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Surface chemistry effects on concentrated suspension rheology
Surface Chemistry Effects on Concentrated Suspension Rheology Y E E - K W O N G L E O N G AND DAVID V. BOGER 1 Department of Chemical Engineering, The University of Melbourne, Parkville 3052, Australia
Received April 24, 1989;acceptedAugust 14, 1989 Two brown coals of different surface charge densities and ionic strengths were evaluated for their rheological properties. Rheological behavior ranging from low viscosity Newtonian to high viscosity pseudoplastic yield, depending on the surface chemistry, was observed. By appropriately varying the ionic strength or surface charge density it was possible to convert the rheological behavior of one brown coal to that of the other. Yield stress behavior occurs when the surface charge density is low or the ionic strength is high. Conversely, the suspension is Newtonian at high surface charge density or low ionic strength. A minimum viscosity occurs when the suspension is close to the point of transition from attractive to repulsive particle interaction (i.e., just dispersed). The knowledge gained from this fundamental investigation was subsequently exploited in the development of a coal-water suspension fuel. © 1990 Academic Press, Inc.
INTRODUCTION
Surface chemistry is known to have a very important effect on rheological properties of concentrated suspensions containing a significant content of colloidal material ( 1 ). In particular, ionic strength and surface potential, or charge density, affect the nature and degree of interaction of the colloidal particles which in turn affects the rheological behavior. For instance, the type of flow behavior and electroviscous effects are properties that are strongly influenced by ionic strength and surface charge density. The importance of surface chemistry on the nature and degree of particle interaction is clearly illustrated by the particle interaction theories such as Deryaquin-Landau-VerweyOverbeek ( D L V O ) (2), constant charge, and charge regulation (3), which were developed for two particles only. These theories predict that an attractive interaction leading to flocculation will occur at high ionic strength and low surface charge density which is qualitatively consistent with experimental observation. However, the quantitative validation of 1 To whom all correspondence should be addressed.
these theories cannot be achieved because of experimental limitations. The ~-potential, which is related to the particle surface potential, may be used to deduce the nature of particle interaction. Rapid flocculation has been observed for both A1203 and TiO2 colloids at ~-potentials of magnitude less than 15 mV (4). In cases where the particle shape and size is ill-defined, such as in coal suspensions, the electrophoretic mobility of the particles can be used to give a reasonable description of the shear layer potential. This mobility parameter was used in the present study to correlate with the rheological properties of brown coal suspensions. When the net particle interaction is attractive, aggregates or flocs of particles are formed in the suspension. With very concentrated suspensions, flocculation of particles can result in the formation of a network structure which may encompass the entire volume of the suspension. Such suspensions are usually characterized by a plastic flow behavior, i.e., the suspension has yield stress and highly shearthinning flow characteristics (5). Tsai and Zammouri (6) illustrated with nonaqueous concentrated suspensions that the shear-thin-
249
0021-9797/90 $3.00 Journal of Colloid and Interface Science, Vol. 136, No. 1, April 1990
Copyright © 1990 by Academic Press, Inc. All rights of reproduction in any form reserved.
250
LEONG
AND
ning or pseudoplastic characteristics of the suspension become more pronounced as the attractive or van der Waals interaction between particles increases. It is important to note that the knowledge gained from this study can be applied to other mineral suspensions that have a high colloidal content. The nature of surface charge group and soluble ions may vary from one mineral to another, but the surface chemistry principles governing the rheological properties will be the same for all concentrated colloidal suspensions. MATERIALS
AND
METHODS
The brown coals used were from Morwell and Loy Yang in the LaTrobe Valley, Victoria, Australia. These brown coals are characterized by a very low ash content and a very porous structure saturated with water. The ash content is less than 2 wt% (dry basis) for the Loy Yang coal and about 5% for the Morwell coal. The amount of water residing in the porous structure is the same for both coals, about 60 wt%. As a result of the high moisture content, the as-mined coal has a density of only 1.14 g/ cm 3 whereas the density of the bone dry coal solids is 1.4 g/cm 3. The surface properties of these coals are tabulated in Table 1. Note that the Morwell coal has seven times more carboxylate content (in terms of milliequivalent per gram of dry coal) and a higher inorganics content than that of the Loy Yang coal. The inorganics are cations of carboxylic acid salts and sodium chloride (7). An acid-form Morwell coal was produced when all the carboxylate groups were converted to carboxylic acid
BOGER
and the inorganics were completely removed by repeated washing with single and triple distilled water. The procedure for preparation of acid-form brown coal has been described elsewhere (8). The functional group content in Table 1, namely, phenolic and carboxylic, was determined by the barium exchange method (911 ). In this method the coal is shaken with a barium-containing extraction reagent (a mixture of barium chloride and barium hydroxide solutions) which reacts with the free carboxylic acids, the carboxylates, and, depending upon conditions, the phenolic groups; i.e., barium is ion-exchanged onto the coal at available sites. The exchanged coal is recovered by filtration, washed, and then heated with perchloric acid to return it to the acid condition. The excess acid is determined by back titration and the acid consumed calculated. By varying the content and pH of the extraction reagent and the washing solution, the phenolics may be included or excluded in the exchange procedure. Hence the phenolic and the total carboxylic (plus carboxylate) contents are determined. The carboxylate content of the coal can be separately determined by reacting the coal (not exchanged) directly with perchloric acid and the acid consumed determined by back titration with alkali. In the determination of inorganics, the cations of carboxylic acid salts were ion exchanged with sulfuric acid. The ion-exchanged solution was collected and the concentration of the different cations in the solution was determined by atomic absorption spectroscopy. Note that the Latrobe Valley brown coals do not contain carbonates
TABLE I Brown Coal Surface Properties Functional groups (meq/g dry coal)
Inorganics (% dry mass basis)
Coals
PhOH
COOH
COO-
Fe2+/3+
Ca2+
Mg2+
Na+
CI
MorweU Loy Yang
3.00 2.88
2.45 2.10
0.7 0.1
0.16 0.01
0.51 0.02
0.33 0.07
0.13 0.06
0.08 0.06
Journal of ColloM and Interface Science, Vol. 136, No. 1, April 1990
CHEMISTRY AND RHEOLOGY OF SUSPENSIONS or sulfates (7). All the chlorine in the brown coal is present as chloride (12). The chlorine content was determined according to British Standard method (British Standard 1016: Part 8 ). The particle size distribution of these suspensions, which has already been reported (13), is fairly broad with size ranging from below 1 to 1000 ~m. The suspensions have a significant content of colloidal material as more than 40% by volume of the particulate were less than 10 ~m. It is this colloidal material which is sensitive to surface chemistry changes and determines the rheological behavior of the suspensions. The p H of the coal suspensions was measured with the Orion p H meter of Model 701A. Before the p H measurement, the p H meter was calibrated with buffer solutions of p H 4.0 and 9.0. The coal suspensions were left standing for at least 2 weeks before any p H or flow measurement was conducted. The coal suspension was well stirred before immersing the p H electrode for the measurement. The flow property (i.e., shear stress-shear rate relationship) was characterized by use of a capillary viscometer (14) especially designed to handle time-dependent and settling suspensions. End and wall-slip (inhomogeneity) effects reported by Tsai and Knell ( 15 ) to be present in their capillary flow data of coal suspensions were absent in our measurements (16). Where the sample size of the coal suspension was small the Haake RV3 concentric cylinder viscometer was used. Both capillary and the concentric cylinder viscometers have been shown to give identical results for the flow properties of the same concentrated suspensions (17). The vane rheometer ( 18, 19) was employed to measure the yield stress of the concentrated brown coal suspensions. The electrophoretic mobilities of the coal particles were determined using the Rank Bro. Mark II microelectrophoresis apparatus. In order that the coal particles for the electrophoretic mobilities determination encounter the same ionic strength as that in the suspension, the water of the coal suspension was used
