Intensification of the Drying Process for Small-Sized Plastic-Molded Products
Abstract Generated abstract
This study examines the drying of small plastic-molded ceramic bodies, treated as capillary-porous colloidal materials whose quality is strongly affected by moisture removal during the initial shrinkage period. Experiments on bauxite tablets compared radiation drying with and without an absorbent powdered clay carrier, measuring drying kinetics, contact moisture exchange, and final density under different temperature regimes. The carrier was found to remove loosely bound capillary moisture in liquid form down to about the maximum molecular moisture capacity, while later impeding vapor removal and reducing gradients. On this basis the authors propose a combined process, preliminary dehydration in a moisture-capacious medium followed by high-temperature drying with the carrier, which shortened thermal drying to about 5 to 7 minutes while maintaining higher product density than radiation drying alone.
Full Text
- P. A. Rebinder. New Methods of Physicochemical Investigations of Surface Phenomena. Publishing House of the Academy of Sciences of the USSR, 1950.
- P. A. Rebinder. “Physicochemical Mechanics—a New Field of Science.” Moscow, Znanie, 1958.
- V. S. Fadeeva. Optimum moisture content for the molding of building products from plastic dispersed masses. Glass and Ceramics, 33, no. 8, 1959.
- Edited by N. P. Bogoroditskii and V. V. Pasynkov. Radioceramics. Moscow–Leningrad, Gosenergoizdat, 1963.
- A. V. Lykov. Theory of Drying. Energia Publishing House, 1968.
- P. S. Kovalev. Author’s abstract of a dissertation. Kiev, 1967.
- E. G. Chapovskii. Laboratory Work in Soil Science and Soil Mechanics. Moscow, Nedra Publishing House, 1966.
Intensification of the Drying Process for Small-Sized Plastic-Molded Products
V. A. Lotov, V. M. Vitlugin
Raw ceramic products obtained by the method of plastic molding, in accordance with A. V. Lykov’s classification, may be assigned to typical capillary-porous colloidal bodies with a coagulation-structured pore suspension. From the standpoint of physicochemical mechanics, the drying process of freshly molded products is a process of transforming the coagulation structure into a condensation structure, accompanied by a significant change in structural-mechanical properties. The quality of the dried products, the principal criterion of which is their density, will to a considerable extent be determined by the regime parameters of the drying process.
In solving the problems of accelerating drying and obtaining defect-free products, the main issue is the correct choice of the maximum value of the permissible drying rate. Of greatest importance for product quality is the drying rate in the initial period, when shrinkage of the products occurs as a result of the removal of capillary moisture. Exceeding the permissible rate of moisture removal during this period may lead to local crack formation as a consequence of the appearance of significant temperature and moisture gradients. One way of eliminating crack formation is the use, during drying, of a moisture-capacious companion medium. K. Laslone [1], studying
The influence of an auxiliary material on the drying process of model capillary-porous colloidal bodies (carrot, potato) established that the auxiliary material (a backing of filter paper) intensifies drying in the initial period.
In the present work, air-dry technical clay earth was used as the auxiliary material; it has a porous structure of individual particles and good absorbency. The drying objects were specimens in the form of tablets 15 mm in diameter and 3 mm thick, molded from finely dispersed bauxite (fraction less than 80 μm) from the Tatar deposit. The initial moisture content of the specimens was 30%. The surface properties of the bauxite and clay earth were evaluated according to the principal indices of water-physical properties presented in Table I.
