Investigation of Structure-Forming Processes in Peat Disperse Systems by the Filtration Method

S. I. Smol’yaninov, G. G. Krinitsyn, V. A. Lyudomir

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

The initial period in the formation of the structure of peat is the formation of a system of coagulation contacts, occurring in peat that has been subjected to dispersion and has not yet lost moisture. The degree of development of the coagulation structure determines the behavior of the material during drying and its physicomechanical properties after the loss of moisture. The introduction of finely dispersed iron oxides into the peat mass affects the properties and structure of the peat material, and these changes should be perceptible at the first stage—during the formation of the coagulation structure. The most widespread method for evaluating the structural characteristics of viscoplastic materials is the filtration method.

Taking peat material as a system of vertical capillaries, the quantities characterizing porosity and kinetic specific surface can be determined by the Kozeny–Carman formula [1]:

\[ V=\frac{m^{3}}{\beta \eta S}\,\frac{\Delta P}{\Delta x}, \]

where \(V\) is the filtration velocity;
\(m\) is the value of active porosity;
\(\beta\) is the pore-shape coefficient;
\(\eta\) is the viscosity of the liquid;
\(\dfrac{\Delta P}{\Delta x}\) is the pressure gradient.

The principal quantities measured in the filtration zone are changes in the concentration of the indicator in the filtrate and the amount of filtrate as a function of time.

Radioactive isotopes [1—3] or solutions of substances having low sorption capacity with respect to peat may serve as indicators. According to data from a number of authors [4—6], sucrose is a good indicator for the investigation of peat.

The investigations were carried out on an FP-1 apparatus designed by M. P. Volarovich and N. V. Churaev [1]. The filtration cassette strictly corresponded to the description. Filtrate samples were taken at equal time intervals using an automatic fraction collector of the 301V type. The indicator was sucrose at a concentration of 11%.

The subject of investigation was sedge lowland peat from the Tagan deposit of Tomsk Oblast, having a degree of decomposition

30–35%, ash-component content 8.7%, working moisture content before the experiment 83.7%; and high-moor cotton-grass–sphagnum peat from the Peschanoe deposit with a degree of decomposition of 15–20%, moisture content 86%, ash content 4.7%. The mineral additives used were iron oxide $\mathrm{Fe_2O_3}$, chemically pure; ferrous oxide $\mathrm{FeO}$, chemically pure; and magnetite concentrate from the Abagur sinter plant [7]. The particle-size composition of the mineral components corresponded to the data summarized in Table 1.

Table 1

Characteristics of the particle-size composition of the iron oxides used in the work

Initially, peat samples without additives and with varying degrees of processing were studied. The results are given in Table 2.

An increase in the degree of dispersion of peat leads to an increase in the number of pores of small radius, and an increase in the kinetic specific surface is observed. A comparison of once- and three-times-dispersed

Fig. 1. Pore-size distribution curves for lowland peat (I) and peat with an addition of 5% iron oxides (magnetite concentrate) (II)

Table 2

Characteristics of the structure of peat and peat-forming materials

peat shows that the active porosity of the latter increased by 25%, and the kinetic specific surface area more than doubled.

The introduction of iron oxides into the peat mass is reflected in the filtration characteristics. The character of the pore-size distribution curves indicates a shift of the predominant radius toward lower values. If the predominant pore size in peat without additives is 1.2 μm, then the addition of 5% brings this value to 0.84 μm (Fig. 1).

When additives are introduced into high-moor peat, the change in the porous structure is characterized by the following values of the predominant pore size. With the introduction of a 5% additive—1.5 μm, 10%—1 μm, 15%—0.6 μm. The reason for the change in pore diameter toward smaller values may be both the intensification of structure formation in the presence of iron oxides and the penetration of finely dispersed particles of mineral additives into the pores, leading to a decrease in the volume of the pore space (Table 2, Fig. 3).

