Cermet Inert Anodes Research Articles

Ultrafast pressureless sintering of high conductivity Ni-based cermets inert anode for Carbon‐free electrolytic aluminum

Cermet is a promising inert anode material to solve the problem of high carbon emission and bearing C-F harmful gas generation of primary aluminum produced from bauxite using molten electrolysis with carbon anode. However, a major challenge for traditional methods is to ensure that the cermet inert anode simultaneously has good conductivity, corrosion resistance, and strength. Herein, a new ultrafast pressureless sintering (UPS) method is developed to greatly improve the conductivity of metal ceramic inert anode materials while ensuring good corrosion resistance and strength. In a departure from conventional practice that metal phase undergoes local enrichment accompanied by irregular formation of ceramic particles at low heating rates, here UPS achieves rapid diffusion and uniform distribution of metal phase at regular ceramic particle interfaces during liquid phase sintering. The new method is applicable to Ni-based cermet and the production of Ni-based cermet inert anode with excellent conductivity in a few tens of seconds has been demonstrated, which is able to improve the quality of cermet inert anode requiring a low energy input. The conductivity of cermet produced by the applied UPS process is 15-fold higher than that of other pressureless sintering processes with low heating rates. These Ni-cermet inert anodes exhibit high corrosion resistance and density with excellent mechanical properties. This rapid sintering technique facilitates the structural design of inert anodes and develops a pathway of new systems with scalable, efficient, and high-quality sintering of composite materials.

Influence of operating conditions on cermet inert anode corrosion in cryolite for aluminium production

A (NixFeyO4–Ni1−x′Fex’O)/(Cux′’Niy’’) cermet candidate as inert anode was investigated in cryolite at 960 °C. Immersion tests were performed and different conditions tested: cryolite ratio, alumina content, electrolyte quality, metallic aluminium content and atmosphere. Then, electrolysis was realised at 0.8 A cm-2 for 4 h in industrial conditions: industrial grade electrolyte, CR 2.2, saturated Al2O3, 960 °C and presence of metallic Al. The initial material composition was still present in the bulk and the phases compositions in the oxidised zone are Ni0.90Fe2.10O4 and Ni0.90Fe0.10O, as observed during immersion tests. The metallic phase is partly oxidised, leading to porosity in the cermet.

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 Ion Structure of NiFe<sub>2</sub>O<sub>4</sub>-10NiO-Based Cermet Anode in the Electrolyte

The problem in the large-scale industrialization for aluminum electrolysis is the corrosion resistance of cermet inert anodes in high temperature molten salts. In this paper, the ion structure of NiFe2O4-10NiO-based cermet anode composition in the electrolyte was investigated. The experiment results show that Al-F and Al-O-F complex ion structures are produced at room temperature and perhaps Me-F (Me=Ni, Cu, Fe), Me-O-F or Me-Al-O-F are also formed. Moreover, Al-F and Al-O-F ion structures exist from room temperature to 700 °C and from 1050 °C to 1200 °C; In the rearrangement and melting process of molten salt at 800 °C -1000 °C, the ionic structures are mainly Al-O-F ions; Me-F and Me-O-F ion structures were not found at high temperature, this indicates that Al in Al-O-F complex ion structures is partially replaced by Fe, Ni or Cu to form MexAlaOyFz(z+2y-2x-3a)-.

Effect of sintering atmosphere on corrosion resistance of Ni/(NiFe2O4–10NiO) cermet inert anode for aluminum electrolysis

A comparative study on the corrosion resistance of 17Ni/(NiFe2O4–10NiO) cermet inert anode prepared in different sintering atmospheres was conducted in Na3AlF6–Al2O3 melt. The results indicate that the corrosion rates of NiFe2O4-based cermet anodes prepared in the vacuum and the atmosphere with oxygen content of 2×10−3 (volume fraction) are 6.46 and 2.71 cm/a, respectively. Though there is a transition layer with lots of holes or pores, a densified layer is formed on the surface of anode due to some reactions producing aluminates. For the anode prepared in the atmosphere with oxygen content of 2×10−3, the thickness of the densification layer (about 50 μm) is thicker than that (about 30 μm) formed on the surface of anode prepared in the vacuum. The contents of NiO and Fe(II) in NiFe2xO4–y–z increase with the decrease of oxygen content in sintering atmosphere, which reduces the corrosion resistance of the material.

