- Email: [email protected]
The optimum flocculant concentration for effective flocculation of china clay in aqueous suspension
Water Research Pergamon Press 1972. Vol. 6, pp. 703-710. Printed in Great Britain
THE
OPTIMUM
EFFECTIVE
FLOCCULANT
CONCENTRATION
FOR
FLOCCULATION OF CHINA CLAY IN AQUEOUS SUSPENSION D. DOLLIMOREand T. A. HORRIDGE
Department of Chemistry and Applied Chemistry, University of Salford, Salford, Lancs.
(Received 2 September 1971) Abstract--The sedimentation and filtration characteristics of aqueous china clay suspensions flocculated by cationic, anionic and non-ionicpolyacrylamides are investigated and discussed. The experimental techniques are shown to be efficient and to give reasonable reproducibility over the polyacrylamide range for effective flocculation. The optimum polyacrylamide concentration for effective flocculation is discussed and it is concluded that the sedimentation rates of the settling floes and the filtration rates through the settled floes, rather than the clarity of the supernatant, determine this optimum concentration.
INTRODUCTION
Tim FLOCCULATIONof mineral suspensions has been discussed by many authors. Lx MEg and SMELLm(1958) have developed a theory of flocculailon which predicts an optimum flocculant concentration by analysing filtration data. Krrcimh'ER and SLXTmt (19663, however, have criticized this theory and concluded that the measurement of the turbidity of the supernatant liquid after standard methods of agitation and sedimentation is a more efficient method of determining the optimum flocculant concentration. The optimum flocculant concentration may well be determined by the process required, i.e. water clarification or the rapid removal of a mineral from suspension. In this study the flocculailon behaviour of china clay-polyacrylamide suspensions was investigated by measuring the sedimentation rate, final settled volume of the floes, turbidity of the supernatant and the filtration rate through the settled flocs. EXPERIMENTAL
All the experiments were carried out in a laboratory maintained at a temperature of 293-2°K q- 0.5°K. Materials
The china clays were supplied by English Clays, Lovering, Pochin & Co. Ltd.; the chemical analysis and particle size distribution are given in T/taLE 1. The anionic and cationic polymers based on polyacrylamide (A and C series respectively) were supplied by B.T.I. Chemicals Ltd., Bradford. From conductometric titrailons the order of anionic character was A 150 > A 130 > A 100 > C 100 > C 110. Molecular weights are believed to be around 3,000,000. The non-ionic polyacrylamide, S.20, was supplied by Cyanamid Inc., and has a molecular weight of 3,000,000. Sedimentation
The sedimentations were carded out in 250 ml stoppered measuring cyfinders of 3-2 cm i.d. KWXTRAet al. (1970), has reported that the optimum ratio of suspension 703
704
D. DOLLIMOREand T. A. HORPODGE TABLE l. CHEMICAL ANALYSISAND PARTICLE SIZE DISTRIBUTION OF CLAYS SPS AND RT Chemical analysis SiOz AI,O3 Fe203 TiOz CaO MgO K20 Na20 Loss on ignition Particle size distribution Finer than 2 #m Above 10 #m 300 mesh residue
SPS percentage
RT percentage
46'02 38'07 0"56 0.09 0"20 0"20 1"01 0"07 13"14
46"07 38"02 -14)0 1.00 1.00 -1-00 12"59
Percentage
Percentage
80.0 0.2 0.01
36.0 30.0
height to diameter (H/D) for the sedimentation of concentrated suspensions is in the range 5-9. In the measuring cylinders a suspension volume of 150 ml had an HID ratio of 5.9 and this was the suspension volume chosen. The weight of china clay in the suspensions was kept constant at 15 g of clay per 150 ml suspension. Floc radii are very dependant on the methods of agitation of the suspension and addition of the flocculant. In order to establish a simple reproducible experimental technique a detailed systematic study on the system was carded out. The methods of agitation investigated were manual agitation and mechanical agitation by means of a magnetic stirrer, a glass stirrer and a table top oscillator. The methods of addition of the flocculant investigated were, (a)by pipetting the required amount from a stock solution into a cylinder containing a clay suspension and then making the total suspension volume up to 150 ml and (b) by adding 50 ml of the flocculant of the required concentration to a 100-ml clay suspension and then immediately agitating the resulting suspension. It was concluded that manual agitation was an efficient method of agitation. For the addition of the flocculant, the results for method (a) showed wide variations whereas the results for method (b) were reproducible. The following standard procedure was then devised. A 100-ml suspension of the clay was prepared in a stoppered measuring cylinder by weighing 15 g of the clay into the cylinder and adding deionized water to the solid. The suspension was dispersed by inverting the stoppered cylinder several times and then left to equilibrate overnight. The suspension settled out overnight and was redispersed the following morning. A stock solution of the polyacrylamide was prepared and the required amount of this solution pipetted into a 50-ml graduated flask which was made up to the mark with deionized water. The clay suspension was then dispersed by inverting the stoppered cylinder several times and the polyacrylamide solution immediately added to the suspension in the cylinder. The resulting 150 ml suspension was then agitated by
Optimum Flocculant Concentration for Effective Flocculation
705
stoppering the cylinder and inverting it once per second for 60 s. The rate of fall of the suspension was then determined. The suspension was then left to settle and the final settled volume recorded when constant.
