The Formation and Disintegration of Rafts from Different Aluminas and Fines

Raft formation is a frequently encountered problem during alumina feeding in the Hall-Héroult-process and will delay alumina from being dissolved into the melt. The mechanisms for the formation and disintegration of rafts are however not thoroughly understood yet. The current study investigates the dissolution behavior and raft structure from three different types of secondary alumina in a lab cell, with a particular attention to effect of fines, and involves both sampling of rafts and video recordings of the feeding. The mass loss rate was calculated to vary between −1.57 and −0.42 g min−1 for regular bulk alumina, and −1.15 and −0.06 g min−1 for fines. Rafts created from bulk alumina were flat with a distinct bulge or crater placed in the center of it, while rafts created from fines had a pellet-shaped structure and traces of undissolved alumina in the middle. The observed structure is due to the initial spreading of powder, confirmed by video recordings.publishedVersio

Influence of Atmosphere and Temperature on Polycyclic Aromatic Hydrocarbon Emissions from Green Anode Paste Baking

Coal tar pitch, a well-known source of polycyclic aromatic hydrocarbons (PAHs), is used as a binder of petroleum coke in prebaked anodes used for electrolysis of aluminum. Anodes are baked up to 1100 °C over a 20-day period, where flue gas containing PAHs and volatile organic compounds (VOCs) are treated using techniques such as regenerative thermal oxidation, quenching, and washing. Conditions during baking facilitate incomplete combustion of PAHs, and due to the various structures and properties of PAHs, the effect of temperature up to 750 °C and various atmospheres during pyrolysis and combustion were tested. PAH emissions from green anode paste (GAP) dominate in the temperature interval of 251–500 °C, where PAH species of 4–6 rings make up the majority of the emission profile. During pyrolysis in argon atmosphere, a total of 1645 μg EPA-16 PAHs are emitted per gram of GAP. Adding 5 and 10% CO2 to the inert atmosphere does not seem to affect the PAH emission level significantly, at 1547 and 1666 μg/g, respectively. When adding oxygen, concentrations decreased to 569 μg/g and 417 μg/g for 5% and 10% O2, respectively, corresponding to a 65% and 75% decrease in emission.publishedVersio

A Study of Bubble Behavior and Anode Effect on the Graphite and Industrial Carbon Anode in a See-Through Furnace During Aluminium Electrolysis

Anode gas bubble behavior and anode effect on graphite and industrial carbon rod-shaped anode in a cryolite melt have been studied using a see-through furnace. The different carbon materials have different properties which can affect bubble behavior and electrochemical properties. Industrial carbon is more inhomogeneous with respect to structure, pore, aggregates and impurities in comparison to the graphite. More bubbles were nucleated on the industrial carbon than on the graphite for the same current density. The time related to the coalescence process for both anodes was found to be in interval 16 to 24 ms and independent of the current densities. Bubbles detached from the horizontal surface of the anode have similar average diameter value for both anodes for current densities 1.0 A cm−2, the average diameter is lower for the industrial carbon anode. The onset of the anode effect occurred faster on the graphite than on the industrial anode. The PFC-containing gas layer appeared to be thicker and more stable on the graphite anode than on the industrial carbon anode.publishedVersio

Electrolysis in reduced gravitational environments: current research perspectives and future applications

Electrochemical energy conversion technologies play a crucial role in space missions, for example, in the Environmental Control and Life Support System (ECLSS) on the International Space Station (ISS). They are also vitally important for future long-term space travel for oxygen, fuel and chemical production, where a re-supply of resources from Earth is not possible. Here, we provide an overview of currently existing electrolytic energy conversion technologies for space applications such as proton exchange membrane (PEM) and alkaline electrolyzer systems. We discuss the governing interfacial processes in these devices influenced by reduced gravitation and provide an outlook on future applications of electrolysis systems in, e.g., in-situ resource utilization (ISRU) technologies. A perspective of computational modelling to predict the impact of the reduced gravitational environment on governing electrochemical processes is also discussed and experimental suggestions to better understand efficiency-impacting processes such as gas bubble formation and detachment in reduced gravitational environments are outlined

Analysis of Polycyclic Aromatic Hydrocarbon Emissions from a Pilot Scale Silicon Process with Flue Gas Recirculation

