A Bio-coke for anode production and the manufacturing method thereof

Mémoire

Asem Hussein

Maîtrise en génie des matériaux et de la métallurgie

Maître ès sciences (M.Sc.)

Québec, Canada

© Asem Hussein, 2014

Résumé Dans l’industrie de l’aluminium, le coke de pétrole calciné est considéré comme étant le composant

principal de l’anode. Une diminution dans la qualité du coke de pétrole a été observée suite à une

augmentation de sa concentration en impuretés. Cela est très important pour les alumineries car ces

impuretés, en plus d’avoir un effet réducteur sur la performance des anodes, contaminent le métal

produit. Le coke de pétrole est aussi une source de carbone fossile et, durant sa consommation, lors

du processus d’électrolyse, il y a production de CO2. Ce dernier est considéré comme un gaz à effet

de serre et il est bien connu pour son rôle dans le réchauffement planétaire et aussi dans les

changements climatiques.

Le charbon de bois est disponible et est produit mondialement en grande quantité. Il pourrait être une

alternative attrayante pour le coke de pétrole dans la fabrication des anodes de carbone utilisées

dans les cuves d’électrolyse pour la production de l’aluminium. Toutefois, puisqu’il ne répond pas

aux critères de fabrication des anodes, son utilisation représente donc un grand défi. En effet, ses

principaux désavantages connus sont sa grande porosité, sa structure désordonnée et son haut taux

de minéraux. De plus, sa densité et sa conductivité électrique ont été rapportées comme étant

inférieures à celles du coke de pétrole.

L’objectif de ce travail est d’explorer l’effet du traitement de chaleur sur les propriétés du charbon de

bois et cela, dans le but de trouver celles qui s’approchent le plus des spécifications requises pour la

production des anodes. L’évolution de la structure du charbon de bois calciné à haute température a

été suivie à l’aide de différentes techniques. La réduction de son contenu en minéraux a été obtenue

suite à des traitements avec de l’acide chlorhydrique utilisé à différentes concentrations. Finalement,

différentes combinaisons de ces deux traitements, calcination et lixiviation, ont été essayées dans le

but de trouver les meilleures conditions de traitement.

v

Abstract In aluminum industry, calcined petroleum coke is considered as the major component in anode

recipe. There is a trend of degrading quality of petroleum coke as the level of impurities is increasing.

This is important for the aluminum industry because these impurities reduce the anode performance

and contaminate the produced metal. In addition, petroleum coke is a fossil source of carbon and

CO2, produced during its consumption in aluminum electrolysis is considered as a greenhouse gas

(GHG) with a well-known role in the global warming and climate changes.

Due to its availability and massive worldwide production, wood charcoal is an attractive alternative for

petroleum coke in production of carbon anode for aluminum smelting process. However, using

charcoal in anode production is a big challenge since it does not meet the specifications required for

anode making. The very porous and disordered carbon structure and its relatively high minerals

content are considered as serious disadvantages. In addition, its density and electrical conductivity

were reported to be lower than those of petroleum coke.

This work explores the effect of heat treatment on properties of charcoal with the aim to bring them

closer to the specifications required for anode making. At high temperature, the structural evolution of

charcoal was detected using several techniques. In addition, various acid leaching conditions were

used to reduce the ash content. Different calcination/acid leaching combinations were performed to

attain the optimum treatment condition. The materials were then characterized for air and CO2

reactivity in order to assess their potential application in anode manufacturing.

vii

Table of contents

Résumé ............................................................................................................................................................... iii

Abstract ............................................................................................................................................................... v

Table of contents ........................................................................................................................................... vii

List of Tables ................................................................................................................................................. ix

List of Figures ................................................................................................................................................ xi

Acknowledgments ............................................................................................................................................. xiii

Chapter 1 : Introduction ................................................................................................................................... 1 1.1 Production of aluminum ............................................................................................................................ 1

1.1.1 Bayer Process ................................................................................................................................... 1

1.1.2 Hall–Héroult process ......................................................................................................................... 2

1.2 Carbon anodes ......................................................................................................................................... 4

1.3 Anode raw materials ................................................................................................................................. 4

1.3.1 Petroleum coke ............................................................................................................................. 4

1.3.2 Coal tar pitch ..................................................................................................................................... 7

1.4 Anode manufacturing ................................................................................................................................ 7

1.5 Anode consumption .................................................................................................................................. 9

1.6 Quality of petroleum coke ....................................................................................................................... 10

1.7 Environmental Impact of aluminum production. ...................................................................................... 11

1.8 Problems ................................................................................................................................................. 12

1.9 A Working Hypothesis ............................................................................................................................. 12

1.10 Project objectives .................................................................................................................................. 13

Chapter 2 : Literature review.......................................................................................................................... 15 2.1 Bio-char (charcoal) .................................................................................................................................. 15

2.2 Structure of carbonaceous materials ...................................................................................................... 15

2.3 Effect of heat treatment on charcoal structure ........................................................................................ 17

2.4 Changes in physical properties of charcoal during heat treatment ......................................................... 22

2.5 Reactivity of charcoal with air and CO2 ................................................................................................... 26

2.5.1 Effect of heat treatment on the reactivity of charcoal with air and CO2 ............................................ 27

2.5.2 Effect of acid leaching on the reactivity of charcoal with air and CO2 .............................................. 29

2.6 Charcoal as a raw material for electrodes production ............................................................................. 31

2.7 Summary ................................................................................................................................................ 32

Chapter 3 : Materials and methods ................................................................................................................ 33 3.1 Introduction ............................................................................................................................................. 33

3.2 Description of raw materials .................................................................................................................... 33

3.2.1 Wood charcoal ................................................................................................................................. 33

3.2.2 Calcined petroleum coke ................................................................................................................. 33

3.3 Experimental procedures ........................................................................................................................ 34

3.3.1 Heat treatment process ................................................................................................................... 34

3.3.2 Acid leaching process ...................................................................................................................... 35

3.3.3 Combination between acid leaching and heat treatment processes................................................ 35

3.4 Characterization techniques ................................................................................................................... 36

viii

3.4.1 Real density ..................................................................................................................................... 36

3.4.2 Specific surface area ....................................................................................................................... 36

3.4.3 Fourier transform infrared spectroscopy (FTIR) ............................................................................... 37

3.4.4 Powder X-ray diffraction (XRD) ........................................................................................................ 37

3.4.5 X-ray fluorescence (XRF) ................................................................................................................ 38

3.4.6 Raman spectroscopy ....................................................................................................................... 38

3.4.7 Air and CO2 reactivities .................................................................................................................... 39

3.4.8 Weight loss during heat treatment ................................................................................................... 40

3.4.9 Scanning electron microscopy (SEM) .............................................................................................. 40

Chapter 4 : Study of the heat treatment effect on the charcoal structural evolution ....................................... 41 4.1 Introduction ............................................................................................................................................. 41

4.2 Weight loss under inert atmosphere........................................................................................................ 41

4.3 Fourier transform infrared spectroscopy (FTIR) ...................................................................................... 42

4.4 X-ray diffraction analysis (XRD) .............................................................................................................. 43

4.5 Raman spectroscopy .............................................................................................................................. 46

4.5.1 Curve-fitting of Raman spectra ........................................................................................................ 47

4.5.2 The ratio between the D and G bands ............................................................................................. 50

4.5.3 The ratio between the D and the (GR + VL+ VR) bands .................................................................... 51

4.6 Conclusions............................................................................................................................................. 52

Chapter 5 : Effects of heat treatment on physical properties and reactivity of charcoal ................................. 53 5.1 Introduction ............................................................................................................................................. 53

5.2 Effect of heat treatment temperature on the specific surface area of charcoal ....................................... 53

5.3 Effect of heat treatment temperature on the real density of charcoal ...................................................... 54

5.4 Scanning electron microscopy analysis .................................................................................................. 55

5.5 Effect of heat treatment on charcoal CO2 and air reactivities .................................................................. 58

5.5.1 Effect of heat treatment temperature on CO2 reactivity of charcoal ................................................. 58

5.5.2 Effect of heat treatment temperature on air reactivity of charcoal .................................................... 59

5.6 Conclusions............................................................................................................................................. 61

Chapter 6 : Effect of a combination between acid leaching and heat treatment on the reactivity of charcoal 63 6.1 Introduction ............................................................................................................................................. 63

