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(1)AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY Faculty of Materials Science and Ceramics Department of Biomaterials. Doctoral thesis. Tomasz Lis Alternative carbon binder systems with low polycyclic aromatic hydrocarbons content (PAHs). Dissertation Supervisor: Prof. dr hab. inż. Stanisław Błażewicz. Kraków 2017.

(2) Acknowledgments. First, I would like to express my gratitude to my supervisor prof. dr hab. inż. Stanisław Błażewicz for his endless guidance, patience, support and shared knowledge.. Moreover, I would like to thank dr hab. inż. Aneta Frączek-Szczypta, mgr inż. Maciej Gubernat, Bronisław Rajca and all of the staff in the Department of Biomaterials, of the Faculty of Materials Science and Ceramics, AGH for the great working atmosphere.. I would also like to thank the SGL Group The Carbon Company for giving me opportunity to undertake a PhD study and financial support of the research. I am also deeply thankful to mgr inż. Janusz Tomala, mgr inż. Patrycja Musioł, mgr inż. Jakub Kawala and Michał Głowiński for great collaboration upon the research.. This work was also supported by the Polish National Centre for Research and Development, project no:PBS2/B5/27/2013 19.19.160.86540..

(3) Contents List of abbreviations ....................................................................................................... 3 Streszczenie...................................................................................................................... 5 Abstract............................................................................................................................ 6 Introduction ..................................................................................................................... 7 Literature part ................................................................................................................ 9 1.. Coal-tar pitch (CTP) ............................................................................................... 9 1.1. Preparation ........................................................................................................... 9 1.2. Chemical composition........................................................................................ 10 1.3. Physical properties ............................................................................................. 12 1.4. Applications ........................................................................................................ 13. 2.. Toxicity of CTP ..................................................................................................... 14 2.1. Law regulations .................................................................................................. 15 2.2. Toxicity aspect .................................................................................................... 16 2.3. Polycyclic aromatic hydrocarbons in CTP ...................................................... 17 2.3.1. Initial CTP toxicity related to methods used ............................................ 18 2.3.2. Initial CTP toxicity related to the softening point ................................... 18 2.3.3. CTP volatiles toxicity .................................................................................. 19 2.3.4. CTP volatiles toxicity – the influence of heating rate .............................. 21. 3.. Less toxic carbon binders – state of the art ........................................................ 23 3.1. Compositions of CTP with carbonaceous component .................................... 23 3.2. Compositions of CTP with polymeric material ............................................... 24 3.3. Modification of CTP by using non-carbon components................................. 26 3.4. Physical modification of CTP ........................................................................... 28 3.5. Other applications of alternative binders ........................................................ 29 3.6. Summary ............................................................................................................. 31. 4.. Summary of the literature part ........................................................................... 34. 5.. Aim of the work and its scope .............................................................................. 37 5.1. Selection of materials ......................................................................................... 38 5.1.1. Wood-derived tars (WTs) .......................................................................... 39 5.1.2. Coking coal (CC)......................................................................................... 40 5.1.3. Hard CTP (CTP243) and modifiers .......................................................... 40.

(4) Experimental part ......................................................................................................... 42 6.. Methods.................................................................................................................. 42 6.1. Methods for determining binder properties .................................................... 42 6.2. Methods for determining electrode properties – laboratory scale ................ 44 6.3. Methods for determining electrode properties – pilot plant scale................. 45. 7.. Materials ................................................................................................................ 47 7.1. Wood tars............................................................................................................ 47 7.1.1. Source of raw materials .............................................................................. 47 7.1.2. Characteristics of raw materials ............................................................... 50 7.1.3. Electrode model experiments – laboratory scale ..................................... 60 7.1.4. Electrode model experiments – pilot plant scale ...................................... 68 7.1.5. Summary and conclusions ......................................................................... 74 7.2. Coking coal (CC) ................................................................................................ 77 7.2.1. Characteristics of raw materials ............................................................... 77 7.2.2. Procedure of the samples preparation ...................................................... 84 7.2.1. Electrode model experiments – laboratory scale ..................................... 88 7.2.2. Summary and conclusion ........................................................................... 94 7.3. Modified CTP243 ............................................................................................... 96 7.3.1. Toxicity of the modified CTP243 ............................................................... 96 7.3.2. Stability of the modified CTP243 .............................................................. 99 7.3.3. Selected properties of the modified CTP243 .......................................... 102 7.3.4. Electrode model experiments – pilot plant scale .................................... 108 7.3.5. Summary and conclusion ......................................................................... 114. 8.. Conclusions .......................................................................................................... 117. 9.. References ............................................................................................................ 120. Supplementary information ....................................................................................... 132.

(5) List of abbreviations Amu - atomic mass unit B[a]P - benzo[a]pyrene B[a]P-eq – benzo[a]pyrene-equivalent CC – coking coal CTP – coal-tar pitch CTP_RT – coal-tar pitch with softening point below room temperature CTP103 – coal-tar pitch with softening point 103°C CTP243 – coal-tar pitch with softening point 243°C CTPs – coal-tar pitches CV – coking value d002 – interplanar graphene distance in carbon and graphite crystallites Diff. – determined by difference DTG - differential thermogravimetry GC- gas chromatography GC-MS - Gas chromatography-mass spectroscopy HPLC – high performance liquid chromatography HT – heat treatment Lc - the stacking heights LC- liquid chromatography Nd - not determined PAH - polycyclic aromatic hydrocarbon PAHs - polycyclic aromatic hydrocarbons QI – quinoline insoluble RT- room temperature SEC – size exclusion chromatography 3.

(6) SEM - scanning electron microscope SP – softening point TEM - transmission electron microscope Tg - glass transition temperature TG – thermogravimetry TI – toluene insoluble Wt% - weight-weight percentage WT – wood-derived tar WTs – wood-derived tars XRD - X-ray diffraction. 4.

(7) Streszczenie Głównym celem niniejszej rozprawy było opracowanie materiału stanowiącego prekursor węglowy, który spełnia wymogi stawiane zarówno spoiwom węglowym stosowanym w technologiach syntetycznych węgli i grafitów, a także wymogi i normy Unii Europejskiej, w zakresie ochrony środowiska i ochrony zdrowia. Na podstawie analizy literaturowej wyłonione zostały trzy rodzaje substancji, które zostały poddane badaniom eksperymentalnym. Zastosowano trzy rodzaje materiałów smoły drzewne, węgiel koksujący oraz wysokotopliwy pak węglowy. Początkowo, każda z substancji analizowana była pod względem jej zdolności przetwórczych. W przypadku smół drzewnych oraz wysokotopliwego paku węglowego dostosowano właściwości materiałów wyjściowych do standardowych warunków stosowanych w tradycyjnych technologiach przetwarzania surowców węglowych. W przypadku węgla koksującego, opracowano optymalne warunki, w których materiał ten wykazuje reaktywność względem wypełniacza węglowego. W dalszej kolejności określono podstawowe parametry charakteryzujące spoiwa węglowe, takie jak skład pierwiastkowy, zawartość najbardziej szkodliwych wielopierścieniowych związków aromatycznych (WWA), stabilność w funkcji temperatury, zmiany strukturalne zachodzących w wyniku obróbki termicznej i inne. Lepiszcza wykorzystane zostały do wytworzenia próbek elektrodowych w skali laboratoryjnej, a także przemysłowej. Materiały scharakteryzowano pod kątem właściwości fizycznych oraz odniesiono do właściwości wyrobów do produkcji, których wykorzystywany jest pak węglowy. W wyniku badań przeprowadzonych w toku realizacji rozprawy opracowane zostały lepiszcza węglowe spełniające obowiązujące kryteria dotyczące toksyczności, które mogą stanowić substytut paku węglowego w wytwarzaniu wyrobów węglowo grafitowych różnego rodzaju.. 5.

