Effect of Crystallinity on the Wettability of Petroleum Coke by Coal Tar

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Effect of Crystallinity on the Wettability of Petroleum Coke by Coal Tar Pitch Arunima Sarkar, Duygu Kocaefe, Yasar Kocaefe, Dipankar Bhattacharyay, Dilip Kumar Sarkar, and Brigitte Morais Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00302 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 26, 2016

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Effect of Crystallinity on the Wettability of Petroleum Coke by Coal Tar Pitch Arunima Sarkar1, ([email protected]) (first author) Duygu Kocaefe1*, ([email protected]) Yasar Kocaefe1, ([email protected]) Dipankar Bhattacharyay1 ([email protected]) Dilip Sarkar1 ([email protected]) Brigitte Morais2 ([email protected]) 1

University of Quebec at Chicoutimi, Dept. of Applied Sciences, 555, boul. De l’Université,

Chicoutimi, Quebec, Canada G7H 2B1 2

Aluminerie Alouette Inc., 400, Chemin de la Pointe-Noire, C.P. 1650, Sept-Îles, Quebec,

Canada, G4R 5M9 ABSTRACT: Under-calcined coke gained interest as raw material for anodes used in aluminum production since it is reported in the literature that anodes produced with this coke might have lower CO2 reactivity in the electrolytic cell. For any anode-grade coke, it is important to identify a suitable pitch which will bond well with that coke during baking and yield dense anodes. The wettability of petroleum coke by molten pitch indicates the quality of bonding between them and influences the final anode properties. In this study, the effect of coke crystallinity on its wettability by pitch has been studied using the sessile-drop test. Also, the chemical and physical properties of coke and pitch have been studied using Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy

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(EDX), and scanning electron microscopy (SEM). It was found that the coke physical and chemical properties depend on its calcination temperature, and thereby greatly affect the wetting properties. The study showed that the wettability of coke by pitch increases with decreasing crystalline length. The presence of porosity, C=C bonds, C=O, COO, and heteroatoms (O and N) are the important factors which control the wetting of coke by pitch. Keywords: Petroleum coke; Coal tar pitch; Wettability; Contact angle; Coke-pitch interactions; Under-calcined 1. INTRODUCTION Petroleum coke and coal tar pitch are the principal raw materials for anode manufacturing. In most smelters, other carbon materials (rejected baked and green anodes, butts) are also added to the recipe for recycling purposes . In this work, since the objective was the study of coke with different crystalline lengths, no recycled material was used to eliminate their interference. The role of pitch is to bind the particles to each other in order to produce a good quality anode. The particulate material is also called the filler and the pitch the binder. Pitch is carbonized (producing pitch coke) linking the particles together during the baking process. Wetting, hence the contact, between coke and pitch plays an important role in attaining the desired anode quality with appropriate physical, chemical, electrical, and mechanical properties. Wettability studies usually involve the measurement of contact angles as the primary data, which indicates the degree of wetting when a solid and a liquid interact. Small contact angles (90°) correspond to low wettability. As first described by Thomas Young

1

in 1805, the contact angle of a liquid drop on an ideal solid

surface is defined by the mechanical equilibrium of the drop under the action of three interfacial tensions (Figure 1):


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γLG cos θ = γSG − γSL

(1)

where γLG , γSG, and γSL represent the liquid-gas, solid-gas, and solid-liquid interfacial tensions, respectively, and θ is the contact angle. γLG is also called surface tension. Equation (1) is usually referred to as Young’s equation. The work of adhesion (Wa) of coke-pitch systems is calculated as a function of the contact angle (θ) and the surface tension (γLG) using the following equation 1. = (1 + cos ) (2) Since the work of adhesion is the work per unit area of interface that must be performed to separate reversibly the two phases, it is a measure of the strength of binding between the phases. The condition for perfect wetting is given by = 2 . This means that the adhesion energy between coke and pitch should be equal to twice the surface tension of the pitch. Nevertheless, it is obvious that wettability is a property that depends not only on the binder pitch (surface composition, surface tension, viscosity) but also on the filler coke. Coke properties change with the calcination temperature. Calcination of coke is the heat treatment of green petroleum coke up to a temperature of approximately 1150-1250°C. Calcination of green coke removes the moisture and the volatile matter (hydrogen, methane, tar) to avoid cracking due to grain shrinkage and devolatilization during the baking of carbon anodes. Calcination is necessary to have good grain stability and to ensure the access of binder pitch to its pores 2. Temperature and soaking (i.e., residence time) have great impact on the calcined coke properties and quality. The X-ray diffraction (XRD) patterns are used to measure carbon crystalline size (Lc – average crystalline length and La – average crystalline thickness). The American Standard Test Method D 5187-91 describes the procedure used to determine Lc. A similar procedure is used to


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determine La. This technique allows measuring the carbon crystalline length Lc (002 band) and thickness La (100 band). Crystalline length is a measure of the rearrangement and alignment of the graphite planes which increases as the calcination temperature increases

3-11

. Oberlin (1984) described that during

calcination from 700°C to 1300°C, the interlayer defects of the basic structural unites of carbon reduce and crystallites start to grow larger, and, the crystallites are largest at the highest calcination temperature

12

. Coke CO2 reactivity decreases with increasing crystallite structure

due to increase in the size of crystallites 7. This decreases the number of highly reactive carbon atoms on the edge with respect to the carbon atoms in the crystal basal plane13. The calcination temperature and the heating rate influence the coke porosity. Fast volatile emission at high temperatures and fast heating rates increase the porosity 14, 15. Tran and Bhatia (2007) found that the void between carbon crystallites decreases with increasing calcination temperature. They explained that the area decreases due to decrease in voids as a result of increase in the graphitization level of coke at higher calcination temperatures 16. Various researchers reported a clear cut decrease in electrical resistivity of coke with increasing calcination temperature 7, 13, 17. Numerous authors have found that final real density of the coke is determined by the final coke calcination temperature and residence time

