Page:Popular Science Monthly Volume 92.djvu/100

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��Popular Science Monthly

��process which practically reverses the long-used wet method of concentration. Instead of sinking the sulphides, they are induced to float and the gangue is allowed to sink. Flotation is the term applied to this revolutionary method.

Thehistoryof flotation, like that of most great industrial processes, is not centered around any one man. Its development, though rapid, has involved a long list of patents taken out by a large number of American and foreign metallurgists.

The first patent which even suggested the process now known as flotation was obtained in 1860 by William Haynes. He knew that sulphides would stick to oil and in a crude way tried to use this principle in separat- ing the metal from the gangue. He was followed by Bradford, whose method involved surface tension concerning which I shall speak later.

���The Floating Spider

The underlying principles governing flotation are too theoretical to admit of satis- factory explanation. The how is more easily explained than the why. Have you never ob- served the trim little water spider go skating across a pool with the greatest ease and agility? And did you make the mistake of believing that he was floating simply because he is so light? Then try floating a needle on the surface of a glass of water. It can easily be ac- complished and you will note that the much heavier needle seems to lie in a sort of depression in the surface of the water and does not readily become wet. This is due to surface tension, supposed to play an important role in flotation.

In surface tension we have a tendency on the part of a liquid to act somewhat like an elastic skin, trying always to con- tract to the minimum area. A drop of water does its best to shape itself into a neat little round sphere instead of spreading out over a large surface. But this is only on surfaces which water does not wet. In contact with paraffined paper, for instance, it maintains the drop form; on the other hand, it cjuickly sinks

��A Bubble Bursts by Crushing Itself

It is believed that surface tension acts like a rubber membrane over the bubble, constantly exerting an in- ward pressure which finally results in its collapse. That tension is reduced by the addition of a delicate film of oil or other viscous substance

��into the meshes of a piece of blotting paper. If a needle is perfectly clean, it will sink; if it is greasy, it can be made to float. The explanation involves two phenomena, surface tension and adhesion.

Surface Tension Is a Force

Surface tension would at first give the impression that an actual film or skin were stretched over the surface of the liquid. In reality, the needle is supported by a force and not by the water itself. This is proved when the needle sinks — the water cannot hold it up.

There exists between the molecules of any body an attraction which holds them together. At the surface of a body of water, the top layer of molecules lacks an attrac- tion from the outside. This lack is compensated by a greater attraction from be- low and from the sides. Thus a horizontal stretching is produced — called surface tension.

This force can be readily upset by bringing about an attraction from above so that the attraction of the top layer of molecules will be more nearly equal in all directions. Now, water molecules are attracted by iron; consequently a clean needle be- comes wet if brought into contact with the water. But water molecules are not attracted by grease; and so a greasy needle does not become wet — which means that a tiny film of air remains around the needle; and the molecules in the top layer of water are still attracted in a horizontal direction (surface tension) so that the needle cannot sink.

Sulphides are not readily wet by water. If crushed into finely divided particles, they tend to float because of surface ten- sion. Gangue, on the other hand, is easily wet by water; consequently it sinks. This may be termed film flotation as distinguished from the newer and far more efficient bubble method known as froth flotation.

A Bubble Bursts by Crushing Itself Consider the surface tension of a

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