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Mingke Technology-a comprehensive collection of flocculants for wastewater treatment, decisive collection after reading!

Release Date: 2019-03-19

In the field of sewage treatment, flocculant is used as a means to strengthen solid-liquid separation. It can be used to strengthen the primary sedimentation of sewage, flotation treatment and secondary sedimentation after activated sludge process. It can also be used for tertiary treatment or advanced treatment of sewage. When used in the conditioning of the remaining sludge, the flocculants and coagulants become sludge conditioners or dehydrating agents.

Q1 What is the role of flocculant?

Flocculant action mechanism: The colloidal particles in water are small, the surface is hydrated and charged to make it stable. After flocculant is added to water, it hydrolyzes into charged micelles and ions around it to form a double-layered micelle.

The method of rapid stirring after administration is used to promote the collision opportunities and times of colloidal impurities in water and micelles hydrolyzed by flocculants. Impurities in the water lose their stability under the action of flocculants, and then condense into larger particles, and then precipitate or float up in the separation facility.

The product GT of the speed gradient G and the stirring time T generated by the stirring can indirectly represent the total number of particle collisions in the entire reaction time. The effect of the coagulation reaction can be controlled by changing the GT value. Generally, the GT value is controlled between 104 and 105. Considering the effect of the concentration of impurity particles 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 between 100 or so.

The process of causing the flocculant to quickly diffuse into the water and mix it with the entire wastewater is mixing. Impurity particles in water interact with flocculants, and the mechanism of compressing the electric double layer and electric neutralization loses or reduces the stability. The process of generating microflocks is called agglomeration. Under the agitation of bridging materials and water flow, the process of agglomeration to form microflocculation particles grows into large flocs through mechanisms such as adsorption bridging and sediment net capture, which is called flocculation. Mixing, coagulation and flocculation are called coagulation. The mixing process is generally completed in the mixing tank, and coagulation and flocculation are performed in the reaction tank.

Q2 What are the types of flocculants?

Flocculant is a kind of material 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 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 polymers thereof.

Organic flocculants can be divided into anionic, cationic, non-ionic, and amphoteric according to the charge properties of the charged monomer group of polymerized monomers. According to their sources, they can be divided into artificial synthesis and natural polymer flocculants .

Q3 What are the types of inorganic flocculants?

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

In general, inorganic flocculants have the characteristics of easy availability of raw materials, simple preparation, cheap price, and moderate treatment effect, so they are widely used in water treatment.

Q4 What are the characteristics of inorganic flocculant aluminum sulfate?

Since the first use of aluminum sulfate in the United States at the end of the 19th century for water treatment and patents, aluminum sulfate has been widely used for its excellent agglomeration and sedimentation properties. Aluminum sulfate is currently the most used flocculant in the world, with an annual output of about 5 million tons of aluminum sulfate worldwide, of which nearly half is used in the field of water treatment.

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

The applicable pH range of aluminum sulfate is related to the hardness of raw water. When treating soft water, the appropriate pH is 5 to 6.6. When treating medium hard water, the appropriate pH is 6.6 to 7.2. For high hard water, the appropriate pH is 7.2 to 7.8. The applicable water temperature range of aluminum sulfate is 20oC to 40oC, and the coagulation effect is poor when the temperature 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.

Q5 What are the characteristics of inorganic flocculant ferric chloride?

Ferric trichloride is another commonly used inorganic low-molecular flocculant. The product has solid black-brown crystals and also has a higher concentration of liquid. It has the advantages of being easily soluble in water, large and heavy alum blooms, good precipitation performance, and wide adaptability to temperature, water quality and pH.

The applicable pH range of ferric chloride is 9 ~ 11, the floc formed is high in density, easy to precipitate, and the effect is still good at low temperature or high turbidity. Solid ferric chloride has strong water absorption, strong corrosiveness, and easy to corrode equipment. It requires high anticorrosion for dissolution and dosing equipment, has an irritating odor, and has poor operating conditions.

The action mechanism of ferric chloride is to use various hydroxy iron ions generated by the progressive hydrolysis of trivalent iron ions to achieve flocculation of impurity particles in water, and the formation of hydroxy iron ions requires the use of a large number of hydroxyl groups in water, so it will be used during the process. A large amount of alkali is consumed. When the alkalinity of the raw water is not enough, it is necessary to supplement alkali sources such as lime.

Ferrous sulfate is commonly known as green alum. It has fast and stable floc formation and short precipitation time. It is suitable for high alkalinity and turbidity, but the color is not easy to remove and the corrosivity is also strong.

Q6 What are the types of 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 throughout the world.

Aluminium, iron and silicon-based inorganic polymer flocculants are actually the intermediate products of their hydrolysis, sol, and precipitation processes, that is, the hydroxyl and oxy polymers of Al (Ⅲ), Fe (Ⅲ), and Si (Ⅳ). . Aluminum and iron are positively charged in cation type and negatively charged in silicon. Their molecular weight in the water-soluble state is about hundreds to thousands, and they can be combined with each other into aggregates with a fractal structure.

