Characterization of carbon anode protected by low boron level: An attempt to understand carbon−boron inhibitor mechanism

Qu Ramzi Ishak,†,‡ Gaetan Laroche,† Jean-Francois Lamonier,§ Donald P. Ziegler,∥ and Houshang Alamdari†

Dép †Department of Mining, Metallurgical and Materials Engineering, Laval University, 1065 avenue de la Médecine, Québec City,Québec G1V 0A6, Canada‡NSERC/Alcoa Industrial Research Chair MACE3 and Aluminum Research Centre-REGAL, Laval University, 1065 avenue de laMédecine, Québec City, Québec G1V 0A6, Canada§UMR 8181, UCCS, Unitéde Catalyse et Chimie du Solide, Université de Lille, CNRS, Centrale Lille, ENSCL, Universitéd’Artois,F-59000 Lille, France∥Alcoa Primary Metals, Alcoa Technical Center, 859 White Cloud Road, New Kensington, Pennsylvania 15068, United States

ABSTRACTSeveral chemical reactions occur during the electrolysis of alumina in the Hall−Heroult process resulting in a significant overconsumption of the carbon anode. Carbon oxidation with oxygen is one of these reactions. The inhibition of this reaction by the application of protective layers on the anode can be an effective technique to reduce carbon consumption. Boron impregnation was shown to suppress this reaction. In this study, very low boron content has been impregnated on the anode, and several characterization methods have been performed to understand the protection mechanism of such a low boron concentration during the oxidation reaction of anodes. An air reactivity test of boron-impregnated anodes has been performed at temperatures between 400 and 600 °C. The samples were characterized using XPS, Raman spectroscopy, XRD, XRF, porosimetry, and thermogravimetric analysis (TGA). TGA revealed that the total number of interactions between oxygen atoms and carbon active sites was reduced, decreasing the pre-exponential factor. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) has been employed as a highly sensitive surface characterization method to identify chemical forms of boron on the anode. It has been confirmed that boron blocks active sites of carbon by creating boron−carbon bonds, thus reducing the interaction of carbon active sites with oxygen.

KEYWORDSAir reactivity, Carbon anode, Impregnation, Boron oxide, ToF-SIMS, Carbon gasification, Aluminum production

CITATIONIshak, R., Laroche, G., Lamonier, J. F., Ziegler, D. P., & Alamdari, H. (2017). Characterization of Carbon Anode Protected by Low Boron Level: An Attempt To Understand Carbon–Boron Inhibitor Mechanism. ACS Sustainable Chemistry & Engineering, 5(8), 6700-6706.

This is the author’s version of the original manuscript. The final publication is available at ACS Link Online via DOI: 10.1021/acssuschemeng.7b00945

1 INTRODUCTION

Inhibiting the reaction of carbon with air or CO2 is of great interest in a number of industrial applications. One of these applications is in the aluminum smelting process where carbon blocks are used as anodes in the electrolysis cell, operating at 960 °C. Anodes are consumed during electrolysis, providing part of the energy for the electrolysis. According to the stoichiometric reaction of aluminum reduction, 334 kg of carbon is theoretically required to produce 1 t of aluminum.1 However, at high temperature, the anode is also reacting with air and CO2, resulting in a considerable overconsumption of carbon, with high-performance plants consuming about 415 kg of anode per 1 t of aluminum.1 Any solution to mitigate this overconsumption is of economic and environmental importance.

Aluminum is conventionally produced by the Hall−Heroult process, involving the electrolysis of alumina, dissolved in liquid cryolite, a molten salt acting as an electrolyte. Prebaked carbon blocks are suspended in the electrolyte, acting as a consumable anode, being replaced every 25 days.2

The anode is made by mixing petroleum coke with coal-tar pitch to form a paste, vibro-compacting or pressing the paste into the form of large green blocks, followed by baking the green blocks to decompose the binder and to increase its mechanical strength.3 Once in the electrolysis cell, a temperature gradient is established between the top and bottom of the anode: the bottom, being immersed in electrolyte at 960 °C, and the top, being exposed to air at ∼500 °C.4

Several attempts have been made to provide a physical barrier on the external surface of the anode, aiming at protecting it against air burning.5,6 Most of these barriers are alumina-based coatings and cannot efficiently protect the anode. In the same context, covering the anode with a mixture of alumina and crushed solidified electrolyte is practiced in almost all smelting plants. This cover may reduce the air-burning rate by generating an oxygen-diffusion layer around the anode, but it has no effect on the CO2 reaction.

