Modelling the electrical resistivity of green carbon anodes for aluminium industry

L’aluminium primaire produit de nos jours est obtenu par l’électrolyse de l’alumine à 960 °C, suivant le procédé proposé par Charles Martin Hall, et Paul Louis-Toussain Héroult, en 1886. Ce procédé, communément appelé Hall-Héroult, du nom de ceux qui l’ont proposé, consiste à imposer un courant électrique au travers un mélange de cryolithe fondue dans laquelle est dissoute l’alumine. Le courant circule entre les anodes et la cathode de carbone. Les anodes sont principalement composées de coke de pétrole calciné et du reste non consommé des anodes usagées, que l’on appelle mégot. Toutes ces particules carbonées sont liées à l’aide de brai de houille (pitch). Une fois le bloc anodique formé, il doit être cuit afin de lui conférer une bonne tenue mécanique. Pour permettre un rendement efficace lors de ce procédé, les anodes, qui amènent le courant, doivent avoir une résistivité électrique la plus faible possible. De plus, ces anodes, consommables, servent aussi d’apport pour le carbone nécessaire à la réaction d’électrolyse, leur composition chimique doit par conséquent être suffisamment pure pour ne pas affecter la qualité de l’aluminium produit. Le projet de recherche présenté ici se focalise sur l’étude et la compréhension du comportement électrique de l’anode avant sa cuisson à des fins de contrôle de qualité en cours de fabrication. Pour déterminer la résistivité électrique d’un matériau composite, le modèle mathématique de Nielsen semble être un outil très intéressant et polyvalent dans différentes applications de modélisation de la résistivité électrique. Pour utiliser ce modèle, il est nécessaire de connaître certaines propriétés des différentes phases constituant l’anode. Dans le cas présent, le matériau anodique sera limité à une fraction de tailles de particules de coke de pétrole et la matrice liante, composée du pitch et de particules fines de coke de pétrole. Les propriétés à connaître sont, pour les particules de coke, leur résistivité électrique intrinsèque, leur rapport de forme, ainsi que la compacité maximale qui puisse être obtenu avec ce matériau. Seule la résistivité électrique de la matrice liante est à connaître nécessairement. Dans l’industrie de l’aluminium, seule la résistivité électrique d’un lit des particules est mesurée pour suivre l’évolution de ce paramètre suivant les différents lots utilisés. Pour implémenter la résistivité électrique du coke dans le modèle de Nielsen, il est nécessaire de déterminer la résistivité électrique intrinsèque du coke. Afin de déterminer cette valeur, des mesures de résistivité électriques ont été faites en utilisant une méthode quatre pointes, qui permet de mesurer la résistivité électrique des matériaux dont la résistivité est faible. Ensuite, le vide inter particulaire présent dans le volume de mesure est retranché afin de ne compter que le volume occupé par le matériau. Finalement, les contacts entre particules ont été estimés par calcul numérique, en utilisant la méthode des éléments discrets. Le nombre, la surface moyenne et la disposition de ces contacts ont été évalués afin d’être implémentés dans un modèle mathématique permettant de calculer la résistivité électrique du coke. Différentes mesures ont permis de proposer une valeur cohérente et acceptable pour la résistivité électrique du coke. Afin de mesurer la résistivité électrique de la matrice liante ainsi que de l’anode, une méthode précise est requise. La méthode standardisée utilisée par l’industrie montre de grands écarts à la moyenne et des valeurs souvent peu reproductibles. La méthode de mesure de la résistivité électrique proposée par Van der Pauw permet d’obtenir des résultats avec une plus grande reproductibilité et un écart à la moyenne fortement amoindri. Cette méthode peut être utile pour mesurer la résistivité électrique des sections coupées dans une carotte d’anode ou de matrice liante moulée. Différentes mesures, effectuées sur des échantillons type anode de différentes compositions ont révélé que la résistivité électrique mesurée en laboratoire ne correspond pas à celle calculée en utilisant le modèle de Nielsen, intégrant les paramètres physiques des matériaux utilisés. Pour expliquer la divergence entre le modèle et les mesures de laboratoire, une analyse en microscopie optique a été effectuée. Il a été révélé que