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Importance of Appropriate Proportion of Carboxyl When Using Amphoteric Polyacrylamide Flocculants

Amphoteric polyacrylamide flocculants are water-soluble polymers with good flocculation properties and are insoluble in most organic solvents. They can reduce the frictional resistance between liquids. According to ionic properties, they are classified as non-ionic, anionic, cationic and amphoteric. Amphoteric polyacrylamide flocculants are widely used in thickening, stabilizing colloids, reducing resistance, bonding, film formation, biomedical materials. etc. In water treatment, they are used as coagulant, flocculant and sludge dewatering agent. In oil drilling, they are used as water-reducing agent and oil repellent agent. Besides, they are used as retention aid and reinforcing agent in paper making process.

1. Characteristics of amphoteric polyacrylamide flocculants in wastewater treatment

The long-chain (linear) molecular structure and polyacrylamide molecules contain a large number of active groups. Polyacrylamide is a linear polymer with long molecular chain, each molecule of which is composed of more than 100,000 monomers. If it is completely straightened, its length is tens of thousands of times longer than that of ordinary molecules (such as sucrose) or ions (such as ca2+). Because its molecules are long and thin, they would bend or curl into irregular curvilinear shapes. This long molecular chain extends many chemically active groups to the outside: amide group and carboxyl group. The amide group is a non-ionic group, forming a secondary valence bond to adsorb and link with the active groups of other substances. Because polyacrylamide molecules are long and thin and have many chemically active groups, they can connect with precipitated particles to form larger flocculants. The structure of these flocculants is like loose, amorphous, interconnected but unstable cotton. There are many spaces and many fine networks inside, and a large amount of liquid is contained, so the specific gravity of the flocculants is close to the liquid itself in which it exists.

Flocculant also contains a variety of particles, that is, particles with different components, different properties, and different sizes. Therefore, good flocculant treatment can completely remove the original particles in the solution, making the solution appear particularly clear, transparent and shiny. Due to the larger size of the flocculant, it settles and filters faster. The flocculation of amphoteric flocculants with colloids in wastewater occurs through two forms of chemical adsorption and physical networks. According to the above mechanism, polyacrylamide with higher molecular weight and longer molecule can absorb more particles. The strong ability to form particle networks produces better flocculation performance.

2. Importance of appropriate proportion of carboxyl when using amphoteric polyacrylamide flocculants

Appropriate proportion of carboxyl groups in the use of amphoteric polyacrylamide flocculants is important. Because most of the colloids in wastewater are negatively charged, polyacrylamide needs an appropriate amount of carboxyl groups to bridge with it through calcium ions. However, if the carboxyl content is too large, the polyacrylamide molecule will be excessively negatively charged, and the repulsive force between the molecules will be too large, which is not conducive to flocculation. The types of polyacrylamide (PAM) are divided into anionic, cationic, nonionic, and amphoteric types. In wastewater treatment, amphoteric flocculants are used to improve the efficiency of processes such as sedimentation, clarification, filtration, and centrifugation during water treatment.

The commonly used polymerization methods for amphoteric polyacrylamide flocculants include bulk polymerization, suspension polymerization, solution polymerization and emulsion polymerization. In the case of free radical polymerization, any of those methods would work. For ionic or coordination polymerization, solution polymerization is generally adopted. For example, ethylene and propylene are polymerized using titanium catalysts. The polycondensation reaction is generally carried out in bulk or in solution, which are called bulk (melt) polycondensation and solution polycondensation respectively, and the polycondensation at the two-phase interface is called interfacial polycondensation. Monomers that are gaseous or solid at the polymerization temperature and pressure can also be polymerized, which are called gas-phase polymerization and solid-phase polymerization.

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