Another strategy to protect carbon is to decrease its intrinsic reactivity by the doping of the anode with boron.7,8 Boron has been reported to suppress the oxidation of graphite-based composites.9,10 Some very useful insights are available in the literature about the effect of boron on the graphite oxidation reaction.10−15 Basically, three mechanisms are proposed to explain the effect of boron on the carbon oxidation reaction, which are summarized as follows.

1.1 Inhibition of the reaction by the redistribution of electron densities on graphite

The substitution of boron in the graphite structure results in electron transfer between carbon and boron (boron being more electronegative) and modifies the distribution of π- and σ-electrons. Radovic et al.10 reported an excellent fundamental analysis of this effect, and their experimental results showed that boron has two opposite effects on the carbon oxidation reaction; at low doping levels, it increases the C + O2 reaction while at high doping levels (1.9 wt %), it inhibits this reaction. Supported by quantum chemistry simulations, they concluded that the electron transfers from carbon to boron and reduces the electron density on the edge-carbon atoms. This results in the reduction of O2 adsorption on the graphite surface. On the other hand, upon the adsorption of O2, boron induces the type b distribution of π-electrons and hence weakens C−C bonds and strengthens C−O bonds. This phenomenon may make the desorption process of the reaction products from the surface difficult, thus decreasing the reaction rate.11 A number of other authors9,12 have also shown the latter effect by means of measuring the activation energy of the C + O2 reaction.

1.2 Effect of Boron on the Graphitization Process.

Boron catalyzes the graphitization reaction.10,12,13 Radovic et al.10 associated this phenomenon with the effect of boron substitution on free valence (FV) data. They obtained the FV data by means of different quantum chemistry models and showed that the FV data increase in the presence of boron promoting the graphite crystallization. Larger graphite crystals with higher

crystallite height (Lc) values (crystallite height along c-axis) exhibit less reactivity with respect to oxygen. This is due to the decrease in the total number of accessible surface-active sites, which are essentially located at the edge of the graphite crystallite.1.3 Formation of Boron Oxide Film and Blockage of Active Sites.

The principal hypothesis behind this postulate is that, as graphite oxidizes, the concentration of boron on the surface may increase, and in the presence of oxygen, it transforms to B2O3.9,16,17 It is thus believed that the B2O3 layer provides an oxygen-diffusion barrier reducing the C + O2 reaction rate. As an example, Kowbel et al.9 investigated the oxidation rate of graphite doped with boron by ion implantation. An extensive surface analysis of samples by XPS confirmed the presence of different boron species (mainly B4C, with some other unidentified peaks attributed to metastable boron carbides) on the surface of ion-implanted samples. The surface composition of the sample after isothermal air oxidation at 500 °C showed that the boron is essentially in the form of boron oxide. Results of kinetic oxidation experiments showed that the ignition temperature of carbon increases by 150 °C by boron doping and that the reaction rate is slow at temperatures lower than 750 °C. However, beyond 750 °C, the reaction rate of doped samples is even higher than those of the undoped samples.

In most reported works on carbon composite fields, the boron addition level is too high (from 1000 ppm up to several atom %). This high level of boron addition is not allowed in anodes, as the boron will most likely reduce in the electrolysis cell and enter the aluminum, causing serious problems in downstream operations. In addition, the anode baking temperature is much lower than the graphitization temperature, and thus no significant graphitization occurs during baking. In boron-doped graphite, boron is always added in the form of elemental boron under inert atmosphere prior to graphitization. Considering the cost of elemental boron, such a high level of addition is justified only for very high-value products.

Impregnation of the anode by a boron-containing solution seems to result in a good protection of the anode with low boron uptake levels. In this context, Moltech and Alcan 18−22 claimed a process of the impregnation coating of boron oxide to protect the anode. The experimental results conducted by Moltech in the laboratory seem to confirm the beneficial effect of this impregnation. Such a positive effect has also been confirmed by other authors.5,23 Contrary to that of the carbon composite materials, the boron uptake level in the anode must be kept extremely low. With the amount of anode required to produce aluminum taken into account, the level of boron in aluminum will be 0.415 times that of boron in anode. Thus, the tolerated level of boron in the anode should be low (order of ppm).

