chiefly oxide of iron. As acid fluxes, slags and sometimes quartz are used; as basic fluxes, refinery cinders, iron ore, roasted matt, &c. The ferruginous substances not only react in the formation of slags, but also in the extraction of lead. Protoxide of iron, as a strong base, expels the oxide of lead from its silicate, which may then be reduced by coal; also one part of the protoxide of iron is always reduced to metallic iron, and has a desulphurising action upon the undecomposed sulphide of lead, and a reducing action upon silicate and oxide of lead; and as no means are given to control this reduction of oxide of iron, the product is apt to be injured, and deposits of iron formed in the hearth of the furnace. This formation is advantageous towards the end of the operation, as it proves that there was sufficient iron in the mixture, which is essential for the yield of lead and silver (Joachimsthal). The reduction of refinery cinders is still less subject to control than the reduction of rich iron ore, as the refinery cinders are difficult to reduce, and nearly all enter the hearth without being decomposed; and as the processes are uncertain with both the substances, metallic iron* is employed in many smelting works, as it not only decomposes sulphide of lead but also reduces oxide of lead. At the same time the sulphide of iron acts upon the silicate of lead, forming metallic lead, sulphurous acid, and silicate of iron. Sulphides of barium and calcium, formed by the reduction of the respective sulphates, act in a similar manner, and sulphide of barium is sometimes added to the mixture in order to produce such a reaction (Vialas). If iron ore and metallic iron are expensive, the fuel may act as a substitute, either in part or wholly; but enough oxidised iron must be contained in the mixture to form an easily fusible slag; and for this purpose the roasting is carried out as completely as possible, and the sulphates formed are chiefly decomposed by silica at a high temperature; the mixture is then combined with coal or coke and slowly smelted in low furnaces, the upper mouth part of them being * B. u. h. Ztg., 1857, p. 67. flaming (Vialas, Spain). Compared with smelting with iron, this process causes a great loss of metal, a large consumption of fuel, and a small production. The chief advantage of the roasting process over the common precipitation process is the non-formation of matt; but the formation of matt is necessary when a loss of silver is to be expected, or when the mixture contains a larger amount of copper (Müsen, Lower Hartz); or again, when for these reasons the roasting must not be carried on so far, and when the roasting process depends on the smelting in the cupola furnaces. Small quantities of matt are dissolved in the slags, and large quantities are allowed to remain in them; thus the slags contain more or less silver (Przibram). If a small quantity of matt is separated, and if it is poor in lead and silver, it is treated like the ores without much cost and loss of metal; but if a greater quantity is obtained, and it is rich in copper, lead, and silver, more elaborate processes are required, causing great expense and loss of metal (Lower Hartz, Müsen, Freiberg). If the roasting process is incompletely carried out with ores containing antimony and arsenic, a formation of speiss takes place in the smelting process, and its working up causes loss of metal. Products rich in lead are frequently added to the mixture, partly in lumps and partly in the form of powder; to avoid a loss of metal the powder should be heated until it cakes, or mixed with lime, as before directed, and worked up separately when containing much lead. The presence of zinc blende in the mixture causes most of the modifications of the smelting process. Low cupola furnaces are best for smelting roasted ores, as they cause a less formation of iron deposits in the hearth, and of metallic soot when smelting ores associated with zinc, thus allowing longer operations. If these inducements do not exist, the application of higher furnaces is advisable, as effecting a saving of fuel and less loss of metal by volatilisation. These furnaces must always be provided with a chamber for condensing the unavoidable smoke. On account of the basic and ferruginous mixture which facilitates the formation of deposits, the furnaces used are mostly constructed like sump furnaces; in rare cases only channel furnaces are used, as in the smelting works on the Rhone, whilst crucible furnaces (Commern, Stolberg, Przibram) are much used. In the latter case, the lower part of the front wall is furnished with a door suspended on hinges, and having its