321.8.FlocculationThe merit of modern polymeric flocculants is their ability to produce larger,stronger flocs than those obtained by coagulation.Flocculants are polymers with highmolecular weight which are soluble in water. It is generally accepted that polymers usedas flocculants aggregate suspensions of fine particles by a bridging mechanismThe bridging is considered to be a consequence of the adsorption of the segments of theflocculant macromolecules onto the surfaces of more than one particle.Such bridginglinkstheparticles intolooseflocs (1.33)Figure 1.33. Schematic illustration of (a) bridging flocculation, (b) restabilization at highconcentrationsbyadsorbedpolymer1.34.Schematic illustration of the conditions needed for a good bridging flocculation
32 1.8. Flocculation Flocculation Flocculation Flocculation The merit of modern polymeric flocculants is their ability to produce larger, stronger flocs than those obtained by coagulation. Flocculants are polymers with high molecular weight which are soluble in water. It is generally accepted that polymers used as flocculants aggregate suspensions of fine particles by a bridging mechanism. The bridging is considered to be a consequence of the adsorption of the segments of the flocculant macromolecules onto the surfaces of more than one particle. Such bridging links the particles into loose flocs (1.33). Figure 1.33. Schematic illustration of (a) bridging flocculation, (b) restabilization at high concentrations by adsorbed polymer. 1.34. Schematic illustration of the conditions needed for a good bridging flocculation
33Fig.1.35.Steric stabilizationatahighflocculantdosage5.004.50-Distillwater4.00$3.50s3.00e1.501.000.500.00501001500200Concentration,mg/LFig.1.36. Effect of A-110 polyarylamide flocculant (M=3.9x10 Dalton; DA=18.1%) onsettlingratefordolomite-illitesuspensionindistilledwater(PengHuan,J.S.LaskowskiHongboZengand QingyeLu, Useofflocculants in highionic strengthprocess waters,COM,Montreal,October2013)
33 Fig. 1.35. Steric stabilization at a high flocculant dosage. Fig. 1.36. Effect of A-110 polyarylamide flocculant(M=3.9x10 6 Dalton; DA=18.1%) on settling rate for dolomite-illite suspension in distilled water (Peng Huan, J.S. Laskowski, Hongbo Zeng and Qingye Lu, Use of flocculants in high ionic strength process waters, COM, Montreal, October 2013)
34The polymers used in flocculation can be classified into coagulants,which arehighly charged, cationic polyelectrolytes with molecular weights in the 50,000 to106Daltons range, and flocculants with molecular weights up to 20x106 Daltons. It is knownthat flocculants are not very effective for treating stable suspensions and as pointed outby Kitchener(1972) the flocculation is much more efficient if the suspension is firstdestabilized by coagulation.This can be achieved by changing pH or by addition of someinorganic coagulants (e.g.lime or alum).Also low molecular weight cationic polymerscan be use to destabilize suspensions as most mineral particles in water carry negativeelectrical charge. In general, destabilization process is strongly dependent on processwaterchemistry,Adsorption of the polymer is generally necessary for flocculation to occur.It isimportant however, to realize that adsorption and flocculation are not separatesequential processes, but occur simultaneously (Hogg, 1999).There is generalagreement as to the basic mechanism involved in the process; the optimum flocculationoccurs at flocculant dosages corresponding to a particle coverage that is significantly lessthan complete. Incomplete surface coverage ensures that there is sufficient unoccupiedsurface available on each particle for the adsorption of segments of the flocculant chainsduring collision of the particles. The bridging takes place at flocculant dosagescorresponding to a particle surfacecoverage that is significantly less than complete,andthus at higher concentrations, the polymers stabilize suspensions by the mechanismreferredtoas steric stabilization.Hogg et al (1993) showed that the appropriate choice of flocculants is determinedprimarily by chemical factors (mineral composition, solution chemistry, etc.), but theperformance of the flocculant depends more on physical variables, such as agitationintensity and the rate of flocculant additionFlocculants.The vast majority of commercial flocculants are based on copolymers ofacrylic acid and polyacrylamide (also referred to as hydrolyzed polyacrylamide):
