Consumable Carbon Anodes Research Articles

A Sustainable Process for Rare Earth Electrowinning in Chloride Based Molten Salts

Neodymium production has become increasingly important due to use of neodymium–iron–boron (NdFeB) magnets in green energy, consumer technology and defense applications. The state-of-the-art practice features a neodymium fluoride and lithium fluoride molten salt with a consumable carbon anode converting dissolved neodymium oxide and carbon to neodymium metal and CO2.1-2 The anode effect results in PFCs and CO emissions, which make the current neodymium electrowinning process unsustainable.3-4 Some attempts have been made to electrolyze the chloride salt of neodymium to produce chlorine at the anode. Chlorine is a value-added product which neither consumes the anode nor produces other deleterious GHGs. Additionally, the chloride of neodymium can be non-carbothermically produced through a spontaneous reaction with hydrochloric acid, producing only water as a side product. However, a common problem in chloride rare earth electrowinning is the low Coulombic efficiency (10-50%) obtained in lab-scale and pilot-scale efforts.5 The neodymium chloride electrolysis process notoriously encounters a two-step reduction producing an intermediate divalent (Nd2+) oxidation state which is stable in the chloride media.6-7 The intermediate neodymium species diffuses away from the cathode leading to Coulombic efficiency loss. Additionally, the kinetics of the chlorine evolution reaction (CER) in chloride molten salts are known to be relatively sluggish, causing significant anodic overpotentials which elevate the specific energy consumption during electrowinning. Furthermore, electrowinning from molten salts often produces spongy or dendritic deposits of metal requiring energy-intensive purification. In this talk, we will address all aforementioned technical hurdles and report on our efforts achieving high Coulombic efficiency (~85%), and compact deposits with superior as-electrowon neodymium metal purity (>99 wt.%). Furthermore, we will report the development of a new anode which catalyzes chlorine evolution, lowering the anodic overpotential considerably during electrolysis at high current densities of practical interest for electrowinning. Specific energy consumption is calculated for some example cases and found to be between 2.1 and 3.5 kWh/kg representing a significant improvement over the conventional oxyfluoride process. Figure 1

(Invited) Magnesium Production By MgO Reduction in Molten CaF2-MgF2 Salt with Reactive Liquid Metal Cathode and Consumable Carbon Anode or Nonconsumable Zirconia Solid Oxide Membrane

A process for magnesium production is proposed beginning with MgO electrolysis in molten MgF2-CaF2 salt with a reactive liquid metal cathode, followed by efficient multiple effect distillation to separate the magnesium from the cathode. Experiments with a liquid tin cathode and carbon anode show over 90% current efficiency; with a yttria-stabilized zirconia (YSZ) solid oxide membrane (SOM) between the bath and anode, current efficiency is above 80%. A detailed mass balance estimates the amount of MgF2 and tin required to maintain steady bath and cathode chemistry with CaO and other impurities in the MgO feedstock. A detailed energy balance describes the relationship between applied potential and cell thermal balance over a range of process temperatures, including minimum energy losses through cell leads. Emissions analysis shows a range of greenhouse emissions depending on the raw material type, MgO separation and calcination method, anode type (carbon or YSZ SOM), and electrical energy source.

Electrical Conductivity of Low-Temperature KF-Naf-AlF3 Melts with CaF2 and Al2O3 Addition

