<p>Rapid dissolution of alumina upon addition to the cryolitic melt is crucial for the modern Hall-Heroult process for aluminium production. The formation of slow - dissolving alumina agglomerates may be detrimental, and irregular dissolution kinetics may cause the loss of process control. So-called anode effects may subsequently ignite, which are a major source of green-house gases from the primary aluminium industry.</p><p>A literature review and the study of the theory of sintering provides the background for discussing the present work. The most probable mass transport mechanism in the transition alumina-fluoride-moisture system studied here is surface diffusion. Surface diffusion is a non-densifying mass transport mechanism that will result in coarsening (alumina grain growth) but only weak interparticle bonding since no macroscopic shrinkage is involved. Rapid mass transport is known to result when there is a simultaneous phase transformation, and this is the case when transition alumina transforms to α-alumina, catalyzed by the presence of fluorides.</p><p>The main experimental techniques used in the present work were powder X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Supporting techniques used have been speciffc area determination by the BET method and simple thermo-gravimetric techniques. An optical furnace was designed and built in order to study the dissolution of tablet alumina agglomerates.</p><p>A preliminary agglomeration study of preformed cylindrical alumina samples served to map some of the most important mechanisms involved when alumina powder interacts with alumina-saturated cryolitic melt. The conditions at the alumina-melt interface were studied, but it is concluded that the experimental method could not provide the necessary parameter control in order to study the agglomeration mechanism in further detail.</p><p>The tablet agglomerate study is the major experimental contribution of the present work. The experimental method provided good control of the sample chemistry and well defined temperature and time variables. It is concluded that liquid cryolitic melt (NaAlF₄) provides an effective mass transport route for the transformation assisted growth of α-alumina platelets. The platelets that initially form will provide the limited mechanical strength necessary for agglomerate formation and their persistence in a cryolitic melt. Alumina agglomeration may therefore take place with only partial, initial phase transformation. It is concluded that differences in the agglomeration behavior of various qualities of alumina may be the rate determining property for alumina dissolution kinetics in cryolitic melts. Differences in the agglomeration behavior may be due to a number of physical properties of alumina. It is argued here that the fundamental, but difficult to measure, alumina nano-structure may be most important.</p><p>The alumina nano-structure is correlated to secondary alumina properties such as the α-alumina content, specific surface area (BET) and moisture content (MOI, LOI). In this study an X-ray diffraction line profile analysis using the Warren-Averbach method shows that there is a significant difference in the nano-structure of the two smelter grade alumina qualities under study. This may explain the different agglomeration behavior that is observed.</p><p>An optical study of tablet agglomerate dissolution in cryolitic melt proved to be largely unsuccessful due to severe corrosion of the quartz crucibles used. However, a proposed mechanism for the tendency for disintegration of alumina agglomerates, thus dissolving as \snow-flakes" is supported. </p><p>The temperature response time in the tablet alumina samples was studied in order to determine the experimental limit of the shortest time period possible in the experiments. The exothermal γ -> α transformation is observed for secondary alumina samples containing adsorbed fluorides. An interesting effect of the carbon content in secondary alumina is also shown.</p><p>The moisture content of smelter grade alumina is a function of the alumina quality, in particular the technology used for the calcination of the aluminium trihydrate precursor. In the current study the moisture content is shown to be a dynamic function of the ambient temperature and relative humidity. The moisture content is an important variable for the study of alumina agglomeration, and for the fluoride emission from the Hall-Heroult process. The kinetics of moisture desorption and absorption for various alumina qualities is studied. The desorption kinetics is concluded to be signifcantly different, while it is also shown that practical absorption kinetics is a function of the sample size and available surface area. </p>

