INFLUENCE OF THE PARTICLE SIZE OF QUARTZ RAW MATERIAL ON THE KINETICS OF THE PROCESS OF SILICON CARBIDE FORMATION

V. V. Shvetsov, V. M. Vityugin, V. I. Kosintsev

(Presented to the scientific-methodological seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production, KhTF)

For the heterogeneous reaction

\[ \mathrm{SiO_2 + 3C = SiC + 2CO} \]

the rate of the process is described, in the general case, by the equation

\[ \frac{dm}{d\tau} = K \cdot l^{E/RT} \cdot f(x) \cdot F_p, \]

where \(\frac{dm}{d\tau}\) is the mass of the component that has reacted per unit time;
\(K\) is the reaction-rate constant;
\(E\) is the apparent activation energy;
\(R\) is the gas constant;
\(T\) is the absolute temperature;
\(f(x)\) is a function of the concentration of the gas phase;
\(F_p\) is the reaction surface of the solid phase.

If the limiting stage is the rate of evaporation of \(\mathrm{SiO_2}\), then the latter depends both on the reaction temperature and on the magnitude of the specific surface area of the quartz grains, that is, on their particle size. This plays an important role in the technological process of obtaining silicon carbide. The process proceeds with the greatest efficiency when the silica vapors have time to be completely absorbed by the carbonaceous material.

At temperatures above \(1400^\circ\mathrm{C}\), the vapor pressure of silica becomes appreciable, and chemical interaction begins between \(\mathrm{SiO_2}\) vapors and solid carbon. From the intensity of carbon monoxide evolution one can judge the rate of the carbide-forming reaction, which, in turn, depends on the rate of evaporation of \(\mathrm{SiO_2}\).

Tashlinski sands, which are used in the practice of producing technical carborundum, have a number of disadvantages [1]. The high dispersity of the sands does not make it possible to conduct the technological process with the greatest efficiency [2]. To improve the technology for obtaining silicon carbide using Tashlinski sands, a method of two-layer granulation of the charge was proposed. Industrial tests showed that this measure is sufficiently effective [3].

The essence of two-layer granulation consists in converting individual quartz particles, with the addition of a binder (sulfite-alcohol stillage), into aggregate granule cores, which are coated with a layer of carbonaceous material. In the course of the furnace process of obtaining silicon carbide, the granule cores sinter into individual globules and conduct

... like monolithic quartz grains. The carbonaceous shell separates the quartz nuclei and prevents their sintering with one another, thereby ensuring stable and uniform gas permeability of the charge.

In the present work, results are given of a comparative study of the kinetics of carbide formation in charges with different silica grain sizes, including for a granulated charge.

A sample containing 6 g of quartzite and 3.6 g of petroleum coke was thoroughly mixed and placed in a quartz tube 20 mm in diameter. To ensure complete absorption of silicon dioxide vapors, the sample was additionally sprinkled on top with a small amount of petroleum coke. The gas evolved was collected in a gasometer filled with a saturated solution of table salt. The amount of gas was measured by the volume of solution displaced. Individual fractions of quartz wastes from the Glukhovets kaolin combine were studied (0–0.5; 0.5–1; and 1–2 mm), as was Tashlinsk sand with a grain size of less than 0.5 mm, and also two-layer quartz–carbon granules in the form of cylinders 3 mm in diameter and 2–3 mm high, prepared from Tashlinsk sand. The grain size of the carbonaceous material in all samples was 0–0.5 mm. The samples were heated at temperatures of 1460, 1500, 1540, and 1580°C.

The studies showed that at constant temperature the rate of gas evolution for all samples, including the granulated charge (Fig. 1), remained constant during the first 30–40 min and then gradually decreased. With increasing temperature, the rate of gas evolution increased.

Fig. 1. Dependence of the degree of conversion of carbon to oxide in a sample containing granules

evolution increased. Thus, in the sample with Tashlinsk sand at 1460°C, during the period of constant rate in the first 10 min, about 15 ml of gas was evolved, whereas at 1580°C, 60–70 ml was evolved. Similar results were obtained for Glukhovets quartzites with a grain size of 0–0.5 mm. In samples containing quartzite fractions of 0.5–1 and 1–2 mm, the amount of carbon monoxide evolved in 10 min was, in the first case, 50–60 ml at 1460°C and about 220 ml at 1580°C, and in the second case, 40–45 and 160–180 ml, respectively. With an increase in the grain size of the quartzite to more than 2 mm, the rate of gas evolution decreased. In samples containing granules, the amount of gas was 70 ml at 1460°C and 200 ml at 1580°C in 10 min.

The reaction-rate constant was calculated from the Kazeev–Erofeev equation [4]:

$$ \alpha = 1 - e^{-K\tau^n}, $$

where $\alpha$ is the degree of conversion of the substance with respect to any component, in fractions of unity;

\(e\) is the base of the natural logarithm;
\(K\) is the reaction-rate constant, in \(1/\mathrm{min}\);
\(\tau\) is the time corresponding to the given degree of conversion;
\(n\) is the reaction order.

Taking the logarithm twice of the Kazeev–Erofeev equation, we obtain the equation of a straight line

\[ \lg[-\lg(1-\alpha)] = n \lg \tau + \lg K. \]

By plotting the dependence \(\lg[-\lg(1-\alpha)]\) versus \(\lg \tau\), one can find the value of \(\lg K\).

To determine the activation energy, the following equation was used:

\[ \lg K = \frac{E}{2.3RT} = \frac{E}{4.575T}, \]

where \(E\) is the activation energy, cal/mol;
\(R\) is the gas constant, cal;
\(T\) is the absolute temperature, °K.

As an example, Table 1 and Figs. 1, 2, and 3 present the results of calculating the reaction-rate constant and the activation energy for the pro—

Table 1

Results of processing experimental data on the carbide-formation process according to the Kazeev–Erofeev equation

Legend: 1580°C; 1540°C; 1500°C; 1460°C.

Fig. 2. Dependence of \(\lg[-\lg(1-\alpha)]\) on \(\lg \tau\). The sample contains granules.

would contain two-layer quartz–carbon granules. For the remaining specimens the final results are given (Table 2).

As is seen from Table 2, the rate constant is minimal for the specimen containing quartz sand with a particle size of less than 0.5 mm. Despite the high specific surface area, the rate of evaporation of silica is insignificant, which can be explained by sintering of the material mass into a continuous monolith. In the case of a granulated charge, sintering of the quartz particles occurs in isolation; therefore the sintered cores of the granules behave like separate coarse grains.

The studies showed that, from the standpoint of silica evaporation,

Fig. 3. Plot of the dependence \(\lg K—10^3/T\) for a granulated charge

Table 2

Rate constants and apparent activation energy of the reaction
\[ \mathrm{SiO_2 + 3C = 2CO + SiC}, \]
obtained by processing the data according to the Kazeev–Erofeev equation

two-layer quartz–carbon granules and the quartz fraction 1–2 mm are approximately equivalent.

The value of the apparent activation energy for materials of different particle size is the same and amounts to 87–91 kcal/mol. Some discrepancy in the value of \(E\) lies within the limits of experimental error. For the granulated material, a certain decrease in the activation energy is observed. Apparently, the highly dispersed carbon formed during pyrolysis of the binder interacts with solid silica. This interaction in the solid phase begins at lower temperatures.

Conclusions

1. The kinetics of silica evaporation in specimens of different granulometric composition has been investigated. It has been shown that the evaporation rate

silicon carbide from granules and a quartz fraction of 1–2 mm is approximately the same.

1. The kinetics of the process of interaction of vaporous silicon oxide with solid carbon is satisfactorily described by the Kazeev–Erofeev equation.

2. The apparent activation energy lies in the range of 73–91 kcal/mol, which indicates that the process proceeds in the kinetic region of reaction.

REFERENCES

1. I. S. Kats. Abrasives, 3, 1970, p. 5.
2. V. A. Rybakov, V. V. Karlin, V. M. Vityugin, V. A. Prokhorovich. Abrasives, 5, 1963, p. 1.
3. V. M. Vityugin, V. V. Shvetsov, V. V. Karlin et al. Abrasives, 1, 1968, p. 3.
4. V. V. Boldyrev. Methods for Studying the Kinetics of Thermal Decomposition of Solid Substances. Tomsk, 1968.