Kinetics of Thermal Decomposition of Talc
Abstract Generated abstract
This study examines the thermal decomposition kinetics of high-quality talc from the Alguy and Onot deposits, a process relevant to the heat treatment of steatite ceramics. Weight loss during isothermal heating was measured with a torsion balance on thin pre-fired samples held for 30 minutes at selected temperatures, allowing dehydration behavior and reaction-rate constants to be determined. The results indicate that talc dehydration follows first-order kinetics, with Onot talc decomposing between 750 and 950°C and Alguy talc between 800 and 1000°C, and an estimated activation energy of about 33 kcal/mol. The comparison suggests that the scaly Alguy talc is more thermally stable than the fine-grained Onot talc, decomposing more slowly at the same temperatures.
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KINETICS OF THE THERMAL DECOMPOSITION OF TALC
P. G. USOV, N. V. SOBORA
(Presented by the scientific seminar of the Department of Silicate Technology)
Talc, a hydrous magnesium silicate, is widely used in various types of ceramic products. For steatite ceramics, in which it is the predominant component of the batch, the processes occurring during heating are of great importance. Therefore, knowledge of the kinetics of talc decomposition during heat treatment is of considerable interest.
The object of the present study was high-quality talc samples from the Alguiskoe and Onotskoe deposits, whose chemical composition differs only slightly from the theoretical composition of the mineral talc (Table 1).
Alguiskoe powdered talc is white in color and has a scaly structure. Under the microscope, the colorless scales exhibit straight extinction and well-defined pseudoabsorption, with \(N_g = 1.580\) and \(N_p = 1.539\). As a mechanical impurity, isometric quartz grains are present in an amount of about 2%.
Onotskii talcite is a dense, massive rock of white color with a greenish tint; in ground form it is pure white. Under the microscope, Onotskii talc—a monomineralic talc variety—is composed of finely scaly aggregates. The refractive index is \(N_g = 1.583\), \(N_p = 1.541\).
The kinetics of talc decomposition during heating was studied by measuring the change in the weight of the substance at a specified constant temperature. The loss in weight was recorded on a torsion balance with an accuracy of 0.25% every 1 minute. The talc samples used, weighing 300–400 mg, formed as thin disks 15 mm in section and about 1.5 mm thick, suspended on a platinum thread, were preliminarily fired at \(600^\circ\text{C}\) and, while hot, were lowered into a furnace preheated to the test temperature. The holding time of the weighed sample in the furnace at each temperature was 30 minutes. During testing, the furnace temperature was maintained automatically with an accuracy of \(\pm 5^\circ\text{C}\).
This experimental method compares favorably with others [2] in that: 1) up to the required temperature no processes occur in the sample; 2) owing to the large area of heat exchange and mass transfer, the dehydration process of talc proceeds more distinctly; 3) at high heating temperatures, the furnace temperature corresponds to the sample temperature.
The experiments showed that the decomposition process of Onotskii talc occurs in the temperature range \(750\text{–}950^\circ\text{C}\). The maximum percentage
Table 1
Chemical composition
| Talc name | Loss on ignition | SiO₂ | TiO₂ | Al₂O₃ | Fe₂O₃ | CaO | MgO | H₂O | Total |
|---|---|---|---|---|---|---|---|---|---|
| Altai | 4,68 | 64,16 | none | 0,31 | 0,04 | 0,22 | 30,65 | 100,06 | |
| Onot | 4,78 | 62,02 | none | 0,95 | 0,45 | 0,42 | 31,64 | 100,26 | |
| Composition of the mineral talc | 63,5 | 31,7 | 4,8 |
the weight loss is 5.06%. Dehydration of the Alguy talc begins at 800° and ends at 1000°C. The total percentage of weight loss is 4.65%.
The experimental results show that the dehydration process of talc is governed by the chemical equations of a first-order reaction. Therefore, for calculating the rate constants we adopted the classical equation:
\[ K=\frac{2.3}{t_2-t_1}\lg_{10}\frac{C_0-x_1}{C_0-x_2}, \]
where \(C_0\) is the initial concentration of water, or the total weight loss in %, and \(x_1\) is the concentration after a time interval.
The values of the reaction-rate constants for different temperatures and for both types of talc are given in Table 2.
Table 2
Value of the reaction-rate constant at different temperatures
| Temperature, °C | \(K\), min\(^{-1}\) — Onot talc | \(K\), min\(^{-1}\) — Alguy talc |
|---|---|---|
| 750 | 0.410 | — |
| 800 | 0.080 | 0.073 |
| 850 | 0.180 | 0.163 |
| 900 | 0.368 | 0.261 |
The activation energy of the thermal decomposition of the talcs is about 33.0 kcal/mol.
Alguy talc, with a scaly crystal form, has higher stability under thermal action than Onot talc, with a fine-grained structure. The temperature at which thermal decomposition of Alguy talc begins is 50°C lower, and the decomposition proceeds at a lower rate than in Onot talc at the same temperatures.
REFERENCES
- V. A. Kireev, Course of Physical Chemistry. 1956.
- P. Murray, J. White. Kinetics of the Thermal Dehydration of Clays. Transas. of the British Ceramic Society, 54, 3, 1955.