Foam separation is a process of removing a component of a liquid mixture using a bubbled gas and a surfactant to collect the material. A surfactant is a chemical that helps produce foam without chemically reacting with the solution. Many materials can be removed from liquids using foam separation methods, including precious metals, proteins, and impurities from water.
Another term for foam separation is adsorptive bubble separation. Adsorption means that a material is deposited on the surface of the foam without a chemical reaction having occurred. The foam has a surface tension, and in some cases a molecular polarity, that attracts the desired material onto the foam surface.
There are a number of chemical methods for extracting gold from ore, but many of them include toxic chemicals and waste products. Gold can be extracted from a liquid mixture using air or nitrogen gas and a foam-producing surfactant. The foam exiting the equipment is enriched in gold, which can then be separated by centrifuges that spin out the gold and remove the foam.
Separation of proteins and oils can be accomplished with foam separation equipment. The surfactants attract the desired product onto the foam interface by preferring the organic or oil molecules to water molecules. Foam systems may use non-reactive nitrogen gas rather than compressed air to prevent any oxidation reactions of air with the desired materials.
Wastewater treatment systems often use foam separation steps to remove solids from the water stream. Larger solids can be removed by gravity or centrifuge operations, but fine solids must also be removed to meet treated water standards. Air can be bubbled along with a surfactant to create a foam layer that rises to the top of the foam tank. The fine solids will be adsorbed onto the foam, which can overflow the tank or be skimmed from the top.
Foam separation can be operated as batch or continuous operations.Batch processes will place the liquid mixture into a processing tank and produce foam until the majority of the product is removed.Product analysis can determine when the processing is complete, and the equipment can be re-filled and the process repeated.
Continuous production systems add the mixture either into the bottom of the vessel or at a point on the side. Air or other gas and surfactant are added at the bottom and are mixed as the gas moves upward. The foam will be carried to the top of the tank or a collection point on the side above the liquid level. Further processing can remove the surfactant from the desired product stream.