16 HYDROMETALLURGY OF SILVER 
these are heavy consumers of chlorine, and the ore therefore re- 
quires more salt. For instance, the ore of the San Francisco del 
Oro mine in Mexico, which is very heavily mineralized, containing 
zinc 24.08 per cent., lead 11.92 per cent., iron 7 per cent., copper 
0.5 per cent., and sulphur 21.35 per cent., required only 3% to 4 
per cent. of salt. 
An excess of salt does not improve chlorination; on the con- 
trary, in many instances I have observed that the chlorination 
already gained was reduced by adding more salt. For this and 
for economical reasons an excess, therefore, should be avoided, 
especially as undecomposed salt in the roasted ore is not advan- 
tageous in the subsequent extraction. 
THE PROPER TIME TO ADD THE SALT 
The generation of chlorine in the furnace does not commence 
until, by the oxidation of the sulphur, metal sulphates have formed, 
which then act on the salt. The first part of the process, there- . 
fore, is an oxidizing process, whether the ore contains salt or not, 
and in this respect it would be immaterial at what time the salt 
were added. That the ore sustains a heavier loss of silver by vola- 
tilization if the salt is added before the oxidizing period is not 
conclusively proved, and actually there is no reason for it. 
Chlorides are not formed until the sulphates are formed, and 
therefore the presence of salt cannot cause a greater volatiliza- 
tion of the silver. An ore which is apt to lose silver on account 
of its arsenic and antimony sustains the larger part of its loss 
during oxidizing roasting. 
There are ores, however, which cannot be chloridized success- 
fully if the salt is added to the ore in the beginning. This is the 
case with ores which contain a large percentage of a dense argen- 
tiferous zine blende, or argentiferous galena, or both, as the prin- 
cipal silver-bearing minerals of the ore. The reason why such 
ores have to be first subjected to an oxidizing roasting before the 
salt is added is the following: 
Zinc blende, if subjected to oxidizing roasting, changes into 
zinc oxide and zinc sulphate, while sulphurous acid escapes. 
The process of oxidizing the zine blende progresses but slowly, 
especially if the mineral is very dense. Iron and copper sulphides, 
on the other hand, oxidize easily and are converted into sulphates 
     
     
    
    
   
   
   
    
    
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