« ForrigeFortsett »
cent. of sulphur, because with poorer ores the unavoidable retention of sulphur in the dross would render the percentage actually saved proportionately too small, and the process would be unprofitable. Rich ores are treated in cast-iron kettles, of not over two cubic meters contents, which are heated by means of a separate fire-place. The heat maintained is over 1110 Centigrade, and must not rise over 150° Centigrade. The nearer the temperature can be kept to the melting-point of sulphur, (1090 Centigrade 228° Fahrenheit,) the better is the result, because at such a temperature sulphur is most liquid and does not burn. The kettles are filled with ore, which is melted down, and occasional additions of raw ore are made, until the kettle is filled with the liquid mass. Meanwhile all the earthy parts which can be reached are taken out with perforated iron ladles. After the kettle is full, the mass is permitted to settle for a short time. The scum on top is then taken off, and the clear sulphur cast into molds, until the sediment at the bottom of the kettle is reached. A new quantity of ore is then introduced, and the process is repeated. After several operations the sediment is taken out of the bottom of the kettle, and either thrown aside or used with poorer ores in pits or furnaces.
b. Eliquation of sulphur in furnaces or pits.-Formerly the sulphur was extracted from the ores of Sicily by means of shaft-furnaces, not over 4 to 5 feet high and 7 to 15 feet wide. They had an inclined bottom, at the lowest point of which a canal communicated with the outside. The largest pieces of ore were put on a bench on the inside of the furnace, near the bottom, and upon these as a base an arch was built, a small hole only being left in the center. Upon this arch smaller ore was thrown, until a small pyramid was formed protruding above the furnacewalls. This was finally covered with fine ore, upon which straw was thrown and ignited. The fire communicated to the sulphur and traveled from the outside toward the inside. After eight or ten hours the liquid sulphur had collected at the bottom, and was tapped into moistened molds or into water. This process furnished only from 40 to 50 per cent. of the sulphur in the ore, and is now nearly everywhere aban doned.
At present pits, or rather stalls, called calcaroni, are almost univer sally used in Sicily, Spain, and elsewhere; the yield in these being, according to Professor B. Kerl, 67 per cent. of the sulphur in the ore.
Mr. H. Sewell, who has had considerable experience with sulphur-ores in Spain, describes this method in the Engineering and Mining Journal as follows:
The governing principle in this method is the working of large masses of ore at low cost. Each calcarone works up per month from 800 to 1,000 tons of ore, the apparatus being constructed of common stone and plaster, and costing $300 apiece. No fuel is required, as one-seventh of the ore is used as combustible for reducing the rest; so that if the ore contained 23 per cent. of sulphur, 20 per cent. net would be produced.* The dimensions of a calcarone differ much, according to the percentage of the ores; that is, the poorer the ore, the larger must be the furnace. When I commenced to use them in Spain, I found that stalls about 15 feet in diameter were the most successfully managed by workmen not versed in the process; but I found, also, that for economy, and a greater production in the liquation, a larger diameter, say 33 feet, gave the best results, and this is the size of the stall in the accompanying drawings.
The height at X, on the front or tapping door, varies from 6 to 8 and 12 feet, (though seldom the latter,) and that at L, the aperture for loading, is about 4 feet. At XX, also, in the ground and vertical plan, an aperture reaching from the bottom to the top of the stall exists. This is also used for loading; but after that operation
*This yield, as claimed by Mr. Sewell, is very high, and at variance with the statements of other authorities. Mr. Sewell seems to allow nothing for the sulphur retained in the ore after treating it.
