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Flocculation-dispersion behavior of quartz in the presence of a polyacrylamide flocculant
JOURNAL
OF COLLOID SCIENCE
16, 609-617 (1961)
FLOCCULATION-DISPERSION BEHAVIOR OF QUARTZ IN THE PRESENCE OF A POLYACRYLAMIDE FLOCCULANT T. W. Healy Mining Department, University of Melbourne, Australia 1 Received March 14, 19G1
ABSTRACT Elcctrokinetic, adsorption, ~nd subsidence studies On quartz dispersions in the presence of ~ commerciM, synthetic, polyacrylamide type floccul~nt are reported. Additional evidence is given for the bridging mechanism of polymer flocculation. This model is extended to include the effect of agitation on the adsorption of polymer and on subsequent floccul~tion. INTRODUCTIO~ Considerable interest is being directed towards understanding the mechanism of adsorption of macromolecules on dispersed inorganic solids and the flocculation-dispersion phenomena associated with this adsorption. Several factors hinder quantitative analysis of the problem, ~mong these being the difficulty of obtaining monodisperse polymer s~mples ~nd the general lack of information on the forces responsible for the adsorption of polymers a t the solid-liquid interface. Furthermore, the stability of aqueous suspensions of insoluble inorganic compounds, in the presence of polymer flocculants, is influenced by many v~riables (1, 2). An attempt has been made in this study to understand the effect of three of the more important variables on the adsorption-flocculation process for the system quartz-Separan 2610. Separ~n 2610 is a partially hydrolyzed polyacrylamide flocculant supplied by the Dow Chemical Corporation. Variables studied were surface charge of the solid-liquid interface and the intensity and time of agitation. It has been suggested, but not proved, that the variation of zeta potential of the solid-liquid interface will have little effect on the mechanism of polymer flocculation (2). Likewise the bridging theory of polymer flocculation, accepted by most workers in this field (1-3, 4) merits further substantiation. Certain aspects of the effect of agitation have been reviewed by Linke and Booth (5) and other workers (2). In these particular studies, no dis1Present address: Chemistry Department, Columbia University, New York 27, New York. 609
610
HEAL¥
tinction was made between time and intensity of agitation; see, however, reference 1. The effect of both these variables is important to the understanding of the mechanism of polymer flocculation. EXPERIMENTAL
Samples of clean reef quartz from Diamond Creek and Buninyong areas in Victoria were used in the preparation of sized fractions of quartz for zeta potential, adsorption, and subsidence measurements. The quartz was stage ground under conductivity water in an Abbe type pebble mill. Samples of --14 + 20 mesh (Tyler) and --48 ~- 65 mesh (Tyler) material were removed, and stored under conductivity water for zeta potential measurements. The quartz used in the adsorption and subsidence work was 95 % --40Omesh (Tyler). One stock solution of this pulp was used for all adsorption and subsidence work. Conductivity water was prepared by passing once-distilled water through a mixed cation, anion exchange resin column. Freshly prepared solutions of the synthetic floceulant Separan 2610, described by Dow as a partially hydrolyzed polymer of acrylamide and other monomers, were used throughout the project. They had an average molecular weight of one million, This was estimated by ionic group titration together with information from the Dow Chemical Corporation. 1. M e a s u r e m e n t of Electrokinetic Potentials
The elcctrokinetic potential (~) of quartz was determined by a streaming potential technique which was a modification of that used by Buchanan and Heymann (6). The electrokinetic potential is defined as the electrical potential at the plane of shear with respect to a point in the bulk liquid which is considered to be at zero potential. With the convention that a negative zeta potential corresponds to a negative surface charge, the zeta potential was calculated from the measured streaming potential by means of the Helmholtz-Smoluchowski equation =
4~rn . E C D RP'
[1]
where n is the viscosity, D is the dielectric constant of the liquid, E is the measured streaming potential when liquid is forced through a plug by a pressure P. Here C / R is the specific conductance of liquid which permeates the plug, C being the plug constant and R the electrical resistance between the electrodes during streaming. No deviation from linearity was observed for the E / P vs. P plot for the pressure range 18-28 cm. Hg. Streaming potentials were measured with a Cambridge valve potentiometer, and plug resistances and solution conductances were determined by means of an a.c. bridge, Philiscope type G.M. 4144.