(8) Abstract The main aim of the work was to develop carbonaceous precursor that meets the demands of both, standard properties of binding materials for carbon & graphite production, as well as the requirements and standards of the European Union referring to environmental and health protection. Based on the literature review and analysis three types of substances have been subjected to experimental study i.e. wood-derived tars, coking coal and hard coal-tar pitch. At the beginning, each of substance was analysed for its processing capabilities. In the case of wood-derived tars and hard coal-tar pitch, the properties of the initial materials were adjusted to the standard conditions applied in traditional synthetic carbon & graphite processing technologies. In the case of coking coal, optimal conditions in which the material exhibits high reactivity towards the carbonaceous fillers have been determined. Next, the basic parameters characterising the carbon binders, such as elemental composition, the most harmful polycyclic aromatic hydrocarbons (PAHs) contents, temperature stability, structural changes under thermal treatment, and other parameters were determined. Selected binders were used to manufacture electrode samples in the laboratory and industrial scale. The heat-treated samples were characterised for their selected physical properties and also were related to the properties of the CTP-based electrode samples. As a result of the investigations conducted in the frame of this thesis work alternative carbon binders which may substitute the coal tar pitch, being in accordance with current criteria of toxicity, that may be used in the manufacture of various carbon & graphite products have been developed.. 6.

(9) Introduction Over the last 50 years, carbon has become one of the most marvellous elements, revolutionising material science. Since 1960, a number of processes have been developed to manufacture new forms of carbons, like various carbon fibres, glass-like carbons and pyrolytic carbons [1–4]. The exploration of new carbon forms encouraged the creation of new ones due to the modifications of processing conditions, new carbon precursors, new techniques of manufacturing, etc. Another breakthrough took place in 1985, when a new carbon form called fullerene was described [5]. In 1996, the Nobel Prize in Chemistry was awarded to Robert F. Curl, Harold W. Kroto and Richard E. Smalley for their discovery of fullerenes [6]. Other carbon nanoforms were subsequently discovered and described in several papers, with the finding of single- and multi-wall carbon nanotubes in 1991 [7]. The high importance of these new carbon nanoforms in the fields of science and technology has been proven by awarding the Nobel prizes in Physics for experiments on a two-dimensional carbon form known as “graphene” [8,9]. On the other hand, it should be emphasised that from an economic point of view, the principal incomes for the worldwide carbon industry are still conventional forms of carbon like artificial carbongraphite blocks, activated carbon and carbon black. Artificial carbon-graphite blocks are widely used in an important branch of industries i.e., in the manufacture of metals and semiconductors. The technology of these kinds of carbon materials began to develop in 1878 and has continuously improved, very often with the use of sophisticated tools peculiar to the more advanced nanomaterials mentioned above. Therefore, the increase in the product quality is often associated with the ability to control its structure, texture and microstructure currently. The carbon-graphite blocks are produced by the hot forming technology of properly prepared mass, generally consisting of coke, anthracite and graphite with a carbon binder. Although pitches can be obtained from several sources, i.e. mostly from coal or petroleum, and from other aromatic compounds, i.e. anthracene oil and polyvinyl chloride waste [10,11], in the minority of cases, the CTP is still an irreplaceable material in many fields of applications of industrial sectors. The current efforts related to material technologies are connected not only to the production of new and innovative materials, or to the optimisation of existing products, but also to a particular attention to the protection of the environment, safety and healthcare. These aspects are particularly relevant for carbon & graphite technology 7.

(10) because they generate high greenhouse gas emissions, as well as mutagenic and carcinogenic substances. The work is divided into 9 chapters and two main parts, i.e., literature and experimental ones. The first part consisting of 4 chapters, presents the literature review on the types of existing binders their drawbacks and advantages including the systems currently investigated in the literature. Taking into account the main goal that have been made in the experimental part of the work, the literature review presents the state of the art on the carbon binders applied currently in the production of carbon and graphite materials, problems of their toxicity and European Union regulations that concern the production and use of carbon & graphite materials regarding the needs for the reduction of PAHs. This part also summarises efforts and the ways to solve the still open problem in this industry. Next chapter 5, preceding the experimental part, performs the goal and scope of the work. The experimental part of the work consists of three chapters, starting from chapter 6 which describes methods for binder analysis and the procedures for manufacture of samples in laboratory and technological scale to verify the effectiveness of the earlier selected binders. The main results of the study are discussed in chapter 7 of the dissertation, followed by a summary and conclusions. The last part of the work contains supplementary data related to the manufactured samples. This work is devoted to the development of new, alternative binders, which are appropriate for the manufacture of carbon & graphite artefacts in a safe and environmentally friendly way.. 8.

(11) Literature part 1. Coal-tar pitch (CTP) In general, according to the recommended terminology of carbon published by the International Union of Pure and Applied Chemistry [12]: coal-tar pitch is a residue produced by distillation or heat treatment of coal-tar. It is a solid at room temperature, consisting of a complex mixture of numerous predominantly aromatic hydrocarbons and heterocyclics, and exhibits a broad softening range instead of defined melting point. This concise definition unequivocally describes the most important features of these materials; nonetheless, for better understanding of its properties, more specific knowledge is essential. This chapter deals with the production, chemical composition, the physical and chemical properties and the industrial significance of CTP.. 1.1. Preparation The industrial preparation of CTP is based on fractioned distillation of the coaltar, which is a by-product of metallurgic coke production [13].The scheme of the CTP production process is shown in Fig. 1. The carbonisation of hard coal to metallurgical coke is performed at around 1000 to 1200°C with a residence time of 14 to 20 h. The main product is coke, with the remainder being comprised of water, light oils, ammonia, coke oven gas and crude coal-tar, which account for 3.5%, relative to feed coal. The pitch preparation from coal-tar is basically with regard to the sequence of distillation steps. In the first distillation column, benzene, toluene, xylenes (BTX) and water are removed. Then, the dehydrated tar is transported to the second column for the recovery of naphtha, naphthalene oil and washing oil. The resulting residue, soft pitch, is fed into a process reactor where it is heated to approximately 400°C and subsequently loaded into a third column (with or without vacuum). There, under the conditions for a controlled distillation process, the pitch is obtained with the required final parameters and the fraction known as anthracene oil. The final yield of that process is about is about 50-55%, relative to crude tar [13,14]. The crude tar and CTP production is directly connected to the engineering development of coke, which is mostly dependent on the metallurgic coke demand for blast-furnace processes. With the worldwide production estimated at 350 million tons of coke and 16 million tons of tar, the production capacity reaches approximately 8-9 million tons. However, the production scale is much smaller and only some of the produced tar 9.

(12) is subjected to distillation [15]. For several years in the US, and in recent years also in EU countries, a noticeable deficit of CTP in the carbon material industry has been observed - mostly because of the diminishing number of by-product coke ovens and environmental restrictions [15–17]. The scheme of the typical process used for CTP production is shown in Fig. 1.. Fig. 1 The scheme of CTP production process. 1.2. Chemical composition CTPs are conglomerates consisting made up of thousands of polynuclear and aromatic compounds, differing from reach other in their molecular weight, functionality, and molecular structure [14,18]. From the results obtained by preparative SEC in conjunction with vapour conjunction osmometry it follows that those compounds range from 150 to more than 3000 amu [19]. On the other hand, despite the very broad distribution of molecular weights, the constituents can be classified into relatively small number of the types compounds [20]: . PAHs;. . Alkylated PAHs;. . PAHs with cyclopentene moieties;. . Partially hydrogenated PAHs;. . Oligoaryls and oligoaryl methanes;. . Hetero-substituted PAHs: NH2, OH;. . Carbonyl derivatives of PAHs;. . Polycyclic heteroatomic compounds. 10.

