Anodic dissolution of metals in oxide-free cryolite melts

The anodic behavior of metals in molten cryolite-alumina melts has been investigated mostly for use as inert anodes for the Hall-Héroult process. In the present work, gold, platinum, palladium, copper, tungsten, nickel, cobalt and iron metal electrodes were anodically polarized in an oxide-free cryolite melt (11%wt. excess AlF3 ; 5%wt. CaF2) at 1273 K. The aim of the experiments was to characterize the oxidation reactions of the metals occurring without the effect of oxygen-containing dissolved species. The anodic dissolution of each metal was demonstrated, and electrochemical reactions were assigned using reversible potential calculation. The relative stability of metals as well as the possibility of generating pure fluorine is discussed

Silver as Anode in Cryolite—Alumina-Based Melts

The anodic behaviour of silver was investigated in cryolite—alumina-based melt. Silver has a lower melting point (ca. 960◦C) than the other metals considered as possible inert materials for aluminium electrolysis. The working temperature used in aluminium industry is approximately 960◦C, depending on the melt composition. Therefore, the stability of silver during the anodic process was tested at 870◦C in an acidic electrolyte consisting of 65.5 mass % Na3AlF6 + 22.9 mass % AlF3 + 5.7 mass % CaF2 + 3.9 mass % LiF + 2 mass % Al2O3 with the melting point ca. 850◦C. The electrolyte without alumina was prepared as well, with the melting point ca. 860◦C. The resulting cryolite ratio (CR = n(NaF)/n(AlF3)) for both electrolytes was equal to 1.6. The behaviour of the silver anode was studied by voltammetry measurements. The electrochemical study showed that an oxidation reaction occurred at a potential below the oxygen evolution potential. Silver was not found to be stable under oxygen evolution. The degradation of the silver anode was apparent after electrolysis

Tracking single bubble in Hall-Héroult aluminium cell : an experimental and numerical study

In simulations of the hydrodynamics of the multiphase flow in gas– liquid systems with finite sizes of bubbles, the important thing is to compute explicitly the time evolution of the gas–liquid interface in many engineering applications. The most commonly used methods representing this approach are: the volume of fluid and the phase field methods. The later has gained significant interest because of its capability of performing numerical computations on a fixed Cartesian grid without having to parametrise these objects (Eulerian approach) and at the same time it allows to follow the interface ( for example bubble’s shape) that change the topology. In this paper, both numerical (phase field method) and experimental results for the bubble shapes underneath a downward facing plane is presented. Experiments are carried out to see the bubble sliding motion underneath a horizontal and inclined anode. It is assumed that the bubble formed under the anode surface is deformed (flattened) due to buoyant field before it goes around the anode corner. The bubble elongates to form a tail-like shape. The change in shape of the bubble is almost instantaneous and has a significant effect on the localised hydrodynamics around the bubble, which could influence the dynamics of the flow patterns in the Hall–Héroult cell. This deformation is the main cause of the bubble wake and the induced flow field in the aluminium cell. Various parameters such as bubble size, deformation and its sliding mechanism at different surface tensions are discussed and compared with experimental results

The Role of Key Impurity Elements on the Performance of Aluminium Electrolysis - Current Efficiency and Metal Quality

Impurities such as phosphorus and silicon mainly enter the aluminium electrolysis process with alumina. These impurities dissolve in the electrolyte and affect the performance of the electrolysis, the emissions from the cells and the quality of the metal produced. In the present work, the behavior of phosphorus and silicon species in the industrial Hall-Héroult cells was investigated. The study was based on the deleterious effect of phosphorus and silicon on the aluminium production process. It is established in the literature that for every 100 ppm of phosphorus in the bath the current efficiency is decreased by 1%. The chemical and electrochemical behavior of phosphorus and silicon compounds in the Hall-Héroult process was studied. Measurements were as well carried out in QATALUM industrial cells by analysis of bath and metal after addition of phosphorus and silicon compounds to the bath. Additions of AlPO4, Na3PO4 and SiO2 to the industrial prebaked cells were performed and the variation of the concentration in both the bath and metal was followed over time. Phosphorus showed to have a much longer retention time in the bath than silicon. Variations during start-up of cells and anode effects seemed to influence the phosphorus and silicon level in the cells. From industrial data trends, it can be observed that the amount of phosphorus in the cells decreases with increasing the operating temperature. It was found that merely small amounts of phosphorus are reduced at the cathode. Unlike phosphorus, large amounts of silicon were primarily reduced at the cathode and ended up in the metal. Mass balance for phosphorus for prebake cells was presented. It was found that a massive amount of the phosphorus entering the cells was transported with the off-gases to the dry scrubbers. Since the secondary alumina is used as feeding for the prebake cells, the phosphorus is recycled back and will end up in the produced metal

