STRUCTURE-FORMING PROCESSES IN RAW TMP STOCK

S. I. Smolyaninov, G. G. Krinitsyn, T. V. Kupchikova

(Presented by the Scientific-Methodological Seminar of KhTF)

Wet, processed peat stock is a typical colloidal substance, which is manifested, in particular, in its capacity to form coagulation structures. Depending on various factors, the character of this process changes, which makes it possible to influence the structure of peat materials, achieving an improvement in their mechanical properties. One method of acting on peat material in order to regularize its structure is the introduction of powder-like materials into the wet peat stock.

This paper presents the results of a study of peat stock when finely dispersed iron oxides are introduced into it.

For the tests, peat from the Tagan deposit was taken, with a degree of decomposition of 20–25% and an initial moisture content of 83.5–87%. From it two samples were prepared: the first was peat stock without additives, and the second was a mixture of peat with iron oxide. The particle sizes of the iron oxide did not exceed 0.1 mm.

The course of the structure-forming processes was judged by the change in the limiting shear stress, which was measured by means of a conical plastometer.

Measurements were carried out at intervals of 24 hours and showed that at the first moment after preparation of the samples the magnitude of the limiting shear stress in both cases differs insignificantly. Over time its value for peat with additives increases substantially, whereas peat stock free of impurities changes its plastic properties to a much lesser degree. The results of the measurements are summarized in Table 1.

Processing of the results showed that, when finely dispersed iron oxides are introduced into the peat stock, the rate of the structuring process increases sharply. If, for peat without additives, the structuring rate is maximal at the first moment after processing, then in the presence of iron oxide the maximum structuring rate is observed after the mixture has been held for 24–36 hours.

To determine the time of the onset of development of the structural network in a mixture of peat with iron oxides, measurements of the limiting shear stress at 15-minute intervals over the course of 2.5 hours were proposed. As a result, the initial moment of development of the coagulation structure in the mixture of peat with iron oxides was assigned to 90–120 min from the moment of mixing.

Table 1

Ultimate shear stress \(\left(\theta \dfrac{\mathrm{g}}{\mathrm{cm}^{2}}\right)\) of raw peat stock \((w = 83.7\%)\)
and of its mixture with iron oxide \((10\% \text{ iron oxide})\) at a temperature of 20° C

Table 2

Ultimate shear stress \(\left(\theta \dfrac{\mathrm{g}}{\mathrm{cm}^{2}}\right)\) of a mixture of peat \((w = 84.3\%)\)
with iron oxide at a temperature of 20° C

A considerable increase in structuring in the presence of iron oxides can be explained by their chemical interaction with the active substances included in the composition of peat, which leads to an increase in the concentration of polyvalent ions in the system. If this assumption is correct, an increase in the temperature of the mixture should lead to an intensification of the coagulation process.

To verify this, a mixture of peat with iron oxide was kept at temperatures of 20° and 50°, at constant moisture content, for five days. Measurements of the ultimate shear stress were made at intervals of 12 hours. At the time of measurement, in order to eliminate the influence of changes in viscosity, the temperature of the stock was brought to 20°C. The measurement results are summarized in Table 3.

Table 3

Ultimate shear stress \(\left(\theta \dfrac{\mathrm{g}}{\mathrm{cm}^{2}}\right)\) of a mixture of peat \((w = 86.4\%)\)
with iron oxide (TPM) as a function of time at 20 and 50° C

Since the growth of the ultimate shear stress of peat stock is due to an increase in the number of coagulation contacts in the system and is proportional to it, it is reasonable to assume that the values obtained when \(\Theta\) changes are directly proportional to the concentration of coagulated particles in the stock, i.e., \(C = \Theta\). With this assumption, the process occurring in the mixture, by analogy with the equation of a chemical reaction of first order, can be represented in the form

\[ \frac{d\Theta}{d\tau} = K\Theta^{n}, \]

where
\(K\) is the reaction-rate constant;
\(n\) is the reaction order;
\(\tau\) is time.

Taking logarithms, we obtain

$$ \lg \frac{d\Theta}{d\tau}=\lg K+n\lg \Theta. $$

Substituting into this equation the values of $\Theta$ obtained experimentally, it becomes possible to calculate the reaction-rate constant that determines the process, and the value of the reaction order.

The calculation gave the following values: for a temperature of 20°C, $K=-4.82$; for a temperature of 50°C, $K=-3.56$. The value of the reaction order is close to first. On the basis of these data, the activation energy of the structuring process can be calculated from the equation

$$ E=4.57\,\frac{\Delta \lg K}{\dfrac{1}{T_1}-\dfrac{1}{T_2}}. $$

Substitution of the values found into the equation gives an activation energy equal to 16,400 kcal/mol.

Conclusions

1. Additions of iron oxides lead to intensification of the structuring process in raw peat pulp.

2. The main kinetic characteristics of the process can be calculated by analogy with first-order chemical reactions.