F

NASA TECHNICAL MEMORANDUM

NASA TM-77363

RHEOLOGICAL STUDY OF CLAY-KAOLIN AQUEOUS SUSPENSIONS

Ro Lapasin and Eo Lucchinl

Translation of "studio Reologico di Sospensioni Acquose di

Argilla/Caolino" Ceramurgia, Volume ii, Number 2, 1981,

pp. 57-61.

(_ASA-T_-773_3) RdEOLGGICAL STUDY OF

CLA¥-KkOLIN K_U_OUS SUSPEhSIO_S (National

_erona_txc_ and Space Administr_tioa) 17 p

HC A02/_ A01 CSC& 07D

G3/25

N8_-23711

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

WASHINGTON, D.C. MARCH 1984
J_

/

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|. Rspe,f No.

NASA TM-77363

Tltlo_d_btltlo

RHEOLOGICAL STUDY OF

AQUEOUS SUSPENSIONS

2. Government Accession No.

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C LAY- K_:OL IN

SlrANDARD TITLE PAGE

3'. Reoipient'e Ceteleo No.

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19a4

,ll •,

4. Per|strain00,9onisoticn Code

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7. _me,(,) R. Lapasin and _. Lucchini.

Institute of _ppl_ed ,and Industrial

Chemistry of the University of Studies

. of Triestet Tri'ester Italy .

9. Pe,form:n 90rgmiaQt,en Ncme and A4drcs8

• L_o Kanner _ssociates

..Redwood City, California 94063

12. ;;9onsorin0 Agency Name and Address" ._ .

National Aeronautics and Space Administration, Washington, D.C. 20546

J ,,

_5. Supplementers, Me_es

Translation .of "Studio Re_logico di Sospensioni

Argilla/Caoli.no", Ceramurgia, Volume ii, Number

pp. 57-61.

L Peflemdn90,0eaiHlien Repo_t Me.

10. Work Un_!He.

||. Con_rect o_ G_ont No.

NASw-3 541

13. _ypeel _epe,t0.dPeriod¢_c,._

Translation

14. Sponsoring; Agency, Co_e

Acquose di

2, 1981,

_. *b,,,_,

Reological characteristics of. clay-kaolin aqueous suspensions

were studied by a rotational v.iscometer .to correlate their

behavior with the'propertieS of ceramic slips for. casting

containing quartz,' f_id_pars, and other nonplastic materiels.

In particular, the'effects of the different amounts of dry

(mixture "lil" of Na2CO3 and Na2SiO3

shear-time-dependent properties of

m_terf_is and deflo'cculant

and o_ temperatures on the

suspensions were e x.amined:

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RHEOLOGICAL STUDY OF CLAY-KAOLIN AQUEOUS SUSPENSIONS

R. Lapasin and E. Lucchini

Institute of Applied and Industrial Chemistry

University of Studies of Trieste, Trieste, Italy

i. Introduction

The property of clay to give colloidal aqueous suspensions is

widely used by the ceramic industry. These suspensions to which kaolin,

fluxes and finely crushed quartz as well as deflocculants are added are

placed in suitable gypsum molds so as to obtain, after hardening and

drying, the object of desired shape.

/5__/7*

The rheological properties of such slips, which are deeply affected

by the dose of the different components, influence considerably the

casting speed. On the other hand, possible changes in their cQmposition,

meant for accelerating this phase of the production process, may alter

the drying speed and some characteristics of the objects such as, for

example, the crude mechanical resistance in the crude slate and the

appearance of the surfaces.

In general the slips are non-Newtonian inasmuch as the viscosity

depends on the speed gradient to which the material is exposed and on

the theological history of the material itself. Consequently it is necessary to know their characteristics depending on shear and time and the

parameters affecting them.

This is extremely useful in planning and determining the dimensions

of the spray dryer, unless drastic simplifications are adopted such as

that of disregarding the losses of load inside the centrifugal nozzles

or resorting to empirical types of relationships whose applications are

difficult to suspend suspensions with different contents of dry material and deflocculant. Finally the paper industry which uses kaolin as

*Numbers in the margin indicate pagination in the foreign text.

kw
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"filler" applied by means of a wet process is involved in very similar

problems.