The protection mechanism of such a low boron level on carbon gasification has not been addressed in the literature. Considering the experimental results reported by Radovic et al.,10 the low boron levels should even catalyze the carbon gasification reaction. It is thus the objective of this work to reveal the effect of boron impregnation on the anode gasification reaction. For this purpose, several characterization methods for carbon-based anodes have been used, especially time-of-flight secondary ion mass spectroscopy because of its high sensitivity for trace elements or compound analysis, on the order of ppm to ppb levels.

2 EXPERIMENTAL SECTION

2.1 Sample preparation

Sample Preparation. Anode samples were first prepared by the mixing of calcined petroleum coke particles (85 wt %) and coal-tar pitch (15 wt %) at 185 °C. Table 1 describes the different coke particle fractions, which were used to prepare anode samples, as described in ref 24. The mixture was then pressed at 150 °C under 60 MPa uniaxial pressure and baked. Baking was performed in a box furnace using the following baking program: from room temperature to 150 °C

with a heating rate of 60 °C/h, from 150 to 650 °C with a heating rate of 20 °C/h, and from 650 to 1100 °C with a heating rate of 50 °C/h. Once the target temperature (1100 °C) was reached, the sample was kept at this temperature for 20 h, and furnace-cooled by the switching off of the furnace.

Table 1. Particle Size Distribution of Calcined Coke (wt %) Used for the Preparation of the Dry Mixture Used for the Fabrication of Prebaked Anodes

The baked anode was crushed and milled for 10 min using a high-energy ball mill, resulting in an average particle size of 34 μm. The particles were then impregnated in a solution of boric acid. The impregnation was performed by the anode particles being mixed with a boric acid solution followed by the mixture being dried. Two boron uptake levels of 62 and 312 ppm were obtained. The samples were labeled anode xB, where x represents the boron content in ppm.

2.2 Sample Characterization.

The level of impurities of the starting samples was determined by X-ray fluorescence spectroscopy (XRF; Axios mAX, Panalytical) according to the standard test method ASTM D4326-06. The mean crystallite height (LC) of the samples was determined by X-ray diffraction (PW 1800, Phillips, Germany), applying the ISO 20203 standard method, which is frequently used in the aluminum industry for the characterization of carbonaceous materials.

For an evaluation of the specific surface area (SSA), between 1 and 3 g of material was first degassed at 200 °C for 4 h under nitrogen atmosphere and then analyzed using a gas adsorption analyzer (Micromeritics, Tristar II 3020). Surface measurements were performed using N2 adsorption at −196 °C. Raman spectra were recorded using a LabRam 800-HR spectrometer (Horiba Jobin-Yvon, France) coupled to an Olympus BX 30 fixed-stage microscope. An internal He−Ne laser (Melles Griot, Carlsbad, CA) set at 633 nm (red line) was used for the acquisition of all the spectra. The laser beam was focused using a 100× objective (Olympus). Before the analysis was started, the Raman spectrometer was calibrated with a silicon line at 521 cm−1. Between 1 and 3 g of material was analyzed. The spectra were recorded in the range 150−2500 cm−1, covering first-order bands. The acquisition time was between 2 and 3 min. All the spectra were analyzed using GRAMS/AI software (version 8.0).

X-ray photoelectron spectroscopy (XPS) experiments were performed using a KRATOS Axis-Ultra spectrometer equipped with a monochromatized aluminum source (Al Κα = 1487 eV) and charge compensation gun.

Time-of-flight secondary ion mass spectroscopy data were acquired using a ToF-SIMS5 spectrometer (ION-TOF GmbH, Germany) equipped with a bismuth liquid metal ion gun. The samples were bombarded with a pulsed Bi+ primary ion beam (25 keV, 1 pA) rastered over a 500 μm × 500 μm surface area. With a data acquisition of 100 s, the total fluence does not amount up to 1012 ions/cm2 ensuring static conditions. Charge effects were compensated by means of a 20-eV pulsed electron flood gun.

