What Are The Classification of Flocculating Agents for Water Treatment?

What are the classification of flocculating agents?

Flocculant is a type of substance that can reduce or eliminate the sedimentation stability and polymerization stability of dispersed particles in water, causing the dispersed particles to agglomerate and flocculate into aggregates for removal.

According to chemical composition, flocculants can be divided into inorganic flocculants and organic flocculants.

l Inorganic flocculant

Traditionally used inorganic flocculants are low-molecular aluminum salts and iron salts. The main aluminum salts include aluminum sulfate (AL2(SO4)3∙18H2O), alum (AL2(SO4)3∙K2SO4∙24H2O), and sodium aluminate (NaALO3 ), iron salts mainly include ferric chloride (FeCL3∙6H20), ferrous sulfate (FeSO4∙6H20) and ferric sulfate (Fe2(SO4)3∙2H20).

Generally speaking, inorganic flocculants have the characteristics of easily available raw materials, simple preparation, low price, and moderate treatment effect, so they are widely used in water treatment.

1. Aluminum sulfate

Commercially available aluminum sulfate comes in two forms: solid and liquid. The solid form is divided into refined and crude forms according to the content of insoluble matter. Alum, a solid product commonly used for drinking water purification in my country, is a compound of aluminum sulfate and potassium sulfate. Salt, but is not widely used in industrial water and wastewater treatment.

The applicable pH value range of aluminum sulfate is related to the hardness of raw water. When treating soft water, the suitable pH value is 5 to 6.6. When treating medium hard water, the suitable pH value is 6.6 to 7.2. When treating high hard water, the suitable pH value is 7.2 to 7.8. The applicable water temperature range of aluminum sulfate is 20oC ~ 40oC. The coagulation effect is very poor when it is lower than 10oC. Aluminum sulfate is less corrosive and easy to use, but the hydrolysis reaction is slow and requires a certain amount of alkali.

2. Ferric chloride

Ferric chloride is another commonly used inorganic low molecular coagulant. Its products are available as solid dark brown crystals and liquids with higher concentrations. It has the advantages of being easily soluble in water, having large and heavy alum flowers, good sedimentation properties, and a wide adaptability range to temperature, water quality and pH.

The applicable pH value range of ferric chloride is 9 to 11. The floc formed has a high density and is easy to precipitate. The effect is still very good at low temperature or high turbidity. Solid ferric chloride has strong water absorption, strong corrosiveness, and is easy to corrode equipment. It has high anti-corrosion requirements for dissolving and dosing equipment, has a pungent odor, and has poor operating conditions.

The mechanism of action of ferric chloride is to use various hydroxyl iron ions generated by the step-by-step hydrolysis of ferric ions to achieve flocculation of impurity particles in the water. The formation of hydroxyl iron ions requires the use of a large number of hydroxyl groups in the water, so it will occur during use. It consumes a large amount of alkali. When the alkalinity of the raw water is not enough, it is necessary to supplement alkali sources such as lime.

3.Ferrous sulfate

Commonly known as green vitriol, forms flocs quickly and stably, and the precipitation time is short. It is suitable for situations with high alkalinity and large turbidity, but the color is difficult to remove and is also highly corrosive.

l Inorganic polymer flocculant

Characteristics of inorganic polymer flocculants

The hydroxyl and oxygen-based polymers of Al(Ⅲ) and Fe(Ⅲ) will further combine into aggregates and remain in the aqueous solution under certain conditions. Their particle size is roughly in the nanometer range. In this way, low-density polymers will be obtained by exerting the coagulation-flocculation effect. The result of high dosage.

If we compare their reaction polymerization speeds, the aluminum polymer reacts more slowly and has a more stable form, while the iron hydrolyzed polymer reacts quickly and easily loses stability and precipitates.

The advantages of inorganic polymer flocculants are reflected in its superior performance than traditional flocculants such as aluminum sulfate and ferric chloride, and its lower price than organic polymer flocculants. Now PAC is successfully used in various treatment processes of water supply, industrial wastewater and urban sewage, including pretreatment, intermediate treatment and advanced treatment, and has gradually become a mainstream flocculant. However, in terms of morphology, degree of polymerization and corresponding coagulation-flocculation effect, inorganic polymer flocculants are still in a position between traditional metal salt flocculants and organic polymer flocculants.