Rapid dissolution of alumina upon addition to the cryolitic melt is crucial for the modern Hall-Heroult process for aluminium production. The formation of slow - dissolving alumina agglomerates may be detrimental, and irregular dissolution kinetics may cause the loss of process control. So-called anode effects may subsequently ignite, which are a major source of green-house gases from the primary aluminium industry. A literature review and the study of the theory of sintering provides the background for discussing the present work. The most probable mass transport mechanism in the transition alumina-fluoride-moisture system studied here is surface diffusion. Surface diffusion is a non-densifying mass transport mechanism that will result in coarsening (alumina grain growth) but only weak interparticle bonding since no macroscopic shrinkage is involved. Rapid mass transport is known to result when there is a simultaneous phase transformation, and this is the case when transition alumina transforms to α-alumina, catalyzed by the presence of fluorides. The main experimental techniques used in the present work were powder X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Supporting techniques used have been speciffc area determination by the BET method and simple thermo-gravimetric techniques. An optical furnace was designed and built in order to study the dissolution of tablet alumina agglomerates. A preliminary agglomeration study of preformed cylindrical alumina samples served to map some of the most important mechanisms involved when alumina powder interacts with alumina-saturated cryolitic melt. The conditions at the alumina-melt interface were studied, but it is concluded that the experimental method could not provide the necessary parameter control in order to study the agglomeration mechanism in further detail. The tablet agglomerate study is the major experimental contribution of the present work. The experimental method provided good control of the sample chemistry and well defined temperature and time variables. It is concluded that liquid cryolitic melt (NaAlF4) provides an effective mass transport route for the transformation assisted growth of α-alumina platelets. The platelets that initially form will provide the limited mechanical strength necessary for agglomerate formation and their persistence in a cryolitic melt. Alumina agglomeration may therefore take place with only partial, initial phase transformation. It is concluded that differences in the agglomeration behavior of various qualities of alumina may be the rate determining property for alumina dissolution kinetics in cryolitic melts. Differences in the agglomeration behavior may be due to a number of physical properties of alumina. It is argued here that the fundamental, but difficult to measure, alumina nano-structure may be most important. The alumina nano-structure is correlated to secondary alumina properties such as the α-alumina content, specific surface area (BET) and moisture content (MOI, LOI). In this study an X-ray diffraction line profile analysis using the Warren-Averbach method shows that there is a significant difference in the nano-structure of the two smelter grade alumina qualities under study. This may explain the different agglomeration behavior that is observed. An optical study of tablet agglomerate dissolution in cryolitic melt proved to be largely unsuccessful due to severe corrosion of the quartz crucibles used. However, a proposed mechanism for the tendency for disintegration of alumina agglomerates, thus dissolving as \snow-flakes" is supported. The temperature response time in the tablet alumina samples was studied in order to determine the experimental limit of the shortest time period possible in the experiments. The exothermal γ -&gt; α transformation is observed for secondary alumina samples containing adsorbed fluorides. An interesting effect of the carbon content in secondary alumina is also shown. The moisture content of smelter grade alumina is a function of the alumina quality, in particular the technology used for the calcination of the aluminium trihydrate precursor. In the current study the moisture content is shown to be a dynamic function of the ambient temperature and relative humidity. The moisture content is an important variable for the study of alumina agglomeration, and for the fluoride emission from the Hall-Heroult process. The kinetics of moisture desorption and absorption for various alumina qualities is studied. The desorption kinetics is concluded to be signifcantly different, while it is also shown that practical absorption kinetics is a function of the sample size and available surface area.

