Thermal Reactivity and Structure of Carbonized Binder Pitches
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Pitches are used on a large scale in the manufacture of carbon anodes for the production of primary aluminium. The role of the pitch is to act as a binder between the petroleum coke grains. The structure of the carbonized pitch binder (pitch coke) has an important impact on the overall performance of the anode. Even though the binder pitch is the minor constituent in an anode, it is impossible to make a good quality anode without a good quality binder pitch. Pitch is an extremely complex mixture of numerous, essentially aromatic and heterocyclic compounds derived from pyrolysis of organic material or tar distillation. Upon heat treatment pitches form cokes in relatively high yields. Physical and chemical properties of the anode such as mechanical strength, electrical resistivity, thermal conductivity and resistance towards oxidation by air and CO2 are dependent on the structure of the aggregate material as well as the carbonized binder pitch. The properties of the pitch coke is in turn mainly dependent on the chemical characteristics of the parent pitch. Coal-tar pitch is the preferred choice of binder material in anode manufacture today. However, the availability of high quality coal-tar is in decline and at least partial replacement by alternative binder sources will become increasingly important in the future. Due to environmental regulations, petroleum pitches are interesting as they generally have lower PAH emissions than coal-tar pitches during baking. Blends of coal-tar pitches and petroleum pitches are in use today on an industrial scale. The aluminium industry must be prepared to meet the challenges involved in adapting binder pitches from new sources which may be of inferior quality to the pitches available on the market today. An increased understanding of the processes involved in the transformation of a pitch into a coke and the link between raw material composition and properties and the final artifact is thus highly relevant. Traditionally, the suitability of a binder pitch for use in anodes, has been defined from parameters like softening point, insolubility in toluene (TI) and quinoline (QI), coke yield, H/C atomic ratio, ash content and density. Although these parameters, which are mostly empirical in nature, give an indication of the pitch quality, more information on the chemical characteristics and carbonization behavior of pitches is certainly valuable. The present work aims to describe and explain the link between “classical” pitch properties, hydrogen transfer properties, information derived from NMR spectroscopy and the structure of the carbonized binder pitch. Coal-tar and petroleum pitches pass through a fluid stage during carbonization. In the early stages of carbonization, free radicals are formed due to thermal rupture of C-C and C-H bonds in reactive components. Polymerization occurs mainly via a free radical mechanism leading to molecular size enlargement (aromatic growth)and the formation of oligomeric systems (mesogens). If the intermolecular reactivity of the pitch constituents is too high, extensive cross-linking and a rapid transformation of pitch molecules through polymerization will occur at a relatively low temperature. In this case, either mesophase will not be formed or the growth and coalescence of mesophase will take place under low fluidity/high viscosity conditions leading to a premature solidification of the pyrolysis system. An isotropic coke or a pitch coke of small optical domains will then be formed. On the other hand, if the pitch has a low thermal reactivity, aromatic growth is constrained and the mesogens will have sufficient mobility to stack parallel to each other and establish a liquid crystal system (mesophase). The growth and coalescence of mesophase take place at a higher temperature where the viscosity of the pyrolysis system is at a low level. Eventually, the system will solidify and an anisotropic coke of large well-developed optical domains is formed. In particular, the presence of alkyl side chains and oxygenated functional groups are considered to lead to an increased thermal reactivity. If free radicals formed by thermal rupture of bonds in reactive pitch species can be stabilized by hydrogen transfer from within the system, extensive cross-linking at a too early stage is prevented. The initiation, growth and coalescence of mesophase are facilitated and consequently a coke of large well-developed optical domains is formed. Hydroaromatic rings and naphthenic rings in hydroaromatic species are considered to be principal hydrogen donor groups. Oxygen acceptor sites are believed to deplete the supply of donatable hydrogen and leave radicals free to recombine. The thermal reactivity of a pitch is thus dependent on both the amount of reactive species and the ability of the pitch to stabilize free radicals by hydrogen transfer. In the present work, the subject of study was five