1. Polyaluminum chloride PAC

Polyaluminum chloride, referred to as PAC, has a chemical formula of ALn(OH)mCL3n-m. PAC is a multivalent electrolyte that can significantly reduce the colloidal charge of clay impurities (mostly negatively charged) in water. Due to its large relative molecular mass and strong adsorption capacity, the floc formed is larger and its flocculation and sedimentation performance is better than other flocculants.

PAC has a high degree of polymerization and rapid stirring after addition can greatly shorten the floc formation time. PAC is less affected by water temperature and works well when used at low water temperatures. It reduces the pH value of water less and has a wide applicable pH range (can be used in the range of pH=5 to 9), so no alkali agent is needed. The dosage of PAC is small, the amount of mud produced is also small, and it is more convenient to use, manage and operate, and it is also less corrosive to equipment, pipelines, etc. Therefore, PAC has a tendency to gradually replace aluminum sulfate in the field of water treatment. Its disadvantage is that it is more expensive than traditional flocculants.

In addition, from the perspective of solution chemistry, PAC is a kinetic intermediate product of the aluminum salt hydrolysis-polymerization-precipitation reaction process, which is thermodynamically unstable. Generally, liquid PAC products should be used within a short period of time (solid products have stable performance , it can be stored for a longer period of time). Adding certain inorganic salts (such as CaCl2, MnCl2, etc.) or polymers (such as polyvinyl alcohol, polyacrylamide, etc.) can improve the stability of PAC and increase the cohesion capacity.

In terms of production technology, one or several different anions (such as SO42-, PO43-, etc.) are introduced during the manufacturing process of PAC, and the polymerization effect can be used to change the structure and morphological distribution of the polymer to a certain extent, thereby improving the The stability and efficacy of PAC; if other cationic components, such as Fe3+, are introduced during the manufacturing process of PAC, Al3+ and Fe3+ are staggered hydrolyzed and polymerized, and a composite flocculant polymerized aluminum iron can be produced.

2. Organic polymer flocculant

Characteristics of organic polymer flocculants

Synthetic organic polymer flocculants are mostly polypropylene and polyethylene substances, such as polyacrylamide, polyethyleneimine, etc. These flocculants are all water-soluble linear polymer substances. Each macromolecule is composed of many repeating units containing charged groups, so they are also called polyelectrolytes. Polyelectrolytes containing positively charged groups are called cationic polyelectrolytes, and those containing negatively charged groups are called anionic polyelectrolytes. Polyelectrolytes containing neither positively charged groups nor negatively charged groups are called nonionic polyelectrolytes.

The most commonly used polymer flocculants at present are anionic, and they can only play a coagulating role on negatively charged colloidal impurities in water. Often it cannot be used alone, but in combination with aluminum salts and iron salts. Cationic flocculants can exert coagulation and flocculation functions at the same time and can be used alone, so they have developed rapidly.

Currently, polyacrylamide non-ionic polymers are mostly used in our country, often combined with iron and aluminum salts. Satisfactory treatment results are obtained by utilizing the electrical neutralization effect of iron and aluminum salts on colloidal particles and the excellent flocculation function of polymer flocculants. In use, polyacrylamide has the characteristics of small dosage, fast coagulation speed, and large and strong floc particles. 80% of the synthetic organic polymer flocculants currently produced in my country are this product.

1. Polyacrylamide flocculant

Polyacrylamide PAM is currently the most widely used synthetic organic polymer flocculant and is sometimes used as a coagulant. The raw material for the production of polyacrylamide is polyacrylonitrile CH2=CHCN. Under certain conditions, acrylonitrile is hydrolyzed to form acrylamide, and acrylamide is then subjected to suspension polymerization to obtain polyacrylamide. Polyacrylamide is a water-soluble resin, and its products are available in granular solids and viscous aqueous solutions of a certain concentration.

The actual form of polyacrylamide in water is random coils. Since random coils have a certain particle size and some amide groups on their surface, they can have corresponding bridging and adsorption capabilities, that is, they have Certain flocculation ability.

However, because the long chain of polyacrylamide is curled into a coil, its bridging range is small. After the two amide groups are connected, the interactions cancel each other out and the two adsorption sites are lost. In addition, some of the amide groups are entangled in the coil structure. The inside of it cannot contact and adsorb impurity particles in the water, so its adsorption capacity cannot be fully exerted.

In order to separate the bonded amide groups again and expose the built-in amide groups to the outside, people try to properly extend the random coils, and even try to add some groups with cations or anions to the long molecular chain. , while improving the adsorption and bridging ability and the effect of electrically neutralizing and compressing the electric double layer. In this way, a series of polyacrylamide flocculants or coagulants with different properties are derived based on PAM.