On the Role of Humic Acids in the Genesis of Coals

Table 1

Generalization of humic acids and their salts

ON THE QUESTION OF THE ROLE OF HUMIC ACIDS IN THE GENESIS OF COALS

K. K. STRAMKOVSKAYA

(Presented by Prof. I. V. Gebler, Doctor of Technical Sciences)

Despite the large number of studies in the field of the genesis of solid combustible minerals, to the present time no exhaustive and convincing answer has been given to the question of the cause of the formation in nature of coals of different technological qualities. Great importance in this question is assigned to humic acids; however, contradictory views on the role of the latter in the genesis of coals have been presented in the literature [1—5].

Existing works on the artificial reproduction of the process of metamorphism by heating plant material [4], dopplerite [6], peat and brown coal [7] (and other works) do not reflect the influence of many factors on the further transformations of humic acids.

The aim of the present work is to clarify the conditions for the selective splitting off of water and carbon dioxide from humic acids and the possibility of the formation thereby of products similar to hard coals.

In the work, the influence on the process of decarboxylation and dehydration of humic acids was tested for temperature (from 170 to 300°), pressure (from 9—90 atm), heating time (from 5 to 150 h), the physical state of the humic acids, and the acidity of the medium.

The object of the investigation was the humic acids of lowland peat from the Tagan Bog, Tomsk Oblast. Dry humic acids, humic acids in the form of an aqueous gel, and also soluble and insoluble salts of humic acids (sodium, ammonium, calcium, and barium humates) were tested. Since it is known that salts of organic acids give off carbon dioxide more readily upon heating, and the presence of water and high pressure hinder dehydration reactions, it is possible that such conditions will favor the selective splitting off of carbon dioxide from the molecule of humic acids.

Humic acids of peat isolated by the usual method were heated at different temperatures: dry—in open test tubes and in an autoclave (in water vapor), and hydrated—in special hermetically sealed steel tubes. In the latter, the humic acids were heated in the presence of water or weak (1—2%) alkali solutions. The products of decarboxylation were washed with water, and in experiments where salts of humic acids were subjected to heating, they were additionally treated with weak hydrochloric acid and washed with water, after which their analysis was carried out. The obtained data of elementary analysis were processed in atomic ratios according to the method of S. M. Grigor'ev [4]. The results of the experiments are given in Table 1.

From the data obtained it follows that the physical state of humic acids strongly affects the further course of reactions. Thus, dry humic acids (sample 1), subjected to heating at a temperature of 170° for 150 hours in water vapor, yielded a product containing almost no acidic substances. In this sample the carbon content on a combustible mass basis increased from 58.37 to 71.20%, while the hydrogen content decreased from 5.12 to 4.0%. Under heating in these conditions, per thousand carbon atoms there was lost—

approximately five oxygen atoms and nine hydrogen atoms were lost, i.e., chiefly dehydration of the humic acids took place. The product obtained did not sinter and gave a yield of volatiles, calculated on the combustible mass, of 44%. When dry humic acids were heated at atmospheric pressure with removal of the reaction products, dehydration reactions likewise proceeded predominantly. The rate of the carbonization process decreases with the duration of heating.

A series of experiments with humic acids swollen in water showed a somewhat different picture. When they were heated in the presence of water, decarboxylation reactions proceeded predominantly. Thus, at a temperature of 170° and a pressure of 9 atm, the humic acids lost chiefly oxygen (12 oxygen atoms per one hydrogen atom, calculated per thousand carbon atoms), and, in addition, the reactions had a high rate. On heating (sample II) during the first ten hours at a temperature of 170°, a product was obtained containing, calculated on the combustible mass, 69.55% carbon, whereas for the carbonization of dry humic acids to the same carbon content at the same temperature, heating for 63 hours was required for sample I and considerably longer for sample II. At a temperature of 300°, after 63 hours of heating, a product was obtained with a carbon content of 84.10%, hydrogen 6.41%, and oxygen 9.45%. Humic acids were already completely absent in this product. The sample was obtained in the form of a dense piece and had a shiny surface at the fracture. The yield of volatiles on the combustible mass was 44.2%; the coke residue was fused. In the amount of hydrogen this product has an analogue in nature. These are the so-called superfat bituminous coals, which contain from 77.6 to 85% carbon and from 5.3 to 6.4% hydrogen [4], but the product obtained possessed negligible caking capacity, although the original humic acids, while they are in the form of a gel, give a caking, dense coke residue.

