INERT ANODES FOR ALUMINUM ELECTROLYSIS AND METHOD OF PRODUCTION THEREOF
An inert anode for Al electrolysis, made of Cu—Ni—Fe—O based materials, comprising Fe in a range between about 10 and 20% by weight, Cu in a range between about 60 and about 80% by weight, Ni in a range between about 20 and about 30% by weight, and oxygen in a range between about 1 and about 3% by weight, and a method for producing the anode, comprising mechanically alloying metallic elements; oxygen doping; and consolidation.
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The present invention relates to inert anodes for aluminum electrolysis. More specifically, the present invention is concerned with a composition of anodes for aluminum electrolysis and a method of production thereof.
BACKGROUND OF THE INVENTIONThe primary aluminum industry is a high producer of greenhouse gases with mean emissions of 5.7 to 19.2 tons of CO2-equivalent per ton of produced Al, depending on the electric power source. A significant contribution, i.e. about 3.7 tons CO2-eq ton Al, originates from the use of consumable carbon anodes in the Hall-Heroult electrolysis process. In this context, the development of alternative cells consisting in a combination of inert anodes, also referred to as O2-evolving anodes, and wetted cathodes is a first R&D priority of primary aluminum producers. Successful research in this field promises significant environmental benefits, energy savings and cost reductions.
Among possible inert anode materials, i. e. metals, ceramics and cermets, metal-based anodes are currently promising candidates because they offer high electrical conductivity, excellent thermal shock resistance, mechanical robustness, ease of manufacture and simplicity of electrical connection to current leads. However, obtaining a metal-based inert anode with a long-term viability, i.e. typically of at least several months, in the highly corrosive conditions of the Al electrolysis, is very challenging. The use of a low-temperature, i.e. between about 700 and about 800° C. instead of a temperature of about 950° C. as standardly used for cryolite electrolyte for Al electrolysis should significantly increase the utility of metal-based anodes and would offer a larger selection of alloys that could be used as inert anodes. However, the decrease of the alumina solubility in low-temperature NaF—AlF3 electrolytes causes operational difficulties. KF—AlF3-based electrolytes have been proposed as a way to operate at lower temperatures due to its relatively high alumina solubility at low-temperatures, in amounts of about 5 wt. % at 700° C.
Metals are chemically unstable in cryolitic bath and, as a result, metallic anodes must be permanently covered by a protective, self-repairing and relatively thin oxide layer during Al electrolysis. For that purpose, the metallic anode composition must be optimized in order to achieve an adequate balance between the oxidation rate of the metal substrate and the dissolution rate of the oxide layer in the electrolyte. Cu—Ni—Fe based alloys have shown promising properties as inert anodes U.S. Pat. No. 5,284,562 to Beck et al., 1994) due to their ability to form an adherent, electronically conducting nickel ferrite plus copper scale during the operation of the electrolysis cell. However, Cu—Ni—Fe alloys present a two-phased microstructure, comprising a Cu-rich phase and a Fe—Ni-rich phase, over a large composition range. This chemical inhomogeneity decreases their corrosion resistance because the iron-rich phase is preferentially corroded upon Al electrolysis inducing the formation of iron fluoride corrosion tunnels in the anode scale as recently shown by Beck et al. (T. R. Beck, C. M. MacRae and N. C. Wilson, Metall. Mat. Trans. B, 42, 807 (2011)).
Homogenization of the alloys through an appropriate thermal treatment is said to improve their corrosion resistance for Al production (T. R. Beck, C. M. MacRae and N. C. Wilson, Metall. Mat. Trans. B, 42, 807 (2011), U.S. Pat. No. 7,7077,945 to Bergsma et al., 2006).
There is still a need in the art for a composition for anodes for aluminum electrolysis and a method of production thereof.
SUMMARY OF THE INVENTIONMore specifically, in accordance with the present invention, there is provided an inert anode for Al electrolysis, made of Cu—Ni—Fe—O based materials, comprising Fe in a range between about 10 and 20% by weight, Cu in a range between about 60 and about 80% by weight, Ni in a range between about 20 and about 30% by weight, and oxygen in a range between about 1 and about 3% by weight.
There is further provided a method for producing metallic inert anodes, comprising mechanically alloying metallic elements; oxygen doping; and consolidation.
Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.
In the appended drawings:
In a nutshell, there is provided an anode composition and a method for producing metallic anodes having an improved resistance to corrosion, by preparing the anodes starting from alloys synthesized by mechanical alloying, optimizing the stoichiometry, and oxygen doping.
In particular, it was found that mechanically preparing Cu65Ni20Fe15 alloys under an oxygen atmosphere yielded nanostructured alloys with a resistance to corrosion in cryolitic environment at 700° C. increased compared to corrosion resistance of Cu65Ni20Fe15 alloys synthesized under inert atmosphere.
