$4 LEAD 



English Process. Treatment by double decomposition. Galena, if placed in a 

 close vessel which protects it from the action of the air, and exposed to a gradually 

 increasing temperature, becomes fused without the elimination of any lead taking 

 place, but ultimately a portion of the sulphur is driven off, and a subsulphide is 

 formed, which at a very elevated temperature is volatilised without change. 



If, however, the vessel be uncovered, and the air allowed to act on its contents, 

 oxygen combines with the sulphur, sulphurous acid is evolved, and the desulphuration 

 of the mineral is slowly effected. 



When galena is spread on the hearth of a reverberatory furnace, and is so placed 

 as to present the largest possible amount of surface to oxidising influences, it will be 

 found that the surface slowly becomes covered with a yellowish-white crust of sulphate 

 of lead. The oxygen of the air, by combining with the two elementary bodies of 

 which galena is composed, will evidently produce this effect. This is not, however, 

 the only chemical change which takes place in the charge under these circumstances ; 

 oxide of lead is produced at the same time as the sulphate, or rather the formation of 

 the oxide is prior to that of the sulphate. 



In fact, during the first stage of the operation of roasting, sulphurous acid is 

 evolved, the sulphur quits the lead, and a portion of that metal remains in a free 

 state. This becomes oxidised by the air passing through the furnace, and subse- 

 quently a part of it combines with sulphuric acid, formed by the oxidation of sulphu- 

 rous acid, and sulphate of lead is the result. In this way, after the expiration of a 

 certain period, both oxide and sulphate of lead are present in the furnace. 



During the early period of the roasting, when the temperature of the furnace is not 

 very elevated, the proportion of sulphate is larger than that of the oxide formed, but 

 in proportion as the heat of the apparatus increases, the production of oxide becomes 

 more considerable, whilst that of the sulphate diminishes. 



The sulphate and oxide thus formed re-act in their turn on the undecomposecl 

 galena, whilst a portion of the latter, by combining with the sulphide of lead, gives 

 rise to the formation of oxysulphide. 



This last compound has no action on galena, except to dissolve it in certain pro- 

 portions, but is readily decomposed by the aid of carbonaceous matter. 



It is therefore evident that the addition of carbon, at this stage of the operation, 

 will have the effect of reducing the oxide and oxysulphide of lead. 



Every process then that has for its object the reduction of lead ores by double 

 decomposition, comprises two principal operations : 1st. The reduction of galena, by 

 the aid of heat and atmospheric air, to a mixture of sulphide, oxide, and sulphate, 

 which mutually decompose each other, with the elimination of metallic lead ; 2nd. 

 The reduction of the oxysulphide by the addition of carbonaceous matter. 



The Reverberatory Furnace. The reverberatory furnace employed for the treatment 

 of galena is composed, like all other furnaces of this description, of three distinct 

 parts, the fire-place, the hearth, and the chimney. 



The hearth has to a certain extent the form of a funnel, of which the lowest point 

 is on the front side of the furnace immediately below the middle door. The molten 

 metal, descending from every side along the inclined bottom or sole, is collected in 

 this receptacle, and is ultimately run off by means of a proper tap-hole. This tap- 

 hole is, during the operation, closed by a pellet of clay. 



The inclination of the hearth is more rapid in the vicinity of the fire-bridge than 

 towards the chimney, in order that the liquid metal may not be too long exposed to 

 the oxidising and volatilising influences of a current of strongly-heated air. 



The dimensions given to these furnaces, as well as the weight of the charge operated 

 on at one time, vary considerably in different localities, but in the north of England 

 the following measurements are usually employed: The fire-grate is 5 feet 9 

 inches x 1 foot 10 indies, and the thickness of the fire-bridge 1 foot 8 inches; the 

 length of the sole is 9 feet, and its average width 7 feet. The depth of the tap is 

 about 2 feet 6 inches below the top of the inclined sole. The height of the roof at 

 the fire-end may be 1 foot 4 inches, and at the other extremity 11 inches. 



The introduction of the charge is in some cases effected by the doors of the furnace, 

 whilst in other instances a hopper, placed over the centre of the arch, is made use of. 



On the two sides of the furnace are placed three doors, about 11 inches x 9 inches, 

 which are distinguished as 1, 2 and 3, counting from the fire-bridge end. The three 

 doors on the one side are known as the front doors, whilst those on the other side are 

 called the back doors. Immediately beneath the door on the front side of the furnace 

 is situated the iron pan into which the molten lead is tapped off. 



The bottom of this arrangement is in most cases composed of fire-bricks, covered 

 by a layer of vitrified slags, of greater or less thickness. In order to form this bottom, 

 the slags arc introduced into the furnace, the doors closed, and the damper raised. 

 An elevated temperature is thus quickly obtained, and as soon as the scoriae have 



