G. N. Khodalevich, I. I. Zharikov, M. I. Dragel’.

On the Solubility of Some Siberian Clays in Acids as a Function of the Temperature and Duration of Calcination and of the Concentration of the Acid Used for Treatment

Communication 1

The question of the chemical nature of white clays, or clays close to white, and of the chemical changes that occur in their composition during calcination operations, despite the numerous investigations carried out in this field, still remains not fully clarified. And this is natural, since the term “clay” is quite broad: clays may be defined as “a mixture of minerals, of which the principal ones are silicates of aluminum, iron, alkalis, and alkaline earths,” as the American clay specialists Aschley and Reis say.

If, with respect to the clays of the central part of the USSR, there are excellent investigations by Prof. Zemyatchensky and the works of a number of other authors, then with respect to the clays of Siberia such investigations are almost absent. Therefore, the question of studying the solubility of a series of samples of Siberian clays, at least in acids as a function of the temperature and duration of calcination and of the concentration of the acid taken for treatment, is quite timely. Working on material little studied from this point of view—Siberian clays—the authors hope that the investigations they have begun may be of interest not only for practice, but may also have theoretical significance, since the accumulation of experimental data concerning the study of the solubility of clays in acids and alkalis, as a function of the temperature and duration of calcination, will undoubtedly be useful for refining our scientific conceptions of such an important raw material in modern industry as clays. The authors consider it necessary to study, from the above-mentioned point of view, not only samples of Siberian clays from various deposits, but also samples of clays from one and the same deposit, taken from its different depths.

As was indicated above, the aim of our work is to study the solubility of a series of samples of Siberian clays in acids as a function of the temperature and duration of calcination of the clay and of the concentration of the acid taken for treatment. We studied the solubility in hydrochloric acid of four samples of Siberian clays, namely: Arinichevskaya A, a sample taken from a depth of 2–12 meters of the Arinichevsky mine; Arinichevskaya B, a sample taken from a depth of 22–23 meters, also of the Arinichevsky mine; Moiskaya, sample No. 9; and Khaitinskaya clay. All four clay samples were preliminarily subjected by us to a total chemical analysis, chiefly with respect to the determinations of silicic acid, aluminum oxide, and iron oxide. Determinations were also made of hygroscopic moisture and of losses on calcination. The results are given in Table No. 1.

The study of the solubility of the indicated clay samples was carried out as follows: a weighed portion of air-dry clay was taken; for each individual—

*1. Izvestia TII, vol. 57, issue II.

Table 1.

For each individual experiment a weighed portion, about one gram in weight, was taken and placed in an electric furnace in a glazed porcelain boat. From the moment the boat containing the weighed portion of the clay under investigation was placed in the furnace, the time at which calcination began and the temperature of the furnace were recorded. The temperature was measured with a pyrometer with a calibrated thermocouple (Pt—Pt/Rh). The calcination of the weighed portion of clay in the furnace was carried out by us chiefly for one hour, and only a few experiments were conducted with a two-hour calcination. The temperatures for calcining the weighed portions of clay in the electric furnace were: 400°, 500°, 600°, 700°, 800°, 900°, and 1000°. After calcination, the boat with the clay was cooled in a desiccator and then weighed on an analytical balance as quickly as possible, in order to avoid absorption of hygroscopic moisture by the clay. The results of the weighings gave the loss on calcination. Then the calcined weighed portion of clay was carefully transferred into a conical flask of Jena glass, and a definite amount of hydrochloric acid of known concentration was added to it, after which the mixture was boiled for a definite interval of time (more often 1 hour). After boiling, the hydrochloric-acid extract was filtered; the extraction residue was washed on the filter until the reaction for the chlorine ion disappeared; and then, by the usual method in silicate analysis, the amount of alumina extracted from the weighed portion of clay was determined.

Having given the scheme of the operations successively carried out by us in studying the solubility of clays, it is necessary to emphasize certain details of these operations.

I. Since we used air-dry clay for the weighed portions, we carried out (despite storing the clay under investigation in glass jars well stoppered with ground-glass stoppers) frequent repeated determinations of hygroscopic moisture.

