Search

Aluminum industry depends on coal-tar-pitch as a binder to produce carbon anodes used for aluminum electrolysis. This binder is nonrenewable and mainly composed of polycyclic aromatic hydrocarbons ...

The present work focuses on the gasification of a single carbon-anode particle with CO2, using a detailed reaction-transport model based on the reaction intrinsic kinetics and transport of gaseous species. The model includes the mass conservation equations for the gas components and solid carbon particles, resulting in a set of nonlinear partial differential equations, being solved using numerical techniques. The model may predict the gas generation rate, the gas compositions, and the carbon consumption rate during the gasification of a carbon particle. Five kinetic models were compared to describe the gasification behavior of carbon particles. It was found that the random pore model (RPM) provided the best description of the reactivity of anode particles. The model also predicted the particle shrinkage during the gasification process. The model was validated using experimental results obtained with different particle size ranges, being gasified with CO2 at 1233 K. The experiments were performed in a thermogravimetric analyzer (TGA). Good agreement between the model results and the experimental data showed that this approach could quantify with success the gasification kinetics and the gas distribution within the anode particle. In addition, the Langmuir–Hinshelwood (L–H) model is used in order to capture the inhibition effect of carbon monoxide on the gasification reaction. The effectiveness factor and Thiele modulus simulated for various particle sizes helped assess the evolution of the relative dominance of diffusion and chemical reactions during the gasification process.

Bio-pitch, driven from biomass, is a potential green alternative of coal-tar-pitch in the production of carbon anodes for the aluminum electrolysis process. Information on the wetting capacity of b...

Coal-tar-pitch (CTP) is a fossil carbon material, currently used as a the binder in carbon anode manufacturing process. Regardless of the technical benefits of coal-tar-pitch, it contains polycycli...

Carbon products such as anodes and ramming paste must have well-defined physical, mechanical, chemical, and electrical properties to perform their functions effectively in the aluminum electrolysis cell. The physical and mechanical properties of these products are assigned during the shaping procedure in which compaction stresses are applied to the green carbon paste. The optimization of the shaping process is crucial to improving the properties of the carbon products and consequently to increasing the energy efficiency and decreasing the greenhouse gas emissions of the Hall–Héroult process. The objective of this study is to experimentally investigate the effect(s) of the strain rate, of the stress maximum amplitude, and of the unloading level on the behavior of a green carbon paste subjected to cyclic loading. To this end, experiments consisting of (1) cyclic compaction tests at different maximum stress amplitudes and strain rates, and (2) cyclic compaction tests with different unloading levels were carried out. The study obtained the following findings about the behavior of carbon paste subjected to cyclic loads. The strain rate in the studied range had no effect either on the evolution of the permanent strain as a function of the cycle number, nor on the shape of the stress–strain hysteresis during the cyclic loading. Moreover, samples of the same density that had been subjected to different maximum stress amplitudes in their loading history did not have the same shape of the stress–strain curve. On the other hand, despite having different densities, samples subjected to the same number of cycles produce the same stress–strain curve during loading even though they were subjected to different maximum stress amplitudes in their loading histories. Finally, the level of unloading during each cycle of a cyclic test proved significant; when the sample was unloaded to a lower level of stress during each cycle, the permanent strain as a function of the cycle number was higher.

