The changes of flocculants that lead to the decline of flocculation performance under adverse conditions are generally referred to as degradation, and are specifically manifested as molecular weight reduction, solution viscosity reduction, poor flocculation performance or even failure. There are many factors that could have had this effect. In this regard, high molecular weight PAM is a rather "squeamish" substance. Moreover, the higher the molecular weight of Polyacrylamide (PAM), the easier it is to produce these changes, and the more sensitive it is to related factors.

We must pay great attention to this problem, otherwise even the best flocculants will not be able to achieve good results. The high molecular weight of modern polyacrylamide products is the basis for its good flocculation properties. However, the macromolecules of this flocculant are easily damaged by external factors, which greatly reduces its performance. Flocculants must be formulated and used carefully to prevent these problem.

The main factors that lead to the reduction of the viscosity and flocculation efficiency of the PAM solution are:

1. Mechanical action: High-speed stirring or strong mechanical shearing in the solution will break the macromolecules. If the PAM solution is stirred in a centrifugal pump for a few seconds, its molecular weight will drop by 75%. If it is dissolved by high-speed stirring or transported by high-speed equipment, its molecular weight and flocculation performance will be significantly reduced.

2. Rust and iron compounds: Add a very small amount (such as 2mg/l) of iron compounds (such as fecl3), or a small amount of rust powder to the PAM solution, and stir it slightly to disperse, and the viscosity and flocculation performance of the PAM solution will be greatly improved. reduce. When the PAM solution is placed in a rusted iron container, the viscosity drops by 78% after 4 hours, and the flocculation efficiency is greatly reduced.

3. The effect of high temperature: PAM macromolecules are very sensitive to high temperature. For example, if a 0.1% PAM solution is placed at 80 °C for 4 hours, the molecular weight will decrease from 21 million to 7.6 million, and it will also drop to 16.9 million when placed at 50 °C; the molecular weight is 1050 10,000 PAM, the molecular weight dropped to 3.3 million after being placed at 80°C for 4 hours. For example, at 30°C, the molecular weight decreases very slowly. If the original molecular weight of PAM is very low, such as 3.7 million, the degradation by heat is very small.

4. The influence of coexisting impurities: if there are suspended impurities in the PAM solution, its viscosity will be reduced. Inorganic ions, especially high-valent ions, also have a great influence. For example, the viscosity of a PAM solution is 191 centipoise, after adding nacl containing na+100mg/l, the solution viscosity drops to 140, and after adding cacl2 containing ca2+100mg/l, the viscosity drops to 30 centipoise.

5. Others: UV irradiation will cause rapid degradation of PAM, and strong irradiation for 4 hours can reduce the molecular weight of PAM from 18 million to 10 million, and the presence of oxidants in the solution will also accelerate the degradation. The degradation of PAM belongs to the free radical chain reaction (free radical chain reaction). The reaction of oxygen and iron can generate free radicals, and ultraviolet rays are the same, so they should be avoided. The performance degradation of PAM solution is partly due to the change of macromolecular morphology: from linearly stretched long chains to contracted and curled spheres. The PAM molecule contains a large number of negatively charged groups, which repel each other and make the macromolecules in a stretched state. The molecules are longer and fully expose the active groups. They are good at bridging and connecting, and have good flocculation performance. However, if there are more cations in the PAM solution, they will form an electric double layer around the negatively charged groups of the macromolecules, which will weaken the repulsion between the negatively charged groups and make the pam macromolecules into a curled state. The higher the ion concentration, the greater this effect. Divalent ions such as ca2+ are not only strongly adsorbed by negatively charged groups, but also may bridge the two negatively charged groups, which further enhances the curling of macromolecules. This not only causes the solution viscosity to drop (the solution viscosity of spherical macromolecules is much lower than that of linear molecules), but also reduces the effective activity of carboxyl groups in PAM molecules, resulting in a significant decrease in flocculation performance.