Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
  • C25C3/12—Anodes

Definitions

• the present invention relates to a material that can be used for structural components in a cell for the electrolysis of alumina dissolved in a fluoride containing molten salt bath by the use of essentially inert electrodes.
• aluminium is produced by the electrolysis of alumina dissolved in a cryolite based molten salt bath by the more than hundred years old Hall-Heroult process.
• carbon electrodes are used, where the carbon anode is taking part in the cell reaction resulting in the simultaneous production of CO 2 .
• the gross consumption of the anode is up to 550 kg/tonne of aluminium produced, causing emissions of greenhouse gases like fluorocarbon compounds in addition to CO 2 .
• the electrolysis cell would then produce oxygen and aluminium.
• the object of the present invention is to identify a material that is stable at an oxygen partial pressure of 1 bar at temperatures above about 680°C and has a sufficiently low solubility in the electrolyte to be used as a material for structural cell components in oxidizing regions of an aluminium electrowinning cell based on substantially inert electrodes.
• the invention is the conclusion of an extensive search for materials capable of fulfilling the requirements for a material for structural cell components in oxidizing regions of an aluminium electrowinning cell based on substantially inert electrodes.
• the stability requirements of such a material are similar to those of an inert anode in said electrowinning cell.
• the choice of possible element oxides for an inert anode is narrowed to: TiO 2 , Cr 2 O 3 , Fe 2 O 3 , Mn 2 O 3 , CoO, NiO, CuO, ZnO, Al 2 O 3 , Ga 2 O 3 , ZrO 2 , SnO 2 and HfO 2 .
• the main requirements for a material intended for use in structural cell components are stability at 1 bar oxygen pressure at temperatures above 680°C and a low solubility in the molten electrolyte.
• the electrical properties are less important, but its electrical conductivity should be far less that the electrical conductivity of the electrodes and the electrolyte.
• the material should either itself fulfil the requirements, or it should upon contact with the molten electrolyte react to form a surface layer of an aluminate that fulfils the said requirements.
• CuO, Ga 2 O 3 , ZrO 2 and HfO 2 are eliminated from the list of possible element oxides, and we are left with: TiO 2 , Cr 2 O 3 , Fe 2 O 3 , Mn 2 O 3 , CoO, NiO, ZnO, Al 2 O 3 , and SnO 2 .
• the first group comprises mixed oxides of the spinel structure with composition (AY u A" u ) x (By v B" v ) y (CVwC"w)- ⁇ 4, in which A' and A" are divalent elements , i. e., Co, Ni, or Zn, B' and B" are trivalent elements, . e., Al, Cr, Mn, or Fe and C and C" are tetrava- lent elements, i. e., Ti or Sn.
• the second group comprises mixed oxides of the ilmenite structure with composition A' ⁇ . s A" s TiO 3 , in which A' and A" are divalent elements, i. e., Co, Ni, or Zn. O is the element oxygen. 0 ⁇ s ⁇ l.
• the third group comprises the divalent oxides of Co, Ni and Zn or solid solutions of these. These will react with dissolved alumina to form surface layers of essentially insoluble aluminates. These materials may be expressed by the formula AY ⁇ A",0. 0 ⁇ t ⁇ l .
• a material suitable as an essentially inert material for structural components in the oxidizing regions of a cell for the electrolytic production of aluminium from alumina dissolved in an essentially fluoride based electrolyte where cryolite is an important ingredient, must be resistant to oxidation and dissolution in the electrolyte.
• MO x + 2x/3AlF 3 MF 2x +2x/6Al 2 O 3 (1)
• reaction 2 (reaction 2)
• MO x + 6yNaF + yAl 2 O 3 Na ⁇ , y MO x+ 3 y + 2yAlF 3 (2)
• elements with the normal valence 2 the only possible elements are thus Co, Ni, Cu and Zn.
• elements with valence 3 one is left with only the elements Cr, Mn, Fe, Ga and Al.
• elements with valence 4 one is left with only the elements Ti, Zr, Hf, Ge and
• Sn. Cu, Ga, Zr, Hf and Ge may be eliminated from the list based on solubility considerations, and we are left with the following list of elements: Co, Ni, Zn, Al, Cr, Mn, Fe, Ti and Sn.
• the possible materials for structural cell components in an aluminium electrowinning cell based on substantially inert electrodes are thus limited to the oxides of the listed elements, or combinations of these oxides in mixed oxide compounds.
• B' Fe, Cr, Mn. This is further illustrated in Examples 3, 4, and 6. These materials are thus possible materials for structural cell components.