Carbon anodes participate in chemical reactions to reduce alumina in the Hall–Héroult process, of which coke aggregates make up a major part. The failure analysis of coke aggregates not only leads to a better understanding of the deformation mechanisms of anode paste under compressive loading but also can identify potential causes of structural defects in carbon anodes, such as horizontal cracks. The coke aggregates are composed of particles with different size distributions and shapes, which may strongly affect the failure behavior of the anode during compaction. In this paper, the effects of particle size distributions and shapes on the mechanical behavior and the failure of coke aggregates are investigated using the discrete element method modeling technique. The numerical results reveal that, although the mechanical behavior of coke mixtures is generally dependent on larger particles, the presence of fine particles in the coke aggregates reduces fluctuations in the stress–strain diagram. In addition, the rolling resistance model is employed as a parameter representing the effect of particle shape. It is shown that the rolling resistance model can be an alternative to the overlapped spheres model, which has a higher computational cost than the rolling resistance model. The second-order work criterion is used to evaluate the stability of the coke aggregates, the results of which indicate that the addition of fine particles as well as increasing the rolling resistance between the particles increases the stability range of the coke aggregates. Moreover, by using the analysis of micro-strain contour evaluations during the compaction process, it is shown that, both by adding fine particles to the coke mixture and by increasing the rolling resistance between the particles, the possibility of creating a compression band in the coke aggregates is reduced. Since the presence of the compaction bands in the anode paste creates an area prone to horizontal crack generations, the results of this study could lead to the production of carbon anodes with fewer structural defects.

In the Hall–Héroult process, prebaked carbon anodes are utilized to produce primary aluminium. The quality of the anode plays a crucial role in the efficiency of electrowinning primary aluminium. In the production of anodes, the anode baking is considered as the stage most frequently causing anode problems. During the baking process, the anode undergoes complex physicochemical transformations. Moreover, the anode at a lower position, imposed by loading pressures from upper anodes, will creep during this process. Thus, the production of high-quality anodes demands efficient control of their baking process. This paper aims to investigate the thermo-chemo-mechanical properties of the anode paste mixture at high temperatures. These properties include kinetic parameters of pitch pyrolysis such as the activation energy and the pre-exponential factor, the thermal expansion coefficient (TEC) and relevant mechanical parameters related to the elastic, the viscoelastic and the viscoplastic behaviours of the anode. For this purpose, experiments consisting of the thermogravimetric analysis, the dilatometry and the creep test were carried out. Based on the obtained results, the forementioned parameters were identified. Relevant mechanical parameters were expressed as a function of a new variable, called the shrinking index, which is related to the volatile released in open and closed pores of the anode. This variable would be used to highlight the chemo-mechanical coupling effect of the anode mixture. New insights into the phenomena such as the expansion due to the increase of the pore pressure and the chemical shrinkage of the anode during the baking process were also gained in this work. These investigations pave the way for modeling the thermo-chemo-poromechanical behaviour of the anode during the baking process.

An in-depth study of the failure of granular materials, which is known as a mechanism to generate defects, can reveal the facts regarding the origin of the imperfections, such as cracks in the carbon anodes. The initiation and propagation of the cracks in the carbon anode, especially the horizontal cracks below the stub-holes, reduce the anode efficiency during the electrolysis process. The failure analysis of coke aggregates can be employed to determine the appropriate recipe and operating conditions in order to avoid the formation of cracks in the carbon anodes. In this paper, it will be shown that a particular failure mode can be responsible for the crack generation in the carbon anodes. The second-order work criterion is employed to analyze the failure of the coke aggregate specimens and the relationships between the second-order work, the kinetic energy, and the instability of the granular material are investigated. In addition, the coke aggregates are modeled by exploiting the discrete element method (DEM) to reveal the micro-mechanical behavior of the dry coke aggregates during the compaction process. The optimal number of particles required for the failure analysis in the DEM simulations is determined. The effects of the confining pressure and strain rate as two important compaction process parameters on the failure are studied. The results reveal that increasing the confining pressure enhances the probability of the diffusing mode of the failure in the specimen. On the other hand, the increase of strain rate augments the chance of the strain localization mode of the failure in the specimen.