Replacing consumable carbon anodes with oxygen evolving inert anodes, in aluminium reduction cells, has long been a goal for the aluminium industry. Using inert anodes would have several advantages, one of them being the elimination of greenhouse gas emission during the electrolysis process. Metallic alloys have proven to be a promising category of material candidates for inert anodes. The industrial electrolyte is a cryolite based fluoride, and the process temperature is around 960°C. This is a very corrosive environment for the anodes, which have failed to be inert. Lowering the electrolysis temperature is key to improving anode performance. The liquidus can be lowered by reducing the NaF ratio, often termed Cryolite Ratio (CR), of the NaF-AlF3 melt, but that is not feasible as it drastically reduces the alumina solubility. If some, or all, of the NaF in the melt is replaced by KF, acceptable alumina solubility is maintained even at the low CR necessary for melting point down to 700°C. Replacing NaF with KF is known to reduce the electrical conductivity, and therefore it is of interest to determine the electrical conductivity of KF-NaF-AlF3-Al2O3 melts with different parameters. In the present study, we determine the electrical conductivity of KF-NaF-AlF3 melt at different parameters such as temperature, cryolite ratio (CR = xNaF+xKF/xAlF3), potassium ratio (xKF/xKF+xNaF) and composition of CaF2 and Al2O3 additives. Continuously varying cell constant (CVCC) technique is used to determine the electrical conductivity of the melt, which is governed by equation (1). (1)Here κ is the electrical conductivity of the melt in S cm−1, A is the cross-sectional area of the conductivity cell in cm2, l is the length of the cell in cm, and Rm is the melt resistance in Ω. The experiments are performed at various cell geometries, as shown in figure 1a.Electrochemical impedance spectroscopy (EIS) is used to determine the resistance (R) of the conductivity cell. AC-technique with a small amplitude of 10 mV and high frequencies ranging from 100 Hz to 100 kHz is used. R = Rm + Rp + Rc (2) Rm is the melt resistance, Rp is the polarisation resistance, and Rc is the resistance related to electrical connections. When EIS is performed at different l values, Rp and Rc remain constant while Rm changes. Thus, the Rp+Rc value can be estimated by extrapolating the line plotted between R and cell constant to 0 (cell constant value) (see figure 1b). Figure 1a. Conductivity cell: 1: Stainless steel rod, 2: Alumina tube, 3: BN tube, 4: Molten electrolyte, 5: Counter electrode electrical connector, 6: Graphite working electrodes, 7: Graphite crucible/counter electrode, 8: k-type thermocouple. 1b. An example graph plotted between the cell resistance and cell constant to estimate the melt resistance. Figure 1

Anodic Behaviour of Ni-Fe-Cu electrode In Molten Fluoride Salts

The industrial process for Aluminium production is the Hall-Héroult process, an electrolysis process where the alumina raw material is dissolved in a cryolite based electrolyte, the aluminium is deposited at the cathode while the oxygen from the alumina reacts anodically with the carbon anodes, which are consumed in the process. The chemical energy in the carbon anodes provides part of the process energy, resulting in CO2 emission of about 1.6 kg/kg aluminium produced. In industrial cells, the electrolyte is modified from the cryolite, Na3AlF6, composition by adding AlF3 to lower the process temperature and improve current efficiency. This modification can be characterized by the Cryolite Ratio, CR, which is the molar ratio of NaF/AlF3. In modern cells, this ratio is often as low as 2.2, as compared to 3 in pure cryolite, which enables an operating temperature of around 960°C, as compared to more than 1009°C for Cryolite.One way to eliminate CO2 process emissions from aluminium electrolysis is to switch from consumable carbon anodes to inert anodes, resulting in O2 emissions instead of CO2. Significant effort has been put into achieving this, but the challenge is to find anode materials that can withstand the harsh conditions in the high temperature cryolite based electrolyte. It would help anode life to reduce the electrolysis temperature from 960 °C to 800 °C or lower. Lowering the CR to 1.3 or lower would achieve this, but for traditional electrolyte components this would lower the alumina solubility to unacceptably low levels. By replacing some of the NaF in the electrolyte with KF, it is possible to operate at this very low CR with acceptable alumina solubility.In this paper, we present a study on anode performance in potassium-rich low CR electrolyte. Both electrodes are vertical, the anodes made of homogenised CuFeNi alloy and cathodes of hot-pressed TiB2. The paper describes the effect of varying the anode composition, anode surface treatment as well as current density on anode stability. Also, the effect of varying the ratio of KR = KF/(NaF+KF), process temperature as well as alumina saturation is addressed.In general, pre-oxidation of the metallic anodes as well as the Cu content greatly affects anode stability. It is possible to operate at low temperature at lower KR, improving the industrial viability of the process, as NaO is added to the cells with the alumina, this reduces the alumina solubility which puts constraints on the process control. The aluminium obtained by electrolysis in this system had less than 0.2% impurities from the electrodes.

