Polyacrylamide (PAM) is a linear water-soluble polymer, one of the most widely used varieties of water-soluble polymer compounds. PAM and its derivatives can be used as efficient flocculants, thickeners, paper enhancers and liquid reducers, which are widely used in water treatment, paper making, petroleum, coal, mining and metallurgy, geology, light textile, construction and other industrial sectors.
The difference between PAM anions and cations
1, From the appearance of words, anions and cations are solid and liquid two forms, solid too similar, generally difficult to distinguish with the naked eye, liquid words or slightly different, anionic liquid white, and cationic will be with a slight blue, careful observation can still be distinguished.
2, Use: Anionic type is mainly used for treating inorganic wastewater, such as coal mine wastewater, sand washing wastewater, etc. These can also be used as thickening agent, binder, etc.; cationic type is mainly used for treating organic wastewater, such as food factory, sugar factory, urban sewage, etc. It is also more often used for sludge dewatering.
3, The indicators are different: the main indicators of anionic type are molecular weight and hydrolysis degree; the main indicators of cationic type are ionicity and molecular weight.
4, The molecular weight is different: anionic general molecular weight is high, ranging from 6 million to 25 million; cationic molecular weight is generally between 6 million to 12 million.
Technical specifications of PAM
The technical indexes of polyacrylamide are molecular weight, hydrolysis degree, ionicity, viscosity, residual monomer content, etc. Therefore, the quality of PAM can also be judged from these indexes!
1、Molecular weight
The molecular weight of PAM is high and has increased considerably in recent years. the molecular weight of PAM applied in the 1970s was generally in the millions; after the 1980s, most high efficiency PAMs have a molecular weight of 15 million or more, with some reaching 20 million. Each such PAM molecule is polymerized from more than 100,000 acrylamide or sodium acrylate molecules (the molecular weight of acrylamide is 71, and the molecular weight of PAM containing 100,000 monomers is 7.1 million). Usually, PAM with high molecular weight has better flocculation performance, with a molecular weight of 71 for acrylamide and 7.1 million for PAM containing 100,000 monomers. The molecular weight of polyacrylamide and its derivatives ranges from several hundred thousand to more than ten million, and can be divided into low molecular weight (less than one million), medium molecular weight (one million to ten million), high molecular weight (ten million to fifteen million), and supramolecular weight (more than fifteen million) according to the molecular mass.
The molecular weight of a polymeric organic substance is not perfectly homogeneous, even within the same product, and the nominal molecular weight is its average value.
2、Hydrolysis degree and ionicity
The ionicity of PAM has a great influence on its use effect, but its suitable value depends on the type and nature of the material to be treated, and there will be different optimal values under different circumstances. If the ionic strength of the material to be treated is high (containing more inorganic substances), the ionicity of the PAM used should be higher, and vice versa should be lower. Usually, the anionicity is referred to as the hydrolysis degree. And ionicity generally refers specifically to cations.
Ionicity = n/(m+n)*100%
Early production of PAM is made of polyacrylamide a monomer polymerization, the original does not contain -COONa group. Before use, NaOH is added and heated to partially hydrolyse the -CONH2 group to -COONa, with the following reaction equation.
-CONH2 + NaOH -→ -COONa + NH3↑ ammonia is emitted in the hydrolysis process. the proportion of amide group hydrolysis in PAM is called the degree of hydrolysis of PAM, it is the degree of anion. The use of this PAM is not convenient, and the performance is poor (heating hydrolysis must make PAM molecular weight and performance significantly reduced), after the 80s has been rarely used.
Modern production of PAM has a variety of different anionicity products, the user can choose the appropriate species according to the needs and through the actual test, without further hydrolysis, after dissolution can be used. However, due to customary reasons, some people still refer to the dissolution process of flocculants as hydrolysis. It should be noted that the meaning of hydrolysis is the decomposition of water, which is a chemical reaction, and the hydrolysis of PAM has ammonia gas emission; while dissolution is only a physical effect, no chemical reaction. The essence of the two is different and should not be confused with each other.
3. Residual monomer content
The residual monomer content of PAM is the amount of acrylamide monomer that is not completely reacted and ultimately remains in the polyacrylamide product during the polymerization of acrylamide into polyacrylamide, and is an important parameter for measuring its suitability for the food industry. Polyacrylamide is non-toxic, but acrylamide has a certain degree of toxicity. Therefore, the residual monomer content in PAM products must be strictly controlled. International regulations stipulate that the residual monomer content in PAM used in the drinking water and food industries should not exceed 0.05%.