has been concluded, the aperture is closed with a cast of plaster of Paris, (or pieces put together,) the thickness being only 2 inches. This thin door is built up new every time, and destroyed for discharging. It is used as a pyrometer, the heat easily piercing it, and indicates to the
smelter how far the sulphur has sweated down. The ore is placed in large bowlders, just as it comes out of the mines, from the middle to the bottom of the furnace, which has a declivity of about 15 to 20 degrees, such being necessary at the end of the operation, in order that the last remnants of melted sulphur shall run toward the tapping-door at point M. In loading the stall, all the smaller-sized ore is reserved for filling near the top, where it is piled into the shape of a cone, as at F F F; and chimneys are left at points DD D, about 2 feet deep. These
are filled with brush-wood, and in this way the ore is made to ignite. These hollows are left while piling the ore and building the cone. The object of placing the small ore at the top is simply to prevent any of the earth and sifted stone from falling through the large crevices that would be left, ff large bowlders were placed at the top. The earth and sifted stone or gravel play an important part in the manipulation. At K K K K we have, immediately in contact with the small ore, a stratum of about 6 inches of small sifted gravel, about the size of a nut, and on this again, at NN NN, we have a coating of earth; this is to make the interior of the stall as impermeable as possible to z the oxidizing action of the air, and this coating is increased or decreased according to the amount of heat required, which in turn depends on the strength of the winds and their direction. The brushwood ignited, the ore commences to burn, and the chimneys are kept open for about twenty hours, at the end of which period the ore has ignited all over the surface of the heap, and to the depth of, say, some 15 or 20 inches. The chimneys are then all closed as follows: bricks are placed over them, as at point P; and should the burning of the ore be too rapid, earth is then thrown over the bricks; but these chimneys are opened toward the middle of the operation, to increase the heat, and closed again, according to circumstances, to decrease it. After forty-eight hours, the melted sulphur begins to trickle down through the interstices of the stone, and congeals, forming, as it were, a conglomerate with the ore; the heat also travels downwards, and so we have remelting and congealing consecutively, till the sulphur arrives at the bottom of the furnace, forming a massive conglomerate of sulphur and ore; for it fills the interstices up to the point where the first tap-hole is drilled, through the thin door of plaster at point Z. The lines across the stall denote the lines or levels of tapping; and this commences naturally at Z, and so downwards, as the congealed sulphur is remelted with the descending heat. Every twenty-four hours a fresh tapping takes place, the former holes are plastered up, and a new hole drilled lower down, and so on till we get down to the lowest point or bottom of the furnace. At the end of the operation, that is, during the last three days, nearly all the chimneys are left open, so that the air shall descend to the lower part of the furnace, and aid the combustion of the ore. The jet of sulphur is received into wooden molds, as at point B. These have been soaked in water, to prevent the sulphur sticking to the wood, and are shaped wheelbarrow fashion, in order that the block of sulphur may easily fall out, without breaking. During the carrying away of a mold that has been filled, and the bringing of an empty one to be filled, the jet runs into a reservoir made for the purpose at A. Öne of the principal reasons for placing large blocks of ore, from the middle of the furnace downwards, is to leave sufficient interstices for receiving the sulphur, otherwise the first tap-hole would be too high, and near the ignited ore, thus setting fire to the stream of sulphur.
Two of the principal things to be guarded against are overheating the apparatus, and, on the other hand, carrying on the process so slowly, by the complete closing of the chimneys, that the operation would take two months instead of four weeks from the commencement. In the former case, instead of the sulphur congealing between
H. Ex. 211-29
the interstices, it would all be in a melted state from the top tap-hole to the bottom, thus not only consuming an unnecessary amount of sulphur in keeping up the heat, but likewise giving, by overheating, a bad chocolate-brown color to the sulphur. This quality would hardly be salable, even for sulphuric acid. Many stalls or heaps, say ten, after having been loaded, can be attended by two men, one in the daytime, the other at night. As soon as the operation is over, which takes about a month, both apertures are opened, to allow a current of air to pass through the apparatus. Otherwise it would not cool for a month; but by this precaution it can be discharged in a few days.
A modification of this process is the following: Before loading the stall, a number of iron bars are set obliquely from the inclined bottom, against the front wall, in which a single tap-hole is located at the lowest point of the pit. To force the liquid sulphur to run to this point, the bottom of the furnace is inclined, from both sides, toward a central line, and from the back toward the front, thus making a sort of trough, dipping forward. These bars form a complete grate, the space underneath remaining empty when the stall is filled with ore. The cone above the walls of the stall is, in this case, made much higher than in the method described above. The smelted sulphur collects continually on the clean bottom beneath the grate, and is from time to time tapped into wet molds, or into a basin with water.
The crude sulphur obtained by any of the above methods must of course be refined, if intended for other use than that of its manufacture into sulphuric acid. But as it is not likely that refined sulphur can be profitably made in our western districts for years to come, I omit treating the subject in this report.