Primary Production of Aluminium and Its Alloys in Molten Salts with Oxygen Evolving Anodes: Overview

Due to environmental and economic concerns, carbon-free aluminium production has been an ultimate target for aluminium industries. For the last few decades, a considerable amount of research has been conducted to find a suitable inert anode that could replace the consumable carbon anodes for aluminium electrolysis. Material types such as metals, ceramics and cermets have been studied extensively. All the anode materials have their advantages and disadvantages. However, metallic alloys have been considered attractive anode material candidates due to their high electrical conductivity, thermal stability and easy fabrication and machining properties. For a successful adaptation of metallic anodes into the aluminium electrolysis cell, an electrolyte with a low-operating temperature and high alumina solubility is required. Another significant component of the carbon-free aluminium electrolysis cell is a wettable cathode. This overview discusses the research progress on inert anodes, wettable cathodes and electrolytes.

Synthesis and Characterization of (Co,Ni)O Solid Solutions As Protective Coatings for Inert Anodes in Aluminum Electrolysis

The industrial production of primary aluminum from alumina ore (Al2O3) is still carried out by the Hall-Héroult process. The process relies on dissolving alumina in an electrolyte consisting mainly of liquid Na3AlF6 at 950-1000 °C. While the reduction of dissolved Al3+ ions in aluminum occurs at the cathode, the complementary reaction occurs at a carbon anode that is consumed to form carbon dioxide, which necessitates its regular replacement (every 25 days). The overall reaction is the following:Canadian aluminum smelters produce annually about 6 Mt of CO2 eq(reported in 2017), which is equivalent to the CO2 amount generated annually by about 2 million cars.The substitution of consumable carbon anodes with inert (O2-evolving) anodes in the electrochemical cells to produce aluminium would have significant environmental benefits because it would eliminate the emissions of carbon dioxide and perfluorocarbons associated with the consumption of the carbon anode. However, the design of inert anodes is a major challenge because of the severe Al electrolysis conditions (cryolithic medium at 960 °C), which requires materials with excellent resistance to corrosion and thermal shock as well as adequate electrochemical properties [1].Single phase Cu65Ni20Fe15 alloy is a promising inert anode for Al production due to its ability to form a protective NiFe2O4 layer upon Al electrolysis. However, its corrosion resistance is still insufficient and a protective layer is required to prevent the penetration of electrolyte into the nickel ferrite layer and the formation of fluorides during Al electrolysis [2-4].In this context, the use of (Co,Ni)O-based protective coatings for metallic anode appears promising [5]. However, it is challenging to prepare a single phase, coherent and crack-free oxide layer as required for industrial Al production. A potentially relevant method to produce (Co,Ni)O coated inert anodes is by direct deposition of (Co,Ni)O oxide compounds by spray deposition techniques such as suspension plasma spray (SPS) and high velocity oxygen fuel (HVOF). These additive manufacturing deposition are well-established technologies for producing protective oxide coatings for various industrial applications (e.g. gas turbines).As a first step toward this goal, pure CoxNi1−xO solid solutions have been prepared over the whole composition range by a two-step procedure that consisted of high-energy ball milling followed by a heat treatment of Co3O4 and NiO powders [6]. Then, the thermal stability and electrical conductivity of (Co,Ni)O solid solutions were determined at temperatures ranging between 700 and 1000 °C. Also, their dissolution rate was measured in K-based cryolite at 700 °C and in Na-based cryolite at 1000 °C.In a second step, (Co,Ni)O powders have been used as raw materials for the thermal spraying (HVOF and SPS) of protective coatings onto Cu-Ni-Fe inert anodes [7]. The morphological and microstructural characteristics of the coating depending on the thermal spray conditions will be presented. The oxidation behaviour of the coated inert anodes under air and argon is presented. Preliminary investigation of the electrochemical behaviour under Al electrolysis conditions of the (Co,Ni)O/Cu-Ni-Fe anodes will be also presented and discussed.