l’épaisseur de la couche de matrice liante entre les particules demeure trop grande pour permettre la création d’un chemin électrique de particule à particule dans le matériau. Ceci implique que la résistivité électrique de la matrice liante impose principalement la résistivité électrique du composite, dans ce cas.Primary aluminium produced nowadays is obtained by electrolysis of alumina at 960 °C, following the process proposed by Charles Martin Hall, and Paul Louis Toussain Héroult, in 1886. This process, named Hall-Héroult, due to the name of its creators, consists in applying an electrical current trough a mix of molten cryolithe in which is dissolved alumina. The current flows between the carbon anodes and the cathode. Anodes are mostly composed of calcined petroleum coke aggregates and remnant particles of used anodes, butt particles. Those carbonaceous particles are tied together using coal tar pitch. Once the anode bloc is formed, it is backed to gain mechanical strength. To allow an efficient yield during the electrolysis process, anodes, through which the current flows, shall have the lowest electrical resistivity. Furthermore, those consumable anodes, are also required to bring the carbon as reactant for the electrolytic reaction, consequently, their chemical composition must be pure enough not to diminish the produced aluminium quality. The research project presented focuses on the study and understanding of electrical behaviour of the anode prior to its backing, for quality control during manufacturing process. To determine the electrical resistivity of a composite material, Nielsen’s model appears as an interesting tool, reliable for multiple electrical modelling applications. Using this model implies knowing several properties of the different phases present in the anode. In the present case, anode material is restricted to a specific size fraction of coke particles and binder matrix, made of pitch and fine particles. The properties to know are, for the coke particles, their intrinsic electrical resistivity, aspect ratio, and the maximal packing fraction that can be reached. Only electrical resistivity is required for the binder matrix. In aluminium industry, only the electrical resistivities of beds of particles are measured to follow the evolution of this parameter depending the batch used. To implement the electrical resistivity of coke particles in Nielsen model, its intrinsic electrical resistivity is required. To obtain this value, measurement were performed using four probes setting, which allows measuring the electrical resistivity of material presenting low resistivity. Then, inter-particles void present in the measurement volume shall be removed in order to only take account of the volume occupied by the material investigated. Finally, the contacts between particles are estimated by numerical calculation, using discrete element method. The number, average surface and disposition of the contacts were assessed to implement them in a mathematical model allowing calculating the electrical resistivity of coke. Several measures lead to the proposition of a consistent and reliable value for electrical resistivity of coke. To measure the electrical resistivity of binder matrix, as well as the anode’s, an accurate method is required. The standardised method used in the industry reveals strong standard deviation and rather not often reproducible values. The method of measurement of electrical resistivity of continuous phases proposed by Van der Pauw allows retrieving highly reproducible results, with a much lower standard deviation. This technique can be useful to measure the electrical resistivity of slices cut out of anode cores or cast binder matrix. Several measurements, performed on anode like samples with different composition revealed that the electrical resistivity measured in laboratory does not fit with the one calculated using Nielsen’s model, using the physical parameters of the characterized materials. To explain the difference, between the model and laboratory measurements, an optical microscopic analysis was performed. It was disclosed that the binder matrix layer between particles remain too thick to allow the creation of an electrical path from particle to particle within the material. This implies that the electrical resistivity of the binder matrix mostly dictates the one of the composite material, in this case

