INVESTIGATION OF ELECTROKINETIC PHENOMENA IN THE PROCESSES OF WET AGGREGATION OF DISPERSE MATERIALS

V. M. VITYUGIN, I. N. LANDSMAN

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

At present the process of pelletizing ore concentrates is finding ever broader application. The objects of granulation are, as a rule, multicomponent polydisperse materials. The fine-grained part of the solid phase, whose particle sizes are of colloidal or nearly colloidal order, has the greatest influence on the pelletizing process.

The aggregate stability of such systems and their capacity for pelletizing are determined to a considerable extent by the electrokinetic properties of the system [1, 2, 3]. In many works on granulation and soil science, assumptions are made that the electrokinetic properties of individual components and of the pelletized system as a whole are of great importance in attaining the specified structure and strength of the final aggregates [4, 5, 6]. From pelletizing practice it is known that finely dispersed materials are pelletized better than coarsely dispersed ones. Thus, quartz sand of ordinary granulometric composition is practically not pelletized, whereas marshallite (finely dispersed SiO₂) is easily pelletized. On the other hand, two components having opposite signs are pelletized better than like-charged ones [6].

To clarify the role of surface phenomena in the pelletizing process, we determined the electrokinetic potentials for components and mixtures used in the pelletizing of iron-ore concentrates at the Sokolovsko-Sarbaiskii mining and concentrating combine (SSGOK).

The electrokinetic potential was determined by the osmotic method in an apparatus of Leningrad State University. Electrical conductivity was measured by the usual compensation circuit of a Wheatstone bridge, where the null instrument was an oscilloscope. The electrokinetic potential \((\zeta)\) was calculated by the well-known Helmholtz–Smoluchowski formula

\[ \zeta = \frac{4\pi^2 \chi \cdot V}{D \cdot I}\,300^2 \ \text{V}, \quad \text{where} \]

\[ \eta \]
— viscosity of the dispersion medium, poise;

\[ \chi \]
— electrical conductivity of the solution, \(\Omega^{-1}\,\text{cm}^{-1}\);

\[ D \]
— dielectric constant;

\[ \frac{V}{I} \]
— electroosmotic velocity, determined from experiment

\[ \frac{V}{I}=\frac{N_{\mathrm{e}}\cdot a}{t\cdot I} \]

\(N_{\mathrm{e}}\) — capillary division value, ml;
\(a\) — number of capillary divisions traversed in time \(t\), sec, at a current \(I\), mA.

The experimental error was about 5%. Measurements were carried out in distilled and industrial water. The experimental data are summarized in Table 1.

Table 1

Value of \(\zeta\)-potentials for components and charges of SSGOK

It follows from the table that the magnitude and sign of the electrokinetic potential depend both on the nature of the solid phase and on the composition of the dispersion medium. Chalk and limestone, when distilled water is replaced by industrial water, change the sign of the charge, whereas for the concentrate the surface remains negatively charged. Consequently, when limestone is added to the concentrate, mutual coagulation of the finely dispersed fraction will occur, which promotes, on the one hand, the convergence of particles and a decrease in the distance between them, and, on the other hand, the formation of readily recoverable coagulation structures. This explanation is confirmed experimentally. From the data of Table 1 it is evident that charges containing concentrate and limestone in their composition have, in industrial water, a \(\xi\)-potential equal to zero; this corresponds to the isoelectric state of the given system. In distilled water, however, owing to the fact that the surfaces are charged with the same sign, the systems remain stable, without losing aggregate stability. For pelletizing iron-ore concentrate with an addition of limestone, the use of industrial water having the hardness permitted by the GOST standard should be considered favorable.

For bentonite, upon transition from distilled water to industrial water, the values of the \(\xi\)-potential change little, which is probably due to the similarity of the ionic composition of the exchange complex of bentonite with the ionic composition of industrial water. From the comparatively high negative value of the \(\xi\)-potential of bentonite it may be judged that, in the pellet, bentonite is present in the form of a sol. Under these conditions it is not inclined toward structure formation and plays the role of a moisture-capacity additive. In addition, ben-

Bentonite increases the viscosity of the pore solution, being uniformly distributed in it. A charge containing bentonite does not reach an isoelectric state, i.e., under these conditions it is not prone to the formation of coagulation structures; this promotes the best packing of particles during granulation, but the distances between particles remain large, so that at the optimum pelletizing moisture content the strengthening effect of the finished pellet is not achieved. A relative increase in strength will occur through binding the amount of moisture in excess of the working pelletizing moisture.

With the combined addition of limestone and bentonite, the system is in an isoelectric state. This is due to the presence of a finely dispersed fraction that mutually coagulates oppositely charged colloidal particles. On the other hand, the joint presence of bentonite and limestone promotes the exchange of the Na ion of bentonite for the Ca ion. All this promotes structure formation in the system.

Conclusions

1. The strengthening effect of limestone during the pelletizing of iron-ore charges in technical water can be explained by the coagulation of oppositely charged colloidal particles of limestone and magnetite.

2. The introduction of bentonite increases the concentration of colloidal particles in the pore solution, i.e., increases the possibility of structure formation.

3. The introduction of bentonite without the addition of limestone at the optimum pelletizing moisture content does not lead to strengthening of the finished pellet. A relative increase in strength can occur only when a charge with an increased moisture content enters the pelletizer, which is explained by an increase in the viscosity of the pore solution and the binding of part of the moisture.

4. The simultaneous introduction of bentonite and limestone is advisable, since the introduction of bentonite increases the concentration of structure-forming elements (colloidal particles), while the introduction of limestone promotes structure formation.

5. Investigation of a technical water supply with a hardness permitted by the GOST standard promotes the pelletizing of iron-ore concentrates with the addition of limestone.

References

1. Collection: Electrokinetic Properties of Capillary Systems, 1955, Publishing House of the Academy of Sciences of the USSR.

2. F. D. Ovcharenko. Hydrophilicity of Clays and Clay Minerals, Publishing House of the Academy of Sciences of the Ukrainian SSR, 1961.

3. A. V. Dumanskii. Lyophilicity of Disperse Systems, 1960, Publishing House of the Academy of Sciences of the USSR.

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

5. V. A. Priklonskii. Soil Science, Part 1, Moscow, 1955.

6. I. S. Lysenko. Theoretical foundations of pelletizing agglomeration charges. Transactions of NIIМ, issue 8, Kharkov, Metallurgizdat, 1964.