Carbon-like materials such as the anode and the ramming paste play a crucial role in the efficiency of the Hall–Héroult process. The mechanical behavior of these materials during forming processes is complex and still ill-understood. This work aimed to investigate experimentally the mechanical behavior of a carbon paste used in the aluminum industry under different loading conditions. For this purpose, experiments consisting of (1) relaxation tests at different compaction levels, (2) quasi-static cyclic tests at several amplitudes, (3) monotonic compaction tests at varied strain rates, and (4) vibrocompaction tests at different frequencies were carried out. The obtained results highlight some fundamental aspects of the carbon paste behavior such as the strain rate’s effect on the paste compressibility, the hardening-softening behavior under cyclic loadings, the effect of cycling amplitude on the stress state and the paste densification, and the frequency effect on the vibrocompaction process. These results pave the way for the development of reliable rheological models for the modeling and the numerical simulation of carbon pastes forming processes.

The Hall-Héroult process uses prebaked carbon anodes as electrodes. The anode’s quality plays a crucial role in the efficiency of the aluminium production process. During the baking process, the anode undergoes complex physicochemical transformations. Thus, the production of high-quality anodes depends, among others, on the efficient control of their baking process. This paper aims to investigate the evolution of some physical properties of the anode paste mixture during the baking process. These properties include the mass loss fraction, real and apparent densities, the ratio of apparent volume, the permeability, and porosities. For this purpose, experiments consisting of thermogravimetric analysis, dilatometry, air permeability, and helium-pycnometric measurements were carried out. The anode permeability at high temperatures was linked to the air permeability through a permeability correlator due to experimental limitations. Moreover, the real density at high temperatures was estimated by combining real densities of the coal tar pitch and coke aggregates. Different porosities, such as the open porosity and the closed porosity related to the pitch binder, were estimated by taking the permeability at high temperatures into account. In this context, the effect of the permeability correlator, which was introduced to link the permeability at high temperatures to the air permeability, was investigated through a sensitivity analysis. These results allow an estimation of the shrinking index, a new variable introduced to reflect the baking level of the anode mixture, which is linked to the volatile that is released in both open and closed pores. Afterwards, the pore pressure inside closed pores in the coal tar pitch was estimated. The obtained results highlight some new insights related to the baking process of the anode mixture. Moreover, they pave the way for better modeling of the thermo-chemo-mechanical behavior of anodes at high temperatures.

This paper reports on the application of an acousto-ultrasonic (AU) scheme for the inspection of industrial-size carbon anode blocks used in the production of primary aluminium by the Hall-Héroult process. A frequency-modulated wave is used to excite the anode blocks at multiple points. The collected attenuated AU signals are decomposed using the Discrete Wavelet Transform (DTW) after which vectors of features are calculated. Principal Component Analysis (PCA) is utilized to cluster the AU responses of the anodes. The approach allows locating cracks in the blocks and the AU features were found sensitive to crack severity. The results are validated using images collected after cutting some anodes.

Carbon anodes used in Hall–Heroult electrolysis cells are involved in both electrical and chemical processes of the cell. Electrical resistivity of anodes depends on electrical properties of its constituents, of which carbon coke aggregates are the most prevalent. Electrical resistivity of coke aggregates is usually characterized according to the ISO 10143 standardized test method, which consists of measuring the voltage drop in the bed of particles between two electrically conducing plungers through which the current is also applied. Estimation of the electrical resistivity of coke particles from the resistivity of particle bed is a challenging task and needs consideration of the contribution of the interparticle void fraction and the particle/particle contact resistances. In this work, the bed resistivity was normalized by subtracting the interparticle void fraction. Then, the contact size was obtained from discrete element method simulation and the contact resistance was calculated using Holm’s theory. Finally, the resistivity of the coke particles was obtained from the bed resistivity.

Aluminium producers are constantly facing challenges regarding the increase in production costs including those related to coal tar pitch (CTP). CTP is used as binder to produce carbon anodes. Alternative sources to CTP capable of addressing the cost and health issues without decreasing anode quality, are thus of great interest. Bio-pitches, produced from bio-oils, could be a good candidate in this regard. The objective of this study is to characterize bio-pitch, obtained from different origins, and to reveal its potential as an alternative for CTP in anode production process. Three bio-pitches were produced from different bio-mass sources under the same pyrolysis conditions. Both physical and chemical properties of bio-pitches were characterized, such as density, softening point, coking value, quinoline insoluble, molecular weight, viscosity, chemical groups and surface composition. The microstructures of the three bio-pitches were also compared to that of CTP.

