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Protection of carbon anode against air burning: A new

approach to apply and understand the inhibiting effect

of boron impregnation

Thèse

Ramzi Ishak

Doctorat en génie des matériaux et de la métallurgie

Philosophiae doctor (Ph. D.)

Québec, Canada

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Protection of carbon anode against air burning: A new

approach to apply and understand the inhibiting effect

of boron impregnation

Thèse

Ramzi Ishak

Sous la direction de:

Houshang Alamdari, directeur de recherche

Gaétan Laroche, codirecteur de recherche

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III

Résumé

L’électrolyse de l’aluminium est un processus consommateur en énergie et en ressources (matières premières, personnel qualifié, temps, etc.). De nombreux projets de recherche sont en cours à travers le monde dans le but d’améliorer l’efficacité du procédé de fabrication de l’aluminium, de diminuer les rejets toxiques de gaz (CO2, CO, CF4, C2F6…) et d’en réduire les coûts de production.

Un des problèmes actuels de l’électrolyse de l’alumine est la consommation excessive des anodes en carbone. En effet, ces anodes, lorsqu’elles sont chauffées à haute température, sont attaquées par l’air ambiant entre 400 et 600 °C, et par le CO2 à 960 °C, ce qui a pour conséquence d’entraîner

une surconsommation de carbone, réduisant ainsi la capacité de fabrication de l’aluminium métallique par kg de carbone consommé. Actuellement, la durée de vie moyenne d’une anode est entre 20 et 30 jours. L’objectif de ce projet est de diminuer la vitesse de réaction à l’air de l’anode. Différentes méthodes ont été élaborées afin d’obtenir une protection efficace et économique contre le phénomène d’oxydation à l’air et au CO2 réduisant ainsi la surconsommation en carbone de

l’anode. L’oxyde de bore étant connu comme inhibiteur de la réaction carbone/oxygène, des essais ont été réalisés dans le but de produire un revêtement sur l’anode et ont permis de confirmer l’effet inhibiteur de l’oxyde de bore sur la réaction d’oxydation à l’air, permettant ainsi la protection des anodes de carbone. L’influence de chacun des paramètres (température, concentration, durée d’imprégnation dans la solution, etc…), ont été également étudiés. La tomographie par rayons X a démontré que l’anode est principalement attaquée sur la surface et que le revêtement d’oxyde de bore créé une barrière physique empêchant l’accès de l’oxygène à l’anode.

Des études plus approfondies ont été réalisées afin de comprendre le mécanisme de protection de l’oxyde de bore avec la réaction carbone-oxygène. Selon la littérature, l’oxyde de bore et l’acide borique peuvent agir de deux façons : soit en se fixant sur la surface de l’anode en bloquant les sites actifs du carbone ou encore en créant une couche vitreuse qui sert de barrière pour l’oxygène. Une étude cinétique a été établie et confirme que le nombre de collisions entre l’oxygène et les sites de carbone diminuent en présence du bore. La technique ToF-SIMS a également démontré que le bore se trouve sous forme d’oxyde sur la surface de l’anode, mais aussi sous forme de liaison

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carbone-IV

bore. Il s’agit donc d’une protection physique d’une part et une protection chimique en bloquant les sites actifs du carbone par les atomes de bore.

La consommation de l’anode en carbone dans la cuve d’électrolyse est contrôlée par les impuretés, par le niveau de graphitisation mais également par le transport de masse à travers sa structure poreuse. La protection des particules de coke avec de l’oxyde de bore pourrait avoir un impact physique sur la porosité et la distribution de celle-ci. Des particules de coke (allant de 4 000 µm à 4 760 µm de diamètre) ont été imprégnées par de l’oxyde de bore afin de révéler la sélectivité des porosités. Les surfaces et les volumes spécifiques différentiels de ces trois tailles de particules gazéifiées à 3 pourcentages (0, 15 et 35%) déterminés par adsorption d’argon et par infiltration de mercure ont permis d’évaluer les contributions des gazéifications sous-critiques et sur-critiques sur la gazéification totale des anodes sous air à 525 °C. La détermination de la taille critique des pores (TC) pour le coke traité et non-traité et la mesure des contributions sous-critique et sur-critique ont

permis de révéler que les pores ayant une taille supérieure à cette taille critique jouerait un rôle prépondérant dans la réactivité à l’air du coke.

Dans cette thèse, une nouvelle méthode de protection des anodes par l’oxyde de bore a été développée. Ceci consiste à traiter les matières premières, avant la fabrication de l’anode. En utilisant une faible concentration d’oxyde de bore (de l’ordre de ppm) dans le but de limiter le niveau d’impureté dans l’aluminium produit. Les résultats montrent que la réactivité à l’air de l’anode diminue de 15%, le charbonnaille de 90% et le dégagement gazeux (CO2 et CO) de 30%. L'influence de

chacun des paramètres (température, concentration, etc.) sur la protection de l’anode a également été optimisée.

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V

Abstract

Aluminum electrolysis is a process that consumes energy and resources (raw materials, qualified personnel, time, etc.). Several research projects are underway around the world to improve the efficiency of the aluminum manufacturing process, to reduce toxic gas emissions (CO2, CO, CF4,

C2F6 ...) and to reduce production costs. One of the current problems of alumina electrolysis is the

excessive consumption of carbon anodes. Indeed, these anodes, when they are heated at high temperatures, are attacked by ambient air between 400 and 600 °C, and by the CO2 at 960 °C which

results in an over-consumption of carbon, thereby reducing the manufacturing capacity of metallic aluminum per kg of carbon consumed. Currently, the average lifetime of an anode is between 20 and 30 days. The objective of this project is to reduce the reaction rate of anode oxidation under ambient air.

Different methods have been developed to obtain an effective and economical protection which would reduce the over-consumption of the carbon anode against the phenomenon of air oxidation. Since boron oxide is known as an inhibitor of carbon/oxygen reaction, several attempts have been made to make a coating on the anode, confirming the inhibitory effect of boron oxide on this reaction, thus allowing protection of the carbon anodes. The influence of each of the parameters (temperature, concentration, duration of impregnation in the solution, etc.) were studied, as well. X-ray tomography showed that the anode is mainly attacked on the surface and that the boron oxide coating creates a physical barrier preventing access of oxygen to the anode.

Further studies have been carried out to understand the inhibitor mechanism of boron oxide on carbon-oxygen reaction. According to the literature, boron oxide and boric acid can act in two ways: either by fixing on the anode surface resulting in blocking the active carbon sites or by creating a vitreous layer which serves as a physical barrier to oxygen. A kinetic study has been established which confirms that the number of interactions between oxygen and carbon sites decreases in the presence of boron. ToF-SIMS has revealed that boron is present as an oxide on the anode surface and also in the form of carbon-boron bond (BC-). Therefore, this acts like a chemical protection while

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The consumption of the carbon anode in the electrolysis cells is controlled by the impurities and the graphitization level as well as the mass transport through its porous structure. The impregnation of coke particle could have an effect on the porosity and its distribution. Coke particles (from 4000 μm to 4 760 μm in diameter) was impregnated with boron oxide in order to reveal its effect on the porosity. The specific surface area and the volumes of 3 conversion rates of particles (at 0, 15 and 35%) were determined by argon adsorption and mercury infiltration in order to evaluate the contributions of subcritical gasification on the total gasification of the anodes under air at 525 °C. To determine the critical pore size (TC) for the treated and untreated coke, the measurement of internal and external

contributions of pores was used. It was revealed that the pore sizes of 0.1-10 µm and larger were the most active pores for the gasification under air. In addition, the volume of only very small pores (0.0004-0.001 µm) was slightly decreased by boron impregnation. However, the contribution of the size range of these small pores to anode gasification is negligible.

In this thesis, a new method for the protection of anodes by boron oxide has been developed. This involves treating the raw materials before anode is formed by using a low concentration of boron oxide (in ppm) in order to limit the level of impurities contained in the produced metal. The results performed with standard equipment showed that the air reactivity of the anode decreased by 15%, the dusting by 90% and CO2/CO loss by 30%. The electrical resistivity of the anode was not affected

by boron oxide at this low level. The influence of each of the parameters (temperature, concentration, etc.) on anode protection was optimized, as well.

