STUDY OF THE EFFECT OF THE CONCENTRATION OF BENTONITE SUSPENSIONS ON SURFACE TENSION AND RHEOLOGICAL PROPERTIES

V. M. VITYUGIN, O. A. FUKS, T. N. SOMOVA

(Presented by the scientific seminar of the Department of General Chemical Technology)

There is still no consensus on the mechanism of bonding of particles in molded capillary-porous bodies. The so-called capillary and molecular mechanisms of bonding have become most widespread. Proponents of the capillary hypothesis give preference to the action of the meniscus forces of water collars at the points of contact of the particles [1], while proponents of the molecular hypothesis explain bond strength as a consequence of the action of van der Waals forces [2].

Analysis of the results of numerous studies of the strength of a raw granule shows that, under actual pelletizing conditions, the simultaneous action of both bonding mechanisms is more probable. The essence of the proposed combined capillary-molecular bonding mechanism is as follows. In the process of granulation, the raw material, which is a loosened three-phase polydisperse system, is transformed into dense capillary-porous aggregates of double structure. Comparatively coarse particles form the framework of the granule, while a suspension of particles of colloidal and semicolloidal size is concentrated in the pore space. During mechanical granulation, formation of a strong granule structure is possible only when the deformation of the aggregates being formed is plastic in character. Such a deformation character is ensured by introducing into the material being pelletized small amounts (0.5–1.0%) of bentonite clays, which, upon contact with water, readily form thixotropic coagulation structures.

The pore bentonite suspension is concentrated near the points of contact of the framework particles, forming a peculiar pseudophase characterized by elevated values of surface tension and plasticity. The strength of this pseudophase is due to the action of molecular van der Waals forces, while the strength at the contacts of the framework grains will evidently be ensured by the action of meniscus forces arising in the collars of the structured bentonite suspension.

The principal factors determining the possibility of formation of a pseudophase from the pore bentonite suspension are the concentration of this suspension and the nature of the bentonite. Below are presented the results of studies of the effect of concentration on the surface tension and rheological properties of bentonite suspensions.

As the object of study, a pink variety of bentonite from the Tagansk deposit (Kazakhstan) was used. The weight concentration of bentonite in the suspensions was varied within the range from 5 to 20%, which

corresponded to the concentration of bentonite suspensions in industrial granulates.

The surface tension was determined in parallel by two methods: the method of maximum pressure of gas bubbles or drops (the Rebinder method) and the ring-detachment method [3], while the rheological properties were determined on an RV-8 rotational viscometer. From the ratio of limiting shear stress to plastic viscosity, the plasticity index of the suspensions studied was calculated.

The averaged results of the studies are shown in the form of graphical dependences in Figs. 1 and 2. As can be seen from Fig. 1, at low concentrations the system is two-phase and its surface tension is close to the surface tension of water; the formation of a coagulation structure is imperceptible. Structuring of suspensions of the bentonite studied, with a contact time with water of 20–30 min, begins at concentrations above 10%. It is noteworthy that the intense increase in surface tension is observed in a comparatively narrow concentration range (12–15%).

Fig. 1. Effect of the concentration of a bentonite suspension on surface tension.

(Vertical axis: Surface tension $\sigma$, dyn/cm. Horizontal axis: Bentonite content in the suspension, % by weight.)

Simultaneously with the increase in surface tension in this concentration range, as can be seen from Fig. 2, an increase is also observed in the limiting shear stress and in the viscosity of the suspension. At the same time, the rheological parameters of the suspension up to a concentration of 12% practically do not change.

For successful conduct of the granulation process, pore bentonite suspensions must possess maximum plasticity. When the pink variety of Taganrog bentonite is used, the maximum plasticity of the pore suspension occurs at a concentration of approximately 16%. The surface tension of a bentonite suspension of such concentration proves to be several times greater than that for water and approaches the surface tension of solid bodies.

In comparative calculations of the effectiveness of the action of capillary and molecular bonding forces of particles in raw granules, it is necessary to use the value of the surface tension not of water, but of the structured suspension, for the liquid phase.

Thus, the role of capillary forces proves to be considerably greater than has been asserted up to now. A practical confirma-

Fig. 2. Effect of the concentration of a bentonite suspension on rheological properties.

A confirmation of the propositions put forward is the fact that satisfactory granulation of iron-ore concentrates is achieved only when a certain amount of bentonite is introduced. In this case, as calculations show, the optimal bentonite addition corresponds to a concentration of the pore suspension in the granules from 10 to 20%, depending on the quality of the bentonite. And, as the above studies have shown, it is in this concentration interval that the formation of a new phase occurs, representing a semisolid, highly plastic body.

REFERENCES

1. D. M. Newitt, J. M. Conway-Jones. A contribution to the theory and practice of granulation. Trans. Inst. Chem. Engrs. 1958, 36, No. 6, 422.

2. V. I. Korotich. Theoretical Foundations of the Pelletizing of Iron-Ore Materials. Metallurgiya Publishing House, 1963.

3. V. A. Kireev. Course of Physical Chemistry. Moscow, State Scientific-Technical Publishing House of Chemical Literature, 1955.