<p>The solubility of divalent iron oxide in cryolite-based melts was studied. Both electrochemical and chemical techniques were employed. To ensure that only divalent iron was present in solution, the melt was contained in an iron crucible under an atmosphere of argon. The experimental work included investigation of the solubility as a function of alumina concentration, temperature and cryolite ratio (CR = NaF/AlF₃ molar ratio). The solubility at 1020 ºC was found to decrease from 4.17 wt% Fe in cryolite to 0.32 wt% Fe in cryolite saturated with alumina. FeO and FeAl₂O₄were found to coexist as solid phases in equilibrium with the melt at 5.03 wt% Al₂O₃; the former being the stable solid phase below this concentration and the latter at high alumina concentrations. The standard Gibbs energy of formation for FeAl₂O₄from its oxide components at 1020 ºC was determined to be -(17.6 ± 0.5) kJ mol^-1. The solubility of FeAl₂O₄ was found to increase from 0.25 wt% Fe at 981 ºC to 0.36 wt% at 1050 ºC in alumina-saturated melts. By assuming Henrian behaviour, the apparent partial molar enthalpy of dissolution of FeAl₂O₄ was found to be (64.8 2.5) kJ mol^-1. Experiments involving varying cryolite ratio in alumina-saturated melts at 1020 ºC showed a maximum solubility of 0.62 wt% Fe at a cryolite ratio of five. Modelling indicated that divalent iron species were present as NaFeF₃in acidic melts (CR < 3), while Na₃FeF₅and/or Na₄FeF₆ dominated in a basic environment (CR > 3).</p><p>The solubility of TiO_{2 i}n cryolite-alumina melts at 1020 ºC was measured. The analytical data showed that the titanium solubility decreased with increasing total oxide concentration, up to a concentration of ~3.5 wt% O, while it increased at higher concentrations. The solubility was found to be 3.1 wt% Ti and 2.7 wt% Ti, respectively, in cryolite and in alumina-saturated melts. Modelling indicated that the most probable titanium species are TiO²⁺ and TiO₃²-, which coexist in the solution; the former dominating at low alumina concentrations and the latter at high alumina concentrations. Unknown amounts of fluoride may also be associated with the titanium atoms. Determination of the solubility of TiO₂ in alumina-saturated melts as a function of temperature showed that the solubility increased from 1.9 wt% Ti at 975 ºC to 2.8 wt% Ti at 1035 ºC. The apparent partial molar enthalpy of dissolution of TiO₂was found to be (88.3 ± 4.1) kJ mol^-1, provided that Henry’s law holds.</p><p>The electrochemistry of divalent iron in cryolite-based melts was investigated by voltammetry, chronopotentiometry and chronoamperometry. A working electrode of copper was found to be best suited for the study of the reduction of Fe(II), while gold and platinum gave the best results under oxidising conditions. The reduction of Fe(II) ions was found to be diffusion controlled. The number of electrons involved was determined to be two. A discrepancy was observed between the diffusion coefficients obtained by the different techniques. The diffusion coefficient of Fe(II) in alumina-saturated melts at 1020 ºC was found to be D_Fe(II) = 3.0 x10^-5 cm² s^-1 by voltammetry. Experiments performed in an electrolyte with industrial composition at ~970 ºC gave a slightly higher value for the diffusion coefficient. The oxidation of Fe(II) on a gold or a platinum wire electrode showed that the process was diffusion controlled, involving one electron. The reversible potential for the redox couple Fe(III)/Fe(II) was found to be more cathodic than the reversible potential for the oxygen evolution by 350 to 400 mV, depending on the solvent composition and on the temperature.</p><p>The electrochemistry of TiO₂ in cryolite-alumina melts was studied by voltammetry. The deposition of titanium on tungsten was found to be a three-electron diffusion controlled process. The deposition peak increased with increasing titanium concentration. In alumina-saturated melts two waves were observed prior to the titanium deposition. The potential difference between the cathodic wave closest to the deposition peak and its corresponding oxidation peak indicated a diffusion controlled process that involved a one-electron charge transfer. However, in cryolite melts a single wave was observed prior to the titanium deposition. It is suggested that these cathodic waves might have been caused by underpotential deposition of titanium, and subsequent alloying with tungsten. It cannot be ruled out that redox reactions take place between tetravalent titanium and the titanium alloyed with tungsten, thereby forming trivalent titanium prior to the metal deposition.</p><p>In order to determine thermodynamic properties of FeAl₂O₄, a solid electrolyte galvanic cell was used. Cryolite was present in the half-cell containing FeAl₂O₄to ensure that alumina of the alpha modification was in equilibrium with FeAl₂O₄. An oxygen ion conducting yttria-stabilised zirconia tube served as the solid electrolyte. The EMF was measured in the rage 1245 to 1343 K. By using literature data at higher temperatures, thermodynamic properties for the reaction Fe(s) + ½O₂(g) + Al₂O₃(s,α) = FeAl₂O₄(s) could be calculated, i.e. ΔHº_1600K = –(270615 ± 1387) J mol^-1 and ΔSº_1600K= -(56.759 ± 0.856) J K^-1 mol^-1. New thermodynamic data for FeAl₂O₄ were also calculated, and a predominance area diagram for solid iron phases at 1293 K was constructed. The standard potential of the redox couple Fe(III)/Fe(II) as a function of the alumina content was derived from the solubility data of Fe(II) obtained in the present work and literature data for Fe(III). When the standard potentials are put into context of the Hall-Héroult process, the results indicate that neither the CO₂/CO anode gas nor the carbon anode itself can oxidise Fe(II) to Fe(III).