POLYELECTROLYTES are water soluble polymer carrying ionic charge along the
polymer chain. Depending upon the charge, these polymers are anionic or
cationic. Polyelectrolytes are available in a wide range of molecular weights
and charge densities. Homo polymers of acrylamide are also included in the
family of polyelectrolytes though they do not carry any charge. These are called
nonionic. Polyelectrolytes have got a wide range of applications right from
water purification, oil recovery, colour removal, paper making, mineral
processing, etc., etc,. Polyelectrolytes are both flocculants as well as
deflocculants depending upon the molecular weight. A flocculant is
essentially a solid liquid separating agent while a deflocculant is a
HOW THE FLOCCULANTS WORK?
Stokes' Law predicts that spherical particles suspended in a fluid medium settle at a rate proportional to the fourth power of the particle radius. Thus large particles will settle much faster than smaller ones. Most particles while suspended in aqueous solution have a net negative surface charge.
This arises from factors such as :
a) Unequal distribution of constituent ions on the particle surface
b) Ionisation of surface groups pH effect
c) Specific adsorption on the particle surface of ions from solution
d) Isomorphous substitution of silicone atoms by aluminum atoms in an alumino silicate mineral lattice (inorganic clays).
The above factors cause an electrical double layer around each particle and colloidal particle in aqueous solution will not settle very quickly. Interparticle interactions will cause repulsion and inspite of Brownian motion trying to bring them together the suspension becomes stable, i.e. the particles do not aggregate unless they are forced. The first phenomenon taking place in the process of flocculation is the neutralisation of the nett charge carried by each particle. Once charge neutralisation takes place several particles come together which will result into coagulation. Flocculation is the stage whereby the destabilised particles are induced to collect into larger aggregates. The aggregation is followed by rapid settling as per Stokes' Law. The various steps in the overall process are shown in Fig.1.
There are two possible mechanisms given for the phenomena of polyelectrolyte induced coagulation and flocculation. These are Charge patch model and Bridging model as shown in Fig. 2 & 3. Polyelectrolyte flocculants can be generally divided into two groups, depending on their molecular character and mode of operation:
1) Primary coagulants (eg., polyamine types)
Have high cationic charge density; satisfy the 'cationic demand' of the negatively charged suspended particles and initiate coagulation and formation of flocs.
Have low to medium molecular weight which allows a slow building of flocs (provided there is an adequately long contact time between the forming floc and the suspended matter) which gives a maximum removal of suspended solids (maximum turbidity reduction).
2) Coagulation aids/flocculants (eg., polyacrylamides)
Have low charge density; are used only for building the size of floc by bridging the primary flocs do not satisfy the 'charge demand'.
Have very high molecular weight ; this is need to produce large, fast settling flocs by bridging many small primary flocs.
The factors affecting the selection of the appropriate polyelectrolyte(s) for a given process are :
a) The nature of the suspended particles (substrate)
* Organic/inorganic content
* Net surface charge density
* Solids content of substrate
* pH of the substrate
* Temperature of the system (Brownian motion).
b) The end result to be achieved
* Rapid separation of the solid matter from the fluid
* Clarity of the separated fluid.
c) Dynamic and shear effects
* Mixing /conditioning of polymer and substrate
* Nature of the shear forces associated with the dewatering equipment used.
TYPES OF POLYELECTROLYTES
Polyelectrolytes are both inorganic and organic exhibiting both flocculation and deflocculation properties. Inorganic polyelectrolytes exhibitingsedimentation properly are coagulants rather than flocculants, whereas organic polyelectrolytes, exhibiting sedimentation, property, are invariably high molecular weight synthetic polymers. Inorganic flocculants are salts of multivalent metals like aluminium and iron. The process of sedimentation these salts exhibit is totally different from the organic types.
As discussed earlier depending on the charge carried by the polymer, polyelectrolyte are classified into anionic (negatively charged), cationic(positively charged) and nonionic (no charge). as shown in Fig.4.
The acrylic acid unit of a polymer ionised to produce a negatively charged polymer backbone. Since the charge carried by the active portion of polymer is negative all such polyelectrolytes are known as anionics. Similarly in the case of cationics the positive charge carrying nitrogen is a part of the polymer. In the case of nonionics there is no charge on the base polymer as there are no ionisable groups present in these.
The common anionic polyelectrolytes are homo polymers and co polymers of NaSalt of acrylic acid with acrylamide which are generally termed as polyacrylamides, eg :
Apart from the above there are several other types of commercially available anionic polyelectrolytes. The important ones are poly styrene sulfonic acids and 2acrylamido2methyl propane sulfonic acids. There are another series of anionics which are derived from polyacrylamides by restricted hydrolysis to produce anionic polyelectrolytes. These resemble copolymers of acrylamide and acrylic acid.
