106 G. D. Hubbard — Colloids in Geologic Problems. 



tion process now practiced in many of our ore concen- 

 tration plants is not really a geologic problem, it is so 

 closely connected with geologic materials that I venture 

 to call attention to it here. 



In the last few years, many of our large concentration 

 plants have installed elaborate facilities for flotation of 

 ores. By the use of this process, the heavy sulphides are 

 separated from the gangue minerals in what might be 

 called a reverse gravity method, because the heavy ores 

 come to the top and are from there removed, while the 

 lighter materials go to the bottom of the medium. But 

 gravity has nothing to do with the process. If galena be 

 wetted with water, and a drop of oil be put upon it, the 

 oil displaces the water. Galena adsorbs oil much more 

 strongly than it does water. On the other hand, if oil be 

 spread over a quartz or calcite crystal and a drop of 

 water be put upon it, the oil is at once displaced by the 

 water. In other words, water is adsorbed much better 

 by these common gangue minerals than is the oil. 



For flotation the mineral is finely ground, usually in a 

 wet condition, fine enough to pass through a 48-mesh 

 sieve. A very small amount of pine, or some other, oil 

 with air is beaten into the water which already contains 

 the pulp of ore and gangue. The beating must not go far 

 enough to make an emulsion or bring the oil into a col- 

 loidal state. The oily froth is made up of films of col- 

 loidal thinness, but is not emulsified. In this mixture the 

 fine particles of sulphides adsorb the oil because it wets 

 their surfaces, float to the top by means of their little 

 coats, disperse themselves in the froth, and are scraped 

 off the flotation tank. Then the froth is beaten out and 

 the ore is free from most of the gangue which has, because 

 wet by water, gone to the bottom of the flotation tank. 

 More than 60,000,000 tons of sulphide ores are thus 

 treated in the United States every year and carried much 

 more cheaply to a higher concentration than was obtained 

 by the old methods. 



In the following ways, then, the subject of colloids 

 touches the flotation process. The oil coat held by 

 adsorption on the sulphides is so thin that the oil is really 

 in colloidal state ; in like manner, films of water of col- 

 loidal thinness wet or are adsorbed to the gangues. In 

 a few cases colloidal kaolins and other clay substances 