Turbidity of the supernatant The turbidity of the supernatant was determined by a method similar to that of KITCt-mNER and SEATER0966). Three minutes after the sedimentation had started a 3-ml aliquot of the supernatant liquid was pipetted off at the 100-ml mark on the cylinder and transferred to a glass cell. The percentage light transmission of the aliquot was then measured on an E.E.L. photo-extinction sedimentometer using distilled water as a blank. If the suspension had not fallen to below the 100-ml mark after 3 min the suspension was not disturbed and the percentage light transmission recorded as zero.
Filtration The filtration rates were recorded on the apparatus described by CHRISTIANet al. (1970).
Reproducibility Typical sedimentation and filtration curves are shown in FIG. 1. For the flocculated suspensions the initial portions of the curves were linear and the rates were calculated from these initial portions of the curves. The reproducibility of the techniques w a s within 5 per cent for the sedimentation and filtration rates, within 3 per cent for the turbidity results and to within 4-0"5 ml for the settled volumes. For the non-flocculated suspensions the initial portions of the curves were non-linear, as the sedimentation and filtration rates were very slow. Accurate sedimentation and filtration rates 150
.~
125 Sedirnentotion
.~.
curve
IO0
"o 75
8
5O -
Filtration
curve
o
_= 25 o
i:~
0
50O
IO00
Time,
t'JO0
2OOO
s
FIo.~I. Sedimentation and filtration curves for SPS--A.100, 65 ppm suspensions.
706
D. DOLLIMOREand T. A. HORRIDGE 70 G; E
60
•j-~ .~ I~
50
o 0
~
02
o') T o
m
o
.
~
~
02
0
20
40
Polyacrylamide
60
I00
80
concentration,
ppm
Flo. 2. Flocculation data for clay SPS-polyacrylamide A.150 suspensions.
65 --
q Eu~
5C
o
0
5C
'00
Polyacryiomide
1~o.
3. F l o c c u l a t i o n
data
15o
2oo
concentration,
2~
~.,o
ppm
for clay SPS--polyacrylamidc
C.110
suspensions.
Optimum FlocculantConcentrationfor EffectiveFlocculation
707
could not be calculated but the final settled volumes of the suspensions were recorded and found to be reproducible to within -4-0.5 ml. RESULTS The flocculation behaviour of clays SPS and RT-polyacrylamide suspensions was investigated and the results plotted as flocculation parameters versus polyacrylamide concentration. The results for clay SPS-polyacrylamide A.150, SPS-CI10 and SPS-S20 suspensions are shown in FIGs. 2 and 4 respectively. A peak in the settled volume curve coincided with a peak in the percentage light transmission curve and a trough in the settled volume curve coincided with peaks in the filtration and sedimentation rate curves. Visual examination showed that large 7O
o~
g,
0
60
;!0 •~
"-~
50
C
C' 120
450
200
250
Polyocrylomide concentration, ppm
FIG. 4. Flocculation data for clay SPS-polyacry]amJde S.20 suspemiom. flocs began to form just prior to the peaks in the settled volume curves. Shortly after the settled volume troughs the suspensions became "sticky" and the results were no longer reproducible. The effective flocculant concentration range for each claypolyacrylamide system was taken to be from the concentration at which large flocs began to form until the concentration at which the results were no longer reproducible. The results at the peaks in the curves are given in TABLES2 and 3 for clays SPS and RT respectively. The settled volumes of clays SPS and RT in deionized water were 62 cm 3 and 51 cm 3 respectively. DISCUSSION The theory of LA MEg and SM~LLm(1958), assumes that the volume of the filter bed is independent of flocculant doseage and that the floe structure is not affected by
708
D. DOLL1MOREand T. A. HORRID~IE TABLE 2. FLOCCULATIONOF CLAYSPS