</p><p>The mass transfer of the impurities Fe, Si and Ti between bath and aluminium in industrial Hall-Héroult cells was investigated. The experiments were performed in several types of cells with prebaked anodes. The impurities were added to the bath in the form of Fe₂O₃, SiO₂ and TiO₂. Bath and metal samples were collected periodically before and after the addition was made. With the criterion that the mass transport was diffusion controlled, a model involving first order reaction kinetics was used to calculate the mass transfer coefficients for transfer into the metal phase. Large scatter were observed in the obtained mass transfer coefficients, but the general trend seemed to be k_Fe > k_Si > k_Ti. By averaging the data obtained, it was found: k_Fe = (10 ± 3) x 10^-6 m s^-1, k_Si = (7 ± 3) x 10^-6m s^-1, and k_Ti = (5 ± 2) x 10-⁶ m s^-1.</p>

The solubility of divalent iron oxide in cryolite-based melts was studied. Both electrochemical and chemical techniques were employed. To ensure that only divalent iron was present in solution, the melt was contained in an iron crucible under an atmosphere of argon. The experimental work included investigation of the solubility as a function of alumina concentration, temperature and cryolite ratio (CR = NaF/AlF3 molar ratio). The solubility at 1020 ºC was found to decrease from 4.17 wt% Fe in cryolite to 0.32 wt% Fe in cryolite saturated with alumina. FeO and FeAl2O4 were found to coexist as solid phases in equilibrium with the melt at 5.03 wt% Al2O3; the former being the stable solid phase below this concentration and the latter at high alumina concentrations. The standard Gibbs energy of formation for FeAl2O4 from its oxide components at 1020 ºC was determined to be -(17.6 ± 0.5) kJ mol-1. The solubility of FeAl2O4 was found to increase from 0.25 wt% Fe at 981 ºC to 0.36 wt% at 1050 ºC in alumina-saturated melts. By assuming Henrian behaviour, the apparent partial molar enthalpy of dissolution of FeAl2O4 was found to be (64.8 2.5) kJ mol-1. Experiments involving varying cryolite ratio in alumina-saturated melts at 1020 ºC showed a maximum solubility of 0.62 wt% Fe at a cryolite ratio of five. Modelling indicated that divalent iron species were present as NaFeF3 in acidic melts (CR &lt; 3), while Na3FeF5 and/or Na4FeF6 dominated in a basic environment (CR &gt; 3). The solubility of TiO2 in cryolite-alumina melts at 1020 ºC was measured. The analytical data showed that the titanium solubility decreased with increasing total oxide concentration, up to a concentration of ~3.5 wt% O, while it increased at higher concentrations. The solubility was found to be 3.1 wt% Ti and 2.7 wt% Ti, respectively, in cryolite and in alumina-saturated melts. Modelling indicated that the most probable titanium species are TiO2+ and TiO32-, which coexist in the solution; the former dominating at low alumina concentrations and the latter at high alumina concentrations. Unknown amounts of fluoride may also be associated with the titanium atoms. Determination of the solubility of TiO2 in alumina-saturated melts as a function of temperature showed that the solubility increased from 1.9 wt% Ti at 975 ºC to 2.8 wt% Ti at 1035 ºC. The apparent partial molar enthalpy of dissolution of TiO2 was found to be (88.3 ± 4.1) kJ mol-1, provided that Henry’s law holds. The electrochemistry of divalent iron in cryolite-based melts was investigated by voltammetry, chronopotentiometry and chronoamperometry. A working electrode of copper was found to be best suited for the study of the reduction of Fe(II), while gold and platinum gave the best results under oxidising conditions. The reduction of Fe(II) ions was found to be diffusion controlled. The number of electrons involved was determined to be two. A discrepancy was observed between the diffusion coefficients obtained by the different techniques. The diffusion coefficient of Fe(II) in alumina-saturated melts at 1020 ºC was found to be DFe(II) = 3.0 x10-5 cm2 s-1 by voltammetry. Experiments performed in an electrolyte with industrial composition at ~970 ºC gave a slightly higher value for the diffusion coefficient. The oxidation of Fe(II) on a gold or a platinum wire electrode showed that the process was diffusion controlled, involving one electron. The reversible potential for the redox couple Fe(III)/Fe(II) was found to be more cathodic than the reversible potential for the oxygen evolution by 350 to 400 mV, depending on the solvent composition and on the temperature. The electrochemistry of TiO2 in cryolite-alumina melts was studied by voltammetry. The deposition of titanium on tungsten was found to be a three-electron diffusion controlled process. The deposition peak increased with increasing titanium