Inert Anode For Aluminum Electrolysis Research Articles

Enhanced thermal insulation and corrosion resistance in Ni-Fe cermet through incorporation of high-entropy spinel oxides

Enhanced thermal insulation and corrosion resistance in Ni-Fe cermet through incorporation of high-entropy spinel oxides

Argon anodic plasma inert anode for Low-Temperature aluminium electrolysis

Argon anodic plasma inert anode for Low-Temperature aluminium electrolysis

Sintering kinetics and properties of NiFe2O4‐based ceramics inert anodes doped with TiN nanoparticles

AbstractSintering kinetics of NiFe2O4‐based ceramics inert anodes for aluminum electrolysis doped 7 wt% TiN nanoparticles were conducted to investigate densification and grain growth behaviors. The linear shrinkage increased gradually with the increasing sintering temperature between 1000 and 1450°C, whereas the linear shrinkage rate exhibited a broad peak. The maximum linear shrinkage rate was obtained at 1189.4°C, and the highest densification rate was achieved at the relative density of 75.20%. Based on the pressureless sintering kinetics window, the sintering process was divided into the initial stage, the intermediate stage, and the final stage. The grain growth exponent reduced with increased sintering temperature, whereas the grain growth activation energy decreased by increasing sintering temperature and shortening dwelling time. The grain growth was mainly controlled by atomic diffusion. NiFe2O4‐based ceramics possessed high‐temperature semiconductor essential characteristics. The electrical conductivity of NiFe2O4‐based ceramics first increased and then decreased with increasing sintering temperature, reached their maximum value (960°C) of 33.45 S/cm under 1300°C, mainly attributed to the relatively dense and uniform microstructure. The thermal shock resistance of NiFe2O4‐based ceramic was improved by a stronger grain boundary bonding strength and lower coefficient of linear thermal expansion.

Effect of TiN content on properties of NiFe2O4‐based ceramic inert anode for aluminum electrolysis

AbstractNiFe2O4‐based ceramic inert anodes for aluminum electrolysis doped with various TiN nanoparticles were prepared by a two‐step cold‐pressing sintering process to investigate how TiN affected the sintering behavior and properties of the composites. The differential scanning calorimetry‐thermogravimetry (DSC‐TG), X‐ray diffraction (XRD), and microstructure analysis results indicated that the Ti and N were evenly distributed in the NiFe2O4 matrix to form a solid solution. The maximum linear shrinkage and linear shrinkage rate were enhanced with the increase of TiN nanoparticles contents, and the sintering activation energy of initial stage was lowered from 382.63 to 279.58 kJ mol−1 with the TiN nanoparticles additive range from 0 to 9 wt%. When the content of TiN nanoparticles was 7 wt%, the relative density, bending strength, and elastic modulus reached their maximum values of 97.24%, 73.88 MPa, and 3.77 GPa, respectively, whereas the minimum static corrosion rate of NiFe2O4‐based ceramic of 0.00114 g cm−2 h−1 was obtained, mainly attributed to the relatively dense and stable microstructure. The electrical conductivity of NiFe2O4‐based ceramics presented a clear ascending trend with increasing TiN nanoparticles content and elevated temperature, attributed to the increased concentration and migration rate of carrier.

Recent progress of inert anodes for carbon-free aluminium electrolysis: a review and outlook

This review introduces the latest research progress of inert anodes for aluminium electrolysis and compares the comprehensive performances of different kinds of materials, including metals, ceramics and cermets.

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.

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.