Table I
Principal water-physical properties of the materials studied
| Material | \(W_{\text{mkv}}, \%\) | \(W_{\text{mmv}}, \%\) | \(W_{\text{mg}}, \%\) | Coefficient of capillary saturation rate, cm\(^2\)/s |
|---|---|---|---|---|
| Technical clay earth | 61.2 | 25.6 | 12.5 | \(17.5 \cdot 10^{-2}\) |
| Tatar bauxite | 52.5 | 21.5 | 8.8 | \(1.1 \cdot 10^{-2}\) |
The procedure for studying drying with an auxiliary material consisted in placing the specimens under investigation, with a known moisture content, in the clay earth. Under the action of capillary forces, the specimen became coated with a layer of clay earth, after which the specimen under investigation was immediately placed in a radiation dryer with a disk metal emitter. After definite intervals of drying time had elapsed, the specimens were cleaned of clay earth with a brush, weighed, and placed in a drying cabinet for final drying at a temperature of 105°C. The moisture content of the specimens was determined from the loss of moisture during the drying process. To determine the influence of the aux-
of the satellite on the drying kinetics of the specimens, radiation drying of specimens without a satellite was carried out under identical conditions.
The results of the studies, presented in Table 2, make it possible to conclude that the satellite intensifies the removal of moisture from the specimens in the first drying period at a temperature of \(100^\circ\mathrm{C}\). In the second drying period the satellite slows the removal of moisture from the specimens. At a temperature of \(150^\circ\mathrm{C}\) the satellite also hinders moisture removal, and at \(250^\circ\mathrm{C}\) it has practically no effect on the drying kinetics of the specimens studied. It is interesting to note that in the initial drying period there is a sharp decrease in moisture content, and to a definite value under the different temperature regimes. This phenomenon is explained by the fact that, as a result of contact moisture exchange, moisture migrates in liquid form from the specimen into the satellite. After the moisture content has decreased to 20–22%, the drying rate with the satellite decreases substantially. To provide an additional check of the influence of the satellite on moisture removal, the kinetics of this process was investigated at a temperature of \(20^\circ\mathrm{C}\), i.e., with practically no temperature gradient. The results presented in Table 3 show that, solely due to contact moisture exchange, the moisture content of the specimens decreases from 30 to 20–21%.
Table 3
Dependence of the moisture content of specimens on the time of contact with the satellite
| Contact time, min | 1 | 3 | 5 | 10 | 15 | 20 | 25 | 30 |
|---|---|---|---|---|---|---|---|---|
| Moisture content, % | 28.2 | 25.1 | 23.6 | 22.2 | 21.5 | 20.7 | 20.3 | 19.8 |
Comparison of this moisture content and the residual moisture content of the specimens during drying with the satellite under different temperature regimes after the first period, with the value of the maximum molecular moisture capacity of bauxite determined by the method of moisture-capacity media [2], shows that the principal role played by the satellite during drying consists in the removal of loosely bound capillary moisture migrating into the satellite in liquid form. The remaining part
Table 2
Dependence of the moisture content (%) of the samples on the drying mode and method
| Drying time, min | Radiation drying, drying temperature, °C: 50 | Radiation drying, drying temperature, °C: 65 | Radiation drying, drying temperature, °C: 100 | Radiation drying, drying temperature, °C: 150 | Radiation drying, drying temperature, °C: 200 | Radiation drying, drying temperature, °C: 250 | Radiation drying with blowing, drying temperature, °C: 50 | Radiation drying with blowing, drying temperature, °C: 65 | Radiation drying with blowing, drying temperature, °C: 100 | Radiation drying with blowing, drying temperature, °C: 150 | Radiation drying with blowing, drying temperature, °C: 200 | Radiation drying with blowing, drying temperature, °C: 250 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 3 | 28,5 | 25,1 | 19,8 | 8,3 | 5,1 | 0,5 | 23,8 | 22,2 | 18,3 | 12,0 | 5,3 | 0,5 |
| 5 | 26,0 | 22,1 | 12,8 | 2,5 | 0,8 | 0,3 | 20,7 | 18,4 | 14,2 | 4,8 | 0,8 | 0,3 |
| 10 | 22,0 | 14,5 | 2,5 | 0,3 | 0,3 | - | 17,2 | 13,7 | 4,9 | 0,3 | 0,3 | - |
| 15 | 18,0 | 8,6 | 0,3 | - | - | - | 14,0 | 10,0 | 1,1 | - | - | - |
| 20 | 14,0 | 4,9 | 0,3 | - | - | - | 11,3 | 7,1 | 0,5 | - | - | - |
| 30 | 6,2 | 0,9 | - | - | - | - | 6,5 | 2,4 | - | - | - | - |
| 40 | 1,7 | 0,3 | - | - | - | - | 3,1 | 0,3 | - | - | - | - |
moisture, held by molecular forces, can move mainly in the form of vapor. The rate of removal will be determined by the rate of evaporation and filtration of water vapor through the structure of the material. The auxiliary agent creates additional resistance to the escape of vapor into the surrounding medium, which explains the decrease in the drying rate with an auxiliary agent in the second period as compared with drying without an auxiliary agent. However, this phenomenon should not be regarded as negative, since the auxiliary agent acts as a kind of moisture-retaining shell and helps reduce moisture and temperature gradients in the second drying period.
