I. V. Gebler and A. I. Shustova.

On the Variability of Osinovsky Coal During Oxidation.

1.

The alteration of any fossil coal during its oxidation, generally speaking, is expressed in the following phenomena: 1) the elemental composition of the coal changes; 2) the yield of volatile substances changes; 3) the amount of humic acids extracted by dilute alkali solutions increases, or, in coals in which humic acids are initially absent, such acids are found after oxidation; 4) the calorific value decreases, and 5) the caking capacity of the coal diminishes, the latter being understood as the expression of the quality of the coal melt in the plastic state in the sense of viscosity and homogeneity.

As regards the coking properties of coal, which determine the quality of the lump coke obtained from it, in this respect coals after oxidation may to a certain extent furnish either better or worse coke in comparison with the original coal, depending on the type of coal. Namely, fat coals with high caking capacity, a great thickness of the plastic layer, and considerable swelling pressure with the corresponding rise of the Treib curve above the zero line—coals which in this state yield fissured, weak coke and, moreover, present difficulties in coking in the sense of danger to the ovens—after moderate oxidation show an improvement in coking capacity. For coals of the opposite type, lean caking coals, with low caking capacity and with a plastic layer of small thickness, the coking properties will decline as oxidation proceeds. On this basis, moderate oxidation of certain components of the charge may lead to the transformation of coals in the desired direction, and has recently been advanced as a factor that can be used for regulating the coking process ¹).

In general, those phenomena which accompany the oxidative process undergone by coals obviously cannot proceed uniformly. For different coals, in connection with their dissimilar nature with respect to petrographic and chemical composition and physical properties, some changes as compared with others may be greater or smaller. Therefore a more or less complete picture of the alteration of coal can be obtained by observing all possible phenomena occurring during the oxidation of coal; moreover, this oxidation must be carried out under conditions close to those that take place when coal is stored in yards, i.e., by laying experimental piles with constant temperature and analytical control and with observation of the meteorological regime ²).

Experiments conducted under such conditions are possible in a special study of coals and are characterized by cumbersomeness, long duration, and expense; they do not always make possible exhaustive conclusions of a general character, since the course of the process is observed only under certain conditions. In this connection, methods of laboratory oxidation are resorted to,

¹) Coke and Chemistry, 1934, No. 8, p. 50.
²) Chemistry of Solid Fuel, 1931, No. 5; 1933, issue 5.

subjecting coal to the action of air or oxygen and observing the changes that the coal undergoes in the process.

In addition, for characterizing coals from the standpoint of their capacity for oxidation—which, in the final analysis, may lead to the spontaneous combustion of coals—there exist a number of methods by which certain characteristic numbers are established for coals, numbers that should indirectly indicate the tendency of coals toward oxidation and spontaneous combustion. Such methods include the following:

1) The critical-temperature method, consisting in finding the temperature point at which the temperature of the coal begins to rise faster than the temperature of the surrounding medium when oxygen or air is passed through the coal (the point of intersection of the curves of coal temperature and ambient temperature);

2) Observation of the rise in coal temperature at a constant temperature of the surrounding medium;

3) Measurement of the amount of oxygen adsorbed by coal under specified conditions;

4) Determination of the adsorptive capacity of coal with respect to certain substances from aqueous solutions (salts, dyes);

5) Chemical methods proper, consisting in the determination of:
a) halogen numbers, which establish the relative degree of unsaturation of certain substances included in the composition of coal, with which its reactivity is associated;
b) oxidation of coals by the wet method with the aid of potassium permanganate or, sometimes, potassium dichromate, whereby the amount of oxygen consumed gives a relative characterization of the coal’s tendency toward oxidation.

Among all these methods, strictly speaking, there is not a single one that is indisputable and firmly established, and they all give only conditional, comparative characteristics; moreover, it is often observed that results obtained by any one of the methods do not agree with those obtained by other methods.

