Popular Science Monthly 



85 



bubble. Surface tension, as just de- 

 scribed, applies to a level water surface. 

 The surface of a quiet body of water is 

 always level since the tendency is to re- 

 duce the area to a minimum. The same 

 holds good in the case of rain drops and 

 bubbles. A drop of water falling through 

 the air becomes spherical since the 

 sphere is the figure of least surface for a 

 given volume. A bubble also assumes the 

 spherical form for the same reason. 



It would at first seem that some in- 

 terior force causes the bubble to burst or 

 explode. This is not so, if the prevailing 

 theories are correct. It is believed that 

 the surface tension acts like a rubber skin 

 over a bubble, constantly exerting an 

 inward pressure which finally results in 

 the collapse of the bubble. If now we 

 reduce this tension, the bubble will exist 

 much longer. For this purpose, oil is 

 added to the water containing the sul- 

 phides and gangue, but in very small 

 quantity. ^ 



There 

 was an ear- 

 lier process 

 using as 

 much as 

 three tons 

 of oil to a 

 ton of ore. 

 In 1898, F. 

 E. Elmore 

 invented 

 the so-call- 

 ed bulk-oil 



process. Strictly speaking, it was not a 

 flotative process at all, the object being 

 to recover the sulphides by using a very 

 large quantity of heavy oil. The buoy- 

 ancy of the oil was the sole agent in 

 floating the metals. Sulphides, being 

 wetted by sufficient oil to overcome the 

 effect of specific gravity, traveled upward 

 into the oil layer, and were consequently 

 floated. 



How the Army of Bubbles Work 



In the froth flotation process only the 

 faintest trace of oil is used. Briefly, the 

 ore pulp, consisting of finely-ground ore 

 particles suspended in a large quantity of 

 water, is brought into contact with a 

 minute quantity of oil. Through agita- 

 tion, countless tiny bubbles are formed 

 which carry the mineral particles to the 

 surface with them, forming a dense froth 

 several inches in thickness. The gangue 

 sinks and is allowed to go to waste. 



-^ Injecting 



bubbles in- 

 to the li- 

 q u id by 

 means of 

 compressed 

 air has been 

 tried gener- 

 ally with 

 less success 

 than when 

 the bubbles 

 are formed 

 within the 



Increasing the Lifting Power of Bubbles with Oil 



Above are shown bubbles of pine oil froth, to which 

 are adhering particles of cassiterite (the oxide of 

 tin). Pine oil is one of the best oils for flotation 

 purpxjses, but it is costly. The creosotes, crude 

 petroleum, and the coal tar oils have favor, some- 

 times with the addition of a small amount of pine oil 



At left: Cresylic acid froth. In this case the agi- 

 tator has been stopped and the bubbles are 

 beginning to coalesce. They are large and thin- 

 filmed. At right is shown a pine oil froth, which 

 usually has small bubbles. After the impeller has 

 ceased its action, the froth coalesces into a scum 



