Langmuir 1995,11, 4678-4684
4678
Influence of Addition of a Polyelectrolyte, Nonionic Polymers, and Their Mixtures on the Rheology of CoaWater Suspensions Th. F. Tadros,**tP. Taylor,? and G. Bognolot Jealott's Hill Research Station, ZENECA Agrochemicals (Formerly part of the ICI Group), Bracknell, Berkshire RG42 6ET, U.K., and ICI Surfactants, Everberg, Belgium Received May 9, 1994. In Final Form: July 31, 1995@ The effect of addition of a polyelectrolyte, a nonionic surfactant; ABA block copolymers, and mixtures of the polyelectrolyte with the block copolymers on the rheology of coallwater suspensions was studied using steady state and oscillatory rheological measurements. The polyelectrolyte was sodium lignosulfonate (Ufoxane3A). The nonionic surfactant (EL1602P) was hexamethylenediaminewith 4 tails of 10 propylene oxide (PO) and 55 ethylene oxide (EO) units. The block copolymers (Synperonic PE) consisted of 55 PO units and two tails of 4-147 EO units per chain. The addition of the polyelectrolyte caused a rapid above -0.1% on the basis ofthe coal. This indicated deflocculation reduction in the complex modulus (G*) of the suspension above this concentration. This deflocculation was caused by high adsorption of the polyelectrolyte that was accompanied by an increase in the negative 5 potential. The deflocculation was also reflected in the sedimentation behavior of the suspension which showed a decrease in sediment volume at the same concentration at which deflocculation became substantial. The results obtained using the nonionic surfactant were significantly different from those using the polyelectrolyte. They showed an initial increase in the modulus, yield value, and viscosity as the concentration of the surfactant was increased. A maximum was reached at a critical concentration, above which there was a rapid reduction in G*and viscosity. These results were explained in terms ofthe adsorption characteristicsofthe surfactant. Initially, the surfactant adsorbs with the PEO chains pointing toward the solid, and this causes flocculation by hydrophobic interaction. At higher surfactant concentration, adsorption occurs via the hydrophobic groups, leaving the PEO tails danglingin solution, and this leads to restabilization. The PEO-PPO-PEO block copolymers showed a gradual decrease in flocculation with an increase in PEO chain length. This was attributed to the increase in adsorbed layer thickness with an increase in PEO chain length. Energydistance curves show an attractive minimum whose depth becomes smaller as the adsorbed layer thickness increases. Addition of the PEO-PPO-PEO block copolymers to coal suspensions stabilized by the polyelectrolyteshowed a small effect when the PEO chain length was 137 units. However, with the largest PEO chain studied (147 units per chain), the addition of the block copolymer caused an initial increase reaching a maximum at an optimum concentration, in flocculation (accompaniedby a rapid increase in G*), after which there was restabilization of the suspension. These results were explained in terms of the orientation of the molecule. Initially, the molecule probably adsorbs with the PEO chain toward the surface (on the hydrophilic batches), resulting in flocculation by screening the charge and possible hydrophobic interaction. At higher concentration a second layer is produced with the PEO chains dangling in solution, resulting in restabilization of the suspension. These mixtures of polyelectrolytesand nonionic block copolymers may find application in the preparation of stable coallwater suspensions.
Introduction In recent years, there has been considerable interest in the preparation of concentrated coallwater suspensions (> 65% by weight) that could be used for transport in pipelines as well as used in alternative fuels. The principles for the preparation of concentrated coallwater suspensions and the role of additives such as surfactants, nonionic polymers, and polyelectrolytes have been discussed before.1,2 For the preparation of concentrated coal/ water suspensions, it is essential to stabilize the particles against flocculation, by either electrostatic or steric repulsion. In the first case, ionic surfactants or polyelectrolytes that adsorb strongly on the particle surface may be used. Steric repulsion may be achieved by using nonionic surfactants or polymers of the AB or ABA block type. In these molecules the B chain should adsorb strongly on the particle surface (the so-called anchoring chain) whereas the A chain(s) should be strongly solvated
* To whom correspondence should be addressed. t
ZENECA Agrochemicals.
