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What Is the Difference Between Coagulants and Flocculants?

2020-11-23

I. Definition of flocculation and classification of flocculants

Flocculation is the process by which the polymer's polymer chains bridge between suspended particles. "Bridge" is the adsorption of different segments of polymer molecules on different particles to promote the aggregation of particles and particles. Flocculants are mostly polymers and have specific electrical properties (ionicity) and charge density (ionicity).

Flocculant is generally divided into organic flocculant and inorganic flocculant. Inorganic flocculants include ferrous sulfate, ferrous chloride, alum, polyaluminum chloride, basic aluminum chloride, aluminum sulfate, calcium chloride, etc .; organic flocculants are mainly polymer flocculants. Among them are sodium polyacrylate, polyacrylamide, polystyrene sulfonate, polyethylene oxide, and the like.

Second, the principle of coagulation and the type of coagulant

When particles suspended in water have a small particle size, the energy of Brownian motion is sufficient to prevent the effect of gravity, so that the particles do not settle. This suspension can remain stable for a long time. Moreover, the surface of the suspended particles is often charged, and often the repulsive force of the same charge between the negatively charged particles makes the particles difficult to merge and grow, thereby increasing the stability of the suspension.

The coagulation process is to add a positively charged coagulant to neutralize the negative charge on the surface of the particles, and to "destabilize" the particles. Therefore, the particles are combined with each other through collision, surface adsorption, and van der Waals gravitational forces to facilitate separation from water.

Coagulants are water-soluble polymers with a low molecular weight and a high positive charge density, most of which are liquid. They are divided into two categories, inorganic and organic. Inorganic coagulants are mainly aluminum, iron salts and their polymers.

3. Mechanism and classification of coagulant

Coagulants are some auxiliary agents added to improve or strengthen the coagulation process. Its working principle is related to specific applications. For the case of algae overproduction, oxidants can be added for pre-oxidation to improve the coagulation effect. Organic high Molecular coagulants increase floc density and improve coagulation and sedimentation effect. For low-temperature and low-turbidity water treatment, due to its large viscosity and poor flocculation sedimentation performance, the coagulant dosage is increased. At this time, organic or inorganic coagulants are added. Polymer coagulants increase floc size, increase floc density, and increase sedimentation speed. For raw water with lower alkalinity, the coagulation process will cause the pH to drop, which not only affects the coagulation effect, but also produces acidic water, which is not good for The water quality of the pipe network is stable, so you need to add alkali to adjust the pH. For organic color water, not only the coagulant dosage is increased, but the sedimentation performance is deteriorated. A certain amount of organic polymer coagulant can be added to improve the sedimentation performance. A certain amount of oxidant can also be added to destroy the stability of colloids by organic matter. For wastewater containing iron and manganese, the oxidant can destroy the organic complexes of iron and manganese, which is beneficial for the removal of iron, manganese and organic matter in water.

Types of coagulation aids: 1. Organic and inorganic polymers, such as activated silicic acid, polyacrylamide, bone glue, etc. 2. pH adjusters such as hydrochloric acid, sulfuric acid and soda lime. / 3. Inorganic particles such as clay and micro sand. / 4. Oxidants such as potassium permanganate, poly aluminum chloride, etc.

Poly Aluminum Chloride

In practical applications, coagulants / flocculants / coagulants are high-molecular substances, and there are large and small molecules in the same product. The so-called "molecular weight" is just an average concept. Therefore, when a certain coagulant, coagulant or flocculant is used to treat sewage, the "electrical neutralization" and "bridging" effects will be intertwined and occur simultaneously. The flocculation process is the result of a combination of factors, and there are still some problems that have not been identified and resolved. As far as we know, the flocculation process is related to the molecular structure, charge density, and molecular weight of the flocculant; it is related to the surface properties, particle concentration, and specific surface area of the suspended particles; it is related to the pH value of the medium (water), the conductance, the presence of other substances in the water, and the water temperature , Agitation and other factors. Therefore, despite the theory and experience to follow, the selection of flocculants by experiments is still indispensable.