All these considerations lead us to implementing a program of

research concerning the rheological behavior of suspensions containing

kaolin and clay; also because a literature search of the study convinced

us that in spite of the fact that there has been considerable scientific activity in the sector, many investigators used instruments which

did not make it possible to characterize completely, from the rheological point of view, the systems examined or they used disputable procedures such as those of hysteresis loop or finally they confined

themselves to investigations of suspensions at very low concentrations

of solid material.

2. Materials and Experimental Procedures

The starting material used in our experiments consisted of a mixture containing 52.5 % kaolin and 47.5 % of a crushed French clay of

the GSG type furnished by the Petra Company. The two components were

characterized from the chemical, mineralogical and granulometric point

of view by the traditional techniques (chemical weight analysis and by

atomic absorption, RX diffractometry, sedimentometric balance). The

results are given in Table i. The deflocculant used is the classic

mixture with a weight proportion 1 to 1 of sodium carbonate and silicate, the percentages added, calculated with regard to dry material,

varied between 0.I and 1%.

The suspensions prepared by this method (altogether 1 kilogram)

were homogenized in a ball grinder with a capacity of 1.4 liter and

with a speed of rotation of 82 rpm.

2
Table 1. Chemical Composition and _anulometric Distribution of the

Materials Used

Chemical _omposition

(weight percentage)

Granulometric Distribution

(weight percentage)

clay kaolin d (_) clay kaolin

l.i. (*) 11.6 12.3 > 9.6 6.6 1.6

SiO 2 50.6 46.0

AI203 33.7 39.3

Fe203 1.5 0.8 9.6 t 3.0 6.7 10.1

TiO 2 i. 4 ---

K20 0.7 0.4 3.0 -. 1.6 6.7 31.1

Na20 0.1 0.4

CaO 0.3 0.i i 1.6 80.0 57.2

MgO 0.1 0.1

*firing loss; (@) equivalent diameter in um.

The experimental tests were carried out with a rotational viscosimeter of the type Rotovisko Haake, RV2, in the velocity gradient range

-i

between i0 and 2,000 s . The measurement system used is of the type

with coaxial cylinders (diameter ratio = 0.96). The experimental procedure adopted consisted of the application of the sequence of constant

velocity gradients; each gradient was applied until a stationary value

of the shear force was obtained. The tests were conducted at 25 + 0._ C.

3. Results and Discussion

At high concentrations of solids and low concentrations of the deflocculant, the experimental procedure makes it possible to reveal a

considerable thixotropic behavior, that is considerable variations of

the shear force in time with constant velocity gradient. These variations of the shear force in time are the result of processes of structural destruction or reconstruction taking place within the material

through the effect of variations of the kinematic conditions imposed.
ORIGINAL PAGE !_

OF POOR QUALrrY

Various approaches may be followed to describe the thixotropic

behavior and to give a theoretical interpretation. In previous publications [i, 2] some examples were given of application of the approaches of Cheng [3] and Camina [4] to the thixotropic behavior of

suspensions of kaolin and clay.

As regards the shear-dependent properties which are the subject

of this study, all the systems show a similar behavior in the range

of velocity gradients studied. This behavior depends to a considerable

extent, in quantitative terms, on the concentration of solids and deflocculant.

Figures I, 2 and 3 give the curves of the equilibrium flux of the

systems at three different concentrations of solid considered. The

above mentioned similarity of theological behavior and the drastic

reduction of viscosity resulting from the addition of deflocculant is

apparent.

I0

i/

Pa.s

0

lo

-1

lO

D

• 0

• o

• U

0 •

0

0

I

I

D

0

O

A •

C D 0.375

• 04

0 04_.

• 045

A05

A0c

006

eJ

ii

dl

G

( A

c.

o

<,

.. .

