CHAPTER XI.

ECONOMICAL RESULTS IN THE TREATMENT OF GOLD AND SILVER ORES BY FUSION.

This chapter was written, at my request, by John A. Church, E. M., of New York City, a metallurgist of much intelligence, to whom I am obliged for the permission to insert here what I think is a very useful and suggestive essay. He desires to make acknowledgment for the information contained in the paper to Dr. Leo Turner, formerly director of the works described, and now at Brixlegg in the Tyrol.

At a time when the treatment of gold and silver ores by fusion, in opposition to the mill-process, is attracting so much attention in this country, it may be useful to consider what is done in a well-conducted foreign works. For this purpose I will ask the reader to accompany me to Lend, in Austria, a small but thoroughly organized establishment. It is situated in the Salzburg Alps, and receives its ore from mines at Rauris and Boeckstein. The former, lying 8,200 feet above the sea, is said to be the highest mine in Europe, some of its openings being made in glacier ice. It was worked by the ancients, who have left the contracted and tortuous workings peculiar to them.

The ore differs in no way, unless in extreme poverty, from countless mines in the West. It consists of gneiss, quartz, and clay-slate, containing the sulphurets of iron, copper, lead, zinc, and antimony, besides arsenical pyrites, gold, and silver. The gold is found in two conditions, free gold and gold alloyed with silver. This alloy for the year 1866 was composed, on the average, of 15.33 gold and. 84.67 silver, which gives a specific gravity of 11.28. Mercury has a specific gravity of 13.6, and as the amalgamation of gold by the Austrian method is looked upon as a proceeding entirely mechanical, the separation being effected solely by the superior gravity of gold over mercury, this alloy, which is lighter than mercury, cannot be amalgamated.* Such is the lesson of long practice, the free or fine gold being extracted from a part of the ore, at least, by amalgamation, while the tailings are smelted to obtain the alloy. The following table will show the proportion of fine to alloyed gold, and also exhibit the extreme poverty of the ore. To the Rauris and Boeckstein ores I have added those from Zell in the same part of the Alps. The ore from this place is not now worked, the point of poverty having apparently been reached at which the auriferous rock ceases to be an ore.

As in 1866 Boeckstein delivered 63 per cent. of the ore, and Rauris 37 per cent., the average value per ton for the year was $10.16,* or 0.004 per cent. gold, and 0.034 per cent. of silver. This does not include the value of the copper and lead, which form, respectively, 2 and 1 per cent, of the ore. The former is extracted; the latter is not sufficient to supply the waste of the process, and lead has to be bought for the works. Even in Europe these ores are considered extremely poor. I am not aware that ores from veins so poor as these have ever been worked in America, but if they have they must have owed their value to the fact that the gold was all fine, and could be amalgamated.

TREATMENT OF THE ORE.

The ore is first sorted to six varieties for the furnace, and one for amalgamation. The former comprise quartzose ore, rich, medium, and poor, compact pyrites, galena and antimonial ore. The ore sent to amalgamation is the poorest kind of pyritiferous rock. It contains merely traces of pyrites, and is amalgamated, because in that process it · undergoes concentration.

Amalgamation.-The ore for amalgamation is crushed under stamps of 220 pounds weight, (total,) through sieves of 1.5 millimeters, (0.06 inch,) the battery-box having a sieve on each side to secure the most rapid discharge of the slime. Two methods of treatment are employed for the slime: first, it is first concentrated, and then amalgamated, or, second, it is first amalgamated, and then concentrated. With ore that contains much pyrites the former is best; with ores very poor in pyrites, the latter.