It is very difficult to correlate the results obtained by these methods with the nature of the changes in coal that it may undergo during storage in air, and in particular with changes in its coking properties, in the sense of the behavior of the coal in the coke oven and the quality of the coke it yields. In this direction, the clearest and most valuable indications, as recent investigations show, are obtained by directly determining, for weathered and oxidized coals, those characteristic quantities by which the coking properties of coal are determined. These include: caking capacity, determined by one or another method, and plastometric parameters, i.e., the thickness of the plastic layer and the magnitude of the final drop of the swelling-pressure curve.

2.

Coal from the Osinovskoye deposit, from the standpoint of oxidizability, was studied by I. Lapteva in the laboratory of the Stalin Coke-Chemical Plant¹. In this work, methods were applied for observing the rise in coal temperature at a constant temperature of the surrounding medium and the critical-temperature method. The Osinovskoye coals were studied for comparison with certain coals of the Prokopyevsk deposit, for which data from analogous investigations by other authors are available.

Coals from run-of-mine samples were characterized by the following data:

Table I.

Oxidation of the coals at a constant bath temperature (150° and 180°) was carried out with oxygen in an Erdmann apparatus for 1 hour. The figures in Tables II--III and the curves (Figs. 1 and 2) characterize the behavior of the coals under these conditions.

Table II.

Bath temperature: 150°

For the Prokopyevsk coals a somewhat smaller rise of the temperature curve is observed in the case of oxidation at 150°, which may indicate their greater stability; however, this difference is not sufficiently sharply expressed in this case. The results of the experiment at 180° do not agree with the preceding ones; part of the Osinovsk coals gave a smaller rise of the temperature curve. In general, all the coals at 180° gave a sharp jump in temperature, and at the same time ashing of the coals was observed. Evidently, a temperature of 180° is too high for oxidation with oxygen; ignition of the coal occurs, which distorts the course of the temperature curves.

¹ Caking capacity is given according to the scale adopted in Kuzbass.
² Sample No. 6 raises doubt as to the correctness of its grading.

Table III.

Bath temperature: 180°

The determination of critical temperatures was also carried out in the Erdmann apparatus; the results of the experiments are given in Fig. 3. The average critical temperatures for the coals investigated are as follows:

As can be seen, the temperatures corresponding to the points of intersection of the curves for all coals fluctuate within very narrow limits, 159,5—167,5°.

In the investigation of certain Kuznetsk coals by the “point of intersection” method under analogous conditions¹, critical temperatures of 180° and 170° were obtained for Prokopyevsk coking coals (seams: Vnutr. III and Vnutr. IV); for Prokopyevsk steam-caking coals (seams: Kharakternyi and Lutuginskii)—167° and 165°; on average, for the coals of the Prokopyevsk deposit, 169°. This figure closely coincides with those obtained for Prokopyevsk coals No. 5 (K) and No. 7 (PS); coal No. 6 shows a slight deviation, giving a figure identical with all the Osinovsky coals, which it also approaches in the content of volatile matter and in caking capacity.

For coals from other deposits the following average critical temperatures are given:

Kemerovo deposit . . . . . . . . 166°
Leninskoye ” . . . . . . . . 164°
Anzhero-Sudzhenskoye deposit . . . . 169°
Khakasskoye ” . . . . . . . . 145°

Fig. 1.

Coals of the Prokopyevsk deposit, according to various data, are considered the most stable during storage in air¹⁾. For them (together with the Anzhero-Sudzhensk coals), the highest value of the critical temperature has been obtained. If one assumes that the stability of coals with respect to their tendency toward spontaneous combustion is the lower, the lower the temperature, correspond-

¹⁾ Khimiya tverdogo topliva [Chemistry of Solid Fuel], 1932, issues 2–3; I. V. Gebler and G. A. Zilberg, “On the question of changes in certain Kuznetsk coals during their oxidation,” Izv. Sib. T. I., vol. 51, issue 1 (1929). Materials on the question of supplying the charge for coke plants of the Urals and Siberia, Giprokoks, 1931.

*2. Izvestiya TII, vol. 57, issue II.

Fig. 2.

...at the corresponding point of intersection of the curves, then, among the coals presented, the Khakass coals will be the most prone to spontaneous combustion. The Osinovsky coals, with a critical temperature of 159°, will occupy the next position, i.e., they will prove more readily oxidizable in comparison with all the other coals except the Khakass coals—provided, of course, that the results of such studies carried out in different laboratories can be compared.

