EFFECT OF AGGREGATION ON THE VOLATILIZATION OF BORIC ANHYDRIDE DURING THE MELTING OF BOROSILICATE GLASSES

V. M. VITOGIN, A. V. PETROV, G. D. SPESHIL, I. N. LANZMAN

(Presented at the scientific final conference of the Chemical-Technological Faculty)

Borosilicate glasses, possessing a complex of very valuable physical properties, are widely used in the production of envelopes and some parts of electrovacuum devices. The content of boric anhydride in them reaches 25 and more percent, and that of alkali oxides up to 5–10%. During the melting of such glasses, alkali borates evaporate from the surface of the melt; as a result, in the surface layer of the glass melt supersaturation with silica is readily attained, with its separation in the form of cristobalite and tridymite. At the same time, a large number of cords and striae are also formed, which leads to extensive rejects during the forming of products.

The volatilization of boron compounds depends on many factors and occurs at all stages of the process of preparing the glass melt, right up to the manufacture of products.

However, many studies [1, 2, 3, 4] have established that the greatest amount of volatiles is released at the first stage, i.e., immediately after the batch is charged into the furnace and during silicate formation.

With regard to questions concerning the dependence of the course of the borosilicate-glass melting process on temperature, batch composition and gas atmosphere, the direction and velocity of gas flows, the design of furnace elements, etc., recommendations have already been developed for improving the production technology of such glasses [5, 6, 7, 8]. Moreover, there is a common opinion that melting glass in electric furnaces should be considered the most rational, since electric melting can proceed practically without losses owing to the reduction to a minimum of the surface area of the glass melt and its protection by a layer of batch [9].

The questions of improving the technology of batch preparation, ensuring the preservation of its homogeneity and, consequently, improving the quality of the glass melt, still remain insufficiently resolved.

All measures currently taken at glass plants to preserve batch homogeneity (moistening, shortening transport routes, eliminating vibration, etc.) reduce segregation only to some extent, but do not eliminate it [10].

Briquetting [11] has the aim, first, of preserving the homogeneity of the batch, which is fundamental in studying this question, and, second, of eliminating dusting in the shop and in the glass-melting furnace, which

which inevitably forms when a loose batch is used. Briquetting of the batch has so far not found application in factory practice, apparently because of the need to install special equipment that complicates the technological process of batch preparation; therefore, for our study we chose a more progressive method—granulation of the batch, which does not require complex equipment and is, moreover, simple and economical.

To evaluate the effectiveness of batch granulation under laboratory conditions, we investigated a series of experiments according to the following procedure. From the factory batch for glass of composition 3S-5K, granules 3–8 mm in diameter and with an average moisture content of 8.7% were prepared on a laboratory disk granulator ($D = 300$ mm). Water served as the binder. The strength of the granules dried after forming ranged from 10 to 12 kg/mm².

Analysis of the component constituents of the batch in the granules showed deviations within permissible limits.

The study of volatilization of the constituents during heating was carried out in parallel with loose and granulated batches under identical conditions. The batches were heated in platinum crucibles in a vertical tubular electric-resistance furnace, at a temperature-rise rate of 7–8° per minute. Temperature was measured with a platinum—platinum-rhodium thermocouple with an automatic potentiometer.

The investigations were carried out:

a) with gradual heating of batch charges together with the furnace to 1000°C;

b) with rapid heating of charges placed in a furnace preheated to 1000°C;

c) by introducing into a furnace heated to 1000°C charges that had previously been held for 30 minutes at a temperature of 180°C.

The results obtained made it possible to establish the following:

1. The loss in weight (due to removal of residual moisture, volatilization of borates, and decomposition products of the batch components), both with slow and with rapid heating, ends mainly at a temperature of 800°C, i.e., upon completion of the silicate-formation stage.

2. For the granulated batch, the weight loss in both cases is smaller: specifically, with gradual heating—by 33%, and with rapid heating—by 30%.

3. With slow heating and holding at a temperature of 180°C, the weight loss of loose and granulated batches is less than with the other heating methods. The results of the average values obtained in the study are summarized in Table 1.

The dynamics of the change in weight is shown in Figs. 1 and 2.

The data in the table confirm the advisability of using a granulated batch. Thus, the preliminary study on batch granulation makes it possible to draw the following conclusions.

1. Batch granulation and its preliminary heating with holding at 180°C significantly reduce losses of boric anhydride in the first stage of glass melting in continuous tank furnaces.

2. During granulation, the homogeneity of the batch is “fixed,” i.e., the constancy of the weight ratios of the introduced raw materials is preserved.

3. Dusting is eliminated, which leads to stabilization of the chemical composition at the investigated stage of glass-melt preparation, an extension of the service life of glass-melting furnaces, and an improvement in production standards.

Fig. 1. Change in batch weight during slow heating

Fig. 2. Change in batch weight during rapid heating

1. The mechanical strength of the dried granules will ensure their normal storage in containers and transportation to glassmaking furnaces without destruction.

Table 1

Average values of weight change during thermal treatment of loose and granulated batches for glass of composition ZS-5K

REFERENCES

1. L. V. Potemkina. Dissertation, 1953.
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3. L. G. Gol'denberg. Report of TsNIIES, 1954.
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5. G. Ya. Ostrovskaya. Reduction of boric-acid consumption in the production of high-boron glasses. Bulletin of Technical and Economic Information, No. 5, 1958.
6. Yu. A. Gastev. Loss of $Me_2O$ and $B_2O_3$ from the glass melt during the melting of borosilicate glasses in continuous tank furnaces. Problems of Radio Electronics. Issue 11, ser. IV, 1964.
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