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VII

Table of contents

Résumé ... III Abstract ... V Table of contents ... VII List of Figures ... XI List of Tables ... XIV Acknowledgments ... XVI Preface ... XIX

Chapter 1: Introduction ... 1

1.1 Aluminum production... 2

1.1.1 The Hall-Héroult process ... 2

1.1.2 Anode manufacturing ... 5

1.2 Problems ... 7

1.3 Project objectives ... 8

Chapter 2: Literature review ... 10

2.1 Carbon anode ... 10

2.1.1 Anode consumption... 10

2.1.2 Dust emission ... 14

2.2 Carbon gasification ... 16

2.3 Inhibition of carbon reaction ... 19

2.3.1 Effect of inhibitors on carbonaceous materials oxidation ... 19

2.3.2 Effect of boron on carbon oxidation ... 25

2.3.3 Carbon protection mechanisms induced by boron ... 27

2.4 Summary ... 31

Chapter 3: Materials and methods ... 32

3.1 Introduction ... 32

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VIII

3.2.1 Calcined petroleum coke ... 32

3.2.2 Coal tar pitch ... 33

3.3 General experimental procedures ... 33

3.3.1 Laboratory scale anode preparation ... 33

3.3.2 Anode and coke impregnation methods ... 36

3.4 Air reactivity tests ... 37

3.4.1 Thermogravimetric analyzer «TGA» ... 37

3.4.2 Fixed Bed Reactor (FBR) ... 38

3.4.3 Standard air reactivity test – (ISO 12989-1) ... 39

3.5 Characterization techniques ... 40

3.5.1 Surface area and pore analyzer (gas adsorption) ... 40

3.5.2 Mercury porosimetry ... 41

3.5.3 Helium pycnometer ... 42

3.5.4 Particles size distribution ... 42

3.5.5 Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) ... 42

3.5.6 Scanning Electron Microscopy (SEM) ... 43

3.5.7 X-Ray Diffraction (XRD) ... 43

3.5.8 X-Ray Fluorescence Spectroscopy (XRF) ... 43

Chapter 4: Application of boron oxide as a protective surface treatment to decrease the air reactivity of carbon anodes ... 45

4.1 Résumé ... 46

4.2 Abstract ... 46

4.3 Introduction ... 47

4.4 Materials and Methods ... 50

4.4.1 Materials ... 50

4.4.2 Air Reactivity Tests... 50

4.4.3 X-ray Computed Tomography ... 50

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4.5.1 Anode Impregnation ... 51

4.5.2 Effect of Impregnation Time ... 53

4.5.3 Boron Loading during Impregnation ... 57

4.5.4 Effect of Boron-Coating on the Anode Reaction Mode ... 60

4.6 Conclusions ... 63

Chapter 5: Characterization of carbon anode protected by low boron level: A try to understand carbon-boron inhibitor mechanism ... 65

5.1 Résumé ... 66 5.2 Abstract ... 66 5.3 Introduction ... 67 5.4 Experimental section ... 70 5.4.1 Sample preparation ... 70 5.4.2 Sample characterization ... 71

5.5 Results and discussion... 73

5.6 Conclusions ... 83

Chapter 6: Effect of boron on the evolution of petroleum coke active pore size under air oxidation 85 6.1 Résumé ... 86

6.2 Abstract ... 86

6.3 Introduction ... 88

6.4 Experimental section ... 89

6.4.1 Materials ... 89

6.4.2 Air reactivity tests ... 91

6.4.3 Characterizations ... 92

6.5 Results and discussion... 92

6.5.1 Impacts of heat treatment on coke properties ... 92

6.5.2 Air reactivity results ... 95

6.5.3 Density evolution during gasification ... 96

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X

6.5.5 Cumulative pore volumes ... 99

6.5.6 Active pore size ... 101

6.6 Conclusion ... 103

Chapter 7: Conclusions and perspectives ... 105

7.1 Introduction ... 105

7.2 Overview of the project... 105

7.3 Effect of surface impregnation parameters on anode air reactivity ... 106

7.4 Effect of low boron content on the carbon active sites ... 107

7.4.1 Kinetic study of carbon-boron reaction ... 107

7.4.2 Identification of Carbon-Boron interaction via ToF-SIMS technique ... 110

7.5 Development of a viable process to protect the entire anode ... 110

7.6 Effect of boron addition of active pores under air oxidation ... 112

7.7 General conclusion ... 112

7.8 Future work ... 112

References ... 117

Appendix 1: Process for manufacturing carbon anodes for aluminum production cells and carbon anodes obtained from the same ... 125

Appendix 2: Evolution of Anode Porosity under Air Oxidation: The Unveiling of the Active Pore Size ... 155

Appendix 3: Formulation without ultrafine coke particles: A way to increase the features of the carbon anode. ... 171

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XI

List of Figures

Figure 1: Greenhouse gas emissions by province and territory, Canada, 1990, 2005 and 2015 [4] . 1 Figure 2: Greenhouse Gas (GHG) emissions generated during primary aluminum production in

Quebec, China Russia and Middle East [6] ... 2

Figure 3: Schematic representation of aluminum electrolysis cell with prebaked anodes [9] ... 3

Figure 4: Schematic representation of the amount of raw materials and energy to produce 1 kg of primary aluminum [12] ... 5

Figure 5: Schematic presentation of pre-baked anode manufacturing process ... 6

Figure 6: Anode consumption [23, 24] ... 7

Figure 7: Temperature variation of carbon anode in the electrolysis cell [28] ... 11

Figure 8: Upper part of anode attacked by air [27] ... 12

Figure 9: Schematic representation of dust emission of anode in the electrolysis cell [14] ... 15

Figure 10: Three zone controlling the reaction rate of porous carbon material [46]... 18

Figure 11: Evolution of carbon structure as function of temperature [14, 58] ... 21

Figure 12. Effect of Lc on the reactivity of petroleum coke [59] ... 21

Figure 13: Effect of tri-chloroethyl phosphate on graphite oxidation under air atmosphere [54] ... 22

Figure 14: Schematic representation of phosphate organic residues adsorbed in the graphite surface after heat treatment [54] ... 23

Figure 15: Anode sample coated with alumina spray [52] ... 23

Figure 16: Cross-section of a sample after heat treatment at 1050°C and air burning test. A continuous layer of boron oxide with a thickness of 100 μm on the surface protects the carbon from air burning ... 26

Figure 17: The effect of boron oxide on graphite oxidation under air atmosphere [55] ... 28

Figure 18: Schematic representation of (BO3)n polymeric residue bonding to the {10-10} of graphite [53, 55] ... 30

Figure 19: Effect of tri- butylborate impregnation on graphite on graphite oxidation under air atmosphere [55] ... 31

Figure 20. Mixer used to make paste samples (Hobart N50 mixer)... 34

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Figure 22. Furnace used for anode baking (Pyradia, Canada) ... 36 Figure 23. Schema of the Fixed Bed Reactor (FBR) ... 39 Figure 24: Air reactivity equipment (R&D Carbon) ISO 12989-1 standard test ... 40 Figure 25: Siemens Somatom Sensation 64 designed for the human body but adapted for material

research. Courtesy of INRS-ETE, Quebec, QC, Canada ... 51 Figure 26: Air reactivity of the samples coated by impregnation ... 53 Figure 27: Gasification vs. reaction time during gasification for a series of samples with different

impregnation times ... 55 Figure 28: SEM micrograph of a sample cross-section showing the crystallization of boron oxide

inside the pores (indicated by arrows) ... 56 Figure 29: Visual observation of the samples; (a) before air reactivity test; (b) after 24 h of

exposure; (c) after 6 days of exposure. The numbers on the samples indicate the

impregnation time ... 57 Figure 30: Gasification vs. reaction time during gasification for a series of samples with different

boron loading ... 60 Figure 31: X-ray tomography images of uncoated samples; (a) before air burning; (b) after 6 h of

exposure at 500 °C ... 61 Figure 32: Superimposed patterns of the fresh (white) and burnt (blue) sample reveal the shrinkage

of the sample after 6 h of exposure ... 62 Figure 33: Computed tomography (CT) scan images of a sample, (a) fresh; and (b) after

impregnation for 15 min and exposure for 56 h at 500 °C. ... 63 Figure 34: Experimental gasification reaction versus reaction time, at three temperatures; analyses performed on a TGA. Each data point is an average of three experiments ... 75 Figure 35: Adsorption-desorption isotherm (BET method) of (a) dried anode sample and (b) heat

treated anode sample... 77 Figure 36: Raman spectrum of anode sample treated with 1000 ppm B at 25 and 225 °C. ... 78 Figure 37: Mass gain as function of reaction time (under pure oxygen atmosphere) of the Anode 0B

and Anode 312B samples. ... 79 Figure 38: Partial positive ToF-SIMS spectra of Anode 0B sample (dashed line) and Anode 312B

sample... 80 Figure 39: Partial negative ToF-SIMS spectra of Anode 0B sample (dashed line) and Anode 312B