Fluid flow hydrodynamics in the aluminum electrolysis cell has been studied through numerical simulations for decades. However, there is little to no available experimental data to validate velocity profiles obtained from these numerical simulations. Velocity measurements inside the electrolysis cell is difficult in practice, due to being chemically aggressive, opaque and at high temperature. A new attempt is undertaken to measure molten aluminum velocity with a drag probe inspired from a proven device first used in wind velocity measurement. The new device is designed to minimize vortex shedding, to increase the drag coefficient and to be applicable in a harsh environment. This paper presents the probe adapted for the electrolysis cell, its calibration and validation method in a water channel at room temperature, and some results obtained in electrolysis cell.

In the industrial aluminum production, anode baking is one of the most cost-intensive manufacturing processes. Different parameters including thermo-mechanical solicitations and internal built-up pore pressure resulting from generated volatiles will have an impact on final anode properties. Hence, evolution of mechanical properties occupies an important position in design of anode block and its baking. In order to capture the evolution of those properties, a reliable experimental procedure needs to be devised to obtain the required data and to calibrate its parameters. This paper presents an improved method for anode sample fabrication, its validation and preliminary results for mechanical properties.

A new methodology was proposed for the characterization of time-dependent behavior of materials in order to develop a constitutive model. The material used for the characterization was ramming paste, a porous material used in an aluminum electrolysis cell, which is baked in place under varying loads induced by the thermal expansion of other components of the cell. In order to develop a constitutive model representing the paste mechanical behavior, it was necessary to get some insight into its behavior using samples which had been baked at different temperatures ranging from 200 to 1000 °C. Creep stages, effect of testing temperature on the creep, creep-recovery, as well as nonlinear creep were observed for designing a constitutive law. Uniaxial creep-recovery tests were carried out at two temperatures on the baked paste: ambient and higher. Results showed that the shape of creep curves was similar to a typical creep; recovery happened and the creep was shown to be nonlinear. Those experimental observations and the identification of nonlinear parameters of developed constitutive model demonstrated that the baked paste experiences nonlinear viscoelastic-viscoplastic behavior at different temperatures.

Carbon anodes, used in Hall–Heroult electrolysis process, are subject to carboxy gasification. It is generally thought that the baked pitch of anode is more chemically reactive than coke and thus it preferentially reacts with CO 2 , resulting in premature disintegration of anode. In this study, the apparent reaction rates of anode and its constituents (coke, pitch and butt) were measured separately. The reaction conditions were set to minimize the mass transport limitations (sample size: 2 mg, CO 2 flow rate: 100 ml/min and particle size: about 30 μm). The activation energies of anode, coke, pitch and butt were found to be 192, 175, 207 and 182 kJ/mol, respectively. The evolution of specific surface areas in micro-, meso- and macropores as a function of gasification percentage was revealed using CO 2 and N 2 adsorptions and the ratio of catalyst/inhibitor was obtained using XPS analysis. It was found that the apparent reaction rate of anode and its constituents increased with the progress of the gasification under CO 2 atmosphere at 960 °C. The reaction rate of 0.033 min −1 at 50% of gasification, was obtained for pitch, which is similar to those of anode, coke and butt, suggesting that a pitch, baked separately, is not the most reactive constituent of anode.

Hall-Héroult cell stability is highly affected by anode changing operations. Upon the insertion of a cold anode in the cell, a layer of molten cryolite freezes under the anode. The thickness, microstructure, and chemical composition of this layer vary as a function of time and its location in the cell. To better understand the evolution of the frozen layer, mandatory for the validation of numerical models, a measurement campaign was conducted on the anodes having a few hours of operation in the cell. The macrostructure of the selected frozen bath samples has been investigated using computed tomography while scanning electron microscope (SEM) has been used to qualify its microstructure. An energy-dispersive X-ray spectroscope (EDS) coupled to the SEM has revealed the chemical content. The results showed not only very different macrostructures between samples, but also significantly heterogeneous structure within the same sample. Nevertheless, for all samples, there is a clear distinction between the frozen cryolite and alumina/dusting phases, with the latter surrounding the cryolite matrix.