coal-tar pitches and four petroleum pitches. In addition, a QI-free coal-tar pitch supplied by GrafTech International was studied. The pitches were characterized by 1H NMR and 13C NMR spectroscopy, hydrogen transfer properties, elemental analysis and the release of volatiles during carbonization. In addition, the pitches were characterized by more “traditional” pitch parameters like insolubility in quinoline (QI), insolubility in toluene (TI), softening point and coking value. The structure of the carbonized pitches was examined by optical microscopy and X-ray diffraction. The hydrogen transfer properties of the pitches were evaluated from their ability to donate hydrogen to an acceptor compound, anthracene, or abstract hydrogen from a donor compound, 1,2,3,4-tetrahydronaphthalene (tetralin). A mixture of pitch and anthracene or tetralin was heat treated in sealed glass tubes filled with argon gas at 400 ºC. Two different heat treatment procedures were tested. In the first, the sample was kept at 400 ºC for 8 hours while in the second, the sample was heated at a rate of 5 ºC/min to 400 ºC with no soaking time. The major hydrogenated products from the reaction between anthracene and pitch were 9,10-dihydroanthracene (DHA) and 1,2,3,4-tetrahydroanthracene (THA). After the reaction, the semi-coke residue was dissolved in carbon disulphide and analyzed by gas chromatography. The hydrogen donor ability (HDa) was calculated from the amounts of DHA and THA formed and expressed as milligrams of hydrogen transferred to anthracene per gram of pitch. For the hydrogen donor ability test, the less severe heat treatment (5ºC/min to 400 ºC, no soaking time) was found to be the most appropriate. The reaction between tetralin and pitch gave one major dehydrogenated product, naphthalene. The hydrogen acceptor ability (HAa) was calculated from the ratio of naphthalene to tetralin as determined by gas chromatography and expressed as milligrams of hydrogen transferred per gram of pitch. For the acceptor ability test, the heat treatment at 400 ºC with 8 hours soaking time was found to be the most appropriate. The release of volatiles during carbonization was studied by thermogravimetric analysis. The amount of volatiles released between 300 and 500 ºC (VM300-500)relative to the total amount of volatiles released at 1000 ºC was selected as a parameter reflecting the thermal behavior of pitches during the critical stages of carbonization. Carbonization of pitches was performed under inert gas pressure (15 bar) and the green cokes obtained at 550 ºC were studied by optical microscopy. Computerized image analysis was performed to quantify the optical texture. The output parameters from the image analysis were the mosaic index, which is a measure of the optical domain size, and the fiber index, which is a measure of the parallel alignment of optical domains. The green cokes were further heat treated to 1150 ºC and the microstructure of the resulting calcined pitch cokes was characterized by X-ray diffraction. The carbon disulphide soluble part of the pitches was investigated by 1H NMR and 13C NMR spectroscopy. Results from elemental analysis of the pitches were used in conjunction with the results obtained from the NMR spectroscopy. The main objective of the NMR analysis was to identify and quantify structures in the pitch which are considered either to increase or decrease the thermal reactivity. The coal-tar pitches were as expected found to be more aromatic than the pitches of petroleum origin. A relationship was found between the aromaticity of the pitches and the H/C atomic ratio as determined from elemental analysis. Elemental analysis is a rapid and convenient method to estimate the aromaticity of pitches. Due to a more hydroaromatic structure, the petroleum pitches were in general found to have a higher estimated concentration of donatable hydrogen which will suppress intermolecular reactivity. However, the petroleum pitches also had a high concentration of alkyl side chains which are generally believed to give increased thermal reactivity. Carbon connected to oxygen could not be distinguished in the NMR spectra. Pitch constituents containing heteroatoms are generally concentrated in the heavier pitch fractions which may not be soluble in carbon disulphide. This could be an explanation for the failure in the detection of aromatic carbon connected to heteroatoms. However,the oxygen content was determined by elemental analysis. The pitches could be distinguished due to their ability to donate hydrogen to anthracene or abstract hydrogen from tetralin. The hydrogen donor ability was not found to correlate with the concentration of donatable hydrogen (NMR) which might have been expected. A likely explanation for this apparent inconsistency is that potential donatable hydrogen in reactive pitches will be preferentially consumed by free radicals and oxygenated acceptor sites instead of being transferred to anthracene. A correlation between the hydrogen donor (HDa) and acceptor