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Application of polyacrylamide as water treatment agent
Application of polyacrylamide as water treatment agent
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1. Introduction of polyacrylamide flocculant
High turbidity water treatment agent "polyacrylamide flocculant (PAM)", also known as "number three" flocculant, is made of acrylamide monomer polymerization of organic polymer, colorless, tasteless, odorless, soluble in water, no corrosion.
Polyacrylamide products according to its purity, there are powder and colloidal two kinds, powder products are white or yellow particles or powder, solid content is generally more than 90%, colloid products are colorless or light yellow through colloid, solid content is 8% ~ 9%. Polyacrylamide products according to its ionic type to points, there are cationic, anionic and non-ionic type 3. Cationic type generally contains trace toxicity, is not suitable for use in water supply and drainage engineering, so we are exposed to water treatment agent polyacrylamide are anionic or non-ionic type.
2. Flocculation mechanism of polyacrylamide
Polyacrylamide has polar amide groups, amide groups are easy to adsorb on the surface of sediment particles by hydrogen bonding. In addition, polyacrylamide flocculant has A very long molecular chain, its length is 100 A°, but the width of the chain is only 1A°, A large order of magnitude of the long chain in the water has A huge adsorption surface area, its flocculation is good, but also can use the long chain in the flocculation particles bridge, the formation of large particle flocculation, accelerate sedimentation.
Water treatment agent polyacrylamide flocculation mechanism is different from ferric chloride, aluminum sulfate, alkali aluminum chloride and other coagulants of the potential coagulation concept, so, polyacrylamide can not be called coagulant, because its machine volume is mainly to adsorption bridge, can only be called flocculant.
Polyacrylamide under the action of NaOH and other bases, it is easy to hydrolysis reaction, so that part of the polyacrylamide into sodium polyacrylate, sodium acrylate molecules in water is unstable, is dissociated into RCOO-Na+. Therefore, polyacrylamide water disintegration is a copolymer of polyacrylamide and sodium polyacrylate, due to the role of RCOO- (hydroxyl), so that polyacrylamide water disintegration into anionic polymer flocculant, and non-hydrolyzed polyacrylamide flocculant for non-ionic polymer flocculant. Partially hydrolyzed polyacrylamide, make its performance from non-ionic to anionic, of the groups in RCCO - (hydroxyl) ion electrostatic repulsion, on the main chain of the polyacrylamide in the molecular chain of curls stretched, increase its adsorption area, improve the ability of bridge, so some water PAM flocculation effect is superior to the disintegration of the nonionic polyacrylamide.
3. Production and application of polyacrylamide
Polyacrylamide treatment of high turbidity water, generally should use part of the water disintegration products, the best degree of hydrolysis (degree of hydrolysis refers to the polyacrylamide molecule, the percentage of amide group into hydroxyl) polyacrylamide sedimentation rate of non-water disintegration of 2 ~ 9 times. But polyacrylamide flocculant hydrolysis is too high or too low products, the flocculation effect is not ideal, because too low hydrolysis, adsorption bridge capacity is not strong, too high hydrolysis will strengthen the anionic properties of the product, increase the rejection ability with soil particles. The best degree of hydrolysis of polyacrylamide flocculant is 25% ~ 35%.
Polyacrylamide flocculant, one of the best flocculants to treat high turbidity water, has been used in China for more than 30 years since the 1960s. Polyacrylamide water disintegration flocculant is the most effective polymer flocculant in the treatment of high turbidity water. It can be used alone or in combination with ordinary coagulant. It has remarkable effect in the treatment of high turbidity water with sand content of 10 ~ 150kg/m3, which can not only ensure the quality of effluent water, but also reduce the dosage of coagulant and the volume of primary sedimentation tank. Generally, it can save 30% ~ 60% of coagulant costs, increase water production by 2 ~ 4 times, and save a lot of capital construction investment. In addition, its biggest economic benefit is to improve the water supply engineering treatment of the upper limit of sand, compared with the previous general water plant treatment sand content of the maximum value of 40 ~ 60 kg/m3, more than this value will reduce water output, water quality is difficult to ensure, bring great impact on production safety operation. After using polyacrylamide flocculant, the maximum sediment content can reach 100 ~ 150 kg/m3.
