EFFECT OF IRON OXIDES ON THE COMPRESSION CHARACTERISTICS OF DISPERSED PEAT

S. I. Smolyaninov, G. G. Krinitsyn

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

Compression indices may be considered among the most important parameters characterizing the physicomechanical properties of peat. The main task of compression tests is to establish the relationship between the porosity coefficient and the external load.

As experimental studies and theoretical developments have shown, the compression curve can, with a high degree of accuracy, be expressed by the equation [1—4]

\[ \varepsilon = C_1 - C \ln(P + P_k), \]

where
\(\varepsilon\) — porosity coefficient under pressure;
\(C\) — compaction coefficient of the compression curve;
\(P_k\) — internal pressure on the peat skeleton;
\(C_1\) — a constant dependent on the type of peat and its initial state.

In view of the small value [2—3], the approximate equation may be used

\[ \varepsilon = A_1 - C \ln P, \]

where \(A\) is the porosity coefficient at a pressure of \(1\ \text{kg}/\text{cm}^2\).

Studies by VNIITP [2] found that the compression curve for peat has a break, where its parameters change their value. It has also been shown that this break is reflected in the strength properties of peat.

N. A. Gal’bin, N. K. Rabotnov, and E. P. Semenskii [5], on the basis of an analogy between thermal drying and mechanical dewatering of peat, established that the point of intersection of two straight lines of the compression equation corresponds to the first critical point (of the drying curve [6—8]) of moisture content, which proved to be independent of the degree of decomposition and botanical composition of the peat and equal to \(2.50\ \text{g}/\text{g}\). The latter is presumably explained by the authors by the equality of the pore sizes of all peats at the critical point and by a change in the mechanism of pressure transmission.

E. T. Bazin and A. S. Korolev [9] showed the change in various forms of moisture with compaction and the change in the compaction mechanism associated with the geometry of the pore space, as well as the presence of a critical point corresponding to different states of peat.

Determination of the compression properties of peat and its mixtures with various-

with varying amounts of iron ore was carried out by us on the apparatus of I. M. Litvinov according to the method of V. M. Veselovskii [10]. The study was performed on peat from the Tagan deposit (sedge lowland peat, degree of decomposition 30%) and on its mixtures with magnetite concentrate, in which the mineral fraction constituted 5, 13, and 20% of the raw mass (moisture content 5.52 g/g) of peat that had been processed three times.

Figure 1 presents the dependence of the porosity coefficient on

Fig. 1. Dependence of the porosity coefficient \((\varepsilon)\) of peat and its mixtures with magnetite on pressure \((P,\ \text{kg/cm}^2)\): 1 — peat, 2 — mixture with 5% magnetite, 3 — peat with 13%, 4 — peat with 20%.

the external load. As can be seen from the data presented, in the pressure range tested, for all samples there are two sections with different parameters of the compression curve \(A(\varepsilon)\) and \(\dot{C}\). The critical points for peat without additive and with increasing additive content correspond, respectively, to pressures of 3.00; 2.82; 2.63 kg/cm² and to moisture contents of 2.3–2.8 g/g.

The same dependence can be constructed using the experimental data of L. S. Amaryan [11]. In this case, for different peats the critical point corresponds to an external load of about 3.25 kg/cm².

However, our processing of the experiments of E. T. Bazin [12] reveals a break point for raised-bog peats \((R = 5–20\%)\) at a considerably lower pressure, about 0.5 kg/cm². A characteristic picture of the relationship between deformation and the structure of peat is given by the curves of the dependence of relative deformation on external load (Fig. 2). Initially,

\[ \lambda = \frac{\varepsilon}{1+\varepsilon_0}, \]

there is a sharp increase in deformation up to a pressure of approximately 0.5 kg/cm²; then it slows down markedly. A similar pattern was noted by L. S. Amaryan [11] for different peats and was explained by him as a reduction in pore size (a slowing of the filtration process) and an increase in the number of particles per unit volume, which raise the resistance of the system to deformation. He also noted-

Fig. 2. Dependence of the relative deformation of peat and its mixtures with magnetite on pressure: 1 — peat, 2 — mixture with 5% magnetite, 3 — peat with 15%, 4 — peat with 20%.

Fig. 3. Dependence of the relative deformation of peat and its mixtures with magnetite on the amount of the mineral component at different pressures (numbers at the curves in kg/cm²): 1 — peat, 2 — mixture with 5% magnetite, 3 — peat with 13%, 4 — peat with 20%.

a linear decrease with increasing degree of decomposition of the peat, which is a consequence of the increase in peat density as a result of mineralization and coagulation of highly dispersed fractions.

