GAS PHASE DURING EXPANDED CLAY FORMATION

A. V. Petrov

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

During accelerated thermal treatment of low-melting clay rocks, predominantly of montmorillonitic and hydromicaceous compositions, the heterogeneous pyroplastic paste that forms swells under the action of the gaseous products released, with the formation of a glass-like material of cellular structure, called expanded clay.

The formation of the gas phase is based on the processes of pyrolysis and volatilization of organic substances present in clays, and on processes of chemical transformation of the constituent components of the clay and their interaction with one another and with the gaseous medium.

An analysis of published works by both domestic and foreign researchers (1, 2, 3) shows that the question of the composition of the gases evolved during expanded clay formation has not yet been studied with sufficient completeness, and contradictory views exist.

It is quite clear that establishing the composition of swelling gases only by studying, in pure form, the various substances that make up the clay material presents great difficulty. In our view, it is simpler to analyze the gases participating in the process of expanded clay formation. The composition of gases in the pores of finished expanded clay was studied on Belarusian low-melting clays (4). The purpose of the present work was a comprehensive study of the gas phase participating in expanded clay formation—determination of the quantity of gases evolved, their composition, and the magnitude of the pressure developed during the formation of the cellular structure. The investigation was carried out by us on cover clay rocks of Quaternary age, which, in terms of the commonality of their compositions and technological properties, are a widely distributed raw material in Tomsk Oblast and belong to one lithological group. The clay component of the loams is polymineral and is composed of ferruginous montmorillonite—beidellite and hydromicas of illite composition. In terms of technological properties, the loams belong to medium- and moderately plastic clay raw material and are low-melting, with refractoriness not higher than 1280°C. From the indicated clay rocks, expanded-clay gravel is obtained with physicomechanical properties satisfying existing requirements.

To determine the composition, quantity, and pressure of the evolved gas phase, we used the method of rapid quantitative phase analysis developed by Berg [5]. The essence of the method conclu-

consists in measuring the volumes of gases released when substances are heated.

The gas phase formed in the reaction vessel (Fig. 1) is first treated with solutions that make it possible to judge qualitatively its constituents. Then, in order to determine the quantitative ratio of the gases in the mixture, samples are taken from the reaction vessel at various temperatures and studied according to the procedure developed for the analysis of gaseous inclusions in glass [6]. The essence of the method consists in measuring the diameter of a gas bubble with the ocular-micrometer scale of a microscope before and after its treatment in various absorbents. For comparison of the data obtained, gases enclosed in the pores of expanded clay were studied. The results are given in Table 1.

Fig. 1. Reaction vessel: 1—thermocouple socket; 2—block; 3—sample; 4—steel cylinder; 5—air cooling

Table 1

The data in the table show that, with increasing temperature and increasing expansion time, the content of carbon monoxide and oxygen decreases; moreover, the quantitative content of the gaseous components released in the reaction vessel at the expansion temperature and in the closed pores of expanded clay obtained at the same temperatures is relatively identical. In all cases an excess amount of oxygen is observed, which should be attributed to the reduction of ferric oxide to ferrous oxide (Table 2) and to the possible decomposition of water vapor at high temperatures.

Calculated data on the excess oxygen present in the pores of expanded clay at the moment of expansion and released from the reaction vessel at the same temperatures are given in Table 3. The decrease in excess oxygen with increasing temperature and increasing expansion time can be explained by the intensification of the oxidation process of the organic impurities present.

Thus, in the formation of expanded clay from loams, as qualitative and quantitative analyses have shown, a gas phase of complex composition takes part, consisting of water vapor from clay minerals,

Table 2

oxygen released during the reduction of ferric iron to ferrous iron and as a result of the decomposition of water vapor at high temperatures, carbon monoxide and carbon dioxide, which are products of the oxidation of organic impurities, and nitrogen (from the air).

Table 3

The volume of gaseous products released was measured in a gas burette (Fig. 2); heating was carried out up to 1200°C.

Fig. 2. Gas burette: 1 — mercury seal; 2 — four-way valve

We have established that the amount of the gas phase released both during rapid (8–10 min) and during slow (50–60 min) heat-

during heating of granules dried to constant weight corresponds to 3–5% by weight; in granules preliminarily fired to a temperature of 850–900°C, the amount of the gas phase is less and is equal to 1–2% by weight. The release of the gas phase ends at a temperature of 1150–1170°C; the pressure developed in this case corresponds to 17–22 mm Hg above atmospheric pressure.

From the available works it is known that, to obtain expanded clay with a bulk density in the piece of 0.6 g/cm³, gas in an amount of 0.022% by weight is required [7]. Our investigations show that the optimum amount of the gas phase directly participating in the process of expanded-clay formation at optimum temperatures should be considered to be 0.06–0.07 percent by weight—a value repeatedly obtained in the study of rocks in a reaction vessel.

In addition, we have confirmed the statements of S. P. Onatskii [3] that the reducing medium created inside the fired clay material is of primary importance in expanded-clay formation. The limiting content of organic impurities for the rocks studied, at which reduction reactions proceed normally and the necessary amount of liquid phase is formed, should be considered 0.6% by weight. In this case expanded clay is formed with a uniform steel-colored structure. Clay rocks containing a smaller amount of organic impurities in the reaction vessel do not swell; a larger amount leads to the formation of pores of nonuniform size, with thin, less strong walls that rupture easily.

Conclusions

1. The formation of the porous structure of the pyroplastic mass occurs owing to the evolved gas phase, which consists of: water vapor, carbon dioxide, carbon monoxide, and oxygen; moreover, the optimum amount of the gas phase for normal expanded-clay formation should be considered 0.06–0.07 percent by weight.

2. The pressure of the gas phase is established as maximum at temperatures of 1150–1170°C and is equal to 17–22 mm Hg above atmospheric pressure.

3. The reducing medium created inside the fired material, and not the external one, is of primary importance in expanded-clay formation; therefore, the rocks studied swell identically in any medium, and the minimum amount of organic impurities should be considered 0.6 percent by weight.

REFERENCES

1. S. Redey. Jouzn Amer Cer Soc, No. 4, 121–128, 1951.
2. E. Ehlers. Bull, Amer, Ceram, Soc, 95–99, 1958.
3. S. P. Onatskii. On the physicochemical nature of the processes of swelling of clays during firing. In: Artificial Porous Aggregates for Lightweight Concretes. Gosstroiizdat, 1954.
4. M. A. Bezborodov, L. K. Petrov, N. P. Grishina. Composition of gases in the pores of expanded clay. Minsk, 1958.
5. L. G. Berg. Rapid quantitative phase analysis. Moscow, 1952.
6. L. A. Afanas’ev, T. E. Golba et al. Practical manual for control of glass production. Gizlegprom, 1939.
7. I. A. Gervids. Expanded clay, a study on technology. Moscow, 1957.