Cationic polyelectrolytes are homopolymers or Copolymers with Acrylamide of three major cationic monomers, viz. :
A wide range of cationic polyelectrolytes are hence available depending on the cationic monomer present, the charge density and the molecular weight.
Another less commonly available cationic monomer is methacrylamido propyl trimethyl ammonium chloride :
Quarternary polyamines which are manufactured from epichlorohydrine and a secondary amine such as dimethylamine are another types of low molecular weight cationics commonly found with very interesting applications.
Poly(dimethyl diallyl ammoniam chloride) and copolymers with acrylamide are yet another type of cationic. Polyethylene imines, which are cationics under acidic conditions, are available as 2030% w/w aqueous solutions. These are generally highly branched and are of low molecular weight in nature.
Similarly in the cationic range, Mannich types are produced by reacting polyacrylamides with formaldehyde and a secondary amine such as dimethyl amine and subsequently quarternising to produce stable cationic polyelectrolytes.
The nonionic polyelectrolytes are by and large homopolymers of acrylamide with a wide range of molecular weights. Though countless polyelectrolytes are theoretically possible depending upon the charge density and molecular weight a few of them have found commercial application. This limits the usage of polyacrylamides of a specific pattern for a specific application irrespective of the source of supply.
Methods of Manufacturing
The conventional method of manufacturing polyelectrolytes is by the solution polymerisation using a redox catalyst. However, the viscosity of the polymer solution produced thus limits the synthesis of high molecular weight polymers.
There are several polymers available in the form of solutions of upto 50% concentration. As the ratio of the acrylic acid to acrylamide goes up in an anionic polyelectrolyte the viscosity built up is considerable which limits the practical use of this method on an industrial scale.
Polyelectrolytes are also manufactured by the emulsion polymerisation of acrylamide and a co monomer like acrylic acid or a cationic monomer.
In this polymerisation process a stable water in oil emulsion of the monomer is prepared and polymerised under conditions which are much more isothermal using the oil phase to dissipate the heat of polymerisation. The emulsion polymers have active polymer content generally ranging from 25 to 50%. Certain additional surfactants are added to enable water in oil emulsion to break and invert to oil in water emulsion at end users' concentration. Wide ranging polyelectrolytes can be manufactured using this emulsion polymerisation technique, however, the major disadvantages associated could be :
i) Shelf life of the emulsions because emulsions are prone to separate especially in high ambient temperatures, and
ii) When used in application which require potable grade of polyelectrolytes because the oil used for making emulsion cannot be removed prior to use.
Acrylamide has a very fast propogation rate and a high exothermic heat of polymerisation. These factors combine to give ato acceleration of polymerisation at high concentration in aqueous solution with subsequent formation of a rubbery gel. The gel is, subsequently granulated and then thermally dried to produce dry granular polyelectrolytes.
DaiIchi Kogyo Seiyaku (DKS) has developed and patented a continuous, photo initiated polymerisation process for manufacturing the full range of Cationic, Anionic and Nonionic polyelectrolytes. In collaboration with DKS, Dai Ichi Karkaria Ltd.,(DIKL) has set up s state of the art plant to manufacture the full range of Polyelectrolyte in solid granular form.
The salient features of the process are :
i) A monomer feed, consisting of various types of monomers, initiators, chain terminators is first made in a kettle.
ii) The above feed is photo polymerised in a specially designed reactor to give a gel sheet.
iii) The gel sheet is further treated to form uniform granules.
iv) Dehydration and blending of dry powders to give a finished product and dry flowable granules.
The major advantages of these processes are :
i) Very easy to manipulate the :
* Molecular weight, from low to very high, by simply changing the feed composition
* The composition of product by incorporating different monomers.
ii) Precise control of charge density as the monomer feed composition is controlled at the initial stages only.
iii) Precise control of molecular weight distribution has the process in a continuous are :
a) No flammable and toxic solvents used
b) No production of waste matter or evolution of obnoxious gases
c) No production of hazardous effluents.
The entire process is very clean and apart from the above advantages, we can obtain product with very low residual monomer contents, and hence can be used for potable applications also. The finished products obtained are in the form of free flowing non dusting granules, which possess a very long shelf life and are easy to sotre and handle.
By far photo polymerisation has gained a prominent position in the industrial method of manufacturing polyacrylamide type polyelectrolytes. The polymer produced is having the same composition as the feed. Precise control of composition, molecular weight and consistency of the product batch after batch are the salient features of the process.
Polymers produced by the photo polymerisation are by far the best type of polymers available in terms of its high molecular weight though medium high molecular weight polymers can be made by emulsion polymerisation technique though there are several draw backs associated with this route. The first among these draw backs are the product stability.
