The solid-liquid interface 163•Q-~s
Figure 6.7lead sulphide to float. In this way the components of a mixed ore can
be separated.
The detailed theory of flotation is a little more complicated than
indicated by the above account. Bubble adhesion is maximum when
there is only 5-15 per cent monolayer coverage by the collector oil
and decreases with further coverage. It is thought that when the
bubble and particle interfaces merge, penetration of the film of
collector oil around the particle by the film of foaming agent
around the bubble occurs. This interlocking between the two films
stabilises the air bubble-particle system and is, therefore, most
favoured when the particles are only partly covered with a film of
collector oil. The function of the foaming agent as such may,
therefore, be of secondary importance as the particles themselves act
as foam stabilisers (see Chapter 10).
Flotation is also used to enrich fuels (e.g. coal and oil) and as a
purification procedure for effluents and chemical processing inter-
mediates.Detergency^84Detergency is the theory and practice of dirt removal from solid
surfaces by surface chemical means. It accounts for the bulk of all
surfactant usage
Soaps have been used as detergents for many centuries. Soap
normally consists of the sodium or potassium salts of various long-
chain fatty acids and is manufactured by the saponification of
glyceride oils and fats (e.g. tallow) with NaOH or KOH, giving
glycerol as a by-product: