SEM study of binder coke in electrode carbon

As part of a study on the use of coal-tar pitches as binders in electrode carbons for the aluminium industry, the object of the present work was to attempt to identify textural components in electrode binder cokes. Accordingly, four experimental carbons, made using the same petroleum coke as filler and four coal-tar pitches of differing characteristics as binders, were examined in a scanning electron microscope (SEM) after embedding in resin, polishing and etching in atomic oxygen. Binder cokes could be distinguished from the filler particles and could be characterized in terms of the content of three components, termed lamellar, intermediate and granular. The textural composition of the binder cokes was dependent upon the character of the pitch and the type of quinoline-insoluble components present. Pitch cokes, made by carbonizing the pitches alone, differed markedly from the corresponding binder cokes. The findings are considered in terms of current ideas on pitch carbonization.     

Influence of pitch composition and surface properties of petroleum coke on their interaction during the preparation of carbon/carbon composites

The influence of pitch composition and surface properties of petroleum coke on the interaction of pitch and coke in the composite is studied. The adsorption results and EPR data indicate that recombination of paramagnetic centers and interaction of oxygen functional groups occur during the preparation of the composite. As a result of interaction between petroleum coke and pitch, polar compounds are concentrated in the adsorption layer. The baking criterion is used for the characterization of the baking ability of the pitches obtained as a result of different oxidation treatments of commercial coal tar pitch. It is determined that physico-chemical properties of the obtained pitches influence the value of the “baking criterion”. The kind of thermo-oxidation treatment of the parent pitch should be chosen hence achieving the maximum value of the baking criterion. For this purpose, the maximal part of the pitch has to take part in the adsorption layer.     

A novel method to obtain a petroleum-derived mesophase pitch suitable as carbon fibre precursor

This paper studies a novel method for the preparation of mesophase pitches suitable as carbon fibre precursors. A series of thermally treated petroleum pitches with mesophase contents ranging from 10.8 to 52.2 vol.% was obtained from pitch A-240. Separation of the phases was initially attempted by hot filtration, a suitable method for separating the phases in thermally treated coal-tar pitches. Although filtration failed for the petroleum samples, the behaviour observed led to the development of a new separation method that consists in the sedimentation of the mesophase. This method, after optimisation, yielded pitches with mesophase contents up to 97 vol.%, which were easily spun into carbon fibres of reasonable mechanical properties.     

Correlations between ability of pitch to penetrate coke and the physical characteristics of prebaked anodes for the aluminium industry

The nature of the interaction between coke and pitch is basic to the final properties of anodes for the aluminium melting industry. One of the significant properties of pitch which contribute to the quality of this binding is its ability to wet coke. The investigations reported here indicate the optimum experimental conditions for the use of an automatic apparatus that monitors pitch penetration into a coke bed. The penetration test described can be used to select pitches with good affinity for petroleum coke and to correlate this quality with the physical properties of electrodes produced therefrom. A particularly important conclusion is that the test suggests the suitability of certain pitches which would not satisfy the usual selection criteria. The temperature-dependent ability of a pitch to penetrate a coke bed, in conjunction with the traditional criteria, assists in the selection of binder pitches used in the aluminium industry.     

Capillary flow of liquid pitch

The capillary flow of liquid pitch into a granular coke bed has been theoretically investigated. An equation relating the flow rate with temperature has been derived considering the pitch as a Newtonian liquid. For a given coke granulometry, the wetting angle and the viscosity of the pitch material are the most important factors governing its flow properties. A comparison with experimental flow measurements indicates that some pitches penetrate into the granular coke bed according to the developed equation of flow.     

Carbonization and liquid-crystal (mesophase) development. 22. Micro-strength and optical textures of cokes from coal-pitch co-carbonizations

This study examines the micro-strength and optical textures of a laboratory coke from a base-blend of Freyming and Pocahontas coal (wt ratio, 1:1) and of cokes from the co-carbonization of the blend, with each of five petroleum pitches in various proportions. Coke pieces, 212–600 μm, from the micro-strength test are assessed in terms of origin and propagation of cracks induced by the test. Always, the addition of pitch to the base-blend improves the strength of the resultant cokes, the pitches behaving differently. A qualitative, subjective appraisal of results indicates that increases in coke strength are associated with relative abilities of pitches to interact with the coals to produce a fluid phase, of solution of coal in pitch, which gives an ‘intermediate’ coke with an optical texture of mozaics. This intermediate coke strengthens the bonding at interfaces. Cracks originate predominantly from the shrinkage cracks in the domains of Pocahontas coke. Mozaic structures tend to resist crack propagation. The coal/pitch system may flow around coal particles so containing incipient crack formation in resultant coke particles.     