• the pure aluminates NiAl 2 O 4 , CoAI 2 0 and ZnAl 2 O 4 are also possible materials for structural cell components.
• This material may in principle be used for structural cell components.
• spinel type materials thus form the second group of materials for structural components of aluminium electrowinning cells.
• Another group of materials for structural components of aluminium electrowinning cells comprise the ilmenite type materials, NiTiO 3 , C0T1O 3 and solid solutions of these. These compositions are given by the formula A' ⁇ . s A" s TiO 3 , in which A' and A" are divalent elements, i. e., Co, Ni, or Zn. O is the element oxygen. 0 ⁇ s ⁇ l.
• A' and A" are divalent elements, i. e., Co, Ni, or Zn.
• O is the element oxygen. 0 ⁇ s ⁇ l.
• Figure 1 Shows a photograph of a sample of a material for structural components in an electrolysis cell before and after the stability test of Example 3.
• Figure 2 Shows a backscatter SEM photograph of the reaction zone of a Ni] ⁇ Cr 2 O 4 material after 50 hours of exposure to molten fluoride electrolyte under anodic polarization.
• Figure 3 Shows a backscatter SEM photograph of a NiFeCrO 4 sample after 50 hours of exposure to molten fluoride electrolyte under anodic polarization.
• Figure 4 Shows a backscatter SEM photograph of a sample of Ni ⁇ 5+x FeTio 5 - x O 4 after the stability test of Example 5.
• Figure 5 Shows a backscatter SEM photograph of a Ni ⁇ o ⁇ Fe 2 O 4 sample after 30 hours of exposure to molten fluoride electrolyte under anodic polarization.
• the sample was exposed to a molten fluoride bath under anodic polarization in order to ensure a partial pressure of 1 bar oxygen on the sample surface.
• the electrolyte was contained in an alumina crucible with inner diameter 80 mm and height 150 mm.
• An outer alumina container with height 200 mm was used for safety, and the cell was covered with a lid made from high alumina cement.
• a 5 mm thick TiB 2 disc was placed, which made the liquid aluminium cathode stay horizon- tal.
• the electrical connection to the cathode was provided by a TiB 2 rod supported by an alumina tube to avoid oxidation.
• a platinum wire provided electrical connection to the TiB 2 cathode rod.
• a Ni wire provided for the electrical connection to the anode.
• the Ni wire and the anode above the electrolyte bath was masked with an alumina tube and alumina cement to prevent oxidation.
• 340 g Al, (99 9% pure), from Hydro Aluminium was placed on the T ⁇ B 2 disc at the bottom of the alumina crucible
• the electiolyte was made by adding to the alumina crucible a mixture of 532 g Na-,A1F 6 (Gieenland cryolite) 105 g AIFj (from Norzink, with about 10 % Al 2 O 3 ) 35 g Al 2 O 3 (annealed at 1200°C for some hours) 21 g CaF 2 (Fluka p a )
• the sample of the mate ⁇ al for structural cell components was suspended above the electrolyte du ⁇ ng heating of the cell The temperature was maintained at 970°C du ⁇ ng the whole expe ⁇ ment
• the sample of the mate ⁇ al for structural cell components was lowered into the molten electrolyte and polarized anodically with a current density of 750 mA/cm 2 based on the bottom end cross sectional area of the sample The real current density was somewhat lower because the side surfaces of the anode were also dipped into in the electrolyte
• ZnO was doped with 0 5 mol% Aid s
• Two Pt wires were pressed into the mate ⁇ al in the longitudinal axis of the ZnO anode and acted as elect ⁇ cal conductois
• the mate ⁇ al was sintered at 1300°C for 1 hour
• the stability test was performed in the same manner as descnbed in Example 1
• the amounts of electrolyte and aluminium were the same
• the temperature was 970°C
• the current density was set to 1000 mA/cm 2 based on the bottom end cross sectional area of the sample
• the electrolysis expe ⁇ ment lasted for 24 hours XRD (X-ray diffraction) analysis of the sample after the electrolysis experiment showed that ZnO had been converted to ZnAl 2 O 4 during electrolysis.
• Example 3 Test of the stability of a Ni 1+x Cr 2 O 4 sample anodically polarized in a molten fluoride electrolyte.
• the starting powder was prepared by a soft chemistry route.