When heated in hermetically sealed steel tubes at a temperature of 170° in the presence of water, the soluble and insoluble salts of peat humic acids behave identically: in all cases exclusively carbon dioxide is evolved. Raising the temperature in this case also increases the rate of the reactions, but, along with acceleration of decarboxylation reactions, dehydration reactions also acquire some significance. Thus, at heating temperatures of 200 and 250°, approximately 7–8 oxygen atoms were lost per one hydrogen atom, calculated per thousand carbon atoms. The product thus obtained (sample II) contained traces of humic acids and had the following elemental composition: carbon 77.55%, hydrogen 6.85%, oxygen 15.5%, i.e., it was strongly enriched in hydrogen; the yield of volatiles from it was 52.6%; the coke residue was fused. In elemental composition this product is analogous to the sapropel-peat coals of the Irkutsk basin, in which the amount of carbon ranges from 85 to 76.1%, and the amount of hydrogen from 5.9 to 7.8% [4].

The data obtained show that heating of hydrated humic acids and their salts in the temperature range 170–300° and at the water-vapor pressures corresponding to these temperatures leads to their predominant decarboxylation. The final product falls, according to S. M. Grigor’ev’s diagram, in the region of bituminous coals with a high hydrogen content.

In testing the influence of the acidity of the medium on the process of decarboxylation and dehydration of humic acids, it was found that, during carbonization of humic acids at a temperature of 200° and the corresponding pressure of water vapor in the presence of hydrochloric acid, the same results were obtained as in experiments 15–17 of the table presented, where the acid

the medium was created by the humic acids themselves, i.e., it was weakly acidic. When the medium changes from an acid to an alkaline reaction, decarboxylation begins to predominate over dehydration reactions.

Conclusions

1. Selective splitting off of carbon dioxide and water from humic acids in the process of metamorphism is apparently possible even without the participation of microorganisms.

2. The physical state of humic acids and the water content of the medium have an enormous influence on the further course of reactions in the coal-formation process.

3. A highly water-saturated medium and the presence of humic acids in the form of salts in a peat bog promote selective decarboxylation in the process of metamorphism and the formation of combustible minerals rich in hydrogen.

4. Dry humic acids in the course of metamorphism are predominantly subjected to dehydration and apparently yield combustible minerals with a low hydrogen content.

5. One of the principal factors of metamorphism—temperature—has an enormous influence on the process of carbonification of humic acids. The higher the temperature, the greater the importance acquired by dehydration reactions, and the poorer the resulting product is in hydrogen.

LITERATURE

1. Stadnikov, G. L. The Origin of Coals and Petroleum. Publishing House of the Academy of Sciences of the USSR, pp. 476, 474, 485, 1937.

2. Khrisanfova, A. I. Transactions of IGI, Academy of Sciences of the USSR, vol. II, 278, 1950.

3. Titov, N. G. Transactions of the Laboratory of Coal Geology. Publishing House of the Academy of Sciences of the USSR, 6, 71, 1956.

4. Veselovskii, V. S. The Chemical Nature of Combustible Minerals. Publishing House of the Academy of Sciences of the USSR, 1955.

5. Grigor'ev, S. M. On the processes of formation and the properties of combustible minerals. Publishing House of the Academy of Sciences of the USSR, 1951.

6. Freund, H.; Neulen-Van-Selms, F. Chemistry and chemical technology. Translations from foreign literature, No. 2, 119, 1957.

7. Zabramnyi, D. T. Transactions of the Laboratory of Coal Geology. Publishing House of the Academy of Sciences of the USSR, 6, 83, 1956.

ERRATA AND CORRECTIONS

Izv. TPI, vol. 102.