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bers connected by iron rods. The stack is 54 feet high, 27 inches square at the top, and increasing slightly toward the bottom.
The charge is about two tons, consisting of different grades of calcined ore and tailings, with sometimes a little raw ore or some rich slags of previous meltings, so mixed as to secure the desired proportions of silica, iron, copper, sulphur, etc. Six or seven hours, and sometimes more, are required for the reduction of each charge. When finished, the slag is raked out through the skimming door and cast in sand-molds; these are broken up and carefully inspected; the portion on the bottom which shows adhering particles of matte must be remelted; that which is sufficiently poor is thrown away. The matte remains in the furnace until, after repeated charges-generally four or five, consuming eight to ten tons of ore-about a ton has accumulated, when it is drawn off through the tap and cast in sand-molds. Under favorable conditions each furnace may yield one ton of matte per day; but this is above the average. If made from the best ore, the matte is rich enough to ship after the first melting; but the greater portion is not up to the standard, and must be remelted. Under existing conditions it is desirable to produce for shipment a matte that contains about 50 per cent. of copper, with 40 or 50 ounces of fine gold, and between 100 and 200 ounces of fine silver, per ton. The loss in smelting for the production of matte is said not to exceed 5 per cent. of the assay value of the ore.
Each furnace runs night and day, requires two men constantly, and four when charging, and consumes daily from ten to twelve cords of wood, costing $6 per cord. The matte is finally broken in the crusher, passed through the rollers, sewed up in small sacks of stout canvas, and shipped to Vivian & Co., of Swansea, Wales. The cost of packages, handling, freight, commissions, etc., not including that of further treatment, are stated at about $120 per ton of matte.*
The prices paid by Professor Hill, previous to January 1, 1870, are shown in the following schedule, which was not, however, invariably adhered to.
In calculating the value of ore according to the above scale, the ounce of fine gold is reckoned at $20, coin, and the unit of copper at $2. The copper unit, however, is reckoned on the English ton; and as the ores are assayed and purchased by the short ton, a deduction of 12 per cent. is made on the copper assay. Thus, if an ore is found to contain 8 per cent. or units of copper, worth, according to the above scale, $16, a deduction of 12 per cent. is made, to adapt it to the English ton. Moreover, the copper is determined by wet assay, from which 14 per cent.
It is expected that the value of the copper will cover this cost, and likewise that of the subsequent gold and silver extraction, leaving the value of the gold and silver as
net return to the works in Colorado.-R. W. R.
is to be deducted for working loss, so that if the percentage of copper contained in an ore does not exceed 14, no account is taken of it in paying for the ore.
In addition to these rates for gold and copper, the silver in the ore was paid for at the rate of 75 cents per ton, after deducting from the number of ounces contained per ton as many ounces as there were units of copper-the rule of the Swansea works.*
Since January, 1870, these works have raised the prices paid for ores. The first shipment of matte was made from these works in June, 1868. Complete statements of the shipments made since that time are not available. They are estimated, up to the close of 1869, at about 25 tons of matte per month, containing, on the average, 40 ounces of fine gold, 200 ounces of fine silver, and 40 per cent. or 800 pounds of metallic copper, per ton. The gross value of these metals would be therefore about $30,000 dollars coin per month, or $570,000 from the date of be ginning to the end of 1869.‡
The following contribution to this report, from the pen of Mr. A. Von Schulz, a very intelligent and well-educated metallurgist, of Central City, is published in full, as interesting and suggestive, though it cov ers, in some particulars, matters already touched upon, and advances views with regard to the metallurgical application of the Colorado coals, and one or two other points, which I am not quite prepared to accept, since, though plausible, they lack experimental proof, so far as I am now informed:
According to the occurrence and behavior of gold in these ores, they are classed as decomposed ores and sulphurets. In the former the gold is disseminated in a free and metallic state, and can therefore be beneficiated by simple amalgamation. In the sulphurets the greatest part of the gold is present in that peculiar form, the nature of which is not yet sufficiently defined. When in this state it cannot be extracted by common amalgamation.