251
for suspending the coal particles for the mobility measurement. This water was collected by filtering the appropriate coal suspension. The electrophoretic mobility was measured on coal particles of size of the order of 1 ~m. The mobilities of coal particles were measured over the p H range between 2 to 8. On the basis of the inorganics content, the ionic strength of the 25-wt% Morwell and the 23-wt% Loy Yang coal suspensions used in the electrophoretic mobilities measurement was estimated to be of the order of 10 -2 and 10 -3 M, respectively. RESULTS AND DISCUSSION
Effects of Brown Coals Figure 1, a logarithmic plot of wall shear stress, Tw ( =~3J~D/4L, where L and D are the length and diameter of the capillary tube and Ap is the pressure difference across it) versus apparent shear rate 8 V/D shows a comparison of the rheological behaviors of as-mined Morwell and Loy Yang coal suspensions at the same solids concentration of 26 wt% or at the same volume fraction of particulates of 0.6 (coal particles and immobilized water residing in the pores). Although the coals have the same particle size distribution and volumetric
10 3
o_ J
S0_ <1
10 2
25.9% wt)
101
I
I
10 2
10 3
Apparent shear rate, 8V/D (s-1 ) FIG. 1. Comparisonof flowpropertiesbetween Morwell and Loy Yang suspensionsat the same solid concentration. Journal of Colloid and Interface Science, VoL 136, No. 1, April 1990
252
LEONG AND BOGER
concentration, their rheological behavior is very different. The Loy Yang suspension exhibits plastic or yield stress flow behavior typical of a flocculated suspension. This is indicated by the flow curve approaching a constant value of shear stress (yield stress) in the low shear rate region. In comparison, the Morwell suspension exhibits Newtonian behavior (constant viscosity or d In rw/d ln(8V/D) = 1.0; the viscosity is defined as the ratio of shear stress to apparent shear rate) and has a much smaller viscosity. In addition to the carboxylate and inorganics content, the natural pH of the suspensions is different, 5.0 for Morwell and 3.6 for Loy Yang. It is important to note that the volume fraction of particulates of 0.6 for both Loy Yang and Morwell coal suspensions was calculated without taking account of coal porosity reduction as a consequence of size reduction by grinding during the suspension preparation. The amount of porosity reduction under such circumstances cannot be determined or estimated accurately (13). Since both brown coals have the same initial porosity and the suspensions produced have the same size distribution, it is therefore reasonable to assume that both coals encountered the same degree of porosity reduction during suspension preparation. The formula of calculating the volume fraction of suspended particulates ~,bsis 4~s = 7.14c/(3.26 - c),
[1]
where c is the weight fraction of solids in the coal suspension. The other parameters required in the derivation of the formula are the solid fraction of as-mined coal which is 0.4 and the densities of the as-mined coal and bone-dry coal solids. The effect of pH on the flow properties for a 25.5 wt% Morwell coal suspension is shown in Fig. 2. At the natural pH of 5.0, the suspension is Newtonian. On decreasing the pH with HC1 (pH = 4.2, 3.4), the suspensions exhibit a pronounced plastic behavior. The yield stress and viscosity increase with decreasing pH as illustrated by the suspensions with pH values of 3.4 and 4.2. On the other Journal of Colloid and Interface Science, Vol. 136,No. 1, April 1990
pH 3.4 10 2 ._1 0 o.0.,
4.2
6.0 101
10 0
Morwell coal (25.5 wt% solids)
I
I
I
10 2
103
10 4
Apparent shear rate, 8V/D (s-1 ) FIG. 2. Effect of pH on the flow properties of a 25.5wt% Morwell suspension.
hand, when the pH is increased with NaOH (pH = 5.8, 6.0), the suspension remains Newtonian but exhibits a higher viscosity. The relationship between rheological properties (yield stress and viscosity) and surface properties (electrophoretic mobility), shown in Fig. 3 for a 25.5 wt% Morwell and a 23 wt% Loy Yang coal suspension, provides a more detailed understanding of the factors responsible for the different rheological properties of the two coals. Extrapolating the mobility-pH curve in Fig. 3 to zero mobility or zero surface charge gives an isoelectric point of about 2.0 for both the MorweU and Loy Yang coals. Woskoboenko (20) also obtained a value of 2.0 for the IEP of Morwell coal. A similar value has been obtained for the Yallourn coal, another LaTrobe Valley brown coal (21 ). An isoelectric point of 2.0 is consistent with the carboxylate group being the main surface charge group (22). The repulsive component of the particle interactions is nonexistent at zero surface charge and the net particle interaction should be attractive. Above the pH of 2.0, the mobility acquires a finite value and increases with pH. The repulsive component is restored and increases with pH. At low pH, or low mobility, the suspension has a high yield
CHEMISTRY AND RHEOLOGY OF SUSPENSIONS I
• i
• Morwell
"~ 80
• Loy Yang
Q_
60 -o 40
>- 20 v
7
'~ o (D
5
3 I
I
I I I I I I
-3
i
.~ > - 2 ~r
o~
•/
I I t t I f O A
F
~"
I Natural slurry pH
',
uJ L
2
I I i
4 pH
I
1
6
8
FIG. 3. Correlation between rheological and surface propertiesfor (•) 25.5-wt%Morwelland (•) 23-wt%Loy Yang suspension. (Apparent viscositycalculatedat 10 sfor the Loy Yang coal and 100 s -~ for the Morwellcoal.)
stress and viscosity. As the repulsive component increases with increasing pH, the suspension exhibits progressive deflocculation as shown by the decrease in yield stress and viscosity with increasing pH. Complete deflocculation, indicated by the absence of a yield stress shown in Fig. 3, occurs at a pH of 4.2 for the Loy Yang coal and 5.0 for Morwell coal. At the deflocculation pH, the viscosity is also at the minimum. For both coals, the too-
253
bility at complete deflocculation is about - 2 . 0 ~m. s -1 • V 1 ° cm. Beyond the deflocculation pH, the viscosity of the suspension increases again but the rheological behavior remains unaffected, i.e., still Newtonian. The mobility-pH curve for the Morwell coal shown in Fig. 3 is below that for the Loy Yang coal. This probably means that the ionic strength of the aqueous phase of the Morwell coal suspension is higher ( 1 ) which is consistent with the higher inorganic content of the Morwell coal shown in Table 1. Therefore, Morwell coal has a higher ionic strength and is not flocculated whereas Loy Yang coal has a smaller ionic strength and is flocculated. For such behavior to be possible, the Morwell coal must have a higher surface charge density in order to compensate for the higher ionic strength. The presence of a higher surface charge density is supported by a higher carboxylate content (seven times that of the Loy Yang coal) and a higher mobility (1) at the natural suspension pH. The mobility is -2.1 t~m- s -1 • V -I • cm for the 25.5-wt% Morwell coal and - 1.8 for the 23-wt% Loy Yang coal. Moreover, the higher deflocculation pH of the Morwell coal is an indication of the higher ionic strength requiring a higher surface charge density to maintain a repulsive particle interaction. Apart from electrophoretic mobility, the nature of particle interactions can be deduced from the properties of the settling suspension, in particular the settled layer (22-24). A flocculated suspension has a settled layer which is soft and loosely constructed, has a large amount of entrapped water and is easily redispersed by shear. On the other hand, the settled layer of an unflocculated suspension is compact, has a small amount of entrapped water, and is difficult to redisperse in shear. The 25.6% Loy Yang coal suspension had a soft settled layer with a solid concentration of only 27 wt%. The net particle interaction for the 25.6% Loy Yang coal suspension is therefore attractive. In contrast, the Morwell suspension had a compact settled layer with a solid concentration of 37 wt% which is very Journal of Colloid and Interface Science, Vol. 136, No. 1, April 1990
254
LEONG AND BOGER O laoy Yang (as mined) • Loy Yang (NaOH added) • Morwell (as mined)
c~
b
dr,\.
101
& 8 m ,>
10 0
pH = 5.6 C = 23.0.
