Water Soluble Polymers - SNF
WaterSolublePolymers
1IntroductionWater soluble polymers cover a wide range ofhighly varied families of products of natural orsynthetic origin, and have numerous uses.Among these families, synthetic polymers, andmore particularly coagulants and flocculants, areused mainly for facilitating the separation ofmaterials in suspension in aqueous media. Theyalso help to dewater sludge from various separation processes.The separation of solids in a liquid medium takesplace rapidly when the density of the particles ismarkedly different from that of the liquidmedium. Either the particles settle out or theyfloat on top of the liquid.Difficulties occur when the particle size allows itto remain in suspension in the liquid medium.In this case, the use of coagulants and flocculantsallows separation to be carried out.
2 Table of contents1IINTRODUCTION1IITABLE OF CONTENTS2IIIBASIC PRINCIPLES OF COLLOID SCIENCE6III-1Colloidal suspensions6III-1-1Hydrophobic colloids6III-1-2Hydrophilic colloids7III-2Measurement of the concentration of colloids7IVCOAGULATION AND FLOCCULATION8IV-1Charge neutralization8IV-2Adsorption9IV-3Suspension destabilization by coagulation10IV-4Suspension destabilization by flocculation10IV-4-1Initiation of flocs10IV-4-2Kinetics of floc development12IV-4-3Consistency of flocs12IV-5IV-6Other suspension destabilization systems13IV-5-1“Encapsulation” systems13IV-5-2Two-polymer-component coagulation systems13IV-5-3Microparticulate systems13Efficiency of flocculation and coagulationWater Soluble Polymers - Flocculation Coagulation14
3Table of contentsIV-7Parameters having an influence on the efficiency of flocculation and coagulation14IV-7-1Influence of the particles14IV-7-2Influence of the polymers15IV-7-3Influence of the polymer mixing in the solution16IV-7-4Influence of the pH and the temperature of the solution17IV-7-5Influence of the organic nature of the suspension17IV-8Advantages of synthetic coagulants17VWATER SOLUBLE POLYMERS18V-1CoagulantsV-28V-1-1Quaternary polyamines18V-1-2PolyDADMAC19V-1-3Dicyandiamide resins20Flocculants20V-2-1Nonionic flocculants20V-2-2Anionic flocculants21V-2-3Cationic flocculants21V-3Amphoteric copolymers22V-4Other polymers22V-5Products in solution23V-6Branched polymers23V-7Thickening agents23
4Table of contentsVIPHYSICAL FORMS OF lamides in bead form27VI-4Products in solution27VII CHEMICAL CHARACTERISTICS OF POLYMERS28VII-1Viscosity29VII-2Measurements of the molecular mass30VII-2-1Light scattering method31VII-2-2Intrinsic viscosity method31VII-3Stability32VII-3-1Chemical and biochemical stability32VII-3-2Mechanical stability32VII-4Ionicity33VII-5Residual polymer content33VII-6Toxicity33VIII LABORATORY TESTS AND INDUSTRIAL TRIALS34VIII-1 Principles34VIII-2 Main laboratory process-water and municipal-water treatment applications36VIII-3 Use of the polymers37VIII-3-1Polymers in powder form and in bead form37VIII-3-2Polymer emulsions37VIII-3-3Polymer solutions37Water Soluble Polymers-Flocculation Coagulation
5Table of contentsIXIX-1GENERAL APPLICATIONS38Separation processes using IX-1-3Belt filters41IX-2Potable water42IX-3Process water43IX-4Industrial effluent44IX-5Municipal sewage45IX-6Thickening and dewatering of sludge46XSPECIAL APPLICATIONS48X-1Mining industry48X-2The paper industry49X-3The petroleum industry50X-4The cosmetic industry51X-4-1Conditioners and film-forming agents for hair and skin products51X-4-2Thickening agents and emulsion stabilizers51X-5X-6The textile industry52X-5-1Synthetic thickeners52X-5-2Sizing agents52X-5-3Fixing agents52Agriculture-Soil conditioners53
Basic principles6 of colloid scienceIII-1 Colloidal suspensionsIn liquid media and more particularly aqueousmedia, organic or inorganic materials are presentin dissolved or solid form.These two forms are distinguished by the size ofthe particles.Dissolved compounds are:- Inorganic compounds, ions and/or organic compounds of low molecular weight with a particlesize of at most 10 -3 µm;- Soluble organic compounds of high molecularweight, such as proteins or polymers, with aslightly larger particle size of between 10 -3 and10 -2 µm.Solid compounds are:- Colloids with a particle size of between about10 -2 and 1 µm;- Materials in suspension with a particle size ofmore than 1 µm.Colloidal particles, invisible to the naked eye, undergohigh-speed Brownian motion - the speed is between0.004 m/s in the case of the largest particles and100 m/s in the case of the smallest.These particles alsohave a very high specific surface area (inversely proportional to the particle diameter). They are therefore moresensitive to surface phenomena than to gravitationalforces.Thus, the largest colloid particles will have a natural settling time over a depth of 1 m of water of abouttwo years.Colloidal particles can therefore form suspensions whichare very stable over time.The stability of a colloid suspension depends on the equilibrium between two types of opposing forces involved.These are Van der Waals’-type force - the universal attractive force between atoms and molecules, - and an electrostatic interaction force.These two forces act differently depending on the intrinsicnature of the colloids, which are either hydrophilic, i.e.water molecules are adsorbed onto their surface, or,conversely, hydrophobic.III-1-1 Hydrophobic colloidsColloids of a hydrophobic nature (for example, clays) formstable suspensions because their surface charge is thesame and they repel each other.These charges may comefrom selective adsorption of an ion onto the surface orfrom the surface itself (crystal lattice defects).The surface of the electrically charged particle is at a different potential from the solvent medium. Electrostaticequilibrium between the surface of the particle and thesolvent medium is achieved by means of two transitionallayers.Water Soluble Polymers-Flocculation Coagulation
Basic principlesof colloid scienceThe first layer is fixed and remains attached to the surface of the particle. The second layer, separated from thefirst by a shear plane, is more diffuse.Figure 1 – Equilibrium layers of a colloidal particleThe charges have an influence on the solubility of hydrophilic colloids. The importance of the role of the chargesdepends on the degree of ionization of the particle’s functional groups. The degree of ionization itself depends onthe pH of the medium. In general, the solubility is a minimum around the isoelectric point, which corresponds to apH of between 4.0 and 6.5.Hydrophilic colloidal suspensions therefore are akin to anactual solution of very large molecules or of aggregates ofsmall molecules, with a very high affinity for the solvent.In some cases, hydrophilic colloids can be adsorbed ontothe surface of hydrophobic colloids, such as clays forexample, and thus impart a hydrophilic nature to thehydrophobic particle. Such colloidal suspensions are thenvery difficult to destabilize.III-2 Measurement of the colloidconcentrationThe potential difference between the shear plane and thesolution is called the zeta potential. This potential difference is about 10 to 200 mV.When it is reduced to zeroor close to zero (the isoelectric point), the particles tend toagglomerate under the influence of the Van der Waals’forces and the colloidal suspension becomes destabilized.III -1- 2 Hydrophilic colloidsColloids of a hydrophilic nature (generally organic substances) owe their stability to the layers of water molecules bonded to the surface of the particle.The bonds areof a chemical nature and the electrostatic charges playonly a secondary role.The layers of water molecules prevent any agglomeration between particles. In this case,there is no real interface between the particle and the solvent medium.There are no direct measurements of the colloid concentration.The colloidal particles can be directly observed with amicroscope.They are visible indirectly by light reflection orthe Tyndall effect, which is why colloidal solutions are turbid.The turbidity may be quantified by nephelometry, a relative measurement expressed in NTU (NephelometricTurbidity Unit) with respect to a calibration solution offormazine in water which has zero turbidity (FrenchStandard NF T 90.053).7
8 Coagulation and flocculationIV-1 Charge neutralizationThe conventional methods of solid-liquid separation, such as filtration, sedimentation, centrifugation and flotation, cannot be used directly onstabilized suspensions. The particles are too finea n d re m a i n s e p a ra t e d f ro m e a c h o t h e r.Coagulation and flocculation destabilize theseIn aqueous media, hydrophobic colloidal particles aregenerally negatively charged. The increase in the cationcontent of the solution reduces the zeta potential andtherefore the thickness of the double layer whichsurrounds the colloidal particle.When the electrical protection of the particles has beenremoved or sufficiently reduced, the moving particles cancollide with each other due to the momentum ofBrownian motion, the movement of the fluid in which theyare contained and the relative movement of the particlesby sedimentation.suspensions and allow solid-liquid separation.The Van der Waals’ forces and the surface adsorptionphenomena then become dominant again. Since theparticle can bond together, the suspension is destabilized.Two mechanisms are mainly involved, namelycharge neutralization and adsorption.This charge neutralization mechanism is reversible.Thereis a limiting cation concentration which allows destabilization to occur. Above this critical concentration, if cationscontinue to be added, a new ionic imbalance between theparticle and the solution may be created, leading to theformation of a new double layer and to the restabilizationof the suspension.The charge neutralization efficiency of an electrolyteincreases with its valency (the Schulze and Hardy rule).This explains the predominant use of bivalent or trivalentelectrolytes for destabilizing colloidal suspensions.Water Soluble Polymers - Flocculation Coagulation
