Wettable Cathode Research Articles

Anodic Behaviour of NixFeyCu5 and NixFeyCu20 in Potassium-Rich Naf-KF-AlF3-Al2O3 Melts

Using inert anode technology to produce aluminium would substantially reduce the aluminium industry's carbon footprint. Replacing the Hall-Héroult process with an alternative process based on vertical electrode cell (VEC) design using inert anodes and wettable cathodes is a promising alternative, but must be developed to be economically viable at scale. Three material types have been explored as inert anodes: metals, cermets and ceramics. Metallic anodes have been widely studied for their attractive properties, such as superior mechanical strength, high electrical conductivity and ease of fabrication. Of metallic anodes, Cu-Al, Ni-Fe and Ni-Fe-Cu-based alloys are the most commonly examined alloy systems for anode applications. It is also necessary to use low-temperature melts during the electrolysis process to ensure that the anodes are not damaged. In NaF-KF-AlF3 melts, increasing the KF content in the mixture would reduce the liquidus temperature and increase the alumina solubility in the melt.In this work, we examine the anodic behaviour of NixFeyCu5 and NixFeyCu20 electrodes, without prior surface treatment, in potassium-rich NaF-KF-AlF3 melts at 770°C. The cryolite ratio (xNaF+xKF/xAlF3) and potassium ratio (xKF/xKF+xNaF) of the melt used for the studies are 1.3 and 0.7, respectively. Electrochemical techniques such as chronopotentiometry, chronoamperometry and linear sweep voltammetry were performed to examine the anodic behaviour of these metallic anodes under an N2 atmosphere.Figure 1 shows the steady-state anodic polarisation curves on NixFeyCu5 and NixFeyCu20 anodes before and after galvanostatic polarisation with an applied current density of 0.5 Acm-2 for 120 minutes. It can be noted that the anodic potential at the same current density is more for the polarised anode compared to the non-polarised in both cases. This is due to the formation of a stable oxide layer during the galvanostatic polarisation of the anode. Oxidation of the anode is expected instead of fluoridation when the melt is saturated with alumina. The stabilisation of anodic current can be seen between 1.9 and 2.4 V. This stabilisation corresponds to the passivation of anodes due to the oxide layer formation. A rapid increase in the current after 2.4 V is due to the oxygen evolution reaction. The anodic current density in the passive region is much lower in anodes with Cu 20%, meaning a more protective layer is formed. After the galvanostatic polarisation of the anode, anodes were examined using scanning electron microscopy to understand the oxide layer composition. Anodes performed stably in potassium-rich melts, and Cu content plays a vital role in the anode's performance. Figure 1. Stationary state polarisation curves of NixFeyCu5 and NixFeyCu20 anodes before and after the polarisation at 0.5 A cm-2 Figure 1

LOW-TEMPERATURE ELECTROLYTIC PRODUCTION OF Al-REM ALLOYS IN CRYOLITE MELTS

The process of electrolytic production of Al–Y and Al–Sc alloys in an electrolyte based on potassium cryolite KF–NaF(10 wt %)–AlF3 with a cryolite ratio (CR) of 1.5, containing Al2O3, Sc2O3, or Y2O3 oxides, in a cell with vertical electrodes has been studied. The Fe–Ni–Cu alloy served as an inert anode. The wetted cathode was a graphite plate coated with the aluminum diboride. The electrolysis was carried out at a cathode current density of 0.2 A/cm2 and a temperature of 830°C. The Al2O3 mass was calculated based on the value of the current efficiency of 60%. The Sc2O3 additive was introduced into the melt in an amount of 1 wt %. The mass of the Y2O3 additive was chosen based on its solubility in the melt under study. For this, the influence of Y2O3 additives on the liquidus temperature of the quasi-binary mixture [KF–NaF(10 wt %)–AlF3 (KO = 1.5)]–Y2O3 was determined and it was found that, in contrast to Sc2O3 additives, which lower the liquidus temperature of the cryolite melt, small additions of Y2O3 lead to its sharp increase. It has been found that the efficiency of the electrolytic reduction of Y2O3 is 10 times higher than that of the aluminothermic reduction. Other things being equal, the efficiency of the electrolytic reduction of Y2O3 is higher than that of Sc2O3. Alloys Al–Y and Al–Sc with a REM content of 0.6 wt % have been obtained. However, the time to reach the maximum recovery of yttrium significantly exceeds the time to recover scandium. Metallographic studies of the obtained alloys indicated the presence of Al3Sc and Al2Y intermetallic compounds. A conclusion is made about the fundamental possibility of low-temperature electrolytic production of Al-REM alloys in cryolite melts based on potassium cryolite in vertical cells with an inert metal anode and a wettable cathode.