0~ <
& 10 0 =k
_ p H =3.6 u~2.6 1% wt)
Coal • • v []
p, >
"~
"°°~
Loy Yang (As-mined, pH = 3.6) Morwell (As-mined, pH = 5.3) Morwe[I (HCl added, pH = 3~4) Morwell (NaCI added)
,-k~vv~ ~ u~
-•.~
~ - ~
10-1
10 -1
o. < C = 25.8 10 -2 i 100
I 101
I 102
I 103
10-2
I 101
100
Apparent shear rate, 8 V / D (s-1 )
I 102
I 103
Apparent shear rate, 8V/O (s -1 )
FIG. 4a. Flow behavior of Loy Yang coal is converted to that of the Morwell coal by increasing the pH. FIG. 4b. Flow behavior of the Morwell coal is converted to that of Loy Yang coal by decreasing the pH and by increasing the ionic strength.
high considering that the as-mined coal solid concentration is only 40 wt%. The net particle interaction for the 25.8% Morwell coal suspension is therefore repulsive. The results above clearly show that the relative amount of the surface charge density to ionic strength determines the rheological behavior of concentrated brown coal suspensions. It should therefore be possible to convert the rheological properties of one brown coal to another by appropriately varying the surface chemistry. The plastic flow behavior of the 23 wt% Loy Yang coal suspension was converted to Newtonian behavior when its surface charge was increased by increasing the p H of the suspension from 3.6 to 5.6. An as-mined Morwell suspension with about the same solid concentration and p H (5.3) is also Newtonian. These results are shown in Fig. 4a. Conversely, flocculation occurs when the surface charge density of the Morwell suspension is decreased by addition of acid. The plastic flow behavior of a 25.5% Morwell suspension with a lower p H of 3.4, shown in Fig. 4b, is similar to that for the 25.6% as-mined Loy Yang suspension (pH 3.6). Apart from decreasing the suspension pH, the coal particles can be flocculated by increasing the ionic strength of the suspension. Figure 4b shows that the 25.9% Morwell SUSJournal of Colloid and Interface Science, Vol. 136, No. 1, April 1990
pension acquires a plastic behavior after NaC1 (0.0056 g / g of suspension) is added.
Effects of Salts Additional results showing the effect of the nature and concentration of added salt on the rheological properties of Morwell suspensions are shown in Figs. 5 and 6. Three salts, CaC12, MgC12, and NaC1, were employed for this study. Note that Ca 2+, Mg 2÷, and Na + arc the three most c o m m o n cations in brown coal. Salt solutions of concentration ranging from 0.01 to 1.0 M w e r e prepared for each salt. The
50
Mg 2+ Ca 2+ "° '--
A--
40 ~
Na +
•
~" 30 ___ •-~
20
>-
10
U-
0.0
0.25
0.50
Morwell coal (24% wt) • Mg 2+ [] Ca 2+
L
I
0.75
1.0
Salt solution concentration (rnol/I or M)
FIG. 5. Dependence of yield stresson NaC1,MgCI2,and
CaC12concentrations for a 24-wt% Morwell suspension.
C H E M I S T R Y A N D R H E O L O G Y OF SUSPENSIONS
255
untreated Morwell coal suspension used had "~ Acid-form J J I an initial solid concentration of 32.8 wt%. The suspension was divided into smaller samples So,,ds 10 2 and a prepared salt solution was added to each ° sample. The a m o u n t of salt solution added 31.5 • was 0.31 g / g of suspension and the resultant suspension obtained had a solids concentration of about 24 wt%. Note that the contriN • bution to the ionic strength of the suspension N from the inherent salt and inorganic content I I I 101 102 103 of the coal is not known, so only the concentration of added salt is quoted. Apparent shear rate, 8V/D (s-1 ) The dependence of yield stress on the conFIG. 7. Dependence of flow properties on the solid concentration of CaC12, MgC12, and NaC1 salts is' centration for the acid-form Morwell coal. shown in Fig. 5. It can be seen from Fig. 5 that the yield stress is absent for the untreated Morwell suspension and also for the suspenthe untreated coal suspension and that treated sion treated with 0.01 M NaC1 solution. For with 0.01 MNaC1 are Newtonian. Suspensions all three salts, it was observed that the yield treated with higher NaC1 concentration, such stress increases rather rapidly with increasing as 0.1, 0.5, and 1.0 M, exhibit plastic behavior. salt concentration and reaches a plateau value The yield stress and viscosity increase with inat a higher salt concentration. The yield stresscreasing NaC1 concentration and this is shown salt concentration curves for CaCI2 and MgC12 by the flow curves moving up the stress axis. are identical and are located above that for Similar behavior was observed for the CaCleNaC1. treated suspension except for two differences. The effects of CaC12 and NaC1 concentraFirst, the suspension treated with 0.01 MCaC12 tion on the flow behavior for a 24-wt% Morsolution is already plastic. Second, the viscosity well coal suspension are presented in Fig. 6. and yield stress of the CaCle-treated suspenConsistent with the absence of a yield stress, sion appear to be always higher than those of the suspension treated with NaC1 of the same equivalent salt concentration; i.e., the conSalt solution centration of CaC12 used should be half that concentration for NaC1. Hence a divalent cation is a more (moles/litre, M) 0.5 effective flocculant than a monovalent cation. 102 . , _ ~ a - , ~ 1.0 & 0.5 This result is consistent with the general prin0.05 ~ 01 "~ ~ 0 . ciples of colloid stability.
/..-'.////
y//,
. ~ 101
0.0
.
_c
Jill .Jw.11"-
Morwell coal (24.0% wt) • Na el added zx Ca CI2added
,/~-
10°
0.01 I
I
I
101
102
103
Shear rate, ~ (s-I) FIG. 6. Dependence of flow behavior on NaC1 and CaC12 concentration for a 24-wt% Morwell suspension.
Acid-Form Brown Coal (Morwell) Suspension An acid-form brown coal was prepared with the objective of obtaining a coal free of inherent inorganic and metal carboxylate. The flow properties of the acid-form Morwell coal suspension, determined as a function of solids concentration, are shown in Fig. 7. The suspension is Newtonian up to a solids concenJournal of Colloid andlnterface Science. VoL 136,No. 1, April 1990
256
LEONG AND BOGER
tration of about 25 wt%. Between the concentrations of 25 to 30% solids, the suspension is pseudoplastic and beyond 30%, the suspension is plastic. Although the pH of the suspension is extremely acidic (pH 2.7), the suspension does not exhibit flow behavior typical of a strongly flocculated suspension generally found for the as-mined coal suspension with low pH values. In fact, the rheological properties are similar to the as-mined Morwell suspension over the same range of concentration despite their large difference in pH (pH 5.0 for as-mined Morwell coal) ( 13, 16). The absence of a flow behavior typical of a flocculated suspension for the acid-form coal suspension (>30 wt%) suggests that the low surface charge density obtained from the dissociation of the carboxylic acid is sufficient to maintain a repulsive particle interaction. From the suspension pH, it was estimated that the carboxylate content obtained from the dissociation of the carboxylic acid is about 0.006 m e q / g of dry coal for a 25 wt% suspension. This is only possible because the ionic strength is very low after the removal of inorganics. By increasing the ionic strength it is expected that the suspension should become flocculated. Flocculation was observed when NaC1 salt was added to a 28% acid-form suspension. NaC1 was added at C1- concentration similar to that present in the as-mined coal. The flocculated flow behavior of the NaC1 modified acid-form coal is illustrated in Fig. 8. The effects of salt and pH on the rheological behavior of the acid-form Morwell coal were also evaluated. In this study a large sample of the acid-form Morwell coal suspension was
103
Acid-formed Morwell suspension treated with Na CI
Solids (wt%) 28.1
E10 2
1
I
102
101 ~,(S -1 )
FIG. 8. Flow behavior of acid-form Morwell coal treated with NaC1. Journal of Colloid and Interface Science, Vol. 136, No. 1, April 1990
pH •
10 3 v(3_
• 4.5
10 2
03
2.4 z~ 2.5 O 3,6 • 5.0 n 5.5 •
101
5.9
[]
/
~ I , ~ / e m I
I
I
101
10 2
10 3
Trueshearrate,~ (s-1) FIG. 9. Effects of salt and pH on the flow behavior of acid-form brown coal (Morwell) coal suspensions.