Further Development on NiFe2O4-Based Cermet Inert Anodes for Aluminum Electrolysis

The new aluminum electrolysis technology based on inert electrodes has received much interest for several decades because of the environment and energy advantages. The key to realize this technique is the inert anode. This article presents China’s recent developments of NiFe2O4-based cermet inert anodes, which include the optimization of material performance, the joint between the cermet inert anode and metallic bar, as well as the results of 20 kA pilot testing for a large-size inert anode group. The problems NiFe2O4-based cermet inert anodes face are also discussed.

Formation law of dense NiFe2O4–NiAl2O4–FeAl2O4 ceramic layer on surface of NiFe2O4 based ceramic inert anode for aluminium electrolysis

To form a dense ceramic spinel layer on the surface of NiFe2O4 based cermet inert anode is one of the important ways to improve the corrosion resistance to high temperature molten salt electrolyte at present. The author examined the formation law of dense NiFe2O4–NiAl2O4–FeAl2O4 ceramic layer on the surface of NiFe2O4 based ceramic inert anode, and the experimental results show that when the alumina concentration is >3% the dense NiFe2O4–NiAl2O4–FeAl2O4 ceramic layer can always form on the surface of NiFe2O4 ceramic, NiFe2O4–10NiO composite ceramic and NiFe2O4–10NiO–15(Cu–20Ni) cermet inert anode after electrolysis for 10 h. Moreover, the atomic ratio of Fe/Ni from the anode surface to the interior is increasing, which indicates that during the dense ceraminc layer formation prior corrosion of the Fe element exists.

Behaviour and Material Properties of the Grain Boundary on the Surface of an Anode during the Formation of a Dense Layer

EPMA was used to investigate the corrosion-evolution mechanism of a dense ceramic spinel layer on the surface of NiFe2O4-10NiO-based cermet inert anodes during electrolysis in a high-temperature molten salt electrolyte. The evolution behaviour of the grain boundary on the surface of a 22(20Ni-Cu)(NiFe2O4-10NiO) anode during the formation of a dense layer was also examined. The experimental results showed that the content of the Al element on the grain boundary at the bottom of both the NiFe2O4,NiAl2O4 and FeAl2O4 was higher than that in the core of the 22(20Ni-Cu)(NiFe2O4-10NiO) anode, and the content of the Al element between the Ni-Cu and the dense layer of NiFe2O4-NiAl2O4-FeAl2O4 was higher than that in both the NiFe2O4,NiAl2O4 and FeAl2O4. It was concluded that the production of the NiAl2O4 was due not only to the reaction of the NiO with the Al2O2F42-, but also to the oxidisation of the Ni, which then reacted with the Al2O2F42-.

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

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.

Corrosion of nickel ferrite refractory by Na3AlF6–AlF3–CaF2–Al2O3 bath

Abstract Based on research on cermet inert anodes for aluminium production, it has been suggested that nickel ferrite spinel might be suitable for use as a sidewall refractory in Hall-Heroult cells. A corrosion resistant sidewall would allow elimination of the frozen bath ledge, and has potentially huge benefits in terms of energy savings and increased productivity. However, little work has been done to assess nickel ferrite's suitability as a refractory. Dense nickel ferrite samples were prepared and characterized, and corrosion tests in cryolite based baths were conducted. Results confirm that the spinel does have good corrosion resistance. The corrosion mechanism is complex, involving grain boundary attack and formation of a Ni–Fe alloy. This alloy could pose a risk in terms of contamination of the aluminium. The use of additives to restrict penetration of grain boundaries may be the key to development of a successful spinel based refractory.

Effects of Compound Additives on Properties of Cermet Inert Anodes

In this paper, a combination of a two-step sintering and a particle size gradation technique was used to prepare cermet samples. The effects of compound additives with various combination of SnO2 and BaO on the performance of cermet were investigated. The results showed that, the performance of cermet could be improved by doping compound additives. The density of cermet sample reached to 5.61g•cm-3, and the porosity fell to 2.01%. Furthermore, we did observe a significant enhancement in its conductivity. The sample with 1% BaO additive has the highest conductivity at 8.02S/cm at 960°C, is increased by about 36% compared with the BaO undoped cermets.

On the development of metallic inert anode for molten CaCl 2–CaO System

Some fundamental aspects related to inert anode development in molten CaCl 2–CaO were investigated based on thermodynamic analysis, electrochemistry of metals and solubility of oxide measurements. The Gibbs free energy change of several key anodic reactions including electro-stripping of metals, electro-formation of metallic oxides, electro-dissolution of metallic oxides as well as oxygen and chlorine evolution was calculated and documented, for the first time, as a reference to develop metallic inert anode in chloride based melts. The anodic behaviors of typical metals (Ni, Fe, Co, Mo, Cu, Ag, and Pt) in the melt were investigated. The results confirmed the thermodynamic stability order of metals in the melts and revealed that surface oxide formation can increase the stability of the electrodes in CaO containing melt. Furthermore, solubility of several oxides (NiO, Fe 2O 3, Cr 2O 3, Co 3O 4, NiFe 2O 4) in pure CaCl 2 or CaCl 2–CaO melts was measured to evaluate the stability of oxide coating or a cermet inert anode in the melt. It was found that the solubility of NiO decreased with increasing CaO concentration, while that of Fe 2O 3 increased. Ni coated with NiO film had much higher stability during anodic polarization.

Effect of additive BaO on corrosion resistance of xCu/(10NiO-NiFe 2O 4) cermet inert anodes for aluminum electrolysis

xCu/(10NiO-NiFe 2O 4) cermet and 1BaO- xCu/(10NiO-NiFe 2O 4) cermet ( x=5, 10, 17) inert anodes were prepared as potential inert anodes for aluminum electrolysis and their corrosion resistance to traditional electrolyte was studied with anodic current density of 1.0 A/cm 2 in laboratory electrolysis. The substantial corrosion of metal Cu was observed, many pores appeared on the surface of anode and electrolytes infiltrated inside anodes during the electrolysis. The wear rates of 5Cu/(10NiO-NiFe 2O 4), 10Cu/(10NiO-NiFe 2O 4), 17Cu/(10NiO-NiFe 2O 4), 1BaO-5Cu/(10NiO-NiFe 2O 4), 1BaO-10Cu/(10NiO-NiFe 2O 4) and 1BaO-17Cu/(10NiO-NiFe 2O 4) are 2.15, 6.50, 8.30, 4.88, 4.70 and 4.48 cm/a, respectively. The addition of BaO to 10Cu/(10NiO-NiFe 2O 4) cermet and 17Cu/(10NiO-NiFe 2O 4) cermet is advantageous because BaO can effectively promote densification and thus improve corrosion resistance. But the addition of BaO to 5Cu/(10NiO-NiFe 2O 4) cermet is unfavorable to corrosion resistance because additive BaO at the grain boundary of anode accelerates possibly the corrosion of cermet.

NiFe2O4-based cermet inert anodes for aluminum electrolysis

Recent developments in the preparation, sintering process, mechanical properties, and thermal shock resistance of cermet inert anodes for aluminum electrolysis are reviewed in this paper. To obtain the desired technologies of low-temperature activated sintering of cermet inert anodes, the effects of material composition, sintering atmosphere, sintering temperature, and sintering aids on the densifi cation and microstructure of NiFe2O4-10NiO- based ceramics and cermets were studied. To obtain the toughening and strengthening technology of the cermet, the effects of material composition including ceramic and metallic phases are discussed. The cermet inert anodes with high density and mechanical properties were prepared through adjustment of material composition and sintering technology and selection of feasible sintering aids.

Cup-shaped functionally gradient NiFe2O4-based cermet inert anodes for aluminum reduction

Application of inert anode and wet-table cathode technology for aluminum reduction will result in significant energy and environmental benefits, so it has been a research focus for several decades. The candidate as inert anode concentrates on oxide ceramic, cermet, and alloy. This paper reviews briefly the research progress and presents the achievements of Central South University, Changsha, China, in developing an NiFe2O4-based cermet inert anode, which includes the preparation and optimization of material performance, the joint between the cermet anode and metallic bar, as well as the results of electrolysis testing for a large inert anode group. At the same time, the problems for NiFe2O4-based cermet inert anode faced are discussed.

Effect of CaO doping on corrosion resistance of Cu/(NiFe2O4-10NiO) cermet inert anode for aluminum electrolysis

The CaO-doped Cu/(NiFe2O4-10NiO) cermet inert anodes were prepared by the cold isostatic pressing-sintering process, and their corrosion resistance to Na3AlF6-K3AlF6-Al2O3 melt was studied. The results show that the relative density of 5Cu/(NiFe2O4-10NiO) cermet sintered at 1 200 °C increases from 82.83% to 97.63% when 2% CaO (mass fraction) is added. During the electrolysis, the relative density of cermet inert anode descends owing to the chemical dissolution of additive CaO at ceramic grain boundary, which accelerates the penetration of electrolyte. Thus, the corrosion resistance to melts of Cu/(NiFe2O4-10NiO) cermet inert anode is reduced. To improve the corrosion resistance of the cermet inert anode, the content of CaO doped should be decreased and the technology of cleaning the ceramic grain boundary should be applied.

Effect of Ni content on corrosion behavior of Ni/(10NiO-90NiFe 2O 4) cermet inert anode

Ni/(10NiO-90NiFe 2O 4) cermet inert anodes with metal Ni content of 0, 5, 10, 15 and 20 (mass fraction, %) were prepared and their corrosion behavior in Na 3AlF 6-Al 2O 3 melts was investigated in laboratory electrolysis tests. The results indicate that the content of metal Ni in anodes has little effect on the steady-state concentration of impurities Ni and Fe in electrolyte and the values range in (114–173)×10 −6 and (287–385)×10 −6, but the content of impurities in the metal aluminum manifolds. There is preferential corrosion for metal Ni in NiO-NiFe 2O 4 based cermet anodes. Considering the corrosion resistance and electrical conductivity, the cermet containing 5%Ni (mass fraction) behaves best among NiO-NiFe 2O 4 based cermet anodes studied, and should be further studied.

Effect of Cu-Ni content on the corrosion resistance of (Cu-Ni)/(10NiO-90NiFe 2O 4) cermet inert anode for aluminum electrolysis

(Cu-Ni)/(10NiO-90NiFe2O4) cermet inert anodes containing metal Cu-Ni 0, 5, 10, 15 and 20 wt pct were prepared and their corrosion resistance to Na3AlF6-Al2O3 melts was investigated. The results indicate that the content of metal Cu-Ni has little effect on the steady-state concentration of Ni in the electrolyte and the values could not be used to effectively differentiate their corrosion resistance. The steady-state concentration of Fe decreases from 304×10−6 to 168×10−6 and that of Cu increases from 21×10−6 to 71×10−6 with the content of metal Cu-Ni increasing from 0 to 20 wt pct. Post-examination shows that metallic phase Cu-Ni is corroded preferentially during electrolysis and many pores are left at the anode surface. Considering the corrosion resistance and electrical conductivity, the cermet containing metal Cu-Ni 5 wt pct should be selected and studied further.

Effect of electrolysis superheat degree on anticorrosion performance of 5Cu/(10NiO-NiFe2O4) cermet inert anode

5Cu/(10NiO-NiFe2O4) cermet inert anodes were prepared by cold-pressing and sintering process, and the effect of superheat degree of melting K3AlF6-Na3AlF6-AlF3 on their anticorrosion performance was studied under electrolysis conditions. The results show that, the fluctuation of cell becomes small with increasing of superheat degree, which is helpful to inhibit the formation of cathodic encrustation; the concentration of impurities from inert anode in bath goes up to certain degree, but it is far smaller than those in traditional high-temperature bath. Increasing the superheat degree of melting K3AlF6-Na3AlF6-AlF3 has unconspicuous effect on the contents of impurities in cathodic aluminum. The total mass fractions of Fe, Ni and Cu in aluminum are 15.38% and 15.09% respectively under superheat degree of 95 and 195 °C. From micro-topography of anode used view, increasing the superheat degree can aggravate corrosion of metal Cu in inert anode, and has negative influence on electrical conductivity of electrode to some extent.

Effect of working condition on thermal stress of NiFe2O4-based cermet inert anode in aluminum electrolysis

Based on the FEA software ANSYS, a model was developed to simulate the thermal stress distribution of inert anode. In order to reduce its thermal stress, the effect of some parameters on thermal stress distribution was investigated, including the temperature of electrolyte, the current, the anode cathode distance, the anode immersion depth, the surrounding temperature and the convection coefficient between anode and circumstance. The results show that there exists a large axial tensile stress near the tangent interface between the anode and bath, which is the major cause of anode breaking. Increasing the temperature of electrolyte or the anode immersion depth will deteriorate the stress distribution of inert anode. When the bath temperature increases from 750 to 970 °C, the maximal value and absolute minimal value of the 1st principal stress increase by 29.7% and 29.6%, respectively. When the anode immersion depth is changed from 1 to 10 cm, the maximal value and absolute minimal value of the 1st principal stress increase by 52.1% and 65.0%, respectively. The effects of other parameters on stress distribution are not significant.