Recovery of Copper from its Solution in Mine Drainage. 313 



their treatment is more varied. The final results to be obtained 

 in this case is either the complete expulsion of the sulphur, or, as 

 is more usual, its partial expulsion and oxidation of the rest, 

 thus converting the sulphide into sulphate. The simplest means 

 by which this latter is accomplished is by weathering or natural 

 oxidation, as it takes place at the mine ; in the waste heaps or 

 " attals" on the surface, and in the " stulls" and workings under- 

 ground.* This is naturally a slow operation, but takes place more 

 rapidly in ores that are not compact, and that contain a high per- 

 centage of iron pyrites, and it is greatly accelerated by the 

 admixture of such salts as chlorides of the alkalies (soda, potash, 

 ammonia) or alkaline earths (lime, &c.), or solutions of ferric sul- 

 phate, hydrochloric acid, or even sulphuric acid. 



Owing to the tardiness of the completion of this natural 

 process, the more rapid method of roasting is generally resorted 

 to. This may be either with or without the addition of some 

 reagent, and after the ore has been finely ground. 



When the ore is roasted at a low heat the sulphides are con- 

 verted to sulphates, and the temperature may be so g^rranged as 

 to decompose the sulphates of iron without affecting those of 

 copper. Owing, however, to the difficulty of adjusting this 

 operation exactly and thoroughly, sulphides of copper remain 

 unoxidized, or oxide, of copper is formed; both insoluble in 

 water, thus causing waste. The introduction in the roasting of 



* The reactions that take place when pyrites is weathering are thus given hj Crookes, 

 and Rohrig. Vol. II., page 231. 



" When decaying, an admixture is formed of iron pyrites, copper pyrites, arsenio- 

 sulphide of cobalt and nickel (Cobalt ores of Seigen), the sulphide of iron is next 

 decomposed : — 



"FeS2+7 = FeO, S O3 + S O3; and 

 "Fe^ S3 + II 0=2 Fe 0, S O3 + S O3. 

 " An admission of air transforms the protoxide of iron into a basic salt of the peroxide 

 and affects the chloride of iron more easily than protosulphate, chloride of calcium is 

 therefore sometimes added to the decaying ore: — 



"2FeO S03 + 0=Fe2 O3SO3+S O3. 

 " The formation of sulphate of copper is then facilitated by the free SO 3 :— ;- 



"Cu, S + 5 + S03=2CuO, SO3. 

 " Finally Co S^, Ni S^, Co As^ and Ni As^ be oxidized also by the action of the free 

 sulphuric acid forming sulphates and arseniates : — 



(Co Ni) S2+7 = (Co Ni) 0, S O3 + S O3. 

 (Co Ni) As^ + ll 0= (Ni Co) 0, As O^ + As 0^. 

 " The latter salts are transformed by basic sulphate of iron thu? : — 

 "(CoNi)O, AsO +2Fe 0, S 03--(Co Ni) 0, S03 + Fe^ 0,, AsO, +Fe„ O3." 