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Figure 40: Partial negative ToF-SIMS spectra of Anode 0B sample (dashed line) and Anode 312B sample... 82 Figure 41: Partial negative ToF-SIMS spectra of Anode 0B sample (dashed line) and Anode 312B

sample... 83 Figure 42: Helium density of coke particles before and after heat treatment (HT) ... 94 Figure 43: Cumulative pore volume measured with CO2 using DFT model for coke particles after

heat treatment process... 95 Figure 44: Carbon conversion of baked coke particles (- 4760 + 4000 µm) as a function of reaction time at 525 °C, under air atmosphere; analyses performed on a Thermo-Gravimetric Analyzer (TGA) ... 96 Figure 45: Helium density versus carbon conversion of impregnated and un-impregnated coke

particles gasified at 3 levels (0; 15 and 35%) under air at 525 °C ... 97 Figure 46: Pore volume distributions measured with carbon dioxide and nitrogen using DFT model

and with mercury intrusion versus the pore size diameter for coke particles gasified under air at 525 °C and at a rate of 0, 15 and 35% in a fixed bed reactor... 99 Figure 47: Cumulative pore volume measured with CO2 and N2 adsorption using DFT model and

with mercury intrusion versus the pore size diameter for coke particles (Coke 0B) gasified at 0, 15 and 35% under air at 525 °C in a fixed bed reactor ... 100 Figure 48: Cumulative pore volume measured with CO2 and N2 adsorption using DFT model and

with mercury intrusion versus the pore size diameter for coke particles (Coke 150B) gasified at 0, 15 and 35% under air at 525 °C in a fixed bed reactor ... 101 Figure 49: Comparison of internal and external carbon contributions versus the carbon conversion

ranges for coke particles gasified under air at 525 °C in a fixed bed reactor, impregnated with boron at two concentrations (0 and 150 ppm) ... 103 Figure 50: Schematic representation of the inhibitor mechanism of carbon-boron ... 109 Figure 51: Boudouard reaction diagram as function of temperature [39] ... 114 Figure 52: Experimental gasification percentage versus reaction time for untreated anode and

treated anode (312ppm of boron), under CO2 atmosphere at 960°C; analyses performed on a

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XIV

List of Tables

Table 1: Electrical generation source in Quebec comparing to China, Russia and Middle East [5] .. 2

Table 2: Particle size distribution in a typical industrial anode ... 6

Table 3: Effect of impurities on the carbon gasification by oxygen [10, 29] ... 11

Table 4: Parameters of the equation 1 [31] ... 13

Table 5: Average values of pre-baked anode properties investigated by R&D Carbon company [14, 31] ... 14

Table 6: Particle size distribution of calcined coke (w. %) used for the preparation of the dry mixture used for the fabrication of prebaked anodes ... 33

Table 7: Properties of coal tar pitch used as binder in anode samples [79] ... 33

Table 8. The weight percentage of the coal tar pitch was 16.2 wt. % of the total weight of the coke particles for all the anodes ... 34

Table 9: ISO 12989-1 standard test parameters ... 40

Table 10: Impregnation conditions of the samples ... 52

Table 11: Impregnation conditions of the samples ... 52

Table 12: Impregnation conditions of the samples ... 54

Table 13: Boron loading on impregnated samples ... 59

Table 14: Particle size distribution of calcined coke (w.t. %) used for the preparation of the dry mixture used for the fabrication of prebaked anodes... 71

Table 15: Chemical composition of the samples measured by XRF and crystallite height (Lc) measured by X-ray diffraction ... 73

Table 16: Results from the application of Arrhenius formula to the three samples at 20% of gasification ... 76

Table 17: Specific surface area (BET method) of untreated anode and anode 312B samples before and after the TGA experiments ... 77

Table 18: Area values of m/z = 26, 27, 42 and 43 peaks ... 81

Table 19: Impregnation parameters of coke particles ... 90

Table 20: Chemical composition of calcined coke ... 90

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À mes parents À mon Frère Elias

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Acknowledgments

Je tiens tout d’abord à remercier les membres du jury, Dr. Frank Feret, Professeur Marc-André Fortin et Dr. Jayson Tessier d’avoir accepté de juger ce travail à titre de rapporteurs et d’examinateurs. Je les remercie pour le temps qu’ils ont consacré à corriger ma thèse et pour la pertinence de leurs commentaires et suggestions. J’aimerais remercier le Professeur Marcel Laflamme, président du jury, pour avoir présidé cette soutenance.

Je souhaite exprimer toute ma gratitude à mon directeur de thèse, Professeur Houshang Alamdari, qui m’a permis de réaliser ce projet de recherche au sein de son laboratoire. Je tiens à le remercier très chaleureusement de m’avoir accordé sa confiance pour la réalisation de ce travail de thèse. Merci pour la très grande qualité de ses conseils et également pour les nombreuses périodes de discussion.

Je souhaite également remercier, mon co-directeur de thèse, Professeur Gaétan Laroche, qui m’a offert son expertise technique. Je le remercie pour ses suggestions, conseils, discussions, et sa disponibilité. J’aimerais remercier mon co-directeur industriel, Donald P. Ziegler, pour ses nombreux conseils pertinents et les discussions tout au long de ce projet.

Je remercie également le directeur du REGAL, Professeur Mario Fafard, pour son support et ces conseils tout au long de ce projet de recherche. J’aimerais remercier Hugues Ferland, Guillaume Gauvin et Donald Picard pour leurs expertise technique et leurs disponibilités qui ont été essentielles dans la réalisation des montages expérimentaux de ce projet. Merci également pour votre bonne humeur quotidienne !

Je voudrais également remercier tous les membres de la compagnie Alcoa pour leur disponibilité et leurs commentaires et notamment Jayson Tessier et Angélique Adams ainsi que l’équipe du laboratoire des matériaux à Deschambault pour leur temps et la grande qualité des résultats fournis. Je suis aussi reconnaissant au Professeur Jean-François Lamonier de l’Université Lille 1 qui m’a accueilli dans son équipe pour un stage de quatre mois et de m’avoir mis à disposition de nombreux moyens et ressources. Je le remercie pour ses suggestions, conseils et discussions. Je profite de

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cette occasion pour remercier Nicolas Nuns et Jean-Charles Morin, Ingénieurs d’études à l’UCCS pour leurs aides, support et conseils pour les caractérisations ToF-SIMS, Raman et IRTF.

Je tiens également à remercier les organismes qui ont aidé au financement de cette thèse sans quoi tout cela n’aurait pas été possible: Le Conseil de Recherches en Sciences Naturelles et en Génie du Canada (CRSNG), de la compagnie Aloca dans le cadre de la chaire de recherche industrielle CRSNG-MACE3 et du Fonds de Recherche du Québec-Nature et Technologies (FRQ-NT) par

l’intermédiaire de centre de recherche sur l’aluminium (REGAL).

