《高等选矿学》课程教学资源（文献资料）Flocculation

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 co￾polymers 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

37103(,6 jw) / [4]a=20102a=0.5a=0.0101105104106107M. / (gmor")Figure1.38.Intrinsicviscosity[|asafunctionofthemolarmassMwfordifferentpolymer-solvent systems (Kulicke and Clasen, 2004). In addition to the experimental data,theoretical possible slopesofthe=Mrelationshiparealso shownIt must be, however, born in mind that the intrinsic viscosity of a polymerincreases with rising solvent quality.This is shown in Fig.1.38 for various polymers invarious solvents. In a good solvent polymer macromolecules are in extended form, butcoil when the solvent quality decreases. This may happen when ionic strength of the

37 Figure 1.38. Intrinsic viscosity [ ] as a function of the molar mass Mw for different polymer-solvent systems (Kulicke and Clasen, 2004). In addition to the experimental data, theoretical possible slopes of the [ ]=M relationship are also shown. It must be, however, born in mind that the intrinsic viscosity of a polymer increases with rising solvent quality. This is shown in Fig. 1.38 for various polymers in various solvents. In a good solvent polymer macromolecules are in extended form, but coil when the solvent quality decreases. This may happen when ionic strength of the

38system is increased, or pH is changed. The exponent a in Eq. 1.39 is a measure of thesolvent quality, and as Figure 17 shows it is large (larger than 0.5) for a good solvent, andsmaller than 0.5 for poor solvents.Since conformation of polymermacromolecules in solvent depends on thesolvent quality,also polymer adsorption onto solid particles depends on it.Adsorption isgenerally higher from a poor solvent than a good solvent (Koral et al., 1958).Klein and Conrad (1980) derived the following empirical equation that can be used todetermine polyacrylamide molecular weight. This reationship holds for polyacrylamidesamples ranging in molecular weight from 5x105 Daltons to 6x10° Daltonswhen measurements are conducted in0.5MNaCl solution at 25C[n] = 7.19 .10~3 . M, 0.7(1.40)Testing flocculation. Solubility and rate of dissolution of high molecular weightflocculants in water are generally low, and preparation ofthe polymer solution is a veryimportant first stage (Brown, 1986). The following step that is mixing of the polymersolution with the suspension is critical (Owen et al, 2009)Since flocculants are either used to enhance solids settling rates to maximizethickener capacity,to enhance dewatering byfiltration, or to improve water clarification,various tests are utilized. They include measurements of solids settling rate, sedimentdensity,filtration characteristics, or supernatant turbiditySeveral techniques have been proposed to determine the settling velocity inlaboratory experiments, the“jar tests" being the most common (Coe and Clevenger1916, Richardson and Zaki 1954, Michael and Bolgers 1962). Jar test involveshomogenization of the suspensions in settling cylinders, introduction of the flocculantand mixing by moving a plunger up and down in the cylinders, or by inverting thecylinders several times. This procedure is claimed not to be satisfactory because of the

38 system is increased, or pH is changed. The exponent a in Eq. 1.39 is a measure of the solvent quality, and as Figure 17 shows it is large (larger than 0.5) for a good solvent, and smaller than 0.5 for poor solvents. Since conformation of polymer macromolecules in solvent depends on the solvent quality, also polymer adsorption onto solid particles depends on it. Adsorption is generally higher from a poor solvent than a good solvent (Koral et al., 1958). Klein and Conrad (1980) derived the following empirical equation that can be used to determine polyacrylamide molecular weight. This reationship holds for polyacrylamide samples ranging in molecular weight from 5x105 Daltons to 6x106 Daltons when measurements are conducted in 0.5 M NaCl solution at 25 oC. 3 0.77 [ ] 7.19 10 M w = ⋅ ⋅ − η (1.40) Testing Testing Testing Testing flocculation flocculation flocculation flocculation. Solubility and rate of dissolution of high molecular weight flocculants in water are generally low, and preparation of the polymer solution is a very important first stage (Brown, (Brown, (Brown, (Brown, 1986). The following step that is mixing of the polymer solution with the suspension is critical (Owen et al., 2009). Since flocculants are either used to enhance solids settling rates to maximize thickener capacity, to enhance dewatering by filtration, or to improve water clarification, various tests are utilized. They include measurements of solids settling rate, sediment density, filtration characteristics, or supernatant turbidity. Several techniques have been proposed to determine the settling velocity in laboratory experiments, the “jar tests” being the most common (Coe and Clevenger Clevenger Clevenger Clevenger 1916, Richardson Richardson Richardson Richardson and Zaki 1954, Michael Michael Michael Michael and Bolgers Bolgers Bolgers Bolgers 1962). Jar test involves homogenization of the suspensions in settling cylinders, introduction of the flocculant and mixing by moving a plunger up and down in the cylinders, or by inverting the cylinders several times. This procedure is claimed not to be satisfactory because of the

