TREATMENT OF ORES. 245 



A small proportion of blende used with calamine is sufficient 

 for the supply of the acid consumed by the lime, the iron, and 

 other foreign materials. 



In the roasting of blende, care must be taken to avoid its 

 being crushed, and the crushing should only be applied to the 

 residual substances, which have remained refractory to a first 

 attack and become more friable after being calcined. 



After this summary preparation, the ores are transformed 

 into sulphates and put to the electrolytic treatment. To that 

 effect they are placed in large tanks, in which a feeble current 

 of water dissolves the sulphate of zinc which has been formed. 

 The sulphated liquor is then directed into a series of tanks, 

 across which it slowly travels, depositing, under the action of 

 the electric current, a portion of the zinc which it contains. 

 The freed sulphuric acid rises to the surface, and runs out by 

 overflowing. The acidulated liquor is cast over the heaps of 

 ores, the acid dissolves the oxides of zinc, and the sulphate of 

 zinc is formed anew. The operation is conducted in a continuous 

 manner, owing to a difference of level between the tanks and a 

 mechanical exertion in one point of the circuit. 



The acid being indefinitely reproduced in the precipitating 

 tanks, it is sufficient that the ores should contain a quantity of 

 sulphate large enough to supply the acid which is absorbed by 

 the foreign matters of the bath. 



Lead, silver, and other matters insoluble in sulphuric acid 

 are collected in the residuum and separately treated. 



M. Letrange indicates the following process for transforming 

 the blende into sulphate. Without any regard to the more or 

 less complete transformation of the sulphurous acid vapours 

 into sulphuric acid, and avoiding the expenditure of nitric acid, 

 the vapours produced by the roasting are simply made to pass 

 through the columns or' chambers containing the zinc ore, 

 which is kept in a wet state by a falling spray of water. The 

 sulphurous acid forms soluble zinc sulphites, which become 

 transformed into sulphates after having been exposed to the 

 atmospheric air for a certain time. 



M. Letrange has calculated that, with the processes actually 

 in existence, the cost of installing a factory capable of annually 