4, 18

In general, under-calcined coke has lower real

density and crystalline length compared to anode-grade standard calcined coke

3, 11, 19

shown in

Figure 2. The degree of calcination of coke influences the chemical, physical and mechanical properties of the anodes. Belitskus (1991) has shown that air oxidation rate decreases with increasing anode baking temperature and decreasing coke calcination temperature, but the effect is typically dependent on the anode baking temperature 7, 11.Also, the total anode consumption reduces with


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decreasing calcination level

17

. Under-calcined coke gained interest in anode production

technology as it is reported that anode produced from under-calcined coke has less and homogeneous reactivity in electrolysis. This resulted in lower carbon consumption and subsequently less dusting in the electrolysis cell

19, 20

. Under-calcined coke (low calcination

temperature) decreases the anode air and CO2 reactivities

11, 21

. This is explained with the fact

that pitch is more reactive than stardard calcined coke. Therefore, it is consumed faster than coke when it is in contact with air or CO2. Decreasing coke calcination temperature increases the coke reactivity, then the coke and pitch reactivities become comparable. This results in a more homogeneous reactivity within the anode and significantly reduces the dusting problem

11, 19-21

.

The effect of under-calcined coke is not significant if the anode baking temperature is too high. The CO2 reactivity of anodes always reduced with increasing baking temperature 11, 21, 22; but at higher baking temperatures, the difference between the reactivities of under and standard calcined coke becomes irrelevant. Also, the crystalline lengths of petroleum coke and pitch coke approach each other. The degree of calcination of coke significantly affects the baked anode density. Various authors stated that the use of undercalcined coke reduces the baked anode density 19, 21, 22; but Lhuissier et al. (2009) also mentioned that the difference in green density is mostly compensated by higher shrinkage rate during baking and results in a slight difference in the baked density 21. This fact is supported by another author 22. As the previous researches show, anodes produced from undercalcined coke could have a future in aluminum production; thus, it is essential to characterize the raw materials for their potential use. In this work, the wettability of coke, calcined at different temperatures, by coal tar pitch was studied using the sessile-drop technique at 170°C to comprehend the interaction between raw materials. A typical coal tar pitch was used as the binder, and four calcined cokes with different


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crystalline lengths (Lc) (from under-calcined to over-calcined) were used as fillers. The calcined cokes were prepared from the same green coke. The effect of the surface chemical composition, the inherent structure, and the level of calcination of four coke samples, calcined at different temperatures, on the wettability by pitch was studied. Different surface characterization techniques such as Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM) were used to establish the relation between surface characteristics of different calcined cokes and their wettability by the binder pitch. 2. MATERIALS AND METHODS 2.1 Materials In this study, four petroleum cokes, calcined at different temperature as received from the supplier, and one industrial coal tar pitch, were used for all the tests. The same green coke was calcined to different temperatures in order to produce the four coke samples with different Lc values. Neither the maximum temperature of calcination for four cokes nor the properties of the original green coke are available. The physical and chemical properties of the cokes and the pitch, as obtained from the supplier, are given in Tables 1 and 2, respectively. 2.2 Sessile drop system The sessile-drop system consists of a tube furnace (Thermolyne 21100), an Inconel tube with a pitch injection system, a graphite sample crucible, a digital video camera (B/W) (APPRO, model KC), and a secondary rotary vacuum pump (GE, Precision Vacuum Pump, Model D25) (see Figure 3. The detailed description of the equipment has been published previously 23, 24.


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The substrate coke powder was placed in the sample crucible. Coke particles were ground, and a particle size of -125µm was used. This particle size was also used by other researchers 25, 26. The particles were compacted in the sample crucible in order to have a smooth coke-bed surface. The experiments were conducted under nitrogen (N2) atmosphere. Pitch was placed in the injection chamber above the sample. This chamber has a small hole at the bottom and is located just above the coke sample during the experiment. The molten pitch droplet was directly deposited on the coke substrate by arranging the position of the injection chamber hole. The sample holder can be removed from the hot region of the furnace by using a specially designed mechanism, and the sample can be quenched for further analysis. A video of the drop was captured for 25 minutes. To measure the contact angle, the FTA 32 software was used. The sessile drop experiments were carried out at 170°C (typical average coke-pitch mixing temperature used in industry). For each experiment, the contact angle was taken as the average of the angles measured at two sides of drop. Each experiment was repeated twice. If the difference of contact angle between two experiments for the same coke-pitch pair differed by more than 5º, the experiment was repeated. 2.3. Characterization of different samples Wettability of coke by pitch is a combination of chemical and physical phenomena

23

. Thus, to

understand the wettability of the four coke samples by the same pitch, the coke samples and the pitch were characterized to identify their chemical functional groups (using FT-IR, XPS and EDX). The physical texture of the coke particles was studied using SEM, and the coke-pitch interface was studied using SEM as well as EDX. In this work, the focus is to identify the role of calcination level (Lc) of coke on the wetting behavior. As the same green coke was used to produce the four coke samples with different Lc values, the same pitch was used for all the


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experiments, and all the wettability experiments were performed under similar conditions, it was possible to study the effect of calcination level, which is related to Lc, on the wettability of coke. 2.3.1. Chemical analysis FT-IR analysis The chemical structures of the four calcined coke samples and the pitch sample were examined by FT-IR spectroscopy at room temperature. During wetting, the chemical functional groups of coke can interact with the complementary functional groups of pitch. The main objective was to identify the functional groups of the coke and pitch samples and study the possibility of new bond formation in the coke-pitch mixture. The IR spectra were collected in the wavenumber range of 500–4000 cm−1, and all the spectra were recorded at 4 cm−1 resolution. Each time, 20 scans were carried out prior to the Fourier transformation. All spectra were collected using a DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) technique (Perkin Elmer Instrument, Spectrum one). Each result was the average of four experiments. Calcined coke has lower absorption characteristics with respect to baseline levels

27

in KBr technique.