39local over-dosing that can occur when the relatively concentrated flocculant solutionmeets the slurry (Kitchener 1978), but more important is that the agitation in this methoddoesnotproducetheoptimumflocculation.Farrow andSwift(1996)demonstratedthatthe jar test has several problems. It is important to realize that adsorption and flocculationare not separate sequential processes, but occur simultaneously (Hogg, 1999).Thecommonlyused improved experimental procedure includesaddition of theflocculanttoavigorously agitated suspension which is immediately stopped after addition of the reagent(Keys and Hogg,1979).Different mixing/polymer addition conditions may result in verydifferent floc sizes and settling rates. Owen et al (2009)showed that mixing of the slurrywith a dilute flocculant solution within the feedwell determines the performance ofcommercialthickeners.Itwas alsoshownthatunder certain conditions intenseagitatiorfor short times may even change the nature of flocculation, from total flocculation to aselective flocculation of only some mineral constituents (Ding and Laskowski, 2007)VARIABLEPeristalticSPEEDMOTORMotoLevelsensor200mmFeed slurryFlocculantDiversioCwhen[samplingComputerSettlingcolumnTowasteFigure 1.39.Schematic illustration of the set-up used to test flocculation(Concha etal,2012)

39 local over-dosing that can occur when the relatively concentrated flocculant solution meets the slurry (Kitchener Kitchener Kitchener Kitchener 1978); but more important is that the agitation in this method does not produce the optimum flocculation. Farrow and Swift (1996) demonstrated that the jar test has several problems. It is important to realize that adsorption and flocculation are not separate sequential processes, but occur simultaneously (Hogg, 1999). The commonly used improved experimental procedure includes addition of the flocculant to a vigorously agitated suspension which is immediately stopped after addition of the reagent (Keys and Hogg, 1979). Different mixing/polymer addition conditions may result in very different floc sizes and settling rates. Owen et al (2009) showed that mixing of the slurry with a dilute flocculant solution within the feedwell determines the performance of commercial thickeners. It was also shown that under certain conditions intense agitation for short times may even change the nature of flocculation, from total flocculation to a selective flocculation of only some mineral constituents (Ding and Laskowski, Laskowski, Laskowski, Laskowski, 2007). Figure 1.39. Schematic illustration of the set-up used to test flocculation (Concha (Concha (Concha (Concha et al., 2012)

40The use of the shear vessel, as described by Farrow and Swift (1996) and Rulyov 1999,2004) in the flocculation tests was recently tested by Rulov et al (2011) and Concha etal (2012). Their set-up is shown in Figure 18. The use of a shear vessel (similar torotational Couetteviscometers)in assessing flocculation efficiencyhas theadvantage ofquantifying the mixing intensity through the shear rate. Using the shear vessel, Rulyov(1999) and Rulyov et al (2004) have demonstrated that the mixing time in flocculationcan be reduced down from minutes to 5-6 seconds by the appropriate hydrodynamictreatment of the suspension at a given shear rate. In the set-up shown in Fig.1.39 theCouette type vessel is rotating in the external vessel, the gap being 1.5 mm. The reactoris fed continuously with the suspension by a measuring peristaltic pump. Before entering theCouette reactor the pulp receives continuously a diluted flocculant solution, at a flow-rate to givea pre-determined dosage. After 6 seconds treatment at a pre-determined shear rate in the Couettereactor,theflocculated suspension is dischargedfromthe ultra-flocculator through a3[mm]inner diameter transparent tube equipped with an opto-electronic sensor which registers thefluctuation of intensity of the light beam passing normally through the tube (in accordance withtechniques proposed by Gregory and Nelson (1984). The electronic signal is processed anddisplayed in a three digital format, thus showing, in relative units, the values of flocculationefficiency (or mean flocs size)and the mean shear ratey.In the tests designed to measure settlingrate of the treated suspension, the suspension from the outlet of the tester is continuously fed to a14 mm diameter settling cylinder and, as soon as the suspension fils the cylinder, the cylinder isone time inverted and the in itial settling rate is measured.The flocculation of flotation tailings from one of the major Chilean copper minesand Orifloc-2010 polycarylamide flocculant in a Couette type reactor was recentlyreinvestigated by Concha et al (2012). By varying the shear rate fom 100 to 2000 [s-l],the solid concentration from 1 to 15 [% by volume], and the flocculant dosage from 0 to20 [g/ton] it was shown that an important interaction exists between these variables. Atthe optimal flocculantdosage,the optimal suspensionconcentration and theoptimalflocculation time,an increaseby50%in the solid fluxdensityfunction is possible whenthe shear rate of =100[s'] is changed to the optimum value of around ~ 400[s']. It