(Cu65Ni20Fe15)100-xOx materials were prepared by mechanical alloying under oxygen atmosphere. Their structural and chemical characteristics were studied at different stages of their preparation and after 20 hours of electrolysis in a low-temperature (700° C.) KF—AlF3 electrolyte. It was shown that oxygen, when added in appropriate amount during the mechanical alloying process, has a significant beneficial effect on the electrode corrosion resistance.
(Cu65Ni20Fe15)100-xOx materials with different O contents were synthesized by grinding powders in two steps as shown in
In a first step, shown in grey in
In a second step, the Cu65Ni20Fe15 powder was oxidized by a subsequent HEBM performed under O2 atmosphere. The oxygen amount in the samples was varied with the number of times (0, 4, 9 and 18 times) that the vial was filled with O2 (
Powder consolidation was then performed to obtain pellet samples for oxidation and electrolysis tests. The as-milled powder was first sieved to select only a powder fraction with a particle size between 20 and 75 μm. Then, it was introduced into a quartz cylinder pre-form and heated from room temperature to 1000° C. under Ar atmosphere for a thermal softening treatment. The resulting sample was cold pressed at 26 tons cm−2 for 10 minutes and then sintered at 1000° C. under Ar atmosphere for one hour. The pellet was removed from the heating zone of the furnace and left to cool down to room temperature. The obtained pellets had a diameter of about 11.3 mm and a thickness of about 5 mm for the electrolysis tests, and of about 1 mm for the oxidation tests. Their porosity was assessed according to the following equation:
where dt is the theoretical density determined from XRD measurements, and dexp is the experimentally measured density obtained by weighing and measuring the thickness of the pellet. The porosity of the samples was thus determined to be of 5±2%. The structure of the consolidated samples was determined by XRD. Backscattered electron (BSE) images of the sample surface were carried out using a JEOL™ JSM-6300F scanning electron microscope (SEM).
Thermogravimetric analyses (TGA) were performed using a Thermax™ 500 equipment. The samples were first heated up from room temperature to 700° C. at 10° C. min−1 under Ar atmosphere. Oxidation experiments were then conducted at 700° C. under Ar-20% O2 with a flow rate of 240 cc min−1. The mass variation of the samples was recorded for 20 hours. The nature of the oxides formed during these oxidation tests was determined by XRD analyses.
For electrolysis tests, a hole was drilled and tapped into the edge of the pellet in order to insert an electrical connection rod protected by an alumina-based cement coating. Electrolyses were performed at about 700° C. under argon atmosphere using a two-electrode configuration cell controlled by a VMP3 Multichannel Potentiostat/Galvanostat (by BioLogic Instruments). The electrochemical reactor contained three electrochemical cells and thus, three electrolysis tests could be conducted in parallel.
The reactor comprises a stainless steel container 4 receiving crucibles 11, provided with a stainless steel cover 3, which temperature was controlled by a furnace controller and a water cooling system.
In the cell geometry and electrode arrangement illustrated in
Current interruption measurements were performed after 0.25, 5 and 20 hours of electrolysis to determine the different voltage components, i.e., Nernst potential, polarization potential and ohmic drop. The current was interrupted for 30 seconds with a voltage sampling rate of 1 ms. The Nernst potential was defined as the voltage measured at 30 seconds after the current interruption. The ohmic drop was defined as the difference between the operating voltage measured before the current interruption and the voltage taken at 1 ms after the current interruption. The polarization potential was defined as the difference between the Nernst potential and the voltage taken at 1 ms after the current interruption.
The figure shows a graphite connecting rod 5 connected to the sintered alumina tubes 2 supported by stainless steel rods 1 connected to a thermocouple 12. As the connection rod 6 was partially immersed in the electrolyte, an undesired part of the contamination of the produced aluminum 10 came from the corrosion of the connection rod 6. Thus, in order to determine the Cu, Ni and Fe contamination levels only coming from the anode corrosion, two series of electrolysis tests were performed: a first series using an Inconel 718 rod for Cu quantification and a second series using an aluminum-bronze (C63000) rod for Ni and Fe quantifications. The Cu, Ni and Fe contents in the produced Al and electrolyte were measured by neutron activation. The experiment was repeated at least twice for each anode composition. The annual wear rate of the anode was calculated according to the equation:
where mb is the mass of electrolyte (g); wb is the mass fraction of contaminants (Cu+Ni+Fe) in the electrolyte (wt. %); mAl is the mass of produced Al (g); wAl is the mass fraction of contaminants (Cu+Ni+Fe) in the produced Al (wt. %); ρa is the anode density (g cm−3); Sa is the geometric surface area of the anode immersed in the electrolyte (cm2); and t is the electrolysis time (h).
