570 



Kenneth A. Mickle 



(Broken Hill) 



Effect of Tarnishing the surface of Sulphides. 



If the surfaces of the sulphides are tarnished, as a rule these 

 sulphides will not float. Taking the Horwood process as an 

 example, the mixture of lead and zinc sulphides is given a 

 slight roast, which is sufficient to cause the galena to become 

 coated with lead sulphate. The zinc blende is not much 

 altered at the low temperature at which the roasting is carried 

 out. When the material is then heated with an oil and acid 

 solution only the zinc sulphide floats. If galena is coated by 

 treating with strong H2SO1 it has a tendency to prevent it 

 floating ; chromic acid has the same effect. Chlorine and nitric 

 acid, which rapidly tarnish the surface of galena, do not in 

 any way prevent flotation, but on the other hand have a 

 beneficial effect. This may be due to the liberation of free 

 sulphur (which does take place) on the surface of the sulphide, 

 sulphur being one of the most easily floated substances. Some 

 ores that will not float in their natural condition will float 

 after being heated, the slight calcining having a beneficial 

 effect on the flotation. This is noticeable with some of the 

 heavy Tasmanian zinc-lead ores. 