ability (HAa) was not found. This indicates that the two parameters represent two separate properties where both are linked to the thermal reactivity of the pitch. The ratio between the hydrogen donor and acceptor ability, HDa/HAa, was used as a parameter reflecting the thermal reactivity of pitches. Pitches which exhibit a high HDa/HAa ratio (low thermal reactivity) are expected to form an anisotropic coke of large optical domains. On the other hand, pitches with a relatively low HDa/HAa ratio are expected to have a high thermal reactivity and form a more isotropic (small optical domains) coke. Despite the higher concentration of donatable hydrogen, the petroleum pitches were not generally considered to have a lower thermal reactivity than the coal-tar pitches expressed by the HDa/HAa ratio. The processes taking place during thermal treatment of pitches are reflected in the release of volatiles. A correlation was observed between the HDa/HAa ratio and the relative amount of volatiles released between 300 and 500 ºC (VM300-500). Thermally reactive pitches exhibiting a low HDa/HAa ratio will have a high activity at low temperatures and release low boiling point molecules and fragmentation species. If on the other hand the pitch has a low thermal reactivity, fragmentation species will be stabilized by hydrogen transfer and retained in the pyrolysis system. The resulting thermally stable molecules of relatively low molecular weight may then act as solvating vehicles maintaining a low viscosity in the system and may also be important as hydrogen shuttling agents. When the system has reached a critical stage for mesophase growth and coalescence, these smaller thermally stable molecules (non-mesogens) are eventually released at higher temperatures. The petroleum pitches developed cokes of relatively large optical domains (coarse mosaic). A correlation was observed between the HDa/HAa ratio and the mosaic index (size of optical texture) for the petroleum pitches. As expected, a high thermal reactivity (low HDa/HAa ratio) resulted in a pitch coke of small optical domains (high mosaic index). The HDa/HAa ratio was, however, not successful in predicting the size of optical texture in the cokes obtained from the coal-tar pitches. This was mainly due to the influence of QI material on the pitch coke structure. It is recognized that particulate matter (primary QI material) hinders the growth and coalescence of mesophase. This was found for the coal-tar pitches. Scanning electron (SEM) and polarized light microscopy images taken at a high magnification revealed how the QI particles were arranged and clustered around smaller anisotropic domains. The detrimental effect of QI material on the development of anisotropic texture in the resulting coke was demonstrated by comparing the structure of the coke obtained from a QI-free coal-tar pitch and a coal-tar pitch containing QI. The QI-free pitch developed a coke of large optical domains whereas the coke obtained from the pitch containing QI material had mainly a fine mosaic texture (small optical domains). However, some large anisotropic domains were present in between the QI clusters. It is also not to be excluded that the QI fraction not only acts physically by obstructing the growth and coalescence of mesophase but may also be chemically active. Findings indicate that the oxygen is concentrated in the QI fraction. Solid QI particles with oxygenated functional groups or heteroatomic structures containing oxygen, which due to their large size are insoluble in quinoline, may act as acceptor sites for hydrogen thus increasing the thermal reactivity. The average coherent stacks of the calcined (1150 ºC) pitch cokes was found to consist of between 7 and 8 graphene layers (Lc divided by d002). The average crystallite size (Lc) was fairly similar for all the calcined pitch cokes but significant differences were found. The coal-tar pitches generally developed cokes of slightly higher average crystallite sizes than the pitches of petroleum origin. The microstructure of the coal-tar pitch cokes is probably influenced by the amount and nature of the QI fraction. For the petroleum pitches there was a tendency that a high average crystallite size was connected to a more well-developed structure (larger domains) at the green coke stage. The evaluation of hydrogen donor and acceptor abilities provides a rapid and relatively simple method to differentiate pitches which can be linked to the development of structure during carbonization. These properties thus reflect the thermal reactivity of pitches and can be connected to the release of volatiles during pyrolysis. However, for coal-tar pitches the QI content was found to be the most influential factor on the development of optical texture and must be considered in addition to the hydrogen transfer properties. Considerations on thermal reactivity from NMR spectroscopy and elemental analysis were found to generally support the results from the hydrogen donor and acceptor ability tests.