Coal Tar Pitch
Coal Tar Pitch
Coal tar pitch, derived from by-product coke ovens, is the preferred material for use as a binder in the manufacture of carbon and graphite electrodes. Coal tar pitch is a coal conversion product. Its IUPAC name is ‘Coal Tar Pitch, High temperature”. Its CAS number is 65996-32-2 and EINECS number is 266-028-2. It is the shiny, dark-brown to black residue produced by distillation of coal tar.
Coal tar pitch contains a large number of substances. It is solid at room temperature and consists of a complex mixture of numerous polycyclic aromatic hydrocarbons (PAHs), their methyl and polymethyl derivatives, and heterocyclics, and shows a broad softening range instead of a defined melting temperature. The hydrogen (H2) aromaticity of coal tar pitch (ratio of aromatic to total content of H2 atoms) varies from 0.7 to 0.9.
Production of coal tar pitch
Coal tar pitch is currently produced from coal tar, which is a byproduct of high temperature coking of coal in the manufacture of metallurgical coke. The tar predominantly contains a mixture of bi- and poly-condensed aromatic hydrocarbons and also compounds with heteroatoms in rings (predominantly nitrogen bases from the quinoline and acridine series, 1 % to 2 %) and phenols (1 % to 2 %). Upon distillation, 8 % to 12 % of a naphthalene fraction, 5 % to 9 % of an absorption fraction, and 21 % to 26 % of an anthracene fraction, which boiled away to 360 deg ?, are separated. The residual part of the tar is the pitch which contains nonvolatile and low volatile substances, whose average yield is around 2 % of the coking coal charge used for high temperature carbonizing.
The industrial production of coal tar pitch consists of the fractional distillation of the coal tar at a temperature around 400 deg C (Fig 1). By this method, the coal tar yields a series of liquid fractions suitable for different industrial applications and a residue which is solid at room temperature, called coal-tar pitch. Two different types of coal tar pitches are usually produced namely (i) binder grade, and (ii) impregnating grade. The main difference between these pitches resides in the quinoline insoluble content (much lower in the impregnating grade coal tar pitch) and in the softening point (around 110 deg C, for binder grade and around 90 deg C, for impregnating grade).
The heaviest coal tar distillation fraction, which distils between around 270 deg C to 400 deg C, is anthracene oil. This fraction, composed of 3 to 5 ring aromatic ring polycyclic aromatic compounds (PACs), has eluded all attempts at polymerization by conventional thermal treatments at atmospheric pressure. Its transformation into a pitch requires the use of specialized forms of treatment that allow the polymerization of low-molecular weight PACs to take place. Industrial Química del Nalón, S.A. in collaboration with INCAR-CSIC has developed an industrial process to transform anthracene oil into pitches. This process involves thermal oxidative condensation and subsequent thermal treatment and distillation until the pitch reaches the desired softening point (Fig 1).
This type of pitch is produced with the specific aim of fulfilling the requirements for binder and impregnations grades, mainly for carbon (C) anodes and graphite electrodes. For this reason, their use in other fields, such as precursors for advanced C materials, normally requires a pre-treatment of the commercial pitches in order to adapt their composition and characteristics for their further utilization.
For example, thermal treatment in an inert atmosphere and air-blowing are the processes generally used to reduce the emission of volatiles during pitch processing and also to increase pitch C value, without altering most of the fundamental characteristics of the pitch (e.g., wetting capacity, fluidity, etc.). Basically, thermal treatment can be considered as an interrupted carbonization (350 deg C – 450 deg C) which involves distillation, polymerization and even the formation of mesophase. The result is pitch which is able to generate carbons with a lower porosity, higher density and pre-graphitic order. All these improvements make thermally treated excellent precursors for matrices of different types of composites, C fibres, self-sintering graphites, etc. On the other hand, air-blowing has similar effects to thermal treatment but at lower temperatures (less than 350 deg C). This is since oxygen (O2) promotes the formation of free radicals which favour polymerization reactions. In this case, polymerization occurs via the formation of planar macromolecules and the formation of cross-linked structures. The latter prevents mesophase development during air-blowing.
Fig 1 Schematics of the production of coal-tar pitch and anthracene oil-based pitch
Properties of coal tar pitch
The aromatic hydrocarbons in coal tar pitch include acenaphthene, fluorene, 2-methylfluorene, 1-methylfluorene, phenanthrene, anthracene, cyclopenta[def]phenanthrene, fluoranthene, acephenanthrylene, pyrene, benzo(a)fluorene, benzo(b)fluorene, benz(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo[e]pyrene, benzo(a)pyrene, perylene, dibenz(a,h)anthracene, indeno(1,2,3-cd)pyrene, benzo(ghi)perylene, and anthantrene. Tar bases/N2 containing heterocycles in coal tar pitch include acridine, and carbazole. The sulphur (S) containing heterocycles in coal tar pitch include dibenzothiophene. The O2 containing heterocycles/furans in coal tar pitch includes dibenzofuran.
At room temperature, pitch appears as a uniform solid, which mainly consists of a mixture of PAHs with four or more aromatic rings. An important qualitative characteristic of pitch is its group composition evaluated based on solubility in different organic solvents namely (i) the concentrations of substances soluble in petroleum ether (gamma fraction), soluble in toluene but insoluble in petroleum ether (beta fraction), and insoluble in toluene (alpha fraction). In turn, the alpha fraction is subdivided into the fractions soluble in quinoline (alpha-2 fraction) and insoluble in quinoline (alpha-1 fraction). The group composition is primarily responsible for the technological properties of pitch such as softening temperature, dynamic viscosity, fluidity at a specific temperature, coking property, and the yield of coke residue.
