What Is Polyacrylamide (PAM)? Properties, Production and Use in Wastewater Treatment
Polyacrylamide (PAM) is the general name for polymers obtained by homopolymerisation of acrylamide, or by copolymerisation of acrylamide with other monomers. It is one of the most widely used water-soluble polymers and one of the most important organic polymeric flocculants used in industry.
Behaviour in solution
At low concentration a PAM solution can be regarded as a network: mechanical entanglement between chains and hydrogen bonding together form the network nodes. At higher concentration the solution contains many chain-to-chain contact points and behaves as a gel. PAM solutions are compatible with many water-miscible organic solvents and are notably tolerant of electrolytes: ammonium chloride, calcium sulfate, copper sulfate, potassium hydroxide, sodium carbonate, sodium borate, sodium nitrate, sodium phosphate, sodium sulfate, zinc chloride, boric acid and phosphoric acid have little effect on them.
Monomer production: three generations of technology
Acrylamide monomer is produced from acrylonitrile, which is hydrated over a catalyst to give a crude acrylamide solution; after flashing and refining, the pure monomer becomes the raw material for PAM.
First generation – sulfuric acid catalysed hydration. Conversion of acrylonitrile and yield of acrylamide are low, by-products place a heavy load on purification, and the corrosive catalyst raises both equipment and production cost.
Second generation – binary or ternary skeleton copper catalysis. Copper ions are carried into the product and interfere with polymerisation, increasing the cost of post-treatment.
Third generation – microbial nitrile hydratase catalysis. The reaction runs under mild conditions at ambient temperature and pressure with high selectivity, high yield and high activity; acrylonitrile conversion reaches 100% with no by-products, and the product contains no copper ions, so ion exchange is no longer needed and the flowsheet is simplified. Gas chromatography shows almost no free acrylonitrile, which makes the monomer particularly suitable for ultra-high-molecular-weight PAM and for the non-toxic grades used in food processing.
Japan built the first 6,000 t/a microbial acrylamide plant in 1985 and Russia followed, and both countries commissioned plants of the 10,000 t/a class in the 1990s. China was the third country to master the technology: the process was developed by the Shanghai Institute of Pesticides over three five-year plans, and the nitrile hydratase catalyst, strain Norcardia-163 isolated from soil samples collected at the foot of Mount Tai and in Wuxi, reaches an activity of 2,857 IU. The technology has been commercialised in Rugao (Jiangsu), Nanchang (Jiangxi), Shengli Oilfield and Wanquan (Hebei), and meets the quality index for ultra-high-molecular-weight PAM.
Polymerisation technology
PAM is produced by polymerising an aqueous acrylamide solution in the presence of an initiator. The resulting gel block is cut, pelletised, dried and milled to give the finished product. Polymerisation is the key step, and the downstream handling must control mechanical cooling, thermal degradation and cross-linking in order to preserve molecular weight and water solubility.
The process has evolved through three stages: tray polymerisation, in which the reaction liquid is held in stainless-steel trays in a heated chamber and the resulting block is cut into strips, pelletised, dried and milled, essentially a workshop process; kneading reactors, in which polymerisation and pelletisation take place together; and, from the late 1980s, conical reactor polymerisation with a rotating cutter at the bottom of the cone, so that the polymer is pelletised as it is extruded and then dried in a rotary dryer and milled.
To stop the gel from sticking to the reactor wall, some processes coat the internal surface with fluoropolymer or silicone, but such coatings tend to flake and contaminate the product. Rotating conical reactors allow the gel block to be tipped out after reaction. Pelletising may be mechanical, by cutting, or wet, in a dispersion, and drying may use through-flow rotary dryers or vibrating fluidised beds. Current practice tends towards fixed conical reactors combined with vibrating fluidised-bed drying.
Formulations also differ in when the alkali is added. Pre-alkali co-hydrolysis is the simpler process, but heat transfer during hydrolysis tends to cause cross-linking and a large loss of molecular weight; post-alkali hydrolysis adds a step but hydrolyses more evenly, with less cross-linking and less molecular weight loss. Azo initiators – azobisisobutyronitrile, azobis-dimethylvaleronitrile, sodium azobis-cyanovalerate and the azobis-amidine salts developed in the 1980s – are used at 0.005–1 ppm, are highly efficient, favour high molecular weight, and are water-soluble and easy to handle.