6.2 Effect of acid concentration on the mineral content ................................................................................ 63

6.3 Effect of acid leaching on the reactivity of charcoal. ............................................................................... 66

6.4 Effect of a combination of acid leaching with heat treatment on the reactivity of charcoal. ..................... 69

6.5 Conclusions............................................................................................................................................. 72

Chapter 7 : General Conclusions and recommendations for future works ..................................................... 73 7.1 General conclusions ................................................................................................................................ 73

7.2 Salient findings and novelties of this study.............................................................................................. 75

7.3 Recommended future work ..................................................................................................................... 75

References ........................................................................................................................................................ 77

ix

List of Tables

Table 1.1: Typical Calcined petroleum coke properties [12] ................................................................................ 6 Table 1.2: Typical prebaked anode properties [16] ............................................................................................. 8 Table 3.1: Properties of raw charcoal ................................................................................................................ 33 Table 3.2: Chemical composition and physical properties of calcined petroleum coke ..................................... 34 Table 4.1: Summary of the Raman band assignment [75] ................................................................................ 48 Table 4.2: Positions of Raman band used for curve fitting process .................................................................. 48 Table 5.1: N2 and CO2 surface area of charcoal as a function of heat treatment temperature and those of CPC

................................................................................................................................................................. 54

Table 6.1: Mineral content of CPC and charcoal untreated and treated with different acid concentrations ...... 64

xi

List of Figures

Figure 1.1: Schematic flow sheet of the Bayer process [1] ................................................................................. 2 Figure 1.2: Electrolysis cell with prebaked anodes [5] ......................................................................................... 3 Figure 1.3: Delayed coking process [9] ............................................................................................................... 5 Figure 1.4: Schematic diagram of the anode manufacturing process [15] .......................................................... 8 Figure 1.5: Anode consumption [17] ................................................................................................................... 9 Figure 1.6: Anode excess consumption air burn (a) and CO2 reactivity (b) [18] .................................................. 9 Figure 2.2: Schematic Representations of (a) Non-Graphitizing Carbon and (b) Graphitizing Carbon [34] ...... 16 Figure 2.1: Structure of graphite [33] ................................................................................................................. 16 Figure 2.3: Schematic representation of a model of non-graphitizing carbon materials [36] ............................. 17 Figure 2.4: Schematic representation of La and Lc ............................................................................................ 19 Figure 2.5: Schematic demonstrating of the quasipercolation model [41] ......................................................... 19 Figure 2.6: Relationship between the bulk density of wood and that of the produced charcoal at 900 °C [50] . 22 Figure 2.7: Bulk density variations as a function of carbonization temperature [51] .......................................... 23 Figure 2.8: Effect of HTT on the real density of different types of charcoal [41] ................................................ 24 Figure 2.9: Resistivity of three carbonized wood-based fiberboard as a function of HTT [51] .......................... 26 Figure 2.10: Effect of different catalysts on charcoal reactivity with CO2 [66] ................................................... 29 Figure 2.11: Effect of leaching conditions on CO2 reactivity of almond shells charcoal [69] ............................. 30 Figure 2.12: Electrical resistivity as a function of HTT for BCE, babassu nut and lignin coke [70] .................... 31 Figure 3.1: Scheme of the heat treatment process ........................................................................................... 34 Figure 3.2: Scheme of the acid leaching Process ............................................................................................. 35 Figure 3.3: NETZSCH STA 449 F3 TGA instrument. ........................................................................................ 39 Figure 4.1: TGA curve of raw charcoal heated under nitrogen to 1000 °C ....................................................... 41 Figure 4.2: FTIR spectra of raw charcoal and charcoal samples heated at 600, 800 and 1000 °C .................. 42 Figure 4.3: FTIR spectra of calcined petroleum coke and charcoal sample heated at 1400 °C ........................ 43 Figure 4.4: XRD patterns of charcoal samples heat treated at different temperatures ...................................... 44 Figure 4.5: XRD patterns of raw charcoal and of charcoal heat treated at 1400 °C ......................................... 44 Figure 4.6: XRD patterns of calcined petroleum coke (CPC) and charcoal treated at 1400 °C ........................ 45 Figure 4.7: Effect of heat treatment temperature on Raman spectra of charcoal .............................................. 47 Figure 4.8: Curve-fitting of a Raman spectrum of the raw charcoal sample. ..................................................... 49 Figure 4.9: Curve-fitting of a Raman spectrum of the charcoal sample heated at 1400 °C. ............................. 50 Figure 4.10: ID/IG band ratio as a function of heat treatment temperature ....................................................... 51 Figure 4.11: ID/ I (GR +VL+ VR) bad ratio as a function of heat treatment temperature ................................... 52 Figure 5.1: Effect of heat treatment temperature on the charcoal real density .................................................. 55 Figure 5.2: SEM images of the raw charcoal sample ........................................................................................ 56 Figure 5.3: SEM images of charcoal samples heated at 1000 °C.(A) and 1300 °C (B, C) ............................... 57 Figure 5.4: CO2 reactivity (conversion-time data) of charcoal samples heated between 1000 and 1400 °C and

of calcined coke ....................................................................................................................................... 58

Figure 5.5: Specific CO2 reactivity of charcoal samples heated between 1000 and 1400 °C and of calcined coke .......................................................................................................................................................... 59

Figure 5.6: Air reactivity (conversion percentage vs time) of charcoal samples heated between 600 and 1400 °C and of calcined coke ........................................................................................................................... 60

xii

Figure 5.7: Specific air reactivity of charcoal samples heated between 600 and 1400 °C and of calcined coke .................................................................................................................................................................. 60

Figure 6.1: Percentage of the removed inorganic minerals for three acid concentrations ................................ 64 Figure 6.2: (a) Backscattered electron image of raw charcoal. (b) EDS spectrum of the mineral found in one

white spot of (a) ........................................................................................................................................ 65

Figure 6.3: Backscattered electron image of HL1 sample ................................................................................. 66 Figure 6.4: Air reactivity data obtained at 525 °C for charcoal samples leached with 0.01, 0.1, and 1 N HCl

and the raw charcoal sample (a) conversion-time data and (b) reaction rate data. .................................. 67

Figure 6.5: CO2 reactivity data obtained at 960 °C for charcoal samples leached with 0.01, 0.1, and 1 N HCl and the raw charcoal sample (a) conversion-time data and (b) reaction rate data ................................... 68

Figure 6.6: Comparison between the air (a) and CO2 (b) reactivities of the HL 1 and CPC .............................. 68 Figure 6.7: Effect of the combination of the acid leaching (at different concentrations) with the heat treatment

on the air reactivity of charcoal (a) conversion-time data and (b) reaction rate data ................................ 70

Figure 6.8: Effect of the combination of the acid leacing (at different concentrations) with heat treatment on the CO2 reactivity of charcoal (a) conversion-time data and (b) reaction rate data ......................................... 70

Figure 6.9: Effect of performing the acid leaching before and after the calcination process on the air reactivity of charcoal (a) conversion-time and (b) reaction rate ............................................................................... 71

Figure 6.10: Effect of performing the acid leaching before and after the calcination process on the CO2 reactivity of charcoal (a) conversion-time and (b) reaction rate ................................................................ 72

xiii

Acknowledgments I express my deepest sense of gratitude to Dr. Houshang Alamdari for his keen interest,

encouragement, constructive suggestions and esteemed guidance throughout this two years project

of this work. I am really thankful to him for his endless support that he provided throughout the

project.

I would like to acknowledge my deep sense of gratitude to Prof. Faïcal Larachi for his support and

valuable advices.

I gratefully acknowledge Prof. Mario Fafard for his helpful advices throughout this work.

Advices, direction and encouragements and the valuable assistance of Dr. Donald Ziegler, Dr. Jayson

Tessier and Mr. Pierre Mineau from Alcoa Inc. were very important and are acknowledged.

The financial support of the National Science and Engineering Research Council (NSERC) and Alcoa Inc.

and of “Fonds de recherche du Québec – Nature et technologies (FQRNT) through "Le Centre de

recherche sur l'aluminium (REGAL) is gratefully appreciated.

