tunnels driven into the hill-side for the purpose, and by purchase from mining companies of their underground waters. Under ordinary circumstances the supply from sources above Virginia City is sufficient for that place, to say nothing of the sources in mines and tunnels lower down. In seasons of drought some inconvenience is experienced. Water is measured by the miners' inch-the quantity that will pass through an orifice one inch square in the side of the measuring-box, under a head, usually of six inches. In California the aperture is usu ally made two inches high, and as long as need be to furnish the desired number of inches, and the water in the measuring-box at one side of the supply flume is allowed to stand about six inches above the middle line of the orifice. But this practice is not uniform, and hence the miners' inch has not an invariable value. The quantity of water that will pass through an orifice one inch square under a head of six inches, determined by multiplying the area of the orifice by the theoretical orifice √2gh, and taking two-thirds of the product as effective discharge, is 0.02633 cubic feet per second, 1.578 cubic feet per minute, or 94.68 cubic feet per hour.† Grinding and amalgamating.-This is performed in pans of various kinds. The objects sought in the different forms of pans are: The most effective form of grinding surface, combining uniform wear with economy of power; free circulation of the pulp; uniform and thorough distribution of the mercury, and the proper degree of heat, insuring favorable conditions for amalgamation; simplicity and cheapness; ease of management and repair; large capacity and economy of time, labor, and material. Probably the highest degree of excellence in all these particulars is not found in any one pan.‡ The most noticeable difference in pans is that of the bottom and grinding surfaces, some being flat, and others conical or curved. Opinions differ as to this feature, but the prevailing opinion seems to favor the flat bottom, though other forms of grinding surface have theoretical advantages, and some pans embodying them, such as Wheeler & Randall's conoidal, and Hepburn & Peterson's conical, are held in high esteem.§ The flat-bottomed pan usually gives more uniform wear, and the various parts of the flat muller are simpler and more easily handled and replaced. The flat muller, carrying its load of thick pulp, requires more power, but, it is claimed, distributes the quicksilver more thoroughly, and thus assists amalgamation. *The average in Colorado is 28 cubic feet of water per cubic foot (125 pounds) of rich ore, or 33 per foot (108 pounds) of poor ore. Per stamp per minute the average is about one-fourth of a cubic foot.-R. W. R. + This is considerably less than the popular estimate of the (not miners') inch, which is 4,032 cubic inches, or 145.86 pounds of water per minute. (See Mr. J. Ross Browne's second report on Mineral Resources, etc., page 184.) Mr. J. Arthur Phillips (Mining and Metallurgy of Gold and Silver, p. 152) agrees exactly with Professor Hague making 60 cubic feet per second equal to 2,280 miners' inches. (See also, for instances of different measurement, my last report, page 477.)—R. W. R. I omit much on Professor Hague's remarks on pans, since the subject was treated at some length in my last report. His general opinions are, however, fairly given in abridged form.-R. W. R. Where the pan is used more for amalgamation than grinding, as in the case of roasted ores, the flat bottom is certainly preferable.—R. W. R. |