On the Solubility of Clays in Acids and the pH of Clays
Abstract Generated abstract
This study examines how mineralogical composition and calcination temperature affect the extraction of aluminum oxide from clays by hydrochloric acid, and how these factors relate to the pH of clay aqueous extracts. Montmorillonite, kaolinite, and hydromicaceous clays were calcined for one hour at temperatures from 300 to 1000°C and treated with 10 percent, and in selected cases 5 percent, hydrochloric acid. The results show that alumina is extracted only with difficulty from montmorillonite, much more readily from kaolinite after calcination at 500 to 800°C, and intermediately from hydromicaceous clay. The authors relate these differences to clay mineral lattice structure and report that pH values also vary with mineralogical composition and heat treatment.
Full Text
On the Solubility of Clays in Acids and the pH of Clays
G. N. KHODALEVICH, T. G. SAKOVICH, O. K. OVECHKINA
(Presented by Professor A. G. Stromberg, Doctor of Chemical Sciences)
Earlier, one of us [1] carried out the first studies of the solubility of Siberian clays in acids as a function of the calcination temperature of the clays and of the acid concentration.
In continuation of these studies, it is of interest to obtain additional data on the extraction of aluminum oxide from clays of various mineralogical compositions1. These data are given in the present article.
For the extraction of aluminum oxide, as in the previous work, 10-percent hydrochloric acid was used, and in a number of experiments 5-percent hydrochloric acid was also used. The extraction procedure was kept the same as before. The results of the experiments are summarized in Table 1.
Table 1
| Calcination temperature, °C | Montmorillonite clay: loss on ignition, % | Montmorillonite clay: extraction in 10% hydrochloric acid, % | Montmorillonite clay: extraction in 5% hydrochloric acid, % | Kaolinite clay: loss on ignition, % | Kaolinite clay: extraction in 10% hydrochloric acid, % | Kaolinite clay: extraction in 5% hydrochloric acid, % | Hydromicaceous clay: loss on ignition, % | Hydromicaceous clay: extraction in 10% hydrochloric acid, % | Hydromicaceous clay: extraction in 5% hydrochloric acid, % | Notes |
|---|---|---|---|---|---|---|---|---|---|---|
| 300 | 7.68 | 1.71 | — | 0.52 | 0.78 | — | 2.68 | 2.04 | — | 1. The calcination time and the time of treatment of the calcined clay samples with acid at boiling in all experiments was 1 hour. |
| 400 | 8.42 | 2.54 | — | 1.01 | 1.35 | — | 4.75 | 3.61 | — | |
| 500 | 11.36 | 2.80 | — | 13.45 | 36.60 | 35.91 | 9.86 | 17.11 | 15.01 | |
| 600 | 15.20 | 2.27 | — | 13.44 | 38.30 | 38.69 | 10.65 | 19.83 | 17.43 | |
| 700 | 15.33 | 2.24 | — | 13.78 | 36.97 | 36.65 | 11.04 | 20.58 | 17.53 | |
| 800 | 15.77 | 1.93 | — | 13.83 | 36.88 | — | 11.28 | 22.14 | — | |
| 900 | 15.70 | 1.77 | — | 13.98 | 1.66 | — | 11.35 | 0.77 | — | 2. Extraction of Al₂O₃, in percent, calculated with respect to [[unclear: calcined/original]] clay. |
| 1000 | 15.95 | 1.84 | — | 14.01 | 0.27 | — | 11.86 | 0.60 | — |
As is evident from the data presented, aluminum oxide is extracted from montmorillonite clay with great difficulty. Aluminum oxide is extracted best of all from kaolinite clay (Prosyanov kaolin)—the extraction of aluminum oxide from a clay sample calcined at \(600^\circ\) is close to 100%. Under the same conditions, only about 57% of the aluminum oxide contained in hydromica clay is extracted (according to the analysis, the clay contains 34.59% aluminum oxide).
It is also clearly evident from the data presented that, from samples of kaolinite and hydromica clays calcined at \(500\text{--}800^\circ\), aluminum oxide is extracted rather well even by a 5-percent solution of hydrochloric acid.
As is known, clay minerals, according to the character of their crystal lattices, are divided into two-layer and three-layer types. Kaolinite belongs to the former, montmorillonite to the latter. The small percentage of extraction of aluminum oxide by hydrochloric acid from montmorillonite clay may be explained by the fact that in montmorillonite the layer of alumino-hydroxyl octahedra is surrounded by two layers of siliceous tetrahedra, whereas in kaolinite there is an alternation of layers of alumino-hydroxyl octahedra and siliceous tetrahedra. The fact is also significant that montmorillonite crystals, upon dehydration, lose a considerable amount of water without destruction of the lattice structure.
The authors of the present work also carried out a series of experiments to determine the hydrogen-ion exponent of clays of different mineralogical composition (both uncalcined and calcined at different temperatures). The pH was determined by various methods. The data obtained are given in Table 2.
Table 2
| Calcination temperature of clays (calcination time 1 hour), deg. | pH value of aqueous extracts: montmorillonite clay | pH value of aqueous extracts: kaolinite clay | pH value of aqueous extracts: hydromica clay |
|---|---|---|---|
| Clay was not calcined | 7.4 | 6.4 | 6.2 |
| 300 | — | 6.6 | 5.8 |
| 400 | 8.3 | 6.6 | 5.4 |
| 500 | — | 6.8 | 5.0 |
| 600 | 10.5 | 7.0 | 5.0 |
| 700 | — | 6.4 | 5.0 |
| 800 | — | 6.4 | 5.0 |
| 900 | — | 6.4 | 5.0 |
| 1000 | — | 6.2 | 5.0 |
From comparison of the measurement data one may conclude that the pH value also depends on the mineralogical composition of the clays.
LITERATURE
- G. N. Khodalevich. Izv. TsNII, vol. 57, issue 11, pp. 1–6, 1937.
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The clay samples were kindly provided to us by the head of the Department of Silicate Technology, Docent N. G. [[unclear: surname]]. ↩