Coal tar pitch is non-explosive and non-oxidizing in nature. It has affinity and binding capacities to other C products (e.g. petroleum coke). It typical physical properties are given in Tab 1.
|Tab 1 Typical physical properties of coal tar pitch|
|Physical state||Black solid||At room temperature|
|Melting point||deg C||65-150||Softening range|
|Boiling point||deg C||Greater than 360||at 1 kg/sq cm|
|Density||grams/cc||1.15-1.40||at 20 deg C|
|Vapour pressure||mm wc||less than 1||at 20 deg C|
|mm wc||less than 100||at 200 deg C|
|Water solubility||mg/l||Around 40||at 22 deg C|
|Flash point||deg C||Greater than 250|
|Auto flammability||deg C||Greater than 450|
Variations in the type of coke oven and coke oven practice and the diversity of sources of supply give rise to variations in pitch quality which affect electrode performance and result, in the case of some pitch binders, in poor performance.
Uses of coal tar pitch
Coal tar pitch and binding agents obtained from it are the most important components in the production of various C materials. The favourable combination of high coking ability and low viscosity in a molten state is mainly responsible for the high level of the physico-mechanical properties of anode pastes obtained on their basis, electrodes for steel furnaces, low ash pitch coke, fireproof materials, tapping hole mixes, C fibers, coal graphite and construction materials for different purposes, electrical products, and roofing and other C materials, which are used in different branches of industry such as in ferrous and non-ferrous metallurgy, electrode industry, the production of semiconductor materials and integrated micro-circuits, chemical apparatus and machine building, electrochemistry, atomic power engineering, aircraft building, and rocket production
Solid coal tar pitch is extruded into small rods and, for this reason, is often referred to as ‘pencil pitch’. In the aluminum industry, coal tar pitch is used as a binder in the manufacture of electrodes which primarily consist of petroleum or anthracite coal. As these electrodes are heated at high temperatures over a period of weeks, the ingredients in the electrodes are converted to C. For this reason, coal tar pitch is also often referred to as ‘C pitch’ or ‘binder pitch’. Additional synonyms for coal tar pitch include ‘roofing pitch’ because of its use in the roofing industry and ‘target pitch’ because of its use in the manufacture of clay targets.
Coal-tar pitch is used in the manufacture of graphite electrodes for steel arc furnaces. It is also used to impregnate and strengthen refractory brick (for lining industrial furnaces), and in surface coatings, such as pipe-coating enamels and black varnishes used as protective coatings for industrial steelwork and as antifouling paints for boats. It is also used as a component in the manufacture of activated C, C refractory for blast furnace lining and tap-hole clays. Hard pitch is used as a binder for foundry cores. It is also used to produce pitch coke, which is used as the C component of electrodes, carbon brushes, and carbon and graphite articles.
Coal tar pitch is an industrial product which is typically dispatched in bulk to customers as either a liquid or a solid. It is often supplied to customers via rail in either tanker cars (liquid pitch) or hopper cars (solid pitch).
Environmental concerns related to coal tar pitch
Coal tar pitch is an industrial product and is not sold directly to consumers or for direct consumer use. Exposure to coal tar pitch is primarily limited to occupational settings, such as tar distillation, roofing and aluminum smelting. In these settings, occupational exposure to liquid, solid, heated or vaporous coal tar pitch is controlled by the use of enclosed processing systems, industrial hygiene controls and personal protective equipment. Each industrial facility is also to have a thorough training program for employees and appropriate work processes, as well as safety equipment in place to limit exposure. Workers are to follow the recommended safety measures in the relevant Safety Data Sheet (SDS).
Long-term toxicity to fish is expected to be minimal. It is also not expected to be toxic at the limit of water solubility.
Coal tar pitch is immobile and does not appreciably leach to groundwater. It is also very poorly soluble in water. Coal tar pitch that is released or disposed in the environment is likely to remain unchanged for many years. When coal tar pitch is seen in soil, it is usually present as distinct pieces or chunks of black, hard material, which is not likely to be contacted in the same way as is soil.
Acute, or short-term, exposure to coal tar pitch has been associated with several health effects. For instance, coal tar pitch is a phototoxic substance. This means that dermal contact with coal tar pitch in the presence of ultraviolet light (sunlight) can result in a skin reaction similar to exaggerated sunburn, as well as blisters. Short-term exposure in the absence of sunlight may also irritate the skin and cause other skin conditions such as dermatitis and acne.
The permissible 8-hour TWA exposure level is 0.2 mg/cum for the volatiles which are emitted from coal tar pitch (coal tar pitch volatiles). This number represents the average airborne exposure in any 8-hour work shift of a 40-hour work week which is not to be exceeded. It is also the highest level of exposure an employee may be exposed to without incurring the risk of adverse health effects.
Additional health effects associated with short-term exposure to coal tar pitch include eye and respiratory tract irritation, severe burns from exposure to hot liquid coal tar pitch, and respiratory difficulty, convulsions, and possible cardiovascular collapse from exposure to airborne concentrations of coal tar pitch mist or vapour significantly beyond the 8-hour time-weighted-average (TWA) workplace exposure limit of 0.2 mg/cum.
Ventilation and automation are the best method for controlling exposure to coal tar pitch volatiles. Respiratory protection to limit inhalation exposure and protective clothing to prevent dermal exposure to coal tar pitch and coal tar pitch volatiles are also recommended, although maintaining exposures below the TWA level is preferable.