,0 ;o ;>

Figure l: Viscosityn against

velocity gradient Y for the systems

with concentration of solid C s

equal to 61%.
u_

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11

lPas'l

0

lO

n

I

c

&

41

A

ii

C: 0. "t

• 0.2

03

• 0 34

_. 0.375

& 0 45

,: C 6

II

II

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10 _ O •

, C I I

i • ,

:% • C, {., C._.,

'_ • • I •

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10 ':L_ 103 ;:' i5'',

ORIGINAL PAGE !8

OF POOR QUALITY

Figure 2: Viscosity q against velocity

gradient # for the systems with solid

concentration C S equal to 50 %.

II

C. _;

• 0.2

• 0.2'75

03

• ,: 45

-.', ',.,b

• 0.8

t, II

II

• II •

• iii • ,II

• "" :'II"

46646 il

lu 10 3 ; { ''- _;

Figure 3: Viscosity r, against velocity

gradient Y for the systems with solid

concentrations C S equal to 40 %.

m
i

OF' POOR QUAL.F_Y

The similarities of the curves of flux makes it possible to qive

a compact representation of the shear-dependent behavior of all the

systems considered, accordinq to a procedure of superimposition by

translation of the curve [2,5] . The flux curve of a system (in the

present case the system with concentrations of solid C S - 61%, and

concentration of deflocculants equal to 0.375 %) is taken as reference

curve. The flux curves of all the other systems are translated over

the reference curve. The components of the translation vector determine

the translation factors a and _. The curve of the reference system

may be taken as characteristic curve of the family of systems (master

curve) in a diagram Logn R - log_ R in which r.R and _R are reduced

variables defined by _k_;;,,a ; _R=_, a

pa

IG z, a_.

16 ......................... -..... , •

0 tO' _t _, x

Figure 4: Characteristic flux curve

(master curve) in terms of reduced

viscosity nR against reduced velocity gradient YR"

Therefore the first result which is obtained with this procedure

is the plotting of the characteristic curve shown in Figure 4 in a diagram icg_ R - log YR" Another advantage deriving from the application

of such a procedure is that in this way it is easy to obtain the characteristic rheological parameters of this sytem. As regards these

parameters it is observed primarily that for each system it is possible

to determine both directly and/or through the above mentioned procedure

a critical velocity gradient @c below which the shear-dependent behavior

is plastic and may be described satisfactorily with the Bingham model.

t

Above Yc the behavior becomes dilating. Figure 4 shows the flux

curve described by the Bingham model and obtained through the fit hing

6
ORIGINAL PAGE I3

OF POOR QUALITY

of all the experimental points concerning the field of velocity gradients

o

less than Yc"

Therefore the shear-dependent rheological behavior of each system

is characterized by three parameters: the limit of slipping TB, the

plastic viscosity np and the critical velocity gradient Yc" The effect

of the concentration of the solid and the deflocculant on the sheardependent rheological properties of the suspensions may therefore be

described by the variations of such parameters.

03 .

i

t, ;

i

.o

1o'

,o

\

,,O

_ _ _- -_2 -_ •

--_- .... -UP"

kL."..

.... -, i,,-- - - ,_'r "

Figure 5: Limit of slipping _B against

concentration of deflocculants C.

_:C, 6_.,: _:C _ 50%; .', :C, _ 40°o) %

(J 02 _,4 I()
,0o

IPa s]

!.,

-1

2

Io

I

%

_,, b

"eL,

, , :-','.)('JR QuAI_r_'_'

-1

lo

16:

t6 _ ,

0 0.2 0'4 0.6 08 C("} 10

D

\

Figure 6: Plastic viscosity rip against

concentration of deflocculant C.

(O:C,=61°/,; [];C,=50oi0: '. c,- 40%).

L

' %m

v

2_

0 0_, 04 06 08 _ . 10

Figure 7: Critical velocity gradient

9 against concentration of defloccuc

lant C.