APPLICATION OF FLOCCULANTS
Major applications of flocculants are their inherent solid liquid separating efficiency. This makes polyelectrolytes a unique class of polymers which find extensive application 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 commercially exploited. The relation between the molecular size and usage is summarised in Fig. 5
Flocculants also behave as filter aids by modifying the filtration characteristics of suspended solids. Many of the difficult to filter slurries 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 are tackled by the use of high molecular weight flocculants.
Colour removal is another area of application of flocculant. The charge carried by the polymer is responsible for the abstraction of dissolved colouring matter from the waste water stream and hence colour removing flocculants have wide applications in the waste water treatment.
Removal of oil and grease especially from the waste water stream is a second major application of speciality polymers. Low molecular weight cationic polymers are extensively used for deoiling oil field effluents, waster water coming out of refineries, engineering industries, etc.
Some of the major areas of applications of polyelectrolytes are in the following industries:
Potable water treatment
Drinking water is produced by treating naturally occurring waters to reduce order, taste, appearance, and sediment to acceptable levels. In general this involves removal of bacteria, viruses, algae, dissolved mineral, dissolved organic matter and suspended solids of the water. Flocculants are used to remove the latter two species. The particle size and physical state are summarised in Fig.6 along with the appropriate method generally adopted. Generally anionic polyacrylamide of low degree of hydrolysis is used for the treatment of portable water. The polymers used are governed by FDA standards that the residual acrylamide monomer content in them be less than 0.05%. Since the turbidity in raw water is mainly due to colloidal particles coagulation sedimentation and filtration are required. Hence low molecular weight cationic flocculants are also popular for this purpose.
Historically, inorganic coagulants based on Aluminium, Iron and Calcium have been used for potable water treatment. Most common out of these is aluminium sulphate (alum). These operate by formation of aluminium hydroxide precipitate which sweeps down or coprecipitates the suspended matter. Apart from the fact that these have to be used in large amounts, coagulants have limited pH range under which hydroxide precipitates is formed.
This increases the dissolved solids in the final drinking water, and may also cause corrosion problems especially with Iron salts and generate excessive amounts of sludges because of the voluminous nature of the metal hydroxide precipitates. Polyelectrolytes can replace in part or whole, the inorganic coagulant to meet the clarity norms at much reduced amounts (At ppm levels) and thereby considerably reducing the sludge formation.
Waste water treatment
Domestic and industrial effluents present a variety of different types of the waste waters. Colour removal from the waste water stream is a challenge to scientists o Water Chemistry. The colour, depending on the source and nature can be removed by the following methods:
1) Chemical destruction
2) Physical removal like adsorption
3) Physicochemical methods
4) Biological methods.
Many of the colouring matters present in the waste water stream can be reduced or oxidised. Oxidation method is preferred when the colouring matter is of organic origin.
Sodium Hypochlorite, Hydrogen Peroxide, etc. are commonly used for this purpose and the treatment is expensive. Physical methods like adsorption has a limited application and generally preferred in conjunction with other methods of treatment as a final polishing step. Flocculant treatment may be called as physico chemical method as both phenomena are involved in this process.
A typical example of this type of product is DK SET CA 60. Treatment with DK SET CA 60 followed by DK SET P 3113 has been found to be an excellent combination for removing colour from coloured waste water from Dyestuff Industries, textile mills, food processing industries, distillery, pulp & paper, etc,. The cost of the treatment is directly proportional to the intensity of the colouring matter.
Municipal Sewage treatment
Municipal effluent is treated in various ways depending on its composition, mainly to remove bioactive materials from the produced water. 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.
The main use of polyelectrolytes as on date is for sludge dewatering. The optimum polyelectrolyte molecular weight tend to increase depending on the type of dewatering equipment used in the following order :
Drying bed << Vacuum Filter
Belt Press < Filter Press < Centrifuge.
As the activated sludge percentage goes up compared to the primary sludge requirement of polymer is for a higher cationic charge. An increase in the proportion of activated sludge to primary sludge decreases the overall dewaterability. At constant molecular weight increasing the polymer cationic charge has very little effect on the dewatering of primary sludge but improves the dewatering of 50:50 primary : secondary sludge quite significantly.
Paper making is a very complex art. Apart from aiding paper mill effluent treatment, polyelectrolytes have numerous applications in the actual manufacture of paper and paper board:
a) Improving the retention on the paper machine of fibres, fillers, dyestuffs and/or sizing chemicals
b) Improving paper machine drainage (on machine dewatering)
c) Improving the 'dry strength' of paper made with waste (recycled) fibres
d) Improvement of 'wet strength' of certain paper grades such as facial tissues and kitchen towels.