Je voudrais remercier tous les membres du département de Génies des Mines, de la Métallurgie et de Matériaux dont notamment les technicien(e)s : Vicky Dodier, Edmond Rousseau, Nathalie Moisan, pour leur gentillesse, leur disponibilité et leur aide. Je remercie également le personnel administratif : Mesdames Andrée Lord, Ginette Cadieux, Martine Demers, Karine Fortin et Julia Lebreux pour leur travail exemplaire et leur bonne humeur. Je remercie également le personnel administratif du REGAL : Valérie Goulet-Beaulieu, Lyne Dupuis et Andréanne Bernatchez pour leur soutien et leur disponibilité.

Je tiens à exprimer toute ma gratitude à Francois Chevarin qui a toujours été là pour m’aider au laboratoire tout au long de mon doctorat (merci pour tout Francois!). Je voudrais remercier aussi les étudiants avec qui j’ai fait une très grande partie de mon doctorat et qui ont toujours été là pour fêter avec moi : Geoffroy, Behzad, Nabil, Kamran, Mousa, Franck, Maryam, Asem, Mahdi, Rémi Martinez, Simon, Pierre, Jean-François, Keven, Remy Averlant, Mounir, Su, Bertrand, Majid H, Deniz, Mahdi; Les étudiant(e)s de biomatériaux : Sébastien et Marie, Vanessa, Eléonore, Ivan, Morgane, Sébastien, Essowè, Carlo, Ludivine, Sergio, Afghany, Francesco… et tous les autres étudiants des départements GMN, GCH, GCI et du groupe de recherche REGAL. Je remercie également toutes les personnes que j’ai pu rencontrer depuis mon arrivée au Canada pour leur support et leur amitié : Lucie, Mario, Sara Z., Sara E., Charles-Henri, Laura, Rabih…

J’aimerais remercier sincèrement mes parents et mon frère qui m’ont toujours soutenu dans mes choix. Merci pour leurs énormes soutiens ainsi que leurs encouragements.

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Je souhaiterais exprimer toute ma gratitude à toutes les personnes qui ne sont pas citées ici mais qui, d’une manière ou d’une autre, m’ont aidé ou soutenu tout au long de ces années.

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Preface

This doctoral thesis is presented to the Department of Mining, Materials and Metallurgical Engineering of Laval University. The project was part of a Collaborative Research and Development (CRD) program between Alcoa and Laval University (MACE3 industrial research chair, Aluminium

Research Centre-REGAL) and was supported by the Natural Science and Engineering Research Council of Canada (NSERC) and Fonds de Recherche du Québec-Nature et Technologies (FRQNT). This doctoral project was carried out under supervision of Professor Houshang Alamdari and co-supervision of Professor Gaétan Laroche and Dr. Donald Ziegler, Program Manager-Modelling at Alcoa Aluminum Center of Excellence.

The works carried out as part of the project under the title of "Protection of carbon anode against air burning" have been reported in this thesis. Chapter 1 presents an introduction of the project, a problematic and the objectives of the thesis. The second chapter of this thesis reports the previous works reported in anode technology literature focusing on the effect of boron on inhibition of carbon reaction with oxygen. Chapter 3 is focused on presenting in detail the research project as well as the methodologies applied. Chapters 4, 5 and 6 present the results and discussions obtained, which generated 3 scientific papers and one patent application.

Chapter 4: Application of boron oxide as protective surface treatment of carbon anodes Authors: Ramzi Ishak, Donald Picard, Gaétan Laroche, Donald Ziegler and Houshang Alamdari Journal: Metals 2017, 7 (3), 79

Ramzi Ishak, Donald Picard and Houshang Alamdari conceived and designed the experiments. Ramzi Ishak and Donald Picard performed the experiments. Houshang Alamdari, Donald Picard, Gaétan Laroche and Donald P. Ziegler contributed in data analysis. Ramzi Ishak wrote the paper and all co-authors commented/corrected it.

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Chapter 5: Characterization of carbon anode protected by low boron level: a try to understand carbon-boron inhibitor mechanism

Authors: Ramzi Ishak, Gaétan Laroche, Jean-Francois Lamonier, Donald Ziegler and Houshang Alamdari

Journal: ACS Sustainable Chemistry and Engineering. 5 (8): 6700-6706

Ramzi Ishak and Houshang Alamdari conceived and designed the experiments. Ramzi Ishak performed the experiments. The scientific revision was done by Houshang Alamdari, Jean-Francois Lamonier, Gaétan Laroche and Donald P. Ziegler. Ramzi Ishak wrote the paper and all co-authors commented/corrected it.

Chapter 6: Effect of boron on the evolution of petroleum coke active pore size under air oxidation

Authors: Ramzi Ishak, Francois Chevarin, Gaétan Laroche, Donald Ziegler and Houshang Alamdari This paper will be submitted to Energy and Fuels journal.

Ramzi Ishak, Francois Chevarin and Houshang Alamdari conceived and designed the experiments. Ramzi Ishak and Francois Chevarin performed the experiments. Houshang Alamdari, Francois Chevarin, Gaétan Laroche and Donald P. Ziegler contributed in data analysis. Ramzi Ishak wrote the paper and all co-authors commented/corrected it.

A general discussion and conclusions will be followed, perspectives for this work are presented in chapter number eight. A patent application was filed during the Ph.D. study period. The patent is included in the appendix as its format is in not applicable to be presented similarly as an article. Appendix 1: Process for manufacturing carbon anodes for aluminum production cells and carbon anodes obtained from the same.

Inventors: Houshang Alamdari, Ramzi Ishak, Gaétan Laroche, Mario Fafard, Donald Picard and Donald Ziegler

US Patent Application N°62/445.308. Applicant: Université Laval and Alcoa Inc.

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Ramzi Ishak and Houshang Alamdari conceived and designed the experiments. The literature review and experiments were performed by Ramzi Ishak. The work plan was discussed with Houshang Alamdari and Donald Ziegler. The results were analyzed by Ramzi Ishak and discussed with Houshang Alamdari, Gaétan Laroche, Donald Ziegler, Mario Fafard and Donald Picard. Houshang Alamdari, ROBIC officers and Ramzi Ishak wrote the patent and all the co-inventors commented/corrected it.

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Chapter 1: Introduction

Aluminum is the most abundant (8.13%) metallic element in the Earth's crust. Most commercial aluminum is produced by electrolysis of alumina through the Hall-Héroult process [1-3]. Among its qualities are excellent thermal properties and light weight, making it a suitable material for the automotive and aeronautic industries. Other benefits are its high corrosion resistance and ease of recyclability. The province of Quebec produces 90% of aluminum in Canada which accounted for 2.8 million tons in 2015. Figure 1 presents the CO2 emissions of Canada provinces from 1990 to 2015.

Quebec province showed stable emission patterns from 1990 to 2005, and a decreasing pattern from 2005 to 2015 [4]. The improvement of energy efficiency has a considerable impact on economy and environment in the province.

Figure 1: Greenhouse gas emissions by province and territory, Canada, 1990, 2005 and 2015 [4]

Greenhouse Gas (GHG) emissions generated during primary aluminum production in Québec are 67 to 76% lower than those emitted in the same production in the Middle East or China and 19% lower compared to Russia (Figure 2) [5].

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Figure 2: Greenhouse Gas (GHG) emissions generated during primary aluminum production in Quebec, China Russia and Middle East [6]

The electrolysis of alumina requires large quantities of electric power. Consequently, the aluminum industry is the largest industrial consumer of energy in Quebec [5]. As a result, aluminum production plants are often located near sources of hydroelectricity. Table 1 summarizes the source of electricity for this industry in the given country or region.