(:__):C 61_o; _'G .: SO°o; " C - 40%]
__m_ -_ _

Figures 5, 6 and 7 give the variations of TB, Fip and _c with the

variation of the concentration of deflocculant. The progressive addition of deflocculant causes a drastic reduction of the parameters,

a reduction which is maximum in case of saturation conditions. This

condition is achieved for different values of concentration at the

three levels of solid concentration studied. Beyond these conditions,

subsequent conditions of deflocculant do not cause considerable effects

of overdoping. Figures 8, 9 and i0 show the effects of the variation

of the solid concentration on T B, np and _c' evaluated in accordance

with three different values of concentration of deflocculant.

/6__£

,0_]

TB !

,Pa,I

q

u

Io

0

I0 ,

/

/

/

/

/ 1

Figure 8: Limit of slipping T B against

solid concentration

Cs (0:C=.4%: _:C=,45°o: O:C 6%).
p •

I0

_Ip

IO

0  ̧

/

,/

,is" •,1_

' 4o _o '_ ,., E,o

Figure 9: Plastic viscosity n

solid concentration C S.

{O:C-_.4°,o: A:C-- ,4S°o: O:C ,6%),

p against

:0

6

O

•_2 --

e

i/

/

Figure 10: Critical velocity gradient

%c against solid concentration C s-

[e :C.. ,4%; A:C - ,4_'_; _;C .6%).

_o_L ...........

40 bo

_,,, L

10
The variation of the parameters with the concentration of deflocculants is derived from the variations of the state of agqregation of

the particles obtained from the absorption of deflocculant and the

variation of potential. Actually the adsorption and the development

of the double layer are the main mechanisms by means of which we explain

the effect of the deflocculant used which frees by hydrolysis Na +

cations and colloidal silica.

The high concentrations of solids and the considerable variations

inside the family of systems studied, of the conditions o_ aggregation

of the dispersed phase makes it difficult to apply extensively the

analyses and models proposed by various authors [6-12] to interpret

the theological behavlor of suspensions of kaolin, clay and similar

materials. At the present time it was considered suitable to reexamine

some interpretations proposed by Firth and Hunter [9] concerning the

flow properties of colloidal suspensions to evaluate the possibility

of extending them to the systems in question.

According to those authors and according to what was already indicated by Thomas [8], it is possible to obtain from the data on plastic

viscosity, an estimate of the effective value of the concentration of

the dispersed phase and its variation with the variation of the concentration of deflocculants.

From the value of the relative viscosity nR (defined by the ratio

np/n s with K s the viscosity of the dispersing medium) and from the

equation of Thomas [13], valid for suspensions of spherical particles

without mutual interaction:

] ' "_ ; _1' ; 1 _" _'J_ 'i'" _ 2.73 1.,, '_',,,,,1_ [']616,I. j

it is possible to obtain the value of the effective volumetric fraction

of the dispersed phase assuming the flux units present in conditions of

high velocity gradients as spherical.

ii
): OHIGINAL PAGE IS

OF POOR QUALITY

The ratio CFp = CEff/¢p, in which CEff is the effective volumetric

fraction calculated by this means and _ is the nominal volumetric

P

fraction, gives a measure of the increase of volume of the dispersed

phase resulting from the aggregation of particles. Table 2 gives the

values of CFp for three different concentrations of solids (C S = solid

concentration, C = concentration of deflocculant. It is apparent that

according to the conditions of maximum reduction of the plastic viscosity of the systems, the deflocculation of the dispersed phase is

not complete, the minimum values of CFp being equal to 1.99, 1.62 and

1.43 for the three different concentrations of solids considered.

/6__!i

Table 2: Values of CFp c al_c_ulated o__n the b__aasis of equations of Thomas

c. - 40

c [(,..)