(Co,Ni)O Coated Anodes for CO2-Free Al Production

Canadian aluminum production is an important source of greenhouse gases (GHGs) with 6 Mt of CO2 eq emitted in 2017, which is equivalent to the amount of GHG generated annually by about 2 million cars. The current technology consumes carbon anodes during the electrolysis of aluminum to form CO2 according to the overall reaction: Al2O3 + 3/2 C = 2 Al + 3/2 CO2. The most effective solution would be to replace the consumable carbon anodes with so-called inert anodes that emit O2 rather than CO2 and that are based on the following overall reaction: Al2O3 = 2 Al + 3/2 O2. This would reduce GHGs by 75 to 100% depending on the type of emissions (CO2, CFx, NOx, SOx, etc.). However, the design of inert anodes is a major challenge because of the severe conditions during aluminum electrolysis that require materials with excellent corrosion and thermal shock resistance while having the same adequate electrochemical properties [1].Among inert anodes studied so far, Cu-Ni-Fe-based alloys appear to be the most promising owing to their ability to form a layer of nickel ferrite (NiFe2O4) on the surface of the anode upon Al electrolysis [2-4]. This nickel ferrite has low solubility in a cryolithic medium. However, the formation of the nickel ferrite protective layer is slow and, depending on the experimental conditions, might not be formed fast enough to provide effective protection of the underlying substrate Cu-Ni-Fe alloy. One strategy to help in the formation of this layer is to coat the Cu-Ni-Fe alloy with a sacrificial layer that would be stable for a period long enough to allow the formation of NiFe2O4 on the surface of the anode.In this context, the use of (Co,Ni)O-based protective coatings for metallic anode appears promising [5]. However, it is challenging to produce coherent and crack-free oxide layer as required for industrial Al production. A potentially relevant method to produce (Co,Ni)O coated inert anodes is by direct deposition of (Co,Ni)O oxide compounds by thermal spray techniques such as suspension plasma spray (SPS) and high velocity oxygen fuel (HVOF). They are well-established technologies for producing protective oxide coatings for various industrial applications (e.g. gas turbines). Additionally, thermal spray could be used on Al production site to restore protective (Co,Ni)O coating on end-of-life inert anodes.As a first step toward this goal, single phase (Co,Ni)O powders with various Co/Ni ratios that could be used as raw materials for the thermal spraying of protective coatings onto Cu-Ni-Fe inert anodes have been produced [6]. The crystalline structure, thermal stability, electrical conductivity and solubility in cryolite media of the produced (Co,Ni)O powders are characterized depending on their Co/Ni ratio. Finally, selected (Co,Ni)O powders have been used as raw materials for the thermal spraying (HVOF and SPS) of protective coatings onto Cu-Ni-Fe inert anodes and the electrochemical behaviour of the coated inert anodes under Al electrolysis conditions is presented.

Synthesis and Characterization of (Co,Ni)O Solid Solutions As Protective Coatings for Inert Anodes in Aluminum Electrolysis

