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same process, 1 ton and 1,216 pounds of concentrated ore. The present working of these machines is not very close. No mineral escapes with the light rock or waste, but a portion of the rock goes with the mineral. This manner of dressing the rock is a success, as will be seen by the following: Stewart charges for reducing this class of ore $35 per ton; three tons, at $35, $105. For dressing three tons into one, at $10, $30 ; reducing, per ton, $35; total, $65; a saving to the mine-owner on every three tons of third-class ore of $40. So far this process is a success. The fact must not be lost sight of, however, that this method of separation can only be applied to that class of ores in which the silver is carried by the heavier minerals, such as zinc and lead. When the silver exists principally as a sulphuret the process cannot be worked so closely, and happily does not need to, as this class of ore is rich enough without concentration.

SMELTING IN SHAFT FURNACES.

The process of smelting in reverberatories, as employed at Professor Hill's works, is acknowledged to be expensive in fuel and labor, but claimed to be necessitated by the nature of the ores and fluxes. Several attempts have been made to smelt in cupola-furnaces, but most of them have failed, because they required a supply of galéna, which could not be obtained. It is a prevalent delusion in Colorado that immense quantities of galena ores can be had by calling for them; but the demand has been repeatedly made in vain. Even the galenas of Clear Creek County are in general so highly charged with zinc-blende, pyrites, etc., as to unfit them for the cupola.

The Western smelting-works, erecting in Black Hawk, at the time of my last visit, under the charge of Mr. William West, a practical smelter, were on a somewhat different plan. The ore was to be desulphurized without crushing, in kilns, such as are used in the manufacture of sulphuric acid from pyrites, then melted in a cupola-furnace, producing a matte, which was to be recalcined and remelted, for concentration. The final separation of the metals was to be effected at the works, sulphuric acid being obtained from chambers to be erected in connection with the kilns. The capacity of the works was intended to be ten tons daily. In this plan, the cupola-smelting is similar to that effected in the copper-furnaces of Ducktown, Tennessee; but the greater proportion of iron-sulphurets in the Central City ores, the different nature of the gangue, and the more serious expense occasioned by short campaigns and "salamanders" render the undertaking more difficult in Colorado. The furnaces were substantially built; and I have since heard of a successful commencement of operations. The experiment is, in my opinion, a hazardous one; but I do not undertake to say it will fail from causes inherent in the metallurgical plan. My latest news, December 3, speaks of the works as running at full capacity.

THE COLORADO COAL.

The most significant single specimen in the fine array of minerals at the Denver Fair was a huge block of coal, said to weigh five and a half tons, from Marshall's mine, near Golden City. Another mine (Murphy's) furnished a single lump weighing two or three tons. The Marshall vein is 14 feet thick, of which 13 feet are workable coal. The question whether this coal can be used in metallurgical operations is an important one, and the answer is, I regret to say, somewhat doubtful. The analysis published of the Marshall specimen (by whom made I do not know) gives but three per cent. of ash and three per cent. of water, the rest being put down as fixed carbon and hydro-carbon. Believing the. coal to be lignite, I cannot believe it to be so nearly anhydrous. All lignites contain considerable water in chemical combination; and I fear

that in this analysis the coal was merely dried, and the loss in weight set down as water, while the chemically-combined water, passing off in the subsequent distillation, was reckoned with the hydro-carbon. The error, if such it is, is a vital one. The water in lignites not only decreases the amount of actual fuel, but by evaporation absorbs heat in the furnace; and it may be consequently difficult, or even impossible, to maintain high smelting temperatures with such fuel economically.

Some experiments already made have resulted both ways; but the favorable results, so far as I can learn, were obtained on too small a scale to be perfectly satisfactory, while the unfavorable ones may possibly be due to the employment of the ordinary grates and fire-bridges used for wood, which are, of course, somewhat unsuitable. Decisive tests have yet to be made; meanwhile, I am inclined to believe that the coal can be used successfully in gas-furnaces with regenerators, and perhaps not otherwise. One thing is certain, it is excellent for all domestic purposes, and for the generation of steam; and I hope that it may soon be furnished so cheaply as to supersede wood for these applications. This should make the supply of wood and charcoal for furnaces last much longer than it will at the present rate of consumption. However, it should be added that there is no lack of wood in the Rocky Mountains. The trouble is that it speedily thins out in the neighborhood of towns and metallurgical works; and the prices of labor and hauling are such as to make it expensive when brought from a distance. I hardly think, nevertheless, that the prices of fuel will rise beyond present figures at this place for some time to come. I believe Professor Hill, at Black Hawk, pays from $5 to $7 per cord for wood, and say 13 to 15 cents per bushel for charcoal.

CHAPTER XII.

THE SPEED OF STAMPS IN COLORADO AND ELSEWHERE.

