402 PRINCIPLES OF CHEMISTRY 



The smelted copper still contains sulphur, iron, and other metallic 

 impurities, from which it is freed by fusion in reverberatory furnaces, 

 with access of air to the surface of the molten metal, as the iron and 

 sulphur are more easily oxidised than the copper. The iron then 

 separates as oxides, which collect in the slag. 4 



4 "Copper ores rich in oxygen are yeryrare"; the sulphur ores are of more common 

 occurrence, but the extraction of tho copper from' them is much more difficult. The 

 problem here n'ot only consists in the removal of the sulphur, but also in the removal of 

 the iron combined with the sulphur and copper. This is* attained by a -whole series of 

 operations, after which "there still sometimes remains the extraction of the metallic silver 

 which generally accompanies the copper, although in but small quantity. These 

 Jprocesses commence with the roasting 1 i.e. calcination of the.ore with access of air, by 

 which means the sulphur is converted into sulphurous anhydride. It should here be j 

 Remarked that iron sulphide is more easily oxidised than copper sulphide, and therefore 

 the greater part of the iron in the residue from roasting is no longer 'in the form of 

 Bulphide but of oxide of iron. The roasted ore is mixed with charcoal, and siliceous fluxes,. 

 and smelted in a cupola furnace. The iron then passes into the slag, because its oxide- 

 gives an easily-fusible mass wifh the silica, whilst the copper, KL the form of sulphide,, 

 fuses and collects under the slag. Tho greater .part of -the iron is removed from, the' 

 mass by this smelting. The resultant coarse metal is again roasted in order to 

 remove the greater part of the" sulphur from the copper sulphide, and to convert the 

 toetal into oxide, after which the mass is again smelted. These processes are repeated 

 several times, according to the richness of the ore. During these smeltings a portion of 

 the copper is already obtained in a metallic form, because copper sulphide gives 

 metallic copper with the oxide (CuS + 2CuO = 8Cu + SO 2 ). We will not here describe 

 the furnaces used. or the details of this process, but the above remarks include the ex- 

 planation of those chemical processes which are accomplished in- the various tech- 

 nical operations which are made use of in the process (for details see works on. 

 metallurgy). 



Besides the smelting of copper there also exist methods for its extraction from 

 solutions in the wet way, as it is called. Recourse is generally had to these methods for 

 poot copper ores. The copper is brought into solution, from which it is separated by 

 means of metallic iron or by other methods (by the action'of an electric current). The 

 sulphides are roasted in such a manner that the greater part of the copper is oxidised 

 into cupric sulphate, whilst at the same time the corresponding iron salts are as-far as 

 possible decomposed. This process is based on the fact that the copper sulphides absorb 

 oxygen when they are calcined .in ..the presence of air, forming cupric sulphate. The 

 roasted ore is treated with water, to which acid is sometimes added, and after lixivia- 

 tion the resultant solution containing copper is treated either with metallic iron or with 

 milk of lime, which precipitates cupric hydroxide from the solution. Copper oxide 

 ores poor in metal may be treated with dilute acids in order to obtain the copper 

 oxides in solution, from which the copper is then easily precipitated either by iron or 

 as hydroxide by lime. According to Hunt and Douglas's method, the copper in the ore 

 is converted by calcination into the cupric oxide, which is brought into solution by 

 the action of a mixture of solutions -.of ferrous sulphate and sodium chloride ; 

 the oxide converts the ferrous chloride into ferric oxide, forming copper chlorides, 

 according to the equation 3CuO + 2FeCl 2 = CuCl 2 + 2CuCl + Fe 2 O 3 . The cupric chloride 

 is soluble in water, whilst the cuprous chloride is dissolved in the solution of sodium 

 chloride, and therefore all the copper passes into solution, from which it is precipitated 

 ty iron. 



The same American metallurgists give the fojlowing wet method foi? extracting the 

 Ag and Au occurring in many copper ores, especially in sulphurous ores : (1) The Cu 2 S is 

 first converted into oxide by roasting in a calciner ; (2) the CuO is extracted by the 

 dilute sulphuric acid obtained in the fourth process, the Cu then passes into solution, 