Hence KBr technique was not used. DRIFTS technique was used with an aperture mask of 2 mm diameter and a reflector angle of 16°. DRIFTS technique is commonly used for rough particles. All spectra were analyzed using the Spectrum version 5.0.1 software. The effective depth of the surface scanning was 0.5-5 microns. The raw diffuse reflectance spectra can appear different from its transmission equivalent. A Kubelka-Munk conversion was applied to the diffuse reflectance spectrum to compensate for the differences. The Kubelka-Munk conversion was done using the Spectrum version 5.0.1 software.


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XPS analysis The surface functional groups of the coke and pitch samples were studied by AXIS Ultra XPS spectrometer (Kratos Analytical) using Mono-chromate Al K[α] (hν = 1486.6 eV) source at a power of 210 W at the Alberta Centre for Surface Engineering and Science (ACSES), University of Alberta. The working pressure in the analytical chamber was lower than 2×10-8 Pa. The resolution function of the instrument for the source in hybrid lens mode was calibrated at 0.55 eV for Ag 3d and 0.70 eV for Au 4f peaks. The photoelectron exit was along the normal of the sample surface with an analysis spot of 400 × 700 µm2. During the analysis, a separate charge neutralizer was used to compensate sample charging. Survey spectra were scanned from binding energy 1100 to 0 eV and collected with an analyzer, a pass energy (PE) of 160 eV and a step of 0.35 eV. For the high-resolution spectra, the PE of 20 eV with a step of 0.1 eV was used. The XP-spectra fitting was performed using the CasaXPS software. The peak area was evaluated and scaled to the instrument’s sensitivity factors after a linear background was subtracted from each peak. The composition was calculated from the survey spectra by taking the sum of all peaks after scaling equal to 100%. High-resolution spectra were used to carry out the spectra fitting and component analysis. The analysed surface depth of the sample was 2-5nm. EDX analysis Using the EDX capability of the SEM equipment, the analysis of the coke, pitch, and coke-pitch interface was carried out. EDX can give an idea of the atomic or weight percentages of different elements present in a certain region. In EDX, the number and energy of the X-rays emitted from a sample is measured by an energy-dispersive spectrometer. The energies of the X-rays emitted by a sample depend on the characteristics of the difference in energy between two shells and the atomic structure of the emitting element. Thus, EDX can measure the elemental composition of a


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specimen 28. It is difficult to study the coke-pitch interface using FT-IR or XPS. EDX is a useful tool to analyze this interface. 2.3.2. Physical characterization SEM analysis The coke samples (125 µm) without polishing were mounted on an aluminum plate with a dimension of 20×30 mm using electrically conducting copper strips. Samples were cleaned using a high-pressure dust-removing air spray in order to remove surface dust and to have strong attachment with the copper strips. Then, the samples were vacuum dried for one day at room temperature prior to SEM analysis. Each coke sample was then sputtered with gold-platinum coating with a plasma current of 10 mA, a chamber pressure of 6×10−2 mbar, and a sputtering time of 140s using a Polaron Range sputter coater. The SEM was also used to study the interface of coke and pitch (a section across the sessile drop and the green anode). The coke-pitch drop obtained by the sessile-drop experiment was cut in a way that the interface could be studied in detail. The samples were polished using a variable speed grinder-polisher (Buehler, Ecomet 4 with Automet 2 Power Head assembly). Initially 340 and 500 grains of silicon carbide per square inch papers and then 3µ and 1µ diamond polishers were used. It was verified that there was no sharp edges which may cause the accumulation of charge during the SEM analysis. In addition, the samples were coated with gold-platinum to ensure that there is no charge accumulation during the SEM measurement. The SEM analysis was done by using JEOL-JSM-6480LV with secondary electron scattering and with a voltage of 20 kV and work distance (WD) of around 10 mm.


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3. RESULTS AND DISCUSSIONS 3.1 Wettability analysis The contact angle measurements of a pitch on a coke substrate are essential for understanding the spreading and penetration characteristics of that pitch, which indicates the degree of interaction between them and consequently the potential impact on anode quality. Figure. 4(a) compares the dynamic contact angles of the coal tar pitch and the four coke samples (from the same source) with different crystalline lengths. Coke 34Lc (over-calcined coke with the highest crystalline length), had a higher contact angle at all the times compared to Coke 24Lc (under-calcined coke with the lowest crystalline length). Coke 28Lc and Coke 30Lc had contact angles in between Coke 24Lc and Coke 34Lc. The contact angles measured for Coke 28Lc and Coke 30Lc were similar up to 80s; and then Coke 28Lc wets slightly better and the pitch completely penetrates at around 120 s whereas a complete penetration was achieved around 140s for Coke 30Lc. It is also clear from Figure 4(b) that there was almost 9° difference in initial contact angle for these two cokes, but soon after the initial few seconds, they approached together. For all cases, the dynamic contact angles of pitch reduced rapidly and penetrated completely between 110 s and 160 s. Figure 4(b) shows initial contact angle data for four different cokes. It is noticeably seen that Coke 24Lc shows the lowest initial contact angle, In the case of Coke 30Lc and Coke 34Lc exhibited the highest and similar value for the initial contact angle. The results also indicated that the initial contact angles for Coke 28Lc and Coke 30Lc differ by about 10°, but after a short period of time they approached each other. The work of adhesion of pitch-coke systems was calculated using the surface tension of pitch at 170ºC and the contact angles measured at 80 s. These conditions correspond approximately to the temperature and the residence time of an industrial kneader in which pitch and coke are


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mixed. The condition for perfect wetting was given by Wa=2γLG 29. This means that the adhesion energy between the coke and the pitch should be twice the surface tension of the liquid which is mainly due to cohesive forces. The surface tension of pitch at 170ºC was calculated using the FTA32 software as 39.33 dyne/cm. The work of adhesion of pitch with Coke 24Lc, Coke 28Lc, Coke 30Lc, and Coke 34Lc after 80s was 78.2, 75.91, 75.79 and 73.84 dyne/cm, respectively. Coke 24Lc had the strongest interface (strongest pitch-coke interaction, hence best wetting) among all the cokes studied. The work of adhesion decreased with increase in Lc values. It shows that the increase in calcination level (Lc) decreases the wettability of coke by pitch. This was further analysed based on chemical and physical characteristics of the coke samples and pitch. 3.2. Chemical analysis There are usually three kinds of chemical interactions possible between coke and pitch

