Page:The American Cyclopædia (1879) Volume X.djvu/263

 LEAD 257 more completely under control. The process as conducted in Flintshire is as follows : The hearth of the reverberator j, which is about 11 ft. long with an average width of 9 ft., is gen- erally concave, and slopes toward the "well" about the middle of the front side. The charge, 21 cwts. of ore, is evenly spread over the fur- nace, and stirred for two hours with an ample supply of air at as high a temperature as pos- sible without causing sintering. The fire is then increased until the charge becomes semi- liquid, and any portion of it which may have run down toward the well is raked back to that which remains on the upper part of the hearth. The temperature is then lowered until the charge becomes thick, when it is pushed back toward the bridge and back part of the furnace. The fire is again raised, and the charge melted down as quickly as possible into the well, when slaked lime is thrown in and raked over the surface of the melted mass. The slag and re- duced portions, being thus rendered sufficiently stiff, are again thrown up or " set up " on the sloping sides of the bed, there left to cool a little, and again remelted. Lime is again added, and the slag is pushed back from the surface of the lead, and left to drain a little, when the lead is tapped off from the well into a re- ceptacle below. The slag is run or drawn out of the furnace in pasty lumps, and is termed " gray slag." The quantity of fuel per charge varies, according to the ore used, from 12 to 16 cwts. The ores treated contain from 75 to 80 per cent, of lead, and the yield is about 14 cwts. of lead from a charge of 21 cwts. Of this yield 91 per cent, is obtained direct from the furnace, and 9 per cent, from the slag and fume. There are numerous variations in the manipulations practised at different works, but in its essential features the process is every- where the same. It is necessary that the ores should be rich and not contain more than 5 per cent, of silica. It is evident that where oxi- dized ores are available in sufficient amount to mix with the galena, the preliminary roasting may be much abridged, or even omitted. The gray slags from this process are rich in lead (over 50 per cent.), and are treated by them- selves in low blast furnaces, where reduction is effected by the fuel alone; the lead thus produced is known as slag lead. The roasting-reductio-n process and the precipitation process are never employed for pure rich lead ores, but are con- fined to ores poor in lead or those containing other metals, as copper, antimony, and nickel. The roasting-reduction process may be con- ducted in a great variety of ways. The roast- ing, which is generally effected in reverberatory furnaces, may be partial or complete. Partial roasting is resorted to where the ore contains, besides lead and silver, the metals mentioned above. In the subsequent smelting of this par- tially roasted ore there is produced, besides lead containing the greater part of the silver, a regulus containing sulphur, iron, lead, and copper. Where arsenic, nickel, and cobalt are present, there is also formed a Speise, which contains these metals combined with iron. But where other metals than lead and silver are not present (iron and zinc excepted), the ore may be roasted completely ("sweet"), the heat -being carried sufficiently high to sinter the mass and convert the oxide of lead into silicate. This agglomerated roasted mass is smelted in a shaft furnace, with the addition of iron, either metallic or as oxide, in order to effect the complete decomposition of the sili- cate of lead. In case the roasting has been effectual and all the sulphur driven off, no reg- ulus is formed. Examples of this latter mode of treating lead ores (i. e., complete roasting) are found at Pontgibaud, Vialas, and La Pise in France, and at Bleiberg, Rhenish Prussia; of the former (i. e., partial roasting), at Sala in Sweden, and Freiberg in Saxony. As the smelting process at Freiberg is a good example of an intelligent metallurgical practice, in- volving the extraction of a number of metals, a brief outline of the process will be given. The ores smelted there are divided into five classes: A, Glanze, or bright lead ores, con- taining- about 30 per cent, of lead; B, llei- iscJie Erze, or leady ores, containing from 15 to 29 per cent, of lead ; 0, Durrerze, or dry ores, consisting chiefly of veinstuff with small quantities of pyrites and galena, containing from 0'05 to O'l per cent, of silver; D, Kup- fererze, or copper ores, containing from 1 to 10 per cent, of copper, averaging 3 per cent. ; and E, Zuschlagserze, or ores containing less than 0'03 per cent, of silver, and chiefly com- posed of pyrites, mispickel, and zinc blende, with some lead and copper, mixed with quartz and calc spar. The mixture for smelting con- sists of about 60 per cent, of A, 20 of B, and 20 of a mixture of and D. The lead in this mixture amounts to from 34 to 38 per cent., and the silver from 0'15 to 0'18 per cent. This ore mixture is roasted in charges of 10 cwts. in a double-hearth reverberatory for 16 hours, until all but 5 per cent, of sulphur has been expelled. It is then smelted in shaft furnaces with roasted reverberatory regulus (Rohsteiri), presently to be described, lime or fluor spar, slags produced in the same process, and cer- tain lead products, such as furnace bottoms. Silver ores of class C, when containing over 0*1 per cent, of silver, are added to the mixture without roasting. The products of this smelt- ing are Werlcblei, work or furnace lead, con- taining 0-5 to 0-6 per cent, of silver, from 0'20 to 0-60 of copper, and about 1*5 of antimony and arsenic ; Bleistein, lead matte or regulus, or blast-furnace regulus, consisting of the sul- phides of iron, lead, and copper, and contain- ing on an average 20 per cent, of lead, 10 of copper, and 0'20 of silver, besides small amounts of nickel, zinc, arsenic, and antimony ; Speise, a compound of arsenic and iron, containing the greater part of the nickel and cobalt of the ore mixture; and Schlacken or slag, consist- ing of silicate of protoxide of iron, containing