Polyelectrolytes are water soluble polymers carrying ionic charge along the polymer chain. Depending upon the charge, these polymers may be anionic or cationic. Homopolymers of acrylamide are also included in the family of polyelectrolytes though they do not carry any charge. These are called nonionic. Polyelectrolyte flocculants can be generally divided into two groups, depending on their molecular character and mode of operation: 1) primary coagulants, and 2) coagulation aids/flocculants.
The primary coagulants have typically high cationic charge density. Thus, they satisfy the “cationic demand” of the negatively charged suspended particles and initiate coagulation and formation of flocks. The primary coagulants have also typically low to medium molecular weight which allows a slow building of flocks, which gives a maximum removal of suspended solids (maximum turbidity reduction). The coagulation aids/flocculants have low charge density and they are used for building the size of flock by bridging the primary flocks. Flocculants have typically very high molecular weight.
There are many factors affecting the selection of the appropriate polyelectrolyte for a given process. Some of the factors causing challenges are high conductivity and high cationic demand of the treated fluid. There is a constant need to develop new polyelectrolyte polymers capable of facing these challenges.
Major applications of flocculants are their inherent solid liquid separating efficiency. This makes polyelectrolytes useful in applications in potable water, industrial raw and process water, municipal sewage treatment, mineral processing and metallurgy, oil drilling and recovery, paper and board production, etc. In all these applications the solid liquid separation property is commonly exploited.
Flocculants also behave as filter aids by modifying the filtration characteristics of suspended solids. Many of the slurries that are difficult to filter are modified by the use of polymeric flocculants at a relatively low dosage so that filtration rates become much faster. The ability of flocculants to dewater slurry especially encountered in municipal sewage treatment, mineral processing industry and metallurgical industry may be tackled by the use of high molecular weight flocculants. Color removal is another area of application of flocculants. The charge carried by the polymer is responsible for the abstraction of dissolved coloring matter from the waste water stream and hence color removing flocculants have wide applications in the waste water treatment.
Municipal effluent is treated in various ways depending on its composition, mainly to remove bioactive materials from the produced water. Various flocculants including polymeric flocculants have been used in order to flocculate and dewater sludge generated from municipal sewage and industrial wastewater. Environmental issues have brought about increasing pressure to remove all such materials including suspended solids from sewage effluent. Polyelectrolytes can be used in some or all of the sedimentation stages of the sewage treatment, but the main use is for sludge dewatering.
In recent years, sludge generated from municipal sewage and industrial wastewater has been increasing because of changes in the recent living environment. Furthermore, the properties of the sludge are becoming worse. Due to increasing demand of cationic polymer flocculants in these application there is a need to develop new cationic block copolymers designed for cost-effective solids/liquid separation processes.
Turbidity in raw water is mainly due to colloidal particles. Therefore coagulation, sedimentation, and filtration are required in water purification. The process typically involves removal dissolved organic matter and suspended solids of the water. Flocculants are used to help the removal of these. For example flocculants are commonly used for the treatment of drinking water. Methods and chemicals used or producing drinking water are, however, strictly regulated by laws and regulations.
In addition to the use as polymeric flocculants, water-soluble polymers, in particular, high-molecular weight water-soluble polymers are used in various technical fields such as retention aids, paper strength agents, and thickeners.
Such water-soluble polymers include homopolymers prepared by polymerizing an anionic monomer such as an acrylate or a methacrylate, a cationic monomer such as dimethylaminoethyl(meth)acrylate quaternary salt, or a nonionic monomer such as (meth)acrylamide and also include ionic polymers such as copolymers of an anionic monomer and a nonionic monomer; copolymers of a cationic monomer and a nonionic monomer; and copolymers of a cationic monomer, an anionic monomer, and a nonionic monomer.
As mentioned above, polyelectrolytes are very useful in paper making. Apart from aiding paper mill effluent treatment, polyelectrolytes have numerous applications in the actual manufacture of paper and paper board, such as a) improving the retention on the paper machine of fibers, fillers, dyestuffs and/or sizing chemicals; b) improving paper machine drainage (on machine dewatering); c) improving the ‘dry strength’ of paper made with recycled fibers; d) improvement of ‘wet strength’ of certain paper grades. Polyelectrolytes used in these applications are often referred to as fixatives, drainage aids, and/or retention aids. They may be added to the pulp before or during the paper production. The term “fixing” generally implies the binding of small particles to pulp fibers. Fixatives build up agglomerates with colloidal material in the water phase and attach them onto fibers so that they end up in the final paper sheet.
It is known that the existing water soluble polymers do not work well under all circumstances. Therefore there is a constant need to find better performing polymers or at least find new alternatives for the existing ones.