The oxidation of a carbon anode with air and CO2 occurs during the electrolysis of alumina in Hall-Heroult cells, resulting in a significant overconsumption of carbon and dusting. Boron is well known to decrease the rate of this reaction for graphite. In this work, the application of boron oxide has been investigated to evaluate its inhibition effect on the air oxidation reaction, and to provide an effective protection for anodes. Different methods of impregnation coating have been explored. Impregnated anode samples were gasified under air at 525 °C according to the standard measurement methods. X-ray tomography was used to obtain the microstructural information of the samples before and after air-burning tests. The impregnated samples showed a very low oxidation reaction rate and dust generation.

Baked anodes quality control is still mostly based on core sampling and characterization. Only a small proportion of the production can be tested by this method due to the costly, time consuming, and destructive nature of the analytical techniques. Furthermore, the core properties are not necessarily representative of those of the whole block. A rapid and non-destructive method for anode quality control based on vibration modal analysis is proposed. A number of baked anodes produced under different conditions at the Alcoa Deschambault smelter were selected. These were excited mechanically at different locations and the vibration signals of the blocks were measured by accelerometers. Principal Component Analysis (PCA) was used to cluster the vibration modes of the anodes. It is shown that the proposed approach allows detecting and distinguishing different types of anode internal defects. Some of the tested anodes were cut and imaged in order to confirm the results visually.

Crack propagation in carbon cathode used in the aluminium industry has been investigated through flexural tests on notched specimens. The main parameters of interest were the geometrical evolution of the crack and the stress intensity factor at the tip ends. The latter, in the case of interfacial fracture in two-dimensional geometries, can be related to normal (mode I) and shear (mode II) stresses. In this work, a new methodology has been applied which optically measures the crack tip displacement field by Digital Image Correlation. The stress intensity factors derived from the experimental data are consistent with results available in the literature. Furthermore, the preliminary results showed that characterizing the mode I (opening) only is somehow challenging due to the heterogeneity of such carbonaceous materials.

As a result of tightening of environmental legislation, there is a renewed interest in the technology of alkali activated concrete also known as geopolymer concrete (GPC). Yet, for GPC to assume its own niche in the construction market, it must display equal or better properties at elevated temperatures than Ordinary Portland Cement (OPC) concrete. Currently there is sufficient evidence that GPC has a better resistance to fire; however not all of its behavioural aspects at high temperatures have been investigated. The aim of this research therefore is to investigate its deformational behaviour, in particular the existence or lack of transient creep, which OPC is known to undergo under first time heating when subject to compressive stresses. It was found that not only GPC undergoes transient creep, but also exhibits both expansive and contractive volume changes during heating. It was also found that GPC maintains its structural integrity at 1000 °C.

The quality of anode electrodes is an important issue in aluminum production plants. Therefore, not only raw material conditions but also the conditions of different production steps could affect the anode quality. Analyzed in this paper is the die compaction of some anode paste samples with specified compositions. Portions of fine coke and coarse coke particles were mixed and some coal tar pitch was added to this mixture in order to make the anode paste sample. The Blaine number of the fine coke portion and the pitch content were selected as two variables to make different anode paste samples. Thereafter, each sample was compacted in a rigid die while applying a uniaxial stress up to 60 MPa. The compaction behavior of different samples was studied and an analytical relationship between the green relative density and the axial pressure applied in the die was suggested. After having identified the parameters of the suggested expression for each sample, the contribution of the rearrangement phenomenon to the material densification was studied. Regarding the good agreement between the suggested relationship and the experimental results, it would be possible to predict the compactability of different anode pastes based on the analytical equation.

