On the Production of Pitch Substitutes

V. S. Shved, N. M. Smolyaninova

(Presented by the scientific-methodological seminar of the KTF)

One of the most important products of by-product coke production is pitch. The bulk of it is used for the production of electrode coke, which is needed in the aluminum industry. In addition, pitch is used as a binder in the production of anode paste, for briquetting, in the roofing-felt industry, for the production of varnishes, for road construction, and for other purposes. Many researchers have shown the positive effect on coke quality of additions of pitch to coal charges.

At present, a shortage of pitch has become clearly evident in the country, despite the planned increase in its production \[1, 2\]. Therefore the question of finding ways to obtain pitch substitutes is of great practical importance.

In the present work, the possibility of obtaining pitch substitutes under the conditions of the Kemerovo by-product coke plant was studied.

At the first stage of the investigation, pitch distillates, an anthracene fraction, crude anthracene from the Kemerovo Coke-Chemical Plant, and their mixtures were used as the starting raw material for obtaining pitch by the oxidation method. The indicated products were oxidized at 300°C with the oxygen of air passed at a rate of 1 l/min. The pitch-like product obtained was analyzed for ash content, yield of volatile substances, and the amount of substances insoluble in toluene. The softening temperature of the pitch according to Krämer-Sarnow and the coke number were also determined \[4\].

For comparison, experiments were carried out in parallel on the oxidation of Chelyabinsk pitch and soft pitch from the Kemerovo plant, taken after the second-stage evaporator in the tar-processing shop. The data obtained are given in Table 1.

From the table it is seen that the yield both of the oxidized product and of the final target product—coke—from all types of tested raw material is small. Oxidation of the anthracene fraction, moreover, is inadvisable for economic reasons as well, since in this case a valuable product of its processing—anthracene oil—is lost. It should be noted that the process of oil oxidation is very lengthy.

Joint oxidation of pitch distillates with soft pitch (experiments 12, 13) gives a significant increase in the yield of oxidized pitch, as well as of coke, calculated on the basis of the distillates themselves.

Table 1

Characteristics of pitches for the production of carbon pitches
Results of oxidation of petroleum feedstocks and characteristics of the pitch

At the second stage of the work, the pitch substitute was obtained by dissolving various grades of Kuzbass coals in pitch distillates, in pitch resin, and in its mixture with pitch distillates, in soft and medium-temperature pitches. The characteristics of the initial raw material are given in Table 2.

The procedure was as follows: a definite amount of solvent (250–800 g, depending on the size of the beaker) was charged into a steel beaker. The beaker was placed in a vertical electric furnace, where it was heated to the dissolution temperature. For coals of grades 2Zh 26, KZh 14, and SS, this temperature was in the range 300–330°; for G6, 1Zh26, K10, and K₂, in the range 350–360°. A weighed portion of air-dry coal was charged into the hot solvent in small portions with stirring over 15–25 min. After charging was completed, stirring was continued for 15–20 min; then the resulting solution was allowed to stand for a specified time (from 1 to 5 h) to precipitate the mineral fraction of the coals, and it cooled until solidification. Then, by rapidly heating the beaker until the thin layer at its walls had melted, the pitch, in the form of a cylinder, was removed from the beaker.

After weighing, the cooled pitch cylinder was split by height into two parts along the visible boundary of separation (the upper part low-ash, the lower part high-ash). Both parts were weighed and analyzed for ash content; the pitch of the upper part was characterized by its softening temperature.

The results of the experiments are given in Table 3. These data show that the yield of coal-tar pitch ranged from 75 to 95%, depending on the type of solvent, the process temperature, and the settling time. The dissolution process proceeded rather rapidly and quite completely. The bulk of the mineral impurities settled out within an hour; increasing the settling time to 4–5 h did not lead to any noticeable decrease in the ash content of the pitch obtained. Determination of ash in different zones over the height of the upper piece of the pitch cylinder showed variations within 0.07–0.10%. Increasing the height of the column of pitch being settled by a factor of 1.5 practically did not reduce its ash content.