The composition and structure of the oxide layers formed on the anode during Al electrolysis were determined by EDX and XRD analyses recorded after polishing the electrode for different times in order to reveal the successive oxide layers. The surface and cross section of the electrodes were observed by SEM.
Moreover, it appears in
This is supported by BSE images of the surface of the consolidated (Cu65Ni20Fe15)100-xOx materials (see
A possible explanation is that the presence of finely dispersed Fe2O3 inclusions in the Cu—Ni—Fe matrix (
The current interruption method was performed after 0.25, 5 and 20 hours of electrolysis in order to determine the different voltage components, i.e., Nernst potential, polarization potential and ohmic drop. For all electrodes, the measured Nernst potential is initially around 2.1 V and reaches a stable value of ca. 2.4 V after a few hours of electrolysis which is in accordance with the theoretical voltage (E=2.37 V) for the decomposition reaction of alumina (Al2O3=2Al+3/2O2) at 700° C. under 1 atm. O2. The polarization potential at the end of the electrolysis is in the range 0.15-0.3V.
The evolution of the ohmic drop with the electrolysis time for the four electrodes is showed in
The schematic representation of these layers determined from EDX and XRD analyses after polishing the electrodes for different times but not shown here is presented for each electrode in
For x=1.4 (
For x=7.2 (
Electrolyte sampling was performed after 0, 1, 2, 4, 15 and 20 hours of electrolysis for the (Cu65Ni20Fe15)98.6O1.4 electrode. The evolution of the Cu, Ni and Fe concentrations in the bath is plotted as a function of the electrolysis time in
As people in the art will now be in a position to appreciate, there is provided an anode composition and a method of production thereof, for inert anodes of a high resistance to corrosion, with an erosion rate of at most 1 cm year−1, during electrolysis of aluminum at low temperature, i.e. at about 700° C. There are provided mechanically alloyed Cu—Ni—Fe—O based materials for inert anodes.
A Cu—Ni—Fe alloy with a composition comprising Cu between about 65 and 70%. Ni—Fe alloy with a composition comprising Cu in the range between 60 and 80 wt. %, Ni in the range between 20 and 30 wt. % and Fe in the range between 10 and 20 wt. % can be considered as appropriate for obtaining an anode with a good corrosion resistance after subsequent O doping in the range between 1 to 3% by weight. For example, an optimized composition was about 15% by weight Fe, about 64% by weight Cu, about 20% by weight, and about 1.5% by weight oxygen.
Oxidization by grinding under oxygen atmosphere after an initial grinding under inert atmosphere to allow a proportion of oxygen between about 1 and 3% by weight is found to be efficient in increasing the anode corrosion resistance (see
As an alternative, Cu—Ni—Fe—O based anodes with dispersed Fe2O3, precipitates can also be produced by ball milling Cu—Ni—Fe alloy with nanometric iron oxide particles, i.e. of a size of at most 100 nm (see
With anodes of (Cu65Ni20Fe15)100-xOx with x comprised in the range between about 1 and about 3, aluminum could be produced with a purity of 99.7%. The rate of corrosion of the anodes is very low, at about 8 mm/year, which is well below current industry target of typically at most 10 mm/year. Moreover, the present anodes have good thermal, and mechanical stability, and low electric resistivity. They also have a stable potential and a low overvoltage for the reaction of oxygen, for example less than 0.4 V at 0.5 A/cm2.
The addition of a small concentration, i.e. at most 5 wt. % and preferably at most 1 wt. %, of rare earth elements (such as Y or Ce for example) to the composition is expected to further increase resistance to corrosion (see for instance, works of R. Cueff et al in Corrosion Science 45 (2003) 1815-10831).
The consolidation procedure for producing nanostructured anodes from ball-milled Cu—Ni—Fe—O powders can be done through a cold pressing-sintering procedure as described hereinabove before. Other techniques characterized by their ability to produce nanostructured bulk materials or coatings from ball-milled powders, such cold spray or spark plasma sintering for example, can also be used.
In order to induce the formation of a protective NiFe2O4-rich layer at the surface of the Cu—Ni—Fe—O electrode before Al electrolysis and then, to prevent the formation of metal fluorides (e.g., FeF2) at the electrode surface during the first minutes of electrolysis, a pre-treatment of the electrode can be performed through an air oxidation step (e.g., oxidation under air atmosphere at 700° C. for 3 h).
Although the present invention has been described hereinabove by way of embodiments thereof, it may be modified, without departing from the nature and teachings of the subject invention as recited herein.
