KINETICS OF THE INTERACTION OF BARIUM CARBONATE WITH THE COMPONENTS OF A STEATITE BODY BATCH

P. G. Usov, E. P. Solomatina

(Presented by the scientific seminar of the Department of Silicate Technology)

This study was carried out because, during the firing of steatite articles, bubbles form both inside and on the surface of the articles, as a result of which they become unfit for use. The bloating process proceeds especially intensively at low overfiring. One of the causes of bloating of the body during firing is considered to be the decomposition reaction of barium carbonate. For example, N. P. Bogoroditskii points out [1] that “the evolution of CO₂ during the decomposition of barium carbonate is often the cause of the formation of internal cavities and blisters on radio components, owing to the difficulty of CO₂ release during the decomposition of BaCO₃.”

The investigation was performed by the gravithermometric method on mixtures of barium carbonate with the components of the batch—kaolin, talc, and quartz glass.

In studying the kinetics of the reaction of barium carbonate with kaolin and talc, we proceeded from the assumption that BaCO₃ interacts with silica formed during the thermal decomposition of talc and kaolin. The data on the change in free energy of the reaction BaCO₃ + SiO₂, calculated by V. B. Glushkova [3], permit the formation of the following compounds.

At temperatures of 700–750° in mixtures of BaCO₃ with SiO₂ of any composition, the formation of barium metasilicate is most probable. At temperatures of 750–800° the thermodynamic probability of formation of barium meta- and orthosilicate is approximately the same. From 800° and higher, the formation of orthosilicate becomes more probable. Practical studies of the reaction rate of BaCO₃ with the batch components were carried out for the temperature range 700–820° with the assumption that one chemical reaction product—barium metasilicate—is formed. All kinetic calculations were performed for this reaction scheme.

In the work, the applicability of Jander’s diffusion equation in the form

\[ I(G) = 1 + (1 - G)^{2/3} - 2(1 - G)^{1/3} = K_{\text{я}}\cdot \tau, \tag{1} \]

obtained by Ginstling’s transformation of the equation

\[ \left(1 - \sqrt[3]{\frac{100 - y}{y}}\right)^2 = K \cdot \tau, \tag{2} \]

was tested.

and the Ginstling equation, which takes into account the degree of conversion as a function of the duration of the process and the particle radius.

\[ I(G)=1-\frac{2}{3}G-(1-G)^{2/3}=\frac{K\cdot \tau}{R^2}=K'\cdot \tau, \tag{3} \]

where \(G\) is the degree of conversion of the starting substance into product, in fractions of unity; \(I(G)\) is the dependence of the degree of conversion of the “coated” reagent on the duration of the process; \(K\) is a constant depending on the properties of the reagents and the process conditions; \(\tau\) is the time of the process in minutes; \(R\) is the particle radius.

Fig. 1. Kinetics of the interaction of BaCO\(_3\) with SiO\(_2\)
\((\mathrm{BaCO_3}:\mathrm{SiO_2}=)\). Curves: 1 — at 700°C, 2 — 740°C, 3 — 780°C, 4 — 800°C, 5 — 820°C.

The reaction of BaCO\(_3\) with silica in the form of quartz glass is described equally well by the Jander equation and by the Ginstling equation. The results of our investigation are presented graphically in Fig. 1.

Kinetic data on the interaction of BaCO\(_3\) with talc calcined at 1000° at various temperatures are given in Table 1 and in Fig. 2.

Data on the interaction of barium carbonate with kaolin calcined at a temperature of 800° are given in Table 2 and in Fig. 3.

The kinetic Ginstling and Jander equations (1 and 3) are equally applicable also for describing the reactions of barium carbonate with calcined talc and with kaolin calcined at 800°.

Fig. 2. Kinetics of the interaction of BaCO\(_3\) with talc calcined at 1000°C (at a ratio of 1 : 1). Curves: 1 — at 700°C, 2 — 740°C, 3 — 780°C, 4 — 800°C, 5 — 820°C.

The reaction-rate constants calculated from the Jander equation always have a slightly larger absolute value compared with the constants obtained from the Ginstling equation.

