Reduction of alumina in the Hall-Héroult aluminium cell requires large amounts of electrical ener... more Reduction of alumina in the Hall-Héroult aluminium cell requires large amounts of electrical energy (about 13 kWh/kg Al), and also involves consumption of carbon anodes. This results in the emission of large amounts of carbon dioxide, particularly if the electrical power is generated from fossil fuels. It should be possible to reduce the energy requirement towards the minimum theoretical requirement by improving the efficiency of the process, that is, by reducing unnecessary energy losses. These losses are detailed in descriptions of the design and operation of the Hall-Héroult cell in the literature. Many of the losses relate to issues in the electrolyte (bath) in which the alumina reduction is carried out.
Jigging is one of the oldest methods of gravity separation and is still widely used in ore proces... more Jigging is one of the oldest methods of gravity separation and is still widely used in ore processing owing to its high separation precision, easy maintenance, cost-effectiveness and high throughput rate. This study investigates solid separation in a jigging device through a series of numerical simulations. The mathematical model adopted consists of computational fluid dynamics (CFD) coupled with discrete element method (DEM), which resolve the liquid and particle flow, respectively. Stratification is heavily influenced by fluid motion through the jig. Therefore, many jigging pulsation profiles have been developed and applied in ore processing. This study explores the trapezoidal pulsation profile adopting variations in amplitude and frequency. This aids in assessing how changes in profile affect jigging behaviour and also helps elucidate the governing concentration mechanics. The initial packing conditions consist of a binary-density particle system where the light particles and he...
Air-water models, as a substitute for CO2-cryolite systems, have been widely applied to study the... more Air-water models, as a substitute for CO2-cryolite systems, have been widely applied to study the complex bubble driven liquid flow that occurs in the aluminium electrolysis process. The argument for using the air-water system is that the kinematic viscosity of water is very similar to that of cryolite (1.005×10 m∙s for water at 20°C and 1.43×10 m∙s for cryolite at 960°C). Thus, similar liquid flow dynamics will result as long as the same volume of gas is used. A recent study [1] showed that for a fixed bubble size, the CO2 bubble in cryolite leads to a larger bubble sliding velocity under the anode and a smaller bubble thickness than the air bubble in water. In this paper, the effect of property differences between the two systems was investigated in detail. The study is based on a computational fluid dynamics (CFD) model with the volume-offluid (VOF) method to predict the bubble shapes. It was found that the contact angle is a dominant factor affecting the bubble dynamics between ...
Two-fluid (Eulerian–Eulerian) simulation of bubbly flow normally assumes that bubble size is smal... more Two-fluid (Eulerian–Eulerian) simulation of bubbly flow normally assumes that bubble size is smaller than the computational cell size, although the formulation should continue to be valid as mesh is refined below bubble size. In some situations, one may need to use a mesh that is finer than the bubble size, but it is argued that the standard twofluid equations fail in such situations. Averaging of the Navier–Stokes equations is analysed to understand this effect, and equations that account for the finite size effect are derived. As an example, finite bubble size effects are shown to be important in a simulation of a water model of an aluminium reduction cell. In this case, bubbles are generated on the lower face of submerged anodes, slide to the edge of the anodes and rise to the liquid surface. The bubble plumes at the side of the anodes are predicted to be too narrow unless finite size effects are taken into account. An approximate method for correcting the usual two-fluid equatio...
ABSTRACT Computational Fluid Dynamics (CFD) models have been developed for many of the unit opera... more ABSTRACT Computational Fluid Dynamics (CFD) models have been developed for many of the unit operations involved in zinc processing, both in the concentrator and in smelting or refining operations. These models are now relatively sophisticated, being capable of accounting for metallurgical effects as well as the underlying multi-phase flow processes, and are therefore powerful tools for optimising process and equipment performance. Examples of unit operation models relevant to the concentrator include flotation, fine grinding and tailings thickeners. Models have also been developed of fluid bed roasters, leach reactors, clarifiers and electrowinning cells as used in a zinc refinery, while smelter processes such as sintering and retorting together with associated equipment such as cyclones and electrostatic precipitators are also amenable to CFD modelling. Although such models exist and have been successfully used in other industries, the application in zinc concentrators and refineries has not so far been widespread. CFD modelling should play an increasingly critical role as operations seek greater gains in recovery because of volatile zinc prices, in energy efficiency because of greenhouse gas issues, and in environmental performance in order to ensure “license to operate”.