4. Preparation and adding of polyacrylamide flocculant
In the use of polyacrylamide flocculant, the biggest problem is the dissolution time of the product, because of the poor solubility of the product, directly affect the use effect, but also easy to block the feeding equipment. Therefore, the preparation method and dissolution time of polyacrylamide flocculant are particularly important. General water disintegration products are easy to dissolve, the dissolution time is 60 ~ 120min, non-ionic products are 90 ~ 240min. In addition, the dissolution time varies with the molecular weight of the product.
Polyacrylamide solution, the general use of mechanical stirring. Mechanical stirring speed has a great influence on the preparation time of the solution, but too large stirring speed will cause the degradation of the polyacrylamide solution, so that part of the polyacrylamide long chain fracture, affecting the settlement effect, so it must be strictly controlled mechanical stirring speed. In the diameter of 1 m stirring drum, the rotational speed should not be greater than 800 RPM, and in the diameter of 1.5 ~ 2 m stirring drum, the rotational speed should not be greater than 600rpm. Increasing the temperature of stirring solution can reduce the dissolution time, but the water temperature is not more than 55℃, otherwise it will cause degradation and affect the use effect.
The preparation of polyacrylamide solution must use special equipment, strictly prevent the common use of other coagulants, or in a pool to add together, otherwise it will make two agents produce copolymerization precipitation, not only affect the effect, but also easy to block the adding equipment.
Because polyacrylamide solution is a drag reducing agent, there is a large gap between the calibration value and clear water. Therefore, polyacrylamide solution metering equipment must be calibrated with solution, can not be calibrated with water. Otherwise, it will increase the amount of polyacrylamide, which will not only improve the processing cost, but also cause unnecessary consequences.
The thinner the concentration of polyacrylamide solution, the better the effect, the thinner the concentration can make the solution in the water rapidly expand, fully mixed, prevent the occurrence of high concentration of colloid protection phenomenon, affect the effect. But too thin a concentration would create a huge feeding facility. Generally, 0.5% ~ 1% is the best concentration, and 2% is the best preparation concentration.
In the treatment of high turbidity water, the flocculation effect of polyacrylamide flocculant is better in batch dosage than in one dosage. The former is more than three times of the latter if the muddy liquid surface settling rate is taken as the comparison value. The so-called batch dosing is to divide the dosage into two parts and add it to water respectively. First, part of flocculant is added and then mixed with water rapidly. After 1 ~ 2min, another part of flocculant is added and then mixed with water rapidly. Because batch dosage can avoid high concentration of flocculant combined with sediment, resulting in the closure of the active group, so it can achieve better results. Batch dosage of the proportion, generally add 60% first, and then add 40% for the best. In the design of water supply engineering, batch dosing should be adopted as far as possible.
When polyacrylamide is used as a coagulation aid, the general adding order is after adding coagulant. If it is used alone as a flocculant to treat high turbidity water, polyacrylamide flocculant should be added first, otherwise it will affect the use effect. In the design of dosing room, measures of dosing sequence change should be taken into account.
5. Safety of the use of polyacrylamide flocculant
Although the use of polyacrylamide flocculant has excellent effect and economic value, but because of the monomer acrylamide of the product, in addition to the control amount has a certain impact on human health, which strictly limits the use value of the product. Practice has proved that polyacrylamide flocculant itself is harmless, that is, the polymerization of polyacrylamide is safe, but the content of acrylamide monomer in the product and the polymerization of incomplete short chain content has a certain impact on human health, polyacrylamide flocculant products, the content of monomer must be strictly controlled. Gb strictly stipulates that the content of monomer must be controlled (including a small amount and short chain polyacrylamide), and unified monomer acrylamide determination method, for the safe use of polyacrylamide flocculant to provide reliable guarantee. Superior product is the numerical value that international standard sets completely, also be our country standard to international standard unify standard. Grade A products are gradually to international standards in line with the transitional products. Qualified product is under the premise of ensuring safety, considering the national conditions of our country, the requirements must achieve the minimum value. In the use of polyacrylamide flocculant in water supply engineering, must buy in accordance with the national standard of monomer content specified indicators of the product, otherwise strictly prohibited use.