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The stamp-batteries of the Port Philip Company, at Clunes, Australía, show, (Ib., p. 678 :)
This is extraordinary efficiency; but the batteries are aided by rockbreakers, and have a double discharge.
The stamp-mills of Cornish pattern in use at the Morro Velho mines, Brazil, (Ib., p. 679,) show:
Comparing the stamp-mills of Colorado with all these examples from other regions, we notice that the speed of their stamps is, on the average, much less, and that, to say the least, the efficiency is no greater than that of more rapid running. But the argument for a higher speed is fairer if the Colorado mills are compared among themselves. Returning, therefore, to the table given on page 381, we notice that of eleven mills exhibiting a greater efficiency than the average of 1.55, five are run at a speed exceeding the average of 30.82, two at 30, and the remaining four at 22, 24, 26, and 28, respectively. The highest efficiency is attained by the Carondelet mill, having the lightest stamps, (350 pounds,) run at the highest speed, (50 drops per minute,) and crushing daily 3.30 tons per horse-power. The Blue mill, on the other hand, develops nearly the same horse-power, but crushes only little over half as much. In the latter case, twice the weight of metal is dropped twothirds as far, four-fifths as often; and while the power is nearly the same, this different application of it appears to be far less advantageous. On the other hand, there are instances in the table which seem to contradict such a conclusion. The slow rate of running insures an immediate and adequate discharge; and much of the advantage of a rapid
rate is lost when the discharge is not ample. The remarkable increase in product secured by the use of a double or even a continuous discharge around the whole battery-box would doubtless influence mill. men to adopt this improvement, were it not for certain difficulties, partly real, partly imaginary, in its use.
If we consider economy as well as efficiency in crushing, the advautage of a high rate is evident. With the same machinery, wages, etc., and, if the mill is well built, with little or no extra repairs, a large increase in capacity is secured. Moreover, the first cost may be reduced by the use of lighter batteries. Probably, also, the increased speed may be attained with less than the proportional increase of fuel.
The objections to higher speeds in Colorado mills are partly set forth in my last report, (pages 365-'66,) in the words of a writer in the Central City Register. His argument is, substantially, that experience has shown different rates of speed to be best for different kinds of ores. Instances are given in which, upon an increase of speed, the yield of gold per ton fell off; and it is claimed that this test should decide what rate is to be adopted in each case. In other words, the rapid running of the stamps, and consequent augmentation of product crushed, causes greater agitation within the battery-box, and requires a larger supply of water to clear the discharge and carry away the greater amount of pulp. The excess of agitation in the battery may prevent the accumu lation of gold on the interior plates, and the excess of current on the aprons may prevent the accumulation of gold there. These objections are most plausible when the gold is most finely divided in the quartz. I propose to consider them briefly.
This reasoning amounts to the confession that the conditions most favorable to economical crushing must be partly sacrificed to secure efficient amalgamation. Is this sacrifice really necessary, or is it merely involved in the method of amalgamation adopted in the Colorado mills? The attempt to catch the greater part of the gold on the interior plates interferes directly with the greatest efficiency of the stamps. The suc cess of the amalgamation at this point is in inverse proportion to the success of the crushing and discharge. There is a certain advantage gained in the force with which the pulp is dashed against the plates; but this force is liable to overdo, and thus undo, its own work, and actually remove the adhering amalgam. The same effect can be more completely secured outside of the battery.
But the arrangements outside are generally poorly adapted for the purpose. The pulp is swept over a small, steep, and smooth amalgamated surface; and it is no wonder that so little gold is caught upon the aprons. The Port Philip, Australia, mills, (see my report of 1870, p. 678) have five distinct steps or drops in the outer plates, where the Colorado mills have none. If this arrangement were adopted, an excess of water would occasion no loss, and the efficiency of amalgamation would be increased.
The principal objection appears to be the clogging of the outside. riffles or steps with pulp, or the removal of amalgam by the falling of the pulp over the steps. But it strikes me that if Australian mills can overcome these difficulties we ought to be able to do the same.
Even retaining the present patterns of outside aprons, the effect of a greater amount of water could be neutralized by spreading the discharge over a wider surface. Let us suppose, for instance, that a twenty-stamp mill is run at a low speed, for fear of losing gold if more quartz and more water were passed through it in a given time; and that ten of the stamps, run at a high speed, would have the same crushing capacity as
the whole mill at present. Why not run ten stamps in this way, and discharge upon the apron surface of the whole twenty? After the pulp is once through the screens, and sliding over the apron, it makes no difference how fast it was crushed. In a word, the conditions of amalgamation should be, and can be, regulated without interfering with the conditions of pulverization. Loss of gold should be, and can be, prevented without crippling the efficiency of the stamps. Power, space, and time are at our disposal; and by a proper use of the two latter we may avoid wasting the first, which is the most costly.
My views on this subject may be summed up as follows:
1. The stamp-mill is the most convenient and practically efficient machine for crushing quartz thus far introduced and proved by experience. It involves little waste of power in gearing; it delivers its power in the most direct and practical manner, namely, by blows, which take advantage of the brittleness of the rock, instead of pressure or friction, which invite the resistance of hardness; its capacities for charging and discharging are ample and easily regulated, both as to quantity and as to fineness of the product; it is subject to few and comparatively inexpensive repairs, and it can be repaired, in most cases, without complete stoppage. These and other excellent features in its construction and operation render it especially suitable for use in mining districts remote from machine-shops, founderies, and centers of skilled labor.
2. To obtain the best results, stamp-batteries should be built and run to secure the highest efficiency and economy in crushing only, without reference to amalgamation. The amalgamating apparatus should be adapted to the batteries, not the latter to the former. If interior plates are employed, they should not be expected to catch the greater part of the gold, nor should the pulp escaping through the screens be swiftly and carelessly manipulated, when a little extra space and time devoted to it, almost without extra labor, would avoid much loss.
3. The efficiency of a stamp may be described as the product of three factors-weight, fall, and speed. The efficiency of a battery of stamps involves a coefficient-the discharge.
4. When the fineness of crushing is regulated by screens, the discharge should be as large as practicable. There may be mechanical objections to continuous screens running around the whole battery; but there are, I think, no valid arguments against the double discharge, in front and rear, when the battery is properly planned with reference to it. Of course a feature of this kind cannot always be successfully added, like a patch, to a battery not duly proportioned for it.
5. Of the three factors of the efficiency of the stamp, the weight and fall determine the force of the blows, and the speed determines their frequency. The height of fall is practically limited by the speed, and by considerations of mechanical convenience.
6. Within certain limits, light blows, frequently repeated, are more efficient than heavy blows at longer intervals. These limits are the following: The stamp must be heavy enough to work steadily, and fall far enough to allow proper feeding and distribution of the ore, and to produce the splash necessary for effective discharge. (In many cases, by the way, more weight might be advantageously put in the stems, and less in the heads.) Again, the blow must be heavy enough to crush the rock upon which it falls. If too heavy, it may waste power in packing the crushed rock; if too light, it may fail to crush, and so may pack. Finally, the speed should not be so great as to prevent proper clearance, or the stamp may strike a second blow upon the rock already crushed. 7. The efficiency of a blow from a heavy stamp with short drop is less
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, 133 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.
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 78,4324 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 tous 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 carlier 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.