Page:Popular Science Monthly Volume 92.djvu/101

 Popular Science Monthly

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��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

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