(13) As mentioned above, the predominant class of compounds is PAHs. It is estimated that around 97% of the carbon occurring in CTP is present in the form of aromatic compounds [21], and about 40% of compounds are those with a molecular weight below 350 amu [14]. Polycyclic heteroatomic systems, derived from i.e. pyrrole, furan, thiophene and pyridine are in lower amounts than PAHs; however, they are of great importance because of their high thermal reactivity [22]. The rest of the CTP compounds occur at much lower concentrations than the classes mentioned previously. Nevertheless, CTP is not a single phase material. It contains dispersed solid matter - called QI – which is classified as primary and secondary QI. The primary QI are partly inorganic (ash) but predominantly of organic nature. They form sphere-like particles, with diameters mainly below 1 µm. Both agglomerates and discrete spheres are present. The second type of QI constitute anisotropic spheres called mesophase. The carbonaceous mesophase is composed of lamellar macromolecules of different molecular sizes, similar to that of liquid crystals. They are between 1 and 60 µm in diameter and form during the thermal treatment of pitches. The primary QI and secondary QI differ in an atomic C/H ratios of about 3.5 and 2.1, respectively. As the composition of pitch is so complex, it is difficult to separate it into individual compounds. The popular method of pitch characterisation, proposed by Jurkiewicz and Mellison [23] and based on the division of their ingredients on a group of structurally-related compounds by solvent fractionation, allows the correct determination of their usefulness and processing properties. The symbols and descriptions of primary fractions are given in Table 1. Table 1 Typical fractions of CTP and their denotations. Fraction type α-resin β-resin γ-resin. Properties Insoluble in quinoline Quinoline soluble, toluene insoluble Soluble in toluene, soluble in quinoline. Denotation QI TI Maltenes. In general, the average molecular weights of the extracted fractions increase in the following order: γ-resin, β-resin, and α-resin [24]. The γ-resin described above determines the rheological properties of binder, while the β-resin improves the binding capability and enhances the carbon yield [25]. A more detailed description of the chemical composition for the above components is provided by Skoczkowski [26]. 11.

(14) 1.3. Physical properties The composition and properties of CTP vary considerably depending on the quality of initial tar, its preparation and on the type and distillation process conditions. However, it is difficult to unequivocally determine the value of these factors [15]. SP, viscosity and CV, which are the primary physical properties determined in a routine way as a measure of the suitability of binder pitches, are described below. The key factor that determines majority of pitch properties is glass transition temperature (Tg), which is typical parameter of polymeric materials. At Tg, the relaxation processes take place, i.e. the molecular rotation and translation. However, it is common practice to specify the pitch rheology, not by measurements of the Tg but by determination of SP [27,28]. There are several methods for SP determination, the most significant include: Kreamer-Sarnow (KS), the Ring-and-Ball (R&B), the Cube-in-Air method (CiA) and Mettler. SP defines the nature of pitch; the general classification by the SP (by Ringand-Ball method) is as follows [29]: . <40°C refined tar;. . 40-60°C soft pitch;. . 60-75°C medium-soft pitch;. . 75-110°C medium-hard pitch;. . >110°C hard pitch.. The temperature of SP is primarily dependent on the distillation; nevertheless, other methods can be used to modify that parameter as well (see chapter 3.4). The viscosity of CTP determines its behaviour during process of homogenisation, (the wettability of another components) and the strength of green product. Isotropic pitches heated well above SP exhibit Newtonian viscosity behaviour [29], whereas nonNewtonian fluid behaviour is observed with those containing mesophase (anisotropic pitches) [30,31]. However, this classification is ambiguous and depends on the sort and SP of pitch. Moreover, it can be assumed that SP and viscosity increases proportionally to the TI and QI fraction content and is inversely proportional to the γ-resin content [26,32]. CV in case of carbonaceous material is a very significant factor of the pitch quality assessment. Basically, it is a measure of quantitative coke forming propensities. The strength of the coke bridge in the material resulting after thermal treatment is dependent 12.

(15) on the amount of residual pitch coke. As with viscosity, CV is proportionally related to TI and QI content [18].. 1.4. Applications CTP is broadly utilised in the production of synthetic carbon & graphite materials which are applied in the production of ferrous and non-ferrous metals. The key products based on pitch are green or baked anodes and cathodes applied in aluminium production, graphite electrodes for arc furnaces and furnace lining applied in pig iron and steel production. In this application, it plays a key role in the different stages of manufacture, in particular as a binder during green product formation, as a matrix after HT, and as a saturant during densification process. Other possibilities for developing carbon materials from CTPs are illustrated succinctly in Fig. 2.. Fig. 2 Traditional and advanced applications of CTPs. Along with uses in metallurgy, there are many other application of CTPs. The main ones are the production of carbon fibres, nuclear graphite, carbon-carbon composites, anode materials for lithium ion batteries [33], mechanical seals or brakes, adsorbents and other porous carbon, and the manufacture of brushes or heat-exchangers [17]. It is also a modifier of the CC production process, among others [13,34]. Moreover, CTP is widely used in the road and building industry, i.e. as a waterproofing material of roofing or as a component of road tars to be used as paving material [29].. 13.

(16) 2. Toxicity of CTP According to Rovinskii [35], the prime source of PAH in the environment is coke industry. It is estimated that more than 20% of the global emissions of PAH originates from this process [36,37]. Hazardous hydrocarbons in particles can be easily transported by rainwater and wind, for example, to nearby water, air and soil sediments, leading to an increased cancer risk for living organisms. As a consequence of environmental aspects, some coking plants have been closed [17]. CTP, which is the coke-industry waste, is thought to be a potential pollutant due to its toxic PAH components; therefore, significant restrictions on its areas of application are currently being observed. PAHs represent a relatively large group of organic compounds containing between two and a dozen aromatic rings in the molecule. As already known, some have mutagenic and carcinogenic activities. It is estimated that 90% of human cancers may be ascribed to chemical carcinogens such as PAH [38]. As a result, some concentrations are also monitored in the air, water, soil, food and living organisms [39]. According to the data in chapter 1.2, about 40% of compounds are those with a molecular weight below 350 amu [14]. Thus, this comprises all PAH representatives. The first experiments to determine the toxicity of pitches, asphalts and bituminous materials, led by the Occupational Safety and Health Administration (OSHA) and the National Institute of Occupational Safety and Health (NIOSH), under the patronage of the Environmental Protection Agency (EPA), were undertaken in the 1970s. The first official information on both tumour-initiating and tumour-promoting activity in mouse skin was published in 1985 and 1987 [40,41]. Nowadays, it is has been convincingly proven in numerous studies and reports that occupational exposure to coal tar or coal tar distillates is associated with skin cancer and cancer of other tissues, including the bladder, kidney, lung, oral cavity, larynx, oesophagus, stomach and digestive tract, as well as leukaemia [42,43]. In spite of the fact that CTP is broadly utilised in carbon & graphite artefact manufacture (see chapter 1.4) the highest demand is related to primary aluminium production. In the statistical report of the International Aluminum Institute, the worldwide production of aluminium by electrolysis was 56.8 million tons in 2015 [44]. Based on that, it can be calculated that about 5 million tons of carbon binder was consumed. The main carbon product used in this technology, which can be divided into prebaked and Soderberg technology, are cathodes and anodes, sidewall blocks and ramming pastes. In 14.

