Journal of Scientific & Industrial Research Vol. 58, August 1999, pp 607-615

Rheological Characteristics of Aqueous Ca-Bentonite Dispersions*

T Tulun and N Gungor

Technical University of Istanbul , Faculty of Science and Letters, Maslak 80626, Istanbul , Turkey

Received: 15 December 1998; accepted: 17 May 1999

The rheological properties of Ca-bentonite suspensions in different volume concentrations have been studied as a function of pH and the concentraiton of added electrolytes. In addition, the conductance of suspensions was determined. The flow behaviour of suspensions with and without containing electrolytes fitted the Bingham plastic flow. The yield stress of suspensions, showed that the transition process from viscous flow to plastic flow required relatively high shear stress for the given volume concentrations. The suspension exhibited a shear-thinning behaviour in their native pH conditions. Below the pH value of 3, the stability of edge to face contact decreased. Apparent viscosity (11 ) values of suspensions were strongly pH dependent as plastic viscosity (l1p) values were nearly constant in the selected pH conditions. Apparent viscosity of suspensions exhibited different behaviour depending on the type of electrolytes. Although the highest increase in the specific conductivity was obtained with large amount of KCI addition, in case of NaCI addition, specific conductivity was increased largely more than in the KCI addition with increasing volume concentrations.

Introduction Rheological characteristics of bentonite dispersions in

water may change ranging from a laminar flow to plastic flow, thus the suspensions can be paste and slurry. These rheological characteristics play an important role in in-dustrial applications . 1-3

Rheological properties of various bentonites differ so widely depending on morphological and chemical char-acteristics of the smectites which are clay minerals with three layers, Smectite minerals45 in Ca-bentonite can form quasicrystals due to the interlayer hydration com-plexes of divalent interlayer cations such as Mg2+ and Ca2+. The thickness of the smectite quasicrystallites in a sus-pension varies closely with the experimental conditions, such as pH6-7 ,exchangeable cations8-9 and hydrodynamic forces 10. Besides, the basal faces of smectite particles carry permanent negative charges due to the ionic substitution within the layers while the electrical charges at edges of smectite platelets vary with pH, the type, the concentra-tion of added electrolytes II and the presence of organic additives l2 .The pH conditions of smectite dispersion in water may modify the charge sign on the edges of a smectite platelet. On the other hand, the electrolyte addi-

"This paper is dedicated to 75 th Anniversary of Republic of Turkey

tion to a smectite suspension compresses the electrical double layers on the particle surface and different ex-changeable cations induce particle association. Due to these effects the structure of flow network may change from true viscous flow to plastic flow. Russel and Israelachvili 13-14 explained that the modes of particle in-teraction that govern the flow network can be either by long-range interaction or association phenomenon .

The aim of the present study is to review the factors such as pH, electrolytes and concentration which greatly affect the viscosity and flow behaviour of Ca-bentonite suspensIOns.

Materials and Methods

A minerologically purified Ca-bentonite sample was obtained from the north-west part of Turkey. Minerological and chemical analysis of samples were carried out by X-ray diffraction (Philips PW 1140/90 instrument) and wet chemistry and atomic absorption spectroscopy. X-ray diffraction data and the result of chemical analYSIS are shown in Figure I and Table 1. Silicon and calcium con-tent of the sample were carried out by gravimetry and EDTA titrations respectively. The rest of the elements were determined by an atomic absorption spectrophotometer (Perkin-Elmer, model 3030). In addition , percentage of

608 J SCIINO RES VOL 58 AUGUST 1999

1750

1500

1000

soo

Figure 1 - X-ray di ffrac tion pattern of Ca-bentonite

Table 1- Chcmical Analysis of Ca-benlonile

Oxides Si02 62.80

AlP ..

9.00

Fep.1 TiOl MgO CaO Nap Kp 0.8

IL

16.1 w/w. % 2.2 0.5 1.8 4.6 1.0

w / w,% of exchangeable cations after removal of 1.1 0.8 0.05

IL: Ignition Loss, Relati ve Precision: 0.09-1.4 ppl soluble salts

exchangeable cations was determined for 100 g Ca-ben-tonite after the removal of soluble sa lts (NaCI, NaN0l' N~S04' KCI) as well as the free carbonates (calcite, dolo-mite) found in the bentonite (Table I) before the addition

of e lectrolytes.

The electro lytes (NaCI, KCI , KOH) and all the other chemicals used were Merck. The weighted amounts of Ca-bentonite were dispersed in the proper amount of dis-tilled water by mechanical stirring in order to prepare sus-pensions in different volume concentration ; then all sus-pensions were a llowed to stand fo r a 24 h period. Con-ductance measurements of suspensions were also carried out with Extech, pH-conducti vity + TDS meter.

