Polymers: Anionic and Cationic
Flocculation and Polymer Principles
The efficiency of solid/liquid separation may be greatly improved by the application of synthetic flocculants, (sometimes misspelled as flocculents) or polymers. This improvement is achieved by bringing dispersed particles together increasing the effective particle size of the solid phase. The stability of the suspension is broken and the liquid phase is then released.
Most anionic and cationic flocculants are man-made polymers based on repeating units of acrylamide and its derivatives, which may contain either cationically or anionically charged species. The term flocculation is often confused with coagulation, although the two refer to quite different processes.
Coagulation Water Treatment
What is coagulation water treatment?
Coagulation water treatment affects any suspended fines which should be removed from the aqueous solution. These principles are in evidence in any water treatment or waste water treatment application.
Coagulation water treatment is an electrical reaction in which particles are able to merge together by a reduction of the repulsive forces of the electrical double layer which kept particles apart from one another or “in suspension.” Flocculation occurs when a polymer, either an anionic flocculant or a cationic flocculant forms a bridge between particles, creating a larger mass of suspended solids. Anionic and cationic flocculants make solids removal possible.
Theory of Flocculation
What is flocculation?
Flocculation is when the suspended particles are physically precipitated to the the bottom of a clarifier vessel by both the flocculant or flocculent and the force of gravity which can function because the flocculant or flocculent has increased suspended solids’ mass by agglomerating fines into larger particles. When that happens, gravity can function without electrical repulsion interference. Anionic and cationic flocculants or flocculents function in this fashion whether the application is in a water treatment or waste water treatment facility.
All particles exert mutual attraction forces, effective only at short distances of separation. When surfaces are brought close together, these forces, known as London-van der Waals forces, take over. This results in overall attraction and natural coagulation takes place. Coagulation/flocculation would happen at once if the suspension were not intact. However, suspensions are rendered stable by repulsion forces, which prevent particles from coming close enough for these attraction forces to take effect. There are two main sources of these repulsion forces: (a) Water is adsorbed from the surroundings onto the surface of the particles which repel one another. (b) Electrical charges – all particles carry an electrical charge on their surfaces, the sign and intensity of which depend on the nature of the surface and on the chemistry of the suspension.
Not only does repulsion prevent particles from coming together, but it also retards settling by keeping the particles in constant motion. Charge repulsion effects are most noticeable with very small particles.
What is an anionic polymer?
An anionic polymer is an long chain molecule with repeating units, of acrylate and acrylamide, which have a net external negative charge.
An anionic polymer is then electrically attracted to a positively charged species such as a mineral, such as iron ore.
What is a cationic polymer?
A cationic polymer is an long chain molecule with repeating units of acrylamide which have a net external positive charge. A cationic polymer is then electrically attracted to a positively charged species such as plant matter, algae, for example.