23

. The

first one is hydrogen bond between the hydrogen atom attached to a highly electronegative atom (O, N) and an another electronegative atom (O,N). Thus, for example, the hydrogen of O-H group can form a hydrogen bond with functional groups containing oxygen (hydroxyl, ether, carboxylic groups) or nitrogen atoms (amine). The second type is acid-base interaction. In this case, acidic functional groups (carboxylic, phenolic) can interact with basic functional groups (amine). The third type of interaction is electrostatic in nature. In this case, the negatively charged pi electron cloud of aromatic rings can form electrostatic bonds with positively charged centres (quartenary ammonium ion). Thus, the objective of the chemical analysis is to find those pairs of functional groups in coke and pitch which can interact with each other.


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3.2.1. FT-IR analysis Petroleum cokes and coal tar pitches have considerable importance in anode industry since raw material properties contribute significantly to the final anode properties. A dependable characterization method is essential in order to determine the suitable coke-pitch pairs. This characterization is difficult as coke and pitch are complex materials containing a large number of constituent compounds with different chemical functional groups and molecular structure. FT-IR is one of the most versatile techniques to study the complex organic compounds such as coal, pitch, and green petroleum coke. It provides information on surface functional groups based on their bond energies and orientation of atoms in space. The results of the present study showed that FT-IR DRIFTS technique can also be used successfully for calcined petroleum cokes. A qualitative analysis was performed to identify surface functional compounds in four petroleum coke samples and pitch. Figure 5 and 6 show the FT-IR spectra of four coke samples, and the pitch sample respectively. Figure 5 shows that the FT-IR spectra of the four cokes were almost similar with regard to functionality although there were some qualitative differences. It can be noted that the functionalities identified using the IR spectra for calcined coke are in accordance with the compounds found using NMR technique by other researchers for green petroleum coke. As calcined petroleum coke has a very low percentage of H, no signal (other than the background) can be detected by the NMR study 30. These studies demonstrated that the green petroleum coke consists of polynuclear aromatic hydrogen-deficient structures with few alkyl side chains as substituents and polynuclear naphthenic molecules such as naphthalene, phenanthrene, anthracene, tri-phenylene, benzo-pyrene, coronene, and pyrene [29, 30]. Thus, the presence of aliphatic and aromatic functional groups in calcined coke may be the result of the presence of the


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polynuclear aromatic and natphthenic molecules in the green coke. Figure 5 shows a peak at 2151 cm-1 which may correspond to alkynes

31

or ketenes (C=C=O) 32. There are no references

available related to the presence of alkyne in coke 27, 32, thus, it is possible that the peak may be due to C=C=O (ketene). Figure 6 shows that all the coke samples contain CHar (around 3000 cm-1) and OH/NH (around 3250 cm-1) groups. Presence of C-O (hydroxyl or ether) groups can be found at around 1100 cm-1. At around 1630 cm-1, peaks from C=O are observed 33. Presence of both C=O and CO can indicate the presence of acidic carboxylic group 23. It is sometimes difficult to predict the presence of COOH as it can show a peak at 1725–1700 cm-1 which falls in the range of C=O (1900-1550 cm-1)

33

. Deconvolution of C1s peak in XPS may help identify the presence of

COOH. The cokes show the presence of substituted aromatic rings (790 cm-1). These substituted aromatic rings (especialy ortho-substituted rings) help in condensation and cyclization reactions. Thus, all four cokes contain functional groups, which can form hydrogen and electrostatic bonds, and undergo acid-base interactions. It is difficult to quantify the functional groups with the FT-IR results. Figure 6 shows the FT-IR spectra of coal tar pitch used. Coal tar pitch is composed of polycyclic aromatic hydrocarbons (PAH), polycyclic hetero aromatic compounds and their methyl derivatives. Also, alkylated PAH, PAH with cyclopenteno moieties, partially hydrogenated PAH, oligo-aryl methanes, hetero-substituted PAH:NH2, OH, carbonyl derivatives of PAH are present 30, 34, 35

. Figure 6 also shows the presence of aromatic rings (3046 cm-1), free moisture/OH (3452

cm-1), C=O (1910 cm-1) 33, C-O (hydroxyl/ether) (1100 cm-1), and substituted aromatic rings (741 and 825 cm-1). As mentioned earlier, the presence of both C-O and C=O shows the possibility of the presence of carboxylic group

23

. C=O can be associated to aldehyde, ketones, carboxylic


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acids, esters or anhydrides. It is possible that the the peak at 1910 cm-1 may correspond to C=O in acid anhydrides. The possibility of reactions between complementary groups can be explained with the following two examples. FTIR study revealed that OH and O/N are present in coke and pitch. H of OH in coke can form hydrogen bond with strongly electronegative O/N atoms of pitch. Similar reaction can happen for the OH group present in pitch. Pi-electron cloud of the aromatic ring can form electrostatic bond with electron-deficient centres such as N+ of quarternary ammonium group. It is difficult to know if N+ is present by the FTIR study. It is difficult to reach a conclusion regarding the wettability of coke by pitch only from the FTIR data; however, this information strongly establishes that there are a number of functional groups available in both coke and pitch which can react during mixing, consequently, coke can be wetted by pitch 27. 3.2.2 XPS analysis XPS technique was used to quantify different elements and functional groups present in coke and pitch samples. The chemical functionality of coke and pitch surfaces was identified by the FT-IR technique. During the XPS analysis, the information obtained from the FT-IR analysis was used to carry out the de-convolution of C1s peak