Their coagulation-flocculation process is a comprehensive manifestation of the two functions of electrical neutralization and adhesion bridging of particles in water. The particle size of the suspended particles in the water is in the nanometer to micrometer range, and most of them have negative charges. Therefore, the positive and negative charges of the flocculant and its form, the electrical strength and molecular weight, and the size of the aggregate are the main factors that determine its flocculation effect. At present, there are dozens of types of inorganic polymer flocculants, and the output also reaches 30% to 60% of the total flocculant output. The most commonly used and widely used is polyaluminum chloride, which is PAC.

Q7 What are the characteristics of inorganic polymer flocculants?

Al (Ⅲ), Fe (Ⅲ) hydroxy and oxy polymers will be further combined into aggregates and kept in aqueous solution under certain conditions, and their particle size is roughly in the nanometer range, so that the aggregation-flocculation effect will be low The result of high dosage.

Comparing their reaction polymerization speeds, the reaction of aluminum polymers is more gradual and the morphology is more stable. The hydrolysis polymer of iron reacts quickly and easily loses stability and precipitates.

The advantages of inorganic polymer flocculants are reflected in that it is more effective than traditional flocculants such as aluminum sulfate and ferric chloride, and it is cheaper than organic polymer flocculants. Now PAC is successfully applied in various treatment processes of feedwater, industrial wastewater and municipal sewage, including pretreatment, intermediate treatment and advanced treatment, and has gradually become the mainstream flocculant. However, in terms of morphology, degree of polymerization and the corresponding coagulation-flocculation effect, the inorganic polymer flocculant is still in the position between the traditional metal salt flocculant and the organic polymer flocculant.

Q8 What are the characteristics of polyaluminum chloride?

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

PAC has a high degree of polymerization, and rapid stirring after addition can greatly reduce the floc formation time. PAC is less affected by water temperature and works well 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 it is not necessary to add an alkali agent. The amount of PAC added is small, and the amount of mud produced is also small, and the use, management, and operation are more convenient, and it is less corrosive to equipment and pipelines. Therefore, PAC has a tendency to gradually replace aluminum sulfate in the field of water treatment. The disadvantage is that the price is higher than that of 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 in 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 at the same time increase the cohesive ability.

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

Q9 What are the types of artificial organic polymer flocculants?

Synthetic organic polymer flocculants are mostly polypropylene and polyethylene substances, such as polyacrylamide and polyethyleneimine. These flocculants are all water-soluble linear high-molecular substances. Each macromolecule is composed of many repeating units containing charged groups, so it is also called polyelectrolyte. A cationic polyelectrolyte containing a positively charged group is an anionic polyelectrolyte containing a negatively charged group, and both a positively charged group and a negatively charged group are called a non-ionic polyelectrolyte.

At present, many polymer flocculants used are anionic, and they can only play a role in coagulation promotion for negatively charged colloidal impurities in water. Often cannot be used alone, but in combination with aluminum and iron salts. The cationic flocculant can be used alone and can develop coagulation and flocculation at the same time, so it develops quickly.

Polyacrylamide-based non-ionic polymers are currently used in China and are often used in combination with iron and aluminum salts. The use of iron and aluminum salts to neutralize the colloidal particles and the excellent flocculation function of polymer flocculants, so as to obtain satisfactory treatment results. Polyacrylamide has the characteristics of less dosage, fast coagulation speed and large and strong flocculent particles in use. 80% of the synthetic organic polymer flocculants currently produced in China are this kind of product.

Q10 What are the characteristics of polyacrylamide flocculants?

Polyacrylamide PAM is one of the most widely used synthetic organic polymer flocculants, 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 then acrylamide is obtained by suspension polymerization. Polyacrylamide is a water-soluble resin. Its products are granular solid and viscous aqueous solution with a certain concentration.

The actual form of polyacrylamide in water is random strands. Since the random strands have a certain particle size, and there are some amide groups on the surface, they can play the corresponding bridging and adsorption capabilities, that is, they have Certain flocculation capacity.

However, because the long chain of polyacrylamide is curled into a string, the bridging range is small. After the two amide groups are connected, it is equivalent to the interactions canceling each other and losing two adsorption sites. In addition, some amide groups are hidden in the string structure. The inside cannot be contacted and adsorbed with impurities particles in water, so its adsorption capacity cannot be fully exerted.

In order to make the linked amide groups separate again and the built-in amide groups can also be exposed to the outside, people have tried to extend the random cords appropriately, and even managed to add some groups with cations or anions to the long molecular chain. At the same time, it improves the adsorption bridging ability and the role of electric neutralization and compression electric double layer. In this way, on the basis of PAM, a series of polyacrylamide flocculants or coagulants with different properties were derived.

For example, by adding a base to a polyacrylamide solution, the amide groups on some of the links are converted into sodium carboxylate, and the sodium carboxylate is easily dissociated into sodium ions in water, so that the COO- groups remain on the branched chain, so a part is generated Hydrolyzed anionic polyacrylamide.