concentration. In alumina-saturated melts two waves were observed prior to the titanium deposition. The potential difference between the cathodic wave closest to the deposition peak and its corresponding oxidation peak indicated a diffusion controlled process that involved a one-electron charge transfer. However, in cryolite melts a single wave was observed prior to the titanium deposition. It is suggested that these cathodic waves might have been caused by underpotential deposition of titanium, and subsequent alloying with tungsten. It cannot be ruled out that redox reactions take place between tetravalent titanium and the titanium alloyed with tungsten, thereby forming trivalent titanium prior to the metal deposition. In order to determine thermodynamic properties of FeAl2O4, a solid electrolyte galvanic cell was used. Cryolite was present in the half-cell containing FeAl2O4 to ensure that alumina of the alpha modification was in equilibrium with FeAl2O4. An oxygen ion conducting yttria-stabilised zirconia tube served as the solid electrolyte. The EMF was measured in the rage 1245 to 1343 K. By using literature data at higher temperatures, thermodynamic properties for the reaction Fe(s) + ½O2(g) + Al2O3(s,α) = FeAl2O4(s) could be calculated, i.e. ΔHº1600K = –(270615 ± 1387) J mol-1 and ΔSº1600K = -(56.759 ± 0.856) J K-1 mol-1. New thermodynamic data for FeAl2O4 were also calculated, and a predominance area diagram for solid iron phases at 1293 K was constructed. The standard potential of the redox couple Fe(III)/Fe(II) as a function of the alumina content was derived from the solubility data of Fe(II) obtained in the present work and literature data for Fe(III). When the standard potentials are put into context of the Hall-Héroult process, the results indicate that neither the CO2/CO anode gas nor the carbon anode itself can oxidise Fe(II) to Fe(III). The mass transfer of the impurities Fe, Si and Ti between bath and aluminium in industrial Hall-Héroult cells was investigated. The experiments were performed in several types of cells with prebaked anodes. The impurities were added to the bath in the form of Fe2O3, SiO2 and TiO2. Bath and metal samples were collected periodically before and after the addition was made. With the criterion that the mass transport was diffusion controlled, a model involving first order reaction kinetics was used to calculate the mass transfer coefficients for transfer into the metal phase. Large scatter were observed in the obtained mass transfer coefficients, but the general trend seemed to be kFe &gt; kSi &gt; kTi. By averaging the data obtained, it was found: kFe = (10 ± 3) x 10-6 m s-1, kSi = (7 ± 3) x 10-6 m s-1, and kTi = (5 ± 2) x 10-6 m s-1.

In the past more and more advanced and fine-tuned processes for steel production have resulted in increased demands for new and more costeffective ferroalloys used as constituents in the processes. Casting techniques and equipment are continually studied for potential improvements. In order to ensure a high and consistent quality in the alloys and the casting equipment, the heat transfer from the alloy to the mould during casting must be understood. Research on free metal flows is scarce and to remedy this a doctoral study at the Norwegian University of Science and Technology was initiated. The study was limited to the region around the impingement point of the metal jet, because this is the critical area for both heat and mass transfer. The flow develops radially, first as a thin film spreading evenly over the surface. At a certain point the thickness of the film increases suddenly - known as a hydraulic jump. Only steady-state conditions on a flat plate (without accumulation of fluid) are studied. The jump develops before the flow reaches the edge of the plate and maintains this position until steady-state conditions are obtained. This system is believed to be a good approximation for the initial conditions during the filling of an open mould. This is also the period when the thermal load on the mould is greatest. Numerous practical and mathematical simulations have been carried out and a relatively simple analytical model depicting the surface rofile of the liquid metal including heat transfer to the surroundings has been developed. The computational fluid dynamics code FLUENT was also used to compute the surface profile with the Volume-Of-Fluid technique, but with little success. The code was instead used to determine the flow and temperature fields inside the already established surface profile.Various laminar and turbulent flow models (variations of the k - εmodel) were tried and compared. Experiments with water were carried out for studying the flow field. Tin was used for heat transfer studies. Finally, these simulations were compared with results from the practical experiments. Introductory experiments were carried out with ferrosilicon with the intent to perform complete experiments with this metal.Measured heat flow usually exceeded predicted values, particularly in the stagnation region. Good agreement is shown between the results from the FLUENT simulations and the new analytic model, which shows good promise of acting as a useful alternative to the much more demanding numerical simulations.