inner side covered with clay; these furnaces allow a better separation of the matt and lead, as it takes place at a higher temperature in the interior of the furnace. For fuel, charcoal, coke, or a mixture of both are employed, the last giving the highest yield of metal; coke produces a high heat in a small space before the tuyere, and charcoal a lower heat in a greater space. When the process is going on properly, less reduction of oxide of lead and volatilisation of lead and silver will take place in the upper part of the furnace, if the smelting masses be less porous or pulverised; on the other hand, a reduction of the oxide of iron now begins if the furnace is not too low, and the grain of the ferruginous flux not too coarse. The masses become softer and gradually fuse as they sink into the furnace. Part of the oxide of lead is reduced by coal and iron, sulphates are decomposed by the fused silicates, and the oxidised lead is extracted from the latter by oxidised iron, which allows the reduction of oxide of lead by coal; undecomposed sulphide of lead, and that reduced from the sulphates, get decomposed by metallic iron; and the sulphide of iron thus formed has a partially decomposing action upon silicate of lead, or forms a matt, if present in larger quantity, together with other sulphides, whilst a fusible slag, poor in lead, is formed at the same time. The action of the coal is stated on page 118. If the ores contain zinc, the previously described reactions take place; the process is effected by a nose from 8 to 12 inches long, and the mouth of the furnace must be kept free from flame. The results of the process are judged satisfactory or not according to the products obtained, which are as follows: 1. Raw Lead, usually more impure and more brittle than that obtained by the precipitation process; it contains an amount of Sb, As, Zn, Cu, Pb2S, Pb,S, &c. It is submitted to Pattinson's process for the extraction of its silver, and, if necessary, previously refined. 2. Lead Matt with varying quantities of Pb, Cu, and Ag. If the amount of these metals is small, it is treated like ore, otherwise the treatment followed is that stated on page 96. 3. Speiss, usually of the composition x(Fe, Ni, Co), As+ y(Fe2S, Cu2S, Pb S) + RS (As, S, Sb S1); it is sometimes worked in order to extract the Pb, Ag, Cu, Ni, and Co contained. 4. Slags of basic nature; the pure slags are thrown aside, the impure ones re-worked. 5. Metallic Smoke, from roasting and smelting, is either mixed up with the smelting mass, or burned, together with from 15 to 20 per cent quartz, in reverberatory furnaces. In this operation part of the lead may be extracted by mixing the smoke with coal. If the ores contain blende, the smoke contains oxide and sulphate of zinc, and is rich in silver. 6. Furnace Ends; these, if carefully worked up, lessen the loss of metal. Illustrations of Smelting Ores Poor in Lead and Associated with Pyrites and Blende. In the three smelting works near Goslar (Lower Hartz),* the lead orest of the Rammelsberg are worked; besides galena they contain a great deal of iron pyrites, some copper pyrites, zinc blende, arsenical pyrites, antimonial ores, baryta, calc spar, quartz, argillaceous clay, &c., so that the average amount of lead in the roasted ores does not exceed from 4 to 10 per cent. LAMPADIUS, Hüttenkunde, ii. Thl., Bd. p. 110; B. u. h. Ztg., 1853, p. 6; 1854, p. 1, 97; 1859, p. 37; 1862, p. 105. JARS, metallurgische Reise, iii. 404. RUSSEGGERS' Reisen, iv., 675. KERL, der Communion Unterharz. Freiberg, 1853. KERL, die Rammelsberger Hüttenprocesse, Clausthal, 1861. B. u. h. Ztg., 1853, p. 6; 1860, p. 14, 439; 1861, p. 391. COTTA, Erzlagerstätten, ii. 103. v. DECHEN, Statists d. zollv. u. nördl. Deutchlands, i. 754. The roasting of the ores is effected as before described. Lead ores associated with much blende (brown ores) are washed with water after the first roasting, and the solution is used for the production of sulphate of zinc. The roasted ores contain from 4 to 10 lbs. of lead per cwt., th ounce of silver, and 1 lb. of gold in about 1,850 tons. As these ores contain a large amount of foreign oxides, chiefly peroxide of iron, a great quantity of siliceous flux is required, and schlich slags of the Upper Hartz are used for this purpose. One mixture for the smelting process consists of35 cwts. of ore. ΙΟ to 99 schlich slags of the Upper Hartz. besides some cwts. of roasted matt, scum, soot, &c., according to necessity; the amount of lead yielded by such mixture is 4 cwts. The furnaces in use are represented by Figs. 42, 43, and 44; they differ from other furnaces of this kind in the singular FIG. 43. FIG. 42. |