Air reactivity tests were performed using a thermogravimetric analyzer (NETZSCH STA 449 F3, Germany). To eliminate the mass transport limitations, the reactions were performed under the chemical regime using appropriate particle sizes, sample mass, and gas flow rates, as already reported in ref 25. A 2 mg sample was loaded in the reaction cell, and the reactivity tests were conducted under isothermal conditions in flowing air at 100 mL/min. The samples were protected from oxidation during the heating period by pure nitrogen with a flow rate of 100 mL/min. When the desired temperature was reached, the sample was held at temperature for 10 min and then a

vacuum pump applied to evacuate the sample chamber to a pressure of 1 mbar. Airflow of 100 mL/min was then introduced into the reaction chamber. The sample was maintained at this temperature for 7 h, followed by cooling to room temperature. During the cooling period, the sample was also protected from oxidation using nitrogen. The use of vacuum prevents the dilution of reactant gas with the nitrogen stream. The sample mass loss was monitored over a range of temperatures from 400 to 600 °C. This temperature range is analogous to the conditions in which the anode is exposed to the air atmosphere during the aluminum smelting process. The percentage of gasification was calculated according to the following:

X ( t )=(1−M (t )

M ( t0 ) )×100

(1)

where M(t) is the mass at time t, and M(t0) is the initial mass of the sample.

3 RESULTS AND DISCUSSION

Table 2 gives the chemical composition as well as the crystallite height of graphite (LC) of the samples. The data confirm that the boron impregnation has no significant effect on the composition (except for boron), and the variations lay within the experimental errors of the analytical measurements.

Table 2. Chemical Composition of the Samples Measured by XRF and Crystallite Height (LC) Measured by X-ray Diffraction

The gasification percentage of the samples as a function of the reaction time for three different temperatures is shown in Figure 1. Each data point is an average of three independent experiments. These results revealed a remarkable effect of low-level boron impregnation on inhibition of sample gasification. Samples were tested at 3 different temperatures (475, 525, and 600 °C) to evaluate the kinetic parameters of the reaction. The curves show that the reaction rate decreases by the increase of the boron concentration. The resistance against air burning of the anode 312B sample was almost doubled compared to that of the anode 0B sample.

Figure 1. Experimental gasification reaction versus reaction time, at three temperatures; analyses were performed on a TGA. Each data point is an average of three experiments.

The kinetic parameters were calculated at 20% gasification, using the Arrhenius equation (eq 2). Table 3 shows the activation energy and the pre-exponential factor obtained for the three samples. Up to 62 ppm, the boron addition does not seem to affect the activation energy. However, by increasing the boron loading, the activation energy decreases in a meaningful way. This observation is in accordance with that reported by Radovic et al.,10 demonstrating the catalytic activity of boron on carbon gasification.

r=A exp(−EaRT )Here, the following abbreviations apply: r is the apparent reaction rate at a specific gasification

percentage, A is the pre- exponential factor, Ea is the apparent activation energy, R is the universal gas constant, and T is the temperature.

Table 3. Results from the Application of Arrhenius Formula to the Three Samples at 20% of Gasification

At first glance, one may expect an increase in the reaction rate by decreasing the activation energy. The opposite observation suggests that the pre-exponential factor plays an important role in the overall reaction rate. The pre-exponential factor decreases monotonically with increasing boron loading. To rationalize these parameters, one can consider the pre-exponential factor as the total rate of interactions between carbon active sites and the reactant species (oxygen for instance), and the exponential factor as the fraction of these interactions leading to the reaction. When the boron loading increases, the pre-exponential factor decreases, indicating that the rate of interactions between oxygen and the carbon active sites decreases. In other words, the number of active sites seems to decrease by boron impregnation. The decrease of the number of active sites could be due either to the decrease in the specific surface area (SSA) or to the occupation of these sites by boron.

