Raw materials and their impact on anode quality

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Raw materials and their impact on anode quality

Raw materials (calcined petroleum coke, coal tar pitch, recycled anode butts, scrapped green and baked anodes) have a great influence on anode quality. The effect of raw materials and formulation has been considered as an essential factor for anode production [5]. Some of the raw material properties to be considered are:
1. Purity, structure, and porosity of coke
2. Binder demand, wetting capability, mixing conditions (especially temperature and time)
3. Recycled material properties

In anode manufacturing, raw materials are required in large quantity and acquired from different sources which might have different properties. This may cause homogeneity problem. Also the impurity level of petroleum coke has been increasing which directly affects the mechanical and physical properties of anodes. In addition, it has been observed that there is a change in QI (quinoline insolubles) content and softening point of the pitches [5]. The publications available in the literature on this specific topic of interest of the current project are summarized below.

Petroleum coke

Coke quality is dependent on the coke production technology. The chemical and structural properties of the green coke are influenced by coking conditions. Physical and mechanical properties of green cokes are controlled by coke microstructure and volatile matter. Coke calcination removes the moisture content and volatile combustible matter and modifies the coke structure.

Green petroleum coke is produced as a by-product of petroleum refining. It is produced by delayed coking or by fluid coking. Petroleum coke has a weak amorphous structure. In green coke, the pores of the matrix are filled with a hardened residuum remaining from the coker feed [5, 14, 15]. Cokes obtained from high asphaltenes feedstock contain higher concentrations of sulfur and metals than cokes produced from feedstock which have high aromatic content [16, 17]. Most of the sulfur in coke exists as organic sulfur bound to the carbon matrix [14]. Other forms of sulfur found in coke include sulphates and pyritic sulfur, but these rarely make up more than 0.02% of the total sulfur in coke [14]. Metals, mainly vanadium and nickel, come from the asphaltenes fraction, and calcium and sodium from desalting process. Some metals are present in coke; but, they are not chemically bonded thus become part of the ash and particulates [5]. Volatile matter content, trace elements, density, and granulometry are the major parameters which characterize green cokes [18]. Calcination of coke is a heat treatment process for green coke up to a temperature of approximately 1150-1250°C. Before using it as anode raw material, green coke is calcined for numerous reasons such as increasing C/H ratio, grain strength, thermal conductivity, and purity, and reducing electrical resistivity, air reactivity, and shrinkage during the baking of anode [19]. Calcination of coke removes the moisture and the volatile matter (hydrogen, methane, tar) to avoid cracking due to grain shrinkage during the baking of the carbon anode and also to ensure the access of binder pitch to its pores during mixing [5]. Cokes from different sources have different volatile content, microstructure, and impurity level; and it is necessary to calcine each coke differently to get the optimum quality for anode-grade coke [12, 18]. Identical green cokes can be calcined under identical conditions, and their various blends can also be calcined under similar conditions [18]. Rotary kiln and rotary hearth are the types of calciner used frequently for green coke calcination process. Processing techniques have a great influence on the mechanical and structural (Lc) properties of the calcined coke [20-22].

Structurally, sponge coke is preferred for anode production because it has a combination of low impurity levels, low air and CO₂ reactivity, a moderate coefficient of thermal expansion (CTE), good density and enough open porosity to allow good interlocking and bonding with a binder pitch. The sponge coke structure is intermediate between the extremes of needle coke and highly isotropic coke. Shot coke is the most isotropic form of coke.

Optical and scanning electron microscopes are useful tools to study coke morphology. During the morphological study with optical microscopy, the material often exhibits optical anisotropy. When coke samples are observed under polarized light, the coke surface looks like a collection of units of varying sizes and colors. This arises due to interaction between the structure and the incident light. The different interfaces between colors probably appear from the grain boundaries [26, 27]. SEM (Scanning Electron Microscopy) can be used as a quick method to examine the coke morphology as it can provide information on the texture of unpolished samples. Detailed structural analysis can be done with SEM because it has large depth of focus, which cannot be done with optical microscope due to its small focus depth [26, 28]. Petroleum coke is composed of mosaic/granular structure, lamella, termed flat or intermediate structure [26, 28]. In 2005, Neyrey et al. [29] studied the effects of coke structure (mostly isotropic coke) on anode properties where this coke was added in varying quantities in the blend and reported that it influenced the coefficient of thermal expansion, reactivity, and density.