The industrial production of primary aluminum from alumina ore (Al2O3) is still carried out by the Hall-Héroult process. The process relies on dissolving alumina in an electrolyte consisting mainly of liquid Na3AlF6 at 950-1000 °C. While the reduction of dissolved Al3+ ions in aluminum occurs at the cathode, the complementary reaction occurs at a carbon anode that is consumed to form carbon dioxide, which necessitates its regular replacement (every 25 days). The overall reaction is the following: Al2O3 + 3/2 C= 2 Al + 3/2 CO2 Canadian aluminum smelters produce annually about 6 Mt of CO2 eq(reported in 2017), which is equivalent to the CO2 amount generated annually by about 2 million cars.The substitution of consumable carbon anodes with inert (O2-evolving) anodes in the electrochemical cells to produce aluminium would have significant environmental benefits because it would eliminate the emissions of carbon dioxide and perfluorocarbons associated with the consumption of the carbon anode. However, the design of inert anodes is a major challenge because of the severe Al electrolysis conditions (cryolithic medium at 960 °C), which requires materials with excellent resistance to corrosion and thermal shock as well as adequate electrochemical properties [1].Single phase Cu65Ni20Fe15 alloy is a promising inert anode for Al production due to its ability to form a protective NiFe2O4 layer upon Al electrolysis. However, its corrosion resistance is still insufficient and a protective layer is required to prevent the penetration of electrolyte into the nickel ferrite layer and the formation of fluorides during Al electrolysis [2-4].In this context, the use of (Co,Ni)O-based protective coatings for metallic anode appears promising [5]. However, it is challenging to prepare a single phase, coherent and crack-free oxide layer as required for industrial Al production. A potentially relevant method to produce (Co,Ni)O coated inert anodes is by direct deposition of (Co,Ni)O oxide compounds by spray deposition techniques such as suspension plasma spray (SPS) and high velocity oxygen fuel (HVOF). These additive manufacturing deposition are well-established technologies for producing protective oxide coatings for various industrial applications (e.g. gas turbines).As a first step toward this goal, pure CoxNi1−xO solid solutions have been prepared over the whole composition range by a two-step procedure that consisted of high-energy ball milling followed by a heat treatment of Co3O4 and NiO powders [6]. Then, the thermal stability and electrical conductivity of (Co,Ni)O solid solutions were determined at temperatures ranging between 700 and 1000 °C. Also, their dissolution rate was measured in K-based cryolite at 700 °C and in Na-based cryolite at 1000 °C.In a second step, (Co,Ni)O powders have been used as raw materials for the thermal spraying (HVOF and SPS) of protective coatings onto Cu-Ni-Fe inert anodes. The morphological and microstructural characteristics of the coating depending on the thermal spray conditions will be presented. Preliminary investigation of the electrochemical behaviour under Al electrolysis conditions of the (Co,Ni)O/Cu-Ni-Fe anodes will be also presented and discussed.

TiB2-Coated Graphite As Wettable Cathode for Al Production

A significant reduction in greenhouse gas emissions from aluminum smelters will only be achieved with the emergence of new technologies. The most effective way to decrease significantly CO2 emissions from this industry is to replace the consumable carbon anodes that are used nowadays with so-called inert anodes that emit O2 instead of CO2 during Al electrolysis [1]. However, the implementation of inert anodes in Al smelters will induce an increase of the cell voltage, which could be compensated by reducing the cell ohmic drop by decreasing the anode-to-cathode distance. For that purpose, the thickness of the molten Al layer presents on the surface of the cathode must be minimized, which requires a high wettability of molten Al on the cathode. To date, the most promising cathode material is TiB2, which has an excellent Al wettability, good electrical conductivity and good chemical stability under Al electrolysis conditions. However, the manufacturing of bulk TiB2 electrodes at the industrial scale poses challenges. Sintering of TiB2 must be performed at high temperature (~2000°C), which induces significant crystal growth that has dramatic consequences on the mechanical properties of the electrode. Faced with these constraints, various consolidation methods operating at lower temperatures have been evaluated (sintering under pressure, spark plasma sintering...), but they are expensive and/or poorly adapted to the production of large electrodes of complex shape as required for Al electrolysis application.In the present study, new avenues based on the elaboration of TiB2 coated graphite cathodes by electrochemical and thermal spray methods are evaluated. The influence of the deposition conditions on the crystalline structure, morphology, mechanical resistance and Al wettability of TiB2 coatings will be presented in this communication.

Effect of metallic content on sintering behavior of NiFe2O4-based cermet inert anode for aluminum electrolysis

In order to overcome the high carbon consumption and serious environmental pollution as a result of adopting consumable carbon anode, NiFe2O4-based cermet inert anode for aluminum electrolysis was prepared by powder metallurgy method using 85Cu-15Ni binary alloy as metallic phase. The metallic phase content was varied from 0 to 20 (mass fraction, %) with 5% intervals. The effect of metallic phase content on sintering behavior and microstructure was investigated by a thermal dilatometer and field emission scanning electron microscopy. The results show that the maximum linear shrinkage and linear shrinkage rates enhance with the increase of the metallic phase content, while the temperature of maximum linear shrinkage rate shows the opposite changing law. The sintering activation energy of the initial stage lowers from 471.13 to 236.47 kJ·mol−1 with the metallic additive range from 0 to 20%. The introduction of metallic phase could effectively enhance the grain boundary bonding strength and increase the grain size step by step.