The nitrogen adsorption isotherms of both untreated and impregnated (312 ppm) samples are shown in Figure 2, and the corresponding BET surface areas are reported in Table 4. Both samples exhibited type II adsorption isotherms with very small hysteresis, which are the typical patterns for the condensation of nitrogen in porous solids with small pore size. The isotherm of the impregnated sample (dried at 120 °C, Figure 2a) shows that this sample adsorbs a lower quantity of nitrogen in all pressure ranges, and the calculated SSA is 5.8 m2/g, which is significantly lower that of the unimpregnated sample (14 m2/ g). Such a behavior was also observed by Allardice et al.26 who reported that boron doping on graphite decreases the specific surface area via the formation of HBO2. Since we first heat- treated the samples at 525 °C before the gasification tests, the adsorption isotherms were also obtained for the samples after a heat treatment at this temperature. As can be seen in Figure 2b, both heat-treated samples exhibit type II adsorption isotherms, almost superposed, suggesting that the effect of boron impregnation on nitrogen adsorption is practically neutralized by heat treatment. The SSA was seen to be almost totally recovered after heat treatment (Table 4).

For evaluation of the effect of heat treatment on the state of the loaded boron, Raman spectroscopy was performed on the impregnated samples before and after heat treatment. Because of the very low loading levels of boron, the corresponding Raman peaks were too weak to be clearly distinguished. Thus, a sample with higher boron loading of 1000 ppm was prepared and heat-treated prior to Raman spectroscopy. As seen in Figure 3, the spectrum of the impregnated

anode sample (before heat treatment) presents typical boron hydroxide (or boric acid) peaks at 210, 500, 880, and 1166 cm−1.27 These peaks disappear after the heat treatment, and boron hydroxide changes to boron oxide represented by a new peak at 808 cm−1.28,29 It can be speculated that boron first impregnates in the form of hydroxide and reduces the SSA by blocking the pores. During heat treatment, boron hydroxide is transformed to boron oxide with much higher density (175%), accompanied by significant transformation shrinkage leading to an unblocking of the blocked pores. These data suggest that the modification of SSA by boron impregnation cannot be considered as a parameter affecting the number of active sites.

Figure 2. Adsorption−desorption isotherms (BET method) of a (a) dried anode sample and a (b) heat-treated anode sample.

Figure 3. Raman spectrum of an anode sample treated with 1000 ppm B at 25 and 225 °C.

Table 4. Specific Surface Area (BET Method) of Untreated Anode and Anode 312B Samples before and after the TGA Experiments

For verification of the second hypothesis, i.e., occupation of active sites by boron, the adsorption of oxygen on both untreated and impregnated (312 ppm) samples was quantified by TGA. TGA analyses were performed on a large sample (1 g) to measure the mass gain and loss

due to oxygen adsorption and desorption. The samples were heated from room temperature to 525 °C with a heating rate of 20 °C/min under nitrogen atmosphere, followed by a 30 min soak at 525 °C. Then, the temperature was decreased to 100 °C with a cooling rate of 20 °C/min and maintained at 100 °C for 50 min. After this degassing step, the nitrogen atmosphere was changed to pure oxygen for 30 min at 100 °C to saturate the surface by oxygen. Figure 4 shows the mass gain of the samples as a function of time. The mass of both samples increased because of the oxygen adsorption, stabilizing after 40 min. It is seen that the mass gain of the untreated sample (0.0085%) is almost twice that of the impregnated sample (0.0040%). This mass gain represents 5.3 and 2.8 μM/g of oxygen, respectively, for untreated and impregnated samples. This result shows that boron impregnation considerably reduces the oxygen adsorption sites on the anode sample, possibly by occupying the carbon active sites.

Figure 4. Mass gain as a function of reaction time (under pure oxygen atmosphere) of the anode 0B and anode 312B samples.

An XPS experiment was carried out to state the chemical form of boron on the surface of the impregnated anode. B 1s binding energies (BEs) of boron and boron oxide are expected to be located between 182 and 192 eV.30 However, no photopeak in this BE range was observed. This result could be explained because of the very low concentration of boron in the samples and the low sensitivity factor for this element (0.159). In comparison with XPS, higher surface sensitivity can be obtained using ToF-SIMS analysis. Indeed, the detection limit of XPS is at best 0.1 atom % for boron, which is much worse than the ppm level detection of ToF-SIMS.31−35

Figure 5 shows partial positive ToF-SIMS spectra of the anode 0B (dashed line) and anode 312B (green line) samples. The analysis in the 9−12 m/z range allows for the highlighting of the presence of boron in the anode 312B sample while no peak was observed for the anode 0B sample. The spectrum of the boron-impregnated sample exhibits two peaks: one at m/z = 11 and the other at m/z = 10, the isotope of boron with a ratio of 20%.36

