Gilde, M.

Szántó, Ferenc

English

Szántó Ferenc

Gilde M.

University of Szeged

RHEOLOGICAL PROPERTIES AND THIXOTROPIC BEHAVIOUR OF KAOLINITE SUSPENSIONS By F. SZÁNTÓ and M. GILDE Institute of Colloid Chemistry, Attila József University, Szeged (Received July 21, 1973) The rheological properties of Ca-Na-kaolinite and of monocationic Na-kaolinite were investi-gated as a function of the concentration of the suspension and of the electrolyte used as coagulating agent. The structure of aggregates of both types is changed by bivalent cations present in the electro-lyte, as reflected by rheological curves and hysteresis loop. Introduction Structure formation of particles and rheological properties of suspensions of clay minerals, such as kaolinite, can be varied. between wide limits by modifying the interactions between the particles. In connection with the formation of structure, VAN OLPHEN [1] supposes aggregated or flocculated states and their combinations. He defines the aggregated state as coming about by face-to-face interaction, whereas in flocculation edge-to-face or edge-to-edge associations are characteristic. He takes into account the double layers of positive charge formed at the edges under suitable conditions. The changes of rheological properties are explained — especially in the case of sols and gels of montmorillonite — by changes in character and strength of the interactions between the double layers of positive charge at the edges and those of negative charge present at basic faces. MICHAELS and BOLGER [2] consider the floes as stable flow units forming larger aggregates. At low shear rates the aggregates form a loose network, which determines the plastic and structural pro-perties of the suspension. At high shear rates the aggregates break down into indivi-dual floes. FLEGMANN, GOODWIN and OTTEWILL [3] stated that the yield stress of monocationic kaolinite suspensions, plotted as a function of pH, changes according to a maximum curve. The maximum observed appeared to occur at a zero point of charge of the kaolinite edges. Materials and methods For preparing the material, Zettlitz kaolin was desaggregated with Na 2 C0 3 . On the basis of our earlier investigations, addition of 0.2 g Na 2C0 3 to 100 g kaolin was found optimal.. The material treated with Na 2 C0 3 solution was dried over water bath. A suspension of 4%. concentration was prepared from the sample and 2 9 2 f . s z á n t ó a n d m. g i l d e the particles of ^m were separated by sedimentation. The fraction to be used was separated from the supernatant containing particles of colloidal dimensions by centrifugating only once. The sample obtained can be considered as Ca-Na-kaolinite, as according to our earlier experiences no total ion exhange occurs with N a 2 C 0 3 [4]. From the Ca-Na-kaolinite, monocationic Na-kaolinite was obtained by acidic and subsequent NaCl treatment as follows. The pH of the suspension was adjusted with HC1 to p H = 4 , thus decomposing the N a 2 C 0 3 . Then the material was treated with 2N NaCl and purified by centrifugation until the Cl~ ion con-centration of the supernatant decreased to 1.5 • 1 0 - 3 mole. The Theological pro-perties and thixotropic behaviour of both Ca-Na- and monocationic Na-kaolinite were studied in suspensions of different concentrations, as well as with constant kaolinite concentration in the presence of different quantities of coagulating electrolyte. The measurements were made with a viscometer type "Rotovisco". The Bingham yield stresses i B were obtained by extrapolating the linear sections of the flow curves in the usual way. We used principally these data for rheological characterization of the materials. Na-kaolinite suspension of relatively lower concentration (20%) exhibits a low yield stress (rB=20 dynes/cm2) without showing a hysteresis loop (Fig. 1). With increasing concentrations the yields stress increases; from 30% concentration this increase is more important and a hysteresis loop appears, the area of the latter also increases with concentration. In the case of Ca-Na-kaolinite the hysteresis loop can be observed already at 20% concentration, but is of opposite sense. The r values obtained with decreasing r.p.m. being, higher then those increasing r.p.m. (Fig. 2), the system exhibits "anti-Results and discussion l i t ml 500 1000 1500 ¿000 2500 Fig. 1. Flow curves of Na-kaolinite suspensions of different concentration r h e o l o g i c a l p r o p e r t i e s a n d t h i x o t r o p i c b e h a v i o u r o f k a o l i n i t e s u s p e n s i o n s 293. thixotropy". With increasing concentration, this phenomenon becomes, more pro-nounced. In suspensions of this type the effect of prolonged shear on flow properties was also studied. After determining the flow curves in the usual way (lower curves in Fig. 3), the suspension was sheared with the highest r.p.m. (D=2620 sec - 1) for 30 minutes, and the flow curves were recorded again (upper curve in the figure). As a result, the T values become significantly higher, whereas the hysteresis loop disappeared. 