Settled volume peak Settled volume trough Effective polymer Polymer Light Polymer Sedimenconeentra-concentra- Settled transmis- concentra- Settled tation Filtration tion range tion volume sion tion volume rate rate (ppm) (ppm) (cm3) (%) (ppm) (cm3) (cm s - t) (cm3 s- t) Polymer A150 AI30 AI00 CI00 CII0 $20
25-80 25-80 30-110 35-210 45--250 145-260
30 30 35 40 50 165
70.0 70.0 68.0 63-0 63-5 63.0
77 76 93 98 I00 98
60 65 65 150 165 210
67.0 67.0 64.0 61.5 61-0 61.0
0.13 0.24 0.31 0.32 0.34 0.18
0.13 0.15 0.25 0.26 0.28 0.12
the pressure applied to the bed to assist filtration. The results of this investigation disagree with the first assumption and the validity of the second assumption is very doubtful. K a T C ~ and SLATER(1966), criticized La Mer's theory and in view of their criticisms concluded that there was no advantage to be gained in using filtration techniques in the study of flocculation. They conclude that the "haze" of suspended solids left in suspension after the polyacrylamide concentrations at which the maximum clarity of the supernatant is recorded, carries a high doseage of flocculant. They state that the "haze" results from rapid sorption of large quantities of flocculant by particles that encounter high local concentration during the addition of the floeculant. For each clay-polyacrylamide system investigated in this study the maximum filtration rate was observed at a higher polyacrylamide concentration than the maximum TABLE 3. FLOCCULATIONOF CLAYRT Settled volume peak Settled volume trough Effective polymer Polymer Light Polymer Sedimenconcentra- concentra- Settled transmis- concentra- Settled tation Filtration tion range tion volume sion tion volume rate rate (ppm) (ppm) (cm3) (%) (ppm) (cm3) (cm s -t) (cm3 s -t) Polymer A150 AI30 AI00 C100 CII0 $20
10-70 10--80 15-95 25-170 25-180 50-210
20 20 30 35 40 100
62"0 62"0 61"0 61-0 60'0 59"0
40 47 82 90 92 98
45 50 55 120 1300 160
57"0 57"0 54'0 54"0 53"0 52"0
0"14 0"27 0-31 0'37 0"55 0"25
0'19 0"28 0"49 0'51 0"52 0'21
clarity o f the supernatant. The explanation of flocculation proposed by Kitchener and Slater, however, implies that results obtained at polyacrylamide concentrations greater than those required to produce maximum clarity of the supernatant cannot be reproducible. If this is true the disagreement between the filtration and clarity results is probably due to either inadequate mixing of the suspensions or the failure of the filtration technique as an accurate measure o f flocculation. The agreement
Optimum Flocculant Concentration for Effective Flocculation
709
between the filtration and sedimentation results, and the reproducibility of the results, however, argue against both these conclusions. The non-reproducibility of the results at high flocculant concentrations may be explained by the existing theories of flocculation (AUDSLEY, 1965). Flocculation is thought to occur via polymer "bridging" in which large loose floes are formed by long chain polymers being adsorbed onto the particles in suspension. To be effective the polymer concentration must be less than that required for complete coverage of the panicles as this would result in particles being enveloped by a polymer instead of being attached to other panicles via the bridging mechanism. This accounts for the sticky appearance of the beds above the optimum concentration range. The sedimentation rate of a concentrated suspension is known to be a measure of particle and hence floc size (RICHARDSONand Z^gl, 1954), and the agreement between sedimentation rates and filtration rates indicates that filtration rates are an accurate measure of flocculation. As KrrcHENT.Rand SLATEg(1966) have reported, the filtration theory of LA MEg and SMELLn~(1958) is of doubtful value but this does not invalidate the actual experimental results. The theory of La Mer and Smellie is based on the Kozery-Carman equation (ORR and DALLAVALLE,1958), equation (1).