Rheological Measurements

Apparent and plastic viscosity of sllspensions were determined with a Fann type viscometer (Baroid, model 35 SA). Figure 2 show the variations in the (11 ) apparent and plasti c ('l p) vi scosities as a function of concentra-tions (v/v, %).

The shear stress ('! ) - shear rate (y) rheograrns of sus-pensions for various concent rat ions (

TULUN & GUNGOR AQUEOUS CA-BENTONITE DISPERSIONS 609

20

18

16 VI

'14 o A(.pa~ v,iscos i ty 9 ni • P ashc vIscosity I a.. / E 12 / - /

.?: 10 /

I VI I 0 u 8 VI > 6

4

2

2 4 6 8 10 12 14 16 18

Concentrat ion, v /"

Figu re 2 - Vari ation of apparent viscosity (11) and pl ast ic viscosit y with (111') volume concentration of suspenslOm.

16000

14 000

a 12000 0. '

E ~. 10 0

100 200 JOO "00 SOO 600 700 800 900 1000

Figure 3 - The shear stress -shear rate rheograms or

610 J SCIIND RES VOL 58 AUGUST 1999

8000

", 6000 0.. E

4.000

2000

o 2 4 6 8 10 pH

Figure 5. Variation of shear stress with pH for CP2' CPv CP4 suspensions, W5.6 %, cpJ:8.4 %, W10.9 %

B o

II> 6 '" Cl.. E

~4

2

0 0 2 4 6 10

pH

Figure 6 - Variation of apparent viscosity (11) with pH cpor CP2' CPJ' CP4 suspensions, CP2:5.6 %, cpJ:8.4 %, CP4: 10.9 %

ticular network . However, Ca-bentonite which contains Ca-smectite as a major mineral, in general, gives face to face network (band-like structure). It can be changed by a small variation in the composition of the medium drasti-cally.

In Figure 2, apparent viscosity and plastic viscosity were plotted as a function of various volume fractions. The

4600

4400

4200

4000

380

3600

3400

3200

3000

'" 2800

Cl.. E 2600 >.

...... 2400

2200

2000

1800

1600

1400

1200

~----r-----r'-----'I-----'I-----'I----2 4 6 a 10

pH

Figure 7 - Variation of yield stress (t y) with pH for cP~, CP" CP4 suspensions, (jJ2:5.6 %, CP3:8.4 %, CP4: 10.9 %

values of apparent viscosity exhibit a linear increase until the volume fraction of 4 (10.9 per cent ), but a deviation from linearity occurs at the volume concentration of 5 ( 13.2 per cent ) due to particle characteristics such as morphology, size, surface area, etc., and palticle interac-tions as we ll as the structure of suspension . Also, the admixed material s certainly exerts an influence, but the factors given above are found to be predominant .

As shown in Table 2 the yield stresses of suspensions increase with increasing volume concentrations. Experi-mental yield values are the extrapolated shear stress of 't=/("y) curve. The calculated values of yield stress were

found with th e equation s of TJ " = e N 2 - e N I and . T y = e NI -71" is where Ni and N2 are 600 and 300 cycle min-i respectively, and 11" is the plastic viscosi ty. The extrapolated and the calculated yie ld stress show a good

TULUN & GUNGOR AQUEOUS CA-BENTONITE DISPERSIONS 61 I

Table 2 - Variation of yield stress with di fferent volume

entl y under the shea'r stress applied.

The viscosi ty of benton i te-water system is strongl y pH dependent. For the bentonite di spers ion in water, Laga ly6 explained that due to the irregular shape of parti cles and lamellae, low pH va lue, that is high concentration of hy-drogen ions, increase the charge density at the particle

Figure 8. Rheogram of

612 J SCI INO RES VOL 58 AUGUST 1999

8

7 III

It) 6 a..

E . 5 It)

c-4

3

2

o --,---.--,--.,---" --~I---'I ------~ 0 "1 234567

Electrolyte , ~ in ~ 2

Figure 9 - 11 " = f (e lectro lyte, %) cur ve of

TULUN & GUNGOR AQUEOUS CA-BENTONITE DISPERSIONS 613

Table 3 - Thixotropy conditions of suspensions

Shear rate, 2' (5.6) 2' (8.4) 2' ( 10.9 )

I S·I e N 11., mPas N 11" mPas N 11., mPas 5.10 1.5 150.0 5.0 500.0 5.3 525.0

10.2 2.0 100.0 6.0 318.0 5.5 275 .0

170.3 3.0 9.0 7.0 21.0 7.5 22.5

340.6 3.5 5.3 8.0 12.0 8.5 12.7

510.9 6.0 6.0 8.5 8.5 12.0 12.0

1021.8 8.0 4.0 11.0 5.5 16.0 8.0

*pH Value of suspensions are 8-8 .2, eN: Dial reading, 11. : Apparent viscosity.