Flue gas recirculation (FGR) is a method used in several industries to control emissions and process conditions, such as NOx reduction and temperature levels, and increase the CO2 concentration in the off-gas, to be better suited for methods of carbon capture. In this study, the influence of FGR, varying levels of flue gas flow and oxygen concentration on the emissions of polycyclic aromatic hydrocarbons (PAHs) was investigated during Si alloy production. In addition, computational fluid dynamics (CFD) modeling was performed using OpenFOAM for combustion of C2H2 and H2 with varying O2 levels to simulate FGR and to gain better insight into the impact of furnace operations on the PAH evolution. Experimental results show that increasing FGR (0–82.5%) and decreasing levels of oxygen (20.7–13.3 vol %) increase the PAH-42 concentration from 14.1 to 559.7 μg/Nm3. This is supported by the simulations, where increased formation of all PAHs species was observed at high levels of FGR, especially for the lighter aromatic species (like benzene and naphthalene), due to the lower availability of oxygen and the reduction in temperature. Residence time was identified as another key parameter to promote complete combustion of PAHs. Benzene oxidation can be prevented with temperatures lower than 1000 K and residence times smaller than 1 s, while complete oxidation is found at temperatures of around 1500 K.publishedVersio

Electrolysis in reduced gravitational environments: current research perspectives and future applications

Electrochemical energy conversion technologies play a crucial role in space missions, for example, in the Environmental Control and Life Support System (ECLSS) on the International Space Station (ISS). They are also vitally important for future long-term space travel for oxygen, fuel and chemical production, where a re-supply of resources from Earth is not possible. Here, we provide an overview of currently existing electrolytic energy conversion technologies for space applications such as proton exchange membrane (PEM) and alkaline electrolyzer systems. We discuss the governing interfacial processes in these devices influenced by reduced gravitation and provide an outlook on future applications of electrolysis systems in, e.g., in-situ resource utilization (ISRU) technologies. A perspective of computational modelling to predict the impact of the reduced gravitational environment on governing electrochemical processes is also discussed and experimental suggestions to better understand efficiency-impacting processes such as gas bubble formation and detachment in reduced gravitational environments are outlined

A Treatise on Interpolar Transport Phenomena

This thesis contributes to the understanding of mechanisms for mass transport in aluminium electrolysis cells. Fundamental studies are undertaken of ow patterns and mass transport in the interpolar region under various operating conditions. A coupled model predicting the turbulent electrolyte ow, under the in uence of both electromagnetism and forces from buoyant gas bubbles, crucial for better prediction of mass transfer mechanisms and voltage oscillations, has been developed from rst principles. The model is validated against experiments performed on a lab scale electrolysis cell. Both modelling and experiments are performed within the scope of this thesis. Experiments on lab- and industrial scale cells have been conducted in order to study the behaviour of anodic gas bubbles under various operating conditions. On industrial scale, bubble related signals show typical frequencies in the range 0.5 to 2 Hz, with amplitudes up to 5% around the mean voltage. Results indicate that the bubble related voltage oscillations increase in both frequency and magnitude with increasing anode age, the latter of which due to the diminishing in uence of slots. No signi cant correlation between anode pairs is identi ed, suggesting that models treating individual anodes are meaningful also on an industrial scale. Due to challenges related to multiple simultaneous phenomena occurring on industrial scales, a series of lab scale measurements have been performed, in order to obtain quantitative data for model validation. The lab scale experimental cell allowed for di erent current densities, interpolar distances and inclination angles, thus spanning ranges typically encountered on the industrial scale. Lab scale frequencies are found to be in the range 0.25 to 0.65 Hz, with magnitude of up to 4% around the mean voltage. The magnitude of the oscillations decreases with increasing anode age, due to increased rounding of the initially sharp anode edges. The traditional voltage measurements have been supplied with high-speed video recordings of the bath surface showing a good correspondence between voltage uctuations and escaping gas bubbles. On average, 0.5 and 2 bubbles were observed per second on lab and industrial scales, respectively, signi cantly higher than frequencies obtained by a FFT of the voltage signal. It is shown that this discrepancy can be due to large variations in the bubble release times, thus violating the assumption of a periodic signal required for an FFT. For industrial anodes, the possibility of overlapping bubbles is investigated as an alternative e ect resulting in the mismatch between observed and calculated frequencies. A phenomenological, coupled, model for the creation and transport of anodic gas bubbles is developed from rst principles. The proposed model is a multiscale approach in which molecular species are produced by Faraday's law and transported by di usion and advection through a supersaturated electrolyte. Sub grid bubbles are allowed to form through nucleation on the anode surface and the resulting bubble population evolves through mass transfer and coalescence. As sub grid bubbles reach a certain size they are transferred to a macroscopic phase which evolution is governed by the volume of uid method, thus allowing for the treatment of complex bubble topology. The model is validated against results from the lab scale experiments in a 2D model, showing that essential features of the voltage signal can be reproduced by the proposed approach. The in uence of various parameters such as bath properties, anode microstructure and mass transfer properties are investigated by means of a factorial design analysis. The factorial design indicates that the contact angle, Sherwood number (and molecular di usivity) and the porosity of the anode that have the most signi cant in uence on the frequencies of the resulting bubble induced voltage uctuations. Furthermore, resulting frequencies appear to be dominated by these selected factors, as coupling is present only at low signi cance. Considering the amplitude of the signal, the dominating factors are the bath viscosity, contact angle and pore diameter. Although the in uence of these factors is large, signi cant coupling between factors is observed, indicating that the physics determining the amplitude of the signal is of a more complex nature than that of the frequencies. The mean voltage is relatively insensitive to the factors studied in this analysis. Simulations are able to reproduce the essential behaviour found experimentally on the lab scale cell, that is: increasing frequencies with increasing current densities and anode inclination, increasing amplitudes with decreasing anode inclination and increasing current densities and nally increasing mean voltages with increasing ACD and current density. Considering individual bubbles, the in uence of electromagnetic forces is small when compared to other forces such as buoyancy and surface tension. However, when considering the system as a whole, the Lorentz forces are found to yield enhanced gas departure rates due to favourable pressure gradients in the bath. This feature is necessarily enhanced further by the signi cantly elevated current densities found in the proximity of large bubbles, as the in uence of the Lorentz forces is found to increase with increasing current densities. Simulations indicate that steady state bubble production on the anode does not imply a direct transfer of all the molecular gas to bubbles. Instead, a balance between bubble production and transport by di usion and advection away from the anode appears to describe this state, resulting in a CO2 supersaturated region greatly extending the bubble layer. The presence of a CO2 enriched region yields a possible explanation to the observed reduction of current eciency if the anode-cathode distance is reduced beyond a critical limit

