184 ECONOMIC GEOLOGY 



and trachite of recent volcanic origin. It is supposed to be of 

 pneumatolytic origin and to provide an example of an inter- 

 mediate type of veins between well-known tin veins with greisen- 

 ized walls and the true veins of galenite with their associated 

 sulphides (Fig. 101.) 



Geological Horizon of Tin. The ores of tin appear to be asso- 

 ciated with the early as well as the late intrusives and to a limited 

 extent with the later crystalline rocks. 



Methods of Extraction. The Smelting Process. Three steps 

 are involved in the process. Calcining, washing, and smelting. 

 The ore is crushed and washed to remove all earthy materials. 

 It is then calcined to remove any volatile constituents that may be 

 in the ore. If the cassiterite is mixed with stannite, the sulphide 

 of tin, copper and iron, the sulphur passes off as sulphur dioxide. 

 This may be reduced to elemental sulphur and sold as a by-prod- 

 uct or converted into sulphurous acid which is so extensively 

 utilized in the manufacture of paper by the sulphite process. If 

 the ore contains arsenic the arsenic is conducted into the condens- 

 ing flues of the reverberatory furnace where it is deposited as the 

 white arsenic of commerce, collected and sold as a by-product. 

 If antimony is present this metal will also be converted into the 

 oxide which may be collected and sold as a by-product. If 

 tungsten minerals are present the ore is treated with sodium 

 carbonate which combines with the tungsten forming sodium 

 tungstate. This compound is also sold as a by-product. In the 

 process of calcining the copper is converted into the sulphate and 

 the iron into its oxide. 



The second step in the process is washing. This removes the 

 copper sulphate as a solution from which the copper may be re- 

 claimed by electrolysis. It also washes out the iron oxides and 

 other minerals of lower specific gravity than the cassiterite. The 

 cassiterite thus concentrated is smelted in a reverberatory furnace 

 with carbon or powdered charcoal when impure metallic tin is ob- 

 tained according to the equation, SnO 2 +2C =Sn+2CO. 



The metal now more or less alloyed with other metals is re- 

 melted, to separate it from the impurities. The molten metal is 

 stirred with bits of green wood which cause a separation of scum or 

 dross from the metal. The reduced metal is drawn into a cast- 

 iron kettle, ladeled into moulds, and the blocks thus formed are 

 known in commerce as block tin or pig tin. 



In the process for the manufacture of tin as carried out at Corn- 



