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What is a flocculant?

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1. What is a flocculant?


As a means of strengthening solid-liquid separation in the field of sewage treatment, flocculants can be used to strengthen the primary sedimentation of sewage, flotation treatment and secondary sedimentation after activated sludge method, and can also be used for tertiary treatment or advanced treatment of sewage. When used for conditioning of excess sludge before dewatering, flocculants and coagulants become sludge conditioners or dewatering agents.


The mechanism of action of the flocculant: the colloidal particles in water are small, the surface is hydrated and charged to make it stable, and the flocculant is hydrolyzed into the charged colloid and its surrounding ions to form a micelle with an electric double layer structure after being added to the water.


The method of rapid stirring after administration is adopted to promote the collision chance and number of micelles formed by the hydrolysis of colloidal impurity particles in the water and the flocculant. The impurity particles in the water first lose their stability under the action of the flocculant, and then agglomerate with each other into larger particles, and then settle or float in the separation facility.


The product GT of the velocity gradient G generated by stirring and the stirring time T can indirectly represent the total number of particle collisions in the entire reaction time, and the coagulation reaction effect can be controlled by changing the value of GT. Generally, the GT value is controlled between 104 and 105. Considering the influence of the impurity particle concentration on the collision, the GTC value can be used as a control parameter to characterize the coagulation effect, where C represents the mass concentration of impurity particles in the sewage, and it is recommended that the GTC value be within 100 or so.



The process of making the flocculant diffuse quickly into the water and mix it with all the wastewater is mixing. The impurity particles in the water interact with the flocculant, and through mechanisms such as compressing the electric double layer and electric neutralization, the stability is lost or reduced, and the process of generating micro flocs is called agglomeration. The process of agglomeration to form micro-flocs under the stirring of bridging substances and water flow, through adsorption bridging and sediment net capture, etc., grows into large flocs, which is called flocculation. Mixing, coagulation and flocculation together are called coagulation. The mixing process is generally completed in the mixing tank, and the coagulation and flocculation are carried out in the reaction tank.


2. Types of flocculants


Flocculants are a class of substances that can reduce or eliminate the precipitation stability and polymerization stability of dispersed particles in water, and make dispersed particles agglomerate and flocculate into aggregates for removal. According to the chemical composition, flocculants can be divided into three categories: inorganic flocculants, organic flocculants and microbial flocculants.


The following mainly introduces the most commonly used inorganic and organic flocculants:


Inorganic flocculants include aluminum salts, iron salts and their polymers.


Organic flocculants can be divided into anionic, cationic, nonionic, amphoteric and other types according to the charge properties of the charged groups of the polymerized monomers. According to their sources, they can be divided into two categories: synthetic and natural polymer flocculants. .


3. Inorganic flocculants


The traditional inorganic flocculants are low molecular aluminum salts and iron salts. Aluminum salts mainly include aluminum sulfate (AL2(SO4)3∙18H2O), alum (AL2(SO4)3∙K2SO4∙24H2O), 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 easy availability of raw materials, simple preparation, low price, and moderate treatment effect, so they are widely used in water treatment.



1. Aluminum sulfate



Since the United States first used aluminum sulfate for water supply treatment and obtained a patent in the late 19th century, aluminum sulfate has been widely used for its excellent coagulation and sedimentation performance. Aluminum sulfate is currently the most used flocculant in the world. The annual output of aluminum sulfate in the world is about 5 million tons, of which nearly half is used in the field of water treatment.


Commercially available aluminum sulfate has two forms: solid and liquid. The solid one is divided into two types: refined and crude according to the content of insolubles in it. Alum, a solid product commonly used in drinking water purification in my country, is the compound of aluminum sulfate and potassium sulfate. Salt, but not much used in industrial water and wastewater treatment.


The suitable pH range of aluminum sulfate is related to the hardness of the raw water. When processing soft water, the suitable pH value is 5-6.6, when processing medium-hard water, the suitable pH value is 6.6-7.2, and when processing high-hard water, the suitable pH value is 7.2-7.8. The applicable water temperature range of aluminum sulfate is 20oC ~ 40oC, and 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 a certain amount of alkali needs to be consumed.




2. Ferric chloride



Ferric chloride is another commonly used inorganic low-molecular-weight coagulant. The products include solid dark brown crystals and liquids with higher concentrations. It has the advantages of being easily soluble in water, large and heavy alum, good precipitation performance, and wide adaptability to temperature, water quality and pH.


The applicable pH range of ferric chloride is 9 to 11. The flocs formed have high density and easy precipitation, and the effect is still very good at low temperature or high turbidity. Solid ferric chloride has strong water absorption, strong corrosiveness, easy to corrode equipment, high requirements for anticorrosion of dissolving and dosing equipment, pungent odor, and poor operating conditions.