treated with CaC12 solution until the C1- content achieved was similar to that present in the as-mined coal. The calcium salt was chosen because Ca 2+ is often the most abundant cation in the brown coal. The suspension was then divided into six equal samples and each sample was treated with different amount of NaOH while maintaining the solids concentration of all samples constant at about 26 wt%. The rheological properties of these suspensions are shown in Fig. 9. In addition, the properties of these suspensions including their flow behavior are tabulated in Table 2. The pH of the suspensions achieved ranged from 2.4 to 5.9. The range ofcarboxylate content calculated from the reaction with N a O H was between 0.043 and 0.86 m e q / g of dry coal. On addition of CaCI2, the suspension pH decreased from 2.7 to 2.4. This is due to the displacement of the acidic proton of the undissociated carboxylic acid by the Ca 2+. Apart from the decrease in pH, the suspension became plastic, i.e., flocculated. On increasing the pH from 2.4 to 4.5, the suspension showed progressive deflocculation. This is indicated by the progressive downward displacement of the flow curves along the stress axis with increasing pH. The suspension with pH of 4.5 is Newtonian although it was observed to form a gel
CHEMISTRY AND RHEOLOGY OF SUSPENSIONS
257
TABLEII Properties of SuspensionsDescribed in Fig. 9 Solids (wt %)
Calculated carboxylate content (meq/g dry coal)
pH
Flow behavior
State (overnight)
26.2 26.2 26.2 26.2 26.1 25.6 26.3
CaC12added 0.043 0.16 0.32 0.43 0.67 0.86
2.4 2.5 3.6 4.5 5.0 5.5 5.9
Plastic Plastic Plastic Newtonian Newtonian Newtonian Newtonian
Gel Gel Gel Gel Settled Settled Settled
on standing, which indicates that the net particle interaction is still attractive. This suspension may still be plastic which should be observable if flow data at low enough shear rates were available. The suspension with pH of 5.0 is Newtonian and has formed a hard or compact settled layer which is a clear indication that the particle interaction now is repulsive (22). Complete deflocculation therefore occurs between the pH of 4.5 to 5.0. The suspension with pH of 5.9 is also Newtonian, has a higher viscosity and a compact settled layer. The minimum viscosity occurs at the transition from attractive to repulsive interaction. The variation of flow behavior with pH shown in Fig. 9 is generally similar to that obtained for the as-mined Morwell and Loy Yang coals. The precise mechanism for the increase in suspension viscosity when the pH was increased from 5.0 to 5.9 is not clear. However, it is a well-known fact that the enhancement in repulsive interaction between particles also results in an increase in the suspension viscosity. This is known as the second electroviscous effect ( 1 ) which describes the increase in suspension viscosity based on double-layer interactions. When the thickness of the double layer increases, the degree of overlap between the double layer also increases, thus resulting in greater repulsive interactions. The doublelayer thickness can be increased by reducing the electrolyte content of the suspension. In the case of brown coal suspensions, the double layer thickness is not increased but the surface
charge density is increased when the suspension pH was raised from 5.0 to 5.9. Nevertheless a higher degree of repulsive interaction between the coal particles is anticipated as a result of the higher surface charge density. Further work is required to fully understand the mechanism for the increase in suspension viscosity of brown coal suspension beyond the deflocculation pH. All previous results showing the dependence of rheological behavior on the relative magnitude of surface charge density and ionic strength were obtained either by changing the suspension pH or by adding or removing soluble ions. The results in Fig. 9 have added significance because they were obtained by adding salt and changing the suspension pH in the same set of experiments. Here, it was observed that the CaCI2 salt flocculates the suspension and that on increasing the pH or carboxylate content, the suspension deflocculates. It is therefore established beyond doubt that the rheological behavior of the concentrated brown coal suspensions is controlled by the relative magnitude of carboxylate to inorganic content. CONCLUSIONS It has been shown that the rheological properties of brown coal suspensions depend strongly on the interparticle interaction. An attractive interaction results in a flocculated suspension which exhibits yield, pseudoplastic Journal of Colloid and Interface Science, Vol. 136, No. 1, April 1990
258
LEONG AND BOGER
behavior. However, a repulsive interaction gives rise to a dispersed suspension with no yield stress and essentially Newtonian flow behavior. The interparticle interaction in brown coal suspensions is dependent upon two factors. First, the surface charge density of the particles due to the presence of carboxylate ions, and second, the ionic strength of the suspending medium due to the presence of inherent soluble salt and the cations associated with carboxylate. These two factors have different values for each coal suspension. A suspension of Loy Yang coal has a low ionic strength but flocculates readily due to a low surface charge density. Thus these suspensions show pseudoplastic and yield stress behavior. In contrast, a Morwell coal suspension of the same concentration has a high ionic strength but is dispersed due to a high surface charge density; thus it displays Newtonian behavior. The rheological properties of a brown coal suspension can be altered by changing the interparticle interaction. This can be achieved by changing the surface charge density, by adjusting the pH of the solution, or by changing the ionic strength of the suspending medium. A minimum in the suspension viscosity is observed when the pH and ionic strength are such that the suspension is just dispersed. Adjustment in the surface chemistry of brown coal suspensions can be exploited for economic gain in the processing of brown coal suspensions (25). ACKNOWLEDGMENTS Y. K. Leong expresses his gratitude to the Coal Corp. of Victoria for funding this work and The University of Melbourne and Eugene Singer for providing scholarships. We thank Professor T. Healy and Dr. F. Grieser of the Department of Physical Chemistry for many helpful discussions and for making available equipment for this work. Finally, we extend our gratitude to Dr. A. Jones of the Department of Chemical Engineering for providing constructive comments on this paper.
Journal of Colloidand InterfaceScience, Vol. 136,No. 1, April1990
REFERENCES 1. Hunter, R. J., in "Zeta Potential in Colloid Science." Academic Press, New York, 1981. 2. Verwey, E. J. W., and Overbeek, J. Th. G., in "Theory of the Stability of Hydrophilic Colloids." Elsevier, Amsterdam, 1948. 3. Healy, T. W., Chan, D., and White, L. R., Pure Appl. Chem. 52, 1207 (1980). 4. Wiese, G. R., and Healy, T. W., J. Colloid Interface Sci. 51, 427 (1975). 5. Wildemuth, C. R., and Williams, M. C., Rheol. Acta 24, 75 (1985). 6. Tsai, S. C., and Zammouri, K., J. Rheol. 32, 737 (1988). 7. Kiss, L. T., and King, T. N., Fuel58, 547 (1979). 8. Murray, J. B., Fuel52, 105 (1973). 9. Schafer, H. N. S., Fuel49, 271 (1970). 10. Schafer, H. N. S., Fuel49, 197 (1970). 11. Van Krevelen, D. W., in "Coal," p. 160. Elsevier, Amsterdam, 1961. 12. Durie, R. A., Fuel40, 407 (1961). 13. Leong, Y. K., Creasy, D. E., Boger, D. V., and Nguyen, Q. D., Rheol. Acta 29, 299 (1987). 14. Want, F. M., Colombera, P. M., Nguyen, Q. D., and Boger, D. V., Hydrotransport 8, 249 (1982). [Johannesburg.] 15. Tsai, S. C., and Knell, E. W., Fuel65, 566 (1986). 16. Leong, Y. K., in "The Rheology of Unmodified and Modified Victorian Brown Coal Suspensions," PhD thesis, The University of Melbourne, Australia, 1988. 17. Renehan, M. J., Snow, R. J., Leong, Y. K., and Boger, D. V., "Chemeca '87," Paper 69, Melbourne, August 1987. 18. Nguyen, Q. D., and Boger, D. V., J. Rheol. 27, 321 (1983). 19. Nguyen, Q. D., and Boger, D. V., J. Rheol. 29, 335 (1985). 20. Woskoboenko, F., in "Rheology of Brown Coal Slurties," PhD thesis, The University of Melbourne, Australia, 1985. 21. Moil, S., Hara, T., Aso, K., and Okamoto, H., Powder TechnoL 40, 161 (1984). 22. Shaw, D. J., in "Introduction to Colloid and Surface Chemistry," Butterworths, London, 1970. 23. Overbeek, J. Th. G., in "Colloid Science" (H. R. Kruyt, Ed.), Vol. 1, p. 337. Elsevier, Amsterdam, 1952. 24. Van Olphen, H., in "An Introduction to Clay Colloids Chemistry." Wiley, New York, 1977. 25. Boger, D. V., Leong, Y. K., Mainwaring, D. E., and Christie, G. B., Inst. Chem. Eng. Symp. Ser. 107, 1(1987).