Table 1: Electrical generation source in Quebec comparing to China, Russia and Middle East [5]

Electrical generation

mode China Middle East Russia Quebec

Hydroelectricity 10% 0% 95% 100%

Charcoal 90% 0% 5% 0%

Natural gas 0% 100% 0% 0%

1.1

Aluminum production

1.1.1 The Hall-Héroult process

The Hall-Héroult process was invented simultaneously by Charles Martin Hall in the United States and Paul Héroult in France in 1886 [7, 8]. The industrial exploitation of this discovery began in France and the United States in 1889. Today, the Hall-Héroult process is the driving principle behind the production of pure aluminum. The metal is obtained in liquid phase in an electrolysis cell by reduction

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of the alumina (Al2O3) dissolved in a molten salt (Na3AlF6) inside carbon-lined cells (anode and

cathode). The electrolyte (Na3AlF6) contained in the electrolysis cell is a mixture of Na3AlF6, AlF3,

CaF2 and Al2O3, maintained at about 960 °C [3]. However, additions of AlF3 and CaF2 lower the

melting point of cryolite to decrease the operating temperature. A powerful Direct Current (DC) electric current between the anode and cathode crosses through the bath and separates the aluminum metal from the chemical solution. Figure 3 shows a simplified schematic representation of the electrolysis cell.

Figure 3: Schematic representation of aluminum electrolysis cell with prebaked anodes [9]

The molten aluminum is a homogeneous viscous fluid that is incompressible at a density of around 2400 kg/m3 (which is higher than the density of the bath). Thus, the formed metal goes to the bottom

of the electrolysis cell which is situated at the bath/metal interface. The overall reaction in the electrolysis cell can be written as the following [10]:

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2 Al2O3 (dissolved) + 3 C (s)  4 Al (l) + 3CO2 (g) (1) The cathode placed at the bottom of the electrolysis cell is made of anthracite or graphite and a pitch as a binder. Cathodes are basically classified in 3 categories: semi-graphitised, graphitic and graphitised. The graphitised coke is heat-treated between 2400 and 3000°C. While the semi-graphitic and semi-graphitic coke are calcined at around 1200 °C before being crushed and sieved. These aggregates are then mixed with coal-tar pitch, extruded and baked. Carbon cathodes require a good electrical conductivity and high resistance to wear.

Anodes are placed in the upper part of the electrolysis bath. A part of anode is exposed to ambient air because they gradually immerse in the bath. They are made of petroleum coke, recycled anodes and coal tar pitch. Carbon anodes are critical for the process of aluminum reduction. They carry the electric charge to the cryolite in the reduction cell. Many parameters can affect the energy efficiency of anodes, such as density, raw materials, porosity and cracks after the baking process and the electrical resistivity. The electrical resistivity of prebaked anodes is between 50 and 60 µΩm which represents 10% of the total cell power [11]. Based on the theoretical reaction (Equation 1), 1 kg of aluminum is produced due to the reaction of alumina with 0.334 kg of carbon [12, 13]. However, over-consumption is generated by secondary reactions known as "parasitic reactions" between the carbon anode and two gases, O2 and CO2 [14, 15]:

 The chemical reaction of the carbon with air at 400 and 600 °C (upper part of the anode which is not immersed in the bath) can be described by the following equations [16,17] (this reaction will be detailed in the following section):

C (s) + O2 (g)  CO2 (g) (2)

 The reaction of CO2 with the surface of the anode immersed in the electrolysis bath at 960

°C, which is known as “Boudouard reaction” [1, 17]:

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As shown in Figure 4, the production of 1 kg of aluminum requires about 2 kg of alumina (about 4 to 5 kg of bauxite is required to obtain 2 kg of alumina), between 0.4 and 0.5 kg of carbon and 13 MWh of electrical energy [12].

Figure 4: Schematic representation of the amount of raw materials and energy to produce 1 kg of primary aluminum [12]

1.1.2 Anode manufacturing

The pre-baked anodes are mainly prepared by a mixture of crushed petroleum coke, recycled anode (butt) and pitch. The conventional anode manufacturing recipe includes the following raw materials [18]:

 65% calcined petroleum coke;  20% of anode butts;

 15% pitch.

First of all, the coke aggregates (calcined petroleum coke and anode butts) are crushed and sieved. The finer coke particles are produced by milling using a ball mill. Another part of the fine coke particles are also collected from the dust throughout the anode plant. Table 2 describes three classes of the coke fraction: coarse, intermediate and fines [19]. Anode butts are classified as coarse having less porosity than the coke [20].

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Table 2: Particle size distribution in a typical industrial anode

Coke Butts and

baked scrap Dry aggregate Coarse Intermediate Fine

Particles size (US Mesh) -1/4 in + 30 -30 + 100 -100 Max 1 in Particles size (µm) -6300 + 600 -600 + 150 -150 Max 25000 The different steps of prebaked anodes manufacturing are described in Figure 5. The second step is mixing the three raw materials to form the anode paste, which is compacted or vibrocompacted. This paste is essential to obtain the desired shape and density of the “green anode” that is baked in an industrial furnace at a maximum temperature of 1100 °C for a cycle of about 3 to 4 weeks. Prior to positioning in the electrolysis cells, aluminum rod connected to a steel stubs are sealed in the prebaked anodes to allow the passage of the electrical current.

Figure 5: Schematic presentation of pre-baked anode manufacturing process

The quality of the produced anodes is a very important factor since it can influence the aluminum production efficiency in the electrolysis cells. A number of criteria were proposed in the literature for making a high-quality anode [21, 22]. The cracks caused by thermal shocks during the baking process is another important factor that can affect the anode quality which inevitably impacts its electrical conductivity. Existence of high amounts of impure contents in the anode could have an

1 2

7 6

4

5 3

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adverse effect on the aluminum production as well as on the carbon over-consumption due to high air and CO2 reactivity [10].

1.2

Problems

As mentioned previously (section 1.2.1), theoretically 334 kg of carbon are required to produce 1 ton of aluminum according to the stoichiometric reaction of aluminum reduction. However, at high temperatures, the anode reacts with ambient air and CO2 in the electrolyte [10]. This results in a

considerable over-consumption of carbon, with high performance plants using about 400-500 kg per ton of aluminum (figure 6). Spending an extra kg of carbon to produce one ton of metal was estimated to be about 2 $ US [21]. As an example, an aluminum smelter that approximately produces 300,000 tons of aluminum per year, uses as many as 150,000 anodes in the same period. The over-consumption of carbon for such a producer is approximately 40 kg per anode and the loss of financial resources could be estimated to be about 12 million $ per year. The high costs caused by the air and CO2 reactivity of carbon and the government laws for CO2 taxes have raised the concerns of the

industry. This has encouraged such producers to develop methods to reduce the reactivity of anodes or their constituents against the gases produced during electrolysis. Successful achievement in the development of the above mentioned methods will lead to the increase of the anode lifetime, reduction of the costs and negative environmental impacts.

Figure 6: Anode consumption [23, 24]

Another important problem which occurs during this reaction is dust emission [25-27]. This phenomenon can be directly related to the increased frequency of anode spikes and loss of current

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efficiency [25-27]. The carbon dust increases the electrical resistance of the bath, leading to a rise in temperature and reduction of the anode-cathode distance [26]. Many attempts have been made to improve anode quality. However, none of these have yet been implemented since producers require a low expense solution with minimal modification of the manufacturing process. Reducing the gasification rate of carbon-oxygen (at the upper part of the anode exposed to air), would lower the amount of carbon consumed per ton of aluminum produced. This will decrease dust emission and also diminish the greenhouse gas emissions.

1.3

Project objectives

One of the main goals of aluminum smelters is to decrease energy consumption in order to stay competitive. One of the great possible improvements for aluminum electrolysis process would be to reduce the anode reactivity to the air surrounding the cells.The protection of carbon with an inhibitor could be an effective way to improve the anode quality. According to the literature, the oxidation can be prevented by coating the anode surface. The principal candidate for this coating is boron oxide which gives carbon a high resistance to oxidation with an advantage of a low cost. The chemical compound cannot be added as a coat to the whole anode surface since it causes contamination of the aluminum produced above acceptable level of boron. Another issue is the localized burn off of the carbon that can be due to improper distribution and thickness of the protective layer if they are uneven or with defects. Addition of a few ppm of boron could help limit the degree of impurity contamination in the metal, and also decrease the oxidation rate of the carbon anodes and raw materials in contact with ambient air at specific temperatures. The work aims to solidify the understanding of the kinetics and inhibitor mechanism of this reaction, as well.