U :
where; U Gc N ps 2 S~, L L, L
:= = = = : =
2Np.r2Sw 2 L ,
(l--e) 2
(1)
The filtration rate The acceleration due to gravity The viscocity of the suspension fluid The density of the suspension fluid The surface area of the bed The length of the bed The effective length of the bed to the suspension fluid
-- The pressure drop across the bed :
The voidage of the bed.
Sw and Gc will be constant for each clay-polyacrylamide suspension and the variations in E and N will be small (AtmSLF~, 1965). Using the filtration tube introduced by CHRISTIANet al. (1970) will be constant and hence the variations in the filtration rates will mainly be due to changes in e, L and Le. The values of L and e, however, are determined by the settled volumes of the suspensions which are at a minimum for the maximum filtration rates (FIGs. 2-4). The filtration rates are dependant on the ionic nature of the flocculant (T~LES 2 and 3) and there is no correlation between the values recorded at the peaks in the filtration rate and settled volume curves. The results suggest, therefore, that variations in the factor Le determine the filtration rates of the suspensions. When the filtration rate is at a maximum the value of L, is at a minimum, indicating the formation of large loose floes. The agreement between the sedimentation and filtration rates reinforces this conclusion. It appears, therefore, that the optimum flocculation of the suspension occurs when the settled
710
D. DOLLXMOREand T. A. HORRIDGE
volumes are at a minimum. At this point the ttocs will be pulled together by the binding force of the polymer via polymer bridging, resulting in a lowering of the settled volume. This binding force will be opposed if the polymer is anionic in nature due to repulsion between the negatively charged clays and the polymer and reinforced if the polymer is cationic in nature. As would be expected the settled volumes of the anionic polymer--clay suspensions are greater than those of cationic polymer--clay suspensions. The peaks in the settled volume curves observed at the commencement of flocculation may be explained as measures of the repulsion between the clay particles and clay particles and the polymers as the polymers are adsorbed. Again, as expected, the settled volumes of anionic polymer-clay suspension are greater than those of cationic polymer-clay suspensions. An alternative explanation of the "haze" of suspended solids can now be postulated. At the polyacrylamide concentrations at which the maximum sedimentation and filtration rates were observed flocculation was at its most effective resulting in the formation of large loose flocs. These flocs were rapidly removed from the suspension allowing fine particles to escape through them, thus producing a "haze" of suspended particles above the settled bed. At the polyacrylamide concentration at which the maximum clarity of the supernatants were observed the flocs were fairly small and were removed fairly slowly from suspension, trapping the fine particles and leaving a clear supernatant. The non-ionic and cationic polyacrylamides produced clearer supernatants than the anionic ones due to the repulsive forces between the essentially negatively charged clays and the anionic polyacrylamides being greater than those between the clays and the cationic and non-ionic polyacrylamides. This resulted in the flocs being farther apart leaving more "escape routes" for the fine particles. The settled volume results reinforce this explanation in that the settled volumes of the anionic polyacrylamide suspensions were greater than those for cationic and non-ionic polyacrylamides-clay suspension (TABLES 2 and 3). The conclusion is therefore, that the optimum polyacrylamide concentration for effective flocculation is determined by the rate of fall of the flocs and the rate of filtration through the settled flocs. Acknowledgement--One of us (T.A.H.) wishes to thank English Clays, Lovering, Pochin and Co.
Ltd. and the Science Research Council for the provision of a maintenance grant. REFERENCES AUDSLEYA. (1965) Flocculation and suspension of solids with organic polymers. Warren Springs Lab., Steveoage, Mincr. Proc. Inform. Note 5. CHRLs'rIANJ. R., DOLLtMO~D. and HOR.RUXJ~.T. A. (1970) An apparatus for the measurement of rates of filtration of flocculated suspensions. J. scient. Instrum. 2, 744-746. Kl-rcrt~Nr.RJ. A. and SLAYERR. W. (1966) Characteristics of flocculation of mineral suspensions by polymers. Discuss. Faraday Soc. 42, 267-275. KWATRAB., AHUJAL. D. and RAMAISRISrINAV. (1970) Cluster formation of kaolin using the hindered settling techniques. J. appl. Chem. 20, 123-125. LAMER V. K. and S~,mLLIER. H. (1958) Flocculation, subsidence and filtration of phosphate slimes-VI. A quantitative theory of filtration. 3". Colloid Sci., 13, 589-599. ORR C. and DALLAVALLEJ. M. (1959) FineParticlc Measurement, p. 136. Macmillan, London. RICHARDSONJ. F. and ZAKXW. N. 0954) Sedimentation and fluidisation. Part l. Trans. Inst. chem. Engrs 32, 35-52.