(17) the smelting technology comparison, Soderberg technology with self-baking anodes generates the higher carcinogenic risk [45]. Nevertheless, the most toxic products, that workers at aluminium plants are exposed to during pot building are ramming pastes. The technology of ramming pastes has been modified since the 1970s, resulting in additional heating of the material to 120-140°C during manual densification, and leading to exposure to strong PAH emissions. Nowadays, ramming pastes are less toxic and less viscous, permitting the densification at much lower temperature, i.e. at 40°C. However, this kind of material is still given special attention due to its direct contact with workers. Due to the aspects described above, the global market of carbon artefacts is facing strong environmental and commercial challenges, particularly with regard to the aluminium industry. Some details related to the toxicology of binder materials used in carbon & graphite technology are discussed in the following part of this work.. 2.1. Law regulations The regulations related to the classification and harmfulness evaluation of CTPbased distillation products are subjected to permanent analyses and modifications. For example, in Europe, CTP was included to the group of substances of unknown or variable composition, complex reaction products or biological (UVCB) materials and consequently carcinogenic substances of category 2, according to the description in regulation no 1272/2008 of the European Parliament and of the Council on 16 December 2008 [46]. On 13 January 2010, it was included in the candidate list for authorisations, following European Chemical Agency (ECHA) decision, because of its persistent, bioaccumulative and toxic (PBT) properties and its very persistent and very bioaccumulative (vPvB) properties concerning criteria of Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulation [47]. On 2 October 2013, the European Commission classified CTP as Carcinogenic 1A-350, Mutagenic 1BH340, Toxic for reproduction 1B-H360FD, Aquatic Acute 1-H400, and Aquatic Chronic 1-H41 [48]. There are many approaches, still being developed to enable the most objective evaluation of CTPs harmful properties. The methods are divided into three main groups, namely the concentration analysis of individual PAH [32,49,50], analysis of the total PAHs selected concentration [39,51] and approximation of the function called B[a]Peq [38]. According to the regulations based on the first approach, the admissible B[a]P concentration cannot exceed 100 ppm in most European countries [46] and i.e. 50 ppm in 15.

(18) Germany [49,52]. On the other hand, for total toxicity determination, the third approach is popularly used. This takes into consideration the equivalent toxicity factor (ETF) which is the risk rating of particular PAHs with respect to B[a]P – the most common and strongest carcinogen. Nevertheless, the definition of hazardous aromatic hydrocarbons varies between countries, and examples of differences were described by Boenigk et al. in [53]. Consequently, B[a]P-eq determination depends on the number of PAHs and assumed ETF. Generally, the standard documents regarding the list of genotoxic PAH include 8 to 18 compounds. In the European Community and Canada, the list contains eight. PAH. compounds:. benzo[j]fluoranthene,. benzo[a]anthracene,. benzo[k]fluoranthene,. chrysene,. benzo[b]fluoranthene,. benzo[e]pyrene,. B[a]P,. and. dibenz[a,h]anthracene.. 2.2. Toxicity aspect CTP composition is far too complex, owing to environmental legislation and from the point of view an occupational hazard, the qualitative and quantitative determination of carcinogenesis and mutagenic compounds in pitches and their volatile fumes is of great importance. In addition, the quantitative identification of individual compounds may also be useful to achieve a greater understanding of pitch structure and properties. For this purpose, a wide range of chromatographic and spectroscopic techniques is generally used. One of the most popular techniques used is capillary gas chromatography (GC) [54,55] and its combination with various kinds of detection methods like: mass spectrometry (GC-MS) [39,55–57] and flame ionisation detector (GC-FID) [58,59]. They are used in the cited works for both qualitative and quantitative analysis of complex organic mixtures such as CTPs and for compound monitoring during chemical reactions; i.e. for observation of the evolution of anthracene oil components during air-treatment under different experimental conditions [60]. One of the critical steps during sample preparation is the extraction of PAH from environmental matrices. Various chemicals, organic solvents such as acetone, methanol, dichloromethane, chloroform, toluene, hexane and cyclohexane and their mixtures are used [61]. These are used in the conditions of directional dissolution or supercritical fluid [62]. In the case of extraction techniques, the Soxhelet extraction is frequently used for PAH mixtures; however, it is a timeconsuming method and requires large volumes of solvents [39]. A lot of alternative methods have been described, i.e. extraction enhanced by ultrasonic [63] and microwave [64] treatment, pressurised liquid extraction [61] and solid phase 16.

(19) extraction [65]. Afanasov et al., in [39], comprised results obtained by common Soxhlet extraction and much shorter-lasting solvent extraction enhanced by ultrasonic treatment. It was shown that there are no significant differences in the case of quantitative PAH analysis of different CTPs, even in comparison to introducing ultrasonic waves in the range from RT to 70ºC. Regardless of the conditions and method analysed, the standard deviation did not exceed 20%. Regarding the GC method, the main disadvantage and limitation is instability of the extracted sample, i.e. partial solubilisation of the sample and volatility problems with certain compounds [55,66]. Despite their wide popularity of this method for CTP components analysis, Xie et al. [67] suggest that this method is more appropriate for the detection of less polar, thermally stable and rather more volatile species. He proved in his work that it is not possible to detect species with molecular masses larger than 280 amu by GC-MS analysis. Other methods can be used to analyse compounds with a molecular weight above the range available to GC or with a more complex structure, or just in the case of the availability of other research equipment. For example, to identify pitch constituents up to 600 amu, chromatography (LC) [36,68], high-performance liquid chromatography (HPLC) with different sorts of detectors i.e. flame ionisation detector (FID) [51,59], spectrofluorimetric detector (SD) [69], and fluorescence detector (FD) [61] can be used. As far as higher ranges are concerned, the laser desorption mass analysis (LIMA) makes it possible to detect components with molecular masses of up to 12000 amu [70,71]; in turn, as for matrix-assisted laser desorption (MALDI) [70,72–74], the method was useful for the trace identification of material up to 200,000 amu. For qualitative and quantitative pitch analysis, UV/VIS spectrometry [32,75–77], SEC [72], supercritical fluid chromatography (SFC) [65] and atmospheric pressure solid analysis probe/time of flightmass spectrometry (ASAP/TOF-MS) [78] are also used.. 2.3. Polycyclic aromatic hydrocarbons in CTP The emission of hazardous compounds is caused by the presence of PAHs in initial CTP itself and also to those formed in pitch thermal treatment during carbon artefact production. Thus, the toxicity is determined by the chemical composition analysis of initial material and the characterisation of volatile fractions during the pyrolysis process. This paragraph comprises the results of PAHs content in different CTPs and the influence of various factors on their most hazardous constituents. 17.

(20) 2.3.1. Initial CTP toxicity related to methods used As there are no physicochemical methods for the determination of individual PAH compound, analyses are preceded by complex fractionation dissolution. Therefore, subjective errors in sample preparation can occur. Nevertheless, the greatest divergence is observed by using a variety of analytical methods. Sidorov [38] collated the B[a]P concentration determined by different methods; e.g. by spectral-fluorescent methods, the B[a]P concentration was in the range from 2.3 to 5.1%, by direct paper chromatography it amounted to 2.6-3.9% and similarly by GC and HPLC, it was 0.8-1.2%. The relatively high overestimation of PAH by spectral-fluorescent methods is caused by very intense fluorescence spectra and, consequently, their possible superposition [38]. When GC-MS and HPLC methods together with MALDI were applied, good agreement with B[a]P concentrations was obtained by Suriyapraphadilok [79]. Regarding the instrumental error, the level of 4% [68] and 20% maximally for HPLC and GC-MS [39] was estimated. 2.3.2. Initial CTP toxicity related to the softening point The dependence of the CTP toxicity in relation to the SP was studied by Boenigk et al. [53] in the range from 100-170ºC, although they did not specify the producers or analytical method. The results are summarised in Table 2. It is evident that with increasing SP and consequently CV, the amount of genotoxic PAHs decrease meaningfully. Therefore, in the case of the analysed material, the increase of SP in the CTP to 170°C cuts the B[a]P-eq twice and reduces total B[a]P emission by 60%. Table 2 Selected properties of CTP with increasing SP [53]. SP (Mettler) [ºC]. 100. 132. 150. 170. CV [%]. 59.9. 64.3. 71.7. 75.5. B[a]P [%]. 1.30. 1.21. 0.82. 0.52. B[a]P-eq [%]. 2.86. 2.69. 2.35. 1.49. On the other hand, the study reported by Andreikov et al. [80] for CTP with lower SP produced at Ural and Siberian coke plants shows distinctly different dependences. The results gathered in Table 3 were determined by GC with a flame-ionization detector. Although each sample was analysed at least three times, the authors did not specify any statistical parameters. Nevertheless, taking into consideration only the SP in the range from 70 to 90.3°C, and without any knowledge of the analysed materials, it is not possible to observe the dependence described by Boenigk (Table 2).. 18.

