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Flocculant and polyacrylamide
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 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.
anionic flocculant will usually react against a positively charged suspension (positive zeta potential). That is the case of salts and metallic hydroxides.
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
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)
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.
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:
- Sulfonated compounds