tricts. Keeping in view their different gangue and varying contents of lead and silver, they are so mixed in quantities of 1,000 cwt. dry weight, that the average contents in the mixture are: of lead, 58 to 60 per cent., and of silver, 0.1 per cent.; while at the same time the gangues are so proportioued as to furnish an easily fusible slag. Such a quantity of 1,000 cwt. is divided into 20 "charges," at 50 cwt. A "charge" is mixed with 25 cwt. of roasted matte from the first smelting, 40 cwt. of copper-slags from the Lower Hartz, 20 cwt. of matte-slag from the Clausthal works, and 16 cwt. of slag from the first smelting. The last two items vary somewhat, according to whether heavier or lighter gangue is preponderating in the ore. This charge is smelted in both the Kast and the Raschette furnaces. In the latter, however, 10 additional cwt. of slag from the first smelting is sometimes mixed with the charge. As seen in the drawings, the Kast furnace is 20 feet high, has a diameter of 3 feet at the tuyeres, and of 5 feet at the top. There are four water-tuyeres, two being in the back wall and one on each side. The distance from tuyere to tuyere is 21 inches, with the exception of the two nearest the front, which are 42 inches apart. It is, however, proposed to put a fifth tuyere into the front wall. The diameter of the nozzles is 12 inches, the pressure of the blast 10 to 12 lines quicksilver. The average results of a month's working, reduced to 100 cwt. of ore, which consumed 51 cwt. of coke, were: 59 cwt. of lead, containing 14 to 15 quints* of silver per cwt. 78 cwt. of lead-matte, containing 2 to 3 quints of silver per cwt. and 7 to 10 per cent. of lead. Slags with 0.08 quint silver and 0.4 per cent. of lead. The quantity smelted in twenty-four hours was 63 cwt. of ore, or 190.65 cwt. of charge, equal to about 9.5 tons. One pound of coke carries 6 pounds of charge. The charge is spread over the whole surface of the furnace. There is hardly ever any trouble in a campaign, the latter being invariably very long, in fact much longer than campaigns have heretofore been made in lead-smelting. There are no accretions, very little dust, and the proportion of fuel used is very small. The yield of the lead is invariably as high as the assay with black flux and iron made of the ore, and is reported to exceed it sometimes. If I should suggest any improvement at all in the construction of the Kast furnace, in its application to the smelting of western ores, I would propose to. straighten out the corners in the inside of the furnace on both sides of the breast. The object of this is simply to make the parts of the furnace immediately behind these corners more easily accessible for the bar and rabble, as in these places, if anywhere, accretions are most likely to occur. It is not to be expected that accretions can be avoided as easily with our western, undressed ores, as with the clean ores of the Hartz. 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 Bocckstein 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. A 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 duf bereitung. |