A two-phase computational fluid dynamics (CFD) model has been developed to simulate the time-aver... more A two-phase computational fluid dynamics (CFD) model has been developed to simulate the time-averaged flow in the molten electrolyte layer of a Hall-Héroult aluminum cell. The flow is driven by the rise of carbon dioxide bubbles formed on the base of the anodes. The CFD model has been validated against detailed measurements of velocity and turbulence taken in a full-scale air-water physical model containing three anodes in four different configurations, with varying inter-anode gap and the option of slots. The model predictions agree with the measurements of velocity and turbulence energy for all configurations within the likely measurement repeatability, and therefore can be used to understand the overall electrolyte circulation patterns and mixing. For example, the model predicts that the bubble hold-up under an anode is approximately halved by the presence of a slot aligned transverse to the cell long axis. The flow patterns do not appear to be significantly altered by halving the inter-anode gap width from 40 mm to 20 mm. The CFD model predicts that the relative widths of center, side and end channels have a major influence on several critical aspects of the cell flowfield.
Processing in the minerals and energy resources industries almost always involves complex multiph... more Processing in the minerals and energy resources industries almost always involves complex multiphase flow systems designed to achieve mixing, reaction or separation. Computational fluid dynamics (CFD) modelling can be a powerful tool to assist in improving these processes or designing new processes and equipment. However proper account needs to be taken for multi-phase interactions, multi-scale effects, and additional non-flow physics and chemistry that can be critical to the process.
Reduction of alumina in the Hall-Héroult aluminium cell requires large amounts of electrical ener... more Reduction of alumina in the Hall-Héroult aluminium cell requires large amounts of electrical energy (about 13 kWh/kg Al), and also involves consumption of carbon anodes. This results in the emission of large amounts of carbon dioxide, particularly if the electrical power is generated from fossil fuels. It should be possible to reduce the energy requirement towards the minimum theoretical requirement by improving the efficiency of the process, that is, by reducing unnecessary energy losses. These losses are detailed in descriptions of the design and operation of the Hall-Héroult cell in the literature. Many of the losses relate to issues in the electrolyte (bath) in which the alumina reduction is carried out.
Jigging is one of the oldest methods of gravity separation and is still widely used in ore proces... more Jigging is one of the oldest methods of gravity separation and is still widely used in ore processing owing to its high separation precision, easy maintenance, cost-effectiveness and high throughput rate. This study investigates solid separation in a jigging device through a series of numerical simulations. The mathematical model adopted consists of computational fluid dynamics (CFD) coupled with discrete element method (DEM), which resolve the liquid and particle flow, respectively. Stratification is heavily influenced by fluid motion through the jig. Therefore, many jigging pulsation profiles have been developed and applied in ore processing. This study explores the trapezoidal pulsation profile adopting variations in amplitude and frequency. This aids in assessing how changes in profile affect jigging behaviour and also helps elucidate the governing concentration mechanics. The initial packing conditions consist of a binary-density particle system where the light particles and he...