Coagulation and flocculationIV-2 AdsorptionAdsorption is a surface mechanism which allows twocontacting particles to be bonded to each other by Vander Waals’ forces or hydrogen bonds.Measuring the amount of polymer adsorbed by a colloidalsuspension as a function of time allows a characteristiccurve, called a Langmuir isotherm, to be plotted:Figure 2 – Adsorption isothermAdsorption is a complex mechanism involving alarge number of parameters relating to:The coagulant:structure of the molecular chainionic charge densitymolecular weightThe colloids:number of available sitessurface chargesspecific surface areaparticle concentrationThe aqueous medium:pHconductivitypresence of other substancesturbulenceshear rate temperaturemixing of the productsThe characteristic curve is called an isotherm since theadsorption is strongly dependent on the temperature andany measurement must be carried out at constanttemperature.Adsorption is an irreversible mechanism which takes placein two steps. Firstly, after a few seconds, most of the reaction has taken place. After a relatively long second step, abalance is achieved, which is limited by the number ofsites available on the surface of the particle.When all theavailable sites are occupied, no further molecule can beattached to the surface of the particle.The complexity of this system explains the difficulties stillencountered at the present time in understanding andmodelling this phenomenon. The experimental approachremains indispensable.9
10Coagulation and flocculationIV-3 Suspension destabilizationby coagulationIV-4 Suspension destabilizationby flocculationCoagulation is the destabilization of a colloidal suspensionusing products with a high ionic charge density.Two typesof mechanisms may occur.Flocculation is the destabilization of a hydrophobic colloidal suspension by bonding between colloidal particlesusing long polymer chains.In a first type, the cations are added to the colloidal suspension in an amount just equal to neutralize the negative charges. Coagulation occurs directly by charge neutralization.This type of coagulation is extensively used in thetreatment of potable water with iron or aluminiumcations.This requires the use of polymers with a high molecularweight (greater than 1 million) and takes place in twomain steps: floc initiation and floc growth.A second type of coagulation is achieved using polymersof Low Molecular Weight (LMW 20,000 to 1 million)with a high cationic charge.When this type of polymer isin contact with anionic particles, its chains may be entirely adsorbed onto part of the surface of the colloidal particles thus forming regions of a cationic nature.Figure 3 – Coagulation using a LMW polymerIV-4-1 Initiation of flocsTo initiate flocs, the polymers essentially act via a mechanism of irreversible adsorption of the colloids along theirmolecular chain.The effect of this mechanism depends onthe concentration of polymer, the concentration of particles and the particle size.If an excess of polymer covers the surface of the particle,the particle will once again be isolated and the suspensionwill restabilize.Figure 4 – Isolation of a particle by a polymerThis adsorption of oppositely charged ions reduces thesurface potential and the protection of the particles.Having become true dipoles, these particles are attractedto each other and collide with each other. The Van derWaals’ forces then come into play, binding the two particles and destabilizing the suspension.Coagulation by this type of mechanism generally hashigher degrees of aggregation than those of the simpleelectrical neutralization mechanism.Water Soluble Polymers - Flocculation Coagulation
Coagulation and flocculationIn the case of a suspension containing particles whichhave a wide particle-size distribution or several particlesize distributions, the suspension may be restabilized fordifferent polymer concentrations corresponding to the different particle-size distributions.Thus, good results may beachieved using a given polymer for one particle size, butnot for another. As a general rule, when this phenomenonoccurs, relatively good flocculation is observed but thesolution remains very cloudy.If the polymer concentration is such that adsorption sitesremain free on the particles, the same polymer chain maybe able to be adsorbed onto two different particles:Figure 5 – Bridging between two particlesThe length of the completely uncoiled polymer chains isabout 1 µm, possibly as much as a few tens of µm in thecase of the longest chains.These lengths should be compared with the dimensions of the particles, which areabout 1 µm.Inter-particle bridging can occur with nonionic, cationic oranionic polymers. In these mechanisms, the charge neutralization phenomena have a secondary influence. Thisexplains why flocculation reactions can occur with polymers carrying charges of the same sign as the colloidalsuspension.The charges may have an influence which promotesbetter uncoiling of the polymer chains due to the effect ofelectrostatic repulsion. The shape of the polymer chainmay also be a key factor in this process.The ionicity of