A new technique for characterizing aerated flocs in a flocculation–microbubble flotation system

A technique named Aerated Flocs Characterization (AFC) was developed and validated to characterize multiphase systems that are formed in flocculation–flotation (with microbubbles) systems at laboratory scale. Synthetic polyacrylamides were used to flocculate dispersed particles models of kaolin, activated carbon and iron hydroxide colloidal precipitate, Fe(OH)3. Aggregation and solid/liquid separations were conducted in a dissolved air flotation (DAF) apparatus, which consisted of a flotation cell connected by a release needle valve to a pressure vessel, employed for water saturation at 4 atm and microbubbles formation. The aerated flocs exhibited very rapid rising rates (> 60 mh−1) as a result of bubbles adhesion, bubble surface nucleation and bubbles growing and entrapped inside the flocs. All these conformations may adhere or lead to the formation of very light (low density) flocs. The number of bubbles attached or entrapped inside the flocs determines the aeration degree in the so-called aeroflocs, property which depends, among others, on solids type (hydrophobicity); this being very noticeable when compared to activated carbon particles with the less hydrophobic model suspensions. The size distribution, up-rising rates, shape factor, fractal dimension and density of the flocs were determined using this image analysis technique. In addition, the bubble positioning, floc structure and bubble size were monitored. The AFC technique was found to be very reliable showing high statistical reproducibility and appears to have a good potential to characterize particles, aerated flocs and bubbles.

Research highlights
► A technique to characterize multiphase systems (water/floc/bubbles) was developed.
► Single, attached and entrapped bubbles in the aerated flocs were characterized.
► The aerated flocs had high up-rising rates.
► Adhesion, nucleation and entrapped bubbles in the flocs were observed.