Flotation of alumina on the surface of the electrolyte in an aluminum electrolysis cell

A model of flotation was developed and applied to alumina in the aluminum electrolysis cell. The conditions of flotation for alumina on the cryolitic bath surface were determined for disc and sphere geometries. The contact angle between alumina and cryolitic bath, which had not been found in the literature, was measured and it was found to be around 30 deg. Experiments with compressed alumina discs on the cryolitic bath surface were conducted, and the results were compared to the model. The experiments showed how impurities on the bath surface influence the flotation; and how a slight asymmetry in the system may accelerate sinking

Experimental determination of the thermal diffusivity of α-Cryolite up to 810 K and comparison with first principles predictions

In aluminum electrolysis cells, a ledge of frozen electrolyte is formed on the sides. Controlling the side ledge thickness (a few centimeters) is essential to maintain a reasonable life span of the electrolysis cell, as the ledge acts as a protective layer against chemical attacks from the electrolyte bath used to dissolve alumina. The numerical modeling of the side ledge thickness, by using, for example, finite element analysis, requires some input data on the thermal transport properties of the side ledge. Unfortunately, there is a severe lack of experimental data, in particular, for the main constituent of the side ledge, the cryolite (Na3AlF6). The aim of this study is twofold. First, the thermal transport properties of cryolite, not available in the literature, were measured experimentally. Second, the experimental data were compared with previous theoretical predictions based on first principle calculations. This was carried out to evaluate the capability of first principle methods in predicting the thermal transport properties of complex insulating materials. The thermal diffusivity of a porous synthetic cryolite sample containing 0.9 wt % of alumina was measured over a wide range of temperature (473–810 K), using the monotone heating method. Because of limited computational resources, the first principle method can be used only to determine the thermal properties of single crystals. The dependence of thermal diffusivity of the Na3AlF6 + 0.9 wt % Al2O3 mixture on the microstructural parameters is discussed. A simple analytical function describing both thermal diffusivity and thermal conductivity of cryolite as a function of temperature is proposed

Perceptual-motor determinants of auditory-verbal serial short-term memory

The role of the compatibility between obligatory perceptual organization and the active assembly of a motor-plan in auditory-verbal serial recall was examined. The classic finding that serial recall is poorer with ear-alternating items was shown to be related to spatial-source localization, thereby confirming a basic tenet of the perceptual-motor account and disconfirming an early account characterizing the two ears as separate input-channels (Experiment 1). Promoting the streaming-by-location of ear-alternating items—and therefore the incompatibility between perceived and actual order—augmented the ear-alternation effect (Experiment 2) whereas demoting streaming-by-location by reducing the regularity of the alternation attenuated it (Experiment 3). Finally, increasing the perceptual variability of an ear-alternating list while demoting the likelihood of streaming-by-location—by adding uncorrelated voice changes—also reduced the ear-alternation effect as did articulatory suppression for that part of the list (pre-recency) associated with motor-planning (Experiment 4). The results are incompatible with theories in which perceptual variability impairs serial recall due to a deficit in encoding items into a limited-capacity short-term memory space and instead point to a central role for perceptual and motor processes in serial short-term memory performance

Méthode de floculation par injection latérale de floculant dans un écoulement à haute vitesse

Les boues rouges sont un résidu du procédé Bayer. Elles sont constituées d'une phase solide en suspension dans une phase liquide. Il est nécessaire de séparer les deux phases à de multiples reprises au cours du procédé. Pour ce faire, les boues rouges doivent subir un processus de floculation avant d'être séparées par décantation. La floculation est présentement réalisée dans un puits d'alimentation, un système fonctionnel mais problématique à plusieurs niveaux. Ce document présente le développement d'une nouvelle technologie visant à remplacer le puits d'alimentation. Le « floculateur » a comme objectif de floculer une suspension de boues rouges aussi efficacement que possible tout en étant robuste par rapport aux fluctuations du système. Tout d'abord, les matières premières sont caractérisées. Ensuite, la théorie pertinente sur le procédé Bayer, la séparation solide-liquide et la CFD est présentée, accompagnée d'une analyse du circuit de lavage à contre-courant. Il y est démontré qu'un floculateur idéal devrait pouvoir traiter une alimentation ayant une concentration de solide aussi élevée que 400 g/1. De plus, une étude détaillée de la sonde Lasentec - le principal instrument de mesure du projet - est réalisée. Les différentes variables permettant de quantifier la distribution granulométrique sont analysées et les meilleures sont sélectionnées. Une revue exhaustive de la littérature sur les puits d'alimentation et de tous les brevets reliés à la floculation au cours des 25 dernières années est aussi incluse. Par la suite, le puits d'alimentation est simulé par CFD. Deux geometries sont considérées : un puits pilote et un puits industriel. Un modèle de floculation est développé et incorporé aux simulations. Les résultats indiquent que l'étape de dispersion du floculant est problématique à l'échelle industrielle. De plus, la dispersion demeure inadéquate peu importe la position de l'injection de floculant. Améliorer l'étape de dispersion devient donc la priorité du floculateur. La solution élaborée consiste à amincir et accélérer l'écoulement afin d'injecter le floculant sur une grande surface. De cette façon, les simulations montrent qu'une dispersion homogène est atteinte pour des temps de mélange de l'ordre du centième de seconde. Un premier prototype du nouveau floculateur permettant de traiter 4 m3/h est conçu, fabriqué et testé sous différentes conditions. Une version améliorée est ensuite construite et testée. Un système d'injection de floculant est également développé. Les résultats montrent une très bonne floculation pour la plupart des conditions d'opérations (médiane de l'ordre de 1000 \xm). Les performances sont excellentes jusqu'à une vitesse de 5 m/s et pour un dosage supérieure à 80 g/t. La concentration de solide de l'alimentation est testée jusqu'à 400 g/l. Les résultats sont ambigus-, mais ils soulèvent la possibilité que le floculateur soit efficace à de telles concentrations. Enfin, un troisième prototype à l'échelle semi industrielle est développé et construit. Il peut traiter un débit volumique de 54 m3/h. Pour terminer, une étude de la transition à l'échelle industrielle est effectuée. Il est montré que le floculateur a la capacité de traiter des débits volumiques allant jusqu'à 1800 m3/h. De plus, une analyse de quelques alternatives simples est présentée. À défaut d'utiliser le floculateur, il est fortement recommandé aux industries d'injecter le floculant dans la conduite de façon aussi uniforme que possible un mètre ou deux avant l'entrée du puits. Un compromis entre le floculateur et le puits d'alimentation est également suggéré