23

. Atomic percentages of different components of

the four calcined coke samples are presented in Table 3 for the survey spectra and de-convoluted C1s spectrum. The de-convoluted C1s spectra of the four calcined cokes and the pitch are presented in Figure 7 and 8, respectively. The deconvolution of the C1s high resolution XPS peak was done by centering the peaks for different functional groups at specific binding energy levels. C=C, C-C, C-O/C-N, C=O, and COO were set to 284.3, 285.1, 286, 287, 288.6 eV, respectively. It is evident from the XPS results that all the coke samples contained high amount of C=C and C-C bonds and trace amounts of oxygen, nitrogen, and sulfur components. C=C


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represents mostly the aromatic compounds which have negatively charged pi electron cloud. C-N represents mostly amines, pyridines or quarternary ammonium ions. The amines and pyridines are basic in nature due to the presence of lone pair of electrons with the N atom and can react with acidic functional groups. The quarternary ammonium ions can form electrostatic bonds with the pi electron cloud of aromatic rings. COO represents the acidic carboxylic group. Presence of C-O/C-N, C=O, and COO shows the possibility of formation of hydrogen bonds. On the other hand, the presence of heteroatoms such as O, N, which are strongly electronegative, indicates the possibility of the formation of hydrogen bond. Presence of N implies the possible presence of amines or pyridines, which are basic in nature. Table 3 shows that the coke and pitch samples contain C=C functional groups. The amount of C=C is maximum in case of coke 24Lc. The amount of C=C decreased with the increase in calcination level. Pitch contains N atom. Thus, there is a possibility of finding quarternary ammonium ions in pitch which can form electrostatic bonds with aromatic rings in coke. As Lc of coke increased, the availability of aromatic rings decreased (C=C decreased). Thus, with increase in Lc there is less chance of formation of electrostatic bonds. The CN/CO/CS do not show a clear trend to explain the wettability results. However, coke 24Lc has the highest amount of CN/CO/CS and shows the best wettability by the pitch. This can be explained in terms of formation of hydrogen bonds and acid-base interaction between coke-pitch pair. It may be noted that pitch has significant amount of acidic COO functional group. C=O shows a clear trend that matches with the wettability of coke-pitch pair. This can be explained in terms of formation of hydrogen bonds. Lower is the Lc, higher is the chance of formation of hydrogen bonds leading to better wettability.


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COO shows a clear trend that matches with the wettability of coke by pitch. This can be explained by acid-base interaction. Lower is the Lc, higher is the chance of acid-base interaction leading to better wettability. The percentage of O shows a trend that matches with the wettability results. The lower the Lc is, the higher the oxygen content is. During the calcination process, increase in calcination level increases the carbon content, and this results in the decrease of the heteroatom content. Coke 24 Lc with high oxygen content was best wetted by the pitch. On the other hand, Coke 34 Lc was least wetted due to the low level of O. N, S, and sum of O,N and S did not show the same trend observed during the wetting experiments. However, the sum of O and N decreased with increase in calcination level. The total of O and N (which are highly electronegative atoms) can lead to the formation of hydrogen bonds between coke and pitch. Thus, increase in O and N levels helped increase wetting through the formation of hydrogen bonds. This explains why increase in Lc resulted in a decrease in the wettability of the coke samples. 3.3 Physical characterization 3.3.1 SEM analysis of coke particles The structure of the coke plays an important role in wetting as well as in final anode quality. Structurally, sponge coke is preferred for anode production because it has a combination of low impurity levels, low air and CO2 reactivities, a moderate coefficient of thermal expansion (CTE), desired density, and enough open porosity to allow good interlocking and bonding with a binder pitch. The sponge coke structure is intermediate between the isotropic (fine grains) and anisotropic (coarse grains) coke 36, 37.


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Figure 9 shows the SEM micrographs of the longitudinal surfaces at a high magnification. This structure in calcined coke is commonly known as the lamella or flow structure. Figure 9 illustrates that how the alignment of the lammela/flow structure improves with the calcination level (Lc values). It can be seen that the lamella structure was rearranged with increasing calcination temperature up to a certain level (up to Coke 30Lc). Above this level, it is difficult to distinguish the change in structure from the images. Coke 30Lc and Coke 34Lc show a better organized structure (see Figures 9 (c) and (d)). A possible reason could be that the interlayer defects of the basic structural unit of carbon reduce, and the crystallites start to grow larger. As seen in Figure 10, the transverse surfaces of the coke samples give an impression about their mosaic/granular structure and their association with pores and other structure. This figure shows the formation of coarse grains with increasing crystalline length which is directly related to the calcination level. At the highest calcination level studied (Lc of 34Å), the crystallites are largest, and as the interlayer defects reduce, less amount of pitch can enter in them. There might be some correlation between the surface texture and wettability. However, it is difficult to conclude any specific correlation between coke Lc and wettability based on the SEM images (Figures 9 and 10). Figure 11 shows the surface porosity for the four coke samples studied. Figure 11 indicates that, with increasing calcination level the formation of micropores or inaccesible pores increased due to further devolatilization. It is possible that larger dimension of pores helped pitch to penetrate in greater amount, consequently contributing to a better pitch wettability for coke 24Lc, 28Lc, and 30Lc. On the other hand, micropores and microcracks were entrapped with air, and the presence of too many micropores created hindrance to the wettability by pitch. Also, it is necessary that the coke is wetted well by pitch so that pitch can enter into the coke pores. The


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stucture can thus influence the wettability by pitch. However, the scanning electron microscopy (SEM) can be used to visualize the surface texture of coke particles at nano-scale. SEM has a limitation in analyzing large coke surfaces. Coke is highly nonhomogeneous and SEM images may not represent the actual pore distribution in the bulk of the coke sample. 3.4. Interface analysis 3.4.1. SEM analysis Scanning electron micrographs of the cross-sectional view of the sessile drops for coal tar pitch on four petroleum cokes are presented in Figure 12. It is evident from the FT-IR and XPS results that coke and pitch had complementary functional groups which can lead to reactions at the coke/pitch interface