The anodes used in aluminum production are formed by compaction of a paste composed of binder matrix and coarse particles of petroleum coke (aggregates). Binder matrix composed of a coal tar pitch and fine calcined petroleum coke is usually characterized by coke and/or pitch content and also by the fineness of the coke particles. Since the coke particles are rigid and assumed to be non-deformable during compaction, the deformation behavior of the binder matrix plays a crucial role in the anode paste compaction process. Compaction of binder matrix with different compositions in a rigid closed die was studied in this work. Binder matrix compositions were compacted to a maximum uniaxial pressure of 70 MPa at 150°C. Different strain rates of 2.9 × 10 − 4 s − 1 and 2.9 × 10 − 3 s − 1 enabled us to evaluate the contribution of viscous behavior of the material to the compaction of binder matrix as a function of its composition and deformation rate. A similar experimental compaction procedure with strain rates of 1.8 × 10 − 4 s − 1 and 1.8 × 10 − 3 s − 1 was applied on paste samples with different pitch contents. This study revealed that the compaction of binder matrix and anode paste with conventional compositions is not significantly a time dependent process. Viscous behavior may therefore not have a significant contribution to the compaction of the material.

article i nfo The quality of anode electrode used in aluminum electrolysis industry and its dependence on the production parameters are studied in this work. Using a calcined petroleum coke and a coal tar pitch, anode pastes were made at laboratory scale at four mixing temperatures and four mixing times. The pastes were compacted in a cylindrical mold at a maximum pressure of 60 MPa. The green samples were then baked at 1130 °C for 12 h. Measurement of density variations as well as porosity and pore size distribution was carried out on the green and baked samples as a function of mixing temperature and time. For the setup used, an optimum mixing time and temperature were suggested, resulting in a better mixing effectiveness, maximum density, and min- imum air permeability and specific surface area.

High quality baked carbon anodes contribute to the optimal performance of aluminum reduction cells. However, the currently decreasing quality and increasing variability of anode raw materials (coke and pitch) make it challenging to manufacture the anodes with consistent overall quality. Intercepting faulty anodes (e.g., presence of cracks and pores) before they are set in reduction cells and deteriorate their performance is therefore important. This is a difficult task, even in modern and well-instrumented anode plants, because lab testing using core samples can only characterize a small proportion of the anode production due to the costly, time-consuming, and destructive nature of the analytical methods. In addition, these results are not necessarily representative of the whole anode block. The objective of this work is to develop a rapid and non-destructive method for quality control of baked anodes using acousto-ultrasonic (AU) techniques. The acoustic responses of anode samples (sliced sections) were analyzed using a combination of temporal features computed from AU signals and principal component analysis (PCA). The AU signals were found sensitive to pores and cracks and were able to discriminate the two types of defects. The results were validated qualitatively by submitting the samples to X-ray Computed Tomography (CT scan).

The long term, visco-elastic behaviour of carbon cathode materials used into the Hall–Heroult cell linings has been investigated. The uniaxial compressive creep tests with free lateral strains have been conducted on three different carbon cathode materials (semi-graphitic, graphitic and graphitized) currently used in aluminium reduction cells. The evolution of mechanical properties caused by damaging material has been estimated through cyclic loading tests. Measurements were made of the axial and the radial strains at ambient temperature. The “key” operating mechanisms behind the observed creep phenomenon are evaluated.

The aim of this work is to simulate the forming process of green anodes. For this purpose, a nonlinear compressible viscoplastic constitutive law is presented. The concept of natural reference configuration is considered. Within an isothermal thermodynamic framework, a Helmholtz free energy is proposed to take into account the nonlinear compressible deformation process occurring between natural reference configuration and current configuration. A dissipation potential is introduced in order to characterize the irreversible aspect of compaction process. The constitutive law is thus formulated through two equations: (1) an expression of Cauchy stress tensor and (2) a differential equation characterizing the evolution of the natural reference configuration. Material parameters are assumed to be a function of the apparent green density. An experimental study is carried out in order to characterize the compaction behavior of the anode paste. A user's material VUMAT subroutine for finite-element dynamic explicit analysis has been developed and implemented in the abaqus commercial software. To evaluate the model predictive capability, numerical simulations of the compaction forming process of anode paste were performed. Simulation results show that the constitutive law predicts the experimental trends and gives insight of physical responses. This constitutes a first step toward characterizing the anode paste behavior and making a benchmark with experimental results on the forming process of anode paste.