Amalgamation takes place in pans, there called "mills." They are 24 inches in diameter at the top, 16 inches at the bottom, and 9 inches high, and made of cast iron, one-eighth to three-sixteenths inches thick. They are not directly conical, but the side forms a step 3 inches wide. In this pan mercury is poured an inch deep, and a wooden block shaped like the pan, and 1 to 1 inches less in diameter, is suspended over it. The upper part of this block is hollowed out like a hopper, with its discharge in the center, and the under side has small pieces of sheet iron placed radially in it, and which just clear the mercury. When this block is revolved, and a stream run into the hopper-like depression on its upper surface, the slime is carried over the mercury from the center to the circumference of the pan, the whole apparatus acting like a "centrifugal" pump. This is the Austrian gold-mill so often described. Great care is taken to prevent too rapid a motion of the stream, which would not allow the gold time to settle and would carry off the mercury. Twelve to thirty-two revolutions a minute is the speed given, depending upon the fineness of the ore, thickness of the slime, and amount of gold present. These mills extract by one operation 75 per cent. of the fine gold, and 15 per cent. more by repeating the process. Each mill passes about one ton of ore in twenty-four hours. Compared with blankets this system does not appear to present any advantages in the first handling of the ores; but I should think the Austrian mill might be substituted with gain in the place of many other amalgamating arrangements now used after the blankets. Compared with the Colorado methods these mills

* Unscientific as the method is, I feel compelled to give these values in American coin, since that is the only expression known to the workers in our mines.

+ Á collection exhibiting these ores, and a full suite of furnace-products, can be seen at the School of Mines, of Columbia College, New York.

‡ Rittinger's is the best account. See his Aufbereitung.

extract 20 per cent. more* than the Colorado amalgamators, though this yield necessarily depends upon the proportion of silver in the gold. They require little watching, except when used immediately after the stamps, when the accumulation of gold might require their cleaning up every two or three days.

Smelting.-For four years the ores delivered for fusion were in the following proportions:

About 66 per cent. of the smelting ore has, therefore, been amalgamated.

From 70 to 75 per cent. of the ore is worthless rock, and this must be removed before adding lead, which would suffer serious loss if charged with so much quartz. The operations are, therefore, as follows:

1. Fusion for raw matte.

2. Roasting of raw matte in stalls.

3. Fusion (without lead) for a more concentrated matte.

4. Roasting of second matte in stalls.

5. Fusion with lead.

6. Cupellation of rich lead.

The first fusion.-Eleven years' experience has proved that the most efficient slag is one approaching the composition of a bi-silicate. The following is an average analysis:

Each year the matte resulting from the previous year's fusion with lead is roasted, analyzed to ascertain the amount of oxide of iron present, and charged in the first fusion as a flux for the quartz; or, if containing above 35 per cent. of copper, it is treated for copper.

The furnace is not new, and contains none of the late improvements, but it does good service. Its dimensions are as follows:

* See Mr. Hague's Report on Mining Industry of the Fortieth Parallel.

From 100 to 120 bushels of charcoal are required to warm the furnace, and then regular charges of 5 cubic feet, or 3 bushels, are made. In "blowing in," the quantity of mixed ore and flux added to this charge of coal is, at first, 56 pounds; then 112 pounds; and when the furnace is thoroughly hot the full charge of 203 pounds, which is the constant burden, to 3 bushels of charcoal. This is usually reached in the first twenty-four hours. Four hours after the first charge of ore and flux, the blast is turned on at first with a pressure of one-third inch and then onehalf inch of mercury; or one-sixth and one-quarter of a pound to the square inch. After eight hours the slag begins to flow. The furnace is, of course, worked with a black throat.

The first matte forms 40 to 45 per cent. of the charge, the difference between this proportion and the 25 to 30 per cent. afforded by the ore being made up by roasted matte from the previous year. Its average composition is

It contains 30 to 40 ounces, troy, of auriferous silver to the ton of 2,000 pounds; or, in American valuation, $100 to $150 in coin. From the fact that the ore is unroasted and the metals are so well "covered" by sulphur, the loss amounts to only 0.25 of 1 per cent. About 38 bushels of charcoal are used to the ton of charge, and 9.75 tons are smelted in twenty-four hours.