THE
OPTIMUM
EFFECTIVE
FLOCCULANT
CONCENTRATION
FOR
FLOCCULATION OF CHINA CLAY IN AQUEOUS SUSPENSION D. DOLLIMOREand T. A. HORRIDGE
Department of Chemistry and Applied Chemistry, University of Salford, Salford, Lancs.
(Received 2 September 1971) Abstract--The sedimentation and filtration characteristics of aqueous china clay suspensions flocculated by cationic, anionic and non-ionicpolyacrylamides are investigated and discussed. The experimental techniques are shown to be efficient and to give reasonable reproducibility over the polyacrylamide range for effective flocculation. The optimum polyacrylamide concentration for effective flocculation is discussed and it is concluded that the sedimentation rates of the settling floes and the filtration rates through the settled floes, rather than the clarity of the supernatant, determine this optimum concentration.
INTRODUCTION
Tim FLOCCULATIONof mineral suspensions has been discussed by many authors. Lx MEg and SMELLm(1958) have developed a theory of flocculailon which predicts an optimum flocculant concentration by analysing filtration data. Krrcimh'ER and SLXTmt (19663, however, have criticized this theory and concluded that the measurement of the turbidity of the supernatant liquid after standard methods of agitation and sedimentation is a more efficient method of determining the optimum flocculant concentration. The optimum flocculant concentration may well be determined by the process required, i.e. water clarification or the rapid removal of a mineral from suspension. In this study the flocculailon behaviour of china clay-polyacrylamide suspensions was investigated by measuring the sedimentation rate, final settled volume of the floes, turbidity of the supernatant and the filtration rate through the settled flocs. EXPERIMENTAL
All the experiments were carried out in a laboratory maintained at a temperature of 293-2°K q- 0.5°K. Materials
The china clays were supplied by English Clays, Lovering, Pochin & Co. Ltd.; the chemical analysis and particle size distribution are given in T/taLE 1. The anionic and cationic polymers based on polyacrylamide (A and C series respectively) were supplied by B.T.I. Chemicals Ltd., Bradford. From conductometric titrailons the order of anionic character was A 150 > A 130 > A 100 > C 100 > C 110. Molecular weights are believed to be around 3,000,000. The non-ionic polyacrylamide, S.20, was supplied by Cyanamid Inc., and has a molecular weight of 3,000,000. Sedimentation
The sedimentations were carded out in 250 ml stoppered measuring cyfinders of 3-2 cm i.d. KWXTRAet al. (1970), has reported that the optimum ratio of suspension 703
704
D. DOLLIMOREand T. A. HORPODGE TABLE l. CHEMICAL ANALYSISAND PARTICLE SIZE DISTRIBUTION OF CLAYS SPS AND RT Chemical analysis SiOz AI,O3 Fe203 TiOz CaO MgO K20 Na20 Loss on ignition Particle size distribution Finer than 2 #m Above 10 #m 300 mesh residue
SPS percentage
RT percentage
46'02 38'07 0"56 0.09 0"20 0"20 1"01 0"07 13"14
46"07 38"02 -14)0 1.00 1.00 -1-00 12"59
Percentage
Percentage
80.0 0.2 0.01
36.0 30.0
height to diameter (H/D) for the sedimentation of concentrated suspensions is in the range 5-9. In the measuring cylinders a suspension volume of 150 ml had an HID ratio of 5.9 and this was the suspension volume chosen. The weight of china clay in the suspensions was kept constant at 15 g of clay per 150 ml suspension. Floc radii are very dependant on the methods of agitation of the suspension and addition of the flocculant. In order to establish a simple reproducible experimental technique a detailed systematic study on the system was carded out. The methods of agitation investigated were manual agitation and mechanical agitation by means of a magnetic stirrer, a glass stirrer and a table top oscillator. The methods of addition of the flocculant investigated were, (a)by pipetting the required amount from a stock solution into a cylinder containing a clay suspension and then making the total suspension volume up to 150 ml and (b) by adding 50 ml of the flocculant of the required concentration to a 100-ml clay suspension and then immediately agitating the resulting suspension. It was concluded that manual agitation was an efficient method of agitation. For the addition of the flocculant, the results for method (a) showed wide variations whereas the results for method (b) were reproducible. The following standard procedure was then devised. A 100-ml suspension of the clay was prepared in a stoppered measuring cylinder by weighing 15 g of the clay into the cylinder and adding deionized water to the solid. The suspension was dispersed by inverting the stoppered cylinder several times and then left to equilibrate overnight. The suspension settled out overnight and was redispersed the following morning. A stock solution of the polyacrylamide was prepared and the required amount of this solution pipetted into a 50-ml graduated flask which was made up to the mark with deionized water. The clay suspension was then dispersed by inverting the stoppered cylinder several times and the polyacrylamide solution immediately added to the suspension in the cylinder. The resulting 150 ml suspension was then agitated by
Optimum Flocculant Concentration for Effective Flocculation
705
stoppering the cylinder and inverting it once per second for 60 s. The rate of fall of the suspension was then determined. The suspension was then left to settle and the final settled volume recorded when constant.
