Flotation is a process where gas bubbles are injected at the bottom of a tank containing a mixture of a liquid and solid particles to separate the solid particles from the liquid. When the gas bubbles move along the liquid mixture, the solids particles stick to the surface of the bubbles. The bubbles form a foam at the surface of the liquid: this foam contains the solid particles, so after the flotation process the solids can be removed just cleaning the liquid from this foam.
Flotation is used to separate sulfide ores from the impurities in the ore (gangue). A detergent is added to the mixture and it is stirred. The sulfide minerals float, while the gangue sinks to the bottom and is removed. This is used in making zinc, where zinc sulfide (sphalerite) is a major zinc ore.