Increasing the concentration of the coal being dissolved led to an increase in the softening temperature of the coal-tar pitch obtained, but at the same time its ash content increased sharply (experiments 24, 25, etc.).

The best results from the standpoint of low ash content and high yield of the low-ash fraction of pitch were obtained in experiments on the dissolution of fat coal, grade Sh. Kapitalnaya-I, in pitch distillates (experiment 3). The pitches obtained by dissolving coals in coal-tar pitches (experiments 26–29) were the most ash-rich, which should be explained by their high viscosity. This circumstance, as well as the small yield of the low-ash fraction, indicates the inadvisability of using coal-tar pitch as a solvent for coals of ordinary ash content, since it does not ensure the production of a raw material for firing high-quality low-ash electrode coke. The latter also applies to pitch resin, the simple oxidation of which also gives a higher yield of high-temperature and, moreover, less ash-rich pitch than when coal is dissolved in it.

Thus, the most suitable solvent for coals is pitch distillates, which at present have no qualified application.

In order to study the possibility of reducing the ash content of coal-tar pitches, a number of experiments were carried out. The use, for dissolution, of a coal fraction larger than 6 mm did not lead to any noticeable decrease in the ash content of the pitch.

Table 2

Characteristics of raw materials for obtaining coal-oil pitches

Table 3

Results of experiments on dissolved coal

Continuation of Table 3

*) The number in parentheses denotes the mine in accordance with Table 2.

Unsatisfactory results were also obtained in investigating the possibility of reducing the ash content of coal–oil pitches with an increased concentration of dissolved coal by mixing them with anthracene or solar oils in order to reduce the viscosity of the solution (to facilitate the settling of the mineral portion of the coal), followed by distillation of these oils from the settled portion of the solution. It was noted, however, that solar oil leads to coagulation of the colloidal solution of coal–oil pitch.

Experiments were carried out on extracting the more readily soluble portion of the coal with a high-boiling solvent: portions of 2Zh26 coal (+6 mm fraction) were loaded into a mesh cylinder, which was then lowered into pitch tar preheated in a beaker to 320°, and held for 5–15 min; the mesh was then removed. The coal residues were poured out and weighed. The amount of coal that passed into solution was one third of that taken for extraction, which corresponded to its ratio to the tar of 1 : 4. The pitch obtained was homogeneous throughout the mass; its softening point was 113°C (by ring and ball), but the ash content was high—0.66%.

Since relatively low-ash coal–oil pitches are obtained when the amount of coal in the initial mixture is no more than 20% and their softening point is not high, these pitches were subjected to additional treatment by oxidizing them with atmospheric oxygen at a liquid-phase temperature of 300°C. The results of the experiments and analyses of the pitches obtained are presented in Table 4.

It should be noted that the oxidation process of coal–oil pitches proceeds in a rather distinctive manner (especially for pitches obtained on the basis of pitch distillates): already after 1.5–2.0 hours the entire mass thickened to such an extent that it was impossible to continue oxidation; moreover, raising the temperature of the liquid phase to 325–330° did not lead to a decrease in the viscosity of the product obtained. Analyses showed, however, that the process of oxidative condensation proceeded only slightly: the softening point rose only to 87–105° (by K. and S.), and the amount of substances insoluble in toluene and the coking value were also low (experiments 1–4, Table 4).

The oxidation process proceeded somewhat more deeply for coal–oil pitches in which the solvent was a mixture of pitch tar and pitch distillates, but in this case as well the oxidized pitches were, in their properties, essentially medium-temperature pitches.