Received April 24, 1989;acceptedAugust 14, 1989 Two brown coals of different surface charge densities and ionic strengths were evaluated for their rheological properties. Rheological behavior ranging from low viscosity Newtonian to high viscosity pseudoplastic yield, depending on the surface chemistry, was observed. By appropriately varying the ionic strength or surface charge density it was possible to convert the rheological behavior of one brown coal to that of the other. Yield stress behavior occurs when the surface charge density is low or the ionic strength is high. Conversely, the suspension is Newtonian at high surface charge density or low ionic strength. A minimum viscosity occurs when the suspension is close to the point of transition from attractive to repulsive particle interaction (i.e., just dispersed). The knowledge gained from this fundamental investigation was subsequently exploited in the development of a coal-water suspension fuel. © 1990 Academic Press, Inc.
INTRODUCTION
Surface chemistry is known to have a very important effect on rheological properties of concentrated suspensions containing a significant content of colloidal material ( 1 ). In particular, ionic strength and surface potential, or charge density, affect the nature and degree of interaction of the colloidal particles which in turn affects the rheological behavior. For instance, the type of flow behavior and electroviscous effects are properties that are strongly influenced by ionic strength and surface charge density. The importance of surface chemistry on the nature and degree of particle interaction is clearly illustrated by the particle interaction theories such as Deryaquin-Landau-VerweyOverbeek ( D L V O ) (2), constant charge, and charge regulation (3), which were developed for two particles only. These theories predict that an attractive interaction leading to flocculation will occur at high ionic strength and low surface charge density which is qualitatively consistent with experimental observation. However, the quantitative validation of 1 To whom all correspondence should be addressed.
these theories cannot be achieved because of experimental limitations. The ~-potential, which is related to the particle surface potential, may be used to deduce the nature of particle interaction. Rapid flocculation has been observed for both A1203 and TiO2 colloids at ~-potentials of magnitude less than 15 mV (4). In cases where the particle shape and size is ill-defined, such as in coal suspensions, the electrophoretic mobility of the particles can be used to give a reasonable description of the shear layer potential. This mobility parameter was used in the present study to correlate with the rheological properties of brown coal suspensions. When the net particle interaction is attractive, aggregates or flocs of particles are formed in the suspension. With very concentrated suspensions, flocculation of particles can result in the formation of a network structure which may encompass the entire volume of the suspension. Such suspensions are usually characterized by a plastic flow behavior, i.e., the suspension has yield stress and highly shearthinning flow characteristics (5). Tsai and Zammouri (6) illustrated with nonaqueous concentrated suspensions that the shear-thin-
249
0021-9797/90 $3.00 Journal of Colloid and Interface Science, Vol. 136, No. 1, April 1990
Copyright © 1990 by Academic Press, Inc. All rights of reproduction in any form reserved.
250
LEONG
AND
ning or pseudoplastic characteristics of the suspension become more pronounced as the attractive or van der Waals interaction between particles increases. It is important to note that the knowledge gained from this study can be applied to other mineral suspensions that have a high colloidal content. The nature of surface charge group and soluble ions may vary from one mineral to another, but the surface chemistry principles governing the rheological properties will be the same for all concentrated colloidal suspensions. MATERIALS
AND
METHODS
The brown coals used were from Morwell and Loy Yang in the LaTrobe Valley, Victoria, Australia. These brown coals are characterized by a very low ash content and a very porous structure saturated with water. The ash content is less than 2 wt% (dry basis) for the Loy Yang coal and about 5% for the Morwell coal. The amount of water residing in the porous structure is the same for both coals, about 60 wt%. As a result of the high moisture content, the as-mined coal has a density of only 1.14 g/ cm 3 whereas the density of the bone dry coal solids is 1.4 g/cm 3. The surface properties of these coals are tabulated in Table 1. Note that the Morwell coal has seven times more carboxylate content (in terms of milliequivalent per gram of dry coal) and a higher inorganics content than that of the Loy Yang coal. The inorganics are cations of carboxylic acid salts and sodium chloride (7). An acid-form Morwell coal was produced when all the carboxylate groups were converted to carboxylic acid
BOGER
and the inorganics were completely removed by repeated washing with single and triple distilled water. The procedure for preparation of acid-form brown coal has been described elsewhere (8). The functional group content in Table 1, namely, phenolic and carboxylic, was determined by the barium exchange method (911 ). In this method the coal is shaken with a barium-containing extraction reagent (a mixture of barium chloride and barium hydroxide solutions) which reacts with the free carboxylic acids, the carboxylates, and, depending upon conditions, the phenolic groups; i.e., barium is ion-exchanged onto the coal at available sites. The exchanged coal is recovered by filtration, washed, and then heated with perchloric acid to return it to the acid condition. The excess acid is determined by back titration and the acid consumed calculated. By varying the content and pH of the extraction reagent and the washing solution, the phenolics may be included or excluded in the exchange procedure. Hence the phenolic and the total carboxylic (plus carboxylate) contents are determined. The carboxylate content of the coal can be separately determined by reacting the coal (not exchanged) directly with perchloric acid and the acid consumed determined by back titration with alkali. In the determination of inorganics, the cations of carboxylic acid salts were ion exchanged with sulfuric acid. The ion-exchanged solution was collected and the concentration of the different cations in the solution was determined by atomic absorption spectroscopy. Note that the Latrobe Valley brown coals do not contain carbonates
TABLE I Brown Coal Surface Properties Functional groups (meq/g dry coal)
Inorganics (% dry mass basis)
Coals
PhOH
COOH
COO-
Fe2+/3+
Ca2+
Mg2+
Na+
CI
MorweU Loy Yang
3.00 2.88
2.45 2.10
0.7 0.1
0.16 0.01
0.51 0.02
0.33 0.07
0.13 0.06
0.08 0.06
Journal of ColloM and Interface Science, Vol. 136, No. 1, April 1990
CHEMISTRY AND RHEOLOGY OF SUSPENSIONS or sulfates (7). All the chlorine in the brown coal is present as chloride (12). The chlorine content was determined according to British Standard method (British Standard 1016: Part 8 ). The particle size distribution of these suspensions, which has already been reported (13), is fairly broad with size ranging from below 1 to 1000 ~m. The suspensions have a significant content of colloidal material as more than 40% by volume of the particulate were less than 10 ~m. It is this colloidal material which is sensitive to surface chemistry changes and determines the rheological behavior of the suspensions. The p H of the coal suspensions was measured with the Orion p H meter of Model 701A. Before the p H measurement, the p H meter was calibrated with buffer solutions of p H 4.0 and 9.0. The coal suspensions were left standing for at least 2 weeks before any p H or flow measurement was conducted. The coal suspension was well stirred before immersing the p H electrode for the measurement. The flow property (i.e., shear stress-shear rate relationship) was characterized by use of a capillary viscometer (14) especially designed to handle time-dependent and settling suspensions. End and wall-slip (inhomogeneity) effects reported by Tsai and Knell ( 15 ) to be present in their capillary flow data of coal suspensions were absent in our measurements (16). Where the sample size of the coal suspension was small the Haake RV3 concentric cylinder viscometer was used. Both capillary and the concentric cylinder viscometers have been shown to give identical results for the flow properties of the same concentrated suspensions (17). The vane rheometer ( 18, 19) was employed to measure the yield stress of the concentrated brown coal suspensions. The electrophoretic mobilities of the coal particles were determined using the Rank Bro. Mark II microelectrophoresis apparatus. In order that the coal particles for the electrophoretic mobilities determination encounter the same ionic strength as that in the suspension, the water of the coal suspension was used