A natural consequence of this geological occurrence was the introduction of stampmills in the course of the development of the Territory. With these the gold con
The complicated system of prices, perquisites, and deductions employed by smelters in calculating their payment for ores, is justly complained of by the miners, as serving merely to bewilder the seller and conceal the profits of the buyer. Many items might be simplified in the interest of fair dealing. The theory, however, is correct, that the prices paid for ores must be graduated, not merely according to their actual contents in valuable metals, but with reference also to the grade and character of ore, as influencing the cost of reduction. It is probable, that on the foregoing schedule Professor Hill lost something in the purchase of the lowest grades, and made it up on the rich ores. Indeed, he would doubtless refuse to purchase 2-ounce gold ores, did he not require them, in the absence of cheaper fluxes, to mix with the others.R. W. R.
At the time of my visit, in the autumn of 1870, I was told that no regular schedule was followed. I heard of instances in which $5 per ton was paid for tailings, containing 1 to 1 ounces of gold. The developments in Grand Island district having made silver ores an important part of the supply of the works, the prices paid for silver had been advanced, possibly to prevent shipments of ore to the East. For 100-ounce silver ores, 80 cents per ounce was paid, and for very rich ores, as high as $1 30 currency-a deduction of $50 per ton being made, however, for the cost of reduction. Probably these works, being well established and commanding large capital, could afford to advance prices still further, if competition should require it; a consideration full of warning to the eager projectors of rival establishments.-R. W. R.
It is estimated that the shipments of 1870 amounted to 520 tons-that is an average of ten tons weekly-worth at least $170 per ton, or $884,000 for the gross value of the shipments for the year. This estimate appears high to me, but it is pronounced low by those who are in a better position to judge; and, as official returns from the works are not available, I have accepted it. The increase is due to the enlargement of the works and the treatment of rich Caribou ores.-R. W. R.
tained in the decomposed ores was extracted according to the method practiced in California, by catching it on amalgamated copper plates. But the more decomposed ores were extracted from the mines and the more sulphurets took their place in depth the more this method appeared insufficient and the want of a better one for the purpose became apparent. This caused all those experiments and enterprises which, inaugurated from 1863 to 1866, never came to a successful conclusion. Instead of imitating the old methods, proved successful in Europe, new ones were desired; instead of common sense and metallurgical knowledge, ingenious schemes, entirely independent of scientific facts, became the order of the day. It was only natural that the loss of the enormous sums invested should be followed by depression. Finally the old stamp-mills, with ja amalgamation on copper plates, were reinstated, or the mines were closed until proper methods for the beneficiation of the refractory sulphurets should come into use. On the whole, those companies which chose the latter course have acted most wisely; for although the cost of running stamp-works is small, they must be decidedly rejected, considering them from the stand-point of national economy, when the beneficiation of rich auriferous sulphurets is desired, as hardly 30 to 40 per cent. of the gold contained in such ores is saved by this treatment.
About four years ago Professor Hill, at Black Hawk, commenced smelting the sulphurets into copper matte, following the old methods universally in use in Europe. He sends this product to England for the separation of the gold, silver, and copper. From the first hour of its commencement to this day, the enterprise has been uniformly successful, and it is now intended to enlarge the capacity of the works for the third time. This exerted a most beneficial influence on mining. New vigor seemed to inspire the whole interest; and the tailings, formerly lost from the mills, have since been caught, concentrated, and sold to the smelting works, where they were welcome as a flux, it being impossible to get lime for the purpose at less cost.
As a drawback to enterprises of this kind for the beneficiation of the sulphurets, it is urged that they require a large working capital, and can only be profitably conducted on a large scale. But this is rather an advantage than otherwise; for large enterprises, conducted with success, benefit directly and visibly the whole community, while small smelting-works hardly ever prosper nowadays.
The greatest advantage of the smelting method over the mill process lies in the fact that by it almost all the gold and silver, as well as the copper, contained in the ores is saved. To be sure, only the rich ores can be treated in this way at present; but as soon as Gilpin County is connected with the coal-fields of Colorado by rail it will also pay well to mine the less rich ores, to concentrate them, and to ship them to the smelting-works. Stamp-mills can, of course, never be dispensed with altogether. They are the cheapest appliances for working the remainder of the decomposed ores; and in the future, after the introduction of a rational system of dressing-works, they will serve for the beneficiation of the tailings from the dressing-works.
