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|Title: ||The efficiency of particle removal by dissolved air flotation|
|Authors: ||Petiraksakul, Anurak|
|Keywords: ||Dissolved air flotation|
|Issue Date: ||1999|
|Publisher: ||© Anurak Petiraksakul|
|Abstract: ||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.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Chemical Engineering)|
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