Electrolytic Production of Aluminum Alloys in Cells with a Low-Consumption Metal Anode and a Wettable Cathode

Electrolytic Production of Aluminum Alloys in Cells with a Low-Consumption Metal Anode and a Wettable Cathode

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

Carbon dioxide and other greenhouse gases are emitted during the production of primary aluminium through the Hall-Heroult process. Moreover, the process requires specific energy consumption of as high as 14.685 kWh/tonne of Al. There is an urgency to replace the existing process with a more environmentally and economically friendly process. One such process involves the use of inert anodes instead of traditional carbon anodes, where Al2O3 → 2Al + 1.5O2 occurs instead of 1.5C + Al2O3 → 2Al + 1.5CO2. The life span of inert anodes can be improved by using them at low operating temperatures, however, alumina solubility and dissolution rate decrease at low operating temperatures. Instead of using a traditional NaF-AlF3 molten solution, NaF-KF-AlF3 or KF-AlF3 could be used as low-temperature electrolytes. Introducing KF salt into the NaF-AlF3 mixture increases the alumina solubility and reduces the liquidus temperature of the molten salt. Another factor that affects the liquidus temperature of the mixture and alumina solubility in the electrolyte is the cryolite ratio , where M = Na or K (or both). It should be noted that the cells with AlF3-rich NaF electrolytes suffer with problems such as high cell voltage, low aluminium purity and unstable frozen side ledges. The addition of CaF2, MgF or LiF to the electrolyte mixture could improve the physical-chemical properties of the electrolyte. For instance, the addition of CaF2 to KF-AlF3 molten salts improves the electrical conductivity of the electrolyte. Specific energy consumption of the cell can be significantly reduced by using a vertical electrode cell with reduced anode-cathode distance (ACD). However, the major problem with low ACD is the contamination of reduced aluminium at cathode by anode products. This problem can be solved by using suspension electrolytes where the electrolyte contains excess about of Al2O3 in the electrolyte which obstructs the transportation of anode products into the aluminium diffusion layer on the cathode, thus maintaining high aluminium purity. In this paper, we summarise the developments obtained in designing the inert anode cells. We mainly focus on three main components: Inert anodes, wettable cathodes, and low-temperature electrolytes. The influence of additives on the properties of electrolytes will be discussed in detail. We also discuss the energy consumption, environmental impact and economic aspects of producing aluminium using vertical electrode cells.

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.

Borated graphite cathodes for low-temperature aluminum electrolysis

Electrochemical boriding of the graphite plates in the potassium cryolite based electrolytes was studied. The boriding were carried out in a cell with vertical electrodes. The procedure included 2 stages: 1) electrolysis in the KF–AlF3–KBF4 melt (CR=1.3) at low current density (0.01–0.02 A/cm2), required for the boron reduction, at 700 and 750 °C; 2) electrolysis in the KF–AlF3–Al2O3 melt at higher current density (0.2 A/cm2), required for the aluminum reduction. The optimal conditions of electrodeposition for obtaining the borated wettable cathodes were determined. According to the SEM data, a continuous AlB2 layer with a thickness of 7–10 μm was formed on the graphite surface. The borated graphite was tested as a wetted cathode during the low-temperature aluminum electrolysis. Prolonged electrolysis in a vertical cell with the graphite anode and the borated graphite cathode was carried out in the KF–NaF(10 wt.%)–AlF3–Al2O3 electrolyte (CR=1.5) at 830 °C. After 100 h of electrolysis, the thickness of the AlB2 layer on the graphite surface was 5 μm, while the cathode surface was completely wetted with aluminum. Thus, we demonstrated the feasibility of using the borated graphite cathode as a wettable dripping cathode in the low-temperature aluminum electrolysis in the vertical cell.

Performance of TiB2 Wettable Cathode Coating

A TiB2 wettable cathode coating was deposited on a graphite carbon cathode material via atmospheric plasma spraying (APS). The microstructure and phase composition of the TiB2 coating were analyzed via scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS). The wettability and corrosion resistance of the coating were studied in a molten-aluminum electrolytic system. The results showed that the surface of the TiB2 coating prepared via plasma spraying was flat and that the main phase of the coating was TiB2. The wettability between the TiB2 coating and liquid aluminum was better than that between graphite cathode carbon block and liquid aluminum. The abilities of the TiB2 coating and graphite cathode carbon block to resist sodium (Na) penetration and prevent molten salt corrosion were compared through a corrosion test. The TiB2 coating was found to have better resistance to Na penetration and better refractory cryolite corrosion resistance than graphite cathode carbon block.