IC1 Surfactants. @Abstractpublished in Advance A C S Abstracts, November 1, 1995. (l)Tadros, Th. F.; Gybels, A. Proceedings of the 3rd European Utilization Conference; 1983;Vol. 3, pp 135-157. (2) Tadros, Th.F. J. Chem. Eng. Symp. 1986, Ser, No. 95, 1-16.
0743-746319512411-4678$09.00/0
by the m e d i ~ m .However, ~ these colloidally stable suspensions tend to settle under gravity, producing closely packed sediments that are dilatant (shear t h i ~ k e n i n g ) . ~ Such systems are difficult to pump in pipelines, and they also cause major problems in storage tanks and on atomization in burning furnaces. To overcome such problems, one usually induces some form of weak flocculation in the system which may produce a threedimensional gel network in the suspension that prevents sedimentation. Flocculation, however, reduces the maximum solid content that can be dispersed, and a compromise has to be reached between stability and flocculation to produce practical systems. One of the major properties of concentrated coallwater suspensions that requires systematic investigation is the flow characteristics or rheology of the suspension. This has to be accurately controlled to allow one to prepare the concentration a t the maximum loading possible, to control its longlterm physical stability (in pipelines or tanks), and for ease of application. Generally one aims a t a pseudoplastic system with sufficiently high low shear viscosity (to prevent sedimentation) and low high shear viscosity for ease of application. This behavior may be (3) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1988. (4)Tadros, Th. F. Adu. Colloid Interface Sci. 1980,12, 141.
0 1995 American Chemical Society
Langmuir, Vol. 11, No. 12, 1995 4679
Rheology of Coal I Water Suspensions Table 1. Coal Comnosition
ultimate analysis (element analysis)
proximate analysis volatiles (%) ashes (%) futed carbon (dim calorificvalue (kcalkg) upper lower
30.15 9.87 59.98 7377 7123
carbon (%) hydrogen (%) nitrogen (%) sulfur (%) ashes (%I oxygen (dim (%I
75.71 4.92 1.58
0.74 9.87 7.17
achieved by the use of suitable additives such as nonionic polymers and polyelectrolytes. In order to arrive a t the optimum dispersant, fundamental investigations of how these molecules affect the rheology of the suspension and its sedimentation characteristics are required. This is one of the objectives of the present study. Some preliminary results have already been p ~ b l i s h e d . ~ In this paper, the effects of addition of various nonionic polymers of the ABA type and a polyelectrolyte on the rheology of concentrated coallwater suspensions has been investigated. The ABA polymers were of the poly(ethy1ene oxide)-poly(propy1ene oxidel-poly(ethy1ene oxide) (PEOPPO-PEO) type (SynperonicPE). The PPO chain length was kept constant (55 PO units), whereas the PEO chain was systematically increased (4 to 147 units). The polyelectrolyte was sodium lignosulfonate. Additional experiments were carried out using a nonionic surfactant (EL1602P1, a hexamethylenediamine derivative with 4 tails of 10 PO and 55 EO units. Mixtures of the ABA block copolymers with the polyelectrolyte were also investigated.
