Page:The American Cyclopædia (1879) Volume X.djvu/265

LEAD 259 formed on the sulphide of lead. The four products of lead smelting, viz., lead, speise, regulus, and slag, separate on standing accord- ing to their relative specific gravities, in the order named. The shaft furnaces used for lead smelting have been until comparatively a late period of small trapezoidal section, nar- rowing from the hearth to the throat, and blown with one or two tuyeres. In modern practice the furnace is generally circular in section, widening toward the mouth, and blown with eight or ten tuyeres. A marked improve- ment in smelting has been the result of this change in furnace construction. The yield and purity of product have been increased, and the consumption of fuel and loss by vola- tilization decreased. The Raschette furnace, of rectangular section and expanding walls, was introduced into the Hartz for lead smelt- ing, with results much more favorable than the old style of furnace, but inferior to those ob- tained in the Pilz furnace, which differs from the Raschette chiefly in its circular section. A modified Pilz furnace has been introduced for silver smelting in Nevada, by Mr. Arendts of the Eureka consolidated works, with good re- sults. A vertical section through the fore hearth and horizontal section through the tu- yeres are seen in the accompanying figures. The hearth of the furnace is 3 ft. wide and 4$- ft. deep ; the breast is open ; the tuyeres, 12 in number, are on the sides and back of the fur- nace. The height of the furnace may be from 8 to 20 ft. above the tuyeres, according to the nature of the ore to be treated. Ordinarily the height is 10 ft. from tuyeres to charging opening. Mr. Arendts has also devised an automatic tap for the lead, consisting of a three-inch wrought- iron pipe in the furnace walls, one end com- municating with the lowest part of the hearth inside the furnace, and the other emptying into a basin on the outside. It is said that by the use of this tap the furnace runs more regularly, the lead obtained is purer, iron sows are pre- vented, and the work of the smelters is light- ened. (For fuller details in regard to smelting argentiferous lead in the western states, see the reports of R. "W. Raymond, United States com- missioner of mining statistics, and vols. i. and ii. of the " Transactions of the American Institute of Mining Engineers.") Owing to the volatility of lead and its compounds, and to the strong draught of the furnaces used for roasting and smelting the ore, there is considerable loss of metal, as is abundantly shown by the white fumes in which lead works are often enveloped. The interception of this lead fume is of great importance both in an economical and sanitary point of view. The methods devised for this purpose may be enumerated as follows: 1, long horizontal flues; 2, condensation cham- bers ; 3, exposure to artificial rain ; 4, forcing through water; 5, intermixture with steam and its subsequent condensation. Experience seems to have proved that the most simple and effective method is the long horizontal flue, terminating in a chimney sufficient to give the requisite draught. In the north of England there are flues nearly a mile in length. But this method is not always applicable, owing to unfavorable situation of the works. The use of steam, although theoretically the most promis- ing of all, has not been found to give good results. The composition of the fume differs with the ores used, and also with the distance from the furnace, where the flue system is employed. It is mainly sulphate of lead mixed with a small amount of the earthy and metallic ingredients of the ore, and varies in color from gray to white. The loss of lead in the fume, when condensing arrangements are not used, has never been accurately determined. Fallize estimates it on an average as 10 per cent, of the assay value of the ore in lead, which is doubtless too high. The amount of lead ob- tained from the fume in proportion to that obtained from the ore and slag, at the Keld Head smelting works in England, for one year's work, was 7'63 to 100. The character of the furnace lead depends on the nature of the ores from which it is produced. It is very frequent- ly contaminated with other metals, notably antimony and copper, to a degree to make it unfit for use in the arts. It must then undergo a process of purification. This process, called indifferently refining, softening, improving, or calcining, is effected by melting the lead in the hearth of a reverberatory furnace, or in a large shallow cast-iron pan in the place of the hearth, and exposing it at a dull red heat to free access of the atmospheric air, or to the action of a blast of air. The foreign metals pres- ent oxidize and rise to the surface in the form of a scum or dross, which also contains a large amount of oxide of lead. This is skimmed off from time to time, and a fresh surface of lead exposed. The operation is continued until a test sample shows that the lead has acquired the proper degree of softness. The length of time required for this process depends on the amount of impurities present and on the extent of surface exposed. Three or more days are often required where the amount of antimony is considerable ; and if present in very large amount, the lead may be completely oxidized before it is softened. The dross or scum, con- sisting mainly of oxides of lead and antimony, is reduced by coal and a second hard lead ob- tained, which is much richer in antimony than the first. If this cannot be profitably softened, it is sold as hard lead, to be used in the manu- facture of bullets, shot, type metal, or pig- ments. The lead is frequently " poled after tapping ; this operation consists in the immer- sion of a piece of green wood under the sur- face of the molten lead ; the escape of moisture and the carbonizing of the wood cause a lively ebullition, exposing a considerable surface to the oxidized influence of the air, and thereby assisting in the removal of the last traces of impurities. The following are analyses of a few varieties of furnace and refined lead :