TiB2 Deposited on Graphite by Suspension Plasma Spray as Al Wettable Cathode

TiB2 Deposited on Graphite by Suspension Plasma Spray as Al Wettable Cathode

On the question of using solid electrodes in the electrolysis of cryolite-alumina melts. Part 1.

This article is aimed at identifying issues associated with the use of solid cathodes in the electrolysis of cryolitealumina melts in order to determine conditions for their practical application. The contemporary technology of using solid anodes and cathodes is reviewed from its inception to the present time. The problems of stable electrolysis are discussed, such as effects of the electrode surface on the technological process. It is shown that all attempts undertaken over the recent 100 years to use solid electrodes, both reactive and inert, have been challenged with the emergence of electrolysis instability, formation of precipitates of varying intensity on the electrodes and impossibility of maintaining a prolonged process at current densities of above 0.4–0.5 A/cm2. Information is provided on the attempts to use purified electrolyte components with different ratios, metal-like and ceramic electrodes with a high purity and a smooth surface in order to approach real industrial conditions. However, to the best of our current knowledge, these experiments have not found commercial application. The authors believe that the most probable reason for the decreased current efficiency and passivation of solid electrodes consists in the chemical inhomogeneity and micro-defects of the bulk and surface structure of polycrystalline cathodes and anodes. It was the physical inhomogeneity of carbon electrodes that directed the development of the nascent electrolytic production of aluminium towards the use of electrolytic cells with a horizontal arrangement of electrodes and liquid aluminium as a cathode. This reason is assumed to limit the progress of electrolytic aluminium production based on the use of inert anodes and wettable cathodes in the designs of new generation electrolytic cells implying vertically arranged drained cathodes. The theoretical and experimental examination of this assumption will be presented in the following parts of the article.

Electrodeposited TiB2 on graphite as wettable cathode for Al production

AbstractTitanium diboride (TiB2) is considered as a promising cathode material for Al production. However, the manufacture of TiB2 cathodes is facing numerous challenges. In this study, electrodeposition of TiB2 on graphite was performed in molten fluoride (FLiNaK) electrolyte at 600°C by using a periodically interrupted current technique for various electrodeposition times (from 10 to 75 minutes) and at two different current densities (−0.12 and −0.5 A/cm2). It is shown that the TiB2 coating morphology/microstructure strongly depends on the applied current density. Denser coatings were obtained at jon = −0.12 A/cm2 with a growth rate of ca. 0.7 µm/min. The thicker films display a preferential crystallographic orientation along the [110] plan. At jon = −0.5 A/cm2, TiB2 coatings are deposited at a growth rate of ca. 6 µm/min with no crystallographic texture. They present a porous and stratified morphology with numerous transversal macrocracks. All TiB2 coatings show excellent wettability for molten Al as confirmed by sessile drop experiments. However, significant molten Al infiltration occurs in the TiB2 coatings, which accumulates at the coating/graphite interface, inducing the coating delamination.

Electrolytic production of aluminium. Review. Part 2. Development prospects

This paper looks at the evolution and the current status of inert electrodes. It also describes attempts to design new-generation aluminium cells with wettable cathodes and vertical electrodes. Numerous laboratory studies and pilot tests demonstrate that aluminium cells equipped with inert electrodes are environmentally safe and can deliver a breakthrough technology enabling to bring the power consumption down below 10 kWt·h/kg Al and at the same time increase the production even in the limited capacity of the electrolytic bath. There exists a number of projects aimed at commercializing the inert electrode technology. And despite the apparent loss of interest in this technology on the part of researchers and aluminium producers, relevant pilot tests are scheduled for 2024. A number of alternative innovative power saving cryolite-alumina bath techniques are being discussed, which should allow to get close to the theoretical level of power consumption and provide a dramatic boost in the cell capacity. The paper examines potential application of thermoelectric generators to reduce heat losses, the use of minimum anode-to-cathode distance and vertical nonwettable electrodes, and a transition to 3D electrodeposition of aluminium with cathode polarization.

