8-t 



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 