A thermoanalytical study of the co-pyrolysis of coal-tar pitch and petroleum pitch

Four pitch blends were prepared at laboratory scale by mixing a coal-tar pitch and a petroleum pitch in several proportions (CTP:PP 85:15, 70:30, 55:45 and 40:60). Single pitches and blends were characterized by standard procedures, infrared spectroscopy and size exclusion chromatography. Pyrolysis behaviour and interactions between the two pitches in the blends were studied by thermogravimetric analysis (TG/DTG) and differential scanning calorimetry (DSC). The results show that blending does not alter the composition of pitches. However, the TG/DTG curves reveal that coal-tar pitch and petroleum pitch interact actively during pyrolysis, modifying the temperature of initial weight loss and the temperature of the maximum rate of weight loss. Primary quinoline-insoluble particles present in coal-tar pitch and transferable hydrogen seem to be the main factors responsible for these modifications. The DSC curves show that the presence of coal-tar pitch in the blends reduces the reactivity of the petroleum pitch and shifts the exothermic peaks observed at the temperature of the cracking/polymerization reactions (>400 °C) to lower temperatures.     

The effect of the substrate on pitch wetting behaviour

The wetting behaviour of a petroleum pitch (modified with a surfactant) and a binder coal-tar pitch was studied using different substrates. The results show that both the pitch and the substrate have a significant influence on wetting behaviour, and consequently, on their mutual interactions during the mixing stage. Low values of surface tension and viscosity in the pitches lead to lower wetting temperatures. It was found that with petroleum coke and magnesia as substrates, the wetting occurs at lower temperatures than with graphite and carbon black for all pitches. Moreover, experiments carried out with amorphous and crystalline alumina revealed that the crystalline order affects the wetting behaviour considerably. Thus, whereas the pitch wets the corundum (crystallized alumina), it does not wet the amorphous alumina (basic, acid and neutral alumina).     

The structure and behaviour of QI material in pitch

The QI material in coal-tar pitches is characterized by optical and scanning electron microscopy. Five main groups of QI are distinguished, primary, secondary, isotropic, foreign inclusions and material of unusual shape. Isotropic QI is considered to consist of asphaltene-like molecules dispersed in the pitch. It is found in air and acid-treated pitches and increases the coke yield compared with untreated feedstock pitches. Mesophase extracted from heat treated A240 pitch is heat treated in isolation and after mixing with fresh A240 pitch. At 375°C, mesophase partially dissolves in the fresh pitch if decacyclene is present. At 400°C, mesophase spheres heated in isolation coalesce only to a limited extent. The presence of fresh pitch is necessary for unhindered coalescence of extracted mesophase spheres and the development of optical texture of maximum size.     

Rheological characteristics of coal tar and petroleum pitches with and without additives

An extensive study of rheological characteristics of coal tar and petroleum pitches with and without additives, namely, petroleum coke, natural graphite and carbon black has been made. It is found that all pitches, pure or mixed with a carbon additive are not Newtonian as reported in the literature, but behave Theologically as Bingham plastics with certain yield stress and plastic viscosity at all temperatures of measurement between 85–180°C. The yield stress and plastic viscosity both decrease with increase in temperature of the pitch. A pure petroleum pitch having the same softening point as that of a coal tar pitch is found to have a lower viscosity compared to that of the latter at all temperatures of measurement. This suggests that the criterion of softening point as a measure of suitability of a coal tar pitch binder in the manufacture of artificial carbon is not sufficient for petroleum pitches. Addition of ten parts of carbon black by weight of pitch results in a considerable decrease in viscosity change with temperature of the coal tar pitch compared to almost insignificant change in the case of the petroleum pitch of the same softening point. However, the addition of petroleum coke or natural graphite makes the pitch more viscous but does not change the temperature dependence of viscosity of either of the two types of pitches. The implications are discussed.     

Bonding of solid additives in cokes from coals: a microscopy study

Cortonwood Silkstone (NCB class 401) and Betteshanger (NCB class 301 a/204) coals were co-carbonized with solid additives such as anthracite, coke breeze, green and calcined petroleum cokes. The resultant carbonization products (cokes) were examined by optical microscopy and SEM was used to investigate polished surfaces etched by chromic acid and fracture surfaces. For both coals only the anthracite and green petroleum coke become bonded to the coal cokes. This probably results from softening and interaction of interfaces of the anthracite and green coke with the fluid coal via a mechanism of hydrogenating solvolysis during the carbonization process. The coke breeze and calcined petroleum cokes were interlocked into the matrix of coal coke.     