FLOCCULATION-DISPERSION BEHAVIOR OF QUARTZ
611
Zeta potentials were reproducible to ±1.0 Inv. at low values, and to =t=2.0 my. at high values. 2. Adsorption Measurements A conductivity method, similar to that of Black (7), was used to determine adsorption of Separan on quartz. The apparatus consisted of a Pyrex glass conductivity cell into which was fixed a rotor blade stirrer. Agitation conditions in the cell were varied by means of a rheostat on the stirrer motor. If 10 units represents one full scale of the rheostat, then conditions of gentle, mild, and violent agitation correspond to scale readings of 1, 3, and 5 units, respectively. For settings above 6, turbulence and whirlpool action in the cell caused air entrapment within the pulp. Residual polymer concentration after adsorption was read off a calibration curve of electrical resistance vs. polymer concentration, due allowance being made for the contribution of soluble silica. 8. Subsidence Measurements Subsidence rates were determined from the slope of the initial constant rate period of the height of subsiding column vs. time plots. (No initiation period was observed, and curves were all of Type II, of Smellie and La Mer (8).) The flocculated pulps were allowed to stand undisturbed in Pyrex glass cylinders, 6 cm. in diameter. A standard procedure for mixing flocculant solution with the pulp was obtained empirically, viz., ten end-over-end rotations of the cylinder at constant speed, during which 100 ml. of the desired flocculant solution was added in three increments, after the second, fifth, and eighth cycles, respectively. By this procedure it was possible to obtain subsidence rates, for a given flocculated sample, which did not vary by more than ±1.0 c m . / m i n RESULTS
1. Electrolcinetic Potentials In the subsequent discussion, the change in zeta potential, calculated from the measured streaming potential by means of Eq. [1], is used as a measure of the variation of adsorption behavior of macromolecules at surfaces and not as a quantitative measure of the charge density in the electrical double layer of quartz. Furthermore, it must be stressed that inherent limitations in electrokinetic techniques and in calculation of zeta potentials from streaming potentials by Eq. [1] do not allow quantitative correlation of zeta potentials with aggregation phenomena (9). The variation of the zeta potential of quartz with Separan 2610 concentration is shown in Fig. 1. After determination of the zeta potential at each concentration, approximately 6 liters of conductivity water was streamed through the plug of quartz, and the zeta potential was redeter-
612
HEALY
-80.0
I
I
I
I
l
-60.0
E -40.0 o ¢-, o m
~.
P'c.X.
-20.0 - O------
0 0
0.1
|
I
I
I
0.2
0.5
0.4
0.5
}.6
Concn. of separan 2610 ( g/I ) FIG. 1. Zeta potential of quartz in aqueous solutions of Separan 2610 at pH 5.9. mined during streaming of the sixth liter of water. The results of these tests are summarized in Table I. For the system noted in these results, --16.5 my. is referred to as the "wash-back" potential (Point X in Fig. I). For concentrations up to and including 0.1 g./liter there is no significant alteration in zeta potential after the wash procedure. However, for concentrations greater than 0.1 g./liter the potential after washing attained a constant value of -- 16.5 my. As washing did not alter the zeta potential of quartz in Separan 2610 solutions up to 0.1 g./liter, the adsorption of polymer %o this stage is therefore considered to be irreversible to washing. Although the potential decreases at concentrations greater than 0.1 g./liter the adsorption is reversed by washing.
2. Adsorption Measurements Adsorption results are reported in Fig. 2 and are summarized in Table II. The time to attain steady-state adsorption, for the above system, was
FLOCCULATION-DISPERSION BEHAVIOR OF QUARTZ
613
TABLE I
Summary of Zeta Potential Data Concentration of Separan 2610
Zeta potential of quartz in test solution
Zeta potential 1 of quartz after plug washing
0 0.001 0.010 0.050 0.100 0.175 0.250 0.500 0.750
--63.0 --43.5 --31.5 --21.5 --17.5 --16.0 --15.5 --13.5 --12.5
--63.0 --44.0 --31.5 --22.0 --18.0 --16.5 --16.5 --16.5 --16.5
(my.)
(g./liter)
(my.)
1 Determined in conductivity water.
O
b5 "5
Ca)
/
o~
E a) .El
¢-
o
/-
/•(b)
/
./.l"
O O. Q) 69
5.0
IO.O
15.0
Residual solution concn, of separan (mg/g of SiO2_) Fx~. 2. Adsorption behavior of quartz-Separan 2610 under conditions of (a) mild, (b) medium, and (c) strong intensity of agitation.
too rapid to measure, i.e., less than 2 sees. after addition of polymer solution to the cell. Prolonged time of agitation, at any one fixed intensity, did not alter the residual polymer concentration in solution. Furthermore, as the intensity of agitation was increased the average floc size of the pulp decreased. Analysis of the shape of the adsorption curves is not possible since assumptions in using the conductimetric method cannot be substantiated
614
I-IEALY
TABLE II
Adsorption of Separan 2610 on Quartz as a Function of Intensity of Agitation Conditions of agitation Polymer added
(mg./g. of quartz) 0.33 0.67 1.67 3.33 6.67
Gentle
Mild Violent Polymer adsorbed (mg./g.quartz)
0.33 0.67 1.58 3.00 6.48
0.27 0.50 1.15 2.30 4.75
0.10 0.16 0.40 0.70 1.65
in the scope of the present study. However, the wide separation of amount adsorbed with change in intensity of agitation is of particular interest. 3. Subsidence Results
The effect of polymer addition on the subsidence rate is shown in Fig. 3. Similar optimum behavior was shown at other pulp densities beteen 3 % and 15 % solids (by weight) and has been observed for other polyacrylamide type floeeulants (5). The effect on the subsidence rate of time of agitation at a fixed intensity is shown in Fig. 4, in which curves 1, 2, and 3 represent Separan 2610 additions of 7.5 X 10-~, 20.0 X 10-5, and 42.5 X 10-5 g./g. of solids, respectively. The greater the initial polymer concentration, the more resistant are the flocs to redispersion. For the highest concentration (No. 3) redispersion was not observed until pre-agitation was eight times the standard value, whereas at low concentrations (No. 1) significant redispersion occurred at twice standard pre-agitation. The pertinent experimental results may be summarized as follows: 1. There is a rapid decrease in zeta potential of quartz with increasing Separan concentration. The potential is not reduced to zero. 2. With increasing intensity of agitation: a. the amount of polymer adsorbed decreases, and b. the average floe size of the pulp decreases. 3. With increasing time of agitation at a fixed intensity: a. the amount of polymer adsorbed does not change, but b. the degree of flocculation (or average floc size) decreases. 4. The resistance of the flocs to redispersion increases as polymer concentration increases. DISCUSSION
From Fig. 1 it can be seen that a"substantial reduction in the magnitude of the zeta potential occurs at low solution concentrations, although with increasing concentration of polymer the zeta potential is not reduced to
FLOCCULATION-DISPERSION
'
'
'
Small F l o c s
20.0
' Large
Haze
E
0
io.o
'
'Small Flocs
Flocs
No H a z e
Haze
/"
/
"~ 15.0
615
BEHAVIOR OF QUARTZ
\
/
(D
co
5.0
/
0
I
0
I0.0
I
I
I
I
!
20.0
30.0
40.0
50.0
60.0
70.0
Concn. of sepGron 2610 (g/g of solids) X 105
FIG. 3. Effect of flocculant addition on the settling rate of a quartz pulp of 5% solids (pH 5.9). zero. Point X (Fig. 1) corresponds to a potential energy barrier of approximately 0.5 kT. This barrier is low enough to allow rapid coagulation to proceed. However, the flocculation shown in Fig. 3 is much more extensive than that shown by, for example, inorganic ion adsorption. Some additional mechanism must be introduced to explain flocculation b y polymeric compounds. It would appear that the zeta potential reduction accompanying polymer adsorption is a subsidiary mechanism to that of bridge formation b y the polymer molecules between adjacent solid particles in the pulp. The significance of the wash-back effect shown in Table I is not completely understood. I t would seem that the adsorption reaction must be agitation controlled. The first polymer chains can attach at m a n y points on the surface. As the polymer concentration increases the surface becomes more and more covered and each additional molecule adsorbed is held at
616
H E A L Y
25.0
I
0
I
O ~
|
1
0
No. 5
\
20.0 c-
E
~
E o.)