Experimental Section Materials. Coal (a highly volatile A bituminous originating from Poland) was supplied by IC1SpecialtyChemicals (Italy). The density of the coal was measured by liquid displacement using a pycnometer and was found to be 1.3g ~ m - This ~. coal has the composition summarized in Table 1. As is clear from this table, the coal has an ash content of -10% which suggests that it has some hydrophilic nature. 5 potential measurements (see below) showed an increase in the potential with an increase of pH, and there is some indication of an isoelectric point of pH x 3. Stock slurries were prepared by ball-millinga 60 wt % aqueous coal suspension for 48 h in the absence of any dispersant. The particle size distribution of the resulting suspension was determined using a Malvern 2600 laser diffraction apparatus. A typical size distribution has been shown b e f ~ r e .This ~ gave a volume mean diameter of -15 pm. The polyelectrolyte sodium lignosulfonate (Ufoxane 3A) was kindly supplied by Borregard (Norway) and used as received. The nonionic dispersants were supplied by IC1 Surfactants. EL1602P, a hexamethylenediamine derivative with 4 tails of 10 propylene oxide and 55 ethylene oxide units, was specially prepared for the present work, and it was used as received. The PEO-PPO-PEO block copolymers (Synperonic PE) were also supplied by IC1 surfactants, and they have the composition and molecular weights given in Table 2. Adsorption Isotherms. A 20 wt % suspension containing a given amount of dispersant was prepared and left to equilibrate for at least 2 days. The concentration of the dispersant in the supernatant liquid was determined using U V absorption for the lignosulfonate or a colorimetric method for the nonionic dispersants. The latter method was described by B a l e a d and Tadros and V i n ~ e n t Basically, .~ a KI-I2 solution (containing 2 ( 5 ) Taylor, P.; Liang, W.; Bognolo, G . ;Tadros, Th. F. Colloids Surf. 1991,61, 147. ( 6 )Van Krevelen, D. W. Coal: Typology-Physics-Chemistry-Constitution, 3rd ed.; Elsevier: New York, 1953. ( 7 )Blom, L.; Edelhausen, L.; Van Krevelen, D. W. Fuel 1967,36, 135. ( 8 )Bale-, B.; C . R. Hebd. Seances Assoc. SOC.Ser C 1976,274. (9) Tadros, Th. F.; Vincent, B. J . Phys. Chem. 1980,84, 1575.
g of KI and 1 g of 12 per 100 mL of solution) is added to the Synperonic PE solution (0.25 mL of reagent is added to 10 mL of solution), and the optical density was measured at 500 nm using a Pye-Unicam SP1700 spectrophotometer. g Potential Measurements. The electrophoretic mobility was measured using the principle of laser velocimetry. For this purpose, a Malvern Zeta sizer 3 was used. The principle of the method is as follows. Two laser beams of equal intensity are allowed to cross at a particular point within the cell containing the suspension. At the intersection of the two beams, which is focused at the stationary layer, interferences of known spacing are formed. The particles moving through the fringes under the influence of the electric field scatter light whose intensity fluctuates with a frequency that is related to the mobility of the particles. Adilute coal suspension was prepared by redispersing a drop of the sediment into the supernatant of the slurry after centrifuging the sample. The instrument is fully automated, allowing one to obtain the mobility distribution of the particles. The average mobility ( u )was converted to the 5 potential using the Smoluchowskiequation (sinceKa 1,where K is the DebyeHiickel parameter and a the average particle radius)
where 17 is the viscosity ofthe medium (taken to be that ofwater), c is the permittivity of the medium, and eo that of free space. Sediment Volume Measurements. A 10 mL sample of a 20 wt % coallwater suspension placed in a 10 mL measuring cylinder and left to stand at room temperature for several days till equilibrium was reached. The relative sediment volume, Vsd, was then measured. Rheological Measurements. Measurements were carried out as a fbnction of both the coal volume fraction and dispersant measurements were carried out, concentration. Two t-of namely, steady state and oscillatory measurements. Both were performed at 25 "C using a Bohlin VOR (Bohlin Reologie, Lund, Sweden) and concentric cylinder platens. In the steady state (continuous shear measurements, the shear stress (a) was measured as a function of the applied shear rate ( 9 ) and the results were analyzed using a Bingham model, = 0s + VplY
where as is the extrapolated (Bingham)yield stress and qPlis the plastic (apparent) viscosity. In oscillatory measurements, a sinusoidal strain ( y ) with frequency v (Hz)or w (rad s-l) (w = 2nv)is applied to the cup and the resulting stress (a)is determined. From the phase angle shift (6 = wAt, where At is the time shift between the maxima in stress and strain amplitudes) and the amplitudes ofstress (a,) and strain (yo),the various rheological parameters are determined. 10
G* = uJyo
(3)
G = G* cos 6
(4)
G" = G* sin 6
(5)
where G* is the complexmodulus, G is the storage modulus (the elastic component of the complex modulus), and G is the loss modulus (the viscous component of the complex modulus). G is a measure of the energy stored elastically in a cycle of oscillation, whereas G is a measure of the energy dissipated as viscous flow in this cycle. In oscillatory measurements, one usually measures the modulus values, at a futed frequency, as a function of strain amplitude in order to establish the linear viscoelastic region. "he latter region is where the modulus values do not show any change with an increase in amplitude. This is maintained up to a critical strainvalue, ym, abovewhich the moduli show changes with a further increase in amplitude (G* and G decrease,whereas G increases above yo). After establishing the linear viscoelastic (10) Ferry, J. D. Viscoelastic Properties ofPolymer Solutions; Wiley: New York, 1980.