Air-water models, as a substitute for CO2-cryolite systems, have been widely applied to study the... more Air-water models, as a substitute for CO2-cryolite systems, have been widely applied to study the complex bubble driven liquid flow that occurs in the aluminium electrolysis process. The argument for using the air-water system is that the kinematic viscosity of water is very similar to that of cryolite (1.005×10 m∙s for water at 20°C and 1.43×10 m∙s for cryolite at 960°C). Thus, similar liquid flow dynamics will result as long as the same volume of gas is used. A recent study [1] showed that for a fixed bubble size, the CO2 bubble in cryolite leads to a larger bubble sliding velocity under the anode and a smaller bubble thickness than the air bubble in water. In this paper, the effect of property differences between the two systems was investigated in detail. The study is based on a computational fluid dynamics (CFD) model with the volume-offluid (VOF) method to predict the bubble shapes. It was found that the contact angle is a dominant factor affecting the bubble dynamics between ...
Two-fluid (Eulerian–Eulerian) simulation of bubbly flow normally assumes that bubble size is smal... more Two-fluid (Eulerian–Eulerian) simulation of bubbly flow normally assumes that bubble size is smaller than the computational cell size, although the formulation should continue to be valid as mesh is refined below bubble size. In some situations, one may need to use a mesh that is finer than the bubble size, but it is argued that the standard twofluid equations fail in such situations. Averaging of the Navier–Stokes equations is analysed to understand this effect, and equations that account for the finite size effect are derived. As an example, finite bubble size effects are shown to be important in a simulation of a water model of an aluminium reduction cell. In this case, bubbles are generated on the lower face of submerged anodes, slide to the edge of the anodes and rise to the liquid surface. The bubble plumes at the side of the anodes are predicted to be too narrow unless finite size effects are taken into account. An approximate method for correcting the usual two-fluid equatio...
ABSTRACT Computational Fluid Dynamics (CFD) models have been developed for many of the unit opera... more ABSTRACT Computational Fluid Dynamics (CFD) models have been developed for many of the unit operations involved in zinc processing, both in the concentrator and in smelting or refining operations. These models are now relatively sophisticated, being capable of accounting for metallurgical effects as well as the underlying multi-phase flow processes, and are therefore powerful tools for optimising process and equipment performance. Examples of unit operation models relevant to the concentrator include flotation, fine grinding and tailings thickeners. Models have also been developed of fluid bed roasters, leach reactors, clarifiers and electrowinning cells as used in a zinc refinery, while smelter processes such as sintering and retorting together with associated equipment such as cyclones and electrostatic precipitators are also amenable to CFD modelling. Although such models exist and have been successfully used in other industries, the application in zinc concentrators and refineries has not so far been widespread. CFD modelling should play an increasingly critical role as operations seek greater gains in recovery because of volatile zinc prices, in energy efficiency because of greenhouse gas issues, and in environmental performance in order to ensure “license to operate”.
A two-phase computational fluid dynamics (CFD) model has been developed to simulate the time-aver... more A two-phase computational fluid dynamics (CFD) model has been developed to simulate the time-averaged flow in the molten electrolyte layer of a Hall-Héroult aluminum cell. The flow is driven by the rise of carbon dioxide bubbles formed on the base of the anodes. The CFD model has been validated against detailed measurements of velocity and turbulence taken in a full-scale air-water physical model containing three anodes in four different configurations, with varying inter-anode gap and the option of slots. The model predictions agree with the measurements of velocity and turbulence energy for all configurations within the likely measurement repeatability, and therefore can be used to understand the overall electrolyte circulation patterns and mixing. For example, the model predicts that the bubble hold-up under an anode is approximately halved by the presence of a slot aligned transverse to the cell long axis. The flow patterns do not appear to be significantly altered by halving the inter-anode gap width from 40 mm to 20 mm. The CFD model predicts that the relative widths of center, side and end channels have a major influence on several critical aspects of the cell flowfield.
Processing in the minerals and energy resources industries almost always involves complex multiph... more Processing in the minerals and energy resources industries almost always involves complex multiphase flow systems designed to achieve mixing, reaction or separation. Computational fluid dynamics (CFD) modelling can be a powerful tool to assist in improving these processes or designing new processes and equipment. However proper account needs to be taken for multi-phase interactions, multi-scale effects, and additional non-flow physics and chemistry that can be critical to the process.
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