The second fusion.-The first matte is roasted three times in stalls containing 28 tons, the roasting not being thorough, but carried only so far as to leave about 40 per cent. of unroasted matte. It is then resmelted with quartz and siliceous slag; and to avoid the use of too much of the flux, a basic slag is made containing about 22 per cent. of silica. This requires very great care in managing the furnace, for the least irregularity of working causes the formation of sows. To secure proper working, whenever the furnace is tapped the hearth is examined by means of a bent bar. If lumps are felt, the front wall is broken out and they are removed; if the sole is slippery, the presence of reduced iron is indicated. A rough, hard, even sole is the proper one.

The pressure of blast is now reduced to one-sixth of an inch, or onetwelfth of a pound to the square inch; the hearth is made 10 to 12 inches larger in diameter than before, and the charge is increased to 222 pounds to 3 bushels of charcoal. These changes have for their object not only the prevention of iron sows, but also of speiss, a compound of arsenic with all the other metals present, and very difficult to utilize. The same precautions are used in blowing in as before. About 30 bushels of charcoal are used to the ton of ore and flux, and 13.5 tons are smelted in twenty-four hours. The second matte contains 52.67 ounces of auriferous silver to the ton, and is worth about $200.

Fusion with lead.-The second matte is roasted as before, but now 50 to 60 per cent. of unroasted matte is left. A stronger roasting would so enrich it that two fusions with lead, instead of one, would be necessary. The slag is again basic, and to keep the heat as low as possible,

the pressure of blast is reduced to one and a half lines of mercury, while the charge is increased to 277 pounds of matte and flux to 4 bushels of charcoal. In order to keep the lead in contact with the matte as long as possible, as well as to decrease the heat, the crucible is made a foot deeper than before. The new slag has an average composition of

Silica..

Oxide of iron.

Lime...
Magnesia.

Alumina

27.45 56.52

10. 19

3. 48

1.25

The loss will not exceed 2.5 per cent. of the lead. When the hearth is full of melted matte it is tapped, the products running into a basin where they are well stirred with poles. The matte is then partially taken off, the lead remaining until 600 to 700 pounds have collected.

For a perfect extraction of the silver it is necessary to charge 120 to 130 pounds of lead for each pound of silver and gold. With this proportion 75 per cent. of these metals is extracted in one operation, and The the matte ought not to contain more than 0.75 per cent. of lead. extraction of 75 per cent. of auriferous silver, means that more than 90 per cent. of the gold and 73 per cent. of the silver have been obtained. A second operation removes so much more that, including amalgamation where the loss is very great, more than 90 per cent. of the silver and 96 per cent. of the gold is obtained. This second operation takes place only when the matte is worked for copper. At other times the gold and silver are obtained by charging the matte back in the first operation. The absolute loss in smelting is but 0.10 of one per cent. From 14 to 16 tons of matte and flux are smelted in twentyfour hours. A certain amount of lead-matte is obtained, and is charged back in the same operation. If the third matte is rich enough it now undergoes a second fusion with lead, but usually it is so poor that it is treated at once for copper. If, however, it contains less than 35 per cent. of copper, it is roasted and returned as a flux to the first fusion for raw matte. At Lend the conditions are such that this takes place every other year, copper being made one year, and only matte the next. Cupellation is performed in a German furnace, with movable hood, made very low so that the heat from the fuel is thoroughly utilized. Inasmuch as none of the side products are sold, and there is no need of having them in great purity, there is, beside the fire-bridge, only one opening to the hearth, through which abzug, abstrich, litharge, and smoke, alike escape. From 6,000 to 7,000 pounds of lead are charged at once, and more is gradually added until about 21,000 pounds (the entire make of a year) have been melted. The blast is slow, and the litharge consequently flows rather cold. Refining follows the brightening of the silver, and metal of .985 to .995 is produced. Usually the loss of lead falls between 4 and 6 per cent., while that of silver and gold seldom reaches 0.10 of 1 per cent. About 3 tons are cupelled in twentyfour hours.

TABLES OF THE OPERATIONS.

The following tables will give at a glance all the foregoing particulars, and also exhibit the amount of material handled. The two fusions without lead are combined in one table.