The effect of additive chemicals on the viscosity of coal-petroleum coke-water slurry fuel for a gasification process

As a preliminary study for the gasification of an anthracite and petroleum coke mixture, viscosity was measured at various temperatures (20–50 °C), slurry concentrations (60–70 wt%) and additive amounts (0–0.8 wt%) by using an LV-II type viscometer. In addition, four types of different additives, sodium naphthalene sulfonate, poly(methyl methacrylate), polypropylene and a polypropylene glycol based additive, were applied to Korean anthracite, petroleum coke and mixtures of these materials, and the viscosity data were compared. Viscosity dependency values for coal, anthracite, bituminous and sub-bituminous coal, were compared, and it was found that a high content of moisture and particularly ash increases CWS viscosity. The four types of additives tested in this research can effectively diminish the viscosity of coal and especially petroleum coke-water slurry by more than 70% to 95%, respectively. Moreover, the sodium naphthalene sulfonate-based additive reduced the viscosity of coal and petroleum coke-water slurry best, especially at concentrations in excess of 65 wt%. Based on these results, highly loaded slurry created by mixing anthracite and petroleum coke with additives was achieved.     

Effects of additives on graphitisability of carbons and graphite

The effect of additives to the green mix on the subsequent graphitisability of various carbons and graphites has been studied using X-ray diffraction techniques. The carbons have been characterised in terms of the structural parameters, L_a, L_c, d₀₀₀₂ and p. It is concluded that metallic iron (or sometimes other catalysing additives) produces little enhancement of graphitisability in materials which are known to be intrinsically graphitising on the Franklin scale (for example chars from PVC, petroleum and pitch). The cooking of petroleum, prior to mixing with a binder, is shown to predetermine the form that the coke will assume as the grist in the graphitised artefact. It is suggested as a consequence that it should be possible to investigate the effect that the grist and binder components have separately on the various physical properties of a heat-treated artefact. It is shown that the addition of metallic impurities substantially enhances the degree of graphitisation and crystallinity attainable in a given heat treatment of classical non-graphitising carbons (furan coke and cellulose). By contrast, for an archetypical graphitising material (PVC char), the crystallinity and structural perfection potentially attainable on heat treatment are considerably decreased as a result of the addition of sulphur.     

Carbonization of pitch: Compatibility of components in carbonization

The carbonization of four kinds of solvent-refined-coal (SRC) pitches was investigated in order to determine the properties required for the formation of needle coke. Although the pitches were free from Ql materials, two of them gave needle cokes, whereas the other two gave mosaic cokes. The BS fractions of all pitches formed needle cokes, and all Bl fractions were infusible and isotropic. A combination of suitable BS and Bl fractions gave a needle coke, whereas another mixture formed a mosaic coke, indicating that the BS and the Bl fractions can be compatible to yield a needle coke. Cocarbonization of the BS fractions from other pitch sources with the Bl from the SRC pitches was further studied to evaluate the compatibility, which has been discussed from the structural viewpoint. Trials to improve the properties were proposed, based on the structural information.     

均四甲苯改性沥青组成与性能的关系

考察了不同压力体系中均四甲苯改性沥青的ＴＳ组分和ＰＩ组分与沥青软化点,焦化值及粘度三种性能参数之间相互关系,发现同一压力体系中所得沥青ＴＳ组分和ＰＩ组分与三种性能参数之间存在良好的线性关系;不同压力体系导致沥青化点波动较大,而焦化值变化相对较小。比较分析了三种压力体系中所得沥青的组成－性能关系。半封闭体系下的改性沥青,其ＴＳ组分和ＰＩ组分对沥青性能的影响较大。通过这些组成－性能关系。可以实现沥青的     

Modification of petroleum coke by different additives and the impact on anode properties

Anodes, which provide the carbon required for aluminum production, are made from dry aggregates (petroleum coke, rejected anodes, and butts) with coal tar pitch as the binder. Good quality anodes require good interaction between coke and pitch, and this relies on good wetting properties. The objectives of this work are to analyze the wetting properties of four different cokes with and without modification using an additive and to test the effect of the modified coke on anode properties. A FTIR study was done to identify functional groups in non‐modified and modified coke samples since they play an important role on coke‐pitch interactions. The wetting tests were done using the sessile‐drop method to measure the contact angle between coke and pitch. The results showed that the additive improved the wettability of all four cokes by pitch. The least wettable coke was chosen to produce anodes. For anode production, the entire dry aggregate is modified. The additive was mixed with the dry aggregate using two different approaches (one day earlier and 5 min before mixing). The anodes were characterized before and after baking. The early treatment with the additive was found to be better for the modification of dry aggregate.