2 9 4 f . s z á n t ó a n d m. g i l d e This difference in behaviour can be explained with the different rates of aggre-gation and differences in the interactions (adhesive forces) between the particles. The bonds between the particles of the Na-kaolinite suspension, in which the adhesion is low, can be more easily interrupted by shearing and regenerate more slowly, thus giving a regular hysteresis loop. In contrary, in the Ca-Na-kaolinite suspensions, the adhesive forces between the particles being higher, the system will be in more aggregated state. On the mechanical impact of the shearing, disaggregation, possibly also orientation of the particles will occur. At the same time, while the number of the particles is increased by the shear, the tendency of the system to rebuild the structure is comparatively high; thus higher x values are obtained in the descending branch of the curve than in the ascending branch. The significant increase of the i values after shearing with the maximum r.p.m. shows that the disaggregation pro-ceeded further. The number of the particles forming the structure having thus reached a maximum, no further increase can be found in the descending branch, and thus the hysteresis loop disappears. The influence of Ca2 + ions on the rheological properties of Ca-Na- and Na-kaolinite was also studied. The Bingham yield stress of 20% suspensions of different CaCl2 concentrations is shown in Fig. 4. The Tb values of the Ca-Na-kaolinite, which is, as we have seen, in a slightly aggregated state, increase steeply in the presence of small quantities of CaCl2, but reach a constant value at 2 mmole/1 CaCl2 concentration. The flow curves show antithixotropy at all CaCl2 concentrations studied. At the same time, the effect of the CaCl2 on a thoroughly washed Na-kaolinite suspension (of 0.3 • 10 - 3 mole C I - ion concentration) is minimal up to 2 mmole/1 concentration added. With further increasing CaCl2 concentrations the Tb values show, however, a steep increase exceeding the maximum observed 1 i i Í 5 6 ? 8 millimolas/l CaCÍ Fig. 4. Bingham yeld stress of 20% Ca-Na-kaolinite and Na-kaolinite suspensions of different CaCl2 concentrations, A Ca-Na-kaolinite, o Na-kaolinite r h e o l o g i c a l p r o p e r t i e s a n d t h i x o t r o p i c b e h a v i o u r o f k a o l i n i t e s u s p e n s i o n s 295. 't 8 •d, iyncis 1000 Soo with Ca-Na-kaolinite. The zB vs. CaCl2 concentration curve of a Na-kaolinite suspen-sion of 40% washed in a usual way (of 1.5 • 10~3 mole CI - ion concentration) shows an essentially similar curve (Fig. 5). Some difference can be observed in the beginning of the curve, a well expressed mi-nimum at low CaCl2 concentra-tions being observable in the lat-ter case. The differences in the effect of the Ca2+ ions on the Theolo-gical behaviour of the samples can be interpreted by taking into account the different states. of aggregation of the starting mate-rial. The strength of the network of the Ca-Na-kaólinite suspén-sions, more aggregated from the beginning, will be increased by the influence of small quantities of CaCI2, resulting in a steep zB curve. The increase of the adhe-sion may be attributed to a de-crease of the thickness of the double layer rather then to ion exchange. In Na-kaolinite sus-pensions the flocculation process expressed by the increase of the yield stress starts at comparati-vely higher CaCl2 concentrations. The minimum of the curve shown in Fig. 5 can be interpreted with changes of the charges ( + ) at the edges and (—) on the faces, caused by electrolyte, as presented by VAN OLPHEN in connection with montmoriilonite. The steep ascension of the curve after the minimum represents the flocculation process as a result of increasing van der Waals forces. We studied also the effect of CaCl2 on the thixotropic behaviour of a 40% Na-kaolinite suspension. The thixotropy found at low CaCl2 concentrations increases at higher, concentrations, as shown by the broadening of the hysteresis loop at 3.3mmole/l concentration (Fig. 6). Maximum thixotropy seems to be connected with a mild flocculating effect. With further increasing CaCl2 concentration, the interactions between the particles become more intense and the breadth of hysteresis loop decreases; finally the loop disappears at 13.4 mmole/l concentration (Fig. 7). So no thixotropy is found in the system with maximum xB values. At CaCl2 concentrations exceeding that corresponding to the maximum of rB, anti-thixotropy appears, similarly as in Ca-Na-kaolinite suspensions. The character of thixotropy or antithixotropy and their degree can be systematically varied by changing the interactions (adhesive forces) between the particles. \ № 15 2.0 Tuillimolfcs/l C a C l t Fig. 5. Bingham yield stress of 40% Na-kaolinite suspensions of different CaCl2 concentration 2 9 6 f . s z á n t ó a n d m. g i l d e Fig. 6. Effect of CaCl2 on the thixotropic behaviour of 40% Na-kaolinite suspension. CaCl3 concen-tration: • 0.67 mmole/1; * 3.3 mmole/1 Fig. 7. Effect of CaCl2 on the thixotropic behaviour of 40% Na-kaolinite suspension. CaCl2 concen-tration: • 6.7 mmole/1; « 13.4 mmole/1; • 26.8 mmole/1 r h e o l o g i c a l p r o p e r t i e s a n d t h i x o t r o p i c b e h a v i o u r o f k a o l i n i t e s u s p e n s i o n s 2 9 7 . References [1] van Olphen, H.: An Introduction to Clay Colloid Chemistry, Chap. 7, John. Wiley,. . New York, 1963. [2] Michaels, A. S., J. C. Böiger: Ind. Eng. Chem. Fundamentals 1, 153 (1962). [3] Flegmann, A. W.,J. W. Goodwin, R. H. Ottewill: Proceedings of the British Ceramic Soci-ety 1969. .[4] Szántó, F., M. Gilde, B. Várkonyi, J. Balázs: Acta Geol. Acad. Sei. Hung. 11, 409(1967). РЕОЛОГИЧЕСКИЕ СВОЙСТВА И ТИКСОТРОПИЯ КАОЛИНЙТОВЫХ СУСПЕНЗИЙ Ф. Санто, М. Гильде Изучены реологические свойства Са-№-каолинита и монокатионного №-каолинита в зависимости от концентрации суспензии и коагулирующего электролита. Структура агре-гатов, образующихся из образцов обеих типов, изменялась под влиянием двухвалентного-катиона прибавляемого электролита. Происходящие изменения отразились в реологическом поведении й на виде гистерезисных петель. 

Rheological properties and thixotropic behaviour of kaolinite suspensions

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Rheological properties and thixotropic behaviour of kaolinite suspensions

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