The question, what is the best proportion among weight, fall, and speed of stamps, is one which has not yet received thorough and systematic examination. In considering the economical application of stamping-machinery, we meet, at the beginning, with serious difficulties in obtaining accurate data for comparison. The weight and fall of stamps vary as the shoes and dies wear out; and this may lead to a change of speed also. Moreover, defects in engines, boilers, or machinery for the transmission of power, may occasion serious losses, which cannot fairly be charged to the arrangements of the stamps proper. Again, the capacity of stamp-mills is directly dependent, in some degree, upon the nature and extent of discharge, fineness of screens, and other peculiarities of the battery. Finally, the hardness and tenacity of the rock crushed varies so much that comparisons between different localities cannot be implicitly trusted. The safest experiments are those made in the same mill, by changing first one and then another condition of working; but this is seldom possible for such conditions as weight and lift of stamps, and only within narrow limits for their speed.

We may eliminate questions of friction, transmission, and generation of power, in the case of stamps, by measuring the power actually developed by their fall. Thus, the weight, multiplied into the fall in feet, and the number of drops per minute, gives us exactly the number of foot-pounds exerted by each stamp. Dividing by 33,000, the number of foot-pounds per minute in one-horse power, we have the horse-power per stamp, from which the effective power of the whole mill may be obtained. Dividing the amount of rock crushed daily by the effective horse-power, gives us the daily amount per horse-power; and this is the best measure that can be obtained for the effectiveness of the stamps. A complete discussion of the subject would require us to determine the exact influence of the discharge, etc., and the exact resistance offered by different classes of rocks, for both of which points the data are wanting. Professor J. D. Hague, in the third volume of the United States Geological Exploration of the Fortieth Parallel, gives a valuable table of the operations of a number of mills in Gilpin County, Colorado. The discussion of this table leads to some interesting results, which I shall briefly set forth. I give a portion of it, rearranged to suit the object in view, and furnished with additional columns.

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Relative efficiency of certain stamp-mills in Gilpin County, Colorado.

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I have taken from the report the names of mills, number of stamps running, weight of stamps, fall in inches, number of drops per minute, and tons of ore crushed per day. To these columns I have added one giving the total horse-power developed and one giving the tons of ore crushed daily per horse-power developed. These figures are obtained by separate calculations for each mill. At the bottom of the table certain totals and averages have been added. The total number of stamps explains itself. The total weight is arrived at by multiplying the number and weight for each mill, and then aggregating these products. The total horse-power, again, is a simple addition. The methods of obtaining averages require more detailed comment. In several columns the numerical differs decidedly from the dynamical average; thus, if we multiply the number of stamps in each mill by their fall, add these products, and divide the sum by the total number of stamps, we obtain a numerical average of the fall; and a similar process gives us a numerical average of the number of drops per minute; but if we should attempt to deduce from the total number of stamps, their average weight and (numerical) average fall and speed, the total horse-power developed, we should obtain a result different from that which is arrived at by simply

* Estimated, generally from maxima and minima given. Thus 15 to 20 is put at 17).

adding the totals given in the column of horse-power developed. The reason is obvious. In taking a merely numerical average we leave out of account the weight of the different stamps; it is therefore necessary to multiply the number and weight of stamps of each mill into the drop and to divide the sum of these products by the aggregate weight of all the stamps of all the mills. In calculating the average speed the drop, as well as the number and weight, must be included. This can be best illustrated by an example, comprising, for the sake of simplicity, only two mills. I take, almost at random, Nos. 2 and 11 from the table, viz: Black Hawk: 60 stamps, 850 pounds, 14 inches, 15 drops, 27 horsepower.

Bates: 8 stamps, 425 pounds, 12 inches, 30 drops, 3.1 horse-power. The totals would be 68 stamps, 54,400 pounds, and 30.1 horse-power. The numerical averages are obtained as follows:

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31.68 horse-power, whereas the aggregate horse-power, as we know by calculating it separately for each mill, is 30.1 horse-power.

The dynamical averages, on the other hand, are obtained as follows: Fall.-60 x 850=51,000 51,000 × 14714,000 3,400 x 12 40,800

8 x 425 3,400

=

54,400

= =

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754,800

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Average speed=11,934,000÷754,800-15.81 drops per minute.

13.87

If now we calculate the total horse-power upon these dynamical averages, we have 54,400 × × 15.81÷33,000=30.1 horse-power, which agrees 12 with the total from the table.

A third set of averages, which I call, for convenience, gross averages, is obtained by disregarding the number as well as the weight of stamps, and considering only the number of mills. Thus, in the case just given, the gross averages would be 637.5 pounds, 13 inches, and 22.5 drops. This has little value for accuracy; but it is the usual manner in which casual observers estimate the matter, and it shows what is the fashion or prevailing custom among owners of mills. Bearing these distinctions in mind, we have the following results, based on a comparison of thirtythree mills:

Total number of stamps, 656; average number in each mill, 19.88; total weight of stamps, 396,110 pounds; average weight, 603.83 pounds; average weight reckoned by mills, without reference to their size, 580.27

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