About a year since another method, Plattner's process of extracting gold by chlorine gas, was introduced in Gilpin County. The enterprise being limited as to capital, and the process hindered by the other metals present, it has so far given satisfactory results only in regard to the fineness of the produced gold. To make the Plattner process adequate to the requirements of Colorado sulphurets, it will have to be modified by adding, after the roasting and chlorination, a process by which the copper can be extracted and the silver separated at small cost.
To render more valuable the ores of medium grade the introduction of ore-dressing is required. Cylinder-crushers, sieves for the separation according to size, and jiggers are the only machinery necessary; and in case of the scarcity of water, that supplied to the jiggers can be used over and over again. The ore should not be crushed finer than to a size of 2 millimeters. The tailings should be treated in the stamp-mills, or in large arrastras, which can very well compete with mills on a large scale, and have, besides, the advantage of a higher yield.
Real and continuous success of the Gilpin County gold mines can only be expected after the completion of the railroad to Golden City, and its coal-fields. In the present state of metallurgy, the lixiviation with sulphuric acid is probably the cheapest and best method for working Colorado gold-bearing sulphurets. If, besides the reverberatories necessary for the production of copper matte, a blast-furnace was added, for leadsmelting, the silver ores from Clear Creek County might at the same time be treated either thus or by the Patera process, according to their larger contents in lead or pyrites.
The products thus gained would be sulphur, copper, lead, silver, and gold, and in the lead-works the lead-matte occurring at Golden and Boulder Cities might also be profitably worked.
After the establishment of such a work those petty dissensions between the different mining companies, which at present hinder so much the development of the Gilpin County mining interest, would also cease. Whenever the contemporaneous execution of the four processes above mentioned is insured, and this in the coal-fields and among the water-powers between Golden and Boulder City; when that region is connected with Gilpin and Clear Creek Counties by rail; when the Territorial mining laws are
revised and improved; then the two counties mentioned, with their great abundance of veins, even if they furnish only poor or medium ores, cannot fail to reach a development such as is known to very few districts on this continent.
Colonel G. W. Baker, of the Central City Herald, published during the summer of 1870 a series of articles on the Colorado treatment of gold ores, which aroused considerable feeling throughout the Territory. So for as their exposition of the losses incurred by the mill-process is concerned, they appear to be well-founded. The plan suggested as a remedy comprises dry crushing, dry separation, and chloridizing roasting in the Stetefeldt furnace, with subsequent amalgamation. For the separation, Krom's dry concentrator is proposed-an excellent machine, and probably the best of that class.
The following account, published by Colonel Baker in July, describes one of several experiments made in Gilpin County, with a view to test Krom's machine in the separation of pyrites from gangue.
HERALD OFFICE, June 26, 1870. GENTLEMEN: Will you please give personal supervision to the separation of some mill tailings now at the Lexington mill, the separation to be done in your presence by the Krom machine, and observe the inclosed instructions, and report according to the schedule to
Yours, very respectfully,
To Messrs. E. E. BURLINGAME, A. VON SCHULZ, Assayers.
G. W. BAKER.
1st. From the pile of mill tailings procure sufficient samples for assay, and then, after weighing the remainder, see it passed through the machine.
2d. Weigh the headings; assay same for per cent. of gangue; assay same for gold and silver.
3d. Take sample of the separated tailings, assay same for metal left; assay same for gold and silver.
4th. Sieve the sample of original tailings for degree of finenes; assay same for per cent. of metal; assay same for gold and silver.
TERRITORIAL ASSAY OFFICE, Central City, June 30, 1870.
DEAR SIR: In accordance with your request Mr. Schulz personally sampled the tailings, weighed the lot, (found to be exactly thirty pounds,) remained whilst they were separated, and brought away the headings and a sample of the separated tailings. The packages were numbered as follows:
1st. Package of mill tailings.
2d. Package of machine-separated headings.
3d. Package of machine-separated tailings.