(21) Table 3 Selected properties of CTP with increasing SP [80]. SP (Mettler) [ºC]. 70.0. 78.1. 87.5. 90.3. 90.3. CV [%]. nd. 57.0. nd. nd. nd. B[a]P [%]. 0.89. 1.01. 1.14. 0.85. 0.88. B[a]P-eq [%]. 2.17. 2.69. 2.93. 2.14. 2.09. The content of B[a]P in individual, extracted fractions (see chapter 1.2) of CTP (SP (Mettler)=68.5ºC, TI=18.4%) was investigated by Zieliński et al. by UV/VIS spectrophotometry [32]. The results are summarised in Table 4. In the analysed CTP, 95% of B[a]P is concentrated in the continuous phase (γ-resin) which generally affects the rheological properties and usually constitutes the fraction with the largest weight fraction. This confirms the results mentioned above. Table 4 B[a]P content in CTP individual ingredient [32]. Type of. Fraction. B[a]P. fraction. [wt%]. [%]. α-resin. 40.1. 0.15. β-resin. 11.6. 0.60. γ-resin. 46.9. 3.95. Moreover, the total B[a]P content calculated by summing the quantities in the individual fraction is higher than the content determined in the initial pitch, i.e. 1.83% (UV/VIS spectrophotometry). Probably, it is the result of acetone reaction with γ-resin and creating a secondary B[a]P. It is worth noting that this phenomenon is not generally taken into account during sample preparation by solvent extraction. The increasing B[a]P content with decreasing SP was also reported by Sidorov [51]. The concentration determined by gas-liquid chromatography for hard CTP with SP 145°C and 152°C was 1.3% and 0.95%, respectively. 2.3.3. CTP volatiles toxicity The carcinogenic properties of the carbonisation products of CTP have been a subject of interest for many studies. This kind of analysis is conducted by chromatographic methods with the use of the solution of condensed PAHs which are released as gaseous products during thermal treatment. Sidorov in his work [81] estimated that if the CTP carbonisation products are analysed, like pitch coke, pitch tar, and pitch coke-gas, the B[a]P is found in the liquid and gas state. Its quantity in the gas phase may 19.

(22) be negligible. Andresen et al. [17], using GC-MS analysis of CTP gaseous products, reported that the volatile phase consists mostly of aromatic compounds with 3 to 6 aromatic rings; however, the predominant compounds are made from 4 member rings. The release of 5 membered ring structure compounds like benzo[j]fluoranthene and B[a]P takes place at about 400°C. A more detailed investigation was conducted by Kwangeui et al. [58], who analysed initial CTP (SP(Mettler)=113.9°C) and additionally condensed volatiles 350°C and 650°C by GC-FID. It was reported that the volatiles are released from pitch without any cracking up to 350°C and are mostly composed of acenaphthalene, fluorine, phentathrene and anthracene, with very small amounts of pyrene and fluoranthene. Thus, the components consist mostly of 3 member ring compounds. The heavier PAHs like benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(e)pyrene, B[a]P, indeno(1,2,3c,d)pyrene, dibenzo(a,h)anthracene and benzo(g,h,l)perylene were detected in coal tar condensed from the carbonisation process at 650°C for 1 hour. They were dominant in comparison to lighter compounds. In addition, the quantity of heavy PAHs found at 650°C was lower in comparison to the initial CTP in contrast to light PAHs. Hence, it can be concluded that the decomposition of heavier compounds occurs at higher stages of the pyrolysis process. The confirmation of these results can be found in the study by Sidorov [51]. During the pyrolysis conducted at the medium-hard pitch annealed up to 900°C, the B[a]P was only detected in the range from 450 to 600°C (results obtained for GC-FID). Nevertheless, it is not a rule that B[a]P and other PAHs are not released above 600°C. This also depends on the kind of pitch and the conditions of the pyrolysis process. For example, they were detectable in medium-soft CTP (SP=77°C) in high-temperature CTP (SP=152°C) annealed up to 800°C, as well as in hard CTP (SP=145°C) [82]. The scheme of PAH liberation in the range from 700 to 800°C proposed by Kurteeva et al. [83] is shown in Fig. 3. According to the authors, within the temperature range from 640-700°C, tarry substances that consist of about 48 wt% of PAH are adsorbed in the porous structure of carbon residue (stage A in Fig. 3). Then, with an increasing temperature from 700 to 750°C, the accumulated tars were desorbed into the gas phase. The desorption process is accompanied by both the enhancement of the carbon residue-specific surface, which is evidence of the liberation pores, and the transition of the carbon residue into the single phase-state. As far as the temperature range from 750 to 800°C is concerned, due to the rupture of C-C bonds during the formation of 20.

(23) carbon residues and the consequent evolution of volatile hydrocarbons, the creation of PAHs occurs due to the gas-phase process.. Fig. 3 A scheme of PAH liberation at the temperature range from 700-800°C [83]. The correlation of the quantity of PAH with different temperature ranges for CTP (SP(Mettler)=111°C) during carbonisation at a rate of 5°C/min was investigated by Marakushina et al. [68]. According to the reported data, the maximum liberation of carcinogens occurs in the temperature range from 270 to 360°C and amounts to 51% of all PAHs. Moreover, this range does not correspond to the maximum mass loss, which was seen at 360-440°C. 2.3.4. CTP volatiles toxicity – the influence of heating rate The. investigation. of. the. composition. of. PAH. release. from. CTP. (SP(Mettler)= 111°C) at different carbonisation rates, in the range from 20 to 1000°C was conducted by Marakushina et al. [68] by using LC technique. The results are summarised in Table 5. Based on the reported data, it is evident that more PAHs are liberated at higher carbonisation rates; however, B[a]P content and B[a]P-eq are lower. Therefore, with an increased heating rate, the thermal decomposition of CTP will be less profound because of the shorter reaction at high temperatures. Accordingly, the authors concluded that this phenomenon is associated with the synthesis of heavy PAHs from light hydrocarbons in the liquid phase of melted pitch. For example, B[a]P can be formed during naphthalene hydration, splitting C-C bonds, condensation reaction and dehydration of the polycyclic products, as stated by Golounin et al. [56].. 21.

(24) Table 5 Compositions of PAHs liberated at different carbonisation rates of CTP [68]. Heating rate [°C/min] 5. 50. 100. 300. Initial pitch. B[a]P [ppm]. 12.3. 10.4. 7.7. 7.8. 8.5. Total (of 17 PAHs) [ppm]. 84.1. 87.4. 94.4. 98.1. 100.8. B[a]P-eq [%]. 35.7. 31.3. 28.6. 27.8. 30.1. 22.

