A surface chemical model of dissolved air flotation

During further development of the model, it was shown that the force of adhesion of water to a surface can be decreased so that air will displace water at the surface.

The preliminary model was validated by evaluating the lack of native flotability of kaolin and bentonite clay in terms of the surface characteristics of the clays. The modified model was validated by using cationic surfactants and inorganic coagulants to decrease the adhesive force of water to the clays, thus rendering them flotable.

The adsorption of the cationic surfactants onto the negatively charged clays was evaluated in terms of the mechanism of adsorption and orientation of the surfactant ions at the surface. As the dose of surfactant was increased, the mechanism of adsorption changed from ion exchange to hemimicelle formation to hydrophobic interaction. The energy of adsorption decreased in the same order.

During ion exchange and hemimicelle formation, the hydrophobic ends of the surfactant ions were oriented toward the water, increasing the flotability of the clay particles. During hydrophobic interaction, the orientation was reversed, decreasing the flotability of the particles. It was possible to monitor the mechanism of adsorption, and thus the flotability of the particles, by measuring the particle zeta potential.

The flotation of the clays using alum and ferric chloride was strikingly similar in many respects to that using surfactants. Again, the flotability of the system could be monitored by measuring the zeta potential. It was concluded that the alum and ferric chloride induced floc was hydrophobic; the floc ensnarement of bubbles was not necessary to explain flotation and probably did not occur.

In general, it was found that good coagulation produced both good settling and good flotation. The inorganic coagulants were effective at slightly lower doses than the surfactants; the efficiency of the surfactants decreased in the order of decreasing effective chain length.

During the course of the experimental work, a new, precise method for measuring the amount of precipitated air was developed. It was found that the amount of air which dissolved at elevated pressures was highly variable with the amount of turbulence and the detention time.

Attempts to measure the size distributions and populations of the colloidal clay systems and the precipitated bubbles with a Coulter Counter met with limited success.