In this connection, the possibility was investigated of oxidizing coal–oil pitches in a mixture with soft coal-tar pitch at a ratio of 1 : 1 and with medium-temperature pitch (from coal tar) at the same ratio. In this case the duration of the oxidation process increased somewhat; in terms of softening point, the oxidation products corresponded to high-temperature pitches, but the coking value and the amount of substances insoluble in toluene were lower than in ordinary high-temperature pitches. Preliminary oxidation of the coal–oil pitch for 1 hour, followed by joint oxidation with medium-temperature coal-tar pitch, produced no substantial change.

The results for joint oxidation and the analyses of the pitches obtained are summarized in Table 5.

For comparison, the same table gives the results for oxidation of soft pitch itself, as well as analyses of high-temperature pitch obtained under production conditions.

Table 4

Oxidation of coal pitches

Table 5

Oxidation of coal-oil pitches in a mixture with coal-tar pitches

Continuation of Table 3

Reducing the air feed rate to 0.2–0.3 l/min considerably lengthens the oxidation process, but the yield of the oxidized product increases.

From the data in Table 4 it should be noted that the pitches obtained by oxidation of the coal–oil pitches themselves have an increased ash content. Oxidizing them in a mixture with pitches from coal-tar made it possible to lower somewhat the ash content of the high-temperature pitch, although the latter remained rather high. Under production conditions, the proportion of coal–oil pitch in the mixture will be significantly smaller (even with complete utilization of pitch distillates for dissolving coal).

This will accordingly reduce the influence of the ash introduced by the coal–oil pitch on the overall ash content both of the high-temperature pitch and of the coke obtained from it. In this case, according to calculation, the average ash content of the pitch coke will not exceed 0.3%, which is the limit for first grade.

In addition, it is known from the literature [1, 3], and is also confirmed by the present study, that the ash of coal–oil pitch contains harmful constituent parts (iron and silicon oxides) in a total amount significantly smaller (in our studies 52.1–56.0%) than the ash of pitch from coal-tar (79.4%) [3].

Thus, the use of coal–oil pitch as a component of the raw material for firing electrode coke is expedient; moreover, under the conditions of the Kemerovo Coke-Chemical Plant this will give a significant economic effect.

Conclusions

1. In the oxidation of pitch distillates, anthracene fraction, and raw anthracene in order to obtain high-temperature pitch, the yield of the pitch obtained is small (48.3–74.6%), while the yield of the final target product—coke—is very low. The oxidation process is distinguished by considerable duration.

2. Oxidation of pitch distillates in a mixture with soft coal-tar pitch leads to a significant increase in the yield of high-temperature pitch and of the coke obtained from it.

3. In obtaining pitch substitute by dissolving Kuznetsk coals of various grades in various solvents, the best results from the standpoint of pitch yield and its ash content were obtained when dissolving coal 2Zh26, mine Kapitalnaya-1, in pitch distillates.

4. Increasing the concentration of the coal being dissolved above 20% in order to raise the softening temperature of the coal–oil pitch leads to a sharp increase in its ash content.

5. It is expedient to obtain coal–oil pitches with an initial coal concentration of up to 20%, followed by their oxidation in a mixture with coal-tar pitch to a high-temperature pitch.

6. Independent oxidation of coal–oil pitches promotes the production of a viscous and poorly transportable product; moreover, according to analytical data, the oxidative condensation process in this case proceeds only to a shallow extent.

Literature

1. V. N. Novikov, B. S. Gurevich, E. I. Yakhnis. Development of ways of using and methods for obtaining chemical coking products with the aim of expanding production of consumer goods. Report of VUKhIN, topic 17, section 111, Sverdlovsk, 1954.

2. M. A. Stepanenko, T. Ya. Gogoleva. On the question of the use of coal–oil pitch. Coke and Chemistry, 12, 1963.

3. B. S. Filippov, G. P. Gorovoi. Interaction of tars with coals of the Kuznetsk Basin. Coke and Chemistry, 8, 1957.

4. M. A. Stepanenko, Ya. A. Bron, N. K. Kulakov. Production of pitch coke. Metallurgizdat, 1961.