In Saxony the furnaces connected with condensers for collecting arsenic are constructed as shown by Figs. 137 and 138. a is the hearth; b, the grate; c, working door; d, firebridge; e, furnace roof; f, opening for charging the ore to be FIG. 138. closed by a clay plate when not in use; g, chimney; h, flue; i, channel conducting to the condensation chambers (k). The flue h is provided with a valve for closing it. The ore is dried upon the roof of the furnace, and roasted in charges of from 12 to 14 cwts. The temperature and the length of the roasting are regulated according to the nature of the ore; for instance, whilst ores poor in arsenic are first roasted at a low temperature, and also for a shorter time (for about six hours), in order to prevent a caking; ores rich in arsenic are at once roasted at a higher temperature, as more white and less grey arsenic are thus formed. The ore must be continually raked, and the fire gradually lessened, till the ore burns dark and commences to become disintegrated. When smoke ceases to be evolved, coal is mixed with the roasting mass, the temperature is increased, the roasting mass properly stirred, and the sulphates and arseniates will then become reduced. Whilst the valve in the flue (h) was before kept open in order to conduct the arsenious acid into the condensation chambers by means of the channel (i), is now closed to prevent any contamination by carbonaceous particles; gases and fume then enter the chimney (g). it the channel is frequently more than 100 feet long, it is provided with several angles to lessen the draught. A charge of 12 cwts. of schlich rich in arsenic, and containing 2 cwts. of tin, requires from 22 to 24 hours' time for roasting, with a consumption of about 144 cubic feet of wood and a production of 5 or 6 cwts. of white arsenic. In Cornwall* furnaces with elliptical hearths of different dimensions are frequently used; for instance, hearths 7 feet long, 4 feet broad at the fire-bridge and 18 inches at the working door, and 5 feet in the middle part. The arched roof is 8 inches above the fire-bridge, 16 inches above the middle of the hearth, and 10 inches above its sides. The grate is 10 inches broad, and lies II inches below the fire-bridge. The charges consist of half a ton of ore or more, which is also dried on the furnace roof, and then spread out on half of the dark red-hot hearth near to the fire-bridge. It is raked every 20 or 30 minutes, and three heaps are thus formed across the hearth; sometimes also a heap is formed alternately length-ways along the hearth. The arsenious acid collects in a channel 2 metres high, 25 metres wide, and several hundred metres long, which is provided with partition walls and leads into a chimney. The channel is cleared every one or two months. The ores are either once roasted and then washed, or are roasted again after washing, according to whether they contain only iron and copper pyrites or arsenical pyrites at the same time. In the latter case, the first roasting takes 12 hours, consuming 260 lbs. of coal, and the second from 8 to 10 hours, consuming about 200 lbs. of coal. According to Dr. Ure's Dictionary of Arts, iii., 910, the machine for calcining tin ore, originally called "Brunton's Patent Calciner," is gradually coming into use in Cornwall, and is adopted in many of the larger smelting works. Its operation may be thus briefly described:-A revolving circular table, usually 8 or 10 feet in diameter, turned by a water wheel, receives through the hopper the tin stuff to be roasted or calcined. The frame of the table is made of cast.iron, with bands or rings of wrought iron, on which rest the fire-bricks B. u h. Ztg., 1859, p. 319. Preuss. Zeitschr., ix., 253. composing the surface of the table. The flames from each of the two fire-places pass over the ore as it lies on the table, which revolves slowly, about once in every quarter of an hour. In the top of the dome over the table are fixed three castiron frames, called the spider, from which depend numerous iron coulters, or teeth, which stir up the tin stuff as it is carried round under them. The coulters on one of the arms of the spider are fixed obliquely so as to turn the ore downwards from one to the other, the last one at the circumference of the table projecting the ore (by this time fully calcined) over the edge into one of the two wrinkles beneath. A simple apparatus called the butterfly, moved by a handle outside the building, diverts the stream of roasted tin stuff as it falls from the table into either one or the other as may be required. Unlike the operations of roasting in the oven previously described, the calciner requires little or no attention; the only care requisite is that the hopper should be fully supplied, and the roasted ore removed from the wrinkles when necessary. The burning house and the calciner are represented by Fig. 139. On page 912 in vol. iii. of Ure's Dictionary will be found the following notes on the action of Brunton's calciners employed at Fabrica la Constanto, Spain, and also the additional suggestions: Diameter of revolving bed, 14 feet. Revolution of bed per hour from three to four, or about I foot of the circumference per minute. Ores introduced by the hopper, at the rate of 1 quintal' to every revolution of table. Quantity of ore calcined per day of 10 hours, 30 to 35 quintals. Salt consumed, generally 6 per cent of the weight of the ore. Fuel consumed per 10 hours, 1200 to 1400 lbs. of pine wood. Power employed to revolve table, half horse. * I Spanish quintal = 46.01 kils. I cwt. (English) = 50.8 kils. FIG. 139. Remarks.-The furnace is charged with ore and salt by means of iron hoppers placed immediately over the centre of each of the hearths. A heap of about 14 quintals of ore, with five or six per cent of salt, is prepared for the supply of each hopper, from time to time, upon a small platform on the top of the furnaces, and a few shovelfuls thrown in occasionally as required, taking care, however, always to have enough ore in the hopper to prevent the ascent of acid vapours, &c., from the furnace. The time the mineral remains in the furnace, and the quantity calcined per hour, must depend on the rapidity of motion of the revolving hearth and the angle at which the iron stirrers are fixed. The average amount passed through each furnace in 24 hours is 84 quintals, or 3 quintals per hour; nearly I quintal is discharged from the furnace at every revolution of the bed. This apparatus effects a considerable saving in labour, and allows a good roasting, as on falling upon the middle of the hearth, the ore gradually rises to a higher temperature, and the continual raking causes the formation of less caked particles, and less ore dust is carried away by the gases. Whilst at Altenberg the cost for roasting 1 ton of black tin is £2 5s. 2d., the cost when using a furnace with a revolving hearth is only £1 6s. 4d., namely, 19s. 4d. for coal, and 7s. for labour. The salt which is sometimes added to the roasting mass, serves to volatilise sulphur, arsenic, &c., and to form soluble chloride of copper. d. Washing the Roasted Schlich.-In order to separate some of the oxides and metallic salts formed by the roasting process, the roasted mass is sometimes washed direct, as in Saxony, which concentrates the ore till it contains from 50 to 70 per cent of tin; the resulting schlich is then sometimes smelted as it is, and sometimes previously submitted to a treatment with acids; if the roasted mass contains a larger amount of copper, it is moistened with water or sulphuric acid, exposed for some time to the atmosphere, the caked particles sifted off, and the fine mass mixed in a tub with dilute muriatic or sulphuric acid; the clear cupriferous liquid is then treated in closed vessels with iron for the precipitation of the copper, and the residue is carefully washed.† The resulting schlich (black tin) must sometimes be roasted and washed again in order to concentrate it to a sufficient richness (50 to 75 per cent of tin and upwards). * B. u. h. Ztg., 1844, p. 656; 1862, p. 295. B. u. h. Ztg., 1859, p. 207; 1862, p. 146. Preuss. Ztschr., ix., 254. |