Conducting filtration experiments with holding for 48 and 84 hours (Fig. 2) showed that, in samples not subjected to any

Fig. 2. Change in the character of pore-size distribution during holding of a peat-ore mixture (10% Fe₃O₄):
1 — mixture after preparation; 2 — holding for 48 hours; 3 — holding for 84 hours.

effects, a change in the porous structure occurs. This circumstance indicates the occurrence in the peat sample of structure-forming processes, the essence of which, apparently, reduces to an increase in the number of contacts in the coagulation network.

Probably, the number of contacts can be estimated through the value of their surface area, proportional to the number of contact interactions. The surface area of the particles of systems can be determined, when the average particle radius and the true specific gravity of the material are known, by the formula [8]

\[ d = \frac{6 \cdot 10^4}{DS}, \]

where \(d\) is the particle diameter;

Fig. 3. Pore-size distribution in samples of high-moor peat with different amounts of mineral additives:
1. Fe₂O₃ — 5%; 2. Fe₂O₃ — 10%; 3. Fe₂O₃ — 15%.

\(D\) — true specific gravity;
\(S\) — particle surface area.

The average particle diameter was calculated from sedimentation-analysis data. The true specific gravity was determined experimentally.

The mean values of the particle diameters determined for peat processed once, twice, and three times are, respectively, \(0.0423\), \(0.0313\), and \(0.0220\) mm. These sizes of globular particles correspond to the following surface areas, at a peat density equal to

\[ 1.45 \ \frac{\mathrm{g}}{\mathrm{cm}^{3}};\qquad 10340 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}};\qquad 13793 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}};\qquad 20689 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}}; \]

The values of the kinetic specific surface obtained in the filtration experiments are, respectively,

\[ 4336 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}};\qquad 5015 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}};\qquad 9849 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}}. \]

The contact area, equal to the difference between the surface values determined by different methods, is, respectively,

\[ 6000 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}};\qquad 8800 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}};\qquad 9849 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}}. \]

The addition of iron oxides leads to a change in the average diameter of the peat particles. The contact area, calculated with allowance for the surface of the mineral particles introduced into the system, is

\[ 6500 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}};\qquad 10540 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}};\qquad 11767 \ \frac{\mathrm{cm}^{2}}{\mathrm{cm}^{3}}. \]

According to these data, mineral additives intensify the process of formation of contacts between particles. The results of the filtra-

tion tests of samples with different contents of \(\mathrm{Fe}_2\mathrm{O}_3\) additives (Fig. 3) confirm this. The presence of an increase in the contact area, according to the concepts of G. I. Kuzman [9] and S. T. Solopov [9, 10], has a favorable effect on the physicomechanical properties of the material.

REFERENCES

1. Volarovich, M. P.; Churaev, N. V. Investigation of the properties of peat and the processes occurring in it with radioactive isotopes. Moscow, “Nauka,” 1960.

2. Churaev, N. V. Methods for studying the water properties and structure of peat using radioactive indicators. In: New Physical Methods for Studying Peat. Moscow–Leningrad, Gosenergoizdat, 1960, p. 125.

3. Volarovich, M. P.; Churaev, N. V.; Minkov, B. Ya. Investigation of the properties of peat using radioactive isotopes. DAN SSSR, vol. 114, no. 5, 1957.

4. Zhitetskaya, F. D. Methods for determining bound water in peat. In: New Physical Methods for Studying Peat. Moscow–Leningrad, Gosenergoizdat, 1960, p. 138.

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8. Karnaukhov, L. P. Globular model of porous bodies. Kinetics and Catalysis, vol. XII, issue 5, 1971, p. 1235.

9. Kuzman, G. I.; Novichkov, S. N. Investigation of the process of drying and moistening fine-lump peat fuel. Engineering Physics Journal, vol. 5, no. 3, 1962, pp. 33–38.

10. Solopov, S. T. Technological principles for the production of high-quality lump fuel in the development of peat deposits of reduced operating moisture content. Scientific Reports of Higher Education. Mining, 1958, no. 1, pp. 41–49.