(25) 3. Less toxic carbon binders – state of the art As previously described, a very restrictive environmental legislation in most countries concerning the emission of hazardous fumes at the workplace has led to the search for new carbonaceous pitches capable of replacing, at least in part, CTPs. In general, the methods which can be used for the reduction of PAH emissions in synthetic carbon & graphite processing can be divided into three main approaches [51]: a) Identification and optimisation of binder carbonisation conditions with minimal atmospheric conditions [66]. b) Maximum sealing of the process line and minimising the dust formation by i.e. installing special afterburners. c) Reduction of PAH quantity in the pitch by various methods. Currently, in order to reduce harmful emissions of gas products, a combination of the following techniques are used: burnout, electrostatic precipitators and fabric filters [84]. As reported by Buchebner et al. [85], combining the second (b) and third (c) option lead to the air-borne concentration of B[a]P to levels below 0.002 mg/m3. Another way of reducing B[a]P content in exhaust gases uses ultraviolet radiation with a mean light-flux density of gas irradiation in the range of 0.001-0.300 J/cm2 [38]. However, the main problem is still connected to the raw materials which are dangerous during transportation, storage and operating in an open system i.e. in the case of ramming paste. Many studies have focused on identifying non-toxic carbon binders. Some are based on both physical and chemical methods of CTP modifications, while others are devoted to completely different substances. Selected methods described in the literature are summarised later in this chapter.. 3.1. Compositions of CTP with carbonaceous component One of the cut-back PAH content method is blending CTP with other coal-based precursors from various sources, namely coal (coal extracted pitch, gasification pitch [86], heavy pyrolytic tar, shale tar [87]) and crude oil (petroleum pitch [88–90], bitumen [38,91]). All of them are composed of hydrocarbons; however, they vary in functional group and molecular size distributions of components. Petroleum pitches are the most popular components for blending with CTP, mainly due to their availability. In this case, the toxicity reduction is achieved according to the law of mixtures. As concluded by Menendes et al. [92], blends of various pitches 23.

(26) did not reveal any additive effects. Two explanations are considered for that phenomenon: (I) CTP and petroleum pitch molecules interact to create a new set of mesogens, which appear in a new position at the cohesion window and (II), blends may have different solubility features compared with single components, thereby affecting the terms in which mesophase appears. It does not matter whether the blend was produced either by simple mixing at elevated temperatures [89] or by the combined distillation of coal tar and heavy cycle oil from catalytic cracking. The second case was conducted in an industrial trial with over 300 T batch [90]. According to the authors, the resulting pitch meets customer requirements and has a lower B[a]P content of 6.8 mg/g, which was 65% that of raw CTP for the proportion of 60 wt% CTP. However, it is not a rule that blends with a less toxic carbonaceous material, leading to the reduction of PAH emissions in a whole processing [93]. As reported by Sidorov [38], the addition of petroleum bitumen to CTP prior to thermal treatment increases the B[a]P content in exhaust fumes, rather than reducing it. The B[a]P content in the exhaust gases formed at 750–900°C during the carbonisation of analysed CTP (containing 0.95% B[a]P) amounted to 18.6 ng/g, and increased to 31.5 ng/g on adding 13.6 wt% of petroleum bitumen (containing 0.06% B[a]P) [82]. Another approach was applied by researchers who analysed the influence of the addition of carbon particulates on the thermal transformation of CTP. By adding 5% of solid modifiers like activated carbon, carbon nanotubes, graphite foam, and carbon nanotubes, a reduction in the quantity of volatile destruction products was obtained [94]. By using mass spectrometric analysis of the adsorption and condensation process, it was concluded that during the thermal treatment, an interaction of aromatic pitch molecules at the surface of carbonaceous particles takes place. Consequently, the reduction of toxic pitch volatiles, including B[a]P, occurs. This effect was the most effective for graphite foam additive; however, no quantitative results were reported.. 3.2. Compositions of CTP with polymeric material In comparison to the above-described method based on petroleum pitch addition, the modification with polymer materials leads not only to a dilution of CTP, but also to physical and chemical changes of its composition as a result of chemical reaction. Several investigations have been made to reduce CTP toxicity by macromolecular material addition, very often aimed at removing B[a]P.. 24.

(27) Zieliński et al. studied the effect of pitch modification with single polymers like thermoplastics [95], polysaccharides and elastomers [96], various polymeric wastes [95– 97], and blends of polymers and catalysts promoting the thermal decomposition of polymers and copolymers, e.g. butadiene and styrene [50]. As reported, the most promising results were obtained for the modification with 4.7 wt% poly(vinyl) chloride with catalyst (B[a]P reduced by about 54%) and 30 wt% of the residue from the glycol recovery step of polyethylene terephthalate waste (B[a]P reduced by about 90%). From the chemical point of view, the changes accountable for B[a]P decrease took place during the homogenisation stage (in case of polyester resin, B[a]P reduced by about 88%), with additional annealing not being as significant (in the case of polyester resin, after annealing at 140°C, B[a]P reduction of about 91% was achieved). It was also reported that due to modifications with polymers, the changes involved mostly the γ-resin. This is particularly interesting because most B[a]P are located in this group (see chapter 2.3.2), so these changes are associated with the reduction of its contents. The authors stated that B[a]P conversions were probably related to the presence of active functional groups ending polymer chains which cause a radical conversion mechanism due to alkylation reaction [32]. However, in the mentioned studies, the modified carbon precursor was designed for road binders, insulating materials and building sealants, etc. Hence, there were no specific requirements, typical for carbon binders for advanced synthetic carbon materials. Additionally, there are many disadvantages to those modification methods, i.e. the complexity of modification processes, and the necessity for huge doses and/or high temperature (mostly in the range from 120 to 270°C); in addition, some polymeric modifiers may generate secondary pollution, i.e. paraformaldehyde can be a source of poisonous methanol [54]. The effectiveness of the CTP modifications by polymers for applications as a binder for aluminium smelting electrodes was studied by Kaushik et al. [49]. In this work, the influence of propylene glycol (PG), unsaturated polyester resin (UPR), styrene and polyethylene glycols with different molecular weight (PEG) was investigated. The results of B[a]P content are presented in Fig. 4. As can be seen, the highest reduction in B[a]P content, of 81%, was obtained with unsaturated polyester resin. The B[a]P content changes were attributed to the alkylating efficiency of the modifier used. It was also reported that the CTP/polymer blend properties did not differ significantly with respect to the initial material, i.e. for the composition with 30 wt% unsaturated polyester resin, 25.

(28) there was an increase in SP by 20%, whereas the other parameters like QI, TI, CV did not vary at levels greater than 1%. An opposite tendency was reported by Brzozowska et al. [75], where the modification of CTP by 10 wt% cumarone-indene resin, polystyrene, poly(ethylene terephthalane), poly(vinyl chloride) and also unsaturated polyester resin and poly(ethylene glycol) significantly influences the physical properties of binders like SP, dropping the ability to penetrate, TI and other parameters. In this work, the influence of unsaturated polyester resin was not so effective; the B[a]P content reduced by only 7%, with a simultaneous decrease in SP of 13%. This reflects the high complexity of the whole modification process, which is dependent on the kind of modified material, the modifier and the conditions in which the process was carried out. As also reported, in comparison to parent CTP-coke, the compositions of pitch/polymer are characterised by a low homogenous optical texture of semi-coke. It means that the constituents segregation occurs with only the exception of poly(vinyl) chloride, which positively affects the development of anisotropy during carbonisation [75].. Fig. 4 Effect of different modifiers on B[a]P content [49]. 3.3. Modification of CTP by using non-carbon components It has been pointed out by Boyd at al. [98] that B[a]P may be converted into nontoxic compounds as some functional groups substitute its active position. A schematic example of such an enzymatic hydroxylation reaction is presented in Fig. 5. The molecular interaction of CTP toxic compounds was reported with a variety of solid ingredients.. 26.

