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Foam flotation separates materials based on their affinity for water using air bubbles. It has been used for over a century in mining and is now used to treat contaminated water. In mining, a reagent is added to enhance hydrophobic properties, and air bubbles carry the desired compounds to the surface. In treating contaminated water, a chemical is added to improve adhesion of contaminants to air bubbles, which are then skimmed off.
Foam flotation is a process that uses air bubbles to separate materials based on their relative affinity for water. Bubbles carry reagents and hydrophobic materials to the top of a tank where they can be removed. Foam flotation has been used for more than a century in mining operations to separate valuable materials from mined ores. More recently, foam flotation is being used for the treatment of contaminated water.
The process used in mining begins with mixing finely ground ore with water in a tank or cell. A reagent is used to enhance the hydrophobic properties of the desired compounds to separate them from residual substances which are more hydrophilic. The mixture is stirred to ensure uniform dispersion.
Air bubbles are introduced to the base of the tank. The tendency of hydrophobic materials to adhere to bubbles carries them all the way to the surface of the tank. At the top, bubbles carrying their mineral load, or foam, are skimmed off. The segregated compounds, which are solids, undergo an additional processing step to separate them from air bubbles and reagent residues. The remaining hydrophilic material mixed with water in the tank, also known as gangue, is drained.
The mining uses of froth flotation include the separation of many different types of compounds including sulfides, silicates, phosphates, coal, and iron ore. The reactants or surfactants are carefully chosen to produce exactly the desired separating effect for a particular mineral or combination of minerals. Many factors influence the quality of the separation; these include the flotation rate, the particle size of the ore, the density of the mineral-water mix, and the amount of air used. A recent use of the process separates ink from recycled paper.
As a method of treating contaminated water, foam flotation is particularly suitable for the separation of waters containing petroleum products. This process is also known as dissolved air flotation. The steps differ slightly from the separation of the mining ore.
The water is first treated with a chemical to improve the adhesion of contaminants to the air bubbles. Some of the water is pumped out and through a holding tank where compressed air is added. The material is recirculated in the flotation cell where the air comes out of the suspension in the form of tiny bubbles; it then carries the oil contaminants and suspended solids to the surface of the cell to be skimmed. The treated water is pumped out and usually sent for further filtration or other treatment.
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