Gas Anodes for Electrowinning of Aluminium

Aluminum is today commercial produced by the Hall-Haroult process using consumable carbon anodes. Consumable anodes have several drawbacks such as CO2 emission, continuous anode-cathode distance adjustments and replacements of anodes. Replacing the consumable anodes with inert anodes has been a topic for many decades without commercial success. Using porous inert gas anodes where natural gas or hydrogen take place in the anode reaction has been shown in laboratory tests to reduce the anode potential and reduce the CO2emission. However, formation of water results in evolution hydrogen fluorides which must be solved. Different type of anodes have been prepared and evaluated as possible gas anodes for this concept. Three different graphite materials with different pore sizes and porosity, commercially available, have been used to fabricate gas anodes. Porous SnO2 based materials have been prepared using either foam preforms or coarse SnO2 based particles sintered together with fine SnO2 based powders. These fabrication routes results in significant different materials. The materials are evaluated as possible porous anodes for this concept. Laboratory experiments using porous depolarized SnO2-based with CH4 and H2-gases have been performed with off-gas analysis and with special attention to hydrogen fluoride evolution. An theoretical mass balance with focus on HF emission has been performed and two different scenarios are presented 1) gas anode process and 2) conventional Hall Heroult cell. In addition we present simulation of the gas transport of a possible industrial gas anode. The challenging side effects of HF emission might be utilized in the process and is discussed.

Development of a niobium-doped titania inert anode for titanium electrowinning in molten chloride salts.

The direct electrochemical reduction of solid titanium dioxide in a chloride melt is an attractive method for the production of titanium metal. It has been estimated that this type of electrolytic approach may reduce the costs of producing titanium sponge by more than half, with the additional benefit of a smaller environmental footprint. The process utilises a consumable carbon anode which releases a mixture of CO2 and CO gas during electrolysis, but suffers from low current efficiency due to the occurrence of parasitic side reactions involving carbon. The replacement of the carbon anode with a cheap, robust inert anode offers numerous benefits that include: elimination of carbon dioxide emissions, more efficient cell operation, opportunity for three-dimensional electrode configurations and reduced electrode costs. This paper reports a study of Nb-doped titania anode materials for inert anodes in a titanium electrolytic reduction cell. The study examines the effect of niobium content and sintering conditions on the performance of Nb-doped TiO2 anodes in laboratory-scale electrolysis tests. Experimental findings, including performance in a 100 h laboratory electrolysis test, are described.

Porous Carbon Anodes for the Supply of Methane during Electrowinning of Aluminium

Currently consumable carbon anodes are utilized in the aluminium production process. The major drawback of this type of anodes is the high amount of CO2 emissions. Furthermore, due to the consumption of the anodes during the electrolysis it is necessary to adjust the anode – cathode distance continuously and finally replace the anode. Introduction of hydrogen or natural gas through the anode can decrease the CO2 emissions significantly and reduce the consumption of the anode. The use of inert porous anodes is interesting since the energy demand will be reduced due to natural gas/hydrogen participating in the anode reaction. However, due to unavailability of a sufficiently inert anode for aluminium electrolysis until now, carbon anodes seem to be more attractive. When natural gas/hydrogen is purged through the porous carbon anode, there will be competing anodic reactions depending on whether carbon from the anode or the CH4/H2 reacts with the oxygen ions from the bath. Previous studies using porous carbon anodes showed 0.3 V depolarization as well as 20% decrease in carbon anode consumption using methane compared to passing argon through the anode1. On the other hand, using CH4/H2 can lead to the formation and evolution of additional hydrogen fluoride. This was studied before and some ideas on how to deal with this extra HF evolution were presented2. In this study, further laboratory experiments were performed and the effect of carbon anodes with different porosities and designs were studied. Besides, off-gas analysis was performed and HF evolution will be discussed.