2 O
15.o
\ %
r°_ G)
,o.o
"0 \ %
5.0
I I0
I 20
I 30
"
I 40
r 50
J 60
70
Number of mixes Fie. 4. Effect of extent of agitation on settling rate of quartz pulps at three flocculant additions. (For Key see text.)
fewer and fewer sites. At any condition of shear there is a critical number of sites at which the polymer chain must be attached for it to remain adsorbed. This model also seems to fit the subsidence data. The optimum effect shown in Fig. 3 will not be discussed in detail. It has been observed by other workers (1, 3, 5) and m a y be summarized as follows: The degree of flocculation, at any particular concentration of polymer depends on (a) the length and number of extended segments, and (b) the available surface onto which extended segments can bridge. This bridging model of the flocculation process can be extended to explain the observations given above. First, since individual chains adsorb at only a fraction of the active sites per molecule (10), the more intense the agitation the more difficult it is for polymer to adsorb a critical number of groups and stay attached; i.e., as intensity of agitation increases, the amount of polymer adsorbed decreases (Fig. 2). Second, increasing time of agitation at a fixed intensity does not change the amount of polymer adsorbed, but the degree of flocculation decreases
FLOCCULATION-DISPERSION BEHAVIOR OF QUARTZ
617
(i.e., redispersion). Also the rate of redispersion at low concentrations of polymer is greater than at high concentrations. Consider the fioc of solid particles bridged together by polymer chains. There will be random movement, partly restricted, of the units of the floc. During this motion the bridging segments of the polymer are reduced in length and number by adsorption on adjacent or individual particles, and the flocs themselves become less resistant to shear. At low surface coverage (low polymer addition), adsorption of extended segments is more rapidly accomplished than at high surface coverage. Furthermore a longer time, at any fixed intensity of agitation, is required at the higher concentrations of polymer, since the probability of an extended segment finding free surface onto which it can adsorb is less than at low surface coverage. Finally it must be stressed that the above discussion is based on observations of the system quartz-polyacrylamide. Other mineral-polymer systems may show divergence from the present system. The bridging model of polymer flocculation would seem adequate at present. Further information on the adsorption forces responsible for initial adsorption of polymer is needed to test the model in more detail. ACKNOWLEDGMENTS The author gratefully acknowledges the award of a Consolidated Zinc Metallurgical Research Scholarship. He also wishes to thank those personnel of the Mining Department, University of Melbourne, who assisted him in many ways in this project. REFERENCES 1. LA MEH, V. K., SMELLIE, R. H., JR., AND LEE, Pvi-Ku~I, d. Colloid Sci. 12,230 (1957). 2. McCARTY, M. F., AND OLSON, R. S., Mining Eng. 11, 61 (1959). 3. LAMER, V. K., AND SiVIELLIE,R. H., JR., J. Colloid Sci. 11,709,711 (1956). 4. RUEHRWEIN, R. A., AND WARD, D. W., Soil Sci. 73, 485 (1952). 5. LINKE, W. F., ANn BOOTH, R. B., Preprint, American Institute of Mimng and Metallurgical Engineers, Annual Meeting, 1959. 6. BUCHANAN,A. S., AND HEYMANN, E., Proc. Roy. Soc. (London) 195, 150 (1948). 7. BLACK, A., M. Eng. Sci. Thesis, Melbourne, Australia, 1952. 8. SMELLIE, R. I~I., JR., AND LAMER, V. K., or. Colloid Sci. 11,720 (1956). 9. BIKERMAN, J. J., "Surface-Chemistry--Theory and Applications." Academic Press, New York, 1958. 10. ULMAN, R., ~4~ORAL,J., AND EIRICH, F. R., Proc. Znd. Intern. Congr. Surface Activity 3, 485 (1957).
