Bitumens

originally only natural products, mainly petroleum and its natural derivatives (most frequently asphalt); they later came to include artificial products similar to asphalt and obtained by processing natural bitumens, petroleum distillation residues, and coal and shale tars—the industrial bitumens. In addition, bitumens have come to include products extracted by organic solvents from peat, lignite, and so on—the solid-fuel bitumens. Extracts derived from sediments and sedimentary rock are called bituminoids.

Man used bitumens even in ancient times. Structures in the Tigris-Euphrates Valley and in Egypt are known to have been built with the use of bitumens as early as 3000 B.C. Bitumens were used as surfacing material for reservoirs and granaries and to cement the walls and floors of palaces and temples; they were used to waterproof a tunnel built under the Euphrates River about 1000 B.C.

Natural bitumens. Natural bitumens are divided into naphthides and naphthoids. Naphthides are very widespread in nature and include petroleum and all its natural derivatives, such as malthas, asphalts, asphaltites, kerites, and ozokerites. Malthas are viscous thickened petroleums that have been weathered. They consist of oils (40–65 percent) and asphalt-tar components (not less than 35 percent). Further changes in malthas under the effect of the same factor lead to the formation of several more substances: solid but easily meltable asphalts, containing 60–75 percent asphalt-tar substances; asphaltites, which are solid but highly fusible products completely soluble in chloroform and carbon disulfide and which contain 60–75 percent asphalt-tar substances; and kerites, which include substances only partially or very weakly soluble in organic solvents. All natural gases based on hydrocarbons are sometimes counted as bitumens (the gaslike naphthides).

Naphthoids are naphthidelike products of the natural sublimation of an organic substance under the influence of magmatic heat. They are considerably rarer than naphthides, as yet poorly studied, and have no industrial importance.

Industrial bitumens. Industrial bitumens are products that usually have a hard or viscous consistency and are obtained chiefly from heavy petroleum residues rich in asphalt-tar substances. The following methods are used: deep-vacuum distillation of heavy petroleum residues (mazuts, soft asphalts, and so on) at a temperature of 300°-350° C (residual bitumens); oxidation by atmospheric oxygen of heavy residues of petroleum distillation (soft asphalts and others) at a temperature of 260°-280° C (oxidized bitumens); and mixing oxidized bitumens with nonoxidized petroleum products (compounded bitumens).

The main components of petroleum bitumens are asphaltenes, tars, and petroleum oils. The asphaltenes determine hardness, the tars determine cementation and elasticity, and oils serve as a liquefying medium for the tars and asphaltenes. The most important technical indexes of bitumens are ductility, or the ability to produce a thread of a given length upon stretching (determined by thickness of the thread formed by stretching a standard-size piece of bitumen to the breaking point); penetration, which indicates the viscosity of the bitumen (determined by the depth of penetration of a steel needle into a layer of bitumen at a pressure of 1 newton [0.1 kg-force] in 5 sec); softening temperature; flash point; and density. Solid, semisolid, and liquid bitumens are produced industrially (see Table 1).

Liquid bitumens (depending on grade) are characterized by the following data: penetration of a residue (after elimination of fractions up to 360° C), 10–30 mm at 25° C; viscosity measured by discharge through an opening 5 mm in diameter, 5–200 sec (at 60° C); flash point 65°-120° C; and not more than 10 percent distillation up to 225° C or not more than 50 percent up to 360° C.

Solid petroleum bitumens are used to produce roll roofing materials (roofing felts) and bituminous mastics; semisolid bitumens are used in waterproofing materials (such as gidroizol, borulin, and bituminized cloths) and coatings, asphalt solutions for construction, asphalts, bituminous plastics, and so on. Liquid bitumens are used in road construction.

The products of thermal processing of solid fuels—high-boiling tar fractions (obtained from coking and low-temperature carbonization of coal), coal tar pitch, and products of the dry distillation of coal, combustible shale, and so on—may serve as substituents of petroleum bitumens.

More than 90 percent of all the bitumen used in the different branches of industry is artificial and is obtained from petroleum. World production of artificial bitumens amounts to tens of millions of tons.

Solid-fuel bitumens. Solid-fuel bitumens are extracted from peat and lignite by means of organic solvents. Their chief components are waxes and resins. The waxes are the most valuable products extracted from solid-fuel bitumens; they are characterized by high melting points, low electrical conductivity, high moisture resistance, and the ability to raise the melting points of other substances when added to them in small amounts. The bitumen yield of bituminous peat is 10–15 percent with extraction by benzene and 6–9 percent with extraction by solvent naphtha. In bituminous lignites (such as the coals of Aleksandriia, Ukrainian SSR) as much as 14–16 percent bitumen is extracted by benzene. The yield of solid-fuel bitumens under conditions of production is usually 80–85 percent of their analyzed content in the fuel. The minimum bitumen content of the solid fuel required to make processing profitable is 8–10 percent.

The process of obtaining solid-fuel bitumens consists in the preparation of the fuel for extraction, extraction by organic solvents, elimination of most of the solvent from the extract, liberation of the bitumens from residual solvent and water, and melting and pouring the bitumens into forms. Bitumens are extracted from solid fuels primarily in banks of synchronized extractors.

Solid-fuel bitumens are applied in various sectors of the national economy, including foundry work (as one of the components of the molds used in precision casting—lost-wax casting), the electrical-engineering industry (as insulating material), and for the finishing of various products made from leather, wood, paper, and so on.