Depolarised gas anodes for aluminium electrowinning

Consumable carbon anodes are used in the electrowinning of aluminium by the Hall-Heroult process. Emissions of CO 2 may be eliminated by introducing an inert oxygen evolving anode, which however will require a higher anode potential. An alternative approach is to use a natural gas or hydrogen gas anode to reduce the CO 2 emissions and lower the anode potential. Preliminary laboratory experiments were carried out in an alternative molten salt electrolyte consisting of CaCl 2-CaO-NaCl at 680 °C. Porous anodes of platinum and tin oxide were tested during electrolysis at constant current. The behaviour of inert anode candidate materials such as tin oxide and nickel ferrite were also studied.

Aluminium production options with a focus on the use of a hydrogen anode: a review

AbstractThe Hall–Héroult process for the production of aluminium uses a consumable carbon anode. Owing to the environmental effects of the consumable carbon anodes, the primary aluminium industry is under increasing pressure to improve on the Hall–Héroult process to make it more environmentally friendly. Inert anodes would provide a major technical revolution, as they may yield significant cost, energy and environmental benefits. However, no fully acceptable inert anode material has been developed to date. Other alternative technologies had been investigated and tested, but with little success so far. The use of a hydrogen anode for the production of aluminium is an interesting concept, as it will have the same reversible voltage as a carbon anode in a conventional Hall–Héroult cell, and it will considerably reduce greenhouse gas emissions. This paper reviews the various process options for aluminium electrowinning with a focus on the application of hydrogen anode in aluminium electrowinning. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.

Development of Inert Anode Materials for Electrowinning in Calcium Chloride Melts

The electroreduction of solid titanium dioxide in a calcium chloride melt is an attractive method for the production of titanium metal. Several groups have proposed versions of this route, but no commercial process yet exists. In laboratory studies, consumable carbon anodes are used, and these can give rise to a number of cell design and operational issues. The replacement of the carbon anode with an inert anode offers numerous benefits that include: elimination of carbon dioxide emissions; more efficient cell operation and reduced electrode costs. We have evaluated a wide range of candidate inert anode materials, including metals, alloys and conductive ceramics. The studies examined the chemical stability of these materials in a calcium chloride-based melt, and also performance in laboratory-scale electrolysis tests. All metals and alloys tested were either dissolved in the melt and/or formed a thick, highly resistive crust. A conductive ceramic, chromium titanate, was the best-performed candidate and gave an acceptable cell voltage during a 24 h electrolysis test. This work was undertaken as part of the CSIRO Light Metals Flagship.

Electrical and Physical Properties of Carbonized Charcoals

Because coal does not conduct electricity and graphite is costly and inert, little attention has been given to the development of a carbon fuel cell (i.e., a battery that utilizes a consumable carbon anode to generate electrical power). In this work we show that a packed bed of carbonized charcoal particles subject to a compressive pressure (ca. 8 MPa) can be a good electrical conductor (σ < 0.2 Ω·cm). Low electrical resistivities σ are manifest by many different charcoals after carbonization at a heat treatment temperature (HTT) of 950 °C. The 5 orders of magnitude decrease in the electrical resistivity of charcoal with increasing HTT from 650 to 1050 °C is not associated with any dramatic change in the carbons' X-ray diffraction spectrum, its Fourier transform infrared spectrum, or its elemental analysis. Our findings cause us to visualize carbonized charcoal to be a macromolecular, cross-linked, three-dimensional, aromatic structure replete with conjugation and π bonds that facilitate the movement of e...

Visualizing gas evolution on graphite and oxygen-evolving anodes

Recent progress in material science might soon allow the replacement of the consumable carbon anode by an inert material. This is likely to induce changes in the overall process, and particularly in the gas evolution. Video recordings of oxygen-evolving anodes (SnO2, Cu, Cu-Ni) and carbon anodes were performed in laboratory electrolysis cells, using direct observation from above, a see-through cell, and radiography techniques. The gas behavior was very different between the two kinds of anodes, and probably linked to the wettability of the material by the electrolyte.