Effect of calcination on different coke properties

Based on earlier discussions, a wide range of publications are devoted to the experimental work aimed at understanding the effect of calcination on different coke properties which is relevant to anode properties.

Numerous researchers reported a clear decrease in electrical resistivity of coke with increasing calcination temperature [22, 30-32]. The specific electrical resistivity of anodegrade calcined coke is around 1000 µΩ.m [33].

Calcination also has an effect on bulk density and real density of coke particles. Calcined coke has a bulk density in the order of 0.8-0.9 g/cm³ . Belitskus (1991) [22] has measured the vibrated bulk densities (VBD) of three different cokes calcined at three different temperatures and have found that coke VBD increases with increasing calcination temperature except at intermediate temperatures.

Calcination temperature and heating rate influence the porosity. Fast volatile evolution due to fast heating rates increases the porosity [34, 35]. Tran and Bhatia (2007) [36] made several important observations on porosity development during calcination and reported that micropore area decreases with increasing calcination temperature. They stated that the the decrease in micro porosity is a result of the increase in the graphitization level of the coke at higher calcination temperatures. They found that laboratory calcined coke has a high graphitized structure with 60-70% organized carbon assembly. They also explained that the reactive micropore volume is the void volume between carbon crystallites and imperfections in crystallites; and due to an increase in calcination temperature, the imperfections reduce and the crystallite growth takes place. Porosity and specific surface area of cokes increase at a certain temperature when desulfurization occurs. This is accompanied by an increase in reactivity which is directly proportional to the surface area [30, 37, 38]. Hume (1993 and 1999) [17, 34] has suggested that it is important to control the calcination conditions in order to have optimum porosity and, consequently, maintain optimum air and CO2 reactivity. Fischer and his co-authors [12] have classified the porosities as open and closed. Open pores are interconnected at the surface and closed pores are inaccessible. Good quality coke contains more open pores (around 0.5 µm-15 µm) than closed pores.

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Table of contents :

1.1 Background
1.1.1 Production of aluminum
1.1.2 Production of carbon anodes
1.2 Statement of the problem
1.3 Objectives
1.4 Scope
1.5 Originality
2.1 Raw materials and their impact on anode quality
2.1.1 Petroleum coke Effect of calcination on different coke properties Effect of various calcined coke properties on anode properties
I. Coke porosity and bulk density
II. Coke crystalline length
III. Coke electrical resistivity
IV. Coke air and CO2 reactivity
V. Coke granulometry Anode consumptions
I. Electrolytic consumption
II. Carboxy attack (CO2 reactivity)
III. Air burn (air reactivity)
IV. Selective oxidation (dusting)
V. Effect of the degree of calcination on anode reactivity
VI. Effect of coke sulfur content on anode reactivity
2.1.2 Recycled anode butt
2.1.3 Coal tar pitch
2.2 Wetting Study
2.2.1. Role of petroleum coke in wetting Porosity and bulk density Particle shape factor Coke surface chemistry
2.2.2. Role of coal tar pitch Pitch surface tension Pitch surface chemistry Other factors
2.2.3 Role of recycled anode butt
2.3 Effect of process parameters on the pitch distribution in anode
2.4 Worldwide anode properties
3.1 Characterizations of raw materials
3.1.1 Chemical analysis FT-IR analysis XPS analysis Proximate analysis of coke
3.1.2 Physical analysis Wettability Tapped bulk density (ISO 10236) Measurement of porosity
3.1.3 Structural analysis SEM analysis Optical microscope analysis
3.2 Preparation and characterizations of laboratory anodes
3.2.1 State of the art of anode production Dry aggregate preparation Mixing Forming Baking
3.2.2 Characterizations of anodes and anode samples Sample preparation Apparent density of anode (UQAC) Apparent density of anode sample [ASTM 5502-00 (2005)] Electrical resistivity of anode (UQAC) Electrical resistivity of anode sample [ASTM D6120-97(2007)] Determination of air (ASTM D6559-00a) and CO2 (ASTM D6558-00a)
reactivities Uniaxial compressive strength of anode samples [C695-91 (2005)] Flexural strength of anode samples (ISO CD 12986) Sulfur analysis of anode
3.3 Characterizations of industrial anodes
3.3.1 Paste analysis
3.3.2 Pitch and pore analysis in green and baked anodes
3.3.3 Measurement of the crystalline length of baked anode samples by XRD
(ASTM D-5187)
3.4 Statistical analysis
3.4.1 Linear multivariable analysis
3.4.2 Artificial neural network analysis (ANN)
3.4.3 Analysis of variance (ANOVA)
4.1. Wettability of petroleum coke from different suppliers by coal tar pitch
4.1.1 General
4.1.2 Materials
4.1.3 Contact angle test
4.1.4 Structural analysis
4.1.5 Chemical analysis
4.1.6 Analysis of coke-pitch interface
4.1.7 Concluding remarks
4.2 Effect of coke crystallinity on the wettability of cokes by pitch
4.2.1 General
4.2.2 Materials
4.2.3 Contact angle test.
4.2.4 Structural analysis
4.2.5 Chemical analysis
4.2.6 Analysis of coke-pitch interface
4.2.7 Concluding remarks
4.3 Wetting characteristic of recycled anode butts
4.3.1 General
4.3.2 Materials
4.3.3 Contact angle test
4.3.4 Structural analysis
4.3.5 Chemical analysis
4.3.6 Analysis of coke-pitch and butt-pitch interfaces
4.3.7 Concluding remarks
4.4 Utilization of different statistical tools to analyze and predict the influence of raw
material composition and impurities on contact angle
4.4.1 General
4.4.2 Materials
4.4.3 Linear multivariable analysis Contact angle at 80 s Contact angle at 1500 s Validation of contact angle predictions
4.4.4 Artificial neural network Contact angle at 80s Contact angle at 1500 s Validation of contact angle predictions
4.4.5 Concluding remarks
5.1 Improvement of anode recipe
5.1.1 General
5.1.2 Materials
5.1.3 Development of ANN model
5.1.4 Characterizations of anodes Green and baked density of anodes Electrical resistivity of green and baked anodes Air reactivity CO2 reactivity Uniaxial compressive strength Flexural strength (Bending)
5.1.5 Concluding remarks
5.2 Influence of coke crystallinity and sulfur content on anode reactivity
5.2.1 General
5.2.2 Materials
5.2.3 Effect of coke crystallinity and baking temperatures on anode properties
5.2.4 Combined effect of coke crystallinity, sulfur content and baking temperatures
on anode reactivity
5.2.5 Concluding remarks
5.3 Effect of green anode cooling times on pitch redistribution in anodes
5.3.1 General
5.3.2 Materials and methodology
5.3.3 Image analysis of anode sections
5.3.4 Concluding remarks
6.1 Pitch distribution in industrial anodes
6.1.1 General
6.1.2 Materials and methods
6.1.3 Image analysis by optical microscope
6.1.4 Image analysis by digital camera Effect of different vibrocompactors and compaction times on pitch
distribution of green anodes Effect of different vibrocompactors and compaction times on
distribution of pitch on baked anode
6.1.5 Concluding remarks
6.2 Effect of different factors on industrial anode CO2 reactivity
6.2.1 General
6.2.2 Materials
6.2.3 Development of the ANN Model
6.2.4 Concluding remarks
6.3 Analysis of industrial anode paste
6.3.1 General
6.3.2 Materials
6.3.3 Calculation of kneader residence time
6.3.4 Performances of industrial kneaders
6.3.5 Concluding remarks

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