Inverse modelling of interfacial tension between ferroalloy and slag using openfoam

The entrainment of molten ferroalloy droplets in slag during tapping operations is strongly related to turbulence and interfacial forces between alloy and slag. Therefore, interfacial phenomena are of great importance for the ferroalloys industry and a better understanding of entrainment mechanisms can reduce ferroalloy losses with slag flow. The interfacial tension plays an important role in the interaction between ferroalloy and slag due to the ability to modify droplets shape and the flow regime. However, the measurement of interfacial tension between two molten phases is challenging due to high temperatures and complex composition. In particular, surface active elements significantly influence the interfacial tension. Available methods for determining the interfacial tension are often based on using complex equipment (e.g. a furnace equipped with an X-ray camera) and tend to have significant uncertainty in measurements. In this study, a methodology for inverse modelling of interfacial tension between ferroalloys and slag was developed and investigated by combining experimental measurements, reduced order modelling and simulations in OpenFOAM. The proposed method relies upon experimental determination of the shape of single droplets, from which surface tension can be determined using numerical procedures such as elliptic fitting and the low-bond axisymmetric drop shape technique. Given relevant material properties for single phases, parameters governing the interactions between the phases, e.g. interfacial tension, can be determined by comparing parametric simulations to experiments in which interactions are present. Simulations are realized using multiphaseInterFoam for a slag droplet at rest on molten metal in an inert atmosphere. The current work describes the modelling strategy and demonstrates its applicability to recent experiments for the FeMn-slag system. The uncertainty and sensibility of the method are assessed by comparing different available simulation settings, resolution and the uncertainty in the experimental data

INVERSE MODELLING OF INTERFACIAL TENSION BETWEEN FERROALLOY AND SLAG USING OPENFOAM

The entrainment of molten ferroalloy droplets in slag during tapping operations is strongly related to turbulence and interfacial forces between alloy and slag. Therefore, interfacial phenomena are of great importance for the ferroalloys industry and a better understanding of entrainment mechanisms can reduce ferroalloy losses with slag flow. The interfacial tension plays an important role in the interaction between ferroalloy and slag due to the ability to modify droplets shape and the flow regime. However, the measurement of interfacial tension between two molten phases is challenging due to high temperatures and complex composition. In particular, surface active elements significantly influence the interfacial tension. Available methods for determining the interfacial tension are often based on using complex equipment (e.g. a furnace equipped with an X-ray camera) and tend to have significant uncertainty in measurements. In this study, a methodology for inverse modelling of interfacial tension between ferroalloys and slag was developed and investigated by combining experimental measurements, reduced order modelling and simulations in OpenFOAM. The proposed method relies upon experimental determination of the shape of single droplets, from which surface tension can be determined using numerical procedures such as elliptic fitting and the low-bond axisymmetric drop shape technique. Given relevant material properties for single phases, parameters governing the interactions between the phases, e.g. interfacial tension, can be determined by comparing parametric simulations to experiments in which interactions are present. Simulations are realized using multiphaseInterFoam for a slag droplet at rest on molten metal in an inert atmosphere. The current work describes the modelling strategy and demonstrates its applicability to recent experiments for the FeMnslag system. The uncertainty and sensibility of the method are assessed by comparing different available simulation settings, resolution and the uncertainty in the experimental data