(29) Fig. 5 Reaction of B[a]P causing conversion of carcinogenic component (III) into substituted non-carcinogenic component (IV) [98]. Reducing the toxicity of CTP may be closely linked to its decomposition during the thermal process. There are various additives used in carbon & graphite technologies, which cause an increase in CV and thereby reduce the emission of volatile products. Such additives include e.g. sulphur, p-toluene sulfonic acid, chloranil, anthraquinone [99], 9,10-bis(chlormethyl)anthracene [100], and iodine [101]. It seems that sulphur is the most effective additive among mentioned hydrogenating agents. Another possibility is the application of selective catalysts for the polymerisation of PAH mixtures, as reported by Bermejo et al. [102]. However, the use of an inappropriate amount of catalyst may cause a puffing phenomenon during graphitisation [99,103,104]. Other additives, defined as additives for catalytic carbonisation [105], such as aluminium chloride and iron(III) chloride, were also used to convert low molecular weight aromatic hydrocarbons to carbon to improve carbon yield, as reported in [103,106,107]. Although, such a modification leads to a higher CV and lower volatiles emission, the level of toxicity reduction was not studied. Wenchao et al. [108] evaluated the influence of potassium permanganate on PAH contents in medium-hard CTPs with SP(Mettler) 87.5°C and 101.5°C. As reported, the highest B[a]P-eq decrease was at the level of 42% in the first CTP and 63% in the second ones. The efficiency of this process was raised by an additional extraction in n-hexane. Additionally, it was roughly determined that the single PAH removal rate rises with increasing ring number, following the order of phenanthrene < fluoranthene < pyrene < B[a]P < benzo(b)fluoranthene < indeno[1,2,3-ed]pyrene < benzo[g,h,i]perylene. However, no technological properties of the product resulting from oxidation have been determined. The influence of catalytic additives i.e. cobalt nitrate, nickel chloride, iron oxalate, and ferrosilicon on the changes of B[a]P emissions during anode production was studied by Barnakov et al. [109]. It was shown that transition metal salts and ferrosilicon suppress the gas-phase emission of organic volatiles, which are captured in the carbon mass of the final product. The B[a]P emission was completely eliminated, in crude medium-hard 27.

(30) CTP, by ferrosilicon and during pyrolysis in the presence of 5 wt% of 3d-metal salts, added to the coke-pitch mixture. This approach had been patented, although was not commercially utilised.. 3.4. Physical modification of CTP The physical methods for diminishing the toxicity of CTP, mentioned in the introduction of chapter 3, are also widely considered as possible mechanisms for the manufacture of low-PAH content carbonaceous binders. These methods are mainly based on the thermo- and oxidative-treatment and its various combinations. In the literature, there are also descriptions of the positive effects of CTP treatment for B[a]P content reduction by ultrasound (600kHz/s) or by high-frequency currents (at ultrashort wavelengths); however, there are no detailed results [38]. Toxic PAH in CTP belongs to the lower boiling constituents (see introduction of chapter 2); therefore, it can be easily removed by a higher temperature distillation of raw materials or additional vacuum-distillation process. During the process, hazardous PAHs vaporise and finally undergo condensation, simultaneously increasing the SP (see Table 2 and Table 3) [53,80,85]. With increasing SP above 70°C and higher than 100°C, the reduction of B[a]P content by 60% [53] and 98.5% [85] was achieved. A much more popular method, used to improve the processing properties in the manufacture of carbon fibres and carbon-carbon composites, is thermooxidation under air, oxygen or ozone atmospheres. As a result of aromatic condensation and cross-linking reactions, the effectiveness of which depends on the chemical structure and oxidation conditions of CTP components, a similar effect to thermopreparation can be obtained, namely an increase of TI content, C/H ratio, CV and molecular size distributions [18,110–114]. As reported by Dominguez et al. [55] the air-blowing process at 275°C for 10, 18 and 25 hours resulted in the reduction of the B[a]P content on the level of 14.6%, 31.3% and 38.1%, respectively. The commercial example of CTP-based binders with low concentration of toxic constituents is Carbores P (Ruetgers Group, Germany). By the high-temperature treatment of the CTP under vacuum, the SP was increased to above 200°C and B[a]P content was reduced to the level of 300 ppm [115]. This material is often investigated in many types of applications and is also studied in more detail in this work (see chapter 7.3).. 28.

(31) However, taking into consideration the literature review related to the selection of the most effective way for the preparation of less toxic pitches, unequivocal assessment is not possible. One of the reasons is that the initial precursors are different. Some differentiation, without taking into account the toxicity problem, has been described in the work [116]. On the basis of TG results, the thermally-treated CTP still contained a considerable number of compounds with a low molecular weight, together with the large planar molecules generated. In contrast, during air-blowing, light components were polymerised to a considerable extent to form cross-linked molecules.. 3.5. Other applications of alternative binders Petroleum pitch is a frequently used component to modify CTPs, and also constitutes an alternative precursor for the preparation of carbon binder. It is less aromatic and consequently contains a low level of genotoxic PAHs. It is assumed that the PAH content in petroleum pitch is up to 10 times lower than in CTP, as defined in [117]. The B[a]P content in the initial pitch and in volatiles produced during HT were estimated to be 0.2% and 0.4%, respectively [90]. Because of the relatively low coking ability resulting in poor mechanical and physical properties of the carbonised and graphitised materials, a pure petroleum pitch is not applicable in its initial form, i.e., “as received compound”. On the other hand, due to its good rheological properties and low QI values, it is wellestablished as an impregnant for high-density products like arc furnace electrodes [53]. Nevertheless, recent investigations have shown that the petroleum pitch is specifically developed for a particular end user by e.g. catalytic modification, thermal or oxidation processes [118–120]. It can be transformed into optimum binders for anodes and electrodes [25,121]. In addition, the study [122] revealed that, in terms of oxidation behaviour, petroleum pitches appear to be a better alternative to CTPs for preparing magnesia–carbon materials due to a higher resistance to oxidation at 600°C. WT, which is generated as a by-product by the charcoal industry, is the subject of research of many scientists from countries where the availability of wood as a raw material is high, e.g. in Brazil. Prauchner and his group analysed the following aspects of eucalyptus tar pitches: elemental composition, chemical structure, chemical structural evolution thermal stability under carbonisation process [123–125]. The study showed that this raw material is characterised by low sulphur and ash content, is toluene insoluble, and that it is possible to achieve a similar SP to standard CTPs. Furthermore, it was claimed that, due to its much more aliphatic hydrogen content, in comparison to CTP, this 29.

(32) pitch is also less toxic. The authors suggest its application as a carbon fibre precursor and binder for carbon electrodes. Attempts at the use of the liquid products of wood pyrolysis with their solid products, charcoal, can be found in works [126,127], where pellets and biocarbon electrodes were prepared [128,129]. In the case of biocarbon electrodes, comparable properties for those products made from petroleum coke and pitch after graphitisation process were obtained. However, because of the publication time of mentioned works, when the toxicity aspects of the carbon binders were not as important, the experimental results do not include data relevant to harmful compounds. This subject is undertaken by the author of this thesis and was partially already published in work [130]. Part of the published results is also included in the experimental section. Beyond the wood pyrolysis products, binding materials may also be obtained from the chemical degradation products, which include cellulose, lignin and lignocellulose. Cellulose. and. its. derivatives,. such. as. microcrystalline. cellulose,. and. carboxymethyl cellulose, are widely used as binders; for example, in pharmacy for the preparation of medications [131] and in the production of lithium-ion batteries [132–134]. The use of lignin, in turn, is still the object of many researches. Its application in the manufacture of lithium ion batteries [135], metallurgical coke [136,137], and carbongraphite electrodes [138,139] were discussed. Cellulose and lignin were mainly used in a solid state and an attempt at their transformation into liquid form was discussed in publications [138,139]. The application of synthetic resins as carbonaceous binder precursors for carbonbased composites was also examined in detail. Tian et al. [140] and Zhou et al. [141] obtained cold ramming paste which can meet industrial requirements and even with superior physical properties, by using phenol-formaldehyde resin instead of conventional binders. According to the authors, no genotoxic PAHs were detected in the gas phase from the heated materials. Furan resin was studied to manufacture TiB2-C [142] cathodes and cold ramming paste [57] with satisfactory results. Based on PAH analysis, it was shown that the B[a]P-eq was only 4.3 times lower compared to typical CTP for cold ramming paste [57]. However, this resin contains some amounts of free phenol and formaldehyde, which are known to be toxic and irritating. Thus, eye and skin contact during tamping should be avoided. The examples of polyimide used for carbon-carbon composites were studied in works [143,144]. As shown, this very high thermally stable polymer losses only 27 wt% after carbonisation up to 1000°C in the compositions with 30.