A Replication of “Motor and Visual Codes Interact to Facilitate Visuospatial Memory Performance (2007; Experiment 1)”

The present study is a replication of Chum, Bekkering, Dodd, and Pratt (2007). Motor and visual codes interact to facilitate visuospatial memory performance. Psychonomic Bulletin & Review, 14, 1189-1193

Impact of the Solidification Rate on the Chemical Composition of Frozen Cryolite Bath

Solidification of cryolite (Na3AlF6)-based bath takes place at different rates along the sideledge, and around alumina rafts and new anodes. The solidification rate has a significant impact on the structure and the chemical composition that determine the thermal conductivity and thus the thickness of sideledge, or the duration of the existence of the temporary frozen bath layers in other cases. Unfortunately, samples that can be collected in industrial cells are formed under unknown, spatially and temporally varying conditions. For this reason, frozen bath samples were created under different heat flux conditions in a well-controlled laboratory environment using the so-called cold finger technique. The samples were analyzed by X-ray Diffractometer (XRD) and Scanning Electron Microscope (SEM) in Back Scattering (BS) mode in order to obtain spatial distribution of chemical composition. Results were correlated with structural analysis. XRD confirmed our earlier hypothesis of recrystallization of cryolite to chiolite under medium heat flux regime. Lower α-alumina, and higher γ-alumina content in the samples obtained with very high heating rate suggest that fast cooling reduces α–γ conversion. In accordance with the expectation, SEM-BS revealed significant variation of the Na/Al ratio in the transient sample

Mass transport by waves on the bath metal interface in electrolysis cell

The movement of waves on the surface of a fluid is always accompanied by a displacement of the fluid particles in the wave propagation direction, this is called the Stokes drift. This phenomenon can be found in aluminum electrolysis cells. More or less regular oscillations animate the interface between the electrolyte bath and the metal in a cell. It is well known that material, such as rafts of undissolved alumina and solidified bath and cryolite snow can float on the metal-bath interface. So these objects can be transported by waves on the interface. The purpose of this paper is to review the literature on the works already done about the surface waves, interface waves and Stokes drift. Then the mass transport induced by these waves will be estimated for the case of the metal bath interface in the electrolysis cells

In Silico Analysis of Pacific Oyster (Crassostrea gigas) Transcriptome over Developmental Stages Reveals Candidate Genes for Larval Settlement

WOS:000459747700197International audienceFollowing their planktonic phase, the larvae of benthic marine organisms must locate a suitable habitat to settle and metamorphose. For oysters, larval adhesion occurs at the pediveliger stage with the secretion of a proteinaceous bioadhesive produced by the foot, a specialized and ephemeral organ. Oyster bioadhesive is highly resistant to proteomic extraction and is only produced in very low quantities, which explains why it has been very little examined in larvae to date. In silico analysis of nucleic acid databases could help to identify genes of interest implicated in settlement. In this work, the publicly available transcriptome of Pacific oyster Crassostrea gigas over its developmental stages was mined to select genes highly expressed at the pediveliger stage. Our analysis revealed 59 sequences potentially implicated in adhesion of C. gigas larvae. Some related proteins contain conserved domains already described in other bioadhesives. We propose a hypothetic composition of C. gigas bioadhesive in which the protein constituent is probably composed of collagen and the von Willebrand Factor domain could play a role in adhesive cohesion. Genes coding for enzymes implicated in DOPA chemistry were also detected, indicating that this modification is also potentially present in the adhesive of pediveliger larvae

Validation of the gravimetric method to properly follow alumina dissolution in cryolitic bath

Alumina dissolution is now one of the greatest concerns about Hall-Héroult process optimization. To tackle alumina dissolution kinetics, a gravimetric method was developed. This method follows the apparent weight of an alumina sample while immersed in a cryolitic bath. Then, from the weight over time curve, it is possible to identify the forces acting on the sample especially the change of gravity and buoyancy forces. The experimental results obtained by this method were compared to four other values: namely the data found in the literature, values computed with a CFD model, the total mass loss of alumina disc samples over time and finally to a sample boundary displacement method. The boundary displacement and the CFD model have also demonstrated the strong dependence of the dissolution rate on the convection pattern around the sample

Mass Transport by Waves: Physical Model with Coalescence, Fragmentation, and Displacement on a Bath-Metal Interface

The behavior of alumina raft at the bath-metal interface in an electrolysis cell has been simulated by a numerical model in a past study. Further improvement on the numerical model led to broader application and a more «cell-like» scenario. The model now considers three distinct bath-metal interface phenomena. First, the transport wave stems from disturbances like a gas bubble and alumina deposit. Second, the stationary wave is produced by the magnetohydrodynamics force in the cell. Third, a well-known permanent deformation of the bath-metal interface (BMI). Also, the new model simulates the fragmentation and coalescence of multiple alumina rafts. The purpose of this paper is to present an in-depth parameter analysis of the improved numerical model and identify scenarios in which alumina draft aggregates along an axis