Coagulation and Flocculation

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Coagulation:
Promoting the natural aggregation of fine particles during inter-particle collisions by enhancing naturally occurring, short range attractive forces between particles.
Typically coagulation aggregates particles in the 0.1 – 1 μm range to 1 – 10 μm. The aggregates formed are dense with minimal water entrained within the structure
Flocculation:
Bringing together small particles or aggregates into much larger aggregations using long chain molecules which bridge between particles and bind them into a macro-structure (floc).
Typically flocculation aggregates particles in the 1 – 10 μm range to 100 – 10,000 μm. The aggregates normally have a large quantities of water trapped within the macro-structure resulting in lower density particles


Coagulation Flocculation
Coagulation – Inorganic Products

Inorganic Coagulants:


Multi-valent metal ions
•Feric (iron) salts - sulphate & chloride
•Aluminium - sulphate (alum)
•Calcium - hydroxide (lime)
Tri-valent cations are general more effective due to higher charge
Poly Aluminium Chloride (PAC)
•poly - aluminium & hydroxyl coordination complex
•activity quoted as % Al2O3 content
•degree of polymerisation - basicity (ratio of Al & (OH) groups
•normal basicity - 1.5 (50%)
•sulphate increases degree of polymerisation
Alternate Names:- Basic aluminium chloride
Aluminium hydroxy chloride
Aluminium chlorohydrate

Dispersed Particles Coagulated Particles


Mechanism - Charge Neutralisation
Coagulation – Organic Polymers

Organic Coagulants
Low Molecular Weight Cationic Polymers
•polyamines
•polydadmacs
•mannichs
•polyacrylate ester quats (eg DMAEMA quat)
•formaldehyde-amine resins
Mol weight - typically 105 to 2 x 106
Physical Form - usually aqueous solutions
Dispersed Particles Coagulated Particles





Mechanism - Patch Coagulation


Flocculation Mechanisms


Flocculation:
The association of small particles or agglomerates
into large macro- structures (”flocs”) which either settle rapidly from suspension or are suitable for de-watering by mechanical means. (eg pressure filtration)



Complex Flocculation:
Coagulants and flocculants used in combination. The coagulant captures fine, well dispersed colloidal particles whilst the flocculant provides rapid separation from the aqueous suspension.
Flocculants:
High molecular weight (typically 5 - 20M) acrylic polymers, usually based on co-polymers of acrylamide. Both the molecular weight and charge density (positive or negative) may be varied to tailor the polymer to individual applications
Flocculation Mechanisms

Dispersed Particles Flocculated Particles


Mechanism - Bridging Flocculation

 

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