However, the difference between the temperatures for the various coals proves to be too small, which indicates insufficient sensitiv-

Fig. 3.

ness of the method, and therefore the corresponding conclusions can be drawn with great caution and only as a first approximation.

Comparing the values of the critical temperatures with the content of volatile substances for various coals according to data from different investigators, and also considering the figures of Prof. Karavaev, who studied Kuznetsk coals by the same method, but under somewhat different conditions, passing not oxygen but air through the coal,¹ one may note the existence of a certain relationship between the critical temperatures and the yield of volatile

¹ Izvestiya Teplotekhnicheskogo instituta, 1930, No. 8 (51).

substances; moreover, a larger amount of volatile substances corresponds to a lower temperature.

Tables IV and V give the average figures from the 1932 work; Table V was compiled from Prof. Karavaev’s data.

Table No. 4.

Table No. 5.

A more distinct difference in temperatures for different coals appears during oxidation by air, and here as well there is a great regularity in their decrease with increasing yield of volatile substances.

For Osinovsky coal, according to the data of I. Lapteva’s work, an attempt to place it in Table IV would result in a deviation (\(V^r\) avg. 28.60%; critical temp. avg. = 159.6), perhaps only owing to not entirely identical conditions of determination in different laboratories; in particular, in I. Lapteva’s work the degree of grinding of the coals during their oxidation with oxygen is not indicated, whereas the degree of grinding and the related relative content of fine grains and their total surface must influence the course of the oxidation process, other conditions being equal.

With respect to some of the same coals from the Osinovsky and Prokopyevskoye deposits, I. Lapteva made a determination of the humic acids formed during oxidation of the coals with nitric acid by Dolgov’s method¹. For these experiments the coals were taken in a fineness corresponding to passage through a sieve of 900 openings/cm². The original coals contained no humic acids. After oxidation with nitric acid the following results were obtained:

Table No. 6.

As can be seen, these figures are, generally speaking, little different and insufficiently characteristic. For coals of one type (No. 3 and No. 4), the difference in the content of humic acids after chemical oxidation proves to be greater than for coals of different types (No. 4, grade GZh, \(V^r\): 28.17%, and No. 7, grade GS, \(V^r\): 22.09%), amounting in the first case to 7.5%, and in the second to 3.3% relative to the mean value. In this connection, on the basis of the results of oxidation of coals with nitric acid, it is difficult to draw any definite conclusion.

3.

For testing with respect to oxidizability, coal from the Osinovsky deposit was delivered to our laboratory with the following certificate: Osinovsky mine, 1st district, central adit, northern wing, 3rd seam. The sample was taken on September 1, 1933.

This coal was subjected to proximate and ultimate analysis and, in addition, the calorific value was determined in a Kroeker bomb, the caking number by the method of I. V. Gebler¹, the thickness of the plastic layer, the yield of primary tar in a Fischer retort, and the humic acids.

To determine the humic acids, 3 g of coal were treated with a 1% solution of caustic soda for 2 hours in a boiling water bath, and in the filtrate, after separation of the coal, the amount of oxygen required to oxidize the extracted humic acids was determined from the consumption of \(1/100\,N\) \(KMnO_4\) solution. Since “standard humic acid” from Merck was not available in the laboratory for a standard solution, the amount of humic acids was characterized conventionally by the number of milligrams of oxygen per 100 g of coal.

The results of all these determinations are given in the following table:—

Table 7.

It is known that some coals are capable of absorbing oxygen even at ordinary temperature². Since the oxidizability of coal is connected with the absorption of oxygen, which, being at first physically adsorbed, subsequently, while in close contact with the coal substance, enters into chemical interaction with it, one may expect oxidation of coal, even if very slow, already at ordinary temperature. Bearing this circumstance in mind, the coal under investigation was left in contact with air for a long time at the laboratory temperature, fluctuating within the limits of 15–22° C. The coal was taken in an amount of 12 kg and spread on a wooden board in a layer of 5 cm, with measures taken to protect the coal from dust by installing screens of gauze. The coal had first been passed through a laboratory crusher, and its granulometric composition proved to be as follows³:

After 5 months, an average sample was taken and analyzed, with all the same determinations made as those indicated for the fresh coal. It turned out that the changes in the figures, compared with those given in Table VII, did not exceed the permissible experimental errors.