Influences of heat treatment on the oxidation and corrosion behavior of Cu–Ni–Fe inert anodes for aluminium electrolysis

Abstract The homogenization heat-treatments were implemented to the experimental as-cast Cux(Ni1.67Fe)100-x alloys with x = 60,52,44 wt%, which have a two-phase structure to obtain a uniform microstructure. Moreover, the oxidation kinetics of the as-cast and as-homogenized Cux(Ni1.67Fe)100-x alloys were investigated by the thermogravimetry at 850 °C under 1atm O2 atmosphere. The thermogravimetry results showed that all the samples approximately follow the parabolic law during oxidation under this condition. The homogeneous alloys have the lower oxidation rate than the as-cast alloys due to the inhibition of outward diffusion of Cu. Furthermore, the as-cast and as-homogenized 52Cu–30Ni–18Fe alloys were tested as inert anode materials in laboratory aluminium electrolysis tests. The as-homogenized 52Cu–30Ni–18Fe alloy anode performs significantly better corrosion resistance due to the formation of NiFe2O4. In the end, there is less impurity contents of Cu in the produced aluminium after electrolysis using the as-homogenized 52Cu–30Ni–18Fe alloy anode than using the as-cast 52Cu–30Ni–18Fe anode.

Electrochemical Behavior of Fe-Ni Alloys as an Inert Anode for Aluminum Electrolysis

Electrochemical Behavior of Fe-Ni Alloys as an Inert Anode for Aluminum Electrolysis

High temperature oxidation and corrosion behaviours of Ni–Fe–Cr alloys as inert anode for aluminum electrolysis

• Oxidation behaviours of Ni–Fe–Cr alloys with different Cr content were investigated at 960 °C in air. • A tri-layer oxide structure of (Ni,Fe) 3 O 4 /(Ni,Fe,Cr) 3 O 4 /Cr 2 O 3 was developed on Ni–Fe–15Cr alloy. • The continuous Cr 2 O 3 inner layer significantly improved the oxidation resistance of Ni–Fe–Cr alloys. • The layered oxide scale exhibited a good electrical property. • (Ni,Fe) 3 O 4 /(Ni,Fe,Cr) 3 O 4 /Cr 2 O 3 was highly corrosion resistant in molten alumina-cryolite at 960 °C. High temperature oxidation and corrosion behaviours of Ni–Fe–Cr alloys with different Cr contents (Cr = 0, 10, 15 wt.%) were investigated as inert anodes for aluminum electrolysis. Oxidation results at 960 °C in air showed that Ni–Fe–15Cr alloy formed a tri-layer oxide structure with an outer layer of (Ni,Fe) 3 O 4 spinel, a middle layer of (Ni,Fe,Cr) 3 O 4 spinel and an oxidation resistant inner layer of Cr 2 O 3 . Simultaneously, the tri-layer oxide scale exhibited good electrical property and corrosion resistance in aluminum electrolysis condition.

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.

The Effect of La on the Oxidation and Corrosion Resistance of $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18}$$ Cu 52 Ni 30 Fe 18 Alloy Inert Anode for Aluminum Electrolysis

The effect of La on the high-temperature oxidation and corrosion performance of ternary $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18}$$ alloy inert anode for aluminum electrolysis in low-temperature KF–NaF– $$\hbox {AlF}_{3}$$ electrolyte was studied. The results indicate that the oxidation kinetics of ( $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18})_{1{-}x}\hbox {La}_{x}$$ (x = 0, 0.5, 1, 2 wt%) alloys at $$850\,^{\circ }\hbox {C}$$ under 1 atm oxygen atmosphere follow the parabolic law. The oxidation scales are stratified state, which contains the Cu oxide as the external layer. While the internal layer of the oxidation products are the mixture of Fe oxide, Ni oxide and nickel ferrite. The Cu outward diffusion oxidation is the dominant factor for the $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18}$$ alloy during high temperature oxidation. After adding La to the $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18}$$ alloy, the oxidation mechanism has become the combined action with outward diffusion of Cu and internal diffusion of O to form the Ni/Fe oxide underneath the outmost CuO layer. The corrosion resistance of $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18}$$ alloy anode during the aluminum electrolysis is further improved by adding 0.5 wt% La. Compared with $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18 }$$ alloy, the corrosion rate of the $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18 }$$ alloy with 0.5 wt% addition of La has reduced from 1.9 to 1.8 cm/a.

