Flocculants may not be unfamiliar to professionals who have been working with water
treatment, but which flocculants are there, and how to distinguish and use them correctly seems a very confusing topic among many people.
Flocculant is a type of substance that can reduce or eliminate the precipitation stability and polymerization stability of dispersed particles in water, and make dispersed particles coagulate and flocculate into aggregates.
The working principle of flocculants
The colloidal particles in the water have the characteristics of tininess, surface hydration and charging, which makes them have strong stability. When the flocculant is added to the water, it will be hydrolysed into a charged colloid and the surrounding ions to form an electric double layer structure.
The method of rapid stirring after pouring is adopted to promote the collision chance and frequency of the colloidal impurity particles in the water and the micelles hydrolysed by the flocculant. The impurity particles in the water first lose their stability under the action of the flocculant, and then agglomerate into larger particles, and finally settle down or float upwards.
The impurity particles in the water interact with the flocculant to lose or reduce the stability through mechanisms such as the compression of the electric double layer and the electric neutralization, and the process of generating micro flocs is called coagulation. The process of agglomerating to form micro-flocs under the agitation of bridging materials and water flow, through adsorption bridging and sediment net catching and other mechanisms to grow into large flocs is called flocculation.
The combination of mixing, coagulation and flocculation is called coagulation. The mixing process is generally completed in the mixing tank, and the coagulation and flocculation are carried out in the reaction tank.
Polyacrylamides are high molecular weight water soluble or swellable polymers formed from acrylamide or its derivatives. Their glass transition temperature is well above room temperature (> 400 K).
Polyacrylamide PAM is currently the most widely used synthetic organic polymer flocculant, and sometimes it is also used as a coagulant aid. The raw material for the production of polyacrylamide is polyacrylonitrile CH2=CHCN. Under certain conditions, acrylonitrile is hydrolysed to produce acrylamide, and acrylamide is then subjected to suspension polymerization to obtain polyacrylamide. Polyacrylamide is a water-soluble resin, and its products include granular solid and a certain concentration of viscous aqueous solution.
The actual form of polyacrylamide in water is random coils. Because the random coils have a certain particle size, and there are some amide groups on the surface, they can play a corresponding bridging and adsorption capacity, that is, it has Certain flocculation ability.
However, because the long polyacrylamide chain is crimped into coils, the bridging range is small. After the two amide groups are formed, they are equivalent to cancel each other out and lose two adsorption sites. In addition, part of the amide groups are trapped in the coil structure. The inside of the water cannot be in contact with and adsorbed by the impurity particles in the water, so its adsorption capacity cannot be fully utilized.
In order to separate the associated amide groups again and expose the amide groups inside, people try to extend the random coils appropriately, and even try to add some groups with cations or anions to the long molecular chain. , At the same time improve the adsorption bridging capacity and the role of electric neutralization and compression of the electric double layer. In this way, a series of polyacrylamide flocculants or flocculants with different properties have been derived on the basis of PAM.
For example, adding alkali to the polyacrylamide solution converts the amide groups on part of the chain links into sodium carboxylate, and sodium carboxylate easily dissociates from sodium ions in water, so that the COO- groups remain on the branched chain, thus forming part of the hydrolysed anionic polyacrylamide.
The COO- group on the molecular structure of anionic polyacrylamide causes the molecular chain to be negatively charged, and repels each other to pull apart the originally associated amide groups, which promotes the molecular chain to gradually extend from the coil shape to the long chain shape, thereby making the frame The scope of the bridge is expanded, the flocculation ability is improved, and its advantages as a coagulant perform better.
2. Factors influencing the effectiveness of PAM
The pH value of the water has a great influence on the use effect of the inorganic flocculant. The pH value is related to the type of flocculant selected, the dosage and the coagulation sedimentation effect. The H+ and OH- in the 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 aluminium 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 hydrolysed into Al(OH)3 in a large amount, mainly in the form of Al3+ ions, and the coagulation effect is extremely Difference. When the pH value is between 6.5 and 7.5, Al3+ is hydrolysed and polymerized into Al(OH)3 neutral colloid with a high degree of polymerization, and the coagulation effect is better. After pH value ﹥8, Al3+ is hydrolysed into AlO2-, and the coagulation effect becomes very poor.
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 of the water is high, you need to add acid to adjust the pH to neutral. In contrast, polymer flocculants are less affected by pH.
Water temperature affects the rate of hydrolysis of the flocculant and the rate and structure of alum 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, the Brownian motion is weakened, the number of collisions between the flocculant colloidal particles and the impurity particles in the water is reduced, and the shear force of the water increases, which hinders the mutual adhesion of the coagulated flocs; therefore, despite the increase With the dosage of flocculant, the formation of flocs is still very slow, and the structure is loose and the particles are small, making it difficult to remove.
Low temperature has little effect on polymer flocculants. However, it should be noted that when using organic polymer flocculants, the water temperature should not be too high. High temperature will easily cause the organic polymer flocculants to age or even decompose to produce insoluble substances, thereby reducing the coagulation effect.
The uneven particle size of impurities in the water is beneficial to coagulation, and small and uniform particles will lead to poor coagulation effects. Too low concentration of impurity particles is often detrimental to coagulation. At this time, refluxing sediment or adding 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 worse, 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 certain anions and surface active substances have an adverse effect on coagulation.
Types of flocculants
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 a colloidal state, inorganic flocculants should be the first choice to make them destabilize and agglomerate. If the flocs are small, it is necessary to add a polymer flocculant or use a coagulant aid such as activated silica gel.
In many cases, the combined use of inorganic flocculants and polymer flocculants can significantly improve the coagulation effect and expand the scope of application. For macromolecules, the greater the amount of charge on the chain molecules, the higher the charge density, the more fully extended the chain, the greater the range of adsorption and bridging, and the better the coagulation effect.
When coagulation is used to treat any wastewater, there is an optimal flocculant and an optimal dosage, which are usually determined by experiments. Excessive dosage may cause the stability of the colloid. Generally, the dosage range of ordinary iron salt and aluminium salt is 10-100mg/L, the dosage of polymer salt is 1/2～1/3 of the dosage of ordinary salt, and the dosage range of organic polymer flocculant is 1～5mg/L. .
Dosing sequence of flocculant
When using multiple flocculants, it is necessary to determine the best dosing order through experiments. Generally speaking, when the inorganic flocculant is used together with the organic flocculant, the inorganic flocculant should be added first, and then the organic flocculant should be added.
When the particle size of the treated impurities is above 50μm, the organic flocculant is often added to absorb the bridge first, and then the inorganic flocculant is added to compress the electric double layer to destabilize the colloid.
In the mixing stage, the flocculant and water are required to mix quickly and evenly. In the reaction stage, it is necessary to create enough collision opportunities and good adsorption conditions to allow sufficient growth opportunities for the flocs, and to prevent the small flocs that have been formed from being Broken, so the stirring intensity should be gradually reduced, and the reaction time should be long enough.
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