I

.2

.25

275

3

45

.6

8

C, = 5_ . c, L;'

C,,. c [°,,) C,, c {",,; C,,,

2 73 1 2 16 .'_,75 1.91

2 44 .2 2.06 .4 1.78

2.36 .3 1.92 425 1.70

2 31 34 1 78 45 1.59

2 26 .375 1 74 5 1.54

2 07 .45 1.60 £, 1 4G

1 99 6 1.62 .8 1.43

2 03 8 1'.65 I 0 1.48

As regards the dependence of iB on CFp, the linear correlation

given by Hunter and Frayne [12] may be considered valid only in the

first approximation. The decrease of Yc with the addition of deflocculation means that in conditions of greater deflocculation, the expansive behavior of the kaolin/clay suspensions is manifested at lower

velocity gradients. We refer to the article by Beazley [.14] for a possible interpretation.

From an examination conducted on a restricted series of systems

within the range from 20 to 30°C, it was found that the variations of

viscosity with temperature are more marked at high velocity gradients.

As regards the characteristic rheological parameters, this is manifested

in a considerable variation of the plastic viscosity np , while the

12
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i

limit of slipping _B remains approximately constant in the cases

studied. If we analyze the data in terms of relative viscosity,

it is observed that under this aspect too no considerable v_riations are observed with the temperature, and the conclusion must

therefore be drawn that in the range of temperature considered

and for the systems studied, the decrease of the viscosity of

the suspensions with the temperature is derived substantially

from the decrease of viscosity of the dispersing agent.

4. Conclusions

a) At high solid concentrations and low concentrations of

deflocculant, a substantial thixotropic behavior may be noted

experimentally. In a subsequent study we will present the results

of a detailed investigation of the effect of the temperature on

the rheological properties of the suspension in question.

b) All the systems studied show a similar shear-dependence

behavior which may be described synthetically through a characteristic flux curve (master curve).

c) It is possible to determine a critical velocity gradient

_C and consequently to differentiate two regions of different

shear-dependent behavior (plastic for gradients less than 9C'

expanding at higher gradients). The plastic behavior may be

described with the Bingham behavior.

d) Three rheological parameters (TB, np and 9C ) are sufficient to characterize the shear-dependence of the systems studied

and to describe the effects of the variations of the contents of

solid and deflocculant.

e) For the three levels of solid contents studied, values

of concentrations of deflocculant were determined beyond which

further additions of deflocculant do not cause perceptible variations of the rheological behavior.

13
f) It is possible to extend partly to the systems examined

the interpretations suggested by Firth et al. for similar systems

with low solid concentration.

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REFERENCES

Lapasin, R., A. Alessandrini and F. Sturzi, Proc. VIII Int.

Congr. on Rheology, Naples. Vol. 2: Fluids, Naples, 1980,

p. 563.

Alessandrini, A., R. Lapasin, E. Lucchini and F. Sturzi,

Proc. 2nd Yugoslav-Italian-Austrian Chemical En@ineerin@

Conference, Bled, 1980, p. 558.

Cheng, D.C.H. and F. Evans, Brit. J. Appl. Phys. 16, 1599 (1965).

Camina, M. and C.G. Roffey, Rheol. Acta i__O0,606 (1971).

Lapasin, R., A. Papo and G. Torriano, Proc. XIV FATIPEC Contr.

Budapest, 1978, p. 653.

12.

6. Michaels, A.S. and J.C. Bolger, Ind. En@. Chem. Fund. _, 24 (1962).

7. Thomas, D.G., A.I.Ch.E. Journal 9, 310 (1963).

8. Thomas, D.G., A.I.Ch.E. Journal i__0, 517 (1964).

9. Firth, B.A., J. Colloid Interface Sci. 57, 248 (1976)•

Firth, B.A., J. Colloid Interface Sci. 57, 257 (1976).

Firth, B.A. and R.J. Hunter, J. Colloid Interface Sci. 5_/7,

266 (1976)•

Hunter, R.J. and J. Frayne, J. Colloid Interface Sci. 71,

30 (1979).

13. Thomas, D.G., J. Colloid. Sci. 2__0, 267 (1965).

14. Beazley, K.M., Trans. Brit. Ceram. Soc. 63, 451 (1964).

Presented at the Fifth International Technical Symposium on the

Production of Ceramics, Rimini, October 1980.

15 )