the suspension on the concentration of its solid phase. Experimental study of this dependence is carried out on an RV-8 rotational viscometer.

With respect to the working-moisture indices of the pelletized system, one can assess the tendency of this system toward pelletizing. 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 index can vary within the range from 0 to 1. The closer the values of \(W_{v.f.}^{p}\) and \(W_{p.l.}^{p}\), the closer the pelletizability index is to unity, 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 optimum moisture content of the pelletized charge \(W_{\mathrm{opt}}^{p}\) 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 indices \(W_{v.f.}^{p}\) and \(W_{p.l.}^{p}\), which are criteria of the water-physical and structural-mechanical properties of pelletized dispersions, reveals 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 Pelletizing 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 and Technical-Economic Studies of Ferrous Metallurgy, Moscow, no. 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, alongside static methods of study...

For studying the shrinkage kinetics of compacted powder bodies during heating, dynamic methods have found application that use special dilatometric devices.

The field of dynamic methods includes the techniques proposed by Dorn, Wulff, and Bannister /1/, and by Ivensen /2/.

The first three methods (Dorn, Wulff, Bannister) /1/ provide for the study of the initial stage of sintering under a nonisothermal regime of heat treatment of the specimen. They make it possible to study more fully and with sufficient accuracy the rapidly occurring deformations of the specimen in the initial period of sintering, which have a significant effect on the subsequent course of compaction. However, the final stages of sintering lie outside the range of applicability of these techniques. The entire process of shrinkage kinetics of metallic specimens was investigated by Ivensen /2/. The research approach proposed by him takes into account the initial density of the specimen and the change in pore volume during its firing, which considerably increases the accuracy of the experiment.

Since the sintering kinetics of corundum specimens, as indicated by E. V. Degtyareva and K. S. Kainarskii /3/, depends on the conditions of heat treatment, it was of interest to study the rate of thermal compaction of corundum materials by the method of V. A. Ivensen.

For this purpose we created a special high-temperature dilatometric installation (SDU) with automatic temperature control and recording of the results. The SDU consists of a specimen-heating system, a system for raising the furnace in a vertical plane along corresponding guides, a system for temperature control and recording, and a system for measuring and recording the shrinkage of the specimen during firing.

Heating of the specimen to a limiting temperature of \(1800^\circ\mathrm{C}\) was carried out by a high-temperature electric furnace with a cylindrical graphite heater. The temperature in the furnace was regulated by a transformer that allowed the voltage at the input of the power transformer to be changed smoothly. The power consumed by the furnace was 20 kW.

By means of a pneumatic lift, the furnace, heated to the required temperature, is raised upward along the guides; in this process the dilatometric tube with the specimen, fixed above the open channel of the furnace, is placed in the firing zone. Control of the rate of

The furnace is moved by automatic switching on and off of the compressor by means of a time-relay unit operating according to a preset program. The compressor operating time is set by an electronic relay within the range from 0.5 to 100 sec, and the holding time by a program relay within the range from 2 to 10 min. When the specimen reaches the firing zone, the lifting of the furnace is automatically stopped and the furnace is fixed at the height by a limit switch.

The rate of furnace lifting determines the rise in the specimen temperature up to the beginning of sintering; further temperature regulation is carried out manually. The temperature monitoring and recording system consists of an ÉPP-09 electronic potentiometer and two PR 30/6 thermocouples, one of which is placed in the bottom part of the furnace, while the other is located inside a hollow cylindrical rod—the indenter—and touches the surface of the specimen with its hot junction.

The change in the linear dimensions of the specimen at the firing temperature is transmitted by means of the indenter and a lever to a PF-2 transducer and is continuously recorded on the chart tape of a secondary VFSM instrument with an accuracy of ±0.5%. The readings of the instrument are checked by means of a micrometric indicator.

Cylindrical specimens with height and diameter equal to 12 mm were pressed in a steel die at a specific pressure of 4000 kg/cm². The specimens were dried to constant weight at 150°C in a drying oven with alumina gel packing. The pressing powder was prepared from activated white electrocorundum powder by sieving it through sieve No. 05, mixing with 0.3% polyacrylamide as a plasticizing and binding substance, and evaporating to a moisture content of 0.5%.

Activation of corundum powder with a grain size of less than 2 μm was carried out by introducing into it various amounts of an acetic-acid solution of zinc, chromium, and magnesium salts, evaporating with stirring, and calcining for 2 hours at 1000°C. Chemical and spectral analyses established the following in the starting material: 0.08% \(Na_2O + K_2O\); 0.2% \(SiO_2\); 0.11% \(Fe_2O_3\); and 0.06% \(CaO + MgO\). The obtained pressing powder contained 0.025; 0.05; 0.075; 0.1; 0.125; and 0.2 wt.% additions of zinc, magnesium, and chromium oxides.

The volume of the specimen and the initial pore volume were calculated from the apparent density determined by the hydrostatic

by weighing the previously paraffin-impregnated specimens (Table I).

