Polymer, Polyelectrolyte,
or flocculants consist of various molecular weight anionic, nonionic and
cationic polymers. They are used to increase the efficiency of settling,
clarification, filtration and centrifuge dewatering operations.
Coagulation and
flocculation are used to separate suspended solids from water. Although
the terms are used interchangeably, they are in fact two distinct
processes.
Finely dispersed solids
(colloids, less than 1 micron in size) are stabilized by their negative electric
charges, which act to suspend themselves in wastewater. The colloids
negative charges act to repel each other, preventing them from forming larger
masses which hinder the small particles from settling. To remove the
colloidal particles both flocculation and coagulation are required.
Coagulation is the
process of destabilization, by introducing common mineral salts such as aluminum
sulfate, ferric chloride, lime, calcium and magnesium chloride which either
reduce, neutralize or invert the electrical repulsion between the colloidal
particles, which give them the opportunity to grow in size or aggregate, to form
a floc.
Flocculation is
used to describe the action of polymeric materials, which carry active charge
groups, which counterbalance the charge of the colloidal particles, which acts
to bridge into long polymer chains. The floc then settles which is then
easily separated. Anionic flocculant will usually react against positively
charge suspension such as salts and metallic hydroxides. Cationic
flocculant will react against negatively charged suspensions like silica or
organic substances. However, some anionic flocculants agglomerate clays
which are electronegative.
Although there are both
mineral (silica, bentonite, etc.) and natural (starch, alginates, etc.)
flocculants, the most commonly used is synthetic polyacrylamide flocculation
polymer, which is a nonionic polymer. Polymers can be given anionic or
cationic character by copolymerizing them with acrylic acid or cationic monomer,
respectively. Every polyacrylamide has a specific amount of ionic
monomer to give a certain degree of ionic character.
Since it is difficult to
prepare highly concentrated polyacrylamide flocculants, they are commonly
dissolved into 0.5% to 0.1% solutions in cold water. High shear devices
can degrade (deteriorate) the polymer chains so avoid using high speed (agitator
shaft output RPM of 3600, 1800, 1200 RPM's) mixers, dispersers, homogenizers,
disintegrators or centrifugal pumps. All polymer solutions will degrade
over time, where degradation time is dependent upon the solutions
concentration.
Polymer Mix
or Day Storage Tanks:
For polymer day tanks from 50 to approximately 5,000 gallons, mixers with
agitator shaft speeds of 350 RPM's are quite common. Typical solution
viscosity will range from 500 to 5,000 centipoise. Although a 100 RPM
mixer will have a lower shear rate than a 350 RPM mixer, which is beneficial to
the efficiency of the very sensitive polymer solution, economics generally don't
consider this option to be feasible. Since 350 RPM mixers are common, 400
RPM mixers are not recommended. Above 1,500 gallons, consideration may be
given to a slower speed mixer, as the initial cost of the mixer may prove
beneficial versus the continual chemical usage cost efficiencies. Since
the polymer solution is sensitive to shear (over mixing), it is recommended that
the customer run the mixer intermittently after the slurry is made. Hand,
Off, Auto Switches, with dual timers, are generally available upon request,
which will automatically mix the tank contents at specified
intervals.
Vertical
tanks, with height to diameter ratios of 1.0 to1,2 are preferred over horizontal
tanks since they take up less space, are easily supported on a concrete slab and
are simpler to mix. Tanks having mixers installed
vertical-on-tank-centerline generally do not require anti-swirl baffles due to
the process viscosity of the application. If you are using a lower
viscosity solution, three (3) anti-swirl baffles installed 120 degrees
apart, maybe required to prevent foaming and other considerations. Contact
your mixer supplier for their recommendation.
As for the mixer design,
the use of an upper impeller maybe recommended, especially if a solid feeding
system such as a funnel eductor is used. In the event of
dewatering, the upper impellers purpose will be to capture and entrain the
dewatered top phase and pump it downward quickly into to lower regions of the storage tank to
achieve a solution. Without the upper impeller, the lower
viscosity water phase may stratify on the surface and take time
to reincorporate into the batch exposing the solution to over mixing.
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