40 The use of the shear vessel, as described by Farrow and Swift (1996) and Rulyov 1999, 2004) in the flocculation tests was recently tested by Rulov et al (2011) and Concha et al (2012). Their set-up is shown in Figure 18. The use of a shear vessel (similar to rotational Couette viscometers) in assessing flocculation efficiency has the advantage of quantifying the mixing intensity through the shear rate. Using the shear vessel, Rulyov (1999) and Rulyov et al (2004) have demonstrated that the mixing time in flocculation can be reduced down from minutes to 5-6 seconds by the appropriate hydrodynamic treatment of the suspension at a given shear rate. In the set-up shown in Fig. 1.39 the Couette type vessel is rotating in the external vessel, the gap being 1.5 mm. The reactor is fed continuously with the suspension by a measuring peristaltic pump. Before entering the Couette reactor the pulp receives continuously a diluted flocculant solution, at a flow-rate to give a pre-determined dosage. After 6 seconds treatment at a pre-determined shear rate in the Couette reactor, the flocculated suspension is discharged from the ultra-flocculator through a 3 [mm] inner diameter transparent tube equipped with an opto-electronic sensor which registers the fluctuation of intensity of the lig ht beam passing normally through the tube (in accordance with techniques proposed by Gregory Gregory Gregory Gregory and Nelson (1984). The electronic signal is processed and displayed in a three digital format, thus showing, in relative units, the values of flocculation efficiency (or mean flocs size) and the mean shear rate γ̇. In the tests designed to measure settling rate of the treated suspension, the suspension from the outlet of the tester is continuously fed to a 14 mm diameter settling cylinder and, as soon as the suspension fills the cylinder, the cylinder is one time inverted and the initial settling rate is measured. The flocculation of flotation tailings from one of the major Chilean copper mines and Orifloc-2010 polycarylamide flocculant in a Couette type reactor was recently reinvestigated by Concha et al (2012). By varying the shear rate from 100 to 2000 [s -1 ], the solid concentration from 1 to 15 [% by volume], and the flocculant dosage from 0 to 20 [g/ton] it was shown that an important interaction exists between these variables. At the optimal flocculant dosage, the optimal suspension concentration and the optimal flocculation time, an increase by 50% in the solid flux density function is possible when the shear rate of 1 γ 100[ ] s − ̇ = is changed to the optimum value of around 1 γ 400[ ] s − ̇ ≈ . It

41is worth pointing out that the ultraflocculator type devices have already been installed atsomecoal preparationplants(Rulyov,2004;Rulyovetal,2009)Testing the use of flocculants in filtrationIn present daypractice, disposal ofthe fine wastefraction (for instance coal flotationtailings)is usually accomplished by:(1) addition of flocculant to the slurry and thickening of the resulting flocs in athickener with the underflow from the thickener pumped to an impoundment area,(2) treatment of the thickener underflow with floccuant and dewatering on amechanical device such as vacuum filter or filter press.The former method requires the availability of enough land environmentally suitablefor construction of impoundments, and in the latter the dewatered material can bemixed with coarsewastefraction and discardedby stacking.The latter methodrequires dewatering by filtrationC..CakeformedCakeformingInitial bridging startscakeformationandfiltrationcommencesEDAirbreakthroughDesaturationbyailCakecompactedFigure 1.40. Schemaic representation of stages in dewatering by filtration (Lokhart andVeal 1996)

41 is worth pointing out that the ultraflocculator type devices have already been installed at some coal preparation plants (Rulyov, Rulyov, Rulyov, Rulyov, 2004; Rulyov et al., 2009). Testing Testing Testing Testing the use of flocculants flocculants flocculants flocculants in filtration. filtration. filtration. filtration. In present day practice, disposal of the fine waste fraction (for instance coal flotation tailings) is usually accomplished by: (1) addition of flocculant to the slurry and thickening of the resulting flocs in a thickener with the underflow from the thickener pumped to an impoundment area; (2) treatment of the thickener underflow with floccuant and dewatering on a mechanical device such as vacuum filter or filter press. The former method requires the availability of enough land environmentally suitable for construction of impoundments, and in the latter the dewatered material can be mixed with coarse waste fraction and discarded by stacking. The latter method requires dewatering by filtration. Figure 1.40. Schemaic representation of stages in dewatering by filtration (Lokhart (Lokhart (Lokhart (Lokhart and Veal, 1996)