USEFUL METALS 123 



is first oxidized to the sulphate according to the equation PbS+ 

 20a = PbSO4. The oxygen acting upon the lead sulphate formed 

 in the presence of a new charge of ore reduces the sulphate to the 

 oxide. The oxide reacts also upon a new charge of ore when 

 some metallic lead is formed and sulphur dioxide is set free ac- 

 cording to the equation 2PbO+PbS = Pb+ SO 2 . The reduced 

 metal sinks to the bottom of the furnace, runs through an inclined 

 trough into an iron kettle from which the metal is dipped into 

 moulds. The process is applicable to galenite that is fairly free 

 from the sulphides of the heavy metals. 



(3) The Precipitation Process. In this process the ores may be 

 charged in the raw state into a blast-furnace or calcined to remove 

 volatile acid radicles or impurities. If the ore is the sulphate, 

 anglesite, it will produce lead oxide and sulphur trioxide, thus: 

 PbSO4 = PbO + SO 3 . If the ore is the carbonate, cerussite, it will 

 yield lead oxide and carbon dioxide, PbCO 3 = PbO+CO 2 . If the 

 ore is the sulphide, galenite, it will yield lead oxide and sulphur 

 dioxide, PbS+30 = PbO+SO 2 If the ore be the oxidation prod- 

 uct, massicot, some volatile impurities may be removed. The 

 oxide is then introduced into a blast-furnace with coke, scrap iron, 

 or the sulphide of iron. At a high temperature the entire mass 

 is melted. The silicate of iron rises to the surface as a slag and may 

 be drawn off as in ordinary copper smelting. The sulphides of iron, 

 tin, antimony, and copper, will be formed provided these metals 

 are present and may be drawn off at a lower level. The sulphides 

 often found are the black sulphide of copper, CuS, the sulphide 

 of antimony, Sb 2 S 3 , of tin, SnS, and of iron, FeS. At the bottom 

 of the furnace is found the metallic lead. An equation represent- 

 ing the reduction to the elemental state in the presence of iron 

 would be PbS + Fe = FeS + Pb. The lead thus obtained is impure 

 and is subsequently refined. If it contains silver in commercial 

 quantities the lead is desilverized by the Pattinson process. 



(4) The Lime-roasting Process. This method is comparatively 

 new and depends upon the treatment of galenite with lime or gyp- 

 sum under conditions favorable for oxidation. The percentage of 

 lead and silver saved by this method is said to be larger than that 

 obtained by the preceding method while the cost of treatment is 

 no greater. 



Uses of Lead. One of the most important uses of lead is in 

 the manufacture of white lead. Lead is used also in the manu- 

 facture of other lead pigments under the name of litharge, red lead, 