After this the coal was left for another 7 months. Analysis of the coal after—

exposure in air under the indicated conditions for one year gave the following results:

Table 8.

No change in most of the indicators is observed in this case. Only the number characterizing the content of humic acids increased by 22.3%, which is connected with a slight increase in the oxygen content (by 2.4%). The technological and energetic properties of the coal, determined by the corresponding indicators (calorific value, volatiles, caking capacity, and plastic layer), evidently did not change.

Thus, at ordinary temperature this coal is very stable with respect to the oxidizing action of air.

The next experiment was carried out at a temperature of 35°--37°. In this case coal of the same degree of pulverization was placed in a desiccator on a mesh in a layer 3 cm thick; air was slowly drawn through the coal, humidified by passing it through water placed in the lower part of the desiccator. These conditions were chosen as more or less close to those that may occur when coal is stored in piles in summertime, bearing in mind the possibility of heating of the coal only due to external heat and insolation to about 35°, and the increased humidity of the air and coal during rainy periods. The desiccator with the coal was placed in an electric thermostat, in which the temperature was constantly maintained within the indicated limits--35--37°.

Control experiments showed very slow variability of the coal. The experiment lasted 1700 hours (~70 days), which exceeds the total number of the warmest days in Siberia. Testing the coal after 1700 hours gave the following figures:

Table 9.

The changes in the coal, as can be seen, are slight, but somewhat greater than in the first experiment, and this is most reflected in the decrease in the caking number, which proved to be an especially sensitive indicator in coal oxidation and was subsequently adopted as the principal criterion in evaluating the degradation of the technological properties of coal in the process of oxidation.

Further experiments on the oxidation of coal by air were carried out at higher temperatures. The coal for these experiments was taken in a size below a sieve with 325 openings/cm, which seemed more appropriate when taking samples for oxidation in small quantities. The composition of the pulverized coal according to sieve analysis:

Coal was poured into crystallizers, which were placed in electric thermostats at temperatures of 75°—78° and 105°—108°¹. At certain intervals of time the caking index was determined, and the following results were obtained:

As can be seen, the dependence of the rate of change in caking capacity on temperature is very sharply expressed, especially upon transition to a temperature of 105—108°. Whereas a decrease in caking capacity at a temperature of 35—37° to 54 is reached after 1700 hours, at a temperature of 75—78° almost the same occurs after 72 hours; however, the subsequent decrease in caking capacity in this case is extremely slowed. At a temperature of 105—108° the decrease in caking capacity proceeds at an even greater rate and weakens little over the entire course of the oxidation process, so that in a comparatively short interval of time the coal is almost deprived of caking capacity.

The course of the change in caking capacity at the indicated temperatures is very clearly seen in the graph (Fig. 4) for one and the same coal sample, which was first placed in a thermostat at a temperature of 75°, and after 500 hours the temperature was raised to 105°. The first curve decreases very little and, in the end, tends to become a straight—

Fig. 4.

there; with increasing temperature the lowering of the second curve immediately becomes sharp and fairly uniform.

A temperature of 105° may be regarded as the temperature at which the degradation of the caking capacity of this coal occurs completely within a comparatively short interval of time. With a further increase in temperature, the rate of change in caking capacity continues to increase, and at 150° the coal had completely lost its caking capacity after 24 hours.

For coal subjected to oxidation at a temperature of 105–108°, samples were taken at separate moments and the volatile matter and plastic layer were determined. The results are given in Table X.

Table X

The change in volatile matter is insignificant. The decrease in caking capacity approximately corresponds to the decrease in the thickness of the plastic layer; both are expressed very sharply (Fig. 5; I—the thickness of the plastic layer, II—the caking-capacity number).