Effect of sintering atmosphere on corrosion resistance of NiFe2O4 ceramic in Na3AlF6–Al2O3 melt

A comparative study on the corrosion resistance of NiFe2O4 ceramic inert anode for aluminum electrolysis prepared in the different sintering atmosphere was carried out in Na3AlF6–Al2O3 melt. The results show that the corrosion rates of NiFe2O4 ceramic inert anodes prepared in the vacuum and the atmosphere with oxygen content of 1×10–2 are 6.08 cm/a and 2.59 cm/a, respectively. A densification layer is formed at the surface of anode due to some reactions which produce aluminates. For the anode prepared in the atmosphere with oxygen content of 1×10–2, the thickness of the densification layer (about 50 μm) is thicker than that (about 20 μm) formed at the surface of anode prepared in the vacuum. The content of NiO and Fe(II) in Ni(II) x Fe(II)1–x Fe(III)2O4 increases with the decrease of the oxygen content of sintering atmosphere, which reduces the corrosion resistance of the material.

Preparation and Properties of a New Ceramet Inert Anode for Aluminum Electrolysis

Preparation and Properties of a New Ceramet Inert Anode for Aluminum Electrolysis

Anodic Corrosion Behavior of NiFe2O4-Based Cermet in Na3AlF6-K3AlF6-AlF3 for Aluminum Electrolysis

A (Cu,Ni)/(10NiO-NiFe2O4) cermet was tested as an inert anode for aluminum electrolysis in Na3AlF6-K3AlF6-AlF3 melt at 1173 K (900 °C), and its corrosion behavior was studied. The results show that the low-temperature Na3AlF6-K3AlF6-AlF3 bath is beneficial, improving the service conditions. With the combined effects of the electrolyte composition and the nascent oxygen during electrolysis, the metal phase (Cu,Ni) at the surface of anode will not be leached preferentially, but be transferred into the aluminates including FeAl2O4, NiAl2O4 and CuAl2O4. This is helpful for the anode to improve its corrosion resistance.

Fabrication and Characterization of Nickel Ferrite Based Inert Anodes for Aluminum Electrolysis

Nickel ferrite-based cermets and their relevant composites have been widely used as inert anodes for aluminum electrolysis due to the good combination of chemical resistance, thermal, and mechanical stability. In this study, various NiO/NiFe2O4 composites consisting 5, 10, and 15% NiO in conjunction with Cu/NiFe2O4 cermets containing 5, 10, and 15% Cu have been prepared by powder metallurgy method. The degradation resistance of developed inert composites has been evaluated under hot corrosion conditions by plunging the samples in the molten electrolyte at 1,000 °C for various holding times. The strength, toughness, hardness, relative density, microstructural observation, phase analysis, and electrical resistivity have been investigated in details by the 3-points bending test, Vickers hardness test, Archimedes method, scanning electron microscope, x-ray diffraction, and conventional direct current four-probe technique, respectively. The experimental results for NiO/NiFe2O4 composites show that a significant improvement of toughness and degradation resistance occurred in conjunction with a moderate decrease in strength by adding NiO content from 5 to 15%, while the relative density has been increased only up to 5%NiO content and then decreased. Moreover, increasing of Cu content from 5 to 15% in the cermet samples, all of the mentioned engineering properties such as strength, toughness and electrical conductivity have been improved considerably, but the degradation resistance has been decreased.

Corrosion of NiFe2O4–10NiO-based cermet inert anodes for aluminium electrolysis

Abstract NiFe2O4–10NiO-based cermet inert anodes for aluminium electrolysis were prepared and their properties were investigated in a lab-scale electrolysis cell. The results show that the inert anodes exhibit good performance during electrolysis in molten salt cryolite at 960 °C, but according to the analyses of phase compositions and microstructures through XRD, SEM/EDX and metallographic analysis, the metal in the anodes is preferentially corroded and many pores are produced on the anode surface after electrolysis. The preferential dissolution of Fe in the NiFe2O4 phase may lead to the non-uniform corrosion of NiFe2O4 grains. Moreover, a dense protective layer of NiFe2O4–NiAl2O4–FeAl2O4 is formed on the anode surface, which originates from the reaction of Al2O3 dissolved in the electrolyte with NiO or FeO, the annexation of NiFe2O4–NiAl2O4–FeAl2O4 to NiO and volume expansion. Thus, the dense NiFe2O4–NiAl2O4–FeAl2O4 layer inhibits the metal loss and ceramic-phase corrosion on the surface of the cermet inert anodes.