251
for suspending the coal particles for the mobility measurement. This water was collected by filtering the appropriate coal suspension. The electrophoretic mobility was measured on coal particles of size of the order of 1 ~m. The mobilities of coal particles were measured over the p H range between 2 to 8. On the basis of the inorganics content, the ionic strength of the 25-wt% Morwell and the 23-wt% Loy Yang coal suspensions used in the electrophoretic mobilities measurement was estimated to be of the order of 10 -2 and 10 -3 M, respectively. RESULTS AND DISCUSSION
Effects of Brown Coals Figure 1, a logarithmic plot of wall shear stress, Tw ( =~3J~D/4L, where L and D are the length and diameter of the capillary tube and Ap is the pressure difference across it) versus apparent shear rate 8 V/D shows a comparison of the rheological behaviors of as-mined Morwell and Loy Yang coal suspensions at the same solids concentration of 26 wt% or at the same volume fraction of particulates of 0.6 (coal particles and immobilized water residing in the pores). Although the coals have the same particle size distribution and volumetric
10 3
o_ J
S0_ <1
10 2
25.9% wt)
101
I
I
10 2
10 3
Apparent shear rate, 8V/D (s-1 ) FIG. 1. Comparisonof flowpropertiesbetween Morwell and Loy Yang suspensionsat the same solid concentration. Journal of Colloid and Interface Science, VoL 136, No. 1, April 1990
252
LEONG AND BOGER
concentration, their rheological behavior is very different. The Loy Yang suspension exhibits plastic or yield stress flow behavior typical of a flocculated suspension. This is indicated by the flow curve approaching a constant value of shear stress (yield stress) in the low shear rate region. In comparison, the Morwell suspension exhibits Newtonian behavior (constant viscosity or d In rw/d ln(8V/D) = 1.0; the viscosity is defined as the ratio of shear stress to apparent shear rate) and has a much smaller viscosity. In addition to the carboxylate and inorganics content, the natural pH of the suspensions is different, 5.0 for Morwell and 3.6 for Loy Yang. It is important to note that the volume fraction of particulates of 0.6 for both Loy Yang and Morwell coal suspensions was calculated without taking account of coal porosity reduction as a consequence of size reduction by grinding during the suspension preparation. The amount of porosity reduction under such circumstances cannot be determined or estimated accurately (13). Since both brown coals have the same initial porosity and the suspensions produced have the same size distribution, it is therefore reasonable to assume that both coals encountered the same degree of porosity reduction during suspension preparation. The formula of calculating the volume fraction of suspended particulates ~,bsis 4~s = 7.14c/(3.26 - c),
[1]
where c is the weight fraction of solids in the coal suspension. The other parameters required in the derivation of the formula are the solid fraction of as-mined coal which is 0.4 and the densities of the as-mined coal and bone-dry coal solids. The effect of pH on the flow properties for a 25.5 wt% Morwell coal suspension is shown in Fig. 2. At the natural pH of 5.0, the suspension is Newtonian. On decreasing the pH with HC1 (pH = 4.2, 3.4), the suspensions exhibit a pronounced plastic behavior. The yield stress and viscosity increase with decreasing pH as illustrated by the suspensions with pH values of 3.4 and 4.2. On the other Journal of Colloid and Interface Science, Vol. 136,No. 1, April 1990
pH 3.4 10 2 ._1 0 o.0.,
4.2
6.0 101
10 0
Morwell coal (25.5 wt% solids)
I
I
I
10 2
103
10 4
Apparent shear rate, 8V/D (s-1 ) FIG. 2. Effect of pH on the flow properties of a 25.5wt% Morwell suspension.
hand, when the pH is increased with NaOH (pH = 5.8, 6.0), the suspension remains Newtonian but exhibits a higher viscosity. The relationship between rheological properties (yield stress and viscosity) and surface properties (electrophoretic mobility), shown in Fig. 3 for a 25.5 wt% Morwell and a 23 wt% Loy Yang coal suspension, provides a more detailed understanding of the factors responsible for the different rheological properties of the two coals. Extrapolating the mobility-pH curve in Fig. 3 to zero mobility or zero surface charge gives an isoelectric point of about 2.0 for both the MorweU and Loy Yang coals. Woskoboenko (20) also obtained a value of 2.0 for the IEP of Morwell coal. A similar value has been obtained for the Yallourn coal, another LaTrobe Valley brown coal (21 ). An isoelectric point of 2.0 is consistent with the carboxylate group being the main surface charge group (22). The repulsive component of the particle interactions is nonexistent at zero surface charge and the net particle interaction should be attractive. Above the pH of 2.0, the mobility acquires a finite value and increases with pH. The repulsive component is restored and increases with pH. At low pH, or low mobility, the suspension has a high yield
CHEMISTRY AND RHEOLOGY OF SUSPENSIONS I
• i
• Morwell
"~ 80
• Loy Yang
Q_
60 -o 40
>- 20 v
7
'~ o (D
5
3 I
I
I I I I I I
-3
i
.~ > - 2 ~r
o~
•/
I I t t I f O A
F
~"
I Natural slurry pH
',
uJ L
2
I I i
4 pH
I
1
6
8
FIG. 3. Correlation between rheological and surface propertiesfor (•) 25.5-wt%Morwelland (•) 23-wt%Loy Yang suspension. (Apparent viscositycalculatedat 10 sfor the Loy Yang coal and 100 s -~ for the Morwellcoal.)
stress and viscosity. As the repulsive component increases with increasing pH, the suspension exhibits progressive deflocculation as shown by the decrease in yield stress and viscosity with increasing pH. Complete deflocculation, indicated by the absence of a yield stress shown in Fig. 3, occurs at a pH of 4.2 for the Loy Yang coal and 5.0 for Morwell coal. At the deflocculation pH, the viscosity is also at the minimum. For both coals, the too-
253
bility at complete deflocculation is about - 2 . 0 ~m. s -1 • V 1 ° cm. Beyond the deflocculation pH, the viscosity of the suspension increases again but the rheological behavior remains unaffected, i.e., still Newtonian. The mobility-pH curve for the Morwell coal shown in Fig. 3 is below that for the Loy Yang coal. This probably means that the ionic strength of the aqueous phase of the Morwell coal suspension is higher ( 1 ) which is consistent with the higher inorganic content of the Morwell coal shown in Table 1. Therefore, Morwell coal has a higher ionic strength and is not flocculated whereas Loy Yang coal has a smaller ionic strength and is flocculated. For such behavior to be possible, the Morwell coal must have a higher surface charge density in order to compensate for the higher ionic strength. The presence of a higher surface charge density is supported by a higher carboxylate content (seven times that of the Loy Yang coal) and a higher mobility (1) at the natural suspension pH. The mobility is -2.1 t~m- s -1 • V -I • cm for the 25.5-wt% Morwell coal and - 1.8 for the 23-wt% Loy Yang coal. Moreover, the higher deflocculation pH of the Morwell coal is an indication of the higher ionic strength requiring a higher surface charge density to maintain a repulsive particle interaction. Apart from electrophoretic mobility, the nature of particle interactions can be deduced from the properties of the settling suspension, in particular the settled layer (22-24). A flocculated suspension has a settled layer which is soft and loosely constructed, has a large amount of entrapped water and is easily redispersed by shear. On the other hand, the settled layer of an unflocculated suspension is compact, has a small amount of entrapped water, and is difficult to redisperse in shear. The 25.6% Loy Yang coal suspension had a soft settled layer with a solid concentration of only 27 wt%. The net particle interaction for the 25.6% Loy Yang coal suspension is therefore attractive. In contrast, the Morwell suspension had a compact settled layer with a solid concentration of 37 wt% which is very Journal of Colloid and Interface Science, Vol. 136, No. 1, April 1990
254
LEONG AND BOGER O laoy Yang (as mined) • Loy Yang (NaOH added) • Morwell (as mined)
c~
b
dr,\.
101
& 8 m ,>
10 0
pH = 5.6 C = 23.0.