24

. The amounts of functional groups present at the coke surfaces studied

were different, and thus it is expected that the degree of reaction at the interface would also be different. The SEM images at higher magnification show some bright regions. It is possible that these bright regions represent the formation of a new reaction phases at the interface. It is difficult to directly confirm the presence of a reaction phase at a coke-pitch interface using an SEM image. It can be observed that the width of the bright region was maximum in case of Coke 24Lc and minimum for Coke 34Lc. If this bright regions represent the interface reaction zone, the width of the bright regions matches with the wettability results. The bright regions can appear due to reaction products or charge accumulation. As the samples were polished, they had no sharp edges and were coated with conducting material; hence, the possibility of charge accumulation is low. Thus, there is a good chance that these bright regions represent the reaction zones, which is an indirect proof of their presence. Thus, EDX was used to analyze the elemental composition of coke and pitch adjacent to a bright region for Coke 24Lc in order to verify whether these regions are the reaction products or not.


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3.4.2 EDX anaysis During the EDX analysis, it was assumed that if the elemental composition of the bright region differs from that of coke and pitch, there is a possibility that the bright spot is a reaction product. It should be noted that it is not possible to examine the reaction-induced zone in detail using EDX (see Figure 13); however, it gives an idea about the elements present in that zone. The EDX and XPS analyses of Coke 24Lc (see Figure 13a and Tables 3 and 4) and pitch (see Tables 3 and 4) indicate the presence of higher amount of carbon and lower amount of heteroatoms (oxygen, sulfur) compared to the reaction-induced phase of the Coke 24Lc/pitch interface. Therefore, this information suggests that there is a possibility of the formation of molecular species due to the interaction between the oxygen/sulfur-containing functional groups of the coke particles and pitch (see Table 4). Table 4 shows that the percentage of C is lower in the bright region compared to those of coke and pitch. On the other hand, O and N increases significantly in the interface. The elemental composition of the bright region differes from that of coke and pitch. Thus, it can be concluded that the bright region is the product of reaction between the surface functional groups of coke and pitch. As this EDX study shows that the bright regions are reaction products, the width of the bright regions in Figure 12 can be correlated to the intensity of interaction of the coke-pitch pair. The lower the Lc of the coke is, the greater the interaction is. A greater level of interaction means the wettability of coke by pitch is better.


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4. CONCLUSIONS There was a substantial difference between the final and initial contact angles of Coke 24Lc and Coke 34Lc. In general, all the cokes were wetted by the pitch, and all the coke/pitch pairs could be stated as wetting. In this study, the contact angle decreased with increasing time. This might be due to the fact that both the extent of reactions between coke and pitch and the penetration of pitch through the pores increased with time. The FT-IR spectroscopy results show that there is no significant difference in the chemical nature of the cokes despite a few qualitative variations, but the results indicated the presence of different complementary functional groups in coke and pitch. The presence of these complementary functional groups suggest that there is a possibility for the occurrence of reactions at the coke/pitch interface. XPS results show that all the cokes were significantly different from each other in terms of their chemical composition. Oxygen content in coke reduced as the crystalline length increased. Oxygen and nitrogen containing functional groups in calcined cokes might be the reason for better wettability as they can easily interact with those of pitch. Similarly, Coke 24Lc contained more carbon double bonds which are reactive. The aomatic C=C decreased with increasing calcination temperature. Coke 24Lc contained higher amounts of C=C, COOH, CN bonds compared to that of coke 34Lc. The presence of higher amounts of oxygen and nitrogen atoms might have played a significant role in wetting of coke by pitch. Micropores in cokes gradually increased with increasing crystalline length. This was probably another reason for the lower wettability of Coke 34Lc. In general, further devolatilization at higher temperatures creates porosity; but, probably most of them are micropores or inaccessible pores (see Table 1). Also, the porosity values of Coke 28Lc and Coke 30Lc are very close, and this may be the reason for them to approach each other during the wetting experiments. In


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addition, the SEM analysis shows that inter-crystallite defects decreased as the crystalline length increased. However, it is difficult to reach a conclusion based solely on the SEM images. It is quite evident that an SEM image of a coke which is highly nonhomogeneous and did not illustrate the actual pore distribution in the bulk of the coke sample, but shows the structural rearrangements occurred as the calcination temperature increased. The SEM images show the presence of reaction at the coke/pitch interface, and the degree of reaction decreased with increasing crystalline length. This study shows that under-calcined coke is most wetted by pitch and has a strong interface with pitch. Increase in Lc of coke decreases the wettability of coke by pitch. The presence of porosity, C=C bonds, C=O, COO,and heteroatoms (O and N) are important factors for better wettability of coke by pitch. The results show that under-calcined coke has potential for use in anode production. Since undercalcined coke is better wetted by pitch, good bonding will form between them. This is likely to result in good quality anodes. 5. AUTHOR INFORMATION Corresponding author Tel: +1 (418) 545-5011 ext 5215 Email: [email protected] Notes: The authors declare no competing financial interest. 6. ACKNOWLEDGEMENT The technical and financial support of Aluminerie Alouette Inc. as well as the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC), Dévelopment