Turbidity of the supernatant The turbidity of the supernatant was determined by a method similar to that of KITCt-mNER and SEATER0966). Three minutes after the sedimentation had started a 3-ml aliquot of the supernatant liquid was pipetted off at the 100-ml mark on the cylinder and transferred to a glass cell. The percentage light transmission of the aliquot was then measured on an E.E.L. photo-extinction sedimentometer using distilled water as a blank. If the suspension had not fallen to below the 100-ml mark after 3 min the suspension was not disturbed and the percentage light transmission recorded as zero.
Filtration The filtration rates were recorded on the apparatus described by CHRISTIANet al. (1970).
Reproducibility Typical sedimentation and filtration curves are shown in FIG. 1. For the flocculated suspensions the initial portions of the curves were linear and the rates were calculated from these initial portions of the curves. The reproducibility of the techniques w a s within 5 per cent for the sedimentation and filtration rates, within 3 per cent for the turbidity results and to within 4-0"5 ml for the settled volumes. For the non-flocculated suspensions the initial portions of the curves were non-linear, as the sedimentation and filtration rates were very slow. Accurate sedimentation and filtration rates 150
.~
125 Sedirnentotion
.~.
curve
IO0
"o 75
8
5O -
Filtration
curve
o
_= 25 o
i:~
0
50O
IO00
Time,
t'JO0
2OOO
s
FIo.~I. Sedimentation and filtration curves for SPS--A.100, 65 ppm suspensions.
706
D. DOLLIMOREand T. A. HORRIDGE 70 G; E
60
•j-~ .~ I~
50
o 0
~
02
o') T o
m
o
.
~
~
02
0
20
40
Polyacrylamide
60
I00
80
concentration,
ppm
Flo. 2. Flocculation data for clay SPS-polyacrylamide A.150 suspensions.
65 --
q Eu~
5C
o
0
5C
'00
Polyacryiomide
1~o.
3. F l o c c u l a t i o n
data
15o
2oo
concentration,
2~
~.,o
ppm
for clay SPS--polyacrylamidc
C.110
suspensions.
Optimum FlocculantConcentrationfor EffectiveFlocculation
707
could not be calculated but the final settled volumes of the suspensions were recorded and found to be reproducible to within -4-0.5 ml. RESULTS The flocculation behaviour of clays SPS and RT-polyacrylamide suspensions was investigated and the results plotted as flocculation parameters versus polyacrylamide concentration. The results for clay SPS-polyacrylamide A.150, SPS-CI10 and SPS-S20 suspensions are shown in FIGs. 2 and 4 respectively. A peak in the settled volume curve coincided with a peak in the percentage light transmission curve and a trough in the settled volume curve coincided with peaks in the filtration and sedimentation rate curves. Visual examination showed that large 7O
o~
g,
0
60
;!0 •~
"-~
50
C
C' 120
450
200
250
Polyocrylomide concentration, ppm
FIG. 4. Flocculation data for clay SPS-polyacry]amJde S.20 suspemiom. flocs began to form just prior to the peaks in the settled volume curves. Shortly after the settled volume troughs the suspensions became "sticky" and the results were no longer reproducible. The effective flocculant concentration range for each claypolyacrylamide system was taken to be from the concentration at which large flocs began to form until the concentration at which the results were no longer reproducible. The results at the peaks in the curves are given in TABLES2 and 3 for clays SPS and RT respectively. The settled volumes of clays SPS and RT in deionized water were 62 cm 3 and 51 cm 3 respectively. DISCUSSION The theory of LA MEg and SM~LLm(1958), assumes that the volume of the filter bed is independent of flocculant doseage and that the floe structure is not affected by
708
D. DOLL1MOREand T. A. HORRID~IE TABLE 2. FLOCCULATIONOF CLAYSPS
Settled volume peak Settled volume trough Effective polymer Polymer Light Polymer Sedimenconeentra-concentra- Settled transmis- concentra- Settled tation Filtration tion range tion volume sion tion volume rate rate (ppm) (ppm) (cm3) (%) (ppm) (cm3) (cm s - t) (cm3 s- t) Polymer A150 AI30 AI00 CI00 CII0 $20
