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Froth flotation is a process using air bubbles to separate materials based on their relative affinity to water. Bubbles carry reagent and hydrophobic materials to the top of a tank where they can be removed. Froth flotation has been used for more than a century in mining operations to separate valuable materials from excavated ores. More recently, froth flotation is being used for treatment of contaminated water.
The process used in mining begins with the mixing of finely ground ores with water into a tank or cell. A reagent is used to enhance the hydrophobic properties of the desired compounds to separate them from the residual substances which are more hydrophilic. The mixture is agitated to assure an even dispersion.
Air bubbles are introduced at the base of the tank. The tendency of the hydrophobic materials to adhere to the bubbles carries them up to the surface of the tank. At the top, the bubbles carrying their load of minerals, or froth, are skimmed off. The segregated compounds, which are solid, go through a further processing step to separate them from the air bubbles and the reagent residue. The residual hydrophilic material mixed with water in the tank, also known as gangue, is drained away.
The mining uses of froth flotation include the separation of many different types of compounds including sulfides, silicates, phosphates, coal, and iron ore. Reagents or surfactants are carefully chosen to produce exactly the separation effect desired for a particular ore or combination of ores. Many factors affect the quality of separation; these include the rate of flotation, the size of the ore particles, the density of the ore and water mixture, and the amount of air used. A recent use of the process separates ink from recycled paper.
As a method of treatment for contaminated water, froth flotation is particularly well suited to the separation of water containing petroleum products. This process is also known as dissolved air flotation. The steps differ slightly from mining ore separation.
The water is first treated with a chemical to enhance the adhesion of the contaminants to the air bubbles. Some of the water is pumped out and through a retention tank where compressed air is added. The material is circulated back into the flotation cell where the air comes out of suspension as very small bubbles; it then carries the petroleum contaminants and suspended solids to the surface of the cell to be skimmed away. The treated water is pumped off and usually sent for additional filtration or other treatment.
I have definitely heard of this being used in wastewater treatment systems but it just dawned on me that people also apply this principle in cooking. A lot of times when you are making soup stock from scratch, especially using meat products, you have to remove the froth from the top. It has always puzzled me why you have to do this but it now makes sense. The froth when you simmer the soup contains denatured proteins and impurities that cloud it.