REFERENCES
- B. Assouli, M. Pedron, S. Helle, A. Carrere, D. Guay and L. Roué, Light Metals, 1141 (2009)
- S. Helle, M. Pedron, B. Assouli, B. Davis, D. Guay and L. Roué, Corros. Sci., 52, 3348 (2010)
- S. Helle, B. Davis, D. Guay and L. Roué, J. Electrochem. Soc., 157, E173 (2010)
- T. R. Beck, Light Metals, 355 (1995)
- T. R. Beck, C. M. MacRae and N. C. Wilson, Metall. Mat. Trans. B, 42, 807 (2011)
- U.S. Pat. No. 5,284,562 (A) Beck et al.
- V. de Nora et al., Light Metals, 501 (2007)
- V. A. Kovrov, A. P. Khramov, A. A. Redkin and Y. P. Zaikov, ECS Transactions, 16, 7 (2009)
- S. Helle, B. Brodu, B. Davis, D. Guay and L. Roué, Corros. Sci., 53, 3248 (2011)
- U.S. Pat. No. 6,692,631 Bergsma et al.
- U.S. Pat. No. 7,077,945 Bergsma et al.
- Aluminum industry technology roadmap, by the Aluminum Association, Aluminum industry technology roadmap, Washington D.C. (2003)
- R. Cueff et al, Corrosion Science 45 (2003) 1815-10831
Claims
1. Inert anode for Al electrolysis, made of Cu—Ni—Fe—O based materials, comprising Fe in a range between about 10 and 20% by weight, Cu in a range between about 60 and about 80% by weight, Ni in a range between about 20 and about 30% by weight, and oxygen in a range between about 1 and about 3% by weight.
2. Inert anode of claim 1, comprising about 15% by weight Fe, about 64% by weight Cu, about 20% by weight Ni, and about 1.5% by weight oxygen.
3. Inert anode of claim 1, further comprising at most 5 wt. % by weight rare earth elements.
4. Inert anode of claim 1, further comprising at most 1 wt. % by weight rare earth elements.
5. Inert anode of claim 1, wherein the rare earth elements are ones of Y and Ce.
6. Inert anodes of claim 1, having a rate of corrosion of at most 1 cm/year during electrolysis of aluminum at a temperature of about 700° C.
7. Inert anodes of claim 1, having a rate of corrosion of about 0.8 cm/year during electrolysis of aluminum at a temperature of about 700° C.
8. Inert anodes of claim 1, having a stable potential and a low overvoltage for the reaction of oxygen.
9. Inert anodes of claim 1, having a stable potential and an overvoltage for the reaction of oxygen less than 0.4V at 0.5 A/cm2.
10. A method for producing metallic inert anodes made of Cu—Ni—Fe—O based materials, comprising:
- mechanically alloying metallic elements;
- oxygen doping; and
- consolidation.
11. The method of claim 10, wherein said mechanically alloying comprises grinding metallic elements under inert atmosphere; and said oxidizing comprises a subsequent grinding of the alloyed elements under O2 atmosphere.
12. The method of claim 10, wherein said mechanically alloying and said oxygen doping comprise grinding metallic elements and iron oxides particles, the iron oxides particles having a nanometric size.
13. The method of claim 10, wherein said mechanically alloying and said oxygen doping comprise grinding metallic elements and iron oxides particles, the iron oxides particles have a size of at most 100 nm.
14. The method of claim 10, comprising synthetizing an alloy with Fe in a range between about 10 and 20% by weight, Cu in a range between about 60 and about 80% by weight, Ni in a range between about 20 and about 30% by weight, and oxygen in a range between about 1 and about 3% by weight.
15. The method of claim 10, comprising synthetizing an alloy with about 15% by weight Fe, about 64% by weight Cu, about 20% by weight Ni, and about 1.5% by weight oxygen.
16. The method of claim 10, comprising synthetizing a metallic CuNiFe alloy by high energy ball milling of Cu, Ni and Fe powders under Ar atmosphere; and oxidizing the CuNiFe alloy by a subsequent high energy ball milling under O2 atmosphere.
17. The method of claim 10, further comprising an air oxidation step.
18. The method of claim 10, further comprising adding at most 5 wt. % by weight rare earth elements.
19. The method of claim 10, further comprising adding at most 1 wt. % by weight rare earth elements.
20. The method of claim 10, wherein said consolidation comprises one of: cold pressing-sintering, cold spray and spark plasma sintering.
Type: Application
Filed: Sep 27, 2012
Publication Date: Oct 30, 2014
Applicant:
Inventors: Sebastien Helle (Montreal), Daniel Guay (Saint-Lambert), Lionel Roue (Saint-Julie)
Application Number: 14/351,961
International Classification: C25C 3/12 (20060101); B22F 3/105 (20060101); B22F 3/02 (20060101);