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économique Sept-Îles, Université du Québec à Chicoutimi (UQAC), and the Foundation of Université du Québec à Chicoutimi (FUQAC) are greatly appreciated. 7. REFERENCES 1. Adamson, A. W., Physical chemistry of surfaces. John Wiley & Sons Inc: 1990. 2. Hulse, K. L., Anode Manufacture:Raw Materials, Formulation and Processing Parameters. R&D Carbon Ltd.: Sierre, Switzerland, 2000; p 416. 3. Neyrey, K., Edwards, L.,Marino, J., Observations on the coke air reactivity test. Light Metals 2013, 1051-1056. 4. Dreyer, C., Anode reactivity influence of the baking process. Light Metals. 1989, 595602. 5. Bopp, A. F., Groff, G. B.,Howard, B. H., Influence of maximum temperature and heatsoak times on the properties of calcined coke. Light Metals. 1984, 869-882. 6. Hardin, E. E.; Beilharz, C. L.; Melvin, L. L., Comprehensive review of the effect of coke structure and properties when calcined at various temperatures. Light Metals. 1993, 501-508. 7. Belitskus, D., Effects of petroleum coke calcination temperature and anode baking temperature on anode properties. Light Metals 1990, 557-563. 8. Belitskus, D., Danka, D. J., Comprehensive determination of effects of calcined petroleum coke properties on aluminum reduction cell anode properties. In Light Metals., 1989; pp 429-442. 9. Rørvik, S., Lossius, L. P., Ratvik, P.A., Determination of coke calcination level and anode baking level - Application and reproducibility of L-sub-c based methods. Light Metals. 2011, 841-846. 10. Dion, M. J., Darmstadt, H.,Backhouse, N.,Canada, M.,Cannova, F., Prediction of calcined coke bulk density. Light Metals. 2011, 931-936. 11. Brym, M. J. C., Gagnon, A.,Boulanger, C., Lepage, D., Savard, G.,Bouchard, G.,Lagacé, C.,Charette, A., Anode reactivity: Effect of coke calcination level. Light Metals. 2009, 905-908. 12. Oberlin, A., Carbonization and graphitization. Carbon 1984, 22, (6), 521-541. 13. Coste, B., Schneider, J. P., Influence of coke real density on anode reactivity consequence on anode baking. Light Metals. 1994, 583-591. 14. Hardin, E. E.; Ellis, P. J.; Beilharz, C. L.; McCoy, L., Comprehensive review of the effects of calcination at various temperatures on coke structure and properties part II. Light Metals. 1994, 571-581. 15. Hume, S. M., Fischer, W. K., Perruchoud, R. C., Welch, B. J., A model for petroleum coke reactivity. Light Metals. 1993, 525-531. 16. Tran, K. N., Bhatia, S. K.,Tomsett, A., Air reactivity of petroleum cokes: Role of inaccessible porosity. Industrial and Engineering Chemistry Research. 2007, 46, (10), 32653274. 17. Dreyer, C., Samanos, B., Vogt, F., Coke calcination levels and aluminum anode quality. Light Metals. 1996, 535-542. 18. Meisingset, H. C., Balchen, J. G.,Fernandez, R., Mathematical modelling of a rotary hearth calciner. Light Metals 1996, 491-497. 19. Sulaiman, D., Garg, R., Use of under calcined coke to produce baked anodes for aluminium reduction lines. Light Metals. 2012, 1147-1151.


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20. Tran, K. N., Berkovich, A. J.,Tomsett, A.,Bhatia, S. K., Influence of sulfur and metal microconstituents on the reactivity of carbon anodes. Energy and Fuels. 2009, 23, (4), 19091924. 21. Lhuissier, J., Bezamanifary, L., Gendre, M.,Chollier, M. J., Use of under-calcined coke for the production of low reactivity anodes. Light Metals. 2009, 979-983. 22. Samanos, B., Dreyer, C., Impact of coke calcination level and anode baking temperature on anode properties. Light Metals. 2001, 681-688. 23. Huang, X., Kocaefe, D.,Kocaefe, Y., Bhattacharyay, D., Wettability of bio-coke by coal tar pitch for its use in carbon anodes. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2016, 490, 133-144. 24. Sarkar, A., Kocaefe, D.,Kocaefe, Y.,Sarkar, D.,Bhattacharyay, D.,Morais, B.,Chabot, J., Coke–pitch interactions during anode preparation. Fuel 2014, 117, Part A, (0), 598-607. 25. Adams, A. N. Characterization of the pitch wetting and penetration behavior of petroleum coke and recycled butts in pre-baked carbon anode. Pennsylvania State University, Pennsylvania, 2004. 26. Rocha, V. G., Blanco, C.,Santamaría, R.,Diestre, E. I.,Menéndez, R., Granda, M., An insight into pitch/substrate wetting behaviour. The effect of the substrate processing temperature on pitch wetting capacity. Fuel. 2007, 86, (7-8), 1046-1052. 27. Menéndez, J. A., Pis, J. J., Alvarez, R., Barriocanal, C., Fuente, E., Díez, M. A., Characterization of petroleum coke as an additive in metallurgical cokemaking. Modification of thermoplastic properties of coal. Energy and Fuels, 1996, 10, (6), 1262-1268. 28. Goldstein, J., Newbury, D. E., Joy,D.C. , C.E. Lyman, D.C. ,Echlin, P., Lifshin, E., Sawyer, L., Michael, J.R., Scanning Electron Microscopy and X-ray Microanalysis. 3rd ed.; Springer Science & Business Media: USA, 2012. 29. Contreras, A., León, C. A., Drew, R. A. L., Bedolla, E., Wettability and spreading kinetics of Al and Mg on TiC. Scripta Materialia 2003, 48, (12), 1625-1630. 30. Suriyapraphadilok, U. Characterization of coal and petroleum derived binder pitches and the interaction of pitch/coke mixtures in prebaked carbon anodes. The Pennsylvania State University, Pennsylvania, 2008. 31. Schmittel, M., Strittmatter, M., Mahajan, A.A., Vavilala, C., Cinar, M. E., Maywald, M.; A., A. W., Steric effects in the thermal C2-C6 diradical cyclization of enyne-allenes ARKIVOC 2007, (viii), 66-84 32. Malekshahian, M., Hill, J., Chemical and thermal modification of petroleum coke. In 8th World Congress of Chemical Engineering, 2009; p 520. 33. Tsakalakos, T., Ovidko, I.A. , Vasudevan, I.A. , Nanostructures:Synthesis, Functional Properties and Applications Spinger. 34. Bo-Hye, K., Arshad, H. W.,Kap, S. Y.,Yun, H. B., Sung, R. K., Molecular structure effects of the pitches on preparation of activated carbon fibers from electrospinning. Carbon Letters. 2011, 12, (2), 70-80. 35. Díaz, C., Blanco, C. G., NMR: A powerful tool in the characterization of coal tar pitch. Energy and Fuels 2003, 17, (4), 907-913. 36. Edwards, L., Backhouse, N.,Darmstadt, H.,Dion, M. J., Evolution of anode grade coke quality. Light Metals 2012, 1207-1212. 37. Edwards, L. C., Neyrey, K. J.,Lossius, L. P., A review of coke and anode desulfurization. Light Metals. 2007, 895-900.