I wish to thank all the faculty members & staffs of the Mining, Metallurgical and Materials Engineering

Department of Université Laval and Regal-Alcoa research group members specially, Lise Lemieux,

Dr. Donald Picard, Guillaume Gauvin, Jean-François Rioux-Dubé, (from Chemistry Department),

Vicky Dodier, Kamran Azari, Francois Chevarin, Behzad Majidi, Geoffroy Rouget, Ramzi Ishak for

their friendship, support and help during the project.

Last but not the least, I would like to express my gratefulness to my parents, my wife Hend and my

daughter Haneen, for their endless support and encouragement, without which I never would have

been able to finish my project.

https://exchange.ulaval.ca/owa/?ae=Item&t=IPM.Note&id=RgAAAABFz4cbGUejSqaeJVYN8fE8BwCYG3KBP8BhSqN%2bpBHMoHhpATgvxGhUAAAyd%2ft1qWbXQ5Wfn%2frBwsUfAIbpLb4VAAAJhttps://exchange.ulaval.ca/owa/?ae=Item&t=IPM.Note&id=RgAAAABFz4cbGUejSqaeJVYN8fE8BwCYG3KBP8BhSqN%2bpBHMoHhpATgvxGhUAAAyd%2ft1qWbXQ5Wfn%2frBwsUfAIbpLb10AAAJ

Chapter 1 : Introduction

1.1 Production of aluminum Aluminum is one of the most abundant elements in the earth’s crust. With 8% of the crust

composition, it ranks third after oxygen (47%) and silicon (26%). Due to its strong chemical affinity

with oxygen, aluminum is not found naturally as a pure element, rather it occurs as oxide (Al2O3) in

bauxite ore and silicates [1].

Aluminum chloride (AlCl3) was the first raw material used to produce the metal commercially. AlCl3

was reduced with sodium by a complicated and expensive process as sodium should first be

produced electrolytically [2, 3].

In 1886, both the American chemist Charles Hall and French engineer Paul Héroult independently

discovered and developed the process by which alumina (Al2O3 ) was dissolved in molten cryolite and

by electrolysis at 980-1000 °C, molten aluminum was produced [2, 3]. Nowadays, pure aluminum

production involves two independent processes to transform the ore, which is bauxite, to the metal.

The first step is the extraction of alumina from bauxite via the Bayer process. The subsequent step is

the Hall-Héroult process in which alumina is electrolytically reduced to molten aluminum.

1.1.1 Bayer Process

Extraction of alumina (Al2O3) from the bauxite ore using Bayer process is the first step to produce

pure aluminum industrially. Figure 1.1 shows a schematic representation of the Bayer process. In the

first step, bauxite is slurred (and stirred) with sodium hydroxide at high temperature to dissolve

alumina as slurry of sodium aluminate (NaAlO2) while the other insoluble oxides like Fe2O3 and

silicates precipitate as red mud. The resulting slurry of sodium aluminate and red mud are then

separated from each other by filtration. The filtrate is cooled down and continuously stirred until the

precipitation of solid alumina trihydrate (Al2O3. 3H2O). In the final step, the precipitated alumina

trihydrate is calcined at 1200- 1300 °C in order to remove the water of crystallization leaving behind

alumina, ready to be used in aluminum smelters [4].

2

Figure 1.1: Schematic flow sheet of the Bayer process [1]

1.1.2 Hall–Héroult process

All modern smelters use the Hall–Héroult process to electrolytically reduce the alumina dissolved in

molten cryolite. In the electrolysis cell, high amperage direct electrical current with low voltage passes

between the carbon anode and cathode through the electrolyte (bath). Oxygen from alumina is

discharged electrolytically and reacts immediately with the carbon anodes, producing gaseous carbon

dioxide (CO2). By this chemical reaction the anodes are gradually consumed. Molten aluminum sinks

under the bath and deposits on the carbon cathode as its density is higher than that of the electrolyte.

The produced pool of molten aluminum acts as the cathode [2]. The overall chemical reaction can be

written as:

2 Al2O3 (diss.) + 3 C (s) = 4 Al (l) + 3 CO2 (g) Eq. 1.1

Depending on the design of the used anode, aluminum reduction cells can be classified into self

baked anode (Søderberg) and prebaked anode cells. Due to the high carbon efficiency and more

convenient working environment with prebaked anodes, most of the smelters which were using

Søderberg technology have already converted to prebaked anode technology.

Figure 1.2 [5] shows commercial prebaked aluminum production cell, consisting of; cathode,

electrolyte and anode. During the electrolysis, the pool of molten aluminum acts as cathode.

However, cathode is a term used industrially to describe the prebaked carbon blocks which are

cemented together with ramming paste. These blocks, forming the bottom cathode and side-walls, are

3

made of graphitic materials and are joined together by ramming paste. Although cathode blocks are

not consumed during electrolysis, sodium penetration and aluminum carbide formation may cause

cathode failure and shut-down of the cell for relining [2, 3].

The electrolytic bath consists mainly of cryolite (Na3AlF6) and alumina. However, several other

additives like CaF2, AlF3 and LiF are also used. The bath is not consumed during electrolysis, but

some loss occurs due to vaporization. As alumina is consumed by the electrolytic reaction, it is

continuously fed to maintain its concentration in the bath between 2-4%. The bath temperature during

cell operation ranges between 940 °C and 970 °C and its height is usually close to 20 cm.

Electrolysis takes place in the gap between the bottom of the anode and the surface of the aluminum

pool (anode-cathode distance (ACD)), which is typically 4-5 cm [2].

Prebaked anodes are made of calcined petroleum coke and coal tar pitch as a binder; these

components are compacted into a specific shape by pressing or vibrocompacting. The green

compacts are then baked at about 1100 °C in a special baking furnace. Finally, an aluminum rod with

iron stub frame is then cast into grooves on the top of the anode block. This frame holds the anode in

its position and conducts the electrical current during the cell operations [2, 3].

Figure 1.2 Electrolysis cell with prebaked anodes. [5] Figure 1.2: Electrolysis cell with prebaked anodes [5]

4

According to the stoichiometric ratio predicted from equation 1.1, 1.89 kg of Al2O3 and 0.33 kg of

carbon are required to produce 1 kg of primary aluminum and 1.22 kg of carbon dioxide. However,

this perfect consumption of raw materials is not met in practice since some of them are consumed in

other side reactions. The typical values are 1.93 kg of Al2O3 and 0.40–0.45 kg of C per kg Al, which

produce about 1.5 kg of CO2 [6].

1.2 Carbon anodes Carbon anodes are considered as a raw material for aluminum production as they are continuously

consumed during electrolysis. Actual anode lifetime is approximately 26-28 days, after this period it

should be replaced by a new one. Since it is not totally consumed during its service cycle, the

remaining parts of the anode, called butts, are cleaned and recycled to be used in new anode

recipes. A typical prebaked anode consists of approximately 65% calcined petroleum coke, 20%

recycled anode butts and 15% coal tar pitch. During cell operations, anodes act as electrical

conductor and as reducing agent in the electrochemical process. High chemical purity, high electrical

conductivity, high mechanical strength and low air and CO2 reactivities are considered the most

important requirements for a high quality anode product [7, 8].

1.3 Anode raw materials

1.3.1 Petroleum coke

Petroleum coke is produced from the heavy residual fractions of crude oil by delayed coking. In

delayed coking process, thermal cracking and condensation/polymerization reactions are responsible

for the partial conversion of viscous liquid to solid coke [9]. The main objective of delayed coking is to

minimize the low-valued residual production and to maximize the high-valued low-boiling point

hydrocarbon converted fractions. Petroleum coke produced is thus considered as a by-product with

low interest because its value represents only 2% of the total value of all the other products [10].

5

Figure 1.3 shows the scheme of delayed coking process. In the first step, the fresh heavy residue is mixed with the recycled fractionator bottom residue and then heated in a furnace at a temperature between 480-515 °C. The heated feedstock is pumped to one of a pair of coking drums and kept at a temperature between 415 and 450 °C and a pressure ranging from 1 to 6 bar (15-90 Psi). A coking drum is usually operated for about 24 h, and then the resulting coke is removed by high-pressure water jets (decoking) and kept into a storage bunker [9]. Depending on the chemical composition of the feed crude oil and the parameters of coking process, three different types of coke are produced: needle, sponge and shot coke. Needle coke is obtained from asphaltenes-free crudes, such as hydrotreated fluid catalytic cracking (FCC) decant oils. Due to its relatively high price, this type of coke is not used as anode raw material in the aluminum industry. So called due to its spongy appearance, the sponge coke is made from feeds that have low to moderate asphaltene concentration. As its properties meet the specifications required by aluminum smelters, sponge coke is called anode grade coke (provided its sulfur content is low enough) and it is used, after calcination, as raw material to produce anodes. Shot coke is produced from crudes rich in asphaltene and oxygen contents. If the temperature of the coke drum is too high, then even feeds with lower asphaltene content can be transformed to shot coke. However, due to its high coefficient of thermal expansion (CTE) and high reactivity, this product is undesirable for anode production [11].

Figure 1.3: Delayed coking process [9]

6

Being characterized by a high volatile content (8-12%), a very weak and amorphous structure, a high

reactivity and a poor electrical conductivity, green coke, produced by delayed coking is not suitable to

be used directly in the anode recipe. Nevertheless, its properties can be improved by heat treatment,

(calcining), at temperature between 1250 and 1350 °C. This way, the rotary kiln and the rotary hearth

calciners are currently used to produce calcined coke with adequate properties [7].

Typical characteristics for calcined “anode grade” petroleum coke are shown in table 1.1 [12].

However, there is a trend of degrading quality of petroleum coke as the level of impurities, especially

sulphur and vanadium, is increasing [7].

Table 1.1: Typical Calcined petroleum coke properties [12]

Properties Method Unit Typical range.

Water content DIN 51904 % 0.0-0.2

Oil content ISO 8723 % 0.1-0.3

Grain stability ISO 10142 % 75-90

Mean bulk density ISO 10236 kg/dm3 0.78-0.84

Density in xylene ISO 8004 kg/dm3 2.05-2.1

Specific electrical resistance ISO 10143 μΩm 460-540

CO2 reactivity loss (1000 °C ) ISO N 802 % 3-15

Air reactivity at 525 °C ISO N 803 %/min 0.05-0.3

Crystallite size (Lc) ISO 20203 A 25-32

Ash content ISO 8005 % 0.1-0.2

Elements S ISO N 837 ppm 0.5 - 3.5

V ISO N 837 ppm 30 - 350

Ni ISO N 837 ppm 50 - 220

Si ISO N 837 ppm 50 - 250

Fe ISO N 837 ppm 50 - 400

Al ISO N 837 ppm 50 - 250

Na ISO N 837 ppm 30 - 120

Ca ISO N 837 ppm 20 - 100

Mg ISO N 837 ppm 10 - 30

7

1.3.2 Coal tar pitch

Pitch is a complex mixture of aromatic hydrocarbons. Coal tar is considered as a by-product of the

production of metallurgical coke by the carbonization of bituminous coal at a temperature between

1100 and 1200 °C. Coal tar is distilled at 350 °C to remove the volatiles and the residual product is

called pitch [13].

Pitch plays two roles during the anode production. As a binder, it covers the coke particles

transforming the dry particles into paste amenable to be formed to the required shape. During the

subsequent baking process, pitch transforms to pitch coke and binds the calcined petroleum coke

particles together [14].

1.4 Anode manufacturing The main anode manufacturing processes are simplified in figure 1.4 [15]. Calcined petroleum coke

and anode butts are crushed and then sieved to a definite size distribution. The desired size fractions

are mixed in a way to obtain the maximum packing density (dry aggregate). This product is then

heated in a preheating screw to a maximum temperature ranging between 150 and 190 °C. The

anode paste is prepared by mixing the preheated dry aggregate with coal tar pitch at high

temperature, usually 50 to 60 °C above the softening point of the pitch [ [8, 14].

Forming of green anodes with specific shape and dimensions is performed by pressing or vibro-

compaction of the anode paste. Subsequently, green anodes are baked at 1200 °C in a large scale

anode baking furnace. During the baking process, pitch is coked acting as a bridge between the coke

particles. The total time of the complete baking cycle exceeds two weeks [8, 14]. In order to be

acceptable as electrodes in the Hall-Héroult cells, the baked anodes should meet the specifications

listed in Table 1.2 [16].

Finally, prebaked carbon blocks are transported to the rodding plant where steel stubs, connected to

an aluminum rod, are centered in the groves situated on the block surface. Molten cast iron with

specific composition is poured in these grooves, making a connection between the carbon blocks and

the stubs [14]. Industrially, the term anode is used to describe the full frame, which consists of

prebaked carbon block, steel stubs and the aluminum rod.

8

Table 1.2: Typical prebaked anode properties [16]

Anode Property Unit Typical Range Typical 2σ* Method

Green Apparent Density kg/dm3 1.55-1.65 0.03 Dimension

Baked Apparent Density kg/dm3 1.50-1.60 0.03 ISO N 838

Baking Loss % 4.5-6.0 0.5 Dimension

Specific electrical

resistance

μΩm 50-60 5 ISO N 752

Air Permeability nPm 0.5-2.0 1.5 RDC 145

Compressive Strength MPa 40-50 8 DIN 51910

Flexural Strength MPa 8-14 4 ISO N 848

Static Elastic Modulus GPa 3.5-5.5 1 RDC 150

Dynamic Elastic Modulus GPa 6-10 2 Grindosonic

Coefficient of Thermal Expansion

10-6/K 3.7-4.5 0.5 RDC 158

Fracture Energy J/m2 250-350 100 RDC 148

Thermal conductivity W/mK 3.0-4.5 1.0 ISO N 813

* Typical 2 σ means the standard deviation

Figure 1.4: Schematic diagram of the anode manufacturing process [15]

9

1.5 Anode consumption Theoretically, 0.33 kg of carbon is required to produce 1 kg of Al. However, since the cell current

efficiency (CE) is lower than 100%, the actual carbon consumption is higher. Other contributors such

as air burn, Boudouard reactions and anode dusting lead to excess carbon consumption (figure 1.5)

[17].

2 Al2O3 + 3 C = 4 Al + 3 CO2

Theoretical consumption 334 kg C/t Al

Electrolytic consumption 334 / CE Excess cons

Net Consumption Butts

Gross consumption

Figure 1.5: Anode consumption [17]

The prebaked anode is partially immersed in the electrolyte. As a result, a temperature gradient

between 400 and 700 °C is established along the block as it appears in figure 1.6 (a). If the anode is

not completely covered by the crushed bath/ alumina mixture, exothermic reaction between the bare,

hot, upper side of the carbon anode and atmospheric oxygen (air burn) takes place.

Figure 1.6: Anode excess consumption air burn (a) and CO2 reactivity (b) [18]

Air a b

10

Boudouard reaction takes place when the CO2 produced by the electrochemical reaction reacts with

the carbon anode at high temperature. This reaction, which produces CO, is endothermic and

happens at the bath temperature (950-960 °C). In addition, under the hydrostatic pressure, CO2 may

sweep through the anode porosity enabling the above reaction to take place in the anode bulk as it is

shown in figure (1.6 b) [18].

Anode dusting is a physical phenomenon that occurs as a consequence of the selective oxidation of

the binder phase. As it is shown in figure (1.5), the net anode consumption is the summation of the

electrolytic consumption and of the excess consumption [17]. Excess anode consumption means

higher aluminum production cost and releasing of more CO2, green house gas, to the atmosphere.

1.6 Quality of petroleum coke In aluminum smelters, anode grade coke is used as filler for anode production. This coke is

characterized by its spongy and anisotropic texture as well as its low sulfur (0.5-4%) and vanadium

(

11

High impurity levels in coke negatively affect the anode performance. Vanadium and other heavy

metals are considered as strong catalysts of both anode airburn and CO2 reactivity [22]. In addition,

increasing their level negatively affects the purity of the produced aluminum. Desulphurization might

happen during the calcination process of high-sulphur coke leading to a decrease in real and bulk

densities of coke. Another desulphurization reaction taking place during the anode baking is known to

have a negative impact on the anode properties [23]. As regards environmental regulations, the use

of high sulfur coke poses problems at smelter sites due to the release of SO2 gas [23, 24].

1.7 Environmental Impact of aluminum production. Scientists classified CO2, nitrogen oxides and chlorofluorocarbons as greenhouse gases (GHG).

These gases have high ability to absorb the heat released into space causing an increase in

temperature of the lower atmosphere (global warming). Among all greenhouse gases, more attention

is given to CO2 as its concentration in the atmosphere is soaring dramatically [25].

As a result of the growing awareness of the importance to reduce GHG emissions and its impact on

the climate change, governments are forced to mitigate greenhouse gases emissions. Global

agreements to limit GHG emissions such as Kyoto Protocol, Montreal Protocol, Clean Development

Mechanism and International Emissions were signed. According to these agreements, the industrial

world is committed to reduce CO2 emissions [26].

Anode in aluminum smelters is generally made of nonrenewable, fossil-based carbonaceous

materials. Thus, CO2 produced during electrolysis is considered as a greenhouse gas. The production

of one ton of aluminum gives rise to approximately 1.5 tons of fossil CO2 (excluding CO2 emissions

that relate to the source of electric energy) [27]. Since 44.9 million tons of aluminum are produced

around the world [28], about 67.35 million tons of CO2, are being discharged into the atmosphere.

With the continuous increment in aluminum production capacity, massive release of CO2 over the

coming years will constitute a serious hurdle.

The environmental concerns such as global warming and climate changes, the current concern over

the rapid depletion of fossil carbon resources, the ongoing price increase of coke combined with an

12

uncertain availability of the required quality, have attracted research efforts toward generating

alternative source of carbon using renewable and sustainable resources such as biochar.

1.8 Problems Although the quality of anode-grade petroleum coke keeps degrading, its price is constantly

increasing. Less than 20% of the petroleum coke annual production meets anode specifications,

which are essentially based on the upper limit of sulfur and heavy metal contents (specially V), the

density, the molecular structure, as well as the mechanical properties like Hardgrove index. Coke

supplies contain more and more sulfur and heavy metals with higher isotropic structure. In addition,

the spectacular increase in aluminum production volume and almost constant level of production of

petroleum coke imbalances the supply and demand equilibrium resulting in its escalating price. In

addition, due to the GHG emissions of carbon consumption, aluminum industry is eagerly looking for

a new and ideally renewable source of carbon, in order to overcome the problems of petroleum coke.

1.9 A Working Hypothesis Being an abundant source of sulfur-free carbon, wood charcoal seems, at first glance, to be an

attractive alternative for petroleum coke. Substitution of 10% of petroleum coke by charcoal in the

anode recipe would reduce the fossil CO2 emissions by the same amount. Furthermore, the very low

content of V and S in charcoal could allow the use of petroleum coke with higher S and V, thus

decreasing the raw material cost. However, charcoal is not recommended to be used directly in the

anode recipe due to its highly amorphous carbon structure and its abundant minerals content. These

minerals act as catalysts during the carbon-oxidative gas reaction at high temperature. Heat

treatment under inert conditions is a useful method to improve the molecular structure of

carbonaceous materials, during this process continuous growth of the more ordered carbon structure

at the expense of the amorphous forms is expected. Consequently, an improvement in the physical

properties and the reactivity of charcoal could occur. In addition, acid leaching is a well known

method to eliminate the inorganic minerals of the charcoal. By performing this process reduction in

both air and CO2 reactivities of charcoal is expected. It is believed that combination between acid

leaching and heat treatment would produce charcoal having lower reactivity and better physical

properties. The pretreated charcoal could be suitable to replace a portion of petroleum coke in the

industrial anode recipe. An incentive worthy of mention in the Canadian context is the concomitant

13

aluminum industrial activity with abundant timber resources. These latter may constitute a hopeful

avenue to counteract the declining pulp and paper activities.

1.10 Project objectives The objective of this work is to improve the properties of charcoal with the aim to bring them close to

the specifications required for anode making. This project targeted three specific parts:

The first one was to explore the effect of heat treatment on the carbon structure and on the

properties of charcoal. In an attempt to determine the optimum calcination conditions, wide

range of temperatures were investigated.

In the second step, the removal of inorganic mineral constituents of charcoal was

investigated. These minerals are known to act as catalyst for carbon oxidation reaction. For

that purpose, various acid leaching conditions were examined with the aim of producing a

low-ash and less reactive charcoal.

Finally, different combinations of calcination and acid leaching were made to determine the

optimum treatment condition. Overall, it was expected that heat treatment and acid leaching

would give rise to a charcoal of higher density and with lower air and CO2 reactivity.

15

Chapter 2 : Literature review

2.1 Bio-char (charcoal) Charcoal is a bio-carbon material obtained from the thermal decomposition of biomass, such as

wood, in the absence of oxygen; this happens at relatively low temperatures, normally above 300 °C

[29]. Charcoal was the first reducing agent used to produce metals from their oxides; this was the

case for copper and iron as early as 3000 and 1200 years BC, respectively. Until 17th century, all iron

production was based on the use of charcoal [30]; it is still used nowadays by Norwegian ferrosilicon

makers to reduce silica to silicon [31].

The first method for charcoal production was pit kiln process where wood is loaded in a shallow pit,

covered with soil and then burnt slowly. Due to its simplicity, this method is still used in developing

countries to produce charcoal [30]. More efficient, industrial charcoal kilns, still in use today, include

the Missouri and the Brazilian Beehive types. In these kilns, carbonization process of wood takes

place via internal heating at near atmospheric pressure. The Missouri kiln, operating on a 7-30 day

cycle, produces charcoal at a 25% yield [31]. Recently, massive production of charcoal was noticed

especially by developing countries in Africa and South America. According to the statistics of Food

and Agriculture Organization of United Nations (FAO), approximately 51.3 million tons of wood

charcoal were produced worldwide in 2012 [32].

2.2 Structure of carbonaceous materials Carbon can be found in three allotropic forms, which are graphite, diamond and fullerene. As it

appears in figure 2.1 graphite which is the most thermally stable form of carbon, is a two dimensional

crystalline solid, composed of a series of stacked parallel layer planes. Within each layer, carbon

atoms are bonded via strong covalent bonds to three other carbon atoms, forming a series of

continuous hexagons. The fourth bonding electron participates in a weak van der Waals bond

between the layers [33].

16

The structure of carbonaceous materials depends on the nature of the starting material and the temperature of preparation. Franklin [34] classified the carbonaceous materials into graphitizing and non-graphitizing carbons, according to their ability to be transformed to graphite upon high temperature heat treatments (figure 2.2). While graphitizing carbons are soft, non-porous and have a relatively high density, non-graphitizing carbon materials have a low density, are hard and porous with high degree of cross-linking. These materials do not ultimately form crystalline graphite at temperatures around 3000 °C. However, portions of graphite-like structure were observed in many non-graphitic carbons upon heating at elevated temperature [34].

Figure 2.2: Schematic Representations of (a) Non-Graphitizing Carbon and (b) Graphitizing Carbon [34]

a b

Figure 2.1: Structure of graphite [33]

17

Harris et al. [35] proposed a model to understand the high resistance to graphitization of non-graphitizing carbons even treated at extremely high temperature. They assumed that the presence of non-hexagonal carbon rings (pentagons) within the graphene sheets (graphene is a one-atom thick layer of graphite) and the tendency of these sheets to be found in curved shapes, as it appears in figure 2.3 [36], were the reasons behind their high graphitization resistance.

According to the Franklin’s classification, coke is considered as a graphitizing material since it passes through a fluid stage during carbonization. This allows large aromatic molecules to align with each other and forming the mesophase precursor of graphitic structure [37]. On other hand, charcoal can be classified as a non-graphitizing material. As it does not pass through a liquid stage during wood carbonization, many chemical bonds are left dangling producing highly porous and extremely reactive carbon material [31].

Several operating conditions, such as heat treatment temperature (HTT), heating rate, soaking time, and the flow rate of the inert gas were reported to influence the structure and the properties of charcoal during its heat treatment [31]. Out of them, HTT was proved to have the most significant impact because all physical changes occurring during heat treatment, such as changes in specific surface area and real density, are temperature-dependent [38-43]. Minor effects were reported for the other parameters; e.g. the heating rate, followed by the inert gas flow rate and finally by the heat treatment soaking time [38].