Acoustic emission techniques to measure the properties of coke particles: a first foray

The performance of the Hall-Héroult aluminium reduction process is strongly influenced by fluctuations of the baked carbon anode properties. The currently decreasing quality and increasing variability of the anode raw materials, coke and pitch, combined with the frequent supplier changes by anode manufacturers to meet their specifications and reduce purchasing costs make it very challenging to produce anodes with consistent properties. Furthermore, the coke quality control scheme traditionally used by aluminium smelters involving infrequent coke sampling and characterization in the laboratory is inadequate for tracking coke variability when formulating the anode paste, and applying timely corrective actions when necessary. Developing new rapid and non-destructive sensors for measuring key coke properties such as density and porosity directly from the production line is highly desirable. This work investigates the possibility of using acoustic emission techniques for measuring physical and/or mechanical properties of coke particles. A set-up was developed for recording the sound made by coke particles dropped on a metal sheet. The potential of the approach was tested on coke samples having different physical properties (several sizes and suppliers). The acoustic signature of each type of coke particle was correlated with their physical properties using regression analysis

Electrical Resistivity Measurement of Carbon Anodes Using the Van der Pauw Method

The electrical resistivity of carbon anodes is an important parameter in the overall efficiency of the aluminum smelting process. The aim of this work is to explore the Van der Pauw (VdP) method as an alternative technique to the standard method, which is commonly used in the aluminum industry, in order to characterize the electrical resistivity of carbon anodes and to assess the accuracy of the method. For this purpose, a cylindrical core is extracted from the top of the anodes. The electrical resistivity of the core samples is measured according to the ISO 11713 standard method. This method consists of applying a 1 A current along the revolution axis of the sample, and then measuring the voltage drop on its side, along the same direction. Theoretically, this technique appears to be satisfying, but cracks in the sample that are generated either during the anode production or while coring the sample may induce high variations in the measured signal. The VdP method, as presented in 1958 by L.J. Van der Pauw, enables the electrical resistivity of any plain sample with an arbitrary shape and low thickness to be measured, even in the presence of cracks. In this work, measurements were performed using both the standard method and the Van der Pauw method, on both flawless and cracked samples. Results provided by the VdP method appeared to be more reliable and repeatable. Furthermore, numerical simulations using the finite element method (FEM) were performed in order to assess the effect of the presence of cracks and their thicknesses on the accuracy of the VdP method

Discrete Element Method Investigation of Bulk Density and Electrical Resistivity of Calcined Coke Mixes

Packing density and electrical resistivity of particles assemblies are important factors for a variety of applications of granular materials. In the present work, a three-dimensional imaging technique is coupled with the discrete element method (DEM) to model anode grade calcined coke particles. Three-dimensional DEM models of samples with different size distribution of particles were studied to obtain the inter-particle contact information. As the content of fine particles increased, a higher inter-particle contact density and smaller average contact radius was observed in the samples. Confronting the DEM data and experimental measurements of electrical resistivity showed the simultaneous effects of packing density and contact density. Samples with higher contact density and smaller contact radius in general held high electrical resistivities. However, if increasing the contact density does not modify contacts between large particles, this will have a positive effect on packing density, so a lower electrical resistivity was obtained

Multifunctional Resistive-Heating and Color-Changing Monofilaments Produced by a Single-Step Coaxial Melt-Spinning Process

Multifunctional coaxial monofilaments were successfully produced by melt-spinning several polymer composites in a single-step. The external layer of the monofilaments was a thermochromic composite having a color-transition at 40 °C (above the ambient temperature) in order to avoid control interferences by the external temperature. The core layer of the monofilaments was a conductive polymer nanocomposite whose resistive heating properties were used to control the monofilament’s temperature and therefore its color using electrical current. The careful selection of the materials and adequate formulation allowed to obtain a trilayer structure with enhanced compatibility between the layers. The mechanical properties of the monofilaments were improved by a solid-state stretching step while also decreasing their diameter. A 64 cm² prototype fabric was woven to characterize the resistive-heating and color-changing properties of the monofilaments. Exceptional thermal output levels were reached, with a temperature rising up to over 100 °C at voltages above 110 V. The reversible color change properties were also successfully demonstrated

Multifunctional Resistive-Heating and Color-Changing Monofilaments Produced by a Single-Step Coaxial Melt-Spinning Process

Multifunctional coaxial monofilaments were successfully produced by melt-spinning several polymer composites in a single-step. The external layer of the monofilaments was a thermochromic composite having a color-transition at 40 °C (above the ambient temperature) in order to avoid control interferences by the external temperature. The core layer of the monofilaments was a conductive polymer nanocomposite whose resistive heating properties were used to control the monofilament’s temperature and therefore its color using electrical current. The careful selection of the materials and adequate formulation allowed to obtain a trilayer structure with enhanced compatibility between the layers. The mechanical properties of the monofilaments were improved by a solid-state stretching step while also decreasing their diameter. A 64 cm² prototype fabric was woven to characterize the resistive-heating and color-changing properties of the monofilaments. Exceptional thermal output levels were reached, with a temperature rising up to over 100 °C at voltages above 110 V. The reversible color change properties were also successfully demonstrated