Concentration by Ion Flotation

Abstract

IT has been found possible to concentrate inorganic ions from aqueous solutions, even if very dilute, by a flotation technique. The principle depends upon the use, as a collector, of a surfactant ion, of charge opposite to the ion to be floated. The surfactant must be introduced in such a way that it exists as a simple ion, not as a micelle. This means that the concentration of the surfactant should not be allowed to exceed the critical micelle concentration, but, also, as soaps have a tendency to age on standing, the soap should be freshly prepared in alcoholic solution, or, preferably, dissolved in a non-polar solvent, such as petrol ether, which is evaporated off, followed by immediate solution in ethyl or isopropyl alcohol. By bubbling a gas, usually air, into the solution, through a fine gas distributor, an extended air–water interface is produced. The surfactant tends to concentrate at the bubble, so orientated that the polar head carrying the charge is on the water side of the water–air bubble interface. There is an attraction between it and the charged ion in the solution, which seems more marked if the ion is polyvalent. The collector and ion are carried to the surface by the bubble, where a froth or scum is produced, depending on whether there is excess surfactant or not. As the froth drains and the bubbles break, the concentration of the collector-ion product increases to form, ultimately, a characteristic scum, often coloured, of insoluble soap, which can be easily removed.