Retention and drainage are often parameters in opposition. Retention aids are generally polyacrylamides anionic or cationic.
Dry strength resins improve the overall strength of the finished paper by Hydrogen bonding between adjacent cellulose fibres particles. These are generally low charge cationic polymers of moderate molecular weight. Use of paper to abosrb moisture (facial tissue or kitchen towels) require the paper to maintain its dimensional structure.
The mining and extraction of coal and inorganic minerals from waste materials involves the use of large quantities of water. Flocculants (invariably anionic polyacrylamides of high molecular weight) are used to dewater the end product and/or the waste products ( tailings ) from aqueous suspensions.
Very highly charged anionic polymers are used to settle ion containing by products (red muds) in alumina production.
Oil field applications
In contrast to the previously described application, areas where the polyelectrolytes are generally employed for their flocculant ability, the use of flocculant in the technology of drilling and exploration of oil reserves from rock ormations also utilise the rheological properties of polyelectrolyte in solutions. Almost all stages of the life cycle of an oil well are potential application areas for polyelectrolytes. They find use during oil (or gas) well drilling, well cementing, well reservoir stimulation, oil or gas production, reservoir profile modification and enhanced oil recovery (Polymer flooding).
The proper functioning of a drilling mud is very crucial to increase the drilling rate and efficiency of the drilling operation without damaging the capacity of the well to yield oil. Polymers are used to modify the mud viscosity as viscosifiers to increase the viscosity or as low molecular weight thinners to decrease the viscosity of the mud under the driling shear conditions.
Low to medium molecular weight polyacrylamides or polyacrylates act as fluid loss additives to prevent synaeresis of mud or water containing components of the mud into the reservoir formation which would block the formation pores and limit egress during the production stage. Conversion of high molecular weight anionic polyacrylamides are used to flocculate the spent mud. Various polyelectrolytes can be used as friction reducers during the fracturing process to stimulate the well.
Injection of suitably formulated slugs of anionic polyacrylamide followed by dosing of multivalent metal ions (particularly chromium) can produce in situ polymer cross linking which results in a solid gel in the porous channels. Fluid flow is therefore diverted to narrower channels since the fluid cannot penetrate the gel. This phenomenon is known as profile modification.
Enhanced oil recovery also known as improved oil recovery employs techniques to extract more oil. High molecular weight polyacrylamides in dilute solution increase the viscosity of water above that of oil. Polymer flooding involves injection of polyacrylamide solution into the well to displace the oil which is of lower viscosity.
POLYELECTROLYTES AS ANTISCALANTS AND DISPERSANTS
Antiscalants either prevent scale formation entirely or permit the scale only to be deposited in such a way that it is easily removed by the fluid flowing along the pipe or heat transfer surface. Dispersants do not stop the formation of scale, but are able to keep the scale particles in suspension in the bulk fluid. Chemically antiscalants and dispersants are low molecular weight anionic polyelectrolytes, being polymers of acrylic acid and its salts, copolymers of acrylamide and acrylic acid. Molecular weights vary in the range of 1000 to 100000 with anti scalants being at the lower end of the range and dispersants, eg., polymers used in laundry detergent formulations at the upper end.
Proper selection of polymer composition molecular weight and molecular weight distribution makes these products excellent for uses in :
1) Boiler treatment
2) Cooling towers and air conditioning systems
3) Desalination of water (brackish or sea water) by either
a) High temperature evaporative processes, such as MultiStage Flash(MSF) Evaporation
b) Ambient temperature processes, such as Reverse Osmosis(RO).
4) Sugar production
5) Mineral processing (kaolinite, calcium carbonate)
6) Papermaking (dispersants for the above minerals used as paper fillers)
7) Oil Drilling mud dispersants ('mud thinners')
8) Oil production.
(DK SET is the brand name of polyacrylamides manufactured by DaiIchi Karkaria Ltd., using the Photopolymerization Technology of DaiIchi Kogyo Seiyaku Co., Japan.)
* Molyneux,P., Water Soluble Synthetic Polymers: Properties and Behaviour,CRC Press, Boca Raton, Florida (2 volumes),1984.
* The Lubrizol Corporation, Amps Monomer, Bulletin 888 36043R1 Ohio, USA, 1988.
* Hoover, M.F., Cationic quarternary polyelectrolytesA literature review J. Macromol, Sci.Chem.,A4(1970)1327.
* Halverson,F&Panzer, H.P.Flocculating agents. In Encyclopedia of Chemical Technology, Volume10,3rd Edition, ed.R.E.Kirk and D.
* DaiIchi Karkaria Ltd. Product brochure of DK SET flocculants.
* Bikales M.M.,Water Soluble Polymers, Plenum Press, NY 1973.