On the Methodology for Extracting Bitumen from Brown Coal

K. K. STRAMKOVSKAYA

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

In the overall balance of reserves of fossil solid fuels, brown coals occupy a significant place. Alongside their use as an energy fuel, they find broad application as a chemical raw material—namely, for obtaining liquid fuel by means of low-temperature carbonization and hydrogenation, and also for gasification. The most bituminous of them are used to obtain montan wax, which is a highly valuable technical product used in a whole range of industrial processes. It is used in the electrical-engineering industry for insulating purposes; in printing, for the manufacture of duplicating papers and typewriter ribbons; in the leather-and-footwear industry, for the production of creams and dressings; in the paint-and-varnish industry; and also in obtaining heavy lubricants for rolling mills, foundry binders for precision casting, and in many other cases.

At present it is believed that the bitumen of brown coal is distributed uniformly throughout its mass, although exceptions are sometimes encountered. Some deposits of brown coals contain entire interlayers of bituminous coals [2].

The yield and, to some extent, the properties of montan wax depend on the origin of the coal, the nature of the solvent, and the conditions of the extraction process [1].

The aim of the present work was to clarify the behavior of the bituminous portion of brown coals when they are treated with weak aqueous solutions of alkalis. Yarsk brown coal from the Tomsk district was investigated; its characteristics were described by us in works [3, 4]. As reagents, one-, two-, and three-percent aqueous solutions of caustic soda, caustic potash, and soda were used.

When a coal sample is boiled with an aqueous alkali solution (at a ratio of approximately 1:10) in an open vessel, a solid film forms on the surface of the liquid. If this film is removed, it forms again and again. Thus, after a certain boiling time the coal can be separated into three products: 1) the film collected on the surface of the liquid; 2) humates soluble in the alkaline solution; 3) residual coal (coal particles insoluble under these conditions). The yield and characteristics of the products are given in Table 1 [5].

It follows from the data in the table that film formation proceeds better in solutions of caustic alkalis: NaOH and KOH, at a two-percent concentration of the solution. As can be seen from the table, in elemental composition

the products obtained differed little. The results of extraction of these products with various solvents are given in Table 2.

The data obtained indicate that all the tested solvents, when extracting the film, gave a higher yield of bitumen. Moreover, the bitumens extracted with the same solvent both from the original coal and from the film have the same elemental composition, similar acid numbers, and the same amount of waxy fraction. Bitumen with a higher content of waxy fraction and with higher dielectric properties is obtained when dichloroethane is used as the solvent.

Table 1

For the purpose of determining bitumens A and C, the film, the humates, and the residual coal were treated with hydrochloric acid and extracted. The yields of extract are given in Table 3.

The data of this table convincingly show that the film was indeed enriched in bitumen, since the residual coal yielded only 5.8% bitumen, whereas 20% was extracted from the film. In this case, alcohol-benzene gave a higher yield of extract, extracting humic acids together with the bitumen. In this case, 73.6% of the alcohol-benzene extract dissolved in a one-percent alkali solution. This confirms the statements of V. S. Veselovskii [6] that bituminous substances can not only themselves pass into solution, but also cause the dissolution of non-bituminous substances by being adsorbed on the surface of their particles and imparting to them induced solubility, or, in other words, serving as a protective colloid.

From the residual coal, approximately the same amount of extract was extracted with alcohol-benzene as with dichloroethane, since humic acids were already absent from the residual coal. The film, enriched with bitumen

Table 2

is also formed in the case when the coal with the alkaline solution is first boiled in a closed vessel, then allowed to settle and, after separating the residual coal, boiled in an open vessel.

The process by which the bituminous part of the coal passes into solution together with the humates soluble in the alkaline solution evidently consists in the fact that, when brown coal is boiled with alkali, the humic acids present in such coals in large quantity pass into solution in the form of humates, carrying with them the bituminous part of the coal adsorbed on their large internal surface. Such an aggregate, in which bitumen is present, evidently becomes lighter and floats to the surface of the liquid, where, as a result of cooling, it solidifies, forming a film.

Table 3

Apparently, under suitable natural conditions, such a route of enrichment of the humus fraction with bitumen at the early stages of coal formation is also not excluded. Thus, for example, Uitman and Strache [1] considered that, in the distribution of bitumen in the humus fraction of coal, its stratification according to specific gravity took place.

The technological properties of the enriched product as a raw material for extracting montan wax are characterized not only by its increased bitumen content. The enriched product dries much more rapidly to the air-dry state and has a considerably higher bulk density. Thus, when ground through a sieve of 400 openings/cm², the enriched product had a bulk density of 0.75 g/cm³, whereas for the original coal of the same fineness and the same moisture content it was equal to 0.36 g/cm³. In addition, the pre-extracted product retains a significantly smaller amount of solvent. Consequently, for extracting montan wax from brown coals they should first be treated with an alkaline solution and a product enriched with bitumen should be obtained.

Conclusions

1. When brown coal is boiled with a two-percent alkali solution in an open vessel, a product enriched with bitumen collects on the surface of the liquid; as a raw material for extracting montan wax, this product has higher technological indices than the original coal.

2. It has been shown that mixed solvents, along with the bituminous fraction, also extract humic acids from the enriched product, and also that the purest bitumen, with a larger amount of the wax fraction and with higher dielectric indices, is extracted by the solvent dichloroethane.

Literature

1. Chemistry of Solid Fuel. Collection 1. Foreign Literature Publishing House, 164, 163, 367, 1951.

1. Stadnikov, G. L. The Origin of Coals and Petroleum. 3rd ed. Publishing House of the Academy of Sciences of the USSR, 1937.

2. Gebler, I. V., and Stramkovsky, V. F. Izv. TPI, 64, 275, 1946.

3. Gebler, I. V., and Stramkovskaya, K. K. Izv. TPI, 64, 291, 1945.

4. Stramkovskaya, K. K. Candidate’s dissertation, Tomsk, 1947.

5. Veselovsky, V. S. The Chemical Nature of Combustible Minerals. Publishing House of the Academy of Sciences of the USSR, 158, 1955.