ON THE MECHANISM OF COAL OXIDATION AND THE CAUSES OF ITS SPONTANEOUS COMBUSTION.

Article 11.

M. Yu. Grigoriev and A. I. Plekhanova.

On the Possibility of Using Ferric Chloride and Ferric Sulfate to Determine the Reducing and Adsorption Capacity of Coals.

The principal criterion used to distinguish between the brown-coal and hard-coal stages of formation of fossil coals is the presence in brown coals of humic acids, which can be isolated by extraction with a weak solution of caustic alkali. It is known, however, that oxidized hard coals also contain humic acids that are readily extractable by alkali and, consequently, by this method they are difficult to distinguish from brown coals. In particular, the question has not yet been resolved as to where many seams of the Cheremkhovo and Moscow Region basins should be assigned: to brown coals or to oxidized hard coals. If one takes into account that methods for the rational utilization of both brown coals and oxidized hard coals have now been developed to a considerable extent, it is clear that assigning the coal of one deposit or another to brown coals or to oxidized hard coals is of enormous importance for the national economy. Recently, a number of methods have been proposed in the literature for determining the degree of oxidation of hard coals, based chiefly on the adsorption capacity of coal¹, ², ³). To these should also be added the method proposed by Vologdin and Komendrovskaya⁴). The essence of this method is as follows: a coal sample of 1 g, passed through a sieve with 900 openings per 1 cm², is treated with 75 cm³ of ferric chloride solution (0.1%), vigorously shaken for 5 minutes, and then left undisturbed for 1 h 25 min. After this it is filtered; the first 10–15 g of filtrate are discarded, and from the remaining portion two samples of 25 cm³ each are taken with a pipette. In the first sample, titration with a centinormal solution of potassium permanganate is used to determine the quantity of reduced iron, which, calculated per 1 g of the organic mass of the coal, gives the number “B” in milligrams of iron. In the second portion of the filtrate, Fe⁺⁺⁺ is reduced by heating with stannous chloride to Fe⁺⁺; the excess stannous chloride is converted to stannic by adding the corresponding quantity (to

until a faint turbidity of HgCl₂ appears) and then Fe·· is titrated with the same centinormal solution of potassium permanganate. The difference between the second and the first determinations, multiplied by 3 and expressed in milligrams of iron per 1 g of the organic mass of the coal, gives the “A number,” characterizing the adsorptive capacity of the coal.

The authors who proposed this method, on the basis of their work, come to the conclusion that the “B number” may be used to characterize the activity of coal with respect to atmospheric oxygen. In addition, since brown coals usually give high “B numbers” and low “A numbers,” whereas oxidized hard coals, on the contrary, give low “B numbers” and high “A numbers,” in the authors’ opinion this method may be proposed for determining the degree of oxidation of coal.

The Kuznetsk Coal Research Institute proposed that we test the action of ferric chloride on coal in order to ascertain: 1) the possibility of using ferric chloride to determine the tendency of coal to oxidation, and 2) the possibility of using ferric chloride to determine the degree of oxidation of hard coal. As objects of the work we took ten different samples of hard coals from the Kuzbass, one brown coal from the Achinsk deposit, and one brown coal from the Yar deposit (Tomsk). For all the samples taken (with the exception of the brown coals), the temperatures at which their vigorous oxidation begins were determined in Erdmann’s apparatus. The determination of the “B numbers” and “A numbers” was carried out by the method described above. The data obtained are summarized in the table.

Table 1

“B” and “A” numbers for certain Kuzbass coals

The “B numbers” and “A numbers” were also determined for certain coals after their oxidation by atmospheric oxygen. Oxidation was carried out as follows: coal, ground under a sieve with 900 openings per 1 cm², was scattered in a thin layer on a watch glass and kept in a thermostat at 75° for a period of

45 days. Two other samples of the same coal were oxidized in a metal retort by a stream of air supplied from below, at temperatures of 150° and 250°. The data obtained are summarized in the table.

Table 2

“B” and “A” numbers for initial and oxidized coals

From the data presented, the following conclusions may be drawn:

1. The “B numbers,” which characterize the tendency of coals toward oxidation, are very small. To determine them, only an insignificant amount of potassium permanganate is used (from 0.2 to 1 cm³), which cannot entail large errors in their determination.

2. For different coals, the “B numbers” are close to one another and, evidently, reflect only slightly the tendency of coals toward oxidation by atmospheric oxygen. If we compare the “B numbers” obtained by us for certain coals with their temperatures of the onset of vigorous oxidation according to Erdman, then no complete parallelism is observed here. Thus, coal from the Kuybyshev deposit, whose temperature of the onset of vigorous oxidation lies at about 173°, has a “B number” greater than coal from the Kemerovo seam, which has a temperature of the onset of vigorous oxidation of about 155°. The same may be said with respect to the Gorely seam and coal of shaft A of the Leninsk deposit, pl. Kharakterny and the same Leninsk coal, etc.

3. The “A numbers” in our experiments for oxidized coals (artificially oxidized) are equal to 0. Of the two brown coals we took, one (from the Achinsk deposit) gave an “A number” greater than the hard coals and a “B number,” on the contrary, smaller.

4. Ferric chloride has weak oxidizing properties and therefore can hardly be used for determining the tendency of hard coal toward oxidation.

5. The use of ferric chloride for measuring the adsorption capacity of coal also meets with objections, since a ferric chloride solution has an acid reaction and, consequently, a chemical interaction between coal and ferric chloride is possible. In our experiments, some coals (in particular, brown and Leninsk coals), after treatment with ferric chloride, gave a brown filtrate.

1. A series of determinations of the numbers “B” and “A,” carried out by us with changes in the experimental conditions (change in the coal charge, temperature, concentration of ferric chloride in the solution, and degree of comminution of the coal), did not yield positive results.

At the same time as ferric chloride, ferric sulfate (oxide) was tested as a reagent. The procedure was the same as with ferric chloride. The reduction of b was carried out according to Folgard. The results obtained, as can be seen from the table, are analogous to the results for the oxidation of coal by ferric chloride.

Table 3

All the conclusions drawn by us with respect to the oxidation of coal by ferric chloride remain valid also with respect to ferric sulfate.

LITERATURE

1. Pentegov B. P. and Nyantkovskaya R. I. Adsorption characteristics of fossil coals. Proceedings of the Far Eastern State University, Vladivostok, 1927.

2. Pentegov B. P. Adsorption as a general characteristic of fossil coal and peat.

3. Syskov K. I. and Ushakova A. A. The adsorption capacity of coals as a characteristic of the degree of their oxidation and as their classification criterion. KhTT, vol. VIII, issue 8.

4. Vologdin M. V. and Komendrovskaya E. A. KhTT, 1936, No. 1.