Consolidation of mechanically alloyed Cu–Ni–Fe material by spark plasma sintering and evaluation as inert anode for aluminum electrolysis

A Cu65Ni20Fe15 powder was prepared by mechanical alloying at semi-pilot scale to form a face-centered-cubic (fcc) phase (γ-phase). This powder was then consolidated by spark plasma sintering (SPS) to form a pellet. The crystallographic structure of the material is not affected by the consolidation process, although there is a slight increase of the crystallite size from 20 to 32 nm, and a slight decrease of the lattice strain from 0.5% to 0.2%. The relative density of the SPS consolidated sample is 95%. Thermogravimetric analysis confirms the good oxidation resistance of the SPS sample with a mass gain of only 0.4% after 20 h of oxidation at 700 °C, which is attributed to the rapid formation of a protective NiFe2O4 layer. The SPS sample was then evaluated as inert anode for Al electrolysis in low-temperature (700 °C) KF–AlF3 electrolyte. After 20 h of electrolysis at an anode current density of 0.5 A cm−2, the cell voltage reaches an unstable value of 5.0 V. The purity of the produced aluminum is 99.4% and the wear rate of the electrode is estimated at 1.8 cm year−1.

Ball-Milled (Cu-Ni-Fe + Fe2O3) Composite as Inert Anode for Aluminum Electrolysis

A (Cu-Ni-Fe + Fe2O3) composite was prepared by ball milling and evaluated as an oxygen-evolving anode for aluminum electrolysis. The material was prepared by first milling elemental Cu, Ni and Fe powders to form a Cu(Ni,Fe) solid solution. Then, the milling operation was resumed for different periods of time (from 30 min to 4 h) in presence of a fixed amount of nanosized Fe2O3 particles to achieve the desired stoichiometry (Cu65Ni20Fe15)98.6O1.4. After 4 h of milling, Fe2O3 precipitates are found to be homogeneously dispersed in the Cu-Ni-Fe matrix. The powder was then heated at 1000°C and pressed to form an electrode for evaluation in low-temperature (700°C) KF-AlF3 electrolyte at an anode current density of 0.5 A cm−2 for 20 h. The cell voltage was stable at ca. 4.5 V and the Cu, Fe and Ni contamination of the produced Al and electrolyte were quite low, resulting in an estimated anode erosion rate of 1.2 cm year−1. This good corrosion resistance is attributed to the formation of a protective NiFe2O4-rich layer on the electrode during Al electrolysis, which is likely to be favored by the presence of the finely dispersed Fe2O3 precipitates acting as nucleation sites for the formation of NiFe2O4.

Preparation by Spark Plasma Sintering and Investigation of Inert Anode for Aluminum Electrolysis

Spark plasma sintering was used to prepare Fe-Ni (Cu-41.32wt.% Cr) alloy in order to enhance the properties as inert anode for aluminum electrolysis. The effect of different sintering processes on the microstructures and properties of Fe-Ni alloy were studied by OM, SEM and EDS. The results show that the sintering process of milled powders can be divided into three stages: particles bonding, sintering neck growing, closed pole gap becoming spherical and narrowing. The SPS process of milled powders has some advantages of reacting at lower temperatures, completing faster and being densified easier than ordinary sintering. The density of Fe-Ni sample is improved and porosity is decreased as the sintering temperature is increased and holding time is extended. 1273K and 5min are the best parameters.