Views:16 Author:Ryde Environment Publish Time: 2019-05-20 Origin:Ryde Environment
PAM is often used in sewage treatment and sludge treatment. In the process of municipal wastewater treatment, it is necessary to add a certain concentration of PAM solution to the wastewater or add to mix with sludge evenly and then enter other equipment such as multi disc screw press or belt filter press.
Usually, flocculants are in a solid state to facilitate transportation and sales. In use, it is necessary to dissolve the solid flocculant into a certain concentration of viscous water solution. The function of Polymer preparation unit is to make the powder or liquid flocculants dissolve or dilute into flocculants solution. It is usually divided into manual dosing device and the integrated of dosing device. The integrated of dosing device is fully automatic operating including automatic dosing, dissolving and feeding.
Polyacrylamide (PAM) is a widely used synthetic organic polymer flocculant, sometimes used as a coagulant aid.
Synthetic organic macromolecule flocculants are polypropylene and polyethylene, such as polyacrylamide and polyethylenimide. These flocculants are water-soluble linear macromolecule substances, each macromolecule consists of many repetitive units containing charged groups, so they are also called polyelectrolytes. Cationic polyelectrolyte containing positive group and anionic polyelectrolyte containing negative group are called nonionic polyelectrolyte, which includes both positive group and negative group.
At present, most of the polymer flocculants used are anionic, and they can only play a coagulant role in the negative colloidal impurities in water. Often can not be used alone, but with aluminium salt, iron salt use. Cationic flocculants can play the role of both coagulation and flocculation and can be used alone, so they have been developed rapidly.
The raw material for the production of polyacrylamide is polyacrylonitrile CH2 = CHCN. Under certain conditions, acrylonitrile is hydrolyzed to acrylamide, and then polyacrylamide is obtained by suspension polymerization.
Polyacrylamide belongs to water-soluble resin. The product has two kinds: granular solid and viscous aqueous solution with a certain concentration.
The actual 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 a corresponding bridge-erecting and adsorption capacity, that is, it has a certain flocculation capacity. However, because the long chain of polyacrylamide is curled into a coil, which makes its bridge-building scope smaller. When the two amide groups are connected, they counteract each other and lose two adsorption sites. In addition, part of the amide groups are coiled in the inner of the online structure, which can not contact and adsorb impurity particles in water, so their adsorption capacity can not be fully developed.
In order to make the bound amide groups separate again and expose the internal amide groups, people try to extend the random coils properly, even try to add some cationic or anionic groups on the long molecular chain, at the same time, improve the adsorption bridging capacity and the role of neutralizing and compressing the double layer. In this way, on the basis of PAM, a series of polyacrylamide flocculants or coagulant aids with different properties have been derived.
For example, adding alkali in polyacrylamide solution can convert the amide groups on some chains into sodium carboxylate. Sodium carboxylate is easy to dissociate sodium ions in water, so that the COO-group is retained on the branched chains, thus forming partially hydrolyzed anionic polyacrylamide. The COO-group in the molecular structure of anionic polyacrylamide makes the molecular chains negatively charged, and repels each other to pull apart the amide groups originally bound together, which promotes the molecular chains to extend gradually from linear cluster to chain shape, thus enlarging the bridge-building range and improving the flocculation ability. As a flocculant aid, the advantages of the COO-group are better.
The use effect of anionic polyacrylamide is related to its degree of hydrolysis, too small degree of hydrolysis will lead to poor coagulation or coagulation aid effect, and too high degree of hydrolysis will increase production cost.
The degree of hydrolysis of anionic polyacrylamide is the percentage of carboxyl group converted from amide group in PAM molecule during hydrolysis. However, it is difficult to determine the number of carboxyl group. In practical application, the ratio of sodium hydroxide dosage to PAM dosage is often used to measure the degree of hydrolysis.
Excessive hydrolysis ratio, high cost of alkali addition and low hydrolysis ratio will lead to insufficient reaction and poor coagulation or coagulation aid effect of anionic polyacrylamide. Generally, the hydrolysis ratio is controlled at about 20% and the hydrolysis time is controlled at 2-4 hours.