Reducing the initial moisture content of the specimens to the value of the maximum molecular moisture capacity makes it possible to intensify the drying process in the second period by increasing the heat loads. On the basis of the investigations carried out, a combined method of moisture removal is proposed, in which freshly molded articles are first dehydrated in a moisture-capacious medium to a moisture content equal to the maximum molecular moisture capacity, after which they can be dried in the presence of an auxiliary agent under a severe temperature regime. Thus, the use of an auxiliary agent for partial dehydration of specimens makes it possible to eliminate from the thermal drying process the most dangerous first period. A comparison of the density of specimens dried by radiation, radiation with an auxiliary agent, and combined methods (Table 4) shows that the use of the combined drying method is the most expedient both from the standpoint of improving the quality of the dried articles and for intensifying the drying process. Thus, to achieve a density of dry moldings of 1.635 g/cm³, radiation drying must be carried out at a temperature of 65°C, radiation drying with an auxiliary agent at 150°C, and combined drying at 250°C. In this case the drying time
Table 4
Dependence of specimen density (g/cm³) on drying temperature
| Drying method | 50 | 65 | 100 | 150 | 200 | 250 |
|---|---|---|---|---|---|---|
| Radiation | 1.675 | 1.67 | 1.655 | 1.635 | 1.62 | 1.60 |
| Radiation with auxiliary agent | 1.70 | 1.69 | 1.68 | 1.67 | 1.66 | 1.65 |
| Combined | 1.71 | 1.705 | 1.70 | 1.69 | 1.68 | 1.67 |
is, respectively, for the radiation method 35 minutes, for the radiation method with a carrier about 10 minutes. With combined drying, the required time of thermal action on the articles being dried is 5–7 minutes.
Conclusions
-
A method has been investigated for drying molded capillary-porous colloidal bodies using powdered mixed-layer clay as a carrier.
-
A scheme has been proposed for a combined drying method with a carrier.
Literature
-
K. Laslone. Investigation of heat and mass transfer in drying processes with a carrier. Author’s abstract of dissertation, Minsk, 1970.
-
A. M. Vasil’ev. Fundamentals of the modern methodology and technique of laboratory determinations of the physical properties of soils. Moscow, 1955.
STUDY OF THE WATER-PHYSICAL PROPERTIES AND CAKING CAPACITY OF SOME POWDERED OXIDES
I. N. Lantsman, G. V. Kocherovskaya, N. F. Stas’, G. G. Savel’ev,
M. S. Lantsman, V. V. Nakhalov, V. M. Vitogin
The practical use of oxide adsorbents for carbon dioxide /1/ without preliminary aggregation will apparently be hampered because of their high dispersity and possible entrainment by the stream of purified gas. The most expedient method of aggregating adsorbents is granulation, since in its process the microstructure of the finely dispersed material is not disrupted. In connection with this, in parallel with studies of adsorption capacity, kinetics, and thermodynamics of adsorption /2, 3/, we have begun investigations on granulation, primarily on determining the water-physical properties and caking capacity of oxides.
The effectiveness of the process of granulating finely dispersed materials is determined by the nature of the material and by the technological regime of the process. The set of natural properties of the material can be preliminarily assessed by the caking-capacity index, which is determined by the ratio /4/: