DEVELOPMENT OF INTERNAL STRESSES IN MOLDED PEAT DURING DRYING

S. I. Smolyaninov, G. G. Krinitsyn, Yu. A. Dvoryankin

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

The drying process of various capillary-porous colloidal bodies, or dispersed systems in general, is accompanied by changes in their structure and volume. Shrinkage deformation is caused mainly by the forces of capillary contraction (the sum of the forces of surface tension on the liquid menisci in the pores of the structure over the entire cross section of the sample) and by the forces of adhesive and cohesive interaction that arise at the points of secondary contacts of particles of the solid phase. The relationship between the forces causing shrinkage of the material and the strength of the structure, characterized by the limiting shear stress, determines the rate and magnitude of shrinkage responsible for the formation of a strong sample. At the same time, a stressed state arises in the material, which is a consequence of the interaction of molecular attraction and repulsion forces. Shrinkage stresses can cause undesirable deformations leading to a decrease in the strength of the samples [1–6].

According to M. S. Ostrikov and G. D. Dibrov [2], shrinkage stress is an experimentally determined quantity representing the resultant of the magnitudes of the forces of capillary contraction and elastic resistance of the structure. The kinetics of the growth of shrinkage stresses has been considered theoretically and determined experimentally for a number of dispersed systems. Curves of shrinkage stresses as a function of time have a characteristic S-shaped form [2, 3].

A fundamentally different approach to determining internal stresses was developed in the work of N. I. Gamayunov and A. L. Koroleva [5]. They measured the internal stress in drying samples of molded peat by means of a spherical sensor 4–6 mm in diameter, converting pressure into an electrical signal. The sensor was placed inside the sample at the moment of molding. Details of the sensor design are not described.

In connection with our study of the effect of iron oxides on structure-forming processes in peat dispersed systems, we investigated changes in internal stresses during the drying of molded peat samples with different amounts of mineral additives.

Lowland sedge–hypnum peat has a degree of decomposition of 25–30%, working moisture content of 87%, and ash content on a dry-matter basis of 11.52%. Before molding, it was subjected to threefold processing in a screw disintegrator.

As mineral additives, reagent-grade iron oxide was used (ground to pass through a 0.05 mm sieve) and magnetite concentrate.

of the Abagur enrichment plant, with an iron content of 62.7% and a degree of grinding of 0.08 mm.

A resistance strain gauge of the PKP-5 type, cylindrical in shape (3×10 mm), was placed at the center of a cylindrical sample (40×60 mm) of peat or of its mixture with iron oxides. Drying was carried out under laboratory conditions at an ambient temperature of 21–23°C and a relative air humidity of 62–68%. The signal from the UTCh-1 strain-gauge amplifier was recorded automatically.

The results of the experiments are presented in Figs. 1–3. As can be seen from the graphs shown (Fig. 1), the method we have developed for determining shrinkage stresses in drying disperse systems gives

Fig. 1. Dependence of internal stress in samples of molded peat and its mixtures with various amounts of iron oxide on moisture content. 1—peat; 2—mixture with 5% iron oxide; 3—the same with 13%; 4—the same with 20%.

a more visually clear picture of the change in this parameter over a wide range of moisture content than those described earlier [2, 5].

For all the samples tested, the characteristic general pattern of change in internal stress during drying is preserved, clearly reflecting the structural changes that occur in the process.

The first segment (up to a moisture content of 2–3 g/g) of the curve is characterized by a relatively low rate of increase in shrinkage stresses and corresponds to physicochemical and structural changes in the system, where coagulation contacts and capillary contraction forces predominate. Then a period begins in which molecular forces prevail. This moment corresponds to the transition from coagulation structures to condensation structures and is characterized by a sharp increase in internal stresses up to a maximum value corresponding to a moisture content of 0.25–0.75 g/g. This moment apparently corresponds to the complete disappearance of capillary menisci and, consequently, of capillary contraction forces; only physicochemically bound moisture remains in the peat, and removal of water ceases.

It is interesting to note a noticeable increase in the values of the maximum internal stresses as a function of the amount of mineral ad-

bundles. This dependence (Fig. 2) approximates a straight line and shows the changes occurring in the peat disperse system under the influence of the filler activity. These changes, as we showed earlier [7, 8], lead to a decrease in the hydraulic radius of the pores, an increase in the limiting shear stress, and an increase in the number of contacts per unit volume and, as a consequence, to an increase in the strength of air-dry molded pieces. As the content of iron oxides increases, the maximum shrinkage stress is reached at a lower moisture content, which, in accordance with what was said above, reflects the corresponding changes in the form of moisture bonding with the peat.

Fig. 2. Dependence of the maximum internal stress in drying specimens of molded peat and its mixtures with magnetite (●) and iron oxides (○) on the content of mineral components.
Axes: internal stress, \(10^5\ \mathrm{N/m^2}\); content of iron oxides, %.

Fig. 3. Dependence of the strength of air-dry specimens of molded peat and its mixtures with iron oxides on the maximum magnitude of internal stresses.
Axes: compressive strength, \(10^3\ \mathrm{N/m^2}\); maximum internal stress, \(10^5\ \mathrm{N/m^2}\).

However, it should be noted that an excessive increase in internal stresses, when they are distributed nonuniformly, can lead to the appearance of weakened places, the formation of cracks, and a decrease in the strength of the body structure or to its complete destruction. A very clear picture in this respect is provided by the graph constructed from our experimental data (Fig. 3), which relates the maximum internal stresses and the compressive strength of the air-dry material. The sharply expressed maximum of strength as a function of the magnitude of shrinkage stresses is, in all probability, a general regularity for disperse systems. This can be successfully used in developing structures with specified properties.

Conclusions

1. A method has been developed for the tensometric determination of shrinkage stresses in disperse structures.

2. It has been shown that, during the drying of peat, shrinkage stresses reflect the physicochemical and structural changes occurring in disperse systems during moisture loss.

3. When iron oxides are added to the raw peat mass, shrinkage stresses increase, which at first leads to an increase in the strength of the structure and then, upon reaching a critical value of this indicator, to a decrease in the strength of air-dry specimens.

References

1. P. A. Rebinder, I. N. Vlodavets. Physicochemical mechanics of porous and fibrous disperse structures. In: Problems of the Physicochemical Mechanics of Fibrous and Porous Disperse Structures and Materials. Riga, “Zinatne,” 1967, pp. 5–43.

1. M. S. Ostrikov, G. D. Dibrov. On the influence of shrinkage stresses on the formation of the structure of drying fibrous and porous materials. Ibid., pp. 205–219.

2. A. S. Avedikov, M. S. Ostrikov, G. D. Dibrov, Yu. M. Kuprin. Shrinkage stress as the resultant of a number of components. Ibid., pp. 225–232.

3. A. I. Fedotov, N. V. Churaev, N. S. Shaban. Investigation of structure-forming processes during drying of viscoplastic peat. Ibid., pp. 399, 406.

4. N. I. Gamayunov, N. L. Koroleva. Investigation of stresses in the process of shrinkage of colloidal materials. Ibid., pp. 439–448.

5. A. V. Lykov. Theory of Drying. Moscow, “Energiya,” 1968.

6. S. I. Smolyaninov, G. G. Krinitsyn. Drying of peat fuel-slab materials. Izv. TPI, vol. 233, (in press).

7. S. I. Smolyaninov, G. G. Krinitsyn. Capillary phenomena in peat. Izv. TPI, vol. 233.