On the Question of Electrokinetic Phenomena in the Process of Aggregation
Abstract Generated abstract
This paper examines the role of electrokinetic phenomena in the aggregation and pelletizing of fine-grained materials, arguing that conventional mechanical descriptions insufficiently account for physicochemical processes in colloidal systems. It relates pelletizability and raw granule strength to the electric double layer, zeta potential, moisture content, electrolyte effects, and the interaction between water and particle surfaces. Drawing on prior experimental observations, the authors suggest that electrokinetic potential can indicate lump-forming ability, help estimate limits of raw granule strength, and guide the selection of optimum moisture and additives for more effective control of the pelletizing process.
Full Text
ON THE QUESTION OF ELECTROKINETIC PHENOMENA IN THE PROCESS OF AGGREGATION
V. M. VITOGIN, I. I. LANDSMAN
(Presented by the scientific seminar of the Department of General Chemical Technology)
The mechanism of the pelletizing process, despite the considerable development of the practice of material granulation, has still been studied only poorly. One important reason for this is the insufficient consideration of the physicochemical phenomena that take place in the pelletizing process. For example, according to Ferst [1], the pelletizing process is likened to mechanical pressing at high specific pressures.
The finer the material, the greater the role acquired by colloidal particles in the pelletizing process. And since, with decreasing coarseness, the pelletizability of materials increases, it is quite obvious that the phenomena taking place in colloidal systems will also characterize the pelletizing process.
Colloidal particles possess specific properties owing to their extremely developed surface and are capable of adsorbing ions and polar molecules. Adsorbed ions, by virtue of electrostatic interaction, attract to themselves ions of the opposite sign. A so-called electric double layer is formed, in which an equilibrium is established between Coulomb forces and forces caused by thermal motion. Depending on which of the forces predominates, the movable boundary is established closer to or farther from the interface between the solid and liquid phases. The potential arising at the movable phase boundary is the quantitative expression of electrokinetic phenomena.
The first attempt to relate the granulatability of materials to electrokinetic phenomena was made by I. S. Lysenko [2]. He showed that particles with different signs of charge are pelletized better than particles charged with the same sign. An example is the fact that concentrate from KMA iron ore, whose liquid phase is negatively charged, is pelletized better when chalk or limestone is added, the liquid phase of which is positively charged. At the same time, a burden with a basicity of 1.0, fluxed with limestone, has a lower potential than a burden fluxed with chalk. Thus, it was established that the electrokinetic potential can serve as a measure of the pelletizability of fine-grained materials.
The magnitude of the electrokinetic potential is affected by the displacement of particles during granulation, as well as by various kinds of mechanical actions that promote, on the one hand, the formation of colloids and, on the other hand, the compression of diffuse layers when particles approach one another.
The strongest influence on the zeta potential is exerted by the introduction of an electrolyte. The ions H\(^+\) and OH\(^-\) are potential-determining. For example, for a molding mixture, a small increase in the concentration of OH\(^-\) ions increases the zeta potential, while the introduction of H\(^+\) ions decreases it [3].
A decrease in the electrokinetic potential is associated with a decrease in the thickness of the diffuse shells and, conversely, an increase—with its reduction [4].
In the process of wet granulation, the principal importance lies in the nature of the interaction between water and the material itself. It may be considered that the water contained in the diffuse layers represents a loosely bound form of water. In this connection, the electrokinetic potential, being a function of the thickness of the diffuse layer, may serve as a measure of the lump-forming ability of the material. From this point of view, the greater the value of the electrokinetic potential of the maximally moistened material, the higher its lump-forming ability.
On the other hand, as follows from the experiments of I. S. Lysenko, the strength of raw granules is inversely proportional to the magnitude of the electrokinetic potential. This is explained by the fact that, during the formation of granules, the distance between particles decreases owing to compression of the diffuse layers. Thus, the electrokinetic potential may be used to establish the upper limit of the strength of raw granules.
On the basis of a study of the electrokinetic potentials of the starting materials and of systems undergoing the lumping process, it is generally possible to establish the optimum moisture content, i.e., the moisture content at which the material lumps sufficiently well and the granules acquire sufficient mechanical strength.
In addition, knowledge of the electrokinetic potentials will make it possible to select additives correctly for improving the lumping process.
Thus, the study of electrokinetic phenomena in the lumping process makes it possible to describe the lumping process with a high degree of accuracy and definiteness and to control the process with the least expenditure of energy.
LITERATURE
-
M. A. Nechiporenko. Lumping of fine concentrates. Proceedings of the Institute of Mechanobr, issue (2), 1958.
-
I. S. Lysenko. Theoretical foundations of the lumping of agglomeration charges. Proceedings of the Ukrainian Scientific-Research Institute of Metals, issue 8, 1962.
-
D. S. Williams. Factors affecting the quality of molding mixtures. Foundry Production, No. 5, 1961.
-
V. A. Priklonskii. Soil Science, part 1, 1949.