The mechanism of action of ferric chloride is to use various hydroxyl ferric ions generated by the gradual hydrolysis of ferric ions to achieve flocculation of impurity particles in water. A large amount of alkali is consumed. When the alkalinity of the raw water is not enough, it is necessary to supplement the alkali source such as lime.


Ferrous sulfate, commonly known as green vitriol, forms flocs quickly and stably, with short precipitation time. It is suitable for situations with high alkalinity and high turbidity, but the color is not easy to remove, and the corrosiveness is also strong.


4. Inorganic polymer flocculants


Inorganic polymer flocculant is a new type of flocculant developed in the world since the 1960s. At present, its production and application have made rapid progress all over the world.


Inorganic polymer flocculants of aluminum, iron and silicon are actually their intermediate products from hydrolysis, sol to precipitation, namely hydroxyl and oxygen polymers of Al(III), Fe(III) and Si(IV). . Aluminum and iron are positively charged by cation, and silicon is negatively charged by anion. Their unit molecular weights in the water-soluble state are about hundreds to thousands, and they can be combined with each other to form aggregates with fractal structures.


Their agglomeration-flocculation process is a comprehensive manifestation of the two effects of electric neutralization and adhesion bridging of particles in water. The particle size of suspended particles in water ranges from nanometers to micrometers, and most of them are negatively charged. Therefore, the positive and negative charges, electrical strength and molecular weight of the flocculant and its form, and the particle size of aggregates are the main factors that determine its flocculation effect. At present, there are dozens of types of inorganic polymer flocculants, and the output has reached 30% to 60% of the total output of flocculants. Among them, the most commonly used and widely used is polyaluminum chloride, namely PAC.




1. Characteristics of inorganic polymer flocculants



The hydroxyl and oxygen-based polymers of Al(III) and Fe(III) will be further combined into aggregates, which will be kept in the aqueous solution under certain conditions, and their particle size will be in the nanometer range. Result of high dosage.


Comparing their reaction and polymerization rates, the reaction of the aluminum polymer is milder and the shape is more stable, while the hydrolyzed polymer of the iron reacts rapidly and easily loses stability and precipitates.


The advantages of inorganic polymer flocculants are reflected in that it is more efficient than traditional flocculants such as aluminum sulfate and ferric chloride, and is cheaper than organic polymer flocculants. Now PAC has been 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.




2. Polyaluminum Chloride PAC



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


PAC has a high degree of polymerization, and rapid stirring after adding can greatly shorten the floc formation time. PAC is less affected by water temperature, and it works well when the water temperature is low. It reduces the pH value of water less, and the applicable pH range is wide (can be used in the range of pH=5~9), so it is not necessary to add alkaline agent. The dosage of PAC is small, the amount of mud produced is also small, and the use, management and operation are more convenient, and it is also less corrosive to equipment and pipelines. Therefore, PAC has a tendency to gradually replace aluminum sulfate in the field of water treatment, and its disadvantage is that the price is higher than that of traditional flocculants.


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


In terms of production process, one or several different anions (such as SO42-, PO43-, etc.) are introduced in the manufacturing process of PAC, and the polymer structure and morphological distribution can be changed to a certain extent by polymerizing, thereby improving the The stability and efficacy of PAC; if other cationic components, such as Fe3+, are introduced in the manufacturing process of PAC to make Al3+ and Fe3+ staggered hydrolytically polymerized, composite flocculant polyaluminum iron can be obtained.


5. Organic polymer flocculant


Synthetic organic polymer flocculants are mostly polypropylene and polyethylene substances, such as polyacrylamide and polyethyleneimine. These flocculants are all water-soluble linear macromolecules, each macromolecule consists of many repeating units containing charged groups, so they are also called polyelectrolytes. Those containing positively charged groups are cationic polyelectrolytes, and those containing negatively charged groups are anionic polyelectrolytes, which contain neither positive nor negatively charged groups, and are called nonionic polyelectrolytes.


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


Currently, polyacrylamide non-ionic polymers are used more frequently in my country, which are often used in combination with iron and aluminum salts. The electric neutralization effect of iron and aluminum salts on colloidal particles and the excellent flocculation function of polymer flocculants are used to obtain satisfactory treatment effects. Polyacrylamide has the characteristics of less dosage, fast coagulation speed, and large and tough flocs in use. 80% of the synthetic organic polymer flocculants currently produced in my country are this product.




Polyacrylamide flocculant



Polyacrylamide PAM is the most widely used synthetic organic polymer flocculant, and is sometimes used as a coagulant. The production raw material 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 the products are granular solid and viscous aqueous solution with a certain concentration.