Tadros et al.
4680 Langmuir, Vol. 11, No. 12, 1995
1 0
Br
i2 0
0.01 0.02
0.05
0.1
0.2
0.5
1
2
[Ufoxane SAI/Yon coal
Figure 1. Variation of the complex modulus and adsorbed amount with the Ufoxane 3A concentration.
region, one may carry out oscillatory measurements as a function of frequency (at constant strain below yo). In the present investigations, measurements were made of the moduli as a function of the strain amplitude at a frequency of 0.1 Hz (strain sweep measurements). The results that will be shown are all in the linear viscoelastic region.
Results and Discussion Influenceof Addition of a Polyelectrolyte (Sodium Lignosulfonate). Figure 1shows the effect of addition of sodium lignosulfonate (Ufoxane 3A) on the complex modulus of a 50 wt % coallwater suspension. For comparison the results of adsorption (I-(mgg-l)) are shown in the same figure. The results show a relatively high modulus (G*) on the order of 38 P a a t low concentrations of Ufoxane 3A ( ~ 0 . 1 % based on the weight of the coal). This high modulus is indicative of a flocculated structure which entraps water, thus resulting in an apparently high volume fraction. The floc structure may consist of chains and cross chains of particles that are able to entrap a considerable amount of liquid. In this case, the ratio between the volume fraction of the flocs, &, to that of the particles, 9, (sometimes referred to as CFPby Hunter and co-workersll), is high, and this results in a high modulus. Above 0.1% Ufoxane 3A, there is a rapid reduction in the complex modulus, reaching a very low value when the concentration ofUfoxane 3A is increased above 0.5%.This effect is due to the deflocculation of the coal suspension as a result ofthe adsorption of the polyelectrolyte. Indeed, the results in Figure 1 show a rapid increase in Ufoxane 3A adsorption a t about the same concentration at which G* shows a rapid decrease. Further evidence for this behavior is shown in Figure 2 which shows the variation of the sediment volume and 5 potential with Ufoxane 3A concentration in the continuous phase. Again for comparison the results of adsorption are shown in the same figure. It can be seen from Figure 2 that there is a n increase in the negative 5 potential and a decrease in the sediment volume with a n increase in the adsorption of Ufoxane 3A. Adsorption of Ufoxane on coal particles probably occurs by hydrophobic interaction between the hydrophobic groups on the Ufoxane molecule and the hydrophobic regions on the coal surface. This leaves the anionic sulfonate groups pointingtoward the bulk solution, and the net effect will be a n increase in the 5 potential. The values of 5 potentials reached a t high Ufoxane 3A concentrations are fairly high (-40 to -60 mV), and this ensures stabilization of the coal suspensions. However, with these complex polyelectrolytes there is also the (11)Firth, B. A.; Hunter, R. J.J.Colloid Interface Sci. 1976,57,248, 257,266.van de Ven, T. G. M.; Hunter, R. J.RheoLActa 1977,16,534.