Table 1

Table 2

For the reaction \(\mathrm{BaCO_3 + SiO_2}\), Jander established a linear dependence of the logarithm of the reaction-rate constant on the reciprocal of the absolute temperature. This dependence is retained in the interaction

Fig. 3. Kinetics of the interaction of \(\mathrm{BaCO_3}\) with kaolin calcined at \(800^\circ\mathrm{C}\) (at a ratio of \(1:1\)). Curves: 1—at \(700^\circ\mathrm{C}\), 2—\(740^\circ\mathrm{C}\), 3—\(780^\circ\mathrm{C}\), 4—\(800^\circ\mathrm{C}\), 5—\(820^\circ\mathrm{C}\).

Fig. 4. On the kinetics of the interaction of \(\mathrm{BaCO_3}\) with \(\mathrm{SiO_2}\), kaolin, and talc. Dependence of \(\lg K\) on the reciprocal of the absolute temperature. 1—\(\mathrm{BaCO_3 + SiO_2}\); 2—\(\mathrm{BaCO_3}\) + talc calcined at \(1000^\circ\mathrm{C}\); 3—\(\mathrm{BaCO_3}\) + kaolin calcined at \(800^\circ\mathrm{C}\).

of barium carbonate with talc and kaolin. The results of processing the experimental data are given in Table 3 and in Fig. 4.

In reactions proceeding in the diffusion region, the reaction rate is determined by the value of the diffusion coefficient. A diffusion process is usually characterized by a low activation energy, and its rate ...

comparatively slowly with temperature. In cases where an increase in temperature sharply increases the reaction rate by three to four times for every 10°, the process is limited by the chemical reaction itself; in the mixtures studied, the temperature dependence of the change in reaction rate is less pronounced. The increase in the reaction rate in them when the temperature is raised by 10° is about 40–50%.

Table 3

The activation energy, calculated from the equation

\[ E = 4{,}57 \cdot \frac{\lg \dfrac{K_2}{K_1}\cdot T_1 \cdot T_2}{T_2 - T_1}, \tag{4} \]

where $K_1$ and $K_2$ are the reaction-rate constants respectively at absolute temperatures $T_1$ and $T_2$, and $E$ is the activation energy.

For the temperature intervals 700–740° and 800–820°, the values of the activation energy are given in Table 4.

Fig. 5. On the kinetics of the interaction of $\mathrm{BaCO_3}$ with talc and kaolin. Character of the relationship between the reaction rate and temperature.
1 — $\mathrm{BaCO_3}$ + talc, calcined at 1000°C;
2 — $\mathrm{BaCO_3}$ + kaolin, calcined at 800°C.

From the data in the table it is seen that the reaction rate of barium carbonate with kaolin and talc changes more strongly with temperature in comparison with the reaction of $\mathrm{BaCO_3}$ with quartz glass.

In the interaction of barium carbonate with quartz glass, the activation energy changes only slightly with temperature. This indicates...

indicates that in the temperature interval 700–820° this reaction is in the diffusion region. In the interaction of barium carbonate with kaolin and talc, a transition is possible from one kinetic region to another, from diffusion to chemical. This is also seen in the curves of the change in the nature of the relationship between the reaction-rate constant and temperature (Figs. 5, 6).

Table 4

Fig. 6. On the kinetics of the interaction of $\mathrm{BaCO_3}$ with quartz glass. Nature of the relationship between the reaction rate and temperature.

Conclusions

The limiting stage of the reaction of barium carbonate with quartz glass, talc, and kaolin is the diffusion of $\mathrm{BaCO_3}$ to the reaction zone through the layer of reaction product. The kinetics of this process is described by Jander’s equation and by Ginstling’s equation using the degree of conversion of the “covered,” and not the diffusing, reactant—in the mixtures studied, $\mathrm{SiO_2}$.

In the interaction of $\mathrm{BaCO_3}$ with kaolin and talc, as the temperature is raised, a transition from the diffusion to the chemical stage of interaction is possible.

REFERENCES

1. Edited by N. P. Bogoroditskii. Radioceramics. Gosenergoizdat, Moscow, 1963.
2. V. B. Glushkova. ZhNKh, vol. 2, no. 10, 1957.
3. P. P. Budnikov, A. M. Ginstling. Reactions in Mixtures of Solid Substances, Gosstroiizdat, Moscow, 1961.