The change in caking capacity and in the thickness of the plastic layer indicates that the processes occurring with sufficient intensity at a tempera—

Fig. 5.

temperatures above 100° affect the bituminous part of the coal; at the same time it remains unknown whether these processes are only oxidative, or whether here there is also, in part, a thermal action, under which one may allow for the possibility of transformation of the bitumens on account of phenomena occurring within their molecules, with a change in the size of the latter under the influence of the constant action of elevated temperature. To clarify this question, an experiment was set up involving prolonged heating of coal in an atmosphere of nitrogen.

Coal in an amount of 20 g was placed in a U-shaped tube, which was installed in a thermostat at a temperature of 105–108°. A current of nitrogen was passed through the tube at a rate of 1 bubble per second for the entire duration of the experiment; at the very beginning of the experiment, to displace the air, a rapid current of nitrogen was passed for \(1/2\) hour. As a control, in the same thermostat an analogous tube with coal was placed, through which a current of air was passed. After 168 hours, the caking capacity was determined for the coals from both tubes; it proved to be:

\[ \begin{aligned} &\text{For coal in a current of air} \ldots\ldots 32\\ &\text{” ” ” nitrogen} \ldots\ldots 45 \end{aligned} \]

Consequently, in a current of nitrogen, all other conditions being equal, the coal undergoes a smaller change with respect to caking capacity; nevertheless, a decrease in caking capacity does occur, which, evidently, must depend on oxidation of the coal by the air originally contained in the coal, which was not completely removed when nitrogen was passed through.

To clarify this question, the following experiment was set up. Coal in an amount of 15 g was placed in a thick-walled flask with a capacity of 50 cu. cm. The flask with the coal was placed on a boiling water bath and connected to a vacuum gauge and a mercury pump, by means of which the air was evacuated from the flask and the coal until a limiting vacuum was established in the flask; after this, nitrogen was drawn into the flask, and it was placed in the thermostat, closed with a stopper with an outlet tube, the end of which was immersed in water when the clamp was subsequently opened. In the thermostat the same temperature, 105–108°, was maintained. Over the course of the same heating period, no change in the caking capacity of the coal occurred.

Thus, when heated in an atmosphere of nitrogen with preliminary removal of air, the coal undergoes no changes; from this it follows that the changes experienced by the coal when heated in the presence of air are due only to oxidative processes; heating of coal in the absence of oxygen up to 105° by itself does not cause changes over the period of time that is sufficient for a considerable transformation of the coal in the case of heating with access of oxygen.

For a more complete characterization of the coal that had undergone changes during oxidation, a more detailed analysis was made of coal oxidized at a temperature of 105–108° for 168 hours. The results are given in Table XI together with the data for the original coal.

Thus, in the oxidized coal we see, along with a sharp decrease in caking capacity and a decrease in the thickness of the plastic layer, a large increase in oxygen and an increase in the number characterizing the content of humic acids. The change in these four indicators is the most characteristic.

The coking capacity of the coal, since it depends on caking capacity and on the thickness of the plastic layer, must also change. For the direct determination of the coking properties, the original and the oxidized coal were subjected to laboratory coking, and the coke obtained was tested

Table XI.

for strength in a laboratory crusher, followed by screening out the fines on a 3 mm sieve. The residue on this sieve characterizes the strength of the coke. It was:

Thus, the coking capacity for the oxidized coal increased greatly, and at an intermediate stage of oxidation it might have proved to be still greater.

A study of the dependence of coking capacity on the degree of oxidation of coal may serve as the subject of a separate work. However, taking into account that the indicated changes for Osinovsky coal proceed with sufficient rapidity at temperatures above 100°, the practical use of the oxidation process to improve the coking properties of coal appears difficult, unless at low external temperature there is a considerable heating of the internal masses of coal in stacks due to the very slowly proceeding oxidation processes at first, at lower temperatures (about 35°).

4.

To study the capacity of coals for oxidation, recourse is sometimes made to the action of certain oxidizing agents, of which potassium permanganate is most often used. The “permanganate numbers” thus obtained, expressed in cubic centimeters of normal potassium permanganate solution consumed in oxidizing coal under specified conditions, relative to a specified quantity of coal, or the numbers indicating the consumption of oxygen in mg corresponding to the amount of permanganate consumed, are a characteristic of the coal’s capacity for self-oxidation.