34 The polymers used in flocculation can be classified into coagulants, which are highly charged, cationic polyelectrolytes with molecular weights in the 50,000 to 106 Daltons range, and flocculants with molecular weights up to 20x106 Daltons. It is known that flocculants are not very effective for treating stable suspensions and as pointed out by Kitchener Kitchener Kitchener Kitchener (1972) the flocculation is much more efficient if the suspension is first destabilized by coagulation. This can be achieved by changing pH or by addition of some inorganic coagulants (e.g. lime or alum). Also low molecular weight cationic polymers can be use to destabilize suspensions as most mineral particles in water carry negative electrical charge. In general, destabilization process is strongly dependent on process water chemistry, Adsorption of the polymer is generally necessary for flocculation to occur. It is important however, to realize that adsorption adsorption adsorption adsorption and flocculation flocculation flocculation flocculation are not separate separate separate separate sequential sequential sequential sequential processes, processes, processes, processes, but occur simultaneously simultaneously simultaneously simultaneously (Hogg, 1999). There is general agreement as to the basic mechanism involved in the process; the optimum flocculation occurs at flocculant dosages corresponding to a particle coverage that is significantly less than complete. Incomplete surface coverage ensures that there is sufficient unoccupied surface available on each particle for the adsorption of segments of the flocculant chains during collision of the particles. The bridging takes place at flocculant dosages corresponding to a particle surface coverage that is significantly less than complete, and thus at higher concentrations, the polymers stabilize suspensions by the mechanism referred to as steric stabilization. Hogg et al (1993) showed that the appropriate choice of flocculants is determined primarily by chemical factors (mineral composition, solution chemistry, etc.), but the performance performance performance performance of the flocculant flocculant flocculant flocculant depends depends depends depends more on physical physical physical physical variables, variables, variables, variables, such as agitation agitation agitation agitation intensity intensity intensity intensity and the rate of flocculant flocculant flocculant flocculant addition addition addition addition. Flocculants. Flocculants. Flocculants. Flocculants. The vast majority of commercial flocculants are based on copolymers of acrylic acid and polyacrylamide (also referred to as hydrolyzed polyacrylamide):
35(-CH2-CH-)m(-CH2-CH-)n11CONHCOONa*As a result of hydrolysis even“nonionic"polyacrylamides contain some anionic groups.This is expressed as“thedegree of anionicity"(the degree of anionicity ofcompletelyhydrolyzed polyacrylamide is 100%, so it is a polyacrylic acid)Another important group of flocculants is polyethylene oxide,(-CH2CH2O-)n C(Rubio, 1981, Scheiner and Wilemon, 1987). Cationic polyelectrolytes such as co-polymers ofacrylamideandquaternaryammonium compoundsare alsoavailable(e.g.Poly-DADMAC). Naturally occurring materials such as polysaccharides (e.gcarboxymethyl cellulose, starch, guar gum, etc.) have also been used as flocculants.According to Kitchener (Kitchener, 1978), the first use of flocculants involved theapplicationof starch in combination with lime forthe clarification ofa coal mine'sefluent (the patent was filed in 1928).The effectiveness of polymers as flocculants depends on their molecular weight,the sign of their charge (e.g. anionic or cationic) and the relative charge density (forpolyacrylamides this is expressed by degree of anionicity). Depending on molecularweight, the same compounds can operate as dispersants (e.g.dextrin, low molecularweight) or flocculants (e.g. starch, high molecular weight). Low molecular weightcopolymers of polyacrylate type are manufactured as disperants (e.g., Dispexmanufactured by Allied Colloids (now CIBA), etc.).Xu and Cymerman's (1999) data confirmed that the best flocculants for theclay-containing wastes (Syncrude tailings) are moderately anionic high molecular weightpolyacrylamides (optimum around 20-30% anionicity).Hamza et al (1988) reported thatanionic polyacrylamides were the best for enhancing the settling rate of fine coal