Computed tomography (CT) is a powerful non-destructive technique providing a large amount of useful data to characterize carbon anodes. Previous works employed this technique to characterize baked anode samples and some relationships have been proposed for apparent density and total porosity as a function of X-ray attenuation coefficients. In this paper, an existing method of crack detection in 2D was applied on CT images of full-scale baked anode to estimate the amount of cracks. The crack detection method has however been modified to improve the termination procedure of the algorithm which was essentially based on the calculation of circularity of the percolated region. The improvement consists of calculating other percolated region properties in order to end adequately the percolation process. The proposed method has been applied to anode slices with and without stub holes.

The study of the deformation behaviour of the collector bar at conditions experienced within the aluminium reduction cell is of great importance to optimizing the efficiency and increasing the life span of the cell. This paper communicates the results of an experimental program carried out on the steel collector bar material (AISI 1006) to investigate its behaviour in relation to its thermal, mechanical and the creep properties. Tests were carried out in compression at low stresses, 0.5 to 2 MPa and high temperature, 900 °C. Different behaviour was observed at low stresses below 2 MPa, which can be characterised by time and applied stress level. For the test at 2 MPa, a conventional creep curve with dominating secondary creep region was obtained. Oxidation and corrosion were factors considered due to the aggressive environment of the test condition. Metallographic inspection showed effect of oxides on tested sample.

To simulate the thermo-mechanical behaviour of ramming paste in aluminium electrolysis cells, experimental data are required to feed 3D constitutive models. Both axial and radial strain measurements are necessary to design constitutive laws and to identify the parameters. However, radial strains are difficult to obtain at high temperature due to limitations of measuring devices. Moreover, samples need to be sufficiently large to acquire significant radial strain amplitude. Hence, the creep behaviour of ramming paste was studied at room temperature so the radial behaviour could then be extrapolated for the high temperature case using the axial strain evolution as a function of the temperature. Mechanical properties of ramming paste, baked at temperatures ranging from 250 °C to 1000 °C, were measured at room temperature. Uniaxial creep tests were performed at three different stress levels for each baking temperature.

To assess the mechanical behavior of ramming paste, representative laboratory samples must be produced for experimental characterization. Experimental data are necessary to feed three-dimensional creep model where radial strain measurements are required. It is thus imperative to perform measurements on a sample large enough to get significant radial strain amplitude. However, the ISO standard fabrication method is restricted to small-size samples. In the present paper, a different compaction process, adapted from the Proctor method, is proposed in order to get larger cylindrical samples of 100 mm diameter and 200 mm height. The ramming paste samples are produced in multilayer by dynamic compaction, using a falling weight in a circular pattern over the tamped surface. The baked apparent density obtained by X-Ray computed tomography, or CT scan, and the baked mechanical properties such as compressive strength, have been characterized as a function of the number of layers and the number of impacts.

Carbon anodes used in the aluminium industry were imaged through destructive and non-destructive testing (NDT) methods. For the latter case, computed tomography (CT), which has previously been used to map the 3D apparent density distribution, was extended to crack detection. Previous work has shown how to overcome technical hurdles related to crack detection by using percolation-based algorithms operating on low-resolution images of full-scale baked carbon anodes. The previous application to 2D images was extended here to the 3D case. The crack detection algorithm has been performed on anode slices containing several independent macro cracks with different morphologies.

