Page:The American Cyclopædia (1879) Volume X.djvu/262

 256 LEAD 1838 ; during these years 3,250,000 Ibs. of lead were smelted. The mining was however so recklessly conducted, being directed solely to the richest masses, most easily reached, that the abandonment of the mine became neces- sary in the following year. Argentiferous galena occurs in the sub-carboniferous lime- stone of Kentucky, but mining operations, which have hitherto been very imperfect and superficial, have not been productive. Ac- cording to Owen (" Kentucky Geological Sur- vey"), the most favorable localities f or ^ de- velopment of lead mines in this formation, judging by analogy with English mines, are in Orittenden and Livingston cos. Lead also oc- curs in the blue limestone formation of central Kentucky. A vein 4 to 6 in. wide in Franklin and Woodf ord cos. has been somewhat worked, but did not prove profitable. Metallurgical Treatment. Ordinarily the ores as they are raised from the mine have to undergo a process of preparation or dressing, to free them from adhering gangue, or to separate the different metallic minerals before they are ready for smelting. The character of this dressing will differ according to the amount and* nature of the associated minerals. Simple breaking and hand sorting often suffices to separate masses of nearly pure galena, while the fine ore and that composed of an intimate mixture of a number of minerals is submitted to various mechani- cal processes, by which the different minerals are separated according to their specific gravi- ties. The processes employed for the extrac- tion of lead from galena are three in num- ber: 1, the roasting-reaction or air-reduction process; 2, the roasting and deoxidizing pro- cess; and 3, the iron-reduction or precipita- tion process. In cases where oxidized ores are smelted, the second process is employed, with the omission, of course, of the roasting. The first process depends on the interesting reaction which takes place when sulphide of lead is heated either with sulphate or oxide of lead, resulting, when the oxygen and sulphur are present in the mixture in the proportion of two molecules of the former to one of the latter, in the production of metallic lead and sulphurous acid gas. Where this proportion does not exist, a residue will remain, consisting of the excess of oxide or sulphide, as the case may be. The reaction is shown in the following equa- tions: PbS + 2PbO=Pb 3 + S0 2, and PbS+Pb O,S0 3 =Pb 2 + 2S0 2. When therefore galena is partially roasted so as to form a certain amount of sulphate or oxide of lead, and the oxide thus formed is heated in intimate mixture with the unaltered sulphide, metallic lead at once separates. This reaction takes place to some extent when galena is placed on the top of an ordinary fire; it is in this way that the "backwoodsman often obtains his lead for bul- lets. Galena has indeed been smelted on the large scale, in localities remote from civiliza- tion, by simply throwing it upon a fire of logs. In the second process the galena is roasted, either completely or nearly so, and the result- ing oxide, with some sulphate, reduced in a shaft furnace by the carbon of the fuel. In the third process advantage is taken of the supe- rior affinity of sulphur for iron at high tem- peratures, so that galena heated with metallic iron is reduced to metal with the formation of sulphide of iron. The selection of the process for the treatment of lead ores depends largely on the presence or absence of other metals, and on the richness of the ores. Where galena alone is treated with but small amount of earthy matters, the roasting-reaction process is generally employed, and the smelting is per- formed either on an ore hearth or in a rever- beratory furnace. During the early part of the last century the ore hearth was almost ex- clusively used in England; it is now mainly confined to the north of England, Scotland, and the United States. The American ore hearth, a modified and improved Scotch hearth, is used in the western lead regions, and was formerly extensively used in the state of New York. It consists of a working plate of cast iron about 3 ft. wide and 2 ft. from front to back, sloping downward and forward about 1 inch in 12. It has a diagonal groove on its upper surface for the flow of lead from the hearth to the cast-iron pot. Enclosing the back half of this working plate on three sides is a hollow casting 14 in. high, forming an air chest, through which the blast passes before it reaches the hearth; this arrangement serves the double purpose of keeping the cast-iron sides cool and heating the blast. The tuyere is on the back plate about 2 in. above the level of the hearth bottom. The operation is an ex- tremely simple one. The fuel used is light wood or charcoal, and the mass of ore and fuel fills up the hearth and slopes down to the front, which is open. Fuel and ore are added in * small quantities at short intervals, and the operation is continued uninterruptedly. The lead accumulates at the bottom of the hearth, and flows out in the groove in the front of the working plate. The yield is dependent to a considerable extent on the skill of the smelter in charging and stirring the mass. At the Rossie works in New York, now abandoned, the average yield of each hearth for 24 hours was about 7,500 Ibs., with a consumption of wood per day of three fourths of a cord ; the direct yield of lead amounted to TO to 80 per cent, of that in the ore. There is also a con- siderable quantity of slag formed, which is subsequently treated in a low blast furnace. The great loss of lead, both mechanically and by volatilization in the ore hearth, has led to the general substitution of the reverberatory furnace in the roasting-reaction process. The chief advantage of the ore hearth is the small expense for plant and the simplicity of the manipulation. In the reverberatory furnace process, called also the English process, the same reactions are involved, but they are effected on a much larger scale, and are much