35 (-CH2-CH-)m(-CH2-CH-)n ǀ ǀ CONH2 COO-Na+ As a result of hydrolysis even “nonionic” polyacrylamides contain some anionic groups. This is expressed as “the degree of anionicity” (the degree of anionicity of completely hydrolyzed polyacrylamide is 100%, so it is a polyacrylic acid). Another important group of flocculants is polyethylene oxide, (-CH2CH2O-)n C (Rubio, 1981; Scheiner Scheiner Scheiner Scheiner and Wilemon, Wilemon, Wilemon, Wilemon, 1987). Cationic polyelectrolytes such as copolymers of acrylamide and quaternary ammonium compounds are also available (e.g. Poly-DADMAC). Naturally occurring materials such as polysaccharides (e.g. carboxymethyl cellulose, starch, guar gum, etc.) have also been used as flocculants. According to Kitchener (Kitchener, Kitchener, Kitchener, Kitchener, 1978), the first use of flocculants involved the application of starch in combination with lime for the clarification of a coal mine’s effluent (the patent was filed in 1928). The effectiveness of polymers as flocculants depends on their molecular weight, the sign of their charge (e.g. anionic or cationic) and the relative charge density (for polyacrylamides this is expressed by degree of anionicity). Depending on molecular weight, the same compounds can operate as dispersants (e.g. dextrin, low molecular weight) or flocculants (e.g. starch, high molecular weight). Low molecular weight copolymers of polyacrylate type are manufactured as disperants (e.g., Dispex manufactured by Allied Colloids (now CIBA), etc.). Xu and Cymerman Cymerman Cymerman Cymerman’s (1999) data confirmed that the best flocculants for the clay-containing wastes (Syncrude tailings) are moderately anionic high molecular weight polyacrylamides (optimum around 20-30% anionicity). Hamza et al. (1988) reported that anionic polyacrylamides were the best for enhancing the settling rate of fine coal
36Polymer molecular weight. The molecular weight of flocculants is commonlycharacterized through viscosity measurements. This is based on the Flory-Huggins (Flory,1953);[n] = KMa(1.39)where [n] is the intrinsic viscosity of the polymer solution (has units of reciprocalconcentration)*, Mis the polymer molecular weight, and Kand aare constants[nl is determined by plotting the reduced viscosity sp/c down to c=0 (using Hugginsequation), sp = rel -1, rel = solution/ solventAfter Henderson and Wheatley (1986) Fig. 1.37 shows the effect ofpolyacrylamide intrinsic viscosity (that is indirectly molecular weight) on sedimentationrate of flocculated tailings for polyarylamides with varying anionicities.SNNONCESN10bPOLYMERINTRINSICVISCOSITY,LgFig.1.37.Effect of polyacrylamide intrinsic viscocity (molecular weight)onsedimentation rate of the flocculated tailings (after Henderson and Wheatley,1986)Because of the relationship between polymer intrinsic viscosity and its molecularweight (Eq. 1.39), what Figure 1.37 shows is the effect of flocculant molecular weight onflocculation
36 Polymer Polymer Polymer Polymer molecular molecular molecular molecular weight. The molecular weight of flocculants is commonly characterized through viscosity measurements. This is based on the Flory-Huggins (Flory, (Flory, (Flory, (Flory, 1953): a [η] = KM (1.39) where [η] is the intrinsic viscosity of the polymer solution (has units of reciprocal concentration)*, M is the polymer molecular weight, and K and a are constants. [η] is determined by plotting the reduced viscosity ηsp/c down to c = 0 (using Huggins equation); ηsp = ηrel – 1, ηrel = ηsolution solution solution solution/η solvent solvent solvent solvent After Henderson Henderson Henderson Henderson and Wheatley Wheatley Wheatley Wheatley (1986) Fig. 1.37 shows the effect of polyacrylamide intrinsic viscosity (that is indirectly molecular weight) on sedimentation rate of flocculated tailings for polyarylamides with varying anionicities. Fig. 1.37. Effect of polyacrylamide intrinsic viscocity (molecular weight) on sedimentation rate of the flocculated tailings (after Henderson Henderson Henderson Henderson and Wheatley, Wheatley, Wheatley, Wheatley, 1986). Because of the relationship between polymer intrinsic viscosity and its molecular weight (Eq. 1.39), what Figure 1.37 shows is the effect of flocculant molecular weight on flocculation