<p>A thorough investigation was performed on the behaviour of Ni, Fe and Cu oxides dissolved in cryolite melts, and the solubility of these species was measured as a function of alumina content, NaF/AlF₃ molar ratio (CR) and temperature. Predominance area diagrams showing the solid phases containing Ni, Fe and Cu, respectively, as a function of the partial oxygen pressure and the alumina activity at 1020 ^oC were constructed. These diagrams were based on present emf and solubility measurements.</p><p>The interpretations of the solubility measurements for the oxides of Ni and Fe gaveconclusive and consistent results. The oxides of Ni and Fe exhibit decreasing solubility with decreasing temperature and with increasing alumina concentration. The Ni(II) concentration decreased from 0.32 wt% in cryolite to 0.003 wt% in alumina-saturated melts, while that of Fe(II) decreased from 4.17 to 0.32 wt% in similar melts. FeO and NiO are stable solid phases at low alumina concentrations, while FeAl₂O₄ and NiAl₂O₄ are stable at high concentrations. The alumina concentrations corresponding to the points of coexistence between FeO and FeAl₂O₄ and between NiO and NiAl₂O₄ were determined to be 5.03 and 3.0 wt% Al₂O₃, respectively, corresponding to the following Gibbs energy of formation from the oxide compounds,∆G⁰_fNiAl2o4 = –28.6 ± 2 kJ/mol and ∆G⁰_{f FeAl2O4} = –17.6 ± 0.5 kJ/mol.</p><p>The solubilities of FeAl₂O₄ and NiAl₂O₄as a function of the CR were investigated in alumina-saturated melts at 1020 ^oC. For both compounds a maximum solubility was found at CR ~5, being 0.008 wt% Ni(II) and 0.62 wt% Fe(II). The results are discussed with respect to the species present in solution. Both Fe(II) and Ni(II) dissolve as fluorides with different numbers of associated “NaF’s”. Ni(II) seems to form Na₃NiF₅ in melts with molar ratios 2 to 12, while Fe(II) is present as NaFeF₃ in acidic (CR 3–10) melts and as Na₃FeF₅and probably some Na₄FeF₆ in basic melts (CR > 3).</p><p>The solubility of both Cu oxidation states Cu(I) and Cu(II) decreases with decreasing temperature. The solubilities of Cu(I) initially decreased with increasing alumina concentration, showing a minimum at a certain alumina concentration followed by an increase. The solubilities were 0.36 wt% Cu(I) and 0.92 wt% Cu(II) in cryolite, and 0.30wt% Cu(I) and 0.45 wt% Cu(II) in alumina-saturated cryolite at 1020 ^oC.</p><p>At 1020 ^oC the solubilities of Cu₂O and CuO were little influenced when changing the CR from 3 to 8 in alumina-saturated melts (~0.30 wt% Cu(I) and ~0.45 wt% Cu(II)), but there was an upward trend for CR < 3. Solubility measurements for CuO in alumina-saturated melts at CR 3.0 to 1.2 clearly showed that the saturation concentration is dependent on both temperature and melt composition.</p><p>Copper ions in solution show a complex behaviour, since they form fluorides and oxycomplexes simultaneously. The extent of co-existence of Cu(I) and Cu(II) in the same melt is also considerable, and it is depending on the alumina activity in the melt. According to thermodynamics the stable copper oxide phases at high alumina activities are the aluminates CuAlO₂ and CuAl₂O₄. However, no clear changes in the solubilities were found for the points of coexistence between Cu₂O and CuAlO₂ and CuO and CuAl₂O₄, respectively, as was the case for Ni(II) and Fe(II). Although there are uncertainties regarding the thermodynamic data available for the formation of copper aluminates, models for the dissolution mechanisms and for the species present in the melt are suggested. Cu(I) seems to form mainly CuF at low alumina contents, while Na₅CuO₃ dominates at higher alumina concentrations. Likewise, Cu(II) seems to form CuF₂, but the concentration of CuF₂ decreases with increasing alumina content. The species that gave the best fit for the cupric oxy-complexes was Na₁₆CuO₉, and the amount increased with increasing alumina content.