The actual existing form of polyacrylamide in water is random coil. Because the random coil has a certain particle size and some amide groups on its surface, it can play the corresponding bridging and adsorption capacity, that is, it has a certain particle size. certain flocculation capacity.


However, because the long chain of polyacrylamide is curled into a coil, its bridging range is small. After the two amide groups are connected, it is equivalent to the mutual cancellation of the interaction and the loss of two adsorption sites. In addition, some of the amide groups are wrapped in the coil structure The inside of it cannot contact and adsorb the impurity particles in the water, so its adsorption capacity cannot be fully exerted.


In order to separate the linked amide groups again and expose the hidden amide groups to the outside, people try to extend the random coil appropriately, 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 electric neutralization and compression of the electric double layer. In this way, a series of polyacrylamide flocculants or coagulants with different properties are derived on the basis of PAM.


For example, adding alkali to the polyacrylamide solution converts the amide groups on some chain segments into sodium carboxylate, and sodium carboxylate easily dissociates sodium ions in water, so that the COO-group remains on the branched chain, thus generating partial Hydrolyzed anionic polyacrylamide.


The COO-group on the molecular structure of anionic polyacrylamide makes the molecular chain have a negative charge, which repels each other and pulls apart the amide groups that were originally linked together, so that the molecular chain is gradually extended from a coil shape to a long chain shape, so that the frame is formed. The bridge range is expanded and the flocculation ability is improved, and its advantages as a coagulant aid are even more outstanding.


The use effect of anionic polyacrylamide is related to its "hydrolysis degree". If the "hydrolysis degree" is too small, the coagulation or coagulation aid effect will be poor. If the "hydrolysis degree" is too high, the production cost will be increased.


6. Factors affecting the use of flocculants


1. The pH of the water


The pH value of water has a great influence on the use effect of inorganic flocculants. The size of the pH value is related to the type of flocculant selected, the dosage and the coagulation and sedimentation effect. The H+ and OH- in water participate in the hydrolysis reaction of the flocculant, therefore, the pH value strongly affects the hydrolysis rate of the flocculant, the existence form and performance of the hydrolyzed product.


Taking the aluminum salt that achieves coagulation by generating Al(OH)3 charged colloids as an example, when the pH value is less than 4, Al3+ cannot be hydrolyzed into Al(OH)3 in large quantities, and mainly exists in the form of Al3+ ions, and the coagulation effect is extremely high. Difference. When the pH value is between 6.5 and 7.5, Al3+ is hydrolyzed and polymerized into Al(OH)3 neutral colloid with a large degree of polymerization, and the coagulation effect is better. After pH value > 8, Al3+ is hydrolyzed to AlO2-, and the coagulation effect becomes very poor again.


The alkalinity of water has a buffering effect on the pH value. When the alkalinity is not enough, lime and other agents should be added to supplement it. When the pH value of the water is high, it is necessary to add acid to adjust the pH value to neutral. In contrast, polymer flocculants are less affected by pH.


2. Water temperature


The water temperature affects the hydrolysis rate of the flocculant and the speed and structure of floc formation. The hydrolysis of coagulation is mostly an endothermic reaction. When the water temperature is low, the hydrolysis rate is slow and incomplete.


At low temperature, the viscosity of water is large, Brownian motion is weakened, the number of collisions between flocculant colloidal particles and impurity particles in water is reduced, and the shear force of water increases, which hinders the mutual adhesion of coagulation flocs; With the dosage of flocculant, the formation of flocs is still very slow, and the structure is loose and the particles are small, which is difficult to remove.


Low temperature has little effect on polymer flocculants. However, it should be noted that when using an organic polymer flocculant, the water temperature should not be too high. High temperature will easily cause the organic polymer flocculant to age or even decompose into insoluble substances, thereby reducing the coagulation effect.


3. Impurities in water


The uneven size of the impurity particles in the water is beneficial to coagulation, and the small and uniform particles will lead to poor coagulation effect. Too low concentration of impurity particles is often detrimental to coagulation. At this time, refluxing sediment or adding coagulation aids can improve the coagulation effect. When the impurity particles in the water contain a large amount of organic matter, the coagulation effect will be poor, and it is necessary to increase the dosage or add oxidants and other agents that play a role in coagulation. Calcium and magnesium ions, sulfides and phosphides in water are generally beneficial to coagulation, while some anions and surface active substances have adverse effects on coagulation.


4. Types of flocculants


The choice of flocculant mainly depends on the nature and concentration of colloids and suspended solids in water. If the contaminants in the water are mainly colloidal


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