Effects on primary energy use, greenhouse gas emissions and related costs from improving energy end-use efficiency in the electrolysis in primary aluminium production

Primary aluminium production is energy- and GHG-intensive in which electrolysis is by far the most energy- and GHG-intensive process. This paper’s aim is to study the effects on (1) primary energy use, (2) GHG emissions and (3) energy and CO2 costs when energy end-use efficiency measures are implemented in the electrolysis. Significant savings in final and primary energy use, GHG emissions and energy and CO2 costs can be achieved by implementing the studied measures. Vertical electrode cells and the combination of inert anodes and wettable cathodes are among the measures with the highest savings in all three areas (primary energy use, GHG emissions and energy and CO2 costs). Direct carbothermic reduction is one of the measures with the highest savings in primary energy use and energy and CO2 costs. For GHG emissions, direct carbothermic reduction is the more beneficial choice in regions with a high proportion of coal power, while inert anodes are the more beneficial choice in regions with a high proportion of low-carbon electricity. Although a company potentially can save more money by implementing the direct carbothermic reduction, the company should consider implementing the vertical electrode cells together with other energy-saving technologies since this would yield the largest GHG emission savings while providing similar cost savings as the direct carbothermic reduction. It may be necessary to impose a price on GHG emissions in order to make inert anodes cost-effective on their own, although further evaluations are needed in this regard. There is a potential to achieve carbon-neutrality in the reduction of aluminium oxide to pure aluminium.

Electrodeposited TiB2 Coating on Graphite As Wettable Cathode for Al Production

Greenhouse gas (GHG) emission from the Canadian aluminum industry was about 6 Mt of CO2 eq in 2017, an amount equivalent to the GHG emitted by about 1.3 million cars [1-2]. The use of inert anodes, like Ni-Fe-Cu alloys [3], instead of carbon anodes to produce O2 rather than CO2 during aluminum electrolysis is paramount to curb GHG emissions from that industry. However, to maintain the energy efficiency of the process, the opposite cathode material must be wetted by molten aluminum to decrease the anode-to-cathode distance and thus reduce the overall cell voltage. TiB2 presents a good wettability by molten Al. Also, it has good electrical conductivity and good chemical stability under Al electrolysis. Accordingly, TiB2 is considered as an interesting cathode material to use in conjunction with inert anodes. However, its production as dense bulk cathode is challenging.In this work, the electrodeposition of TiB2 on graphite substrate was performed by periodically interrupted current method in a molten LiF-NaF-KF salt at 600°C containing K2TiF6 and KBF4 as titanium and boron precursors, respectively [4]. The impact of the electrodeposition parameters on the crystalline structure, morphology, adhesion strength, Al wettability and Al penetration resistance of the TiB2 deposits will be presented.[1] 2017 sustainable development report. Aluminium Association of Canada.[2] Greenhouse Gas Emissions from a Typical Passenger Vehicle. United States Environnemental Protection Agency, 2018[3] S. Helle, M. Pedron, B. Assouli, B. Davis, D. Guay, L. Roué, Structure and high-temperature oxidation behaviour of Cu–Ni–Fe alloys prepared by high-energy ball milling for application as inert anodes in aluminium electrolysis. Corros. Sci. 52 (2010) 3348.[4] G. Ett, E.J. Pessine, Electrochim. Acta. 44 (1999) 2859. Figure 1: (A) SEM cross-section image of electrodeposited TiB2 coating on graphite substrate. Optical images of Al droplet at 1000 °C on (B) TiB2-coated and (C) pure graphite substrates.Fig Figure 1

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.

Electrolytic properties and element migration in Ni–TiB2/Al2O3 composite cathode

With alumina sol as binder and Ni metal as sintering aids, the Ni–TiB2/Al2O3 composite cathode material for aluminum electrolysis was prepared by cold-pressed sintering. The mechanical properties of the composite cathode material were measured. Its electrolytic properties were identified by a 20-A electrolysis test. Cathode samples before and after electrolysis test were measured by energy-dispersive spectroscopy (EDS). The migration behavior of various elements in the electrolysis process was studied by phase analysis. The result shows that Ni metal can effectively fill the gap between the aggregate during the sintering process, which can improve the sintering density of the composite cathode material significantly. The voltage of the 20-A electrolysis test is stable. The impurity of aluminum liquid is 0.42%. The aluminum liquid can wet the cathode surface effectively, and the Ni–TiB2/Al2O3 composite is an ideal wettable cathode material. In the process of electrolysis, the alkali elements in the electrolyte penetrate the electrode, where K goes deeper than Na. Al generated on the cathode surface will also penetrate the cathode through the gap of the composite material, while Ni in the electrode will spread into the aluminum liquid layer.

