Bubble Floc Collision and Attachment in the Mixing Zone

There are three possible mechanisms for forming aggregates of bubbles and particles:

1. entrapment of preformed bubbles in large floc structures (floc size much larger than bubble size scale)

2. growth of bubbles whose nuclei formed on particles or within flocs

3. particle collision with adhesion to preformed bubbles

For DAF processes, the third mechanism is the most important.

Classically, the contact angle between the adsorbed bubble and particle has been used to characterize the extent of bubble-floc adhesion. Here the contact angle must be finite and large enough that the energy of adhesion of water to the solid particle is less than the energy of cohesion of water. A larger contact angle indicates both hydrophobicity and good adhesion. The magnitude of the contact angle, however, depends on the size of the bubbles and particles. A different view of particle-bubble attachment of colloidal particles by small bubbles is that a finite contact angle need not form. In this heterocoagula-tion model, the stability of charged particles and bubbles is described. Attachment requires reduction in electrical charge interactions and attraction by London-van der Waals forces as particles are transported to bubble surfaces.

Both the contact angle and the heterocoagulation models predict experimentally observed trends that two conditions are necessary for favourable flotation: charge neutralization of the particles and production of hydrophobic particles. Bubble attachment to particles requires hydrophobic particle surfaces or hy-drophobic regions on the particles. For many particles, hydrophobicity can be increased by reducing the negative charge. Other particles, such as freshly precipitated or amorphous Al(OH)3, have polar surface groups that make them hydrophilic. This hydrophilic effect may be reduced by charge neutralization, but aluminium hydroxide particles have a polymolecular coating of water which hinders bubble adhesion.

Solar Panel Basics

Solar Panel Basics

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