Changes in the Rheological Properties of Peat Mass upon Incorporation of an Iron Ore Component
Abstract Generated abstract
This paper examines how adding an iron ore component alters the moisture content and rheological behavior of peat mass intended for peat ore material production. Using established relations between moisture content and limiting shear stress, together with calculations based on carbon to iron requirements and observations on Tagan peat mixtures, the authors estimate practical additive limits and discuss the colloid chemical processes occurring during dispersion, holding, and heating. They conclude that iron ore additions of about 6.5 to 20% of the wet peat mass can substantially affect structural development, moisture capacity, electrokinetic properties, aggregation, and shear resistance, while mixtures with small additions may initially resemble dispersed peat without additives. The findings indicate that molding should be carried out immediately after dispersion when developing production technology.
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CHANGE IN THE RHEOLOGICAL PROPERTIES OF PEAT MASS UPON THE ADDITION OF AN IRON-ORE COMPONENT
S. I. SMOLYANINOV, G. G. KRINITSYN
(Presented by the scientific-methodological seminar of the organic departments of KhTF)
Rheological properties can be characterized by the viscosity of a mass or by the limiting shear stress. According to S. S. Korchunov, the following relationship exists between the limiting shear stress and moisture content [1]:
\[ \ln \Theta = \ln \Theta_0 + \lambda (W_0 - W), \]
where \(\Theta\) is the limiting shear stress;
\(\Theta_0\) is the limiting shear stress at full moisture capacity;
\(\lambda\) is the structural hardening coefficient;
\(W\) is the moisture content of the mass;
\(W_0\) is the full moisture capacity.
The introduction of mineral additives changes the moisture content of the peat mass. To establish the limits of the change in moisture content, and consequently the change in the rheological characteristics of the peat mass, it is necessary to determine the limits of the content of mineral components in it.
The purpose of obtaining a peat-ore mixture is the rational use of fuel. This is achieved through the interaction of volatile products of the thermal processing of dried peat and iron oxides. The content of nonvolatile carbon of the peat and oxides in the mixture must be in a definite ratio. Thus, for the use of peat-ore material in the blast-furnace process, the ratio \(C : Fe\) should be from \(1 : 1\) to \(0.7 : 1\). The amount of nonvolatile carbon may be taken (in accordance with the peats under study) as 20% of the dry mass.
Thus, the addition of ore with an Fe content of 60% to the wet mass in this case should amount to 6.5%. The change in the moisture content of the mass under the influence of such an amount of additives should be
\[ W_1 = \frac{100 W}{\mu + \mu_1}, \]
where \(W_1\) is the moisture content of the mixture;
\(\mu\) is the weight of the initial peat;
\(\mu_1\) is the weight of the mixture.
In the case of a moisture content equal to 85%, the addition of such an amount of mineral impurities will reduce the moisture content to 81%. Such a change in moisture content is very substantial and, according to Lazovskaya [2], leads to a change—
... of the limiting shear stress from \(7\ \text{dyn}/\text{cm}^2 \cdot 10^{-3}\) to \(12\ \text{dyn}/\text{cm}^2 \times 10^{-3}\).
An increase in the content of the iron-ore component in the peat mass contributes to an even greater increase in the limiting shear stress. Increasing the amount of iron ore to 20% leads to a total moisture content of the system of 70%, which corresponds to approximately 60% mineral impurities on a dry-weight basis; such an amount of impurities may be considered limiting.
Thus, the amount of additives in our case should not exceed 6.5–20% of the wet mass of the peat. Naturally, the limiting shear stress in the case of 20% additives is high. Nevertheless, it differs from the value of the limiting shear stress obtained for these same peat samples without additives when they are dried to 70%.
Studies carried out with Tagan peat and its mixtures with iron oxides indicate that an artificial reduction in peat moisture content through a change in the solid phase does not lead to the expected changes in the limiting shear stress. Thus, when the amount of additives is 5–10%, the mixture obtained by intensive dispersion of peat in the presence of mineral particles, in terms of limiting shear stress, practically does not differ from peat without additives of the same degree of dispersion at the first moment after dispersion. The reason for this lies in the great intensity of dispersion in the presence of mineral impurities and in the release of a large amount of intracellular water and the low rate of solvation of ore particles; the influence of polyvalent metal ions and the removal of the stability factor from individual areas of the surface of peat particles are also manifested.
Thus, the processes occurring in the peat mass when it is mixed with mineral additives are colloidal and must obey the corresponding theoretical principles of colloid chemistry; but at the same time, as the data show, chemical interaction between the substances constituting the peat and the mineral additive is possible in the system. The result of this is an increase in the concentration of free metal ions, which must affect the colloidal process. Holding the peat mass at an elevated temperature leads to an acceleration of the development of the structural network. The structural parameters of peat without mineral additives change only slightly when it is held at elevated temperature. The electrokinetic potential of peat with additives decreases when the mass is held. An analogous phenomenon is observed when small amounts of iron ions are introduced into peat.
The results of these phenomena include a decrease in the average particle size, an increase in the adhesion forces between them, the formation of enlarged loose aggregates, and an increase in the amount of weakly bound water that can be released from the molding. In connection with the fact that syneresis processes occur in the peat mass, part of the moisture may be removed from the peat mass during their course.
The change in the structure of peat after mineral additives are introduced into it can be traced by the influence of these changes on the total moisture capacity. Measurements carried out with peat into which 15% additives had been introduced and which initially had a total moisture capacity equal to \(5.1\ \frac{2}{2}\) showed that at first there is a certain increase in moisture capacity to \(5.4\ \text{g}/\text{g}\), and then its gradual decrease to \(2.4\ \text{g}/\text{g}\).
If it is assumed that the chemical properties of the peat mass change, then the principal changes in moisture capacity occur as a result of changes in the size...
particles and in their shape. Thus, the changes initially amount to the formation of a large number of spherical particles, which then form aggregates of larger size. This is consistent with the results of measurements of viscosity and limiting shear stress.
All these considerations must be taken into account when developing technologies for the production of TPM. According to the data presented, molding should be carried out immediately after dispersion.
REFERENCES
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Korchunov S. S. Study of the physicomechanical properties of peat. Proceedings of VNII TP, issue 12, Gosenergoizdat, 1953.
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Lazovskaya N. V. Rheological studies of peat. Proceedings of MTI, issue 5, 141–172, Moscow, 1957.