</p><p>Cermet anodes were prepared with a NiFe₂O₄-based oxide phase mixed with a ~20 wt% copper-rich metal phase. The electrical conductivity for these materials was measured as a function of temperature, showing semiconductor behaviour in the temperature range from room temperature to 1050 ^oC. The highest electrical conductivity measured was ~30 S/cm at 1000 ^oC, which is on the low side for use as an anode material for aluminium production.</p><p>Three cermet anodes were tested by electrolysis for 48 hours. After the experiments the anodes were examined with SEM. There was no metal phase present in the outer 100 µm of the anode, not even pores were observed that could indicate where the metal grains had been. A copper-rich phase was found in one case ~2 mm from the outer surface, and it is believed that copper diffuses out of the anode.</p><p>The cermet anodes dissolved slowly in the electrolyte during electrolysis. The steady state concentrations of Fe and Cu in the electrolyte were below the saturation concentrations, while the concentration of Ni was 3 - 4 times above saturation. The dissolution of the anode does not fit a first order mass-transport model, but it can probably be explained by a controlled dissolution mechanism with some additional disintegration/spalling of the anode material. Further work is needed to draw a firm conclusion. In general, correct solubility data for the anode constituents are needed to make a proper evaluation of various anode materials. Perhaps the first order mass-transport model agrees for some materials, but based on the present results it seems untenable for cermet materials made of NiFe₂O₄ with a copper-rich metal phase.</p><p>The solubilities of the oxides of Ni(II) and Fe(III) are very low for the alumina-saturated melt used during electrolysis, which make them promising candidates for inert anodes. However, if nickel aluminate, which is an insulator, is formed and deposited on the anode surface, it is a cause of concern. Fe(II) aluminate is not expected to form on the anode surface, since Fe(III) is the stable oxidation state in the presence of oxygen gas. However, solid Fe(II) aluminate may be formed in the bulk of the electrolyte where the partial oxygen pressure is lower.</p>

A thorough investigation was performed on the behaviour of Ni, Fe and Cu oxides dissolved in cryolite melts, and the solubility of these species was measured as a function of alumina content, NaF/AlF3 molar ratio (CR) and temperature. Predominance area diagrams showing the solid phases containing Ni, Fe and Cu, respectively, as a function of the partial oxygen pressure and the alumina activity at 1020 oC were constructed. These diagrams were based on present emf and solubility measurements. The interpretations of the solubility measurements for the oxides of Ni and Fe gaveconclusive and consistent results. The oxides of Ni and Fe exhibit decreasing solubility with decreasing temperature and with increasing alumina concentration. The Ni(II) concentration decreased from 0.32 wt% in cryolite to 0.003 wt% in alumina-saturated melts, while that of Fe(II) decreased from 4.17 to 0.32 wt% in similar melts. FeO and NiO are stable solid phases at low alumina concentrations, while FeAl2O4 and NiAl2O4 are stable at high concentrations. The alumina concentrations corresponding to the points of coexistence between FeO and FeAl2O4 and between NiO and NiAl2O4 were determined to be 5.03 and 3.0 wt% Al2O3, respectively, corresponding to the following Gibbs energy of formation from the oxide compounds,∆G0fNiAl2o4 = –28.6 ± 2 kJ/mol and ∆G0f FeAl2O4 = –17.6 ± 0.5 kJ/mol. The solubilities of FeAl2O4 and NiAl2O4 as a function of the CR were investigated in alumina-saturated melts at 1020 oC. For both compounds a maximum solubility was found at CR ~5, being 0.008 wt% Ni(II) and 0.62 wt% Fe(II). The results are discussed with respect to the species present in solution. Both Fe(II) and Ni(II) dissolve as fluorides with different numbers of associated “NaF’s”. Ni(II) seems to form Na3NiF5 in melts with molar ratios 2 to 12, while Fe(II) is present as NaFeF3 in acidic (CR 3–10) melts and as Na3FeF5 and probably some Na4FeF6 in basic melts (CR &gt; 3). The solubility of both Cu oxidation states Cu(I) and