• the appropriate amounts of Ni(NO 3 ) 2 , and Cr(NO 3 ) 3 were complexed with citric acid in dilute nitric acid. After evaporation of excess water, the mixture was pyrolysed and calcined at 900°C for 10 hours. The sample was cold isostatically pressed at 200 MPa, then sintered at 1440°C for 3 hours. The material was found by XRD to possess the spinel structure.
• the stability test was performed in the same manner as described in Example 1, but a platinum wire provided electrical connection to the sample.
• the platinum wire to the sample was protected by a 5 mm alumina tube.
• the electrolysis started the anode was dipped approximately 1 cm into the electrolyte.
• a photograph of the sample before and after electrolysis is shown in Figure 1.
• the electrolyte, temperature and current density were the same as described in Example 2.
• the stability test lasted for 50 hours. After the experiment the sample was cut, polished and examined in SEM (Scanning Electron Microscope). A reaction zone could be seen between the Ni[. ⁇ Cr 2 O 4 - material and the electrolyte.
• Figure 2 shows the backscatter SEM photograph of the reaction zone. On the photograph one can see a reaction zone that has propagated along the grain boundaries of the NiuCr 2 O 4 material. The white particles are NiO.
• reaction product consisted of a material where the chromium atoms were partly exchanged with aluminium atoms as described by the formula NiCr 2 . x Al x O 4 where x varies from 0 to 2.
• the reaction product forms an insulating coating.
• the starting powder was prepared by a soft chemistry route.
• the appropriate amounts of Ni(NO 3 ) 2 , Fe(NO 3 ) 3 and Cr(NO 3 ) 3 were complexed with citric acid in dilute nitric acid. After evaporation of excess water, the mixture was pyrolysed and calcined at 900°C for 10 hours. The sample was cold isostatically pressed at 200 MPa, then sintered at 1600°C for 3 hours. The material was found by XRD to possess the spinel structure.
• the stability test was performed in the same manner as described in Example 3. The amounts of electrolyte and aluminium were the same. The current density was set to 1000 mA/cm 2 based on the cross sectional area of the rectangular sample. The experi- ment lasted for 50 hours. Examinaton of the sample after exposure to molten fluorides under anodic polarization showed a several micron thick reaction layer where Cr in the material was partly exchanged with Al atoms. A backscatter SEM photograph of the reaction layer is shown in Figure 3. Light grey areas consist of original NiFeCrO 4 material. Medium grey area contains almost no Cr atoms and a much lower content of Fe.
• NiFeCrO 4 material reacts with alumina in the electrolyte and forms a dense, essentially insoluble, insulating layer of NiFe,. x Al ⁇ +x O 4 .
• Ni(NO 3 ) 2 , Fe(NO 3 ) 3 and TiOsH ⁇ Cio (titanyl acetylacetonate) were complexed with citric acid in dilute nitric acid. After evaporation of excess water, the mixture was pyrolysed and calcined at 900°C for 10 hours. The sample was cold isostatically pressed at 200 MPa, then sintered at 1500°C for 3 hours. The material was found by XRD to possess the spinel structure.
• the stability test was performed in the same manner as described in Example 3. The amounts of electrolyte and aluminium were the same. The current density was set to 1000 mA/cm 2 based on the cross sectional area of the rectangular sample. The experi- ment lasted for 30 hours. After the experiment the sample was cut, polished and examined in SEM. The backscatter photo in Fig. 4 shows the end of the sample facing the cathode. In this experiment no reaction layer was detected on the Ni ⁇ . 5+x FeTio.5- ⁇ 4 anode after 30 hours.
• the starting powder was prepared by a soft chemistry route.
• the appropriate amounts of Ni(NO 3 ) 2 , and Fe(NO 3 )3 were complexed with citric acid in dilute nitric acid. After evaporation of excess water, the mixture was pyrolysed and calcined at 900°C for 10 hours. The sample was cold isostatically pressed at 200 MPa, then sintered at 1450°C for 3 hours. The material was found by XRD to possess the spinel structure.
• the stability test was performed in the same manner as desc ⁇ bed in Example 3
• the amounts of electrolyte and aluminium were the same
• the current density was et to 1000 mA/cm 2 based on the cioss sectional area of the rectangular anode
• the expe ⁇ - ment was stopped after 30 hours After the expe ⁇ ment the sample was cut, polished and examined in SEM Figuie 5 shows a backscatter photograph of the sample at the end facing the cathode An approximately 10 micron thick reaction layer is seen
• Element Atom % of element in the interior of Atom % of element in the reaction the anode shown in Figure 5 and layer as shown in Figure 5 and analysed with line scan EDS: analysed with line scan EDS:

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