Utilization of natural substances as sedimentation aids forclarifying effluents in the textile industry long predates written history. Thefirst specific mention of lime and aluminus earth for this purpose appeared inthe latter part of the first century AD. Use of such aids in large industrialplants appears to have started in the first part of the nineteenth century. In1951, ushered a new era in solid liquid separations by chemical reagents.They were the first of a wide range of flocculants tailored to meet many needssuch as clarification of water, treatment of municipal sewage and industrialwastes, textile, pulp and paper mills effluents etc.


Theory of Flocculation


In the market place, selection of a flocculants depends onoptimizing the cost to performance ratio that is achieving desired performanceat a minimum cost. Although a systems point of view predominates {includingflocculant availability, reproducibility, handling, storage, tolerance tofluctuations in treatment plant loading, while meeting out put specifications,equipment in place and necessary modifications etc }, the delivered cost perunit weight of individual flocculants always enters as one factor.Consequently, a relatively few monomers suitable for incorporation in to watersoluble polymers and produced on a sufficiently larger scale to have low cost,are the major building blocks for commercially important synthetic polymericflocculants. In the absence of flocculating agent, the energy of interactionbetween a pair of particles { a factor in formation of aggregates }traditionally is formulated as arising from two components, a term a risingfrom overlap of double layers leading to repulsion, which leads to van derWalls attraction. Since dispersion forces are short range, the particles mustbe close together before a significant attractive force develops. The distanceof closest approach for colloidal particles is strongly influenced by themagnitude of the double layer, which therefore, influences the stability of asuspension. Flocculating agents enhance aggregation either by modifying theseenergy terms or by introducing new interaction terms, there by destabilizingthe suspension and effective solids liquids separation. Emphasis onelectrostatic stabilization of suspended particles in an aqueous substrate doesnot imply the absence of other kinds stabilization. The surface characteristicsof particles range from hydrophobic to hydrophilic. Suspensions of hydrophobicparticles such as minerals, tend to be stabilized primarily by surfacecharge.Hydrophylic particles, such as sewage sludes are stabilized by solvationas well as charge. For example, the hydrophilic surface may result from acoating of firmly anchored polysaccharide or similar polymeric species withloops of the polymer extending out from the particle surface in to the aqueousphase. When two particles approach one another and the polymer



segments interpenetrate . Solvent molecules are forced outof interpenetration volume. There is free energy associated with the solventexclusion called steric stabilization, which provides the thermodynamic barrierto close approach of particles. The flocculants currently in commercial use canbe conveniently classified as inorganic, synthetic organic and those derivedfrom natural products { naturally occurring organic polymers which may havebeen modified chemically. These flocculants, polyelectrolytes are they arecalled, have very high molecular weights and are fully soluble in water. Theyhave the ability to bridge between suspended particles.


 

There appears to be five general modes by which flocculating agents bring about destabilization of aqueous suspensions, double layer compression, specific ion adsorption, sweep flocculation- enmeshment, polymer charge patch and polymer bridging. More than one mode of action may be involved at the same time in any real flocculating system. Flocculation would be spontaneous if not for factors which impart stability to suspensions. All particles exert mutual attraction forces effective only at short distances. This results in overall attraction and natural coagulation takes place. However, suspensions are rendered stable by repulsion forces, which prevents particles from coming close enough for these attraction forces to take effect.


There are two main sources of these repulsion forces.

1:Water is adsorbed from the surroundings on to the surface of the particles forming solvation layers, which repel one another.

2: Electrical charges : All particles carry an electrical charge on their surface, the sign and intensity of which depend on the nature of the surface and on the chemistry of the aqueous suspending medium In general aqueous suspensions of Ph 4 and above carry a negative charge. Positively charged suspensions occur in particular in strong acid.


Factors affecting flocculation


1: Polymer charge

2: Ionic strength

3: Ph

4: Slurry solids

5: Flocculant dilution

6: Shear

7: Molecular weight

8: Process conditions




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