0~ <
& 10 0 =k
_ p H =3.6 u~2.6 1% wt)
Coal • • v []
p, >
"~
"°°~
Loy Yang (As-mined, pH = 3.6) Morwell (As-mined, pH = 5.3) Morwe[I (HCl added, pH = 3~4) Morwell (NaCI added)
,-k~vv~ ~ u~
-•.~
~ - ~
10-1
10 -1
o. < C = 25.8 10 -2 i 100
I 101
I 102
I 103
10-2
I 101
100
Apparent shear rate, 8 V / D (s-1 )
I 102
I 103
Apparent shear rate, 8V/O (s -1 )
FIG. 4a. Flow behavior of Loy Yang coal is converted to that of the Morwell coal by increasing the pH. FIG. 4b. Flow behavior of the Morwell coal is converted to that of Loy Yang coal by decreasing the pH and by increasing the ionic strength.
high considering that the as-mined coal solid concentration is only 40 wt%. The net particle interaction for the 25.8% Morwell coal suspension is therefore repulsive. The results above clearly show that the relative amount of the surface charge density to ionic strength determines the rheological behavior of concentrated brown coal suspensions. It should therefore be possible to convert the rheological properties of one brown coal to another by appropriately varying the surface chemistry. The plastic flow behavior of the 23 wt% Loy Yang coal suspension was converted to Newtonian behavior when its surface charge was increased by increasing the p H of the suspension from 3.6 to 5.6. An as-mined Morwell suspension with about the same solid concentration and p H (5.3) is also Newtonian. These results are shown in Fig. 4a. Conversely, flocculation occurs when the surface charge density of the Morwell suspension is decreased by addition of acid. The plastic flow behavior of a 25.5% Morwell suspension with a lower p H of 3.4, shown in Fig. 4b, is similar to that for the 25.6% as-mined Loy Yang suspension (pH 3.6). Apart from decreasing the suspension pH, the coal particles can be flocculated by increasing the ionic strength of the suspension. Figure 4b shows that the 25.9% Morwell SUSJournal of Colloid and Interface Science, Vol. 136, No. 1, April 1990
pension acquires a plastic behavior after NaC1 (0.0056 g / g of suspension) is added.
Effects of Salts Additional results showing the effect of the nature and concentration of added salt on the rheological properties of Morwell suspensions are shown in Figs. 5 and 6. Three salts, CaC12, MgC12, and NaC1, were employed for this study. Note that Ca 2+, Mg 2÷, and Na + arc the three most c o m m o n cations in brown coal. Salt solutions of concentration ranging from 0.01 to 1.0 M w e r e prepared for each salt. The
50
Mg 2+ Ca 2+ "° '--
A--
40 ~
Na +
•
~" 30 ___ •-~
20
>-
10
U-
0.0
0.25
0.50
Morwell coal (24% wt) • Mg 2+ [] Ca 2+
L
I
0.75
1.0
Salt solution concentration (rnol/I or M)
FIG. 5. Dependence of yield stresson NaC1,MgCI2,and
CaC12concentrations for a 24-wt% Morwell suspension.
C H E M I S T R Y A N D R H E O L O G Y OF SUSPENSIONS
255
untreated Morwell coal suspension used had "~ Acid-form J J I an initial solid concentration of 32.8 wt%. The suspension was divided into smaller samples So,,ds 10 2 and a prepared salt solution was added to each ° sample. The a m o u n t of salt solution added 31.5 • was 0.31 g / g of suspension and the resultant suspension obtained had a solids concentration of about 24 wt%. Note that the contriN • bution to the ionic strength of the suspension N from the inherent salt and inorganic content I I I 101 102 103 of the coal is not known, so only the concentration of added salt is quoted. Apparent shear rate, 8V/D (s-1 ) The dependence of yield stress on the conFIG. 7. Dependence of flow properties on the solid concentration of CaC12, MgC12, and NaC1 salts is' centration for the acid-form Morwell coal. shown in Fig. 5. It can be seen from Fig. 5 that the yield stress is absent for the untreated Morwell suspension and also for the suspenthe untreated coal suspension and that treated sion treated with 0.01 M NaC1 solution. For with 0.01 MNaC1 are Newtonian. Suspensions all three salts, it was observed that the yield treated with higher NaC1 concentration, such stress increases rather rapidly with increasing as 0.1, 0.5, and 1.0 M, exhibit plastic behavior. salt concentration and reaches a plateau value The yield stress and viscosity increase with inat a higher salt concentration. The yield stresscreasing NaC1 concentration and this is shown salt concentration curves for CaCI2 and MgC12 by the flow curves moving up the stress axis. are identical and are located above that for Similar behavior was observed for the CaCleNaC1. treated suspension except for two differences. The effects of CaC12 and NaC1 concentraFirst, the suspension treated with 0.01 MCaC12 tion on the flow behavior for a 24-wt% Morsolution is already plastic. Second, the viscosity well coal suspension are presented in Fig. 6. and yield stress of the CaCle-treated suspenConsistent with the absence of a yield stress, sion appear to be always higher than those of the suspension treated with NaC1 of the same equivalent salt concentration; i.e., the conSalt solution centration of CaC12 used should be half that concentration for NaC1. Hence a divalent cation is a more (moles/litre, M) 0.5 effective flocculant than a monovalent cation. 102 . , _ ~ a - , ~ 1.0 & 0.5 This result is consistent with the general prin0.05 ~ 01 "~ ~ 0 . ciples of colloid stability.
/..-'.////
y//,
. ~ 101
0.0
.
_c
Jill .Jw.11"-
Morwell coal (24.0% wt) • Na el added zx Ca CI2added
,/~-
10°
0.01 I
I
I
101
102
103
Shear rate, ~ (s-I) FIG. 6. Dependence of flow behavior on NaC1 and CaC12 concentration for a 24-wt% Morwell suspension.
Acid-Form Brown Coal (Morwell) Suspension An acid-form brown coal was prepared with the objective of obtaining a coal free of inherent inorganic and metal carboxylate. The flow properties of the acid-form Morwell coal suspension, determined as a function of solids concentration, are shown in Fig. 7. The suspension is Newtonian up to a solids concenJournal of Colloid andlnterface Science. VoL 136,No. 1, April 1990
256
LEONG AND BOGER
tration of about 25 wt%. Between the concentrations of 25 to 30% solids, the suspension is pseudoplastic and beyond 30%, the suspension is plastic. Although the pH of the suspension is extremely acidic (pH 2.7), the suspension does not exhibit flow behavior typical of a strongly flocculated suspension generally found for the as-mined coal suspension with low pH values. In fact, the rheological properties are similar to the as-mined Morwell suspension over the same range of concentration despite their large difference in pH (pH 5.0 for as-mined Morwell coal) ( 13, 16). The absence of a flow behavior typical of a flocculated suspension for the acid-form coal suspension (>30 wt%) suggests that the low surface charge density obtained from the dissociation of the carboxylic acid is sufficient to maintain a repulsive particle interaction. From the suspension pH, it was estimated that the carboxylate content obtained from the dissociation of the carboxylic acid is about 0.006 m e q / g of dry coal for a 25 wt% suspension. This is only possible because the ionic strength is very low after the removal of inorganics. By increasing the ionic strength it is expected that the suspension should become flocculated. Flocculation was observed when NaC1 salt was added to a 28% acid-form suspension. NaC1 was added at C1- concentration similar to that present in the as-mined coal. The flocculated flow behavior of the NaC1 modified acid-form coal is illustrated in Fig. 8. The effects of salt and pH on the rheological behavior of the acid-form Morwell coal were also evaluated. In this study a large sample of the acid-form Morwell coal suspension was
103
Acid-formed Morwell suspension treated with Na CI
Solids (wt%) 28.1
E10 2
1
I
102
101 ~,(S -1 )
FIG. 8. Flow behavior of acid-form Morwell coal treated with NaC1. Journal of Colloid and Interface Science, Vol. 136, No. 1, April 1990
pH •
10 3 v(3_
• 4.5
10 2
03
2.4 z~ 2.5 O 3,6 • 5.0 n 5.5 •
101
5.9
[]
/
~ I , ~ / e m I
I
I
101
10 2
10 3
Trueshearrate,~ (s-1) FIG. 9. Effects of salt and pH on the flow behavior of acid-form brown coal (Morwell) coal suspensions.
