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Flocculation by definition means a process in which individual
particles of a suspension form aggregates. In the water
treatment industry, the terms coagulation and flocculation
imply different mechanisms. Flocculants consist of various
molecular weight anionic, nonionic and cationic polymers.
They are used to increase the efficiency of settling, clarification,
filtration and centrifugation operations.
FLOCCULATION
FLOCCULATION
is used to describe the action of polymeric materials which
form bridges between individual particles. Bridging occurs
when segments of a polymer chain adsorb on different particles
and help particles aggregate. Flocculants carry active groups
with a charge which will counterbalance the charge of the
particles. Flocculants adsorb on particles and cause destabilization
either by bridging or charge neutralization.
An
anionic flocculant will usually react against a positively
charged suspension (positive zeta potential). That is the
case of salts and metallic hydroxides.
A
cationic flocculant will react against a negatively charged
suspension (negative zeta potential) like silica or organic
substances.
However
the rule is not general. For example, anionic flocculants
agglomerate clays which are electronegative.
Three
groups of flocculants are currently used
1.1
- MINERAL FLOCCULANTS
They
are colloidal substances. Adsorption and charge neutralization
play some part in the flocculation mechanism. They are:
-
activated
silica.
-
certain
colloidal clays (such as bentonite),
-
certain
metallic hydroxides with a polymeric structure (alum,
ferric hydroxide)
1.2-
NATURAL FLOCCULANTS
They
are water soluble anionic, cationic or nonionic polymers.
Nonionic polymers adsorb on the suspended particles. The
most common natural flocculants are:
- the starch
derivatives: mostly pregelatinized hence water-soluble.
They are corn or potato-starches. They can be natural
starches, anionic oxidized starches or amine treated
cationic starches. The use of this class of products
has decreased in water treatment but remains important
in the paper industry.
- the polysaccharides:
usually guar gums and mostly used in acid medium.
- the alginates:
anionic and used in potable water treatment.
1.3-
SYNTHETIC FLOCCULANTS
Polyacrylamides
The
most common polymers are those based on polyacrylamide,
which is a nonionic polymer. Their effect is due to bridging
between particles by polymer chains.
Polymers
can be given anionic character by copolymerizing acrylamide
with acrylic acid. Cationic polymers are prepared by copolymerizing
acrylamide with a cationic monomer. All available acrylamide
based polymers have a specific amount of ionic monomer giving
a certain degree of ionic character.
They
have a specific average molecular weight (i.e. chain length)
and a given molecular distribution.
For
each suspension, a certain degree of anionic, cationic or
nonionic character is beneficial. Usually, the intrinsic
flocculating power increases with the molecular weight.
Polyacrylamides
have the highest molecular weight among the synthesized
industrial chemicals in the range of 10-20 millions. Other
polymers display specific properties and are used under
specific conditions.
They
are mostly:
- Polyethylene-imines
- Polyamides-amines
- Polyamines
- Polyethylene-oxide
- Sulfonated
compounds
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