The negative ions have also been detected in the 25.5−28 m/ z range for the untreated and treated anodes (Figure 6). The peaks at m/z = 26 and m/z = 27 are assigned to 12CN− and 13CN−

species. In comparison with the m/z = 26 peak intensity, it can be seen that the intensity of the m/z = 27 peak for the treated anode is higher than that for the untreated one. In ToF- SIMS analyses, some of the fragment ions that form different molecules can have the same or very similar masses, which can cause peak overlapping. The increase in the m/z = 27 peak intensity could be explained by the contribution of BO− ions overlapping. The ratios of peak areas (13CN−/12CN−) are 1.5% and 7.3%, for the anode 0B and anode 312B samples, respectively (Table 5). Considering that the ratio of carbon isotopes (13CN−/12CN−) is 1.0%, the BO− peak most probably overlaps with the 13CN− peak in the anode 312B sample.37

Figure 5. Partial positive ToF-SIMS spectra of the anode 0B sample (dashed line) and anode 312B sample.

Figure 6. Partial negative ToF-SIMS spectra of the anode 0B sample (dashed line) and anode 312B sample.

Table 5. Area Values of the m/z = 26, 27, 42, and 43 peaks

Figure 7 shows the negative ToF-SIMS spectra of the anode 0B sample (dashed line) and of the anode 312B sample (green line) in the 41.5−43.5 m/z range. Since the representative isotopic compositions for boron 10B and 11B are 19.9 and 80.1 mol %, respectively, the negative ion peaks at m/z = 42 and m/ z = 43 can be attributed to the presence of 10BO2

− and 11BO2− ions, respectively,

in the anode 312B sample.

Finally, Figure 8 shows evidence of boron interaction with carbon on the impregnated sample. The peaks at m/z = 22 and m/z = 23 can be attributed to 10BC− and 11BC− ions, respectively, with intensity ratio in agreement with the isotopic compositions for boron. This analysis suggests that part of the impregnated boron interacts with carbon sites, thus occupying them and hindering the oxidation reactions.

Figure 7 shows the negative ToF-SIMS spectra of the anode 0B sample (dashed line) and of the anode 312B sample (green

Figure 8. Partial negative ToF-SIMS spectra of anode 0B sample (dashed line) and anode 312B sample.

4 CONCLUSIONS

The objective of the present study was to reveal the protection mechanism of boron, at very low concentrations, for the air reactivity of carbon anodes. The effect of up to 312 ppm of boron on the apparent reaction rate of the anode in the chemical regime was evaluated. It was shown that boron decreases the apparent reaction rate. The kinetic analysis revealed that boron decreases the activation energy of the oxidation reaction. In other words, it catalyzes the carbon oxidation reaction. However, it was also observed that boron decreases the pre-exponential parameter, suggesting that it reduces the total number of interactions between oxygen and carbon active sites. On the basis of the TGA results, it was concluded that oxygen adsorption on the impregnated sample is reduced by half, as compared to that of an untreated sample, suggesting that half of the carbon sites are occupied by boron. While XPS was not sensitive enough, ToF-SIMS analysis allowed the identification of boron at very low concentrations in the carbon-based anode. Boron

interacts with oxygen (BO− and BO2− ions) but also with carbon (BC− ions). The presence of

boron−carbon interactions on the impregnated samples is a strong indication that the carbon sites are occupied by boron, preventing their participation in oxidation reactions. The impregnation of the carbon anode by a dilute boron solution could thus be considered as an effective way to prevent anode air burning during the aluminum electrolysis process.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the financial support of NSERC, FRQNT, and Alcoa. The authors would like to express their sincere thanks and appreciation for the assistance of Alcoa (Deschambault plant, Canada) and to Mr. Pierre Mineau and his colleagues for conducting the chemical and crystallite size analyses. The authors would also like to extend their appreciation to Mr. Nicolas Nuns at Universite Lille 1 for the ToF-SIMS analyses and also to Lise Lemieux, Guillaume Gauvin, and Hugues Ferland at Universite Laval for their technical support.

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