Metallurgy development: Discovery and utilization of aluminum through history

Since the early human civilizations, the discovery and use of new materials and the development of new technologies have changed culture and influenced the development of the modern human environment. At the same time, innovations based on scientific discoveries and technological advances are connected to increasingly complex social and political structures and international relations, which have an impact on economic growth and social benefits. Unfortunately, the human capacity for technological and strategic innovation has most often been demonstrated under stressful conditions inspiring the phrase “necessity is the mother of invention”. The goal of this review is to examine the necessity that compelled mankind to search for metals, primarily focusing on the discovery of aluminum and the challenges represented by the complex nature of its minerals. Although men’s first contact with metal initiated with native copper and meteoritic iron, bronz was the first metallic material significantly impacting human society. The experiments with its chemical composition led to the development of metallurgical processes such as smelting, refining, and casting as well as mechanisms of economics and communication. The accidental discovery of iron in the process of copper ore refining gave mankind greater control over its environment, resulting in increased population and expanded settlements. Aluminum, as a brilliant white metal, was introduced to the world through the works of Wöhler and Deville. However, it became commercially available after electrolysis was discovered by Charles Martin Hall on February 23rd 1886 in a woodshed using home-made battery. A few months later, the similar results were obtained by Paul Louis Toussaint Héroult, so the process for electrolytic production of aluminum bears both of their names. As a symbol of modernity aluminum is used today in the automotive, aerospace, railway, marine, electric, and architectural applications. At the end of this work, it is superfluous to ask whether humanity would have been able to explore the universe and reach the stars if it had been restricted by stone, bones and wood

Étude de la formation de boues dans la cellule Hall-Héroult

La production d’aluminium comme un procédé métallurgique énergivore, joue un rôle majeur dans l’économie du Québec. Un facteur qui peut augmenter la consommation d'énergie correspondante est la formation de dépôts résistifs (des boues) à la surface de la cathode. Par conséquent, l'objectif de cette thèse est de clarifier davantage la nature complexe de la formation de boues dans le procédé Hall-Héroult, en tant que le procédé principal de production d'aluminium. Les objectifs de recherche sont: a) étudier l’effet possible de la nuance de cathode sur la formation ou la dissolution des boues; b) déterminer l'importance relative de trois paramètres opérationnels, à savoir la température d’opération, le ratio de cryolite (CR) ainsi que la nuance de cathode; c) étudier de façon expérimentale des boues induites chimiquement (c'est-à-dire précipitation des espèces cryolitiques dues au changement de la chimie du bain comme la consommation d'alumine ou la polarisation de concentration de sodium). La méthodologie expérimentale comporte une configuration expérimentale à l'échelle du banc d’essai, cinq nuances cathodiques différentes, une conception de plan expérimental factoriel complet ayant trois variables et deux niveaux pour chaque paramètre, à savoir la température d’opération (940 et 960 ° C), le ratio de cryolite (2,2 et 3,8) et deux nuances cathodiques. Les résultats de cette recherche sont: i. une combinaison de porosité ouverte et de perméabilité à l'air des cathodes affecte l'épaisseur des couches de carbure d'aluminium; ii. parmi plusieurs propriétés, la conductivité thermique et la porosité ouverte des cathodes affectent la tendance à la formation de boues en raison de la perte de chaleur au fond de la cellule et des phénomènes interfaciaux; iii. l'influence significative de la température d’opération par rapport au ratio de cryolite et à la nuance de cathode; iv. la précipitation des espèces cryolitiques solides est accélérée grâce à la polarisation de concentration plus élevée et le taux d'épuisement de l'aluminium plus rapide; v. le temps de réalimentation des boues est plus élevé pour les cathodes à porosités ouvertes plus élevées en raison de l'interaction des boues, avec des couches de carbure plus épaisses et des espèces carbonées au fond de la cellule