(33) carbon-based fillers. Gaseous compounds evolved during pyrolysis were found to show low toxicity, although their specification was not shown. It should also be emphasised that phenolic resins are relatively more expensive than other binding materials, what makes it impossible to use in the production of conventional carbon & graphite artefacts [145]. The commercially available, new ramming pastes called NeO2 CleO2, which are currently products of Carbone Savoie, were introduced by Allard et al. [146,147]. These materials were tested at Emirates Global Aluminium (EGA) sites, which is one of the primary aluminium producers [148]. As reported, neither PAH nor BTEX could be measured in a crude paste and during baking up to 1000°C. Only dichloromethane was detectable, although at the detection limit. NeO2 was characterised by a higher apparent density, though it had a lower compressive strength, thermal and electrical conductivity, in comparison to CleO2 [148]. Another, non-conventional method is based on the thermo-chemical processing of a raw coal, which may be an alternative process instead of the highly toxic process of CTP production. By specific treatment of the structure of coal with various chemicals at elevated pressure, and temperature, and whilst changing other processing variables, the intermolecular bonds between the oligomers are destroyed, and coal extracts suitable for the subsequent processing to obtain a pitch binder can be manufactured. Rahman et al. [149] studied hard coal and they showed that such a coal at the intermediate stage of metamorphism is suitable for this process. Andresen et al. reported [16] that anthracite may also be used as a raw material. In all cases, it was possible to obtain CTP-like products,. with. good. processing. properties,. i.e.. with. SP. (determined. by. Thermomechanical Analysis) in the range between 100°C to 260°C [150]. Kuzentsov et al. [151] determined a reduced level of B[a]P (0.46-0.64% of pitch), which was two to three times lower than in CTP with a similar SP [53]. The B[a]P-eq of extracted pitches was determined on the level of 1.36-1.6% i.e., it was also lower than for CTPs, for which this level amounts to 2.5 to 4.5%.. 3.6. Summary The above-described analysis of the literature strongly emphasises the differences in characterising the toxicity of carbon binders. Some studies have investigated the content of B[a]P in the initial materials, while a significant part of the publications is based on analysing the content of the gases emitting during HT. Occasionally, some studies take into account both approaches. As reported by Andreikov et al. [82], “The 31.

(34) most important consideration is the content of toxic polycyclic aromatic hydrocarbons in the exhaust gases. These are often the primary emissions from pitch processing and must be regarded as a hazardous atmospheric pollutant. Thus, in our experiments, we measure the benz[a]pyrene content in the exhaust gases from the laboratory coking of hightemperature pitch…”. This approach is used in many works [152]. However, such a method does not take into account the differences in the conversion conditions of the whole HT of pitch (which often constitutes less than 20 wt% of the total green material). This applies to: . a significant decrease in the gaseous products as a result of a chemical reactions with the carbonaceous materials;. . the inhibition of gaseous products liberation by the porous structure of a material;. . a much longer time, often several dozen days of HT, which significantly changes the pitch degradation. Additionally, if the use of a material for the production of cold ramming paste is. intended, the B[a]P content in the raw material is particularly important. The discrepancies are also in work presented by Bechebner at al. [85]. It was proved that the release of hazardous PAHs can be prevented during the production stage to the level recommended by i.e. the healthy air regulations, actually in force in Germany, by appropriate gas processing equipment, which is currently required in many countries. Another point considered on the basis of the literature analysis relates to the evaluation of toxicity which frequently corresponds to the content of B[a]P, without analysing the other compounds, even more harmful, like dibenz[a,h]anthracene, which is included in B[a]P-eq. An inconsistency is also found in research devoted to synthetic polymeric resins in which the toxicity evaluation is performed regarding B[a]P and other PAHs, without taking into account the other hazardous compounds present in the proposed non-toxic binders that may be released from the raw material. It can also be mentioned that, in the case of CTP modified by chemical or physical methods, the level of B[a]P content reduction to levels required by current regulations (described in chapter 2.1) should be higher than 99%, which was not achieved in any of the published works. The solution to use petroleum pitch does not lead to the intended purpose as well. The only satisfactory outcome was obtained by Barnakov et al. [109] by 32.

(35) using the transition metal. However, the authors do not take into consideration the toxic PAH content in raw pitch and the changes of the functional parameters of binding properties and resulting properties of products obtained with the modified pitch due to relatively high additive content. To summarise this chapter, one can conclude that none of the proposed binders with reduced toxicity meet the requirements set by Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH), excluding the NeO2 and CleO2 pastes. Nevertheless, the technological requirements referring to cold ramming paste completely differ from those posed for other branches of carbon & graphite material products, e.g. carbon anode, graphitised cathode, carbon-carbon composite matrix, etc. For this reason, there is still a need to search for non-toxic binders that will satisfy the requirements related to the toxicity and physical and mechanical characteristics of the product for which it is designed.. 33.

(36) 4. Summary of the literature part Current issues regarding carbon binders bring up two basic questions. As a consequence, they generate a demand for the development of new, alternative precursors for carbon materials which could function as binders and saturants in both conventional and advanced carbon & graphite product technologies. The first of the issues is the deficiency of CTPs, which might be caused by four reasons: 1. Rise of more and more stringent, environmental requirements for coke plants, which in effect, leads to shutting them down. 2. Declining demand for metallurgy coke through PCI (Pulverised Coal Injection) technology implementation. 3. Modification of coke processing towards limiting it to two products only, i.e. coke and electrical energy, without capturing by-products such as coal tar. 4. Growing demand of the metallurgy market caused by continuous aluminium and steel annual production growth rate. The second issue is environmental requirements for the raw materials utilised in the technologies. It applies to the countries that are particularly pursuing environmental quality improvement - The United States and European countries are good examples. As PAH condensates, CTPs have received a lot of attention recently. Continuous tightening up of the environmental regulations by numerous organisations defining environmental standards, might, as a consequence, focus not only on reduction of harmful PAH compositions to the defined level but a total ban on using substances containing PAHs for any applications is to be expected. According to the applicable European Union regulations, a material is deemed to be genotoxic if the sum of the concentrations of PAHs,. such. as. benzo[j]fluoranthene,. benzo[a]anthracene, benzo[k]fluoranthene,. chrysene,. benzo[b]fluoranthene,. benzo[e]pyrene,. B[a]P,. and. dibenz[a,h]anthracene, exceeds 1000 mg/kg. B[a]P is a key compound with a maximum permissible concentration of 100 ppm. The research works on the development of binders with decreased levels of PAH are based on varied toxicity reduction methods. Based on the results mentioned in paragraph 3 it can be concluded that for CTP with B[a]P exceeding 1%, the dilution method cannot cause the concentration of B[a]P to reach 100 ppm. Accordingly, the required level can only be reached by changing the chemical composition by thermal or chemical preparation, namely by removing the most toxic aromatic hydrocarbons from. 34.