To meet the project goals, three specific objectives had to be satisfied. The first one was to develop a protective coating of boron oxide while meeting the industry restrictions (few ppm of boron impurities in the metal pad) in order to decrease the air reactivity of industrial anodes. In this regard, the impregnation parameters that affect the efficiency of the process were investigated. The behavior of the protected anode during different gasification stages was also determined in order to understand the reaction on the surface and inside the electrode. The second objective was to understand the inhibitor mechanism of a low boron level (in the order of ppm) on carbon gasification

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by revealing the effect of boron impregnation on the kinetics of anode gasification reaction. The characterization of the surface chemistry of the impregnated electrodes should be determined in order to define the inhibitor mechanism. The third objective was to validate if boron could affect the porosity of the raw materials (coke particles) by blocking the gas access. For this purpose, the effect of boron on the active pore size of petroleum coke during gasification was evaluated. The behavior of impregnating coke particles with boron-based solutions at low concentrations prior to anode paste mixing was investigated. Additionally, the effect of several parameters were studied, especially the helium density, pore volume and pore size distribution. The role of porosity of untreated and impregnated coke particles in gasification was revealed.

Finally, the overall goal of this project was to define a novel approach in applying boron in carbon anodes with a low cost and minimum modification of the manufacturing process. The purpose of this work is to provide an applicable, robust and optimized fabrication process to produce carbon anodes with a longer life while considering the technological, environmental and economical aspects. Improvement of the anode quality involves several standard characterizations such as air reactivity, electrical resistivity, apparent density, physical and chemical properties.

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Chapter 2: Literature review

This chapter presents a literature review on the carbon consumption of carbon anodes once introduced in the electrolysis cells. The dust emission of anodes and their effect on the bath and the metal produced is also detailed. When air attacks the anode, several chemical reactions occur that affect the gasification rate of the anode. These reactions might be inhibited in a number of ways that eventually lead to a decrease in the reaction rate of the anode in order to increase its lifetime, decrease the dust emission, and minimize the GHG emissions. These will be considered in the following subsections.

2.1

Carbon anode

2.1.1 Anode consumption

In recent years, studies on the characterization of the temperature of an anode during the production of aluminum have shown that it can vary from 400 to 960 °C depending on the depth in the electrolytic bath [28]. According to Fischer and Perruchoud [28], the temperature of the anode gradually begins to decrease from the bottom to the top of the anode (outside of the bath). Figure 7 shows a schematic representation of the temperatures variation as a function of the bath depth [28]. Thus, the temperature on the surface of the partially immersed anode in the bath (electrolytic surface of the anode) is between 600 and 960 °C, while the surface exposed to air presents a lower variation between 400 and 600 °C.

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Figure 7: Temperature variation of carbon anode in the electrolysis cell [28]

The anode quality is a very important factor from an economic and environmental point of view. During the last few years, several efforts have been made to study this consumption and to find ways to reduce it. Consumption is linked to a series of variables, including the temperature of the electrolyte, the air permeability of the anode, and its thermal conductivity. Carbon anodes contain metallic impurities that can influence the reaction of the anode with air. Houston and Oye [10], and Hume [29] determined the effects of several impurities on the oxidation of carbon by oxygen (Table 2).

Table 3: Effect of impurities on the carbon gasification by oxygen [10, 29]

Reaction C (s) + O2 (g) → CO2 (g)

Catalysts Fe, V, Ni, Na, S, Ca, Pb, Cu, Zn, Cr, Ti, Al Inhibitors S, AlF3, B, P

The upper part of the anode is covered with a layer of protection mix in the electrolytic cell during the first week of placement. However, some part of the anode cannot be fully covered due to the design of the cell and its little space. As a result, the air can attack this part of the anode that is outside the bath. This has a negative influence on the lifetime of the anode, as shown in figure 8. Indeed,

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oxidation of the upper part of the anode surface which is in direct contact with air, is one of the important factors that initiates the over-consumption of the anode [13].

Figure 8: Upper part of anode attacked by air [27]

Fischer [30] correlated the physicochemical data of the cells with those of the anodes (Equation 4) in order to determine the efficiency of electrolysis cells and carbon anodes. The anode reactivity and gas permeability measurements were determined by R&D Carbon Company. Table 4 shows the impact of anode properties on net carbon consumption, taking into account the cell design and operating parameters [31]:

NC = C + 334

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Table 4: Parameters of the equation 1 [31]

Abbreviation Unit Factors Values

NC [kgC/tAl] Net carbon consumption 400-500

C [-] Energy factor of the bath 270-310

CE [%] Current efficiency 0.82- 0.95

BT [°C] Bath temperature 945-980

CRR [%]
 CO2 reactivity residue 75-90

AP [nPm] Air permeability 0.5-5

TC [W/mK] Thermal conductivity 42800

ARR [%] Air reactivity residue 60-90

The prebaked anode undergoes various critical steps which influence its performance in the cell. For example, the anode may be subjected to thermal shock when it enters the electrolysis cell. Therefore, a heat wave penetrates the anode which generates thermal stresses that can cause cracks in it. R&D Carbon company [14, 31] has characterized a large number of industrial anodes (several hundred anodes from sixty different plants) in order to obtain a range of variation of different pre-baked anode properties which are presented in Table 5.

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Table 5: Average values of pre-baked anode properties investigated by R&D Carbon company [14, 31]

Properties Unit Method Average

Apparent Density kg/dm3 ISO 12985 1,50-1,62

Specific Electrical Resistance μΩm ISO 11713 51-74

Flexural Strength MPa ISO 12986-1 42839

Compressive Strength MPa ISO 18515 30-65

Static Elastic Modulus GPa RDC 150 3,0-6,5

Coefficient of Thermal Expansion 10-6K-1 ISO 14420 3,6-4,6

Fracture Energy J/m2 RDC 148 100-260

Thermal Conductivity W/mK ISO 12987 42799

Crystallite Size Å ISO 20203 25-32

Air Permeability nPm ISO 15906 0,3-4,0

CO2 Reactivity Residue % ISO 12988-1 75-96 Dust % 0-10 Loss % 42840 Air Reactivity Residue % ISO 12989-1 55-95 Dust % 42747 Loss % 4-35 S % ISO 12980 0,8-3,0 V ppm ISO 12980 30-350 Ni ppm ISO 12980 70-220 Na ppm ISO 12980 100-1000 Fe ppm ISO 12980 100-800 Si ppm ISO 12980 50-300 P ppm ISO 12980 42765 2.1.2 Dust emission

The dust on the surface of the bath is a result of a selective oxidation of pitch and coke due to the air attack, involving the detachment of carbon particles of the anode [32]. These particles interfere with the electrolysis, causing an increase in the temperature of the bath and then an increase in the electrical resistance of the bath [33]. According to Sadler [34], the air attack on the top of the anodes would produce less dust comparing to the attack of CO2 (in the bath). As described in figure 9, the

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in the bath [14] that results in a loss of current efficiency [25-27]. This "dusting" phenomenon is responsible for over-consuming of the raw materials.

Figure 9: Schematic representation of dust emission of anode in the electrolysis cell [14]

According to the literature [14, 25, 35, 36], the dust emission in the electrolysis cells would be caused by several phenomena such as:

 The preferential reactions of air and CO2 on the binder matrix (fine coke and pitch)

 The wear of the cathode (0.5 to 1 kg of dust consumed per ton of aluminum)

 The wear of protective collars of steel bars supplying current to the anode (0.5 to 6 kg of dust consumed per tonne of aluminum)

 Carbon in recycled alumina (between 0.15 and 0.5% by weight)

It was estimated that for a medium-sized aluminum smelter producing about 300000 tons of metal per year, 9000 tons of aluminum per year (3% of annual production) would be lost from the production because of the dusting phenomenon [14]. Considering that the price of aluminum is about US $ 1920/tAl [37], the above mentioned loss represents about 12 million $US per year [38]. However, some modifications in raw materials or manufacturing conditions have made it possible to reduce the dust emission temporarily. Aluminum smelters are using an Anode Cover Material (ACM) to decrease

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the reaction rate, to control heat losses on the top, and to minimize fluoride emission produced during the process [12, 39]. The ACM are constituted of recycled Recovered Anode Cover Material (RACM), grab pure bath and alumina to avoid thermal loss [12]. Therefore, the carbon loss would then be estimated at around 3 million $ US per year [38].