Computed tomography has been used in recent years to gather information on carbon anodes which can be used to calibrate numerical models dedicated to simulate the anode forming process. To this end, samples with diameters varying from 50 mm up to 300 mm and cored from an industrial anode have been scanned in a Somatom Sensation 64. A correlation was established between the CT scan results and the apparent density. To validate the correlation, an extended campaign was performed on 50 mm diameter samples cored in 20 different anodes with the advantage of using possibly different raw materials. In addition to the CT scan results, the apparent and real densities have been experimentally measured to estimate the porosity level. Similarly to the apparent density, a correlation between the CT scans results and the porosity has been proposed.

Creep of ramming paste was studied from ambient to operational temperature in order to characterize its mechanical behaviour as used in the peripheral joint of aluminium electrolysis cells. Two types of uniaxial creep test over a specified stress level were performed on two inch samples: tests at room temperature for samples baked at different temperatures and test at temperatures close to the sample baking temperature. It is concluded that at certain baking temperatures and given stress level, three types of creep, called primary, secondary and tertiary, take place successively. In addition, at lower baking temperatures (200 °C), the creep level is larger in comparison with creep at higher baking temperatures. Also, for specified baking temperature, creep strain obtained by high temperature testing is larger than creep strain obtained by room temperature testing. These results give new insights on the ramming paste behaviour in aluminium electrolysis cell.

1.1 Content The characterization of materials behaviour is of importance in many research fields. From an analytical point of view, the mechanical behaviour of materials could be described either by using empirical laws based on experimental observations or by using a framework to develop constitutive laws. In the latter case, the thermodynamic of irreversible processes could be used as the framework. These constitutive laws are generally dedicated to complex problems and are thus developed in a three-dimensional context. Materials behave in different ways under loading but, under specific conditions, they all will generally exhibit instantaneous and time-dependant deformations. Instantaneous deformation could be elastic, plastic and so forth, while time-dependant deformation generally refers to the viscosity of the material. Moreover, as for the Navier-Stokes equation in fluid mechanics which is only valid for Newtonian fluids, constitutive laws are developed considering the nature of the materials. Hence, in this chapter, targeted materials are isotropic granular materials such as concrete, carbon block materials and ramming paste (Sorlie and Oye, 2010). Contrary to metals, this implies that the tensile behaviour is different from the compressive one. Models are also meant to represent various phenomena such as creep and relaxation, which are mainly those of interest in this chapter. The upcoming sections will give a brief overview of the mechanical deformations such as creep/relaxation that are involved in viscoelasticity. Some basic principles governing the development of constitutive law modelling within a thermodynamic framework will be discussed, as well.

The current study investigates the effects of coke particle characteristics and paste formulation on the flowability and the compression behavior of anode pastes. Shape factor and texture of different fractions of cokes were characterized using an image analysis system where the characteristics of each coke were correlated to its vibrated bulk density (VBD). A compression test was designed to study the effects of particle characteristics and paste recipe on the compactability of pastes. The test was applied on four anode pastes, prepared from different coke types, particle size distributions and pitch contents. It was observed that the compression test is significantly sensitive to any changes in raw materials characteristics and formulations. Consequently, the compression test may be used as a tool for evaluating anode quality in relation with material variations.

In order to adequately simulate the behavior of a Hall–Héroult electrolysis cell, a finite element model must take into account the properties of each material forming the cell structure and those contained in it. However, there is some lack of full knowledge of the mechanical behavior of these materials, e.g., the long term viscoelastic (creep/relaxation) behavior of the carbon cathode. In this present paper, a three-dimensional viscoelastic model is devised and proposed, being ready to be implemented in a finite element code. This 3D viscoelastic model was developed from the thermodynamics of irreversible processes, where the selection of the model’s internal variables was based on a phenomenological approach. The model has been developed at a particular reference state; therefore, the model parameters are represented by constant constitutive tensors. The model’s particular parameters were identified for three different types of cathode carbon, i.e., semigraphitic, graphitic, and graphitized.