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Figures

Figure 1 Schematic representation of the wetting principle


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Standard Calcined Coke Real Density(g/cc): 2.00-2.05 Lc(Ǻ):25.3-32.7

Over Calcined Coke Real Density(g/cc): >2.08 Lc(Ǻ):>32.7

Under Calcined Coke Real Density(g/cc): 2.00-2.05 Lc(Ǻ):19.7-25.3

Figure 2 Ranges of under-calcined, standard-calcined and over-calcined cokes based on their real density and crystalline length.


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Figure 3 Schematic diagram of Sessile Drop Experimental Set-up at UQAC


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(a)

(b)

Figure 4. (a) Dynamic contact angles and (b) initial contact angles of pitch and four calcined cokes with different Lc


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Figure 5. FT-IR spectra of four calcined cokes with different Lc.


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Figure 6. FT-IR spectra of coal tar pitch


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(a)

(c)

(b)

(d)

Figure 7. De-convoluted C1s peak of the four cokes (a) Coke 24Lc (b) Coke 28 Lc (c) Coke 30 Lc (d) 34 Lc


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Figure 8. De- convoluted C1s peak of the coal tar pitch


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(a)

(c)

(b)

(d)

Figure 9. SEM images of the longitudinal surfaces of the four coke samples with different Lc values: (a) Coke 24Lc (b) Coke 28Lc (c) Coke 30Lc (d) Coke 34Lc


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(a)

(b)

(c)

(d)

Figure 10. SEM images of the transverse surfaces of the four coke samples with different Lc values: (a) Coke 24Lc (b) Coke 28Lc (c) Coke 30Lc (d) Coke 34Lc


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Figure 11 SEM images of the surface porosity of the four coke samples with different Lc values: (a) Coke 24Lc (b) Coke 28Lc (c) Coke 30Lc (d) Coke 34Lc


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(a)

(b)

Coke

Coke

Reaction Products

Pitch Reaction Products

Pitch

(c)

(d)

Coke

Pitch Coke Pitch Reaction Products

Figure 12. SEM images of coke/pitch interface (a) Coke 24Lc (b) Coke 28Lc (c) Coke 30Lc (d) Coke 34Lc


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(a)

(b)

(c)

Figure 13. EDX analysis of (a) Coke 24Lc, (b) coal tar pitch, and (c) reaction products at the Coke 24Lc/pitch interface


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Tables Table 1. Physical and chemical properties of coke Calcined Petroleum Coke

Properties

Coke 24Lc 0.893

Coke 28Lc 0.901

Coke 30Lc 0.909

Coke 34Lc 0.893

Sp. Electrical Resistance (ohm-inch)

0.044

0.0396

0.0367

0.0359

Real Density(g/cc)

2.038

2.058

2.075

2.070

CO2 Reactivity (%)

9.6

5.2

7.0

3.6

Porosity for 125µm Particle** (%)

8.1

6.5

6.1

4.9

24.14

28.14

30.73

34.14

Ash Content*** (%)

0.2

0.2

0.13

0.2

Moisture Content (%)

0.1

0.045

0.03

0.013

C (wt%)

85.55

92.23

90.53

88.25

N (wt%)

0.95

0.83

0.76

0.75

H (wt%)

2.19

1.66

1.05

0.9

S (wt%)

2.84

2.78

2.81

3.21

Bulk Density* (g/cc)

Crystalline Length(Å)

*Measured by ASTM D4292-10; **Measured by ISO 1014:1985; ***Measured on dry basis.


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Table 2. Physical and chemical properties of pitch Properties

Coal Tar Pitch

Ash at 900°C (%m/m)

0.12

β Resin (%m/m)

22.2

Density at 20°C (g/ml)

1.320

Quinoline Insoluble (%m/m)

6.9

Toluene Insoluble (%m/m)

29.1

Coking Value (%m/m)

59.1

Softening Point (°C)

119.6

Dynamic Viscosity 170°C (mPa.s)

1390

Surface Tension (dyne/cm) at 170°C

39.33


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Table 3.Atomic percentage of different components of the four different calcined cokes and the coal tar pitch

Coke Type

C(%)

Carbon components C=C

C-C

CN/CO/CS

C=O

O(%)

N(%)

S(%)

O+N (%)

O+N+S (%)

COO

Coke 24Lc

94.2

79.99

7.57

5.84

3.66

2.94

3.23

1.86

0.70

5.09

5.79

Coke 28Lc

95.52

76.43

17.14

3.18

1.63

1.62

2.85

1.3

0.63

4.15

4.78

Coke 30Lc

96.69

74.50

18.68

4.10

1.26

1.46

2.43

0.06

0.36

2.49

2.85

Coke 34Lc

96.94

67.44

26.43

4.92

1.21

-

1.99

0.43

0.72

2.42

3.14

Pitch

97.77

74.48

20.72

2.74

-

2.06

1.16

1.07

2.23

2.23


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Table 4. Atomic percentages of reaction products formed at the Coke 24Lc/pitch interface and 24Lc coke Coke 24Lc/Pitch Coke 24Lc

Pitch interface

Element

Atomic %

Atomic %

Atomic %

C

97.27

95.61

81.04

O

2.29

4.39

16.04

Si

-

-

1.59

S

0.44

-

0.72


Effect of Crystallinity on the Wettability of Petroleum Coke by Coal Tar

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