1. Ya. Z. Tsypkin. Fundamentals of the Theory of Learning Systems. Moscow, Nauka, 1970.
2. Collected volume, Signal Reception in the Presence of Noise. Moscow, Izd. IL, 1960.
3. A. A. Kalinin, A. N. Pokrovskaya, B. A. Kubrak, M. V. Krasheninnikov. Chemistry and Chemical Technology. Tomsk, Tomsk Polytechnic Institute Press, 1973.

On the Theory of Pelletization of Wet Dispersed Materials

V. M. Vityugin

The widespread introduction of granulated dispersed materials into metallurgy and chemical technology urgently requires the creation of a solid theoretical basis for the directed improvement of this process. The possibility of application and the effectiveness of the granulation process are determined, first of all, by the propensity of each specific dispersed material to pelletization, i.e., by its pelletizability. Pelletizability, in the final analysis, also determines two main techno-economic indicators of the process: the specific productivity of the equipment and the quality of the granulate. Thus, the elucidation of the causes determining the pelletizability of dispersed materials and the development of a scientifically substantiated method for assessing pelletizability are primary tasks of the theory of granulation.

The criteria for assessing pelletizability must take into account both the properties of the dispersed material and the specific features of the pelletization process in mechanical drum- and pan-type granulators. In this sense, the greatest importance is acquired, respectively, by the water-physical characteristics of the dispersed material and the structural-mechanical properties of the pelletized system. The water-physical properties, expressed through characteristic moisture capacities, predetermine the strength of the green granulate /1/. The plasticity of the pelletized material, in turn, exerts a decisive influence on the rate of rolling, since the main feature of this process consists in the great freedom for the development of bulk deformations of the forming granules; a stable process will be effective only in the case of the predominant development of plastic deformations /2/.

It is natural that the water-physical and structural-mechani-

the properties of pelletized materials are closely interrelated, and an increase in pelletizability simultaneously leads both to an improvement in the quality of the raw granulate and to an increase in the specific productivity of granulators. However, as in any other process, these final techno-economic indicators have a contradictory character with respect to one another. When the properties of the pelletized system remain constant, an improvement in granulate quality is limited by a decrease in the specific productivity of granulators, and vice versa. This predetermines the extremal character of the influence of pelletizability on the efficiency of the granulation process as a whole. Thus, the optimum value of the pelletizability index will be determined by a strict relationship between the water-physical and structural-mechanical properties of the pelletized system.

To establish numerical values of the pelletizability index, it is necessary to express the set of water-physical and structural-mechanical properties of the pelletized material through identical indicators. Obviously, such indicators may be the optimum values of the working moisture content of the raw material, established, on the one hand, from water-physical properties and, on the other, from structural-mechanical properties.

Numerous studies and the practice of pelletizing various dispersed materials have established that the moisture content of the material being pelletized must strictly correspond to its moisture capacity. Thus, for well-pelletizing cement raw mixes, the optimum working moisture content coincides with the value of the minimum capillary moisture capacity (MCMC).

However, as pelletizability worsens, the optimum working moisture content substantially exceeds the MCMC value, while aggregation of coarse-dispersed, practically non-pelletizing materials is possible only at moisture contents close to the maximum capillary moisture capacity (MCC). The working value of the moisture content of the pelletized feed according to water-physical properties \((W_{в.ф}^{p})\) may be determined from the difference

\[ W_{в.ф}^{p} = \mathrm{MKB} - \mathrm{HKB} \tag{1} \]

This moisture content will be the lower limit of the optimum value, determining the possible maximum strength of the granulate.

