PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
NAMED AFTER S. M. KIROV, HOLDER OF THE ORDER OF THE OCTOBER REVOLUTION
AND THE ORDER OF THE RED BANNER OF LABOR

Volume 257
1973

EFFECT OF AIR HUMIDITY ON THE STRENGTH OF GLASS BATCH GRANULES

L. G. Lotova, V. A. Trofimov, V. M. Vitogin, V. V. Ivanov

(Presented by the scientific seminar of the Department of General Chemical Technology)

The melting of glass using loose batches made from fine-grained materials is associated with many disadvantages (segregation, dusting, disruption of the homogeneity of the glass melt, destruction of the refractory lining of furnaces, deterioration of the atmospheric conditions in the working area, etc.), which ultimately lead to a reduction in the productivity of glass plants. In order to eliminate these disadvantages, a loose batch can be agglomerated by briquetting or granulation. From the economic point of view, preparation of the glass batch by the granulation method is of the greatest practical interest. Unlike briquetting, granulation does not require expensive, complex, and at the same time low-productivity equipment; the method is relatively simple and economical [1, 2, 3].

Mastering granulation technology on an industrial scale would make it possible to centralize the preparation of high-quality glass batch, which is of enormous national-economic importance. From this point of view, the strength of granules during their transportation and the moisture content during charging into furnaces acquire special importance. Solving the question of the minimum strength of granules is also of great significance when choosing the conditions for their storage. In substantiating the required minimum strength of granules, it should be taken into account that, for glass melting, preservation of the shape and dimensions of the granules is not essential. Partial destruction of granules during transportation and storage is not dangerous if it does not lead to disintegration of the granules and the formation of fines and dust capable of segregation.

In the granulation of alkali-containing glass batches, the setting and hardening processes of crystalline hydrates of sodium carbonate and sulfate have a great influence on the strength of the granules. During the formation of the granule structure, unstable decahydrate crystalline hydrates are capable of weathering, decomposing with a decrease in the number of molecules of water of crystallization, and gradually transforming into the monohydrate or thermonatrite (with a moisture content of 15%). The processes taking place cannot fail to affect the strength of the granules.

To determine the effect of air humidity on the strength and moisture content of granules, a study was carried out on granules from the glass batch of the Angero-Sudzhensk Plant of the following composition: sand — 54.82%, feldspar — 5.23%, dolomite — 17.30%, soda — 15.92%, sodium sulfate — 5.28%, sodium silicofluoride — 0.69%, fuel oil — 0.76%. The granules were obtained in a dish-type laboratory granulator. The batch was rolled at an initial temperature of 50°C and a moisture content of 15%. For the experiments, granules 5–7 mm in diameter were selected. Samples of the raw granules, immediately after production, were placed in desiccators with sulfuric-acid solutions of different concentrations, providing the specified relative air humidity from 40 to 90% (\(t_{\mathrm{в}} = 20–22^\circ\mathrm{C}\)). At definite time intervals, the crushing strength and moisture content of the granules were determined.

From the data obtained (Table 1) it is evident that, at a relative air humidity of 40 to 70%, the strength of the granules initially increases with holding time, passes through a maximum, and then decreases.

The moisture content of the granules decreases in this case, ultimately reaching 2–3%, which corresponds to the presence in the granule structure of monohydrates of soda and sodium sulfate. At a relatively high air humidity—80–90%—the strength of the granules at first increases slightly and then falls to very low values (0.33; 0.24 kg/granule). The moisture content of the granules in this case rises to a value exceeding the initial one; cracks appear on the surface of the granules.

Discussion of results.

At a low relative air humidity (40–70%), strengthening of the freshly obtained (moist) granule occurs through the formation of crystalline bridges. At the same time, moisture is removed from the surface of the granule, causing a redistribution of chemical bonds inside the granule. The process of moisture redistribution proceeds until monocarbonate forms in the granule. The strength of the granules in this case remains still considerable (from 1 to 1.2 kg/granule).

Table I

Values of the moisture content and strength of granules as a function of air humidity

Note: the initial moisture content of freshly obtained granules is approximately 14%; their strength 15 minutes after formation is about 0.7 kg/granule.

At high relative humidity of the air (80–90%), simultaneously with the crystallization hardening of the granules, a process occurs in which the granule absorbs moisture from the atmosphere. Mechanically admixed water (adsorption water) has a wedging effect on the structure that has formed; cracks appear, and the strength of the granules falls to a minimum (0.2–0.3 kg/granule).

The data obtained (Table 1) are mainly of practical significance. Under production (plant) conditions, the residence time of the batch from the moment of its preparation (including the granulation processes) until loading into the furnaces is determined chiefly by the correspondence between the productivity of the granulator and the capacity of the reserve bunkers, and may usually vary within the range of 8–12 hours.

When granules are stored under ordinary conditions, i.e., at a humidity of 40 to 70% and a temperature of about 20°C, their strength and moisture content (7–8%) after 10–12 hours from the moment of preparation fully meet the requirements of glass production.

Transporting granules over long distances in centralized production does not require special storage conditions, except for maintaining an air humidity of 40–70%.

References

1. M. F. Guryanova, Ya. A. Frolov, Granulation of glass batch with coarse sand. Steklo i keramika, 1961, No. 10.

2. B. K. Demidovich. Abstract of a dissertation for the degree of Candidate of Chemical Sciences. Minsk, 1967.

3. Brandenberg H. Granulation of glass batch with heating. Silikattechnik, No. 1, 1964.