Figure 2.3: Schematic representation of a model of non-graphitizing carbon materials [36]

http://en.wikipedia.org/wiki/Graphite

18

Structural evolution of charcoal during heat treatment can be detected by several characterization

techniques. Among them X-ray diffraction (XRD) is widely used to detect the structural changes in

carbon structure by following two peaks appearing on the XRD spectrum. The first one, which

appears at 2θ =26°, refers to the inter layer distance between graphene sheets. The second one

refers to the presence of graphene sheets and appears at 2θ = 44 °. Increase the intensity and the

sharpness of these two peaks give an evidence for the presence of more ordered carbon structure

during the heat treatment [39]. The two peaks detected in Raman spectra are used to determine the

structural development of charcoal and to confirm the XRD results; they appear at Raman shift =1360

and 1600 cm-1 [42]. By increasing the treatment temperature, H/C and O/C ratios in charcoal

decrease progressively indicating continuous improvement in the degree of aromaticity, therefore, by

looking at the presence or the absence of chemical functional groups (O and H-containing groups)

during heat treatment, it is also possible to follow the structural evolution of charcoal, using Fourier

transform infrared (FTIR) [43]. Finally, transmission electron microscopy (TEM) is very helpful to

visualize any arrangements in carbon structure as heating temperature increases [45].

In order to reveal the structural changes in non-graphitizing carbons as a function of heat treatment

temperature, the structural evolution of carbonized cellulose was studied at temperatures between

900 and 1600 °C [39]. At 900 °C, the sample had an amorphous carbon structure consisting of short

and distorted aromatic layers. Upon heating between 1000 and 1400 °C there was a continuous

elimination of hydrogen and structural defects, resulting in a lateral growth of carbon layers (La).

Within the same temperature range, the interlayer spacing (Lc) remained the same. Although at HTT

higher than 1400 °C a slight change in Lc was detected, a significant improvement in Lc was observed

at 1600 °C [39]. Similar findings were reported for carbonized wood monoliths [40] showing that the

degree of carbon crystallinity was continuously improved as HTT was increased. However, the most

significant improvement in Lc and La was found between 1500 and 2500 °C [40]. Figure 2.4 shows

schematic representation of La and Lc.

19

In an attempt to explain the structural evolution during charcoal carbonization, Kercher and Nagle [41] built up their quasipercolation model based on XRD analysis of carbonized medium-density fiberboard at different temperatures and up to 1400 °C [41]. According to their model, the structural evolution of charcoal occurs as a result of continuous transformation of low density and disordered carbon into denser graphene sheets with increasing carbonization temperature. Increasing in the heat treatment temperature resulted in gradual growth of the graphene sheets (La); however, the number and thickness (Lc) of these sheets remained constant. Not all graphene sheets grew with the same rate; some of them have seen their growth stopped by the structural defects [41]. In figure 2.5, which shows a schematic illustration of the quasipercolation model, light gray, medium gray, dark gray and white areas respectively represent disordered carbon phase, graphene sheets present at low temperatures, growth of graphene sheets at higher temperature and pores [41].

Figure 2.5: Schematic demonstrating of the quasipercolation model [41]

Graphene La

Lc

Figure 2.4: Schematic representation of La and Lc

20

Paris et al. [42] studied, by XRD and Raman spectroscopy, the structural evolution of European

spruce wood treated at temperatures ranging from 25 and 1400 °C. The structural changes were

studied in three different temperature regions: below 250 °C, between 250 and 350 °C, and from 350

to 1400 °C. Up to 250 °C, typical of torrefaction treatment temperatures, water evaporation and

dehydration were found to be responsible for the slight changes in sample mass and dimensions. At

this relatively low carbonization temperature, no structural changes in the crystallinity, shape and size

of cellulose were detected. However, at this temperature range, changes in both hemicellulose and

lignin (the other main components of the wood cell) could occur. The major change in the sample

mass was observed between 250 and 350 °C. At this temperature range, complete depolymerization

of cellulose structure took place via thermal decomposition and evaporation of the lighter molecules,

resulting in mainly amorphous carbon structure. At approximately 320 °C, aromatic arrangement was

observed for the first time using Raman spectroscopy. Between 350 and 1400 °C the amorphous

carbon structure continuously decreased as a result of the continuous growth of the polyaromatic

graphene sheets. Although the graphene extension La increased with the carbonization temperature,

graphene stacking height Lc remained constant up to 1400 °C. According to this work, a sequence of

three processes, dehydration, pyrolysis and graphene nucleation and growth, would be responsible

for the structural evolution of charcoal during heat treatment [42]. The results so obtained are in

accordance with the quasipercolation model [41].

From a chemical point of view, continuous loss of certain functional groups was believed to be

responsible for the structural ordering during the charcoal heat treatment. Using FTIR at different HTT

between 650 and 850 °C, Antal et al. [43] followed the changes in charcoal chemistry. Untreated

charcoal was observed to have an alkyl aromatic structure with many oxygen-containing functional

groups like carbonyl (C=O), phenols (C-O-H) and ether (C-O-C). At 650 °C, the hydroxyl, carbonyl

and aliphatic C-H groups were greatly reduced and the first sign of the presence of aromatic structure

(aromatic C-H group) started to appear. At 750 °C, aromatic C-H group was almost eliminated due to

the continuous condensation of the aromatic rings. Only a weak aromatic C-H peak at about 880 cm-

1, referring to hydrogen atoms located on the edges of the condensed aromatic sheets, still remained.

In addition, the hetero atoms, such as oxygen, present within the aromatic structure, were responsible

for the skeletal stretching and bending broad band between 1000 and 1700 cm-1. At 850 °C, this

broad band and aromatic C-H group became weaker. It is worth noting that the formation and growth

21

of aromatic sheets (having low-energy electron excitations) were the reasons behind the considerable

shift in the base line of FTIR spectrum towards higher adsorption, as the HTT increased between 650

and 850 °C [43].

X-ray photoelectron spectroscopy (XPS) was used to follow the changes in the chemistry of charcoal

produced from the carbonization of cellulose [44]. Although the oxygen content was observed to

continuously decrease, as the HTT was increased, it was not completely eliminated even at 2000 °C.

The remaining oxygen plays a role in the cross-linking of carbon structure by preventing its complete

graphitization at elevated temperature [44].

Using High-resolution transmission electron microscopy (HRTEM), Miser et al. [45] were able confirm

the presence of graphene sheets in the carbonized tobacco at high temperature. Their study was

performed at temperature varying from 350 and 1200 °C, and they noticed a progressive reduction in

hydrogen and oxygen contents and also an increase in aromatic nature of char, while the heat

treatment temperature was increased. Graphene sheets, surrounded by amorphous carbon, were

observed for the first time at 550 °C close to cellulosic cell walls [45].

Although HTT is known to have a significant effect on the charcoal structure and properties, the

contribution of the heating rate and the nature of charcoal precursor should not be ignored. This way,

Guerrero et al. [46] studied the effect of heat treatment at different heating rates on two types of

charcoal (rice husk and eucalyptus) up to a final HTT of 900 °C. Their findings showed that, structural

changes of eucalyptus charcoal were more sensitive to heat treatment than that of rice husk charcoal

and that the use of lower heating rate (10 °C/min) resulted in more ordered carbon structure.

In addition to the effect of the heat treatment temperature, the pressure also affects the carbon

structure of charcoal. At the same HTT, the crystallinity of radiata pine charcoal was improved as the

pressure was increased. XRD results showed that both La and Lc of charcoal treated at 850 °C and

under pressure of 20 bar were found to be higher than those of charcoal which was treated at the

same temperature but at atmospheric pressure [47].

22

Beside the structural evolution, several morphological changes also occur during heat treatment of

charcoal. Due to its botanic origin, charcoal has a very porous carbon structure which appears as

continuous and aligned honeycomb-like pores. While at low heating rate no significant morphological

changes were observed in the produced charcoal, due to the regular release of volatiles, at high

heating rate, the produced char coal had completely different morphology, due to melting and

complete damage of cell structure. By increasing the applied pressure during heat treatment,

charcoal particles were melted and fused together producing larger and smooth particle clusters [48,

47].

2.4 Changes in physical properties of charcoal during heat treatment The bulk (apparent) density of the porous carbon is the mass per unit volume including both solid and

voids [49]. The effect of heat treatment on this property was studied by Byrne et al. [50] for a variety

of wood species. In all cases, the bulk density was lower after carbonization. They found that the

nature of wood precursor could also affect the bulk density of the resultant charcoal, and presented a

linear relationship between the bulk density of the produced charcoal at 950 °C and the density of the

original wood samples (figure 2.6). For all samples tested, the bulk density of the produced charcoal

was 80% of that of the starting material.

