The efficiency of particle removal by dissolved air flotation

The efficiency of flotation processes may be improved through an understanding of the flotation models. Two mathematical models, particle trajectory and mixing zone models, have been modified and used to describe flotation results obtained from a semi-continuous flotation rig. Two types of clay suspensions, kaolin and Wyoming bentonite, were used as representative raw materials treated with a cationic surfactant, hexadecyltrimethylammonium bromide (HT AB), and/or coagulants i.e. alum, ferric chloride and polyaluminium chloride (PAC). HT AB concentrations were varied in the range of I x 10-6 to 3 x 10-5 mol/L. Alum at a concentration of 40 mg/L, ferric chloride at 40 mg/L and PAC at 10 mg/L were the selected coagulant dosages to be used in flotation tests. For the clay-HT AB-coagulant system, a HT AB concentration of I x 10-s mol/L was used in the flotation tests. Suspension flow rate was chosen at 2 Llmin and recycle ratios were varied in the range of 6-40% at room temperature. Two categories, suspensions with and without flocs, have been considered. The trajectory model gave a good match even taking account of the decreases In flotation efficiency at high recycle ratios where flocs had been broken by shear gradients. This model included hydrodynamic and surface forces i.e. electrostatic, van der Waals and hydrophobic forces and was calculated by a Runge Kutta technique. The effect of the shear force on a size reduction was determined from particle size measurements (Lasentec apparatus) in a mixing tank and the results showed a decrease of floc sizes with increasing agitator speeds. Bubble zeta potentials obtained using a modified rectangular cell in a Rank Brothers' apparatus gave points of zero charge at concentrations of 1.61 x 10-9 mollL for HTAB, 1.69 x 10-8 mol/L for tetradecyltrimethylammonium (TTAB) and 2.37 x 10-7 mol/L for dodecyltrimethylammonium bromide (DTAB) at 2SoC respectively. Van der Waals and hydrophobic or hydration forces were obtained from contact angle measurements on solid surfaces. The hydrophobic forces were increased by increasing HT AB concentrations while the hydration effects occurred upon the addition of coagulants to the suspensions. A flocculation model using the extended-DLVO theory showed a good correlation when compared to experimental results. For the mixing zone model, an attachment efficiency for the aggregation of a particle and a bubble was proposed from a ratio between the energy barrier (E1) and the maximum free energy at equilibrium. When particle size is constant, the attachment efficiency values rise with increasing hydrophobic force levels. On the other hand, when floc sizes are increased, the attachment efficiencies are decreased due to the increase in the repulsive long range van der Waals force.