Reactive Metals as Energy Storage and Carrier Media: Use of Aluminum for Power Generation in Fuel Cell-Based Power Plants

In recent years, the energy production sector has experienced a growing interest in new energy vectors enabling energy storage and, at the same time, intersectoral energy applications among users. Hydrogen is one of the most promising energy storage and carrier media featuring a very high gravimetric energy density, but a rather low volumetric energy density. To this regard, this study focuses on the use of aluminum as energy storage and carrier medium, offering high volumetric energy density (23.5 kWh L$^{-1}$), ease to transport and stock (e.g., as ingots), and is neither toxic nor dangerous when stored. In addition, mature production and recycling technologies exist for aluminum. Herein, the performance of power systems driven by aluminum powder in terms of electrical efficiency (η$_{(I)}$) and round‐trip efficiency (RTE) is analyzed. Along with the additional advantages relating to high volumetric energy density, and safety and management aspects, the aluminum‐based technology appears to outperform the power‐to‐power systems based on hydrogen and liquid fuels

L'elettrolisi: un'occasione per riflettere sulla Chimica, ma anche sulla storia, l'economia, l'ambiente

Pensiamo che partire da esempi tratti dal mondo che ci circonda, in cui le procedure della Chimica, sia teoriche che sperimentali, appaiano in tutto il loro intreccio con le attività umane, possa costituire un metodo efficace per fornire a vari livelli un’immagine della disciplina corretta. Ciò può aiutare ad approcciarsi ad essa senza paure e senza trionfalismi. Nell’articolo in particolare si tratteranno alcuni significativi processi di elettrolisi, evidenziando il loro sviluppo storico e le loro implicazioni dal punto di vista produttivo e del rapporto con uomo e ambiente.We believe that an effective way to provide, at various levels, a correct image of Chemistry could be based on examples from the world around us, in which the procedures of the discipline, both theoretical and experimental, appear in all their interweaving with human activities. This may help to approach it without fear and without triumphalism. In the article in particular we discuss some significant electrolysis processes, highlighting their his- torical development and their implications from the point of view of production and the rela- tionship with man and environment

Metallurgy development: Discovery and utilization of aluminum through history

Since the early human civilizations, the discovery and use of new materials and the development of new technologies have changed culture and influenced the development of the modern human environment. At the same time, innovations based on scientific discoveries and technological advances are connected to increasingly complex social and political structures and international relations, which have an impact on economic growth and social benefits. Unfortunately, the human capacity for technological and strategic innovation has most often been demonstrated under stressful conditions inspiring the phrase “necessity is the mother of invention”. The goal of this review is to examine the necessity that compelled mankind to search for metals, primarily focusing on the discovery of aluminum and the challenges represented by the complex nature of its minerals. Although men’s first contact with metal initiated with native copper and meteoritic iron, bronz was the first metallic material significantly impacting human society. The experiments with its chemical composition led to the development of metallurgical processes such as smelting, refining, and casting as well as mechanisms of economics and communication. The accidental discovery of iron in the process of copper ore refining gave mankind greater control over its environment, resulting in increased population and expanded settlements. Aluminum, as a brilliant white metal, was introduced to the world through the works of Wöhler and Deville. However, it became commercially available after electrolysis was discovered by Charles Martin Hall on February 23rd 1886 in a woodshed using home-made battery. A few months later, the similar results were obtained by Paul Louis Toussaint Héroult, so the process for electrolytic production of aluminum bears both of their names. As a symbol of modernity aluminum is used today in the automotive, aerospace, railway, marine, electric, and architectural applications. At the end of this work, it is superfluous to ask whether humanity would have been able to explore the universe and reach the stars if it had been restricted by stone, bones and wood