LEAD 53 



The next most important ore of lead is the carbonate, which is a brittle mineral, of 

 a white or greyish-white colour, having a specific gravity varying from 6'46 to 6'50. 

 Its composition is 



Carbonic acid . 16'05 



Oxide of lead 83'56 



99-61 



Large quantities of this substance occur in the mines of the Mississippi Valley in 

 the United States of America, where they were formerly thrown away as useless, but 

 have since been collected and smelted. Vast deposits of this substance have also been 

 found in the Bunter sandstone, near Duren in Prussia, and at Freyung in Bavaria. 



The extraction and mechanical preparation of ores is the business of the miner, 

 and not of the metallurgist who receives them from the former freed as perfectly as 

 possible from foreign matters. 



The metallurgic processes, by the aid of which lead is obtained from galena, may 

 be divided into two classes. The first of these is founded on the following reactions : 

 If one equivalent of sulphide of lead and two equivalents of the oxide of the same 

 metal are fused together, the result is three equivalents of metallic lead and one 

 equivalent of sulphurous acid, which is evolved. 



This reaction is represented by the following equation : 



PbS + 2PbO = 3Pb + SO*. 



When, on the other hand, one equivalent of sulphide of lead and one equivalent of 

 sulphate of lead are similarly treated, two equivalents of lead are obtained, and two 

 equivalents of sulphurous acid are evolved. Thus : 



PbS + PbO.SO 3 = 2Pb + 2S0 2 . 



The process, founded on the foregoing reactions, and which we will distinguish as 

 the method by double decomposition, consists in roasting the galena in a reverberatory 

 furnace until a certain amount of oxide and sulphate has been formed, and subse- 

 quently, after having intimately mixed the charge, and closed the doors of the furnace 

 causing the whole to enter into a state of fusion. 



During this second stage of the operation, the reaction between the sulphides, sul- 

 phates, and oxides takes place, and metallic lead is eliminated. The roasting of the 

 ore is, in some cases, conducted in the same furnace in which the fusion is effected, 

 whilst in others two separate furnaces are employed. 



The process by double decomposition is best adapted for the richer varieties of ore, 

 and such as are least contaminated by siliceous or earthy impurities, and is con- 

 sequently that which is almost universally employed for smelting the ores of this 

 country. 



By the second method, which we will call the process by affinity, the ore is fused 

 with a mixture of metallic iron, which by combining with the sulphur liberates the 

 metallic lead. This reaction will be understood by reference to the following eqtia- 

 tion : 



PbS + Fe = Pb + FeS. 



In practice, however, metallic iron is not always employed for this purpose ; cast 

 iron is also frequently used, and in some instances the ores of iron and hammer-slags 

 are substituted, as are also tap-cinder and other secondary products containing a con- 

 siderable percentage of this metal. None of these substances are, however, found to 

 be so efficacious as metallic iron, since cast iron requires to be decarburised before it 

 can readily decompose the sulphide of lead, and the ores of iron require the intro- 

 duction of various fluxes, and the consequent expenditure of an additional amount of 

 fuel. In all cases, however, it is judicious to subject the ore to a preliminary roasting, 

 iu order to eliminate a portion of the sulphur, and thereby reduce the expenditure of 

 iron, as well as to agglutinate the ore and render it better adapted for its subsequent 

 treatment in the blast-furnace. 



"We will not attempt to describe the different forms given to roasting furnaces em- 

 ployed for the ores treated by this process, but would remark that they frequently 

 resemble the kilns used for the preparation of lime, whilst in some instances the ores 

 are roasted in heaps interstratified with wood or other fuel. 



The method of treating ore by affinity is particularly adapted to those varieties that 

 contain a considerable amount of silica, since such minerals, if treated by double de- 

 composition, would, by the formation of oxide of lead, give rise to silicates, from 

 which it would be exceedingly difficult to extract the metal. 