Cu(II) decreases with decreasing temperature. The solubilities of Cu(I) initially decreased with increasing alumina concentration, showing a minimum at a certain alumina concentration followed by an increase. The solubilities were 0.36 wt% Cu(I) and 0.92 wt% Cu(II) in cryolite, and 0.30wt% Cu(I) and 0.45 wt% Cu(II) in alumina-saturated cryolite at 1020 oC. At 1020 oC the solubilities of Cu2O and CuO were little influenced when changing the CR from 3 to 8 in alumina-saturated melts (~0.30 wt% Cu(I) and ~0.45 wt% Cu(II)), but there was an upward trend for CR &lt; 3. Solubility measurements for CuO in alumina-saturated melts at CR 3.0 to 1.2 clearly showed that the saturation concentration is dependent on both temperature and melt composition. Copper ions in solution show a complex behaviour, since they form fluorides and oxycomplexes simultaneously. The extent of co-existence of Cu(I) and Cu(II) in the same melt is also considerable, and it is depending on the alumina activity in the melt. According to thermodynamics the stable copper oxide phases at high alumina activities are the aluminates CuAlO2 and CuAl2O4. However, no clear changes in the solubilities were found for the points of coexistence between Cu2O and CuAlO2 and CuO and CuAl2O4, respectively, as was the case for Ni(II) and Fe(II). Although there are uncertainties regarding the thermodynamic data available for the formation of copper aluminates, models for the dissolution mechanisms and for the species present in the melt are suggested. Cu(I) seems to form mainly CuF at low alumina contents, while Na5CuO3 dominates at higher alumina concentrations. Likewise, Cu(II) seems to form CuF2, but the concentration of CuF2 decreases with increasing alumina content. The species that gave the best fit for the cupric oxy-complexes was Na16CuO9, and the amount increased with increasing alumina content. Cermet anodes were prepared with a NiFe2O4-based oxide phase mixed with a ~20 wt% copper-rich metal phase. The electrical conductivity for these materials was measured as a function of temperature, showing semiconductor behaviour in the temperature range from room temperature to 1050 oC. The highest electrical conductivity measured was ~30 S/cm at 1000 oC, which is on the low side for use as an anode material for aluminium production. Three cermet anodes were tested by electrolysis for 48 hours. After the experiments the anodes were examined with SEM. There was no metal phase present in the outer 100 µm of the anode, not even pores were observed that could indicate where the metal grains had been. A copper-rich phase was found in one case ~2 mm from the outer surface, and it is believed that copper diffuses out of the anode. The cermet anodes dissolved slowly in the electrolyte during electrolysis. The steady state concentrations of Fe and Cu in the electrolyte were below the saturation concentrations, while the concentration of Ni was 3 - 4 times above saturation. The dissolution of the anode does not fit a first order mass-transport model, but it can probably be explained by a controlled dissolution mechanism with some additional disintegration/spalling of the anode material. Further work is needed to draw a firm conclusion. In general, correct solubility data for the anode constituents are needed to make a proper evaluation of various anode materials. Perhaps the first order mass-transport model agrees for some materials, but based on the present results it seems untenable for cermet materials made of NiFe2O4 with a copper-rich metal phase. The solubilities of the oxides of Ni(II) and Fe(III) are very low for the alumina-saturated melt used during electrolysis, which make them promising candidates for inert anodes. However, if nickel aluminate, which is an insulator, is formed and deposited on the anode surface, it is a cause of concern. Fe(II) aluminate is not expected to form on the anode surface, since Fe(III) is the stable oxidation state in the presence of oxygen gas. However, solid Fe(II) aluminate may be formed in the bulk of the electrolyte where the partial oxygen pressure is lower.