OF COLLOID SCIENCE
16, 609-617 (1961)
FLOCCULATION-DISPERSION BEHAVIOR OF QUARTZ IN THE PRESENCE OF A POLYACRYLAMIDE FLOCCULANT T. W. Healy Mining Department, University of Melbourne, Australia 1 Received March 14, 19G1
ABSTRACT Elcctrokinetic, adsorption, ~nd subsidence studies On quartz dispersions in the presence of ~ commerciM, synthetic, polyacrylamide type floccul~nt are reported. Additional evidence is given for the bridging mechanism of polymer flocculation. This model is extended to include the effect of agitation on the adsorption of polymer and on subsequent floccul~tion. INTRODUCTIO~ Considerable interest is being directed towards understanding the mechanism of adsorption of macromolecules on dispersed inorganic solids and the flocculation-dispersion phenomena associated with this adsorption. Several factors hinder quantitative analysis of the problem, ~mong these being the difficulty of obtaining monodisperse polymer s~mples ~nd the general lack of information on the forces responsible for the adsorption of polymers a t the solid-liquid interface. Furthermore, the stability of aqueous suspensions of insoluble inorganic compounds, in the presence of polymer flocculants, is influenced by many v~riables (1, 2). An attempt has been made in this study to understand the effect of three of the more important variables on the adsorption-flocculation process for the system quartz-Separan 2610. Separ~n 2610 is a partially hydrolyzed polyacrylamide flocculant supplied by the Dow Chemical Corporation. Variables studied were surface charge of the solid-liquid interface and the intensity and time of agitation. It has been suggested, but not proved, that the variation of zeta potential of the solid-liquid interface will have little effect on the mechanism of polymer flocculation (2). Likewise the bridging theory of polymer flocculation, accepted by most workers in this field (1-3, 4) merits further substantiation. Certain aspects of the effect of agitation have been reviewed by Linke and Booth (5) and other workers (2). In these particular studies, no dis1Present address: Chemistry Department, Columbia University, New York 27, New York. 609
610
HEAL¥
tinction was made between time and intensity of agitation; see, however, reference 1. The effect of both these variables is important to the understanding of the mechanism of polymer flocculation. EXPERIMENTAL
Samples of clean reef quartz from Diamond Creek and Buninyong areas in Victoria were used in the preparation of sized fractions of quartz for zeta potential, adsorption, and subsidence measurements. The quartz was stage ground under conductivity water in an Abbe type pebble mill. Samples of --14 + 20 mesh (Tyler) and --48 ~- 65 mesh (Tyler) material were removed, and stored under conductivity water for zeta potential measurements. The quartz used in the adsorption and subsidence work was 95 % --40Omesh (Tyler). One stock solution of this pulp was used for all adsorption and subsidence work. Conductivity water was prepared by passing once-distilled water through a mixed cation, anion exchange resin column. Freshly prepared solutions of the synthetic floceulant Separan 2610, described by Dow as a partially hydrolyzed polymer of acrylamide and other monomers, were used throughout the project. They had an average molecular weight of one million, This was estimated by ionic group titration together with information from the Dow Chemical Corporation. 1. M e a s u r e m e n t of Electrokinetic Potentials
The elcctrokinetic potential (~) of quartz was determined by a streaming potential technique which was a modification of that used by Buchanan and Heymann (6). The electrokinetic potential is defined as the electrical potential at the plane of shear with respect to a point in the bulk liquid which is considered to be at zero potential. With the convention that a negative zeta potential corresponds to a negative surface charge, the zeta potential was calculated from the measured streaming potential by means of the Helmholtz-Smoluchowski equation =
4~rn . E C D RP'
[1]
where n is the viscosity, D is the dielectric constant of the liquid, E is the measured streaming potential when liquid is forced through a plug by a pressure P. Here C / R is the specific conductance of liquid which permeates the plug, C being the plug constant and R the electrical resistance between the electrodes during streaming. No deviation from linearity was observed for the E / P vs. P plot for the pressure range 18-28 cm. Hg. Streaming potentials were measured with a Cambridge valve potentiometer, and plug resistances and solution conductances were determined by means of an a.c. bridge, Philiscope type G.M. 4144.