The second factor determining the value of the optimum working moisture content, as indicated above, is the plasticity of the pelletized...

of the material itself. The coarse-grained portion of real dispersed materials is, by itself, incapable of coagulation structure formation and does not possess plasticity. Only hydrophilic colloidal and semicolloidal particles form coagulation structures in an aqueous medium. These active fractions are concentrated in the pore space of the granules being formed, ensuring plastic deformations in the pelletized system. From this standpoint, it is expedient to divide the dispersed material into a coarsely dispersed nonplastic framework part and an active plastic pore suspension-paste. As is known, the plasticity of coagulation structures \((\psi)\) can be expressed through the ratio of the limiting shear stress \((\theta)\) to the plastic viscosity \((\eta_{\mathrm{pl.}})\).

\[ \psi = \frac{\theta}{\eta_{\mathrm{pl.}}} \tag{2} \]

As studies have shown /3,4/, plasticity is in an extremal dependence on the concentration of the solid phase in the suspension. The position of the maximum of plasticity on the curve of its dependence on the concentration of the solid phase of the suspension or, correspondingly, on the moisture content of the suspension, is determined by the colloid-chemical properties of the finely dispersed part of the pelletized material and by the nature of the liquid phase.

All the water of the pelletized system \((W^{p})\), with the exception of the firmly bound \((M\Gamma_k)\) hygroscopic water of the granule framework, participates in the formation of the pore suspension. Thus, the moisture content of the pore suspension can be calculated from the difference:

\[ W_{\mathrm{p.s.}} = \frac{W^{p} - M\Gamma_k(1-x)}{x}, \tag{3} \]

where \(x\) is the fraction of the solid phase contained in the pore suspension of the granulate. The working moisture content of the pelletized medium corresponding to the maximum plasticity of the pore suspension \((W_{\mathrm{pl.}}^{p})\), in this case, is calculated from the equation

\[ W_{\mathrm{pl.}}^{p} = M\Gamma_k(1-x) + W_{\mathrm{p.s.}}\cdot x, \tag{4} \]

where \(W_{\mathrm{p.s.}}\) is the moisture content of the pore suspension at the concentration of the solid phase corresponding to maximum plasticity. Obviously, \(W_{\mathrm{pl.}}\) is the upper limit of the moisture optimum of the pelletized materials. The numerical value of \(W_{\mathrm{p.s.}}\) is determined from the experimental extremal dependence of the plasticity of the pore

suspension on the concentration of the solid phase in it. An experimental study of this dependence is carried out on an RV-8 rotational viscometer.

With respect to the working-moisture indicators of the pelletized system, one can evaluate the propensity of this system for pelletization. The pelletizability index \((K)\) is expressed by the relation

\[ K=\frac{W_{v.f.}^{p}}{W_{p.l.}^{p}} . \tag{5} \]

The numerical value of this indicator may vary within the limits from 0 to 1. The closer the values of \(W_{v.f.}^{p}\) and \(W_{p.l.}^{p}\), the closer to unity is the magnitude of the pelletizability index and the better the material is pelletized.

Knowledge of the parameters \(K\), \(W_{v.f.}^{p}\), and \(W_{p.l.}^{p}\) makes it possible, without carrying out complex and lengthy direct pelletizing experiments, to calculate the optimal moisture content of the pelletized burden \(W_{\mathrm{opt}}^{p}\) from the equation

\[ W_{\mathrm{opt}}^{p}=W_{v.f.}^{p}+\left(W_{p.l.}^{p}-W_{v.f.}^{p}\right)(1-K). \tag{6} \]

Moreover, the study of the factors determining the magnitude of the indicators \(W_{v.f.}^{p}\) and \(W_{p.l.}^{p}\), which are criteria of the water-physical and structural-mechanical properties of pelletized dispersions, opens practical ways of increasing pelletizability and, consequently, the efficiency of the wet granulation process.

References

1. V. M. Vityugin, A. S. Bogma. Proceedings of Higher Educational Institutions. Ferrous Metallurgy, no. 4, pp. 18–22, 1969.

2. V. I. Korotich. Theoretical Foundations of Pelletization of Fine-Grained Iron-Ore Concentrates. Moscow, Metallurgizdat, 1966.

3. A. A. Bagrov. Colloid Journal, no. 4, p. 486, 1968.

4. V. M. Vityugin, I. N. Lantsman, P. N. Dokuchaev. Bulletin of the Central Research Institute of Information on Ferrous Metallurgy, Moscow, 22, p. 32, 1971.

Study of the Rate of Thermal Compaction of Corundum Materials by a Dynamic Method

P. G. Usov, A. T. Dobrolyubov

At present, along with static methods of investi-