2.2

Carbon gasification

The temperature of the top of the anode reaches 400°C after a few days of its insertion in the cell, while the combustion occurs at 450 °C. The oxidation reaction of carbon with air is exothermic and the reaction rate is sufficiently high to produce excess heat, CO2 and CO as shown in the equations

5 and 6:

C (solid, anode) + O2 (g) → CO2 (g) G = -395 kJ (at 500 °C) (5)

C (solid, anode) + ½ O2 (g) → CO (g) G = -185 kJ (at 500 °C) (6)

Marsh and Kuo [40] have presented a very complex reaction mechanism to explain the phenomenon of carbon oxidation in the presence of oxygen. However, another simpler mechanism of this reaction has been proposed in the literature [41]:

Cf + O2 → C(O) + O (7)

Cf + O→ C(O) (8)

C(O) → CO + Cf (9)

Where Cf: Free active carbon atom, C(O): Oxygen atom chemisorbed on carbon active site f

(complex).

It is possible to determine the equilibrium constants of the oxidation reaction for temperatures between 450 and 650 °C, at a pressure of 1 atm, by calculating the free enthalpy of the reaction at 298 K (25 °C):

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Both reactions (5 and 6) are heterogeneous (gas + solid) and produce gases. Thus, they are gasification reactions. The gasification rate X (transformation of the solid into CO-type gas, etc.) per mass unit is defined as [42, 43]:

𝑋(𝑡) = 1 − 𝑀( ) 𝑀( )

× 100 (11)

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

This gasification rate depends on many factors such as [44]:  The correlation rate of O2 on the surface of graphite;

 The O2 partial pressure;

 The surface available for the oxidant on the graphite;  The dissipation rate of the reaction products;

 The amount of carboxyl deposits on the surface of graphite pores;  The amount and distribution of catalytic impurities in graphite;  Temperature;

 The diffusion coefficient of gas.

The kinetics of carbon gasification reactions involve three distinct physicochemical phenomena [45]:  Gas transfer;

 Diffusion;

 Chemical reaction.

Three phenomena are controlled essentially by the temperature; Walker et al. [46] define three temperature zones at which several phenomena occurs, as described in figure 10:

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Figure 10: Three zone controlling the reaction rate of porous carbon material [46]

Chemical regime: In this temperature range (400-600 °C), there is a homogeneous concentration of the gas around and in the solid due to the diffusion of the gases. The reaction rate is low compared with the penetration rate of the gas into the pores (diffusion). The chemical reaction is the limiting step, and the activation energy is measured directly using the Arrhenius diagram, thus the chemical regime is occurring [46].

Diffusion regime: The second part is situated in intermediate temperature range (600-920 °C). The reaction rate is influenced by the diffusion of the gas on the solid surface and in the pores as well as the chemical reaction. A gas concentration gradient (O2) is created as a function of the depth of the

sample. Therefore, the concentration of the gas is high on the surface of the material and it decreases gradually once it penetrates inside the material [46].

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Gas transfer regime: This last part corresponds to reactions at high temperature (> 920 °C). The reaction rate is very high and completely controlled by the gas transport to the external surface of the carbon. At these temperatures, the activation energy is lower than the chemical regime and diffusion regime zones [46].

Bonal and Robin [47] have determined these limits for graphite under air oxidation. Based on their results, the temperature range is similar to the upper part of the anode in the electrolysis cell (between 450 and 600 °C), the oxidation reaction should take place approximately in the first part of the graph (chemical regime).

In aluminum smelters and analytical laboratories, carbon anodes are characterized by a number of analytical techniques (mechanical tests, impurity levels, etc.). The anode air reactivity is also evaluated. In the field of aluminum production, the reactivity of the anodes is quantified by the mass loss generated by the attack of air on the anode for a defined time and at a given temperature. These mass losses are in the form of dust and gas which can be measured with the equipment designed by R&D Carbon company [48] (ISO 12989-1). Both of these air reactivity techniques are able to predict the anode behavior in electrolysis cells qualitatively. However, these apparatuses do not allow us to describe the physicochemical transformations taking place on the surface and inside the anodes. A better understanding of the phenomena occurring on the surface of the anode would make it possible to better interact with the industrial process which could ultimately reduce the over-consumption of raw materials generated by the reactivity between gases and carbon.

2.3

Inhibition of carbon reaction

2.3.1 Effect of inhibitors on carbonaceous materials oxidation

Among several factors that can influence the gasification rate of carbon are temperature, gas flow, carbon type, pore volume and their distribution, crystallite size (LC), active surface area, catalysts,

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A. Reduction of the quantity of catalysts, essentially the metallic one, by extraction of metallic impurities [25, 34];

B. Reduction of the number of active sites by increasing the degree of graphitization (increase of Lc) [49];

C. Limitation of gas penetration using a coating on the carbon surface [50-52];

D. Prevention of the chemical reaction by formation of chemical complexes at carbon reactive sites [53-57].

Regarding the problem related to the oxidation of the anode, the first two approaches do not seem conceivable. In fact, since the amount of impurities in the raw materials depends on the nature of the pitch, coke or anode butts, it is difficult to interact with the quantity of impurities during the anode manufacturing process [10, 14, 30]. On the other hand, the crystallinity (LC)

(Figure 11) of the anode depends on the baking time and temperature [14, 58]. These two parameters are more or less fixed, and therefore difficult to be manipulated without modifying the physical characteristics of the anodes. Encasing the carbon surface with a coating was another approach studied by a number of researchers because of the simplicity of the application and its efficiency [50-52].

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Figure 11: Evolution of carbon structure as function of temperature [14, 58]

Another relationship has been proposed by Lavigne and Castonguay [59] to predict the reactivity of calcined petroleum coke. The study involved 39 cokes of commercial calcined oils. As presented in figure 12, it should be mentioned that the calcined cokes having a large LC value react less with air.

Figure 12. Effect of Lc on the reactivity of petroleum coke [59]

Based on the literature, phosphoric acid and phosphate have an inhibitor effect on the air reactivity of carbon (Figure 13) [56]. McKee [54] revealed that phosphorus has a good resistance to carbon

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oxidation at high temperatures (400-600°C) under air atmosphere. The thermal decomposition of the phosphorus compounds at 200 to 600 °C results in the formation of a hydrophilic residue which is strongly adsorbed on the graphite surface (Figure 14), and more specifically on carbon active sites where the oxidation occurs [54]. However, it was shown that the addition of phosphorus in the carbon anode could reduce the current efficiency in the cell and contaminate the produced aluminum [50].

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Figure 14: Schematic representation of phosphate organic residues adsorbed in the graphite surface after heat

treatment [54]

The CSIRO Light Metals Flagship [52] has developed an aluminum spray coating and a surface treatment containing of alumina with a small quantity of a silicate-based binder (figure 15). These coatings have been tested in industrial scale in order to evaluate air burn via the lifetime of the anode in the electrolysis cells. The net carbon reduction of the alumina-silicate based binder coating was estimated to be about 0.02 Kg C/Kg Al. However, these techniques have not been applied in aluminum smelters, possibly due to the higher cost [51].

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Boron oxide has been recognized as an inhibitor of the reactivity of graphite to air [51]. Boron ore is extracted with high concentrations, mainly in the United States and Turkey. Their mineralization takes the form of borax (Na2B4O5(OH)4,8H2O), kernite (Na2 B4O5(OH)4,2H2O), Colemanite

(Ca2B6O11,5H2O) and Ulexite (NaCaB5O9,8H2O) [60].