FLOCCULATION-DISPERSION BEHAVIOR OF QUARTZ
611
Zeta potentials were reproducible to ±1.0 Inv. at low values, and to =t=2.0 my. at high values. 2. Adsorption Measurements A conductivity method, similar to that of Black (7), was used to determine adsorption of Separan on quartz. The apparatus consisted of a Pyrex glass conductivity cell into which was fixed a rotor blade stirrer. Agitation conditions in the cell were varied by means of a rheostat on the stirrer motor. If 10 units represents one full scale of the rheostat, then conditions of gentle, mild, and violent agitation correspond to scale readings of 1, 3, and 5 units, respectively. For settings above 6, turbulence and whirlpool action in the cell caused air entrapment within the pulp. Residual polymer concentration after adsorption was read off a calibration curve of electrical resistance vs. polymer concentration, due allowance being made for the contribution of soluble silica. 8. Subsidence Measurements Subsidence rates were determined from the slope of the initial constant rate period of the height of subsiding column vs. time plots. (No initiation period was observed, and curves were all of Type II, of Smellie and La Mer (8).) The flocculated pulps were allowed to stand undisturbed in Pyrex glass cylinders, 6 cm. in diameter. A standard procedure for mixing flocculant solution with the pulp was obtained empirically, viz., ten end-over-end rotations of the cylinder at constant speed, during which 100 ml. of the desired flocculant solution was added in three increments, after the second, fifth, and eighth cycles, respectively. By this procedure it was possible to obtain subsidence rates, for a given flocculated sample, which did not vary by more than ±1.0 c m . / m i n RESULTS
1. Electrolcinetic Potentials In the subsequent discussion, the change in zeta potential, calculated from the measured streaming potential by means of Eq. [1], is used as a measure of the variation of adsorption behavior of macromolecules at surfaces and not as a quantitative measure of the charge density in the electrical double layer of quartz. Furthermore, it must be stressed that inherent limitations in electrokinetic techniques and in calculation of zeta potentials from streaming potentials by Eq. [1] do not allow quantitative correlation of zeta potentials with aggregation phenomena (9). The variation of the zeta potential of quartz with Separan 2610 concentration is shown in Fig. 1. After determination of the zeta potential at each concentration, approximately 6 liters of conductivity water was streamed through the plug of quartz, and the zeta potential was redeter-
612
HEALY
-80.0
I
I
I
I
l
-60.0
E -40.0 o ¢-, o m
~.
P'c.X.
-20.0 - O------
0 0
0.1
|
I
I
I
0.2
0.5
0.4
0.5
}.6
Concn. of separan 2610 ( g/I ) FIG. 1. Zeta potential of quartz in aqueous solutions of Separan 2610 at pH 5.9. mined during streaming of the sixth liter of water. The results of these tests are summarized in Table I. For the system noted in these results, --16.5 my. is referred to as the "wash-back" potential (Point X in Fig. I). For concentrations up to and including 0.1 g./liter there is no significant alteration in zeta potential after the wash procedure. However, for concentrations greater than 0.1 g./liter the potential after washing attained a constant value of -- 16.5 my. As washing did not alter the zeta potential of quartz in Separan 2610 solutions up to 0.1 g./liter, the adsorption of polymer %o this stage is therefore considered to be irreversible to washing. Although the potential decreases at concentrations greater than 0.1 g./liter the adsorption is reversed by washing.
2. Adsorption Measurements Adsorption results are reported in Fig. 2 and are summarized in Table II. The time to attain steady-state adsorption, for the above system, was
FLOCCULATION-DISPERSION BEHAVIOR OF QUARTZ
613
TABLE I
Summary of Zeta Potential Data Concentration of Separan 2610
Zeta potential of quartz in test solution
Zeta potential 1 of quartz after plug washing
0 0.001 0.010 0.050 0.100 0.175 0.250 0.500 0.750
--63.0 --43.5 --31.5 --21.5 --17.5 --16.0 --15.5 --13.5 --12.5
--63.0 --44.0 --31.5 --22.0 --18.0 --16.5 --16.5 --16.5 --16.5
(my.)