25-80 25-80 30-110 35-210 45--250 145-260
30 30 35 40 50 165
70.0 70.0 68.0 63-0 63-5 63.0
77 76 93 98 I00 98
60 65 65 150 165 210
67.0 67.0 64.0 61.5 61-0 61.0
0.13 0.24 0.31 0.32 0.34 0.18
0.13 0.15 0.25 0.26 0.28 0.12
the pressure applied to the bed to assist filtration. The results of this investigation disagree with the first assumption and the validity of the second assumption is very doubtful. K a T C ~ and SLATER(1966), criticized La Mer's theory and in view of their criticisms concluded that there was no advantage to be gained in using filtration techniques in the study of flocculation. They conclude that the "haze" of suspended solids left in suspension after the polyacrylamide concentrations at which the maximum clarity of the supernatant is recorded, carries a high doseage of flocculant. They state that the "haze" results from rapid sorption of large quantities of flocculant by particles that encounter high local concentration during the addition of the floeculant. For each clay-polyacrylamide system investigated in this study the maximum filtration rate was observed at a higher polyacrylamide concentration than the maximum TABLE 3. FLOCCULATIONOF CLAYRT Settled volume peak Settled volume trough Effective polymer Polymer Light Polymer Sedimenconcentra- concentra- Settled transmis- concentra- Settled tation Filtration tion range tion volume sion tion volume rate rate (ppm) (ppm) (cm3) (%) (ppm) (cm3) (cm s -t) (cm3 s -t) Polymer A150 AI30 AI00 C100 CII0 $20
10-70 10--80 15-95 25-170 25-180 50-210
20 20 30 35 40 100
62"0 62"0 61"0 61-0 60'0 59"0
40 47 82 90 92 98
45 50 55 120 1300 160
57"0 57"0 54'0 54"0 53"0 52"0
0"14 0"27 0-31 0'37 0"55 0"25
0'19 0"28 0"49 0'51 0"52 0'21
clarity o f the supernatant. The explanation of flocculation proposed by Kitchener and Slater, however, implies that results obtained at polyacrylamide concentrations greater than those required to produce maximum clarity of the supernatant cannot be reproducible. If this is true the disagreement between the filtration and clarity results is probably due to either inadequate mixing of the suspensions or the failure of the filtration technique as an accurate measure o f flocculation. The agreement
Optimum Flocculant Concentration for Effective Flocculation
709
between the filtration and sedimentation results, and the reproducibility of the results, however, argue against both these conclusions. The non-reproducibility of the results at high flocculant concentrations may be explained by the existing theories of flocculation (AUDSLEY, 1965). Flocculation is thought to occur via polymer "bridging" in which large loose floes are formed by long chain polymers being adsorbed onto the particles in suspension. To be effective the polymer concentration must be less than that required for complete coverage of the panicles as this would result in particles being enveloped by a polymer instead of being attached to other panicles via the bridging mechanism. This accounts for the sticky appearance of the beds above the optimum concentration range. The sedimentation rate of a concentrated suspension is known to be a measure of particle and hence floc size (RICHARDSONand Z^gl, 1954), and the agreement between sedimentation rates and filtration rates indicates that filtration rates are an accurate measure of flocculation. As KrrcHENT.Rand SLATEg(1966) have reported, the filtration theory of LA MEg and SMELLn~(1958) is of doubtful value but this does not invalidate the actual experimental results. The theory of La Mer and Smellie is based on the Kozery-Carman equation (ORR and DALLAVALLE,1958), equation (1).
U :
where; U Gc N ps 2 S~, L L, L
:= = = = : =
2Np.r2Sw 2 L ,
(l--e) 2
(1)
The filtration rate The acceleration due to gravity The viscocity of the suspension fluid The density of the suspension fluid The surface area of the bed The length of the bed The effective length of the bed to the suspension fluid
-- The pressure drop across the bed :
The voidage of the bed.