Boron oxide (B2O3) is known as diboron trioxide, boric anhydride, or anhydrous boric acid, and has

a molar mass of 69.62 g/mol [61]. Boron oxide is a hygroscopic material at room temperature. It absorbs moisture from the air, thus forming boric acid according to equation 12 [61]:

B2O3 (glass) + 3H2O (l) → 2B(OH)3 (aq) (12)

This reaction is exothermic with a H298 = -75.94 kJ/molmay be reversible by heating the boric acid

which undergoes multi-stage dehydration with temperature [61]. At high temperature (>1000°C), molten boric oxide easily reacts with water vapor, forming metaboric acid in the vapor state.

The following three equations explain the behavior of boric acid as a function of temperature [61]. At 170.9°C, boric acid transforms to metaboric acid (equation 13):

B(OH)3 →HBO2 + H2O (13)

Continuing heating to 236°C, the metaboric acid undergoes further dehydration to form the tetraboric acid (equation 14):

4 HBO2 → H2B4O7 + H2O (14) Finally, the tetraboric acid converts to boron oxide at a temperature above 300°C [61]:

H2B4O7 → 2 B2O3 + 3 H2O (15) Boron oxide can be found in liquid form in the electrolysis cell once reaching the melting point that is about 450°C. However, boron oxide is a good candidate to protect carbon anodes against air oxidation, thus increasing their lifetime. Nevertheless, adding several ppm of boron oxide to the carbon anode does not affect the process.

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2.3.2 Effect of boron on carbon oxidation

Several studies are available in the literature on the inhibitor effect of boron on graphite oxidation. McKee [62] studied the effects of impregnating graphite powder with B2O3 and organo-boron

compounds and evaluated the air oxidation. The results showed that organo-borates added a higher resistance to graphite air oxidation at a high temperature range (600-1000°C). In a later paper [63], McKee studied the inhibitory effects of several boron compounds (such as ammonium borate and boric acid) on the reactivity of carbon fibers and carbon/carbon composites against oxygen. The liquid organo-borates seep into the pores and then transform to boron oxide at higher temperatures forming a coating layer on the composite surface. However, according to McKee, “this coating was less effective above 1000°C because of the chemical volatilisation of borate species and the presence of water vapor”.

Another study of McKee [64] aimed to investigate the dispersed refractory effect of metal compounds as oxidation catalysts. The author found that the borides of Zr, Si, Cr and Al exerted an inhibiting effect while V and Mo played the role of a catalyst of carbon/carbon composite oxidation under air. McKee [65] later showed that the air oxidation reactions of carbon were controlled by gas diffusion. However, boron oxides form a crystalline layer on the graphite particle preventing carbon oxidation.

Several attempts have been made to decrease the over-consumption of carbon anode used in aluminum smelters [66-73]. Inhibiting carbon oxidation caused by air attack would reduce the amount of greenhouse gas emissions (CO2, CO) emitted by aluminum smelters and increase the yield per

kilogram of carbon per tonne of aluminum. For this purpose, boron oxide has been applied as an inhibitor of carbon anodes gasification in aluminum process [66]. These techniques are based on the addition of inhibitors to the coke-pitch mixture and the application of protective coatings to the anode surface [50]. Berclaz et al. [66], investigated the impregnation of the upper part of carbon anode used in the aluminum industry in a solution of boron salts. The impregnation was performed under vacuum using a low boron concentration to minimize metal contamination. The authors compared their results to aluminum spray treatment. It was found that boron impregnation reduced the oxidation as well as dust emission. In addition, the boron treatment cost was lower than aluminium spray. Given that the boron concentration should be controlled in order to limit the metal contamination.

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Applying a layer on the anode surface by a vitreous ceramic showed an efficient protection against air burning, especially by keeping the same microstructure parameters and composition [52]. The protection methods developed for anodes in recent years have been based on applying boron oxide in the form of a paste on a substrate. For this purpose, two approaches can be envisaged: one is to extend hydrated boron oxide on the substrate and the other to immerse the carbon substrates in molten boron oxide. One of the main disadvantages of these two methods is the production of thick and irregular coatings. In addition, a large amount of boron oxide could contaminate the aluminum produced. The molten boron oxide is highly viscous with low wettability with the anode, resulting in poor adhesion to the anode surface. The application of boron oxide under these conditions implies the formation of air bubbles in the coating (Figure 16, a test conducted at Laval University). Therefore, it is necessary to heat the sample very slowly up to 1050 °C in order to form a continuous and adherent coating. This method is effective against oxidation in air atmosphere. However, it is far from being applied in industry because it requires a large amount of boron oxide to form this coating layer on the anode surface.

Figure 16: Cross-section of a sample after heat treatment at 1050°C and air burning test. A continuous layer of boron oxide with a thickness of 100 μm on the surface protects the carbon from air burning

Tosta et al [51] tried to decrease the resistance of the anode coating by adding a small quantity of silica gel to the boric acid solution. Results showed an improvement on coating adherence of anode

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samples as well as the air oxidation. The authors revealed that the addition of silica gel in the solution modified the microstructure of boric acid deposit, thus having a better covering of the pores on the anode surface. However, it should be mentioned that the boron concentration was still high.

2.3.3 Carbon protection mechanisms induced by boron

According to the literature, boron seems to be an effective candidate to protect the carbon towards oxygen. Therefore, revealing the inhibitor mechanism of boron on carbonaceous materials was a big challenge for researchers. However, several hypotheses were proposed in the literature to describe this mechanism. Boron oxide can provide protection of the anode against air reactivity by the following mechanisms:

 By inserting it into the graphite crystal structure at a very high temperature (about 2500°C) and substituting carbon atoms [74];

 By occupying carbon active sites of the surface, thus it inhibits their reactivity [55,74,75];  By creating a physical barrier to the diffusion of oxygen when it is in liquid or molten form

[52].

X. Wu et al. [74], studied the inhibitor effect of high temperature (2500°C) boron-doping on the catalytic oxidation of carbon composites. Boron was substituted in the carbon structure in order to enhance its graphitization. Boron doping in carbon composites has supressed the catalytic activity of calcium during the gasification under air atmosphere, as well. Radovic et al. [76] specified that boron-doping also had an inhibitor effect on carbon materials by decreasing the total electron density of the reactive atoms of carbon atoms, thus reducing oxygenchemisorption. Moreover, Radovic et al [76] reported that the distribution of  and -electrons was modified due to the substitution of boron in the graphite structure. In other words, electrons are localized on carbon atoms due to their higher electronegativity compared to boron atoms. This technique could not be applied to carbon anodes or their raw materials because of the high temperature used for boron doping.

The reduction of the oxidation rate may be either due to a blocking mechanism of the carbon active sites or to the formation of diffusion barriers that limits the accessibility of the oxygen on the surface

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of the sample [76]. McKee [55] studied the effect of boron oxide (5 to 20 wt. %) impregnated on the graphite powder after graphitization. This powder was dried at 200°C in order to evaporate the water. Thereafter, an air reactivity test was performed for a temperature range between 600 and 900°C (figure 17). In the presence of 5 wt. % B2O3, the activation energy increased from 46.2 to 51.4

Kcal/mol. A negligible change in activation energy was observed with the subsequent increase of boron oxide concentration to 20 wt. %. The fact that the apparent activation energy is not affected by the presence of boron oxide may explain that the decrease in oxidation rate is due to the decrease in the number of active sites [75]. The B2O3 levels used in the McKee graphite study were much

higher than the allowable 150 ppm for carbon anodes.

Figure 17: The effect of boron oxide on graphite oxidation under air atmosphere [55]

The vitreous boron oxide has a random structure composed of polymeric residue (BO3)n where each

boron atom is bonded to three oxygen atoms. The terminal of oxygen atoms can bind to the carbon atoms on the surface or impurities corresponding to the carbon active sites. McKee [55] specified

Figure

Figure 1: Greenhouse gas emissions by province and territory, Canada, 1990, 2005 and 2015 [4]
Table 1: Electrical generation source in Quebec comparing to China, Russia and Middle East [5]
Figure 3: Schematic representation of aluminum electrolysis cell with prebaked anodes [9]
Figure 4: Schematic representation of the amount of raw materials and energy to produce 1 kg of primary  aluminum [12]
+7

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