     

Molten storage and carbonization characteristics of premium grade coal-tar pitch: Influence of crude tar type and distillation conditions

A growing trend towards the transport and use of molten rather than solidified pitch has caused tar distillers to examine the relative stability of pitches while held in storage tanks. The scope of laboratory storage-stability tests can be extended to higher temperatures as a method of studying the earliest stages of pitch polymerization/carbonization. Some indications of the ultimate performance of the pitch in the end-use process can be gained. Crude tar has a strong influence on the ultimate properties and performance of a pitch. Seven tars have been made into 120 °C Mettler softening-point pitches by two distillation methods to study their subsequent storage and carbonization behaviour. The behaviour exhibited by each pitch allows a judgement on overall ‘reactivity’ and on the sensitivity to thermal conditions in the distillation of each tar. Two distillation schemes were adopted: batch distillation followed by heat treatment; and batch distillation at a pressure which was progressively reduced. The results show that time at elevated temperature has a major effect on the properties and performance of the resulting pitch. Vacuum distillation allows lower processing temperatures. Larger amounts of secondary quinoline-insolubles were formed under the ‘distillation with heat treatment’ scheme. Complete wetting of coke by pitch took place at lower temperatures for the vacuum-distilled pitches. Heat-treated pitches increased more rapidly in softening point and lost more mass when thermally soaked. Distillation under vacuum conditions resulted in significantly modified pitch carbonization characteristics.     

Formation of thin mesophase layers at the interface between filler and binder in prebaked anodes. Effect of mixing on mesophase

Prebaked anodes used for aluminum electrolysis are prepared by mixing petroleum coke as a filler with coal-tar pitch as a binder. Pitches which contain too large amounts of mesophase spheres are not suitable for preparing high quality anodes since mesophase microstructure is destroyed by mixing. Both the well extended parallel preferred orientation and the spherical shape disappear. It is observed that disorganized mesophase forms a shell around the coke particles. Such shells reduce the wettability of the coke by the pitch. This transformation was reproduced experimentally by strong grinding. When a disorganized mesophase is heat-treated at 700°C, it recovers a local molecular orientation, the extension of which widely varies from one area to the next one. The smaller the orientation extent, the lower the graphitizability. A mixture of non graphitizing microporous carbon and of partially graphitizing macroporous carbon is thus obtained. Such porous carbons are also found in anodes after baking as shells around the filler.     

Influence of the coke filler on the carcinogenicity of pitch–coke mixtures on carbonization

The emissions of benzo[a]pyrene at different temperatures and its concentration in the exhaust gases are measured in laboratory experiments on the carbonization (at temperatures up to 850°C) of coalpitch and petroleum-pitch binders and their mixtures with roasted petroleum and pitch coke. These pitch–coke mixtures are similar in composition to the anode mass used in aluminum production. The experiments confirm that the total benzo[a]pyrene emissions are much greater in the carbonization of petroleum pitch produced by cracking (T _so = 100°C) than for electrode pitch (T _so = 89°C) and other coal pitch. In most experiments, the benzo[a]pyrene emissions in the carbonization of pitch–coke mixtures is markedly less than for individual binder pitches. It is found that the benzo[a]pyrene emissions in the carbonization of a mixture based on pitch coke are much less than for a mixture based on petroleum coke in the high-temperature region that presents the greatest environmental hazard.     

Pitch and petroleum coke additions to coke oven charges

Several pitch materials and a petroleum coke were added to coke oven charges in an attempt to make good metallurgical coke from Canadian coal of poor coking quality. Coal and petroleum pitches were added to a low fluid western Canadian coal of medium volatile bituminous rank, and the blends coked in a technical-scale moveable wall test oven having a 230-kg charge capacity. Pitches improved coke tumble test indices, the principal coke quality parameter related to blast furnace performance. Varying levels of petroleum coke were added to an eastern Canadian coal of high volatile bituminous rank, and the blends, some partially briquetted, were carbonized in a test oven. Tumble indices of coke from the partially briquetted charges approached an acceptable level. These investigations confirm that petroleum products as well as coal derivative can play a useful part in the production of a metallurgical strength coke from poor or non-coking coals.