Figure 2.6: Relationship between the bulk density of wood and that of the produced charcoal at 900 °C [50]

23

The bulk density of charcoal produced by carbonization of different types of wood-based fiberboard

was also measured by Kercher et al. [51, 41] between 600 and 1500 °C. As presented in figure 2.7, a

maximum bulk density of 0.62 g/cm3 was found at 900 °C. This behavior was explained by the

continuous conversion of the amorphous phase into denser graphene sheets (quasipercolation

model) and by the volumetric shrinkage occurring at this temperature. Upon further heating above

1000 °C, a slight reduction in the bulk density was observed due to the limited volumetric shrinkage

along with the continuous weight loss.

Figure 2.7: Bulk density variations as a function of carbonization temperature [51]

Since the bulk density of porous carbon depends on its porosity, Guo et al. [52] measured the bulk

density of powder charcoal samples by a mercury intrusion porosimeter. They found the bulk density

to continuously decrease with increasing HTT up to 800 °C. However, when the temperature was

increased up to 900 °C, the bulk density increased. This behavior was explained by the sintering and

the shrinkage of the charcoal particles at that temperature.

Real (skeletal) density is the density on the molecular level or the density of carbon skeleton. It is

calculated by dividing the mass by the volume of the carbon excluding the micro pores [49]. Helium

pycnometer is used to determine the real density. This method depends on the ability of helium to

enter inside the open pores and gives a measure of the solid volume. Unlike the bulk density, the real

density of charcoal does not depend on the density of the starting material however; it is affected by

the HTT [41]. As it is clearly seen in figure 2.8 significant change in real density of charcoal occurs as

24

HTT is increased [40, 41]. A maximum real density of 2.0 g/cm3 was reached at 1000 °C; this value

was close to 2.25 g/cm3 which is the real density of graphite. Kercher et al. [41] postulated that the

continuous increase in the real density was a consequence upon the growth of the ordered carbon at

the expense of the amorphous phase as HTT was increased. However, at 1400 °C a significant

reduction in the real density was observed. Closing of some small pores during the structural ordering

and vapor deposition processes was believed to be responsible for the reduction in the real density of

charcoal at elevated temperature [41]. According to Brown et al. [53] the heating rate has no

significant effect on the real density of charcoal.

Figure 2.8: Effect of HTT on the real density of different types of charcoal [41]

Having a very porous carbon structure, charcoal contains different types of porosity. Based on their

diameter, pores can be classified into micropores, mesopores and macropores [54]. Micropores have

a diameter below 2 nm; they contribute most to the surface area and are responsible for the high

absorptive capacity of charcoal [55]. The diameter of mesopores is ranging between 2 and 50 nm,

while that of macropores being greater than 50 nm [54].

Gas adsorption is used to determine the surface area of porous materials. Both N2 and CO2 are

generally used for this purpose. Since it is used at very low temperature (-196 °C), N2 does not give

precise measurement of the micropores; it has not the required kinetic energy to enter inside the

25

microporous network. On the other hand, CO2 used at 0 °C, has enough energy to penetrate deeper

into the micropores, giving more precise results regarding microporosity. Consequently, a sample

presenting a surface area having a high value with CO2 but a low value with N2 can be suspected to

possess a high microporosity. In addition, if the sample has identical CO2 and N2 surface areas, it

might contain large pores [47].

Nitrogen adsorption at – 196 °C was used by Lua et al. [38] to follow the changes in porosity and

surface area of pistachio-nut shells charcoal during heat treatment. The results showed a progressive

augmentation in the surface area as the HTT was increased from 250 to 500 °C. This behavior was

attributed to the rise in the volatilization rate as a function of HTT. A maximum surface area of 778

m2/g was observed at 500 °C. However, a reduction in the surface area was detected at 800 °C due

to the melting of the charcoal structure which resulted in a porosity closure. At temperatures higher

than 900 °C, two phenomena occurred resulting in an increase of the surface area. The first was

volatilization of some of the melt formed at lower temperature and reopening of the previously closed

pores; in the second one, new pores were developed due to volatilization of higher molecular weight

volatiles. The surface area reduced again when the HTT was increased up to 1000 °C; this

temperature was sufficiently high to ensure that most of the volatiles have evolved leaving larger

pores with lower surface area [38]. Guerrero et al. attributed the significant decrease in the surface

area of charcoal, as the HTT exceeded 800 °C, to the structural arrangements and to the

coalescence of micropores, which yielded the development of meso and macropores with a lower

surface area [46]. Upon heating the cellulose carbon up to 1300 °C, a significant reduction in its

surface area was observed. This behavior could be explained by the conversion of open micropores

to closed micropores as a result of the structural ordering at elevated temperature, and of the

increasing size of open micropores [39].

Because of its molecular structure, graphite has a high electrical conductivity. Carbon atoms in

graphite have sp2 hybridization and each of them has one unhybridized free p orbital electron which is

able to move within the conjugated planes, providing a means of conducting electricity similar to

conduction band in metals [56]. On the contrary, the charcoal prepared by carbonization of wood at a

relatively low temperature has a highly amorphous carbon structure resulting in a high electrically

resistant material [51]. The electrical property of carbonized wood is closely related to the final HTT;

26

as it can be seen in figure 2.9, a considerable reduction in electrical resistivity of carbonized wood

was detected by increasing the HTT. The electrical resistivity of the produced charcoal at 1400 °C

was found to be close to that of the polycrystalline graphite [51]. This behavior can be explained by

the previously discussed quasipercolation model [41]. According to this model, higher carbonization

temperatures increased the amount of the conductive phase in the carbonized wood by continuously

converting the amorphous phase into conductive and more ordered graphene sheets. Furthermore,

the volumetric shrinkage, which occurred at elevated temperature, reduced the distance between the

graphene sheets which then became close enough to provide a way for electron movement and a

subsequent electrical conduction [41].

Figure 2.9: Resistivity of three carbonized wood-based fiberboard as a function of HTT [51]

2.5 Reactivity of charcoal with air and CO2 The reactivity with air and CO2 of carbon materials used in anode production is one of the most

important parameters to predict the behavior of the anode during electrolysis. Several factors were

reported to affect the reactivity of the carbonaceous materials with the oxidizing gases [57]:

27

The concentration of active sites. Oxidizing gas does not react evenly with the whole sample;

some sites (active sites) have higher reactivity than others. The prismatic edges of the

carbon layers and the defects in the carbon crystals, such as vacancies and dislocations, are

considered to be active sites during carbon-gas reaction.

The total surface area accessible for the oxidizing gas and the number of active sites per

unit surface area.

The presence of impurities which can act as catalysts during the reaction.

The presence of oxygen heteroatoms within the carbon structure.

In addition to its very large surface area, charcoal has a highly disordered carbon structure, which

means a high concentration of active sites; it has an abundant content of catalysts as alkali and

alkaline earth metals. Furthermore it has oxygen heteroatoms within its carbon structure.

Consequently, charcoal is considered as a high reactive material with the oxidizing gases.

2.5.1 Effect of heat treatment on the reactivity of charcoal with air and CO2

Reactivity of carbonaceous materials can be reduced by increasing HTT. Lu et al. [58] attributed the

reduction in carbon reactivity to the structural ordering during heat treatment. Chars, prepared at high

temperature, presented a continuous structural evolution. As HTT was increased, the concentration of

the more ordered and condensed carbon structure, which is reasonably less reactive, increased at

the expense of the high reactive amorphous phase. In addition, surface area of the carbonaceous

materials decreased due to the structural ordering, resulting in a reduction of its reactivity [58].

However, the reduction in the active surface area was found to have the most significant effect. Due

to the structural ordering, the concentration of edge carbon atoms (high reactive form of carbon

atoms) decreases, resulting in lesser amount of active sites and thus lowering their reactivity with

oxidative gases [59].

Kumar et al. [60] have studied the effects of several heat treatment parameters such as final HTT,

heating rate and soaking time, on CO2 reactivity of wood charcoal. Their results showed that by

increasing HTT and the soaking time, the reactivity decreased due to the increase in structural

ordering. In addition, using low heating rate during heat treatment gave more opportunity to the light

hydrocarbons to deposit rather than volatilize; these deposits have more ordered carbon structure

28

than that of chars. Furthermore, using lower heating rates resulted in a carbon structure with lower

remaining heat stresses and consequently being less reactive.