Sw and Gc will be constant for each clay-polyacrylamide suspension and the variations in E and N will be small (AtmSLF~, 1965). Using the filtration tube introduced by CHRISTIANet al. (1970) will be constant and hence the variations in the filtration rates will mainly be due to changes in e, L and Le. The values of L and e, however, are determined by the settled volumes of the suspensions which are at a minimum for the maximum filtration rates (FIGs. 2-4). The filtration rates are dependant on the ionic nature of the flocculant (T~LES 2 and 3) and there is no correlation between the values recorded at the peaks in the filtration rate and settled volume curves. The results suggest, therefore, that variations in the factor Le determine the filtration rates of the suspensions. When the filtration rate is at a maximum the value of L, is at a minimum, indicating the formation of large loose floes. The agreement between the sedimentation and filtration rates reinforces this conclusion. It appears, therefore, that the optimum flocculation of the suspension occurs when the settled
710
D. DOLLXMOREand T. A. HORRIDGE
volumes are at a minimum. At this point the ttocs will be pulled together by the binding force of the polymer via polymer bridging, resulting in a lowering of the settled volume. This binding force will be opposed if the polymer is anionic in nature due to repulsion between the negatively charged clays and the polymer and reinforced if the polymer is cationic in nature. As would be expected the settled volumes of the anionic polymer--clay suspensions are greater than those of cationic polymer--clay suspensions. The peaks in the settled volume curves observed at the commencement of flocculation may be explained as measures of the repulsion between the clay particles and clay particles and the polymers as the polymers are adsorbed. Again, as expected, the settled volumes of anionic polymer-clay suspension are greater than those of cationic polymer-clay suspensions. An alternative explanation of the "haze" of suspended solids can now be postulated. At the polyacrylamide concentrations at which the maximum sedimentation and filtration rates were observed flocculation was at its most effective resulting in the formation of large loose flocs. These flocs were rapidly removed from the suspension allowing fine particles to escape through them, thus producing a "haze" of suspended particles above the settled bed. At the polyacrylamide concentration at which the maximum clarity of the supernatants were observed the flocs were fairly small and were removed fairly slowly from suspension, trapping the fine particles and leaving a clear supernatant. The non-ionic and cationic polyacrylamides produced clearer supernatants than the anionic ones due to the repulsive forces between the essentially negatively charged clays and the anionic polyacrylamides being greater than those between the clays and the cationic and non-ionic polyacrylamides. This resulted in the flocs being farther apart leaving more "escape routes" for the fine particles. The settled volume results reinforce this explanation in that the settled volumes of the anionic polyacrylamide suspensions were greater than those for cationic and non-ionic polyacrylamides-clay suspension (TABLES 2 and 3). The conclusion is therefore, that the optimum polyacrylamide concentration for effective flocculation is determined by the rate of fall of the flocs and the rate of filtration through the settled flocs. Acknowledgement--One of us (T.A.H.) wishes to thank English Clays, Lovering, Pochin and Co.
Ltd. and the Science Research Council for the provision of a maintenance grant. REFERENCES AUDSLEYA. (1965) Flocculation and suspension of solids with organic polymers. Warren Springs Lab., Steveoage, Mincr. Proc. Inform. Note 5. CHRLs'rIANJ. R., DOLLtMO~D. and HOR.RUXJ~.T. A. (1970) An apparatus for the measurement of rates of filtration of flocculated suspensions. J. scient. Instrum. 2, 744-746. Kl-rcrt~Nr.RJ. A. and SLAYERR. W. (1966) Characteristics of flocculation of mineral suspensions by polymers. Discuss. Faraday Soc. 42, 267-275. KWATRAB., AHUJAL. D. and RAMAISRISrINAV. (1970) Cluster formation of kaolin using the hindered settling techniques. J. appl. Chem. 20, 123-125. LAMER V. K. and S~,mLLIER. H. (1958) Flocculation, subsidence and filtration of phosphate slimes-VI. A quantitative theory of filtration. 3". Colloid Sci., 13, 589-599. ORR C. and DALLAVALLEJ. M. (1959) FineParticlc Measurement, p. 136. Macmillan, London. RICHARDSONJ. F. and ZAKXW. N. 0954) Sedimentation and fluidisation. Part l. Trans. Inst. chem. Engrs 32, 35-52.