Interfacial wetting mechanisms of Al liquid on cathode carbon blocks of aluminum reduction cell for developing wettable cathode materials

Molecular dynamics simulation had been performed to investigate the wetting properties of Al droplets on amorphous carbon surfaces and graphite surfaces. The effects of temperature, graphitization of substrate and roughness were considered. Our results show that the contact angles can be improved effectively with the increased temperature. Furthermore, the Al droplet on graphite surface has a better wettability than that on amorphous carbon surface. Similarly, the contact angle will also reduce with the increased degree of graphitization and the wetting state will change from the transition wetting state to the Cassie state when the roughness increases. In addition to the contact angles on the rough surface, the remaining contact angles are less than 90°, which is different from the previous reports. The free energy and thermodynamic properties analysis were applied to character the solid-liquid interface properties and explain the wettability. Another interesting finding is that the study reveals the reason of the poor wettability between Al liquid and cathode carbon blocks of aluminum electrolytic cell. These findings improve our understandings of the wetting behaviors of Al droplets on cathode carbon block surfaces at the atomistic level, which is profitable to develop the wettable cathode materials for aluminum electrolysis.

Progress in Application of Energy-Saving Measures in Aluminum Reduction Cells

The Hall–Herault process is the only methodology applied in the industrial production of aluminum. Energy saving has long been the focus of the aluminum industry since its invention. Nowadays, the pursuit of high productivity, low DC energy consumption and large-scale cell adds new urgency to energy saving, among which the improvement of magnetic-hydro-dynamic (MHD) stability in aluminum electrolysis cells plays a very important role. This article reviewed the developments and applications of the measures in improving MHD stability in terms of structure, effectiveness, simulation, pilot and industrial results. Also some studies were conducted on a TiB2 wettable cathode. At the end, some suggestions on further reduction of energy consumption in the future are proposed.

Coalescence and wetting mechanism of Al droplets on different types of carbon for developing wettable cathodes: a molecular dynamics simulation.

So far, there have been few studies on the microscopic wetting behavior of aluminum liquid on cathode surfaces, which is critical for developing wettable cathode materials. In the present study, an investigation on the coalescence and wetting mechanism of Al droplets on different carbonaceous substrates has been performed via molecular dynamics (MD) simulation for developing wettable cathodes. The growth rate of liquid bridge, the mean squared displacement, the balanced contact angle, and the time of full coalescence were calculated to describe the coalescence and wetting of the Al droplets. The results illustrate the sequence of full coalescence time for the Al droplets: DG < HCNT < VCNT ≈ AC and the corresponding balanced contact angles were 47.98°, 53.32°, 55.02°, and 63.12°, respectively. Furthermore, the presence of defects on DG will increase the time of coalescence and the contact angle but the directions of defects have little influence. The free energy analysis indicates that the defects reduce the solid-liquid interaction and the work done for removing the Al droplet from the substrates so that the wettability is weaker than that for perfect graphene, which also explains the balanced wettability of Al droplets on the other substrates. In addition, the surface roughness increases the contact angle of Al liquid on AC (from 62° to 113°-120°) and hence, the wettability is changed from good to poor. In general, our results can improve the understanding of the wetting of AC and graphene by Al liquid at the atomic level, which can provide direction and theoretical guidance for further research on wettable cathodes.

Exploring inexpensive processing routes to prepare dense TiB2 components

Titanium diboride (TiB2) is considered one of the most suitable candidate materials for applications as wettable cathodes in redesigned aluminium smelting cells. In this work, titanium diboride compacts have been produced from powders using a wet colloidal processing approach combined with pressureless sintering for densification. The approach has good potential to reduce the manufacturing cost compared to hot-pressing. Owing to the elevated cost of the raw materials, the possibility of mixing powders with different particle sizes (more expensive fine and less expensive coarse powder) has been explored. The outcome of this study is very promising, since using two different particle sizes improved the particle packing of the green material resulting in sintered densities around 80% of the theoretical density at 2000°C.

Microstructure and Property of Plasma Sprayed TiB&lt;sub&gt;2&lt;/sub&gt; Wettable Coatings on Carbon Cathodes

In this work, fine-lamellar structured TiB2 ceramic wettable cathode coatings were deposited on carbon cathodes of aluminium oxide reduction cells by atmosphere plasma spraying (APS). Cathode carbon blocks consisting of coatings 800 μm thick of plasma sprayed TiB2 finepowder on carbon substrate were fabricated. In order to investigate the results of coating, the microstructure and phase composition of the wettable cathode coating were characterized by scanning electron microscopy (SEM). It is found that TiB2 fine-powders were partly oxidized during the plasma spraying process. The coating was mostly formed by a matrix of fully molten particles where the presence of semi-molten feedstock agglomerates. The APS method is believed to be a good choice to deposit TiB2 wettable cathode coating with high performance and reliability.