4、Viscosity
PAM solutions are very viscous. The higher the molecular weight of PAM, the more viscous the solution. This is because PAM macromolecules are long and thin chains, which have a high resistance to movement in solution. The viscosity of solutions of various polymeric organic compounds is high and increases with higher molecular weights. Determination of polymer organic molecular weight of a method, is to determine the viscosity of a certain concentration of solution under certain conditions, and then according to a certain formula to calculate its molecular weight, called "viscosity average molecular weight".
Selection of PAM
Polyacrylamide can be divided into four types according to ionic characteristics: non-ionic, anionic, cationic and amphoteric polyacrylamide. According to the molecular weight to be divided into different specifications of molecular weight, ionicity, etc. derived from many models, the face of the market cluttered specifications system, for their own sewage system preferential selection of the best polyacrylamide model is really difficult, how a few moves to deal with the sewage or sludge polyacrylamide selection of common problems.
1. Understanding the sources of sludge
Sludge is an inevitable product of wastewater treatment and first we should understand the source, nature, composition and solids content of sludge. Sludge can be divided into organic sludge and inorganic sludge according to the main components it contains.
Generally speaking, cationic polyacrylamide is used to treat organic sludge and anionic polyacrylamide is used to treat inorganic sludge. It is not easy to use cationic polyacrylamide when the alkalinity is very strong, while anionic polyacrylamide is not suitable when the acidity is very strong.
2、Molecular weight selection of polyacrylamide
The molecular weight of polyacrylamide refers to the length of the molecular chain in the molecule, the molecular weight of polyacrylamide is between 5-18 million, generally speaking, the higher the molecular weight of polyacrylamide products, the greater the viscosity, however, when using, not the higher the molecular weight of the product, the better the use effect, specifically in use, according to the actual application industry, water quality, processing equipment and other conditions, to determine The suitable molecular weight of polyacrylamide.
Polyacrylamide is a high molecular coagulant, the product can be divided into three categories according to its average molecular weight: low molecular weight (<1 million), medium molecular weight (2-4 million) and high molecular weight (>7 million).
Polyacrylamides are used in wastewater treatment, with molecular weights ranging from several million to tens of millions of high molecular water-soluble organic polymers. The domestic polymer polyacrylamides are: non-ionic polyacrylamide (abbreviated NPAM, molecular weight 6-12 million), anionic polyacrylamide (abbreviated APAM, molecular weight 6-25 million), cationic polyacrylamide (abbreviated CPAM, molecular weight 6-12 million, ionicity 10%-80%).
If used as a coagulant, the higher the molecular weight, the tighter the flocs and the more economical the drug, but the molecular weight of anionic polyacrylamide is not recommended to exceed 20 million.
If in sludge dewatering. Use belt filter press, the molecular weight generally can not be too high, if the molecular weight is very high may lead to filter cloth clogging, affecting the dewatering effect; then for example, you use centrifugal filter press, the molecular weight requirements should be higher, because the centrifugal filter press requires the floc to be able to resist shear as much as possible, so choose a relatively high molecular weight products.
3、Ionicity selection of polyacrylamide
For the sludge to be dewatered, flocculants with different ionicity can be screened through small experiments to select the best suitable polyacrylamide, so that the best flocculant effect can be obtained and the dosing amount can be minimized to save costs. The key to selecting ionicity is.
(1) Size of flocs
Floc size: too small flocs will affect the speed of drainage, too large flocs will bind more water and reduce the mud biscuit degree. The floc size can be adjusted by choosing the molecular weight of polyacrylamide.
(2) Floc strength (water content)
Strength of flocs: Flocs should remain stable under shear without breaking. Increasing the molecular weight of polyacrylamide or choosing a suitable molecular structure can help to improve the floc stability.
(3) Mixing of polyacrylamide with sludge
The polyacrylamide must react fully with the sludge at a certain location in the dewatering equipment and flocculation must occur. For this reason, the viscosity of the polyacrylamide solution must be suitable, to be able to mix fully with the sludge under the existing equipment conditions, and whether the two are mixed evenly is a key factor for success. The viscosity of the polyacrylamide solution is related to its molecular weight and the concentration of the formulation.
(4) Dissolution of polyacrylamide
The dissolution is good in order to give full play to the flocculation effect. As mentioned before, the dissolution process of polyacrylamide is actually the ripening process of polyacrylamide, sometimes it is necessary to speed up the dissolution speed, and then the concentration of polyacrylamide solution can be considered to increase.
So the best product choice has to be determined by laboratory beaker experiments.
It can be seen from the above that the choice of molecular weight and ionicity is not absolute, it is best to do the selection test before choosing polyacrylamide, and it is best to operate the test on the machine, so that the data obtained is the most accurate, and the preferred polyacrylamide is more cost-effective.
