Mechanism of flocculation of polyacrylamide

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2015-11-21 10:54:05

PAM has two characteristics: a long chain (line) - like molecular structure and a large number of active groups.

PAM is straight chain polymer, because each molecule is composed of more than one hundred thousand monomer polymerization, the molecular chain is quite long. If it is fully extended, the length of molecular than general (such as sucrose) or ion (such as Ca2+) long tens of thousands of times. Because of its long, thin, curved or curled into irregular curve shape. The long molecular chain out of many chemical active groups to the outside: the amide and carboxyl - COO - CONH2.

Non ionic groups is amide group, but also good at secondary valence bond formation and other substances active groups of adsorption and link up. Simple polyacrylamide can be used in the general water treatment, so that the suspended sediment in the water. The carboxyl group is electronegative groups, it is the key factor of particle flocculation in syrup. Because the particles in the main flocculation syrup produced by bridging calcium ions. Proof of Bennett of suspended particles in juice and most of the colloidal material with a negative charge, their surface often adsorption syrup calcium ion. Due to Ca2 + two unit positive charge, and colloids and particles on the surface of a charged by often only a negative charge (i.e. monovalent ions such as - COO -), so these adsorbed calcium ion also remaining a unit of positive charge, and other negative groups combined. In this way, the calcium ion in between plays a bridging role and connect them. Connection with calcium phosphate particles or colloids is by this action, flocculant and particle link mainly through the effect, namely through the carboxyl groups of the flocculant - COO - and the role of calcium and with various calcium salt deposits and various with charged particles are connected to each other. Phosphate and calcium phosphate are stored in solution, also can through the calcium phosphate and phosphate bridge and other particles on the surface of the calcium ion connection. Many PAM molecules are associated with many calcium salt precipitation and calcium phosphate precipitate particles to form a thick, flocculation group. Its size can reach several millimeters or more. According to the Bennett study, the flocculation formation of sugarcane juice after adding PAM approximately includes 105 to 107 of the original particles.

Due to PAM molecules are long and thin and has many active reaction groups they can and precipitation particles produced many connections are formed larger flocs, the floc structure like cotton, loose, amorphous, are connected to each other but not very stable, lots of space and lots of fine network, wrapped in a large amounts of liquid and the proportion of flocs are quite close to the liquid which exists in itself. It also network a variety of particles, which will be a variety of different ingredients, different nature, different size of the particles together. Therefore, a good flocculant treatment can be the original particles in the solution network completely removed, so that the solution is special clear transparent and glossy. Due to the size of the large size of the flocculation, it is relatively fast sedimentation and filtration.

Flocculant and particle effect is through the two forms of chemical adsorption and physical network generated. According to the above principles, higher molecular weight, molecular longer to PAM, adsorb more particles, form an network ability. Therefore, the flocculation efficiency was good. Similarly, the proportion of the carboxyl group of the PAM molecules are properly is very important, from the majority of particles in juice of negatively charged, PAM needs amount of carboxyl group by calcium ion bridge and its role. But if too many carboxyl content, PAM molecules themselves charged is too strong, the repulsion between the molecules themselves force is too large, is not conducive to the flocculation.

In addition carbonate method, (sugar cane juice and syrup) and PAM are after adding phosphoric acid and lime milk neutralization. They reflect the calcium phosphate precipitation is floc, capable of trapping various suspension liquid particles to form larger particles. This is called the first flocculation. On the basis of this, the formation of a larger PAM, called the second flocculation. A good flocculation can significantly improve the effect of PAM two times, and reduce the number of PAM required; because a flocculation has been a small number of particles, greatly reducing the total number of particles, thereby reducing the burden on PAM. Do a good job of flocculation is the basis for the best effect of PAM.

PAM solution is very thick. The higher the molecular weight, the higher the viscosity of PAM solution. This is because the PAM molecular chain is long and thin, the movement in the solution of great resistance. The essence of viscosity is the size of the friction force in the solution, also known as the internal friction coefficient. The viscosity of the polymer solution is higher and higher with the increase of the molecular weight. One way to determine the molecular weight of polymer organic matter is to determine the viscosity of a certain concentration of solution under certain conditions ". There follows an exponential function relationship between PAM solution of characteristic viscosity [ETA] with molecular weight M: [ETA] = 3.73 * 10 - 4 * m 0.66

The practical experience of the prove PAM flocculation performance and its solution viscosity has a direct relationship, high viscosity performance better; if its viscosity by some factors of influence to reduce, its flocculation properties inevitable decline. The viscosity of the PAM solution to be measured with a special instrument.

According to our years of experience, we can also use two simple methods to observe.

1, A glass rod into the PAM solution in a stirred gently pulls, mucus silk observation glass rod formed by the length of the end. Concentration was 0.05 ~ 0.1% PAM solution, good can form the appearance of 10 ~ 15 cm or longer, such as spider silk filaments, with the air flowing; and poor formation of filaments very short, can not even filamentation.

2,With a small bottle of PAM solution, bottle tilting solution slowly flows, then stop pouring, observing the liquid flow formed at the end of the viscose rayon. A good PAM solution can form a long filament. Using this observation method, the dissolution of PAM has been observed. Not completely homogeneous solution, to pour out can clearly see the liquid stream suddenly suddenly coarse fine, or pearl shaped or fusiform effluent. This is not completely dispersed the use effect of the uniform of PAM solution is not good and is easy to adhere on the surface of the cloth or equipment, resulting in side effects.