72 LEAD 



blast of air, or finally, by carefully measured additions of nitrate of soda. The theory 

 of the reaction is as follows : By simple solution in water, soda abandons all the 

 oxides which it holds in solution or suspension, and is evaporated and dried for use, 

 in the operation, almost without loss. The metals oxidise in the melted alkaline bath 

 in the order of their affinity for oxygen, an order modified, however (1) by their 

 particular affinity for soda ; (2) by the action of affinity exercised by the largest mass 

 present. Thus, tin and the metals of platinum, although much less oxidisable than 

 lead or copper, are attacked very rapidly ; and before the latter in the soda-bath, by 

 reason of their propensity to act as electro-negative elements. Hence, also, in an 

 alloy very rich in lead the copper oxidises first. Another phenomenon of not less 

 importance is that the solutions of the oxides in the soda-bath act chemically in pre- 

 sence of the re-agents exactly as do the metallic salts dissolved in water. It is thus 

 in this igneous solution all the metals are precipitated one after the other, in the 

 inverse order of their solubility, and in the direct order they preserve each other from 

 oxidation. In this respect even insoluble reducing agents, such as charcoal, may be 

 employed in the bath. The principal applications in the process are its adaptation, 

 not only to the refining of lead and the extraction of silver by the zinc process from 

 lead and argentiferous scoriae, but the purification of argentiferous copper and old 

 complex alloys; the treatment of ores of platinum, gold, silver, &c., of ores of 

 chromium, &c. Since March last the inventors have constructed a plant, and have 

 carried on the process at Marseilles ; and we learn that the hard leads of Greece 

 (containing 2J per cent, antimony, 1 per cent, arsenic, per cent, copper, and 1 to 2 

 per cent, iron and sulphur), hard Spanish lead, and other forms of the metal contain- 

 ing large quantities of foreign substances, have been successfully treated. A company 

 has been formed for the fusion of ores, separation of metals, and then refining by the 

 process of natro-metallurgy. 



Reducing. The reduction to the metallic state of the litharge from the refinery, 

 the pot-dross, and the mixed metallic oxides from the calcining furnace, is effected in 

 a reverberatory apparatus, somewhat resembling a smelting furnace, except that its 

 dimensions are smaller, and the sole, instead of being lowest immediately below the 

 middle door, gradually slopes from the fire-bridge to near the flue, where there is a 

 depression in which is inserted an iron gutter, which constantly remains open, and from 

 which the reduced metal flows continuously into an iron pot placed by the side of the 

 furnace for its reception, whence it is subsequently laded into moulds. 



The litharge, or pot-dross, is intimately mixed with a quantity of small coal, and is 

 charged on that part of the hearth immediately before the fire-bridge. To prevent 

 the fused oxide from attacking the bottom of the furnace, and also to provide a sort 

 of hollow filter for the liquid metal, the sole is covered by a layer of bituminous 

 coal. 



The heat of the furnace quickly causes the ignition of this stratum, which is rapidly 

 reduced to the state of a spongy cinder. The reducing gases present in the furnace, 

 aided by the coal mixed with the charge itself, cause the reduction of the oxide, which, 

 assuming the metallic form, flows through the interstices of the cinder, and ultimately 

 finding its way into the depression at the extremity of the hearth, flows through the 

 iron gutter into the external cast-iron pot. The surface of the charge is frequently, 

 during the process of elaboration, turned over with an iron rake, for the double pur- 

 pose of exposing new surfaces to the action of the furnace, and also to allow the 

 reduced lead to flow off more readily. 



Fresh quantities of litharge, or pot-dross, with small coals, are from time to time 

 thrown in, in proportion as that already charged disappears, and at the end of the 

 shift, which usually extends over 12 hours, the floor of cinder is broken up, and after 

 being mixed with the residual matters in the furnace is withdrawn. A new floor of 

 cinders is then introduced, and the operation commenced as before. A furnace of 

 this kind, having a sole 8 feet in length and 7 feet in width, will afford, from litharge, 

 about 5J tons of lead in 24 hours. 



The dross from the calcining pan, when treated in a furnace of this description, 

 should be previously reduced to a state of fine division, and intimately mixed up with 

 small coal and soda-ash. In many cases, however, the calcined dross is treated in 

 the smelting furnace. The hard lead obtained from this substance is again taken to 

 the calcining furnace, for the purpose of being softened. 



In the establishment from which the foregoing data were obtained, the cost of slack, 

 delivered^ at the works, was only 2. lid. per ton, which is cheaper than fuel can be 

 obtained in the majority of the lead-mills of this country. In North Wales the cost 

 of small coal is generally about 4s., and at Bristol 5s. 6d. per ton. 



Figs. 1354 and 1355 represent a vertical section and plan of a reducing-furnace. 

 A, fire-place ; B, ash-pit ; c, fire-bridge ; D, hearth ; s, working-door ; f, iron spout for 