O
O
I
i
- 60
[Ufoxane 3A1 e q /%
Figure 2. Variation of the relative sediment volume, 5 potential, and adsorbed amount with the Ufoxane 3A concentration. possibility of a n additional steric repulsion as a result of the presence of long dangling tails a t the solidholution interface. l2 The sediment volume experiments also showed interesting behavior. At low Ufoxane 3A concentrations, the relative sediment volume was fairly high, the resulting structure in the sediment was relatively “soft”, and it was possible to redisperse the suspension by shaking the tubes. This is due to the presence of a soft three-dimensional floc structure. As the concentration of Ufoxane 3A was gradually increased, these sediments became “harder”, and ultimately a very hard dilatant sediment was produced. This was indicative of a highly deflocculated structure. The repulsive forces between the particles (electrostatic and steric) allow them to roll over each other to form a densely packed ~ t r u c t u r e .The ~ latter can be easily felt using a glass rod since any penetration results in disturbance of the close packed array with the appearance of dilatancy (shear thickening). Although the polyelectrolyte is very effective in deflocculating the coal suspension, allowing one to reach high volume fractions, it suffers from the disadvantage of producing dense sediments that are difficult to redisperse. This will cause problems in pipelines whereby buildup of the sediment will increase the pressure drop, and ultimately blockage of the pipes may take place. It is also a disadvantage in storage tanks, even if these are kept in mild agitation conditions. To overcome such problems, it is essential to induce some weak flocculation in the system or add a n antisettling agent such as a thickener (e.g., high molecular weight polysaccharide).
Influence of Addition of a Nonionic Surfactant (EL1602P). The influence of the addition of a nonionic surfactant, a hexamethylenediamine derivative with 4 tails each of 10 PO and 55 EO units is shown in Figure 3. In this figure the plasticviscosity (r,,~), the extrapolated yield value (OD), the complexmodulus (G”), and the relative sediment volume are plotted as a function of EL1602P concentration (based on the coal). These results are quite different from those obtained with the polyelectrolyte. They show an increase in all rheological parameters with an increase in the surfactant concentration, reaching a maximum at -0.1% (corresponding to about one-third coverage), afier which there is a rapid decrease in rheological parameters with a h r t h e r increase in the surfactant concentration. Thisbehavior may be explained on the basis of the orientation of the adsorbed surfactant molecules a t the solid/liquid i n t e r f a ~ e .On ~ a molecular (12)Heath, D.;Tadros, Th. F. Colloid Polym. Sci. 1983,261,49.
Rheology of Coal I Water Suspensions
Langmuir, Vol. 11, No. 12, 1995 4681
15 r loo
[EL1602Pl/% o n c o a l
Figure 3. Variation of the plastic viscosity, yield value, complex modulus, relative sediment volume, and amount adsorbed with the EL1602P concentration. Table 2. Composition of Synperonic PE Block Copolymers ~_____ Synperonic ;MPO o PE polymer nEO npo MEO LlOl 4 55 361 3250 P104 25 55 2167 3250 P105 37 55 3250 3250 F108 147 55 13000 3250
scale, the coal surface is heterogeneous, containing both hydrophilic (e.g., -C=O and phenolic group^^,^) and hydrophobic regions. Initially the surfactant molecule may preferentially adsorb on the hydrophilic groups via hydrogen bonding between the PEO chains and the -C=O or phenolic groups. This adsorption may be preferred since the adsorption energy in this case is higher than that of the hydrophobic interaction between the hydrocarbon chains. This results in the hydrocarbon chains pointing toward the solution. Hydrophobic interaction between these alkyl groups on various particle surfaces results in flocculation of the suspension, and this results in an increase in rheological parameters. m e r complete saturation of the hydrophilic groups on the particle surface, further addition of surfactant results in their adsorption on the hydrophobic regions of the surface, now with the alkyl groups facing the surface and the PEO chains dangling in solution. There is also the additional possibility of the formation ofbilayers of surfactant molecules. The latter occurs through hydrophobic bonding between the alkyl groups on the surfactant chains, again leaving the PEO chains danglingin solution. This results in steric stabilization, and all rheological parameters show a rapid decrease after the maximum is reached. Thus, a t high nonionic surfactant concentration a deflocculated system is formed. This behavior is also reflected in the sediment volume results shown in Figure 3. These results show a decrease above the surfactant concentration a t which the maximum in rheological parameters is found. Influence of Addition of PEO-PPO-PEO Block Copolymers. The effect of addition of the PEO-PPOPEO block copolymers (composition shown in Table 2) on the stability of the suspensions was investigated from rheological measurements as a function of the volume fraction (4) of the coal. In these experiments, the dispersant concentration was kept constant a t 1%based on the weight of the coal. Figure 4 shows the variation of the complex modulus (G*)with 4 for the various block
o
9 90 -1 l80 0 4
60
50
40 30 20
10
-
i .I4 F108
Ufoxane3a
copolymers studied. For comparison, the results obtained using the polyelectrolyte Ufoxane 3A are shown in the same figure. All the results show the trend expected for concentrated suspensions, namely, a rapid increase in the complex modulus above a critical volume fraction of coal ($J~~). All suspensions were more elastic than viscous ( G > G). q5cr increases with an increase in the EO chain length of the block copolymer. In addition, at any given 4, there is a reduction in the complex modulus with a n increase in the EO chain length of the block copolymer. This behavior shows a systematic reduction in flocculation of the suspension with a n increase in the EO chain length of the block copolymer. The most likely orientation of the block copolymer on the particle surface is with the PPO chain adsorbing on the surface (the PPO chain length of all block copolymers is the same), leaving the two PEO tails dangling in solution. It should be mentioned that the PPO chain has significant polar character, allowing it to adsorb on both hydrophilic and hydrophobic regions
4682 Langmuir, Vol. 11, No. 12, 1995
300
Tadros et al.
3 U f o x a n e 3A
1
200 L l O l P104 P i 0 5 FlOB
(0
----A
P
\
$00 0
I
*
50
20
10
5
2 0.25
0.35
0.45
0.55
@
0.65
Figure 5. Variation of the plastic viscosity with the volume fraction of coal for the various block copolymers and Ufoxane 3A.
of the surface. One would expect the adsorbed layer thickness to increase with a n increase in the PEO chain length. Energy-distance calculations show an attractive minimum whose depth depends on the adsorbed layer thickness.13 With the shortest PEO chain (L101 with two PEO chains of only four units), this minimum would be sufficiently deep to cause extensive flocculation. This explains the rapid increase in the complex modulus a t @ > 0.3. As the PEO chain length is increased, the depth of this minimum becomes smaller and the extent of flocculation is reduced. This is clearly shown from the results with the longer PEO chains, with F108 (which contains 2 chains of 147 EO units) giving the maximum deflocculation. Indeed the G*-@ curve approaches that for the polyelectrolyte. The latter is expected to give the maximum deflocculation as a result of the combined effect of electrostatic and steric repulsion (see above). Figure 5 shows the viscosity (qpl)-@curves for the block copolymers as well as for the polyelectrolyte. These results are distinctly different from those of the complex modulus. The curves are very close to each other, showing a rapid increase in qpl above 4 x 0.45-0.5. Indeed the results for the polyelectrolyte show a rapid increase in qpla t a @value that is lower than that for F108. This behavior is consistent with the effect of shear on the flow units that are produced in a flocculated system. At high shear rates, the flocs are broken into smaller units, and it is likely that these units are very similar for all suspensions. This explains the smaller variation in viscosity between the various systems when compared with the results for the complex modulus. The latter gives a measure of the flocculated structure a t low deformation. Under these conditions, larger differences in the flocculated structure may be distinguished. This illustrates the powerful use of low deformation measurements in assessing the state of the suspension. When measurements are made in the linear viscoelastic region, the floc structure is maintained to a large extent. Influence of Addition of PolyelectrolytdBlock Copolymer Mixtures. In these experiments a 52 wt % (13)Tadros, Th.F.The Effect of Polymers on Dispersion Stability; Academic Press: London, 1982; pp 1-39.
1
l
0
0.1
*
I
0.2
~
0.3
l
'
/
I
'
0.4
0.5 [Synperonic PEI /%
Figure 6. Variation of the complex modulus with the weight fraction of PEO-PPO-PEO block copolymers.
coallwater suspension containing 0.5% (based on the weight of the coal) Ufoxane 3A was used. To this suspension various concentrations of nonionic block copolymers were added. Figure 6 shows the variation of the complex modulus with the weight percent of the nonionic block copolymer. With the exception of F108 (with the longest PEO chain), all other block copolymers have little effect on the complex modulus of the suspension containing the polyelectrolyte alone. The block copolymer F108 that contains 2 PEO chains of 147 units shows a remarkable (and unexpected effect) on the rheology of the suspensions that contain the polyelectrolyte. As shown in Figure 4, both Ufoxane 3A and F108 when used separately are effective in deflocculating the coal suspensions. Addition of F108 to an already stable suspension containing Ufoxane 3A seems to induce flocculation. This effect is also reflected in the sediment volume results shown in Figure 7 which show a n increase in the sediment volume with an increase of the weight fraction of F108. All the other block copolymers show a small effect on the sediment volume. This enhanced flocculation by the F108 block copolymer must be attributed to the chain length of the PEO chain. All block copolymers adsorb on the coal particles, and they show some displacement of the Ufoxane 3A. This is illustrated in Figure 8 which shows the amount adsorbed as a function of the F108 concentration. The adsorption value of Ufoxane 3A decreases from 4.2 to 3.0 mg g-' as the F108 concentration is increased from 0% to 2%. On the other hand, the adsorption of F108 increases rapidly with a n increase of its concentration, reaching a plateau value of 5.8 mg g-l when the polymer concentration is greater than -0.8%. These results imply that the coal surface is not fully saturated with Ufoxane 3A and there must be some uncovered patches on the coal surface. The amount of Ufoxane used (0.5%)was lower than that required for plateau adsorption. Assuming these patches are predominantly hydrophilic in nature, it is likely that the F108 chains adsorb with the PEO chains on the surface, leaving the PPO chain exposed to the bulk solution. This orientation of the block copolymer will cause flocculation as a result of two effects. The first effect is due to the
,
Rheology of Coal I Water Suspensions
Langmuir, Vol. 11, No. 12, 1995 4683
1 L l O l P104 P105 FlO8 ++-A--A-
"sed
0.95
0s25r
0.9
0.20
0.5
.
Po I
1.5 /%
[Floe1 on c o a l
0.85
2
-40
Figure 9. Effect of the concentration of F108 on the relative sediment volume and 1; potential of coal suspensions containing Ufoxane 3A. 0.8
0
0.75
Io
0
0.1
0.2
0.3
0.4
*'a
0.5
0.5%
300
-
1
\
Ufoxane 3A
L
o b
-
a
[Synperonic PEI /%
Figure 7. Variation of the relative sediment volume with the weight fraction of PEO-PPO-PEO block copolymers.
'
400
' I
0.5 '
"
"
"
1 '
"
[ F l o e 1 on
"
1.5 '
c o a l /%
"
"
2"
Figure 8. Effect of the concentration of F108 on the adsorption of Ufoxane 3A and F108.
Figure 10. Complex modulus versus the coal volume fraction at various weight fractions of F108.
screening of the charges on the polyelectrolyte and a shift in the shear plane toward the bulk solution. This effect is confirmed by 5 potential measurements as shown in Figure 9. The 5 potential decreases rapidly as the concentration of F108 is increased. The second effect that causes flocculation of the suspension is the possible hydrophobic interaction between the PPO chain on the particle surfaces. I t should be mentioned, however, that with a further increase in the F108 concentration some restabilization of the suspension occurs. This is shown in Figure 6 which shows a reduction in the modulus after reaching a maximum with a further increase in the F108 concentration. It is more clearly shown in Figure 10, whereby the concentration of F108 was increased to 2%. There is a definite maximum in the sediment volume a t -0.3% F108 (corresponding to about half coverage), after which is continues to decrease with a further increase in the polymer concentration, and a limitingvalue is reached above 1%polymer concentration. This restabilization could be accounted for by the formation of a second layer of polymer which now adsorbs through the PPO chain (by hydrophobic interaction), leaving the PEO chains dangling in solution. "his mechanism of flocculationhestabilization
is not applicable to the other block copolymers with the shorter PEO chains. The shorter PEO chains do not give sufficient layer thickness for screening the charge on the polyelectrolyte or shifting the shear plane. The stability in this case is dominated by the contribution from the polyelectrolyte which gives sufficiently high 5 potentials for electrostatic repulsion and some possibility of steric repulsion from the dangling chains. Further evidence of the above mechanism is obtained from plots of G* versus 4 as shown in Figure 10 at various concentrations of the copolymer. In the absence of the copolymer, the modulus values are low, and they only show a gradual increase with a n increase in 4 At 0.33% and 0.5% F108, G* shows a rapid increase when q5 exceeds -0.4. This rapid increase is more pronounced a t 0.5% F108. However, a t a weight concentration of 0.75%, the G*-# curve is shifted to much lower values and it closely approaches that of the polyelectrolyte. These trends are indicative of flocculationhestabilization of the coal suspension as the block copolymer concentration is increased. The state of flocculation is not, however, reflected in the high shear viscosity measurements shown in Figure 11. The ~ ~ 1 curves -4 a t various block copolymer concentra-
Tadros et al.
4684 Langmuir, Vol. 11, No. 12, 1995 1200 0.75%
m 111
a
I‘
.: 1000 CI
cn
]
pension, but suffers from the problem of rapid sedimentation and formation of dilatant structures. This problem may be alleviated to a large extent by addition of a block copolymer with the optimum chain length and concentration.
Conclusions 800
600
400
200
0
0.35
0.4
0.45
0.5
0.55@ 0.6
Figure 11. Plastic viscosity versus the coal volume fraction at various weight fractions of F108.
tions are close to each other; as discussed above, the flocs are broken into smaller units under high shear conditions. The above results using a mixture of a polyelectrolyte and nonionic block copolymers with long PEO chains could find u s e h l application in optimizing the physical stability of coavwater suspensions. As mentioned before, using the polyelectrolyte alone results in a deflocculated sus-
The flocculated nature ofthe coal slurries a t low Ufoxane concentrations (or in water alone) shows that the surface of the coal was predominately hydrophobic. However, 5 potential measurements as a function of pH demonstrated that the surface was populated with dissociable hydrophilic groups. These results show that the surface of the coal used is heterogeneous. Rheological investigations showed that polyelectrolytes are able to produce colloidally stable coavwater suspensions. Nonionic dispersants have been found to produce either flocculated or relatively well dispersed suspensions, depending on both their concentration and their molecular weight. The flocculated systems showed relatively weak structures that are broken up under shear. Nonionic polymers with sufficiently high molecular weight produce weak flocculation when added to coavwater suspensions stabilized by a polyelectrolyte. These effects may be of some use in the production of practical coavwater slurries (of nature similar to that used in the present paper) which are sufficiently fluid under shear for transport through pipelines, but have sufficient structure under rest to prevent settling.
Acknowledgment. The work described in this paper was supported by the EEC (European Coal and Steel Community) under Contract No. 7220EM202. The authors gratefully acknowledge their financial help. LA9403845
