Anionic polyacrylamide APAM

1. Anionic PAM is mainly used for water treatment, papermaking, petroleum exploration and coal washing in coal mines. Obviously, APAM can play an important role in the treatment of micro-polluted water, source water, urban sewage, industrial water, and even the dewatering of excess sludge.
2. Because anionic polyacrylamide flocculant has the characteristic of large specific surface area, linear molecular chains and electrical groups are usually used as adsorbents or flocculants in water treatment. As an important coagulant aid, anionic polyacrylamide is usually used in combination with inorganic polymer flocculants such as polyaluminium chloride PAC and polyferric sulfate PFS in water plants and sewage treatment. Due to the large number of anionic groups in the molecular chain, APAM has a large negative potential. Therefore, for industrial wastewater, especially metal-containing wastewater, APAM can show high treatment efficiency through electric neutralization. But in the process of sludge dewatering, sludge particles will be adsorbed on the molecular chains with larger specific surface area. Adsorption performance of Anionic PAM plays an important role in this process, and the dehydration efficiency can reach 75%.
3. Anionic polyacrylamide is a popular soil flocculant, often used to prevent soil erosion in cultivated land.
4. Anionic polyacrylamide polymer can be used for silt management on construction sites. It can control erosion, clarify sandy runoff, and remove wet sediment during pond cleaning. They are usually be used in conjunction with other best management practices as part of a multi-barrier approach to minimize soil loss and improve the settlement of suspended sediments.

Since the molecular chain of the polymer has more charge, it can be stretched more in water, which will increase the adsorption capacity and the bridging ability to remove suspended particles. The interaction between APAM and suspended particles is mainly electrostatic, hydrogen bond or covalent bond. Anionic PAM has high molecular weight and good solubility, and is an important flocculant. Because of its good flocculation performance, it has been widely used in the water treatment.

1. On the one hand, unstable particles with opposite charges can be completely neutralized by anionic chemical groups.
2. On the other hand, these unstable particles will be captured by molecular chains of APAM and aggregate into large and dense flocs, thereby flocculating.
3. In the process of solid-water separation, the main mechanisms involved are charge neutralization, bridging, and electrostatic repair.

According to the development history of APAM synthesis in these years, there are six different synthesis technologies: homopolymerization post-hydrolysis method, homopolymerization co-hydrolysis method, copolymerization method, inverse emulsion polymerization method, precipitation polymerization method and radiation polymerization method.

Homopolymerization posthydrolysis process. The hydrolysis process after homopolymerization can be divided into two steps. The first is acrylamide radical polymerization. The second step is completed by adjusting the pH value to be greater than 7 under alkaline conditions. In this step, non-ionic polyacrylamide hydrolysis will occur. Finally, APAM can be obtained through these two steps. The biggest advantage of this process is that the first polymerization reaction is carried out at low temperature, which is beneficial to increase the molecular weight of APAM. However, because the polymerization process has one more step than other processes, the polymerization conditions are not easy to control. Secondly, the hydrolysis process may be heterogeneous, and it is difficult to meet the control requirements of solubility.

Homopolymerization cohydrolysis process. Based on the hydrolysis process after homopolymerization, the production process is proposed after simplification and improvement. Sodium is added to the reaction system before polymerization, which is the biggest difference from the hydrolysis process after homopolymerization. In this way, the polymerization reaction and the hydrolysis reaction will occur simultaneously. The hydrolysis reaction can produce hydroxyl anions. Finally, APAM is obtained through drying and granulation. This method can overcome the general problem of uneven hydrolysis caused by the post-hydrolysis process. However, adding sodium before polymerization may introduce impurities into the reaction system. These impurities can exist in the polymer through polymerization, thereby affecting the properties of the polymer.

Copolymerization method. The core idea of the copolymerization method is that acrylamide (AM) is polymerized with one or two other anionic monomers under initiation. Many small molecular organic compounds can be used as anionic monomers, such as sodium acrylate, 2-amido-2-methylpropane sulfonic acid and so on. The advantages of the copolymerization process are short production cycle, fewer processes, and no ammonia emissions. More importantly, the degree of hydrolysis can be controlled by copolymerization. The APAM produced has better solubility and higher molecular weight.

Inverse emulsion polymerization. Using the monomer aqueous solution as the dispersed phase and the water-insoluble organic solvent as the continuous phase, reverse emulsion polymerization is carried out in a water-in-oil emulsion. Inverse emulsion polymerization is commonly used to prepare hydrophilic polymer particles such as acrylamide, acrylate and n-isopropylacrylamide. Compared with bulk polymerization or other polymerization techniques, the inherent advantage of inverse emulsion polymerization is that this process can achieve high molecular weight and high reaction rate during the polymerization process. In addition, the reaction is easy to control, keeping the molecular weight in a relatively narrow range. The main problem of this technology is difficulty in emulsification and separation of oil and water. In addition, the solvent recovery process is also very complicated.

Precipitation polymerization method. Precipitation polymerization is a unique method for preparing cross-linked polymer particles of uniform size and shape without adding any stabilizers. In the polymerization mechanism, due to the non-swelling ability of the cross-linked primary particles, it is proposed that the growth of the particles is carried out by the effective precipitation of nucleated particles. For precipitation polymerization, the temperature won’t increase excessively with the heat dissipation of the reaction system, which is of great significance to the stability of the reaction. In addition, in the late stage of the reaction, the rest of the monomer will not gather together and can diffuse freely, which conducive to increase the conversion rate and molecular weight. Because most of the remaining monomers remain in the solvent, it plays a positive role in preparing APAM with low toxicity and high purity. Due to the precipitation in the solvent, the polymer is easily separated or filtered.

Radiation polymerization. Radiation polymerization can be carried out under the irradiation of high-energy ionizing radiation, electron beams, alpha rays, beta rays, gamma rays (especially Co-60 gamma) and x-rays. The most important mechanism is to generate ions or free radicals as active centers.