the solution may also influence chain uncoiling by limiting or eliminating the repulsion effect. Thisphenomenon is known as the counterion effect or salteffect.In this mechanism, the polymer allows part of its molecular chain to uncoil in the solution beyond the particle’sdouble layer.The free end of the molecular chain is in turnadsorbed onto the surface of a second particle, whichthus creates bridging between the two particles.In general, it is found that the optimum polymer concentration must be such that more than half the adsorptionsites remain available on the particles.For a given polymer, bridging mainly depends on twoparameters: the number of sites available for adsorptionon the surface of the particle and the rate of collision ofthe particles.11
12Coagulation and flocculationIV-4-2 Kinetics of floc developmentDepending on the mechanisms described above, development of the flocs takes place in several sequential steps:IV-4 -3 Consistency of flocsFlocs can be of two types consistencies, namely “soft”,which are reversible, and “hard”, which are irreversible.- Dispersion of the polymer in the mediumWhen a floc provides good retention of fine particles forlow shear rates in a quiescent zone, as the shear rateincreases the retention of the fine particles decreases. Onreturning to the initial shear conditions, if the floc resumesits initial structure, it is called a soft floc. Soft flocs aregenerally obtained with polymers of low molecular weight.- Diffusion of the polymer towards the solid-liquid interface- Adsorption of the polymer onto the surface of a particleCollision of particles carrying an adsorbed flocculant withanother particle- Adsorption of the flocculant onto a second particle inorder to form a bridge and a microfloc- Growth of the microflocs by successive collisions andadsorptions- Breaking of the flocs formed, by shear.Each step takes place according to its own kinetics andthe final result, in terms of the floc, depends on the relative rates of the various steps. Thus, for example, if theadsorption phase is much more rapid than the growthphase, there will be many small flocs whereas, if thegrowth rate is higher, the flocs will be larger and fewer innumber.In general, adsorption reactions are extremely rapid. Thelimiting step in the development of flocs is mainly due, tothe frequency of collision between particles and collusionswith flocs already formed.A hard floc, is stronger, maintains good retention of fineparticles over wider turbulence and increased shear.However, when it is subjected to high turbulence for anexcessive length of time, the retention of fine particlesdecreases.The floc then becomes soft.The assumed reasons for this phenomenon stem from theconfiguration of the polymer chains on the surface of theparticle.A hard floc is composed of particles connected bybridging via molecular chains which uncoil in the spacearound the particle:Figure 6 – Hard floc by double flocculation1. Initial adsorption2. Initial flocculation3. Change of conformation4. ReflocculationAfter shear, the molecular chains, due to the influence ofthe ionic forces, have a tendency to cover more of the surface of the particle, giving, by bridging, a floc of soft consistency.Water Soluble Polymers - Flocculation Coagulation
Coagulation and flocculationIV-5 Other suspensiondestabilization systemsThree other systems are also encountered - stillemploying the two basic mechanisms, namelycharge neutralization and adsorption.Figure 7 – Two-component coagulationIV -5-1 “Encapsulation” systemsThe bridging system described above is based on the primary interaction between a polymer chain and the particle in order to initiate flocculation. As regardsencapsulation mechanisms, these occur when two ormore polymer chains react first of all with each other dueto the influence of electrostatic interactions or of hydrogenbonds. This reaction produces a network of crosslinkedpolymer chains which mechanically trap the particle. Inthis mechanism, which is not very well understood, theelectrostatic interactions between the particles and thepolymer do not play an important role.IV-5-2 Two-polymer-componentcoagulation systemsThe sequential addition of two polymers of opposite charge allows very good coagulation to be achieved. In practice, a cationic polymer of low or moderate molecularweight is firstly introduced into the solution, followed by ananionic polymer of high molecular weight.The low-molecular-weight cationic polymer is adsorbedonto the particle to form cationic regions onto which thehigh-molecular-weight anionic polymer may be fixed. Theanionic molecular chain tends to uncoil in the solution dueto the effect of charges of the same type carr
Water soluble polymers cover a wide range of highly varied families of products of natural or synthetic origin,and have numerous uses. Among these families, synthetic polymers, and more particularly coagulants and flocculants,are . 12. water. IV-7- 2 Influence of the polymers 15.
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