Our data (Fig. 3) show a linear dependence of relative deformation on the content of iron oxides in mixtures with peat.

E. T. Bazin and A. S. Korolev [9] showed that the graph of the dependence of the relative compression modulus, expressed by the straight line

\[ E = \frac{P}{\lambda}, \]

has a characteristic point of inflection (critical point) at a pressure of about 5 kg and a moisture content of 2.5—2.6 g/g and, in their opinion, corresponds to the transition of peat from a plastic state to a solid one, with a specific change in the mechanism of moisture squeezing at these stages.

The data presented in Fig. 4 show that, on the graph of the indicated dependence, there may apparently be two inflection points. The first is at a pressure of about 1 kg/cm² and the second at 2.5—3.0 kg/cm².

It should be noted, however, that the above-mentioned authors in one case observed a sharp change in the absolute compression modulus at a moisture content of about 6.0 g/g (high-moor complex peat with a degree of decomposition of 20%). As our experiments showed, the critical points are expressed most clearly on the curve of the dependence of the compression coefficient (compaction coefficient) \(a\) on pressure \(P\)

\[ a = \frac{d\varepsilon}{dP}. \]

From the compression coefficient found by graphical differentiation, a graph was constructed in semilogarithmic coordinates (Fig. 4), which clearly revealed two critical points: one at a pressure of 1.10—0.90 kg/cm² (moisture contents 3.10; 2.26; 1.76 and 1.38 g/g for peat and its mixtures with 5, 13 and 20% magnetite, respectively) and the second at a pressure of 3.00—2.65 kg/cm² (moisture contents 2.30; 1.70; 1.35 and 1.05 g/g).

As is known [3, 4], for soils all of whose pores are completely filled with water, the compression curve can be represented as a curve of the relationship between pressure and moisture. Moreover, it co-

Fig. 4. Dependence of the relative compression modulus \((E)\) and the compaction coefficient \((a,\ \mathrm{cm}^2/\mathrm{kg})\) of peat and its mixtures with magnetite on pressure: 1 — peat, 2 — mixture with 5% magnetite, 3 — peat with 13%, 4 — peat with 20%.

is preserved both in the static and in the dynamic state.

Figure 5 shows a dependence plotted in semilogarithmic coordinates from our experimental data.

As can be seen from the graph, all experimental points lie well on a straight line described by the equation

\[ \lg P = \lg P_0 - BW, \]

Fig. 5. Dependence between pressure \((P,\ \mathrm{g}/\mathrm{cm}^2)\) and the moisture content of peat and its mixtures with magnetite. 1 — peat, 2 — mixture with 5% magnetite, 3 — peat with 13%, 4 — peat with 20%.

Fig. 6. Effect of magnetite additions on the total moisture capacity of peat \((W_n)\).

where $\lg P_0$ is the limiting value of this quantity, obtained by extrapolating the straight line to $W=0$;

$B$ is the angular coefficient.

It is interesting to note that for all samples $\lg P_0$ is the same and equals 4.90 (pressure is expressed in g/cm²). Extrapolation of the straight line to $W=0.18$ g/g, which corresponds to a pressure of 1.5 g/cm², gives a moisture value that may be interpreted as the full moisture content ($W_{\text{п}}$), since the height of the sample in the standard method of determining it [13] is 3 cm.

It follows from this that the indicated dependence is expressed in the same way as the well-known equation of S. S. Korchunov [2] (bond energy of moisture in peat—moisture content), and for our particular case has the following expression:

\[ \lg P = 4.90 - 4.70 \frac{W}{W_{\text{п}}}. \]

Using the data mentioned, it proved possible to establish the dependence of the full moisture capacity of peat on the content of iron oxides in the mixture (Fig. 6).

Thus, compression tests made it possible to reveal characteristic changes in the structural properties of peat and the influence on them of additions of iron oxides.

When iron oxides (magnetite) are introduced into the peat dispersed system, the general regularities of the change in the compression curves are preserved; however, their absolute values change toward an increase in resistance to deformation, linearly with the content of the mineral component.

On the curves of the dependences of the compression parameters on the external load, characteristic sections with critical inflection points and a different mechanism of deformation were revealed, determined by the geometry of the pore space and by the change in the number of particles per unit volume.

Particles of the finely dispersed filler penetrate into the pore space of the peat, which reduces the pore sizes of the solid phase (peat organic matter plus iron oxides) and the moisture capacity of the system.

REFERENCES

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