treated with CaC12 solution until the C1- content achieved was similar to that present in the as-mined coal. The calcium salt was chosen because Ca 2+ is often the most abundant cation in the brown coal. The suspension was then divided into six equal samples and each sample was treated with different amount of NaOH while maintaining the solids concentration of all samples constant at about 26 wt%. The rheological properties of these suspensions are shown in Fig. 9. In addition, the properties of these suspensions including their flow behavior are tabulated in Table 2. The pH of the suspensions achieved ranged from 2.4 to 5.9. The range ofcarboxylate content calculated from the reaction with N a O H was between 0.043 and 0.86 m e q / g of dry coal. On addition of CaCI2, the suspension pH decreased from 2.7 to 2.4. This is due to the displacement of the acidic proton of the undissociated carboxylic acid by the Ca 2+. Apart from the decrease in pH, the suspension became plastic, i.e., flocculated. On increasing the pH from 2.4 to 4.5, the suspension showed progressive deflocculation. This is indicated by the progressive downward displacement of the flow curves along the stress axis with increasing pH. The suspension with pH of 4.5 is Newtonian although it was observed to form a gel
CHEMISTRY AND RHEOLOGY OF SUSPENSIONS
257
TABLEII Properties of SuspensionsDescribed in Fig. 9 Solids (wt %)
Calculated carboxylate content (meq/g dry coal)
pH
Flow behavior
State (overnight)
26.2 26.2 26.2 26.2 26.1 25.6 26.3
CaC12added 0.043 0.16 0.32 0.43 0.67 0.86
2.4 2.5 3.6 4.5 5.0 5.5 5.9
Plastic Plastic Plastic Newtonian Newtonian Newtonian Newtonian
Gel Gel Gel Gel Settled Settled Settled
on standing, which indicates that the net particle interaction is still attractive. This suspension may still be plastic which should be observable if flow data at low enough shear rates were available. The suspension with pH of 5.0 is Newtonian and has formed a hard or compact settled layer which is a clear indication that the particle interaction now is repulsive (22). Complete deflocculation therefore occurs between the pH of 4.5 to 5.0. The suspension with pH of 5.9 is also Newtonian, has a higher viscosity and a compact settled layer. The minimum viscosity occurs at the transition from attractive to repulsive interaction. The variation of flow behavior with pH shown in Fig. 9 is generally similar to that obtained for the as-mined Morwell and Loy Yang coals. The precise mechanism for the increase in suspension viscosity when the pH was increased from 5.0 to 5.9 is not clear. However, it is a well-known fact that the enhancement in repulsive interaction between particles also results in an increase in the suspension viscosity. This is known as the second electroviscous effect ( 1 ) which describes the increase in suspension viscosity based on double-layer interactions. When the thickness of the double layer increases, the degree of overlap between the double layer also increases, thus resulting in greater repulsive interactions. The doublelayer thickness can be increased by reducing the electrolyte content of the suspension. In the case of brown coal suspensions, the double layer thickness is not increased but the surface
charge density is increased when the suspension pH was raised from 5.0 to 5.9. Nevertheless a higher degree of repulsive interaction between the coal particles is anticipated as a result of the higher surface charge density. Further work is required to fully understand the mechanism for the increase in suspension viscosity of brown coal suspension beyond the deflocculation pH. All previous results showing the dependence of rheological behavior on the relative magnitude of surface charge density and ionic strength were obtained either by changing the suspension pH or by adding or removing soluble ions. The results in Fig. 9 have added significance because they were obtained by adding salt and changing the suspension pH in the same set of experiments. Here, it was observed that the CaCI2 salt flocculates the suspension and that on increasing the pH or carboxylate content, the suspension deflocculates. It is therefore established beyond doubt that the rheological behavior of the concentrated brown coal suspensions is controlled by the relative magnitude of carboxylate to inorganic content. CONCLUSIONS It has been shown that the rheological properties of brown coal suspensions depend strongly on the interparticle interaction. An attractive interaction results in a flocculated suspension which exhibits yield, pseudoplastic Journal of Colloid and Interface Science, Vol. 136, No. 1, April 1990
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behavior. However, a repulsive interaction gives rise to a dispersed suspension with no yield stress and essentially Newtonian flow behavior. The interparticle interaction in brown coal suspensions is dependent upon two factors. First, the surface charge density of the particles due to the presence of carboxylate ions, and second, the ionic strength of the suspending medium due to the presence of inherent soluble salt and the cations associated with carboxylate. These two factors have different values for each coal suspension. A suspension of Loy Yang coal has a low ionic strength but flocculates readily due to a low surface charge density. Thus these suspensions show pseudoplastic and yield stress behavior. In contrast, a Morwell coal suspension of the same concentration has a high ionic strength but is dispersed due to a high surface charge density; thus it displays Newtonian behavior. The rheological properties of a brown coal suspension can be altered by changing the interparticle interaction. This can be achieved by changing the surface charge density, by adjusting the pH of the solution, or by changing the ionic strength of the suspending medium. A minimum in the suspension viscosity is observed when the pH and ionic strength are such that the suspension is just dispersed. Adjustment in the surface chemistry of brown coal suspensions can be exploited for economic gain in the processing of brown coal suspensions (25). ACKNOWLEDGMENTS Y. K. Leong expresses his gratitude to the Coal Corp. of Victoria for funding this work and The University of Melbourne and Eugene Singer for providing scholarships. We thank Professor T. Healy and Dr. F. Grieser of the Department of Physical Chemistry for many helpful discussions and for making available equipment for this work. Finally, we extend our gratitude to Dr. A. Jones of the Department of Chemical Engineering for providing constructive comments on this paper.
Journal of Colloidand InterfaceScience, Vol. 136,No. 1, April1990
REFERENCES 1. Hunter, R. J., in "Zeta Potential in Colloid Science." Academic Press, New York, 1981. 2. Verwey, E. J. W., and Overbeek, J. Th. G., in "Theory of the Stability of Hydrophilic Colloids." Elsevier, Amsterdam, 1948. 3. Healy, T. W., Chan, D., and White, L. R., Pure Appl. Chem. 52, 1207 (1980). 4. Wiese, G. R., and Healy, T. W., J. Colloid Interface Sci. 51, 427 (1975). 5. Wildemuth, C. R., and Williams, M. C., Rheol. Acta 24, 75 (1985). 6. Tsai, S. C., and Zammouri, K., J. Rheol. 32, 737 (1988). 7. Kiss, L. T., and King, T. N., Fuel58, 547 (1979). 8. Murray, J. B., Fuel52, 105 (1973). 9. Schafer, H. N. S., Fuel49, 271 (1970). 10. Schafer, H. N. S., Fuel49, 197 (1970). 11. Van Krevelen, D. W., in "Coal," p. 160. Elsevier, Amsterdam, 1961. 12. Durie, R. A., Fuel40, 407 (1961). 13. Leong, Y. K., Creasy, D. E., Boger, D. V., and Nguyen, Q. D., Rheol. Acta 29, 299 (1987). 14. Want, F. M., Colombera, P. M., Nguyen, Q. D., and Boger, D. V., Hydrotransport 8, 249 (1982). [Johannesburg.] 15. Tsai, S. C., and Knell, E. W., Fuel65, 566 (1986). 16. Leong, Y. K., in "The Rheology of Unmodified and Modified Victorian Brown Coal Suspensions," PhD thesis, The University of Melbourne, Australia, 1988. 17. Renehan, M. J., Snow, R. J., Leong, Y. K., and Boger, D. V., "Chemeca '87," Paper 69, Melbourne, August 1987. 18. Nguyen, Q. D., and Boger, D. V., J. Rheol. 27, 321 (1983). 19. Nguyen, Q. D., and Boger, D. V., J. Rheol. 29, 335 (1985). 20. Woskoboenko, F., in "Rheology of Brown Coal Slurties," PhD thesis, The University of Melbourne, Australia, 1985. 21. Moil, S., Hara, T., Aso, K., and Okamoto, H., Powder TechnoL 40, 161 (1984). 22. Shaw, D. J., in "Introduction to Colloid and Surface Chemistry," Butterworths, London, 1970. 23. Overbeek, J. Th. G., in "Colloid Science" (H. R. Kruyt, Ed.), Vol. 1, p. 337. Elsevier, Amsterdam, 1952. 24. Van Olphen, H., in "An Introduction to Clay Colloids Chemistry." Wiley, New York, 1977. 25. Boger, D. V., Leong, Y. K., Mainwaring, D. E., and Christie, G. B., Inst. Chem. Eng. Symp. Ser. 107, 1(1987).