(g./liter)
(my.)
1 Determined in conductivity water.
O
b5 "5
Ca)
/
o~
E a) .El
¢-
o
/-
/•(b)
/
./.l"
O O. Q) 69
5.0
IO.O
15.0
Residual solution concn, of separan (mg/g of SiO2_) Fx~. 2. Adsorption behavior of quartz-Separan 2610 under conditions of (a) mild, (b) medium, and (c) strong intensity of agitation.
too rapid to measure, i.e., less than 2 sees. after addition of polymer solution to the cell. Prolonged time of agitation, at any one fixed intensity, did not alter the residual polymer concentration in solution. Furthermore, as the intensity of agitation was increased the average floc size of the pulp decreased. Analysis of the shape of the adsorption curves is not possible since assumptions in using the conductimetric method cannot be substantiated
614
I-IEALY
TABLE II
Adsorption of Separan 2610 on Quartz as a Function of Intensity of Agitation Conditions of agitation Polymer added
(mg./g. of quartz) 0.33 0.67 1.67 3.33 6.67
Gentle
Mild Violent Polymer adsorbed (mg./g.quartz)
0.33 0.67 1.58 3.00 6.48
0.27 0.50 1.15 2.30 4.75
0.10 0.16 0.40 0.70 1.65
in the scope of the present study. However, the wide separation of amount adsorbed with change in intensity of agitation is of particular interest. 3. Subsidence Results
The effect of polymer addition on the subsidence rate is shown in Fig. 3. Similar optimum behavior was shown at other pulp densities beteen 3 % and 15 % solids (by weight) and has been observed for other polyacrylamide type floeeulants (5). The effect on the subsidence rate of time of agitation at a fixed intensity is shown in Fig. 4, in which curves 1, 2, and 3 represent Separan 2610 additions of 7.5 X 10-~, 20.0 X 10-5, and 42.5 X 10-5 g./g. of solids, respectively. The greater the initial polymer concentration, the more resistant are the flocs to redispersion. For the highest concentration (No. 3) redispersion was not observed until pre-agitation was eight times the standard value, whereas at low concentrations (No. 1) significant redispersion occurred at twice standard pre-agitation. The pertinent experimental results may be summarized as follows: 1. There is a rapid decrease in zeta potential of quartz with increasing Separan concentration. The potential is not reduced to zero. 2. With increasing intensity of agitation: a. the amount of polymer adsorbed decreases, and b. the average floe size of the pulp decreases. 3. With increasing time of agitation at a fixed intensity: a. the amount of polymer adsorbed does not change, but b. the degree of flocculation (or average floc size) decreases. 4. The resistance of the flocs to redispersion increases as polymer concentration increases. DISCUSSION
From Fig. 1 it can be seen that a"substantial reduction in the magnitude of the zeta potential occurs at low solution concentrations, although with increasing concentration of polymer the zeta potential is not reduced to
FLOCCULATION-DISPERSION
'
'
'
Small F l o c s
20.0
' Large
Haze
E
0
io.o
'
'Small Flocs
Flocs
No H a z e
Haze
/"
/
"~ 15.0
615
BEHAVIOR OF QUARTZ
\
/
(D
co
5.0
/
0
I
0
I0.0
I
I
I
I
!
20.0
30.0
40.0
50.0
60.0
70.0
Concn. of sepGron 2610 (g/g of solids) X 105
FIG. 3. Effect of flocculant addition on the settling rate of a quartz pulp of 5% solids (pH 5.9). zero. Point X (Fig. 1) corresponds to a potential energy barrier of approximately 0.5 kT. This barrier is low enough to allow rapid coagulation to proceed. However, the flocculation shown in Fig. 3 is much more extensive than that shown by, for example, inorganic ion adsorption. Some additional mechanism must be introduced to explain flocculation b y polymeric compounds. It would appear that the zeta potential reduction accompanying polymer adsorption is a subsidiary mechanism to that of bridge formation b y the polymer molecules between adjacent solid particles in the pulp. The significance of the wash-back effect shown in Table I is not completely understood. I t would seem that the adsorption reaction must be agitation controlled. The first polymer chains can attach at m a n y points on the surface. As the polymer concentration increases the surface becomes more and more covered and each additional molecule adsorbed is held at
616
H E A L Y
25.0
I
0
I
O ~
|
1
0
No. 5
\
20.0 c-
E
~
E o.)
