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 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 matte ought not to contain more than 0.75 per cent. of lead. The 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. 291 6,820 7,111 1.8 65.2 Labor: Thirty-nine 12-hour shifts, 5 men to each shift=195 days. Charcoal: For warming furnace, bushels. Charcoal: For smelting, bushels.. Labor: Per ton of ore,* days... Charcoal: Per ton of ore,* bushels... Table of third fusion, 1866. ........... ... Labor: Ten 12-hour shifts, 5 men in each shift=50 days. Charcoal: For warming furnace, bushels.. Charcoal: For smelting, bushels.... Labor: Per ton of ore,* days......... Charcoal: Per ton of ore, bushels.. Lead charged per ton of ore, pounds........ 100 1,710 1,810 0.46 16.6 218 * In calculating this it is to be remembered that 28 tons of matte from the previous year were smelted, which must be counted as ore in calculating the expense of charcoal and labor. Labor: 26 days per ton of ore,* days. Wood: 7.52 cords=per ton of ore, cords. Charcoal: 40 bushels per ton of ore, bushels.. Table of cost per ton of ore in units of labor and material. 0.1476 0.0180 0.24 0.69 0.37 To this must be added a small quantity of wood, or refuse charcoal, and labor used in roasting the matte. The above is the cost for ores of the richness above given. With richer ores there is more matte to treat, and the expense of fuel, labor, and lead is therefore greater, and the cost per ton is more; but proportionately richer ores are cheaper to treat than poor. The following table gives the relative cost for various ores, the poorest being taken as unity : The milling ore of Colorado The Lend ore falls under the first class. is worth from $15 to $30 a ton, and comes under the same category. The Colorado "smelting ore" so called is probably mostly in the second and third ranks. Losses. By reference to the above tables it will be found that the following is the loss and gain of the year: * In calculating this it is to be remembered that 28 tons of matte from the previous year were smelted, which must be counted as ore in calculating the expense of char coal and labor. |