Table I

Amount of activating additive and properties of the specimens
before firing

The specimens were fired in an SVEDU installation in a continuous stepwise mode up to a temperature of 1650°C. The temperature difference between steps was 50°C. At each heating step the specimen was held for 2 h under isothermal conditions, with simultaneous continuous recording of shrinkage. The transition time from one isothermal holding temperature to another was 10 min. From the obtained dependences of the reduction in specimen length on time it should be noted that sintering of specimens with a combined additive of zinc, chromium, and magnesium oxides proceeds more intensively beginning at a temperature of 1400°C. The values of the shrinkage rates of the specimens at the end of the isothermal hold at temperatures of 1300, 1500, and 1550°C are given in Table 2.

From the data in Table 2 it is evident that the shrinkage rate of specimens of pure electrocorundum at the end of a two-hour hold depends little on temperature, whereas for the activated specimens it remains still rather high, which apparently will require an increase in the sintering time under the optimum regime

Table 2

Rates of shrinkage of specimens with different contents of the activating additive

firing mass. The composition containing 0.075% additive stands out clearly. The shrinkage rate of specimens of this composition proved to be higher than for specimens with a larger or smaller amount of additive.

In connection with this, it was of interest to calculate the activation energy of sintering by the method of /2/.

On the basis of the data obtained, the dependence of the relative change in pore volume on time was constructed,

\[ \frac{V_c}{V_n}=\psi(\tau) \]

determined from the equation

\[ \frac{V_c}{V_n}=\frac{d_k \frac{V_c}{V_n}-d_n}{d_k-d_n}, \]

where \(d_k\) is the density of electrocorundum, \(3.99\ \text{g/cm}^3\);
\(d_n\) is the apparent density of the pressed specimen, \(\text{g/cm}^3\).

\[ \frac{V_c}{V_n}= \frac{\pi(d_n-\beta \Delta l)(l_n-\Delta l)}{4V_n}, \]

where \(V_n\) is the volume of the pressed specimen, \(\text{cm}^3\); \(V_c\) is the volume of the fired specimen, \(\text{cm}^3\); \(d_n, l_n\) are the diameter and length of the specimen before sintering, cm; \(\beta=\frac{\Delta d_k}{\Delta l_k}\); \(\Delta d_k, \Delta l_k\) are the final changes in diameter and length, cm.

The activation energy of the “volume flow” of the specimen during corundum sintering was calculated by the formula

\[ E=4.58\,\frac{\lg \dot{v}_2-\lg \dot{v}_1}{\frac{1}{T_1}-\frac{1}{T_2}} \]

The quantities \(\dot{v}_2\) and \(\dot{v}_1\)—the rates of change of pore volume—were estimated from the graph of the dependence

\[ \frac{V_c}{V_n}=\psi(\tau) \]

at temperatures

$T_1$ and $T_2$, respectively, and were reduced to $1\ \mathrm{cm}^3$ of volume, since during the transition from one temperature to another a certain change in volume occurred. $T_1$ and $T_2$ are absolute temperatures, °K. The activation energy of sintering of corundum with a 0.05 wt.% addition of zinc, chromium, and magnesium oxides proved to be:

for the temperature interval $1573{-}1623^\circ\mathrm{K}$ — 331 kcal/mole;

for the temperature interval $1773{-}1823^\circ\mathrm{K}$ — 559 kcal/mole.

Conclusions

1. To implement the dynamic method, a special high-temperature dilatometric setup (SVDU) was created. With the aid of the SVDU, the dynamics of the sintering rate were traced, and it was established that the selected oxide composition accelerates the densification process of electrocorundum powder during firing; the optimal holding time exceeds 2 hours.

2. The activation energy of sintering of a corundum specimen with 0.05% zinc, chromium, and magnesium was calculated; at $1573{-}1623^\circ\mathrm{K}$ it is 331 kcal/mole. At temperatures of $1773{-}1823^\circ\mathrm{K}$ the activation energy reaches 559 kcal/mole.

3. The limiting shrinkage rate of specimens with small additions of magnesium, zinc, and chromium is observed at a concentration of 0.075 wt.%. Deviation of their content in either direction from the optimal concentration by 0.025% significantly reduces the densification rate of corundum powder during firing.

Literature

1. J. L. Woolfrey, M. J. Bannister. Methods of nonisothermal investigation of the initial stage of sintering. Glass, Ceramics and Refractories, Express Information, 1973, No. 7.

2. V. A. Ivensen. Kinetics of Densification of Metallic Powders during Sintering. Moscow, Metallurgizdat, 1971.

3. E. V. Degtyareva, I. S. Kainarskii. Doklady of the USSR Academy of Sciences, 156, 4, 937, 1964.

SIMULTANEOUS DETERMINATION OF CHROMIUM AND IRON IN CORUNDUM

A. V. Voznyak, V. V. Kolyin

The grain products of chromic electrocorundum (CE) currently obtained contain impurities of metallic chromium and