In addition to the effect of HTT, the size of charcoal particle was also reported to have a significant

effect on its reactivity [61]. Compared to the larger particles, the smaller ones had higher

concentration of carbon with ordered structure. This could be attributed to the unequal heating rate or

temperature gradients between surface and core of particles. Also, the concentration of inorganic

minerals (known to act as catalyst) is higher in the amorphous carbon than in the condensed and

ordered carbon structure. In raw charcoal, minerals such as Na and Ca are found bounded to the

carboxylic groups and to the alkyl chains. These groups are extensively abundant within the small

aromatic ring systems (amorphous carbon). Upon heat treatment, most of these groups are lost

during the polymerization reaction resulting in more condensed carbon structure and lower mineral

content. Consequently, the smaller particles, which had less reactive form of carbon and less

concentration of catalysts, showed a lower reactivity than the bigger ones [61].

Reactivity of carbonaceous materials is affected by the presence of catalysts. Indeed, inorganic

impurities, found in carbonaceous material, increase the rate of carbon oxidation. These elements

catalyze the reaction by acting as carriers for the oxidative gas and/or by making surface

topographical changes (pitting or channeling) during gasification [57]. The reactivity of graphitic

carbon with CO2 was found to be higher by addition of small amounts of CaCO3, MgCO3, BaCO3 and

SrCO3 [62].

Charcoal is known to contain a range of inorganic minerals at different concentrations and in different

forms; these elements are used by plants as nutrients. The most abundant elements in the charcoal

precursor are nitrogen, phosphorus, potassium, sodium, calcium, magnesium and silicon [63]. Alkali

and alkaline earth metals are particularly effective catalysts for charcoal reactivity. Cellulosic charcoal

loaded with calcium and potassium showed an increase in its reactivity with air [64, 65].

An attempt was made by Huang et al. [66] to determine which elements have the most significant

catalytic effect on charcoal reaction with CO2. Fir tree sawdust was doped with a metal solution and

heated to 550 °C in order to obtain charcoal. Thermogravimetric analysis (TGA) was used to study

29

the reactivity of the metal impregnated charcoal. As it can be seen in figure 2.10 the reactivity of the

charcoal was increased through the addition of metal catalysts in the order K > Na > Ca > Fe > Mg.

Figure 2.10: Effect of different catalysts on charcoal reactivity with CO2 [66]

2.5.2 Effect of acid leaching on the reactivity of charcoal with air and CO2

The process of demineralization involves washing the charcoal with a given solution in an attempt to

reduce its content of inorganic minerals. Jenkins et al. [67] used only water as a leaching agent. They

employed various washing techniques such as spraying water over the sample, pouring tap or

distilled water through the sample spread over a fine stainless steel screen and finally, soaking

samples in water for 24 hours. The last washing method was the most effective. Since potassium,

sodium and chlorine found in the biomass were largely removed by water leaching, it suggested that

these elements are present in a water-soluble form. On the contrary, calcium, magnesium, silicon,

and nitrogen contents appeared to be unaffected by any of the water washing treatments.

Das et al. [68] studied the effect of leaching with water, with a 5 M HCl solution, and with an HF

solution (concentration ranging from 0.5 to 3%) on the mineral content in sugarcane bagasse

samples. The amount of mineral matters removed at each step of the different treatments was

quantified by Inductively Coupled Plasma Atomic Emission Spectra instrument (ICP-AES). The

results showed that simple water leaching was effective in removing of alkalis like Na and K wherein,

30

5 M HCl leaching further removed other metals like Mg, Ca and Al. However, HF treatment removed

almost all the ash elements. Although the last treatment showed to be the most significant de-ashing

treatment, it is not recommended in industrial scale due to its environmental impact.

Iniesta et al. [69] studied the effect of the demineralization process on the ash content and its

subsequent impact on the CO2 reactivity of the produced charcoal. Almond shells were leached by

sulfuric acid (10 wt %), sodium hydroxide (2 wt %) and a mixture of H2SO4 and NaOH solutions. The

effect of the leaching time was also examined. Samples treated with acid during 3 and 24 hours

showed the most significant ash reductions of 62.9% and 71.0%, respectively. The pretreated

samples were then carbonized at 850 °C and their CO2 reactivity was measured by TGA. Figure 2.11

shows the effect of different chemical pretreatment on the reactivity of charcoal with CO2. The

samples which were washed by acid only showed the least reactivity. On the other hand, samples

treated with basic solution (NaOH) showed higher reactivity due to the addition of more ashes

(catalysts) to the raw sample.

Figure 2.11: Effect of leaching conditions on CO2 reactivity of almond shells charcoal [69]

31

2.6 Charcoal as a raw material for electrodes production Little efforts have been exerted to use charcoal as a raw material for electrode production; Coutinho

et al. [70] prepared electrodes from biochar, obtained by carbonization of Eucalyptus wood at 1000

°C under nitrogen atmosphere. The volatiles released during the carbonization process were

condensed to produce bio-pitch, which was used as a binder material. Charcoal and biopitch were

mixed together and then formed in a cylindrical shape to give green anodes which were calcined at

1000 °C and further subjected to an additional heat treatment at 2700 °C (graphitization). The

authors found significant improvements in the physical properties of the bio-anodes as the treatment

temperature was increased up to 2700 °C. The anodes so obtained showed an enhancement in the

mechanical properties such as Young’s modulus and cold crushing strength; these values were

comparable to those of commercial reference anodes. Moreover, a pronounced improvement in the

electrical properties was achieved. As it is clearly shown in figure 2.12, the specific electrical

resistance decreased by nine orders of magnitude and reached its minimum value of 10-4 Ω.m at 800

°C, being close to that of commercial graphite. The results of this work pointed out that it is possible

to use bio-carbon materials to produce an electrode with adequate mechanical and electrical

properties. However, the extenside heat treatment temperature of 2700 °C makes this process

difficult to apply for anode production.

Figure 2.12: Electrical resistivity as a function of HTT for BCE, babassu nut and lignin coke [70]

32

An attempt to investigate the possibility of using charcoal as a substituent of petroleum coke in

anodes for aluminum production was done by Monsen et al. [71]. Charcoal produced by carbonization

of different wood precursors was milled to destroy its very porous carbon structure. This powder was

then used to replace different percentages of the fine coke fractions in the anode recipe. Reference

anode, fully made of petroleum coke, was used for comparison. Mechanical, electrical and reactivity

analyses were carried out to assess the effect of such substitution on the final anode properties. The

apparent baked density, specific electrical resistivity and compressive strength were found to be

negatively affected by charcoal addition. They attributed such a behavior to the high porosity and to

the low density of charcoal, and also to the inefficient filling of the open porosity by coal tar pitch

during the mixing process. The porous structure together with high alkali and alkaline earth metal

contents (e.g. Na, K and Ca) make charcoal extremely reactive with respect to air and CO2.

Consequently, deterioration in both air and CO2 reactivities of the anodes was observed. They finally

concluded that addition of charcoal to the anode recipe had deleterious effect on the final anode

properties [71].

2.7 Summary

The literature related to the effect of heat treatment and acid leaching on the final properties of

charcoal were reviewed in this chapter. Many authors have investigated the impact of heat treatment

temperature on the evolution of charcoal carbon structure. A strong relationship was found between

the reactivity of charcoal and the heat treatment conditions and they were stated as the only key

parameter for charcoal reactivity. However a few publications have considered the effect of acid

leaching and its combination with heat treatment on charcoal reactivity. Upon our knowledge only two

papers discussed the possibility of using wood charcoal as a raw material for electrode production.

33

Chapter 3 : Materials and methods

3.1 Introduction Due to several disadvantages, the commercially available wood charcoal is not a suitable raw

material to be used for the anode production. Amongst them, its carbon structure is highly amorphous

with a considerably high porosity and surface area, its high content of alkali and of alkaline earth

metals (AAEM) such as Na, K and Ca. Wood charcoal is thus characterized by its low density, its low

electrical conductivity and its extremely high reactivity with air and CO2. In this work several attempts

were done to overcome those disadvantages. In order to improve its amorphous structure, heat

treatment (calcination) under inert environment at elevated temperatures was investigated and the

effect of acid leaching on its AAEM content was stu