The COO-groups on the anionic polyacrylamide molecular structure make the molecular chain have a negative charge, repel each other and pull the original amide group together, which promotes the molecular chain to gradually extend from a string shape to a chain shape. The scope of the bridge is expanded and the flocculation ability is improved. As a coagulant, its advantages are even more outstanding.

The use effect of anionic polyacrylamide is related to its "degree of hydrolysis". If the "degree of hydrolysis" is too small, it will lead to poor coagulation or coagulation aid effect. If the "degree of hydrolysis" is too large, it will increase the production cost.

Q11 What are the factors that affect the use of flocculants?

(1) pH value of water

The pH value of water has a great influence on the use effect of inorganic flocculants. The value of pH value is related to the type of flocculant used, the dosage and the effect of coagulation and sedimentation. 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 hydrolysate.

Take the aluminum salt that achieves coagulation by generating Al (OH) 3 charged colloid as an example. When the pH value is ﹤ 4, Al3 + cannot be hydrolyzed into Al (OH) 3 in a large amount. It mainly exists in the form of Al3 + ions. difference. When the pH value is between 6.5 and 7.5, Al3 + hydrolyzes and polymerizes into a highly polymerized Al (OH) 3 neutral colloid, which has a better coagulation effect. After pH ﹥ 8, Al3 + is hydrolyzed to AlO2-, and the coagulation effect becomes poor again.

The alkalinity of water has a buffering effect on the pH value. When the alkalinity is insufficient, lime and other agents should be added to supplement it. When the pH of the water is too high, you need to add acid to adjust the pH to neutral. In contrast, polymer flocculants are less affected by pH.

(2) Water temperature

Water temperature affects the rate of hydrolysis of the flocculant and the rate and structure of alum bloom formation. The coagulation hydrolysis 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, the Brownian motion is weakened, the number of collisions between flocculant colloid particles and impurities particles in water is reduced, and at the same time, the shear force of water is increased, hindering the adhesion of coagulated flocs; The amount of flocculant added, the formation of flocs is still very slow, and the structure is loose, the particles are small, and it is difficult to remove.

The effect of low temperature on polymer flocculants is small. However, it should be noted that when using organic polymer flocculants, the water temperature must not be too high, and the high temperature easily causes the organic polymer flocculants to age or even decompose to generate insoluble substances, thereby reducing the coagulation effect.

(3) Impurities in water

The uneven size of impurities in the water is beneficial to coagulation, and small and uniform results in poor coagulation. Too low concentration of impurity particles is often detrimental to coagulation. At this time, refluxing the sediment or adding a coagulant can improve the coagulation effect. When the impurity particles in the water contain a large amount of organic matter, the coagulation effect will be deteriorated, and it is necessary to increase the dosage or add an oxidizing agent such as an coagulant. Calcium and magnesium ions, sulfides, and phosphides in water are generally beneficial to coagulation, while certain anions and surface-active substances have adverse effects on coagulation.

(4) Type of flocculant

The choice of flocculant mainly depends on the nature and concentration of colloids and suspended solids in water. If the pollutants in the water are mainly in the colloidal state, inorganic flocculants should be preferred to destabilize and aggregate. If the floc is small, it is necessary to add a polymer flocculant or use a coagulant such as activated silica gel.

In many cases, the combination of inorganic flocculant and polymer flocculant can obviously improve the coagulation effect and expand the scope of application. For polymers, the greater the amount of charge on the chain-like molecule, the higher the charge density, the more the chain can be fully extended, the larger the range of adsorption bridging, and the better the coagulation effect.

(5) Dosage of flocculant

The best flocculant and the best dosage for the treatment of any wastewater using the coagulation method are usually determined by experiments. Excessive dosage may cause the colloid to re-stabilize. Generally, the dosage range of ordinary iron and aluminum salts is 10 ~ 100mg / L, the polymerization salt is 1/2 ~ 1/3 of the ordinary salt, and the dosage range of organic polymer flocculant is 1 ~ 5mg / L. .

(6) Flocculant dosing order

When multiple flocculants are used, experiments need to be performed to determine the optimal dosing sequence. Generally speaking, when inorganic flocculant and organic flocculant are used together, the inorganic flocculant should be added first, and then the organic flocculant should be added.

When the size of the impurity particles is more than 50 μm, the organic flocculant is usually added to the adsorption bridge, and then the inorganic flocculant is added to compress the electric double layer to destabilize the colloid.

(7) Hydraulic conditions

In the mixing stage, the flocculant and water are required to be mixed quickly and uniformly. In the reaction stage, it is necessary to create sufficient collision opportunities and good adsorption conditions to allow the floc to have sufficient growth opportunities, and to prevent the small flocs that have been generated from being trapped. Break, so the intensity of the stirring should be gradually reduced, and the reaction time should be long enough. >>> Sewage Detection Kit

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