The following is the result of tests made according to instructions:
1st. 44 65-100 per cent. of the mill tailings (package No. 1) passed through sieve, 120 meshes to linear inch, or 14,400 to the square inch; 85 per cent. passed through sieve, 80 meshes, or 6,400 to the square inch.
2d. An acid assay of sample (package No. 1) showed contents to consist of, gangue matter, 58 3-10 per cent.; pyritous matter, 41 7-10 per cent.
3d. Separated headings (package No. 2) weighed 9 7-10 pounds; acid assay of sample resulted in leaving 8 2-10 per cent. gangue; pyritous, 91 8-10 per cent. of mass. 4th. Machine-separated tailings (package No. 3) by acid assay left, of gangue matter, 90 6-10 per cent.; contained pyritous, 9 4-10 per cent.
5th. From the thirty pounds 32 3-10 per cent. of pyritous matter was separated by one operation, leaving 9 3-10 unseparated.
Assay of package No. 1, mill tailings, $6 20 gold; $4 81 silver-Total $11 01.
G. W. BAKER, Editor Herald.
E. E. BURLINGAME,
A. VON SCHULZ,
The mill tailings operated upon as above were from Roderick Dhu ore, of low grade milling quality. The yield is reported to have been about three ounces per cord. At seven tons per cord, the result per ton was $8 57. By the above assay, package No. 1, there was left in the tailings $11 01 per ton, gold and silver. If the tailings represented the whole original tons of ore, it would show that the mill saved considerably less than one-half of the precious metals. There are two considerations, however, which prevents such a calculation. The first is, that a portion of the original ore is carried off in suspension in the water. Hence the number of tons of tailings will not equal the number of tons milled. The second consists in the fact that gold is also thus taken off in suspension by the same water. As there are no means of getting at either of these, as to quantity, no actual statement can be made as to what proportion the mill did save of the whole value. Suffice it for our purpose, here is a waste represented by the number of tons of mill tailings worth, in precious metal, $11 per ton.
The test shows an exceeding fineness of stamping or pulverization. The quantity of perfectly atomic particles of metal would most likely cause a loss in water concentration, using the utmost care and best contrivance, of not less than 40 or 50 per cent. of the pyritous matter. The preparation of ores intended to be concentrated does not permit the making of so large amount of fine particles, hence the separation as made by Krom's separator, under these circumstances, is most astonishing, leaving of this exceedingly minute matter only 9 per cent. of the mass. When we take into consideration the fact that, although this last amount was left, it carried no gold whatever, we get at a just appreciation of the value of this experiment. All the metal containing gold was obtained by the separation. That which was left in the mass had been so completely comminuted that no gold remained associated with it. This is a fact of extraordinary importance. When it is considered that in concentrating sulphurets of silver by water, under the best conditions, a loss of from 35 to 45 per cent. cannot be prevented, the small amount in value of silver shown by assay of package No. 3 dwindles into insignificance.
The assay of package No. 2, machine headings, shows a larger value in gold and silver than the assay of package of No. 1, mill-tailings, would justify by $2 68. How this originates we cannot say. It requires but a small particle of gold to be present in the one case, or absent in the other, to make a large difference in the result, comparatively. This is all that can be said about such discrepancies. The facts as they are must be taken as the only basis obtainable in such matters.
The results may be summed up
1st. The Krom machine separated nearly 92 per cent. of the metal from the mass, and left nothing of value.
2d. The ore operated upon was in a condition that demanded the most extreme perfection in the machine. The result was most surprisingly successful.
This experiment was tried upon tailings; that is, upon material the very lightest portion of which, together with the finest free gold liable to loss, had been already swept away by water. There seems to have
been no test made for quicksilver and amalgam, which would certainly be present, and, by its superior gravity, increase the apparent efficiency of the machine.
With regard to the value of tailings generally, the assays have been made by Messrs. Burlingame and Von Schulz, showing:
Undressed tailings, 45 samples, average value.
Dressed tailings, 38 samples, average value
Experiments with Krom's concentrator were subsequently made upon ore of the second-class (mill-rock) with the following results, as published in the Herald: