Flotation of Minerals, Part II. 



313 



true adsorption of the oil by the mineral. When more oil is 

 added it is probable that a secondary envelope of oil becomes 

 attached to the primary film. When this oil film is thin, there is 

 still the attraction of the oil for the sulphide, but as the film 

 becomes thicker the subsequent overlapping envelopes of oil 

 are held by the attachment of one oil film for another. These 

 overlapping films are less persistently held than the primary 

 films, and some of the oil can be separated mechanically, as by 

 pressure or centrifugal force. The point at which the secondaiy 

 film is thickes't and still stable is represented in the above tests, 

 when no excess of oil shows on the surface of the solution. 



The various stages betwen the primary adsorption and the 

 putty-like product represent the gradual thickening of the 

 secondary oil envelope. The strength of the attachment of the 

 secondary film depends largely on the cohesion of the oil itself, 

 the more viscous oils forming more coherent products and the 

 thinner oils less coherent products. 



In the test on sized galena crushed to pass an 80 sieve but 

 retained on a 120-mesh sieve (linear inch), it was found that 

 all the oil (5 gms. oleic acid) was adsorbed by 60 gms. of the 

 galena. From these data, the approximate thickness of the 

 oil film at this stage in the adsorption can be calculated. As- 

 suming that the average size of the particles is yj^ inch, and that 

 the particles are cubes, the thickness of the oil film would be 

 0.003 centimetre. 



Diagrammatically these successive oil attachments may be 

 fihown as follows : — 



Diagrammatic representation of oil adsorptions showing the 

 gradual thickening of the secondary film of oil from the primary 

 adsorption to the oily magma. The concentric rings represent 

 the oil films round the mineral particles. 