II. As our first experiments in determining the loss on calcination of clay samples at various temperatures showed, this operation proved to be extremely delicate in the sense of obtaining well-agreeing data for two parallel experiments. A. M. Sokolov¹ also points this out, emphasizing that “the agreement of the results of repeated investigations of one and the same material is directly dependent on the degree, duration, and uniformity of calcination.” On this basis, in order to obtain quite acceptable data from two parallel experiments for determining loss on calcination, we had not only to keep careful watch over the constancy of the temperature and the time of calcination, but even to ensure that the weighed portion being calcined was in boats as nearly identical as possible in the thickness of their walls, capacity, etc. The best results are given by parallel experiments carried out consecutively in one and the same boat.

III. Boiling the weighed portion of clay with hydrochloric acid was carried out, as already indicated above, in conical flasks of Jena glass,

¹ Proceedings of the Petersburg Technological Institute for 1913.

closed with rubber stoppers (previously treated with concentrated hydrochloric acid), with reflux condensers inserted into them. The capacity of the flasks was 100–200 cm³. Boiling was carried out on asbestos screens covered with a layer of asbestos cardboard and heated over an adjusted small flame of a Bunsen burner. To avoid overheating of the liquid, which is always possible when boiling liquids mixed with a precipitate, we periodically shook the contents of the flasks.

For extracting alumina from the clays we chose 10% hydrochloric acid (A. M. Sokolov worked with 6% hydrochloric acid, Otto Krause¹ with 12%, American investigators with 20%, etc.), since previous investigations had shown good solubility of calcined clays in hydrochloric acid of a concentration close to that chosen by us. Hydrochloric acid was added to the weighed sample at the rate of 25 cm³ per 1 g of air-dry clay. Several experiments were also carried out with 20% hydrochloric acid and with 2-hour calcination, without changing the other conditions.

The results of the experiments carried out are placed in Table No. 2 and are shown graphically (see the appended graphs). Table No. 2 gives the mean values of two parallel experiments, which served as the basis for constructing graphs expressing the changes in the solubility of alumina of the clays as a function of the calcination temperature, as well as the changes in losses on calcination as a function of the calcination temperature.

On the basis of the available data, the following preliminary conclusions may be drawn.

1. The extraction of alumina from the investigated clay samples by 10% hydrochloric acid depends on the calcination temperature of the clay.

2. The highest percentage of alumina extraction from calcined clays (with one-hour calcination and treatment with 10% hydrochloric acid) corresponds to temperatures of 700°–800°. More prolonged calcination (see the results of the 2-hour experiments) gives an increase in the extraction of alumina from the clays, especially noticeable for the temperature 400°.

3. For the sample “Moiskaya clay, sample No. 9,” it is interesting to note a solubility of alumina close to the maximum when weighed portions of this sample are calcined in the temperature zone 500°–600°.

4. Replacing 10% hydrochloric acid with 20%, while preserving the other experimental conditions, also gives a certain increase in the percentage extraction of alumina. Moreover, at a temperature of 400°, as in the experiments with more prolonged calcination, this difference in extraction is most noticeable.

5. Determination of the losses on calcination within the temperature range 400°–1000° shows an increase in losses on calcination up to 900° (3 samples) and up to 1000° (1 sample). The temperature interval 400°–500° for all the investigated clay samples is characterized by a sharp rise in the curve of losses on calcination (see graph). This is a characteristic feature of the Siberian clays investigated by us.

6. A sharp decrease in the extraction of alumina from the clays is observed for all the investigated clay samples in the temperature interval 800°–900°; and for samples calcined at 1000°, extraction of alumina from the clay ranges from 1% to 3%, i.e., becomes practically impossible.

Note. A detailed list of the literature used will be placed in the second communication, in connection with the final conclusions of the work.

¹ O. Krause u. Wöhner. Berichte d. Deutsch. Keram. Gesellschaft, 1932.

Table No. 2.

of average values of losses during calcination at various temperatures and of the percentage extraction of aluminum oxide as a function of the calcination temperature.