Treatment of coal with a solution of potassium permanganate salt is carried out either in an acid solution (J. Kreulen¹), or in an alkaline solution (H. Heathcoat²), always under strictly specified conditions.

With respect to the Osinovsky coal under investigation, several experiments on oxidation with potassium permanganate salt were carried out. The coal for oxidation

¹ Br.—Chemie, 1920, S. 397.
² Fuel XII, 1933, I.

was taken in the degree of comminution corresponding to passage through a sieve with 900 openings/cm². A coal sample of 0.5 g was treated with a \(^{1}/_{10}\) N solution of KMnO₄ at ordinary temperature (\(\sim 20^\circ\)C) for 1 hour. During the experiment the contents of the flask were stirred with a stirrer.

During oxidation in an alkaline medium, the consumption of oxygen per 1 g of dry coal was 16.3 mg; during oxidation in an acidic medium—35.0 mg.

In the work cited above,¹ larger figures are given, namely:

Correspondingly, one might have concluded that Osinovsky coal has a lesser capacity for oxidation. However, in doing so it is necessary to bear in mind the circumstance that permanganate numbers depend to a great extent on the granulometric composition of the coal, which was observed by us in work with permanganate and proved in a special experiment.

For this purpose the coal under investigation was taken in three degrees of comminution, corresponding to passage through sieves with numbers of openings per cm² of 400, 900, and 4900, and each of these samples was subjected to oxidation by permanganate in an alkaline medium at a temperature of \(\sim 20^\circ\), under completely identical conditions, for greater expressiveness for 3 hours. The following was obtained:

Table XII

i.e., the numbers characterizing the capacity for oxidation of one and the same coal, depending on the degree of comminution, differ extraordinarily among themselves.

In the work mentioned, the degree of comminution of the coals is not indicated; it is only stated that a finely comminuted sample was subjected to oxidation. This creates uncertainty in the conditions of the experiment, and the corresponding figures lose their clarity.

It must be noted that, when coals are comminuted by ordinary methods, different coals, owing to their unequal hardness, give products with unequal granulometric composition. Thus, when two coal samples, Osinovsky and “PZh,” were comminuted under completely identical conditions—

¹ Chemistry of Solid Fuel. 1932, issue 2–3.

and of Prokopyevsk coking coal, until passage through a sieve with 400 holes/cm², their screen composition proved to be as follows:

Table 13.

The amount of the finest fraction in the less hard Osinovsky coal is 10% greater.

In other cases the screen composition may prove still more different. This circumstance should be borne in mind when testing coals for oxidizability by means of permanganate.

CONCLUSIONS:

1. The coal studied from the Osinovsky deposit, with respect to its capacity for self-oxidation, should be classified as a stable coal, in accordance with its rather high critical temperature and low permanganate numbers.

2. At ordinary temperature, coal in contact with air remains practically unchanged over the course of one year.

3. At a temperature of 35–37°, such as coal may experience in summer due to external heat, a certain change in the coal is observed, which is expressed practically in a slight decrease in the caking number; this was established over a period of about 70 days.

4. Very vigorous and rapid oxidation of coal proceeds at a temperature somewhat above 100°; in this case, within as little as 400 hours, a sharp change in the properties of the coal occurs.

5. Oxidation of coal at a temperature above 100° (~105°) is accompanied chiefly by a change in the technological properties of the coal, which is expressed in a decrease in caking capacity, in the thickness of the plastic layer, and in the yield of primary tar.

6. The coking capacity of the coal increases during oxidation, since the coal, being initially fat, with high caking capacity and yielding a weak coke, becomes leaner in the course of oxidation.

7. The method proposed by the author for determining the caking capacity of coals is very sensitive for monitoring the variability of coal during oxidation, while at the same time being very simple and rapid in execution.

1. Under these same conditions the experiment was also carried out at a temperature of 35—37°; the result proved to be the same as in the preceding experiment (at a temperature of 35—37°) with coarser coal and with humidification of the air. ↩↩↩↩↩

2. Fuchs, W. Die Chemie der Kohle. 1931. ↩

3. Losses in sieve analysis: 1.00%. ↩