22

20

18

16

VI 14 ~

Q..

E 12 ~

c' 10

8

6

4

KCI

NaCI

KOH

O+-~--.--.--.--.--.-.-----o 1 2 345 6 7

;

Electrolyte, % in ~)

Figure 10 - 11" = f (e lectrolyte , %) curve of (j)) suspens ions «j)): 8.4, %)

particle size, increases the interaction between particles, and then the gelation proceeds. As a consequence, in the present study such large increment of the apparent vis-cosity indicates that KCI addition to the system is more effective on gelation than other electrolytes .

The difference between the effect of KCI and KOH electrolytes on the apparent viscosity arises from the dif-ferent characters of their anions. Due to the higher mobil-ity of C}- ions than OR ions, dissociation of particles into

14

12 KCI NaC!

VI

:u 10 Q..

E 8 ru

c-6

4 KOH

2

0 0 2 3 4 5 6 ' 7

Electrolyte, % in q,4

Figure II - 11" = f (electrolyte, %) curVe of (j)4 suspensions (W 10.9, % )

small platelets increases with above explained reason, and the tendency of gelation increases with C}- ions.

The acid addition to the system during pH adjusment also exhibits the same effect as electrolytes do due to the adsorption of protons and anions on the particle sUlface and edge respectively. The adsorbed ions cause screening effect on the electrostatic forces, diminishing the charge, and thus the decreasing attraction between the particles and destroying card-house structure .

614 J SCI IND RES VOL 58 AUGUST 1999

. X ms .c m-I

10000

9500

9000

8500

8000

7~iOO

7000

6S00

6000

5500

50 00

4500

40 00

3500

3000

2500

2000

1500

0 --r- I

2 3 4 I 5

I 6

-7 Electrol yte, %

Figure 12 - Spec ific conducti vity K = f (electrolyte, %) curve of

TULUN & GUNGOR AQUEOUS CA-BENTONITE DISPERSIONS 615

pensions with KCl addition of 7 per cent. The increment in conductivity becomes 5.26 fold of the given suspen-sion of without KCI.

The NaCI addition to suspensions in all concentrations increases the specific conductivity largely as the volume concentration increases. The reason is that Na+ ions ex-change cations between the layersl9 and thus the free ex-changeable cations increase the conductivity. The pres-ence of free cations in liquid medium causes the particle attraction, and therefore the card-house structure is main-tained6. '7.22 . The increment of specific conductivity with

increasing per centage of NaCI in suspension is not as much as KCI. The mobility of K+ ions is larger than Na+ ions . The stepwise hydration reaction of the both ions is exothermic and the ionic radius of K+ ions is larger than Na+ ions. Standard enhalpy changes for the hydration of K+ ions is higher than that of Na+ ions4 . Therefore, K+ ions displace the exchangeable cations found between the layers more easily than Na+ ions, and finally increasing free cations enhance the conductivity of liquid medium.

Conclusions Hoffman22 attributed the plastici ty of clay particles

dispersed in water med ium to a card-house structure. The stmctural network between the particles of a sllspension is a lways in question depending on the composition of the medium so that even small alteration in the composi-tion, the flow behaviour, and hence the paIticle network of a suspension, can change drastically5 .

In the present study, the flow behaviour of suspensions ex hibits Bingham features and their network structure depends greatly on the pH conditions of the system as well as the volume concentration of solid particles .Tn the concentrated suspensions, increas ing pH decreases the edge to face contact, and thus the stability of card-house network diminishes.

The viscosity of the studied system is strongly pH de-pendent. The linear increase of apparent viscosity and yield stress values between the certain pH values sug-gests that the stability of edge to face contact decreases and possibly band-like stmcture begins to increase with increasing pH values. The increase in pH seems to frag-ment the card-house network. Particularly for Ca-smectite particles, aggregation begins to occur with more face to face association than edge to face association of parti-cles.

I The Bingham body of suspens ions is maintained with the electrolyte addition and also the size and number of

aggregates are enhanced. The most pronounced increase in the apparent viscosity is obtained by KCl addition be-cause of the high mobility of Cl- ions as well as K+ ions. With increasing particle concentration face to face ag-gregation progresses, and the gelation tendency of the system increases due to, mainly. the increased thickness of the face to face associated sheets of particles.

Specific conductivity of suspensions decreases with in-creasing particle concentration in liquid medium due to the charge interaction between the particles.

As a consequence, a stability is reached in the flow behaviour of the studied system. This stability occurs even though the network structure of flow changes with the addition of different electrolyte, and a considerable plas-ticity is encountered with increasing pH. An easy flow is obtained in the deflocculated condition so that in oil well drilling, easy flow is required for the proper ci rcu lation with the available pumping equipment at relative ly high clay concentration.

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