2 O
15.o
\ %
r°_ G)
,o.o
"0 \ %
5.0
I I0
I 20
I 30
"
I 40
r 50
J 60
70
Number of mixes Fie. 4. Effect of extent of agitation on settling rate of quartz pulps at three flocculant additions. (For Key see text.)
fewer and fewer sites. At any condition of shear there is a critical number of sites at which the polymer chain must be attached for it to remain adsorbed. This model also seems to fit the subsidence data. The optimum effect shown in Fig. 3 will not be discussed in detail. It has been observed by other workers (1, 3, 5) and m a y be summarized as follows: The degree of flocculation, at any particular concentration of polymer depends on (a) the length and number of extended segments, and (b) the available surface onto which extended segments can bridge. This bridging model of the flocculation process can be extended to explain the observations given above. First, since individual chains adsorb at only a fraction of the active sites per molecule (10), the more intense the agitation the more difficult it is for polymer to adsorb a critical number of groups and stay attached; i.e., as intensity of agitation increases, the amount of polymer adsorbed decreases (Fig. 2). Second, increasing time of agitation at a fixed intensity does not change the amount of polymer adsorbed, but the degree of flocculation decreases
FLOCCULATION-DISPERSION BEHAVIOR OF QUARTZ
617
(i.e., redispersion). Also the rate of redispersion at low concentrations of polymer is greater than at high concentrations. Consider the fioc of solid particles bridged together by polymer chains. There will be random movement, partly restricted, of the units of the floc. During this motion the bridging segments of the polymer are reduced in length and number by adsorption on adjacent or individual particles, and the flocs themselves become less resistant to shear. At low surface coverage (low polymer addition), adsorption of extended segments is more rapidly accomplished than at high surface coverage. Furthermore a longer time, at any fixed intensity of agitation, is required at the higher concentrations of polymer, since the probability of an extended segment finding free surface onto which it can adsorb is less than at low surface coverage. Finally it must be stressed that the above discussion is based on observations of the system quartz-polyacrylamide. Other mineral-polymer systems may show divergence from the present system. The bridging model of polymer flocculation would seem adequate at present. Further information on the adsorption forces responsible for initial adsorption of polymer is needed to test the model in more detail. ACKNOWLEDGMENTS The author gratefully acknowledges the award of a Consolidated Zinc Metallurgical Research Scholarship. He also wishes to thank those personnel of the Mining Department, University of Melbourne, who assisted him in many ways in this project. REFERENCES 1. LA MEH, V. K., SMELLIE, R. H., JR., AND LEE, Pvi-Ku~I, d. Colloid Sci. 12,230 (1957). 2. McCARTY, M. F., AND OLSON, R. S., Mining Eng. 11, 61 (1959). 3. LAMER, V. K., AND SiVIELLIE,R. H., JR., J. Colloid Sci. 11,709,711 (1956). 4. RUEHRWEIN, R. A., AND WARD, D. W., Soil Sci. 73, 485 (1952). 5. LINKE, W. F., ANn BOOTH, R. B., Preprint, American Institute of Mimng and Metallurgical Engineers, Annual Meeting, 1959. 6. BUCHANAN,A. S., AND HEYMANN, E., Proc. Roy. Soc. (London) 195, 150 (1948). 7. BLACK, A., M. Eng. Sci. Thesis, Melbourne, Australia, 1952. 8. SMELLIE, R. I~I., JR., AND LAMER, V. K., or. Colloid Sci. 11,720 (1956). 9. BIKERMAN, J. J., "Surface-Chemistry--Theory and Applications." Academic Press, New York, 1958. 10. ULMAN, R., ~4~ORAL,J., AND EIRICH, F. R., Proc. Znd. Intern. Congr. Surface Activity 3, 485 (1957).