Inverse suspension polymerisation
Inverse suspension polymerisation is the most widely used and technically mature route to PAM microspheres. Monomer or monomer mixture is dispersed by vigorous stirring in an organic medium to form fine droplets, and polymerisation then proceeds in the presence of initiator and dispersing stabiliser. After azeotropic dehydration, separation and drying, a particulate product is obtained. Typical specifications are a solids content above 90%, conversion above 95%, residual monomer below 0.5% and a particle size of 10–500 µm, with good water solubility.
The route is simple, easy to control, allows the heat of polymerisation to be removed readily, and gives a pure, uniform and stable product that separates, washes and dries easily, so it is easy to scale up. It does, however, have drawbacks: it is very sensitive to stirring speed and prone to agglomeration and gel formation, the system is unstable during azeotropic distillation, dewatering takes a long time, the particle size distribution is broad, large volumes of organic solvent are needed, and both safety and cost are less favourable, which is why it is rarely used for PAM production in China.
Solution viscosity and what affects it
The viscosity of a PAM solution mainly reflects the internal frictional resistance between liquid molecules as they flow or move relative to one another. That resistance depends on polymer structure, solvent properties, concentration, temperature and pressure: the higher the value, the more viscous the solution.
Temperature. Temperature reflects the intensity of random molecular motion. Molecules must overcome intermolecular forces – hydrogen bonding, internal friction, diffusion, chain orientation and entanglement – all of which directly affect viscosity, so the viscosity of a polymer solution changes markedly with temperature. PAM solution viscosity falls as temperature rises, because the entangled network formed by the dispersed polymer particles is more easily broken at higher temperature.
Hydrolysis time. Viscosity also changes with hydrolysis time. When hydrolysis is short the polymer has not yet had time to build its network, so viscosity is low; when hydrolysis is excessive the structure loosens and viscosity falls again. Partially hydrolysed PAM dissociates in water into negatively charged macromolecules, and electrostatic repulsion between molecules, and between anionic segments of the same molecule, makes the chain extend and entangle, which is why partially hydrolysed PAM raises solution viscosity so effectively.
Salinity. Where cationic groups outnumber anionic groups the net charge and polarity are high, and because water is a polar molecule the polymer dissolves well and intrinsic viscosity is high. As mineral content rises, the positive charges are progressively surrounded by an ionic atmosphere, polarity falls and viscosity drops. At higher salt concentration, intra- and intermolecular hydrogen bonding between charged groups reduces solubility, while the added salt ions screen the charges and break up the ionic associations; the two effects compete, so viscosity stays low above about 0.06 mol/L.
Practical guidance for plant use
Dissolution. Only a well-dissolved polymer develops its full flocculating effect. Where faster dissolution is needed, raising the solution concentration is one option, but it has to be balanced against dosing accuracy and pumpability.
Ionic grade. For sludge dewatering, flocculants of different ionic charge should be screened in bench tests to find the grade that gives the best dewatering at the lowest dose, which is where most of the chemical cost is saved.
Combined use with PAC. For some wastewaters a single flocculant is not enough and two reagents must be combined; PAC and PAM together usually perform better, but the order of addition matters. PAC, a coagulant, first neutralises charge and destabilises the colloids to form small flocs; PAM then builds those flocs into larger, faster-settling aggregates. Because the PAC reaction is very fast it needs vigorous mixing, while PAM needs a longer and gentler stage: strong mixing first to disperse, then gentle mixing to avoid breaking the flocs. The usual order is coagulant first and PAM second, but the order, dosing point, dose and mixing intensity should always be confirmed by jar tests, and the two reagents should never be mixed together before dosing.
Where PAM is used
In China the main PAM applications are oil recovery, water treatment, papermaking, superabsorbent resins, metallurgy and coal washing. In 2012 the consumption split was approximately 81% oil recovery, 9% water treatment, 5% papermaking, 2% mining and 3% other uses. Oil recovery remains the largest outlet, while water treatment is the second largest and keeps growing as municipal and industrial wastewater treatment rates rise. In papermaking, PAM serves mainly as a binder for pulp fibres and additives and for effluent treatment. The product is covered by the Chinese national standard GB/T 13940-1992.
