Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
  • C25C7/02—Electrodes; Connections thereof
  • C25C7/025—Electrodes; Connections thereof used in cells for the electrolysis of melts
• • 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
• • 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
  • C25C3/04—Electrolytic production, recovery or refining of metals by electrolysis of melts of magnesium
• • 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
• • 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
  • C25C3/125—Anodes based on carbon
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• This invention relates to an electrolytic cell and to the electrolytic formation of an electrode product using a molten salt.
• Hall-Heroult process In the electrolytic decomposition of alumina in molten salts, the Hall-Heroult process is commonly employed. Examples of Hall-Heroult cells are shown in U. S. Patent Nos. 3,839,167 3,996,117, and 4,269,673.
• This invention enables the use of molten salts with limited reactant, e.g.alumina, solubility to be used as electrolytes for producing a desired product such as aluminum.
• An advantage is that some of these salts have low liquidus temperatures, e.g. iii the range 300 to 900°C, preferably in the 500 to 800°C, such that the molten salt operating temperature can be lower than usual Hall-Heroult cell operating temperatures, for instance from 900°C down to the melting point of aluminum (or even below the melting point of aluminum if it is desired to produce solid aluminum) .
• These salts are not as corrosive as those conventionally used in the Hall-Heroult process, are lower in density, and have lower alkali metal actLvity.
• the reduced corrosion eliminates the need for frozen electrolyte to protect the materials used in cell construction.
• the lower density of the salt improves cell stability because of the greater density difference between the metal and the salt, requiring greater force differences to create the same amplitude waves at the bath-metal interface.
• the lower alkali metal activity improves current efficiency, and eliminates swelling of carbon materials in the cell, enhancing cell life.
• the invention additionally provides the potential for the use of graphitic cathodic floors, rather than the usual carbon floors; graphitic floors are currently not feasible, because of high intercalation of alkali metal species into them, leading to premature failure.
• an electrolytic cell e.g. a Hall-Heroult cell
• an electrode having an extended surface area effective for the selective evolution of the desired electrode product
• a molten salt with limited rea ⁇ tant solubility.
• reactant solubility is ⁇ lwt%. This allows the use of all broader ranges of molten salt, at lower operating temperatures with benefits in both physical properties and lessened chemical reactivity of the molten salt with cell components such as the materials of construction (for instance the refractories used for the sidewalls and the floor) and the electrodes.
• the anode and cathode are in close proximity to one another, i.e. 0.25" - ' 1.25 ft , and the outside walls of the cell are thermally insulated sufficiently to maintain the electrolyte temperature at the decreased power levels. Operation can be without a frozen sidewall; some suspended solid reactant will usually be present.
• Percentages herein are on a weight basis, unless indicated otherwise. Reactant concentrations are based on total weight of molten salt plus reactant, although such is not an essential point in view of the low reactant concentrations.
• the invention comprises an improvement concerning the electrolytic decomposition of a substance in a molten salt electrolyte, e.g., a chloride and/or fluoride electrolyte, which typically has low solubility for an oxide whose decomposition is desired.
• a molten salt electrolyte e.g., a chloride and/or fluoride electrolyte
• an electrode is employed having an extended or substantially increased surface area effective for the electrolysis of the desired reactant, e.g. alumina, and the evolution of a desired electrode product, such as oxygen and/or carbon oxides, rather than halogen or halogen compounds, such as C ⁇ F (e.g. CF 4 ) .
• an extended surface area anode results-in selective electrolysis of a metal oxide at low concentrations in an electrolyte.
• the use of an anode with a plurality of holes or channels to increase the surface area was found to decompose aluminum oxide (alumina) in preference to chloride electrolyte in which it was contained.
• the size of the hole channel for gas evolving electrodes should be large enough to avoid gas bubbles blocking the electrical flow of current or providing a path of high resistance.
• An important feature of the invention is that despite low reactant solubility nevertheless appreciable current densities are achieved.
• superficial anode current densities greater than 1, 2, 3, 4, and even 5 or 6 amperes/square inch (0.15, 0.3, 0.45, 0.6, and even 0.75 or 0.9 amperes/square centimeter) are achieved.
• Superficial anode current density is determined by dividing the cell current by the cross sectional area of the bottom of the anode assuming there are no holes or channels in it. This area is referred to herein as the "superficial area" Brief Description of Drawing
• Fig. 1 is a side view of a laboratory electrolytic cell.
• Fig. 2 is a cross sectional view along the cutting plane II-II of Fig. 1.
• Fig. 3 is an end view of half of a production cell with a vertical cutting plane through an anode for showing internal structure of the anode.
• 1 is a cylindrical anode
• 2 is one of the channels in the anode
• 3 is molten salt
• 5 is a molten metal cathode.
• the perimeter of the anode, for test purposes, is shielded with a non-conductor 4 to prevent this area from taking part in the electrochemical reaction.
• the anode is suspended in a quartz vessel 6, and 7 is a graphite liner for the cathode. Gas bubbles 8 are shown rising from the channels 2.
• the anode and cathode may be reversed.
• FIG. 2 shows the end view of the anode illustrating a typical hole pattern drilled into the anode to extend its surface area.
• a toroidally shaped circulation pattern is set up in the molten salt due to the gas-lift action of the bubbles 8 rising in the channels 2, with the salt rising in the channels and then falling down the outer sides of non-conductor 4, thence to sweep across the upper surface of the cathode 5, and again up through the channels.
• This circulation acts to suspend undissolved alumina particles and to incorporate into the molten salt the replenishment alumina particles as such is fed from the top of the cell into the molten salt.
• Fig. 3 shows the half of a production cell left of centerline 24, where 11 is an anode, 12 is one of the channels in the anode, 13 is the molten salt bath, and 14 is a carbonaceous, electrically conductive floor.
• Molten metal (e.g. aluminum) cathode 25 rests on floor 14.
• Insulation is provided by bottom lining 15, sidewall 16 and lid 17,18.
• Rod 19 is an anode collector bar for providing d.c. electrical current to the anode 11.
• the cell lid is attached to a superstructure 21 via elbow 20 and rests on the sidewall 16. Current is removed from the cell through cathode collector bar 22.
• Sleeve 23 protects the connection between the anode collector bar and the anode from molten salt.
• a larger anode can be employed, because there is no frozen electrolyte to interfere with its positioning.
• the anode and cathode may be reversed.
• the circulation pattern executed by the molten salt in the cell of Fig. 3 will be influenced both by the gas-lift action of the evolved anode product and by electromagnetic phenomena, and the resulting circulation pattern executed by the molten salt will be the result of those combined ef ects.
• Electromagnetic effects become more important in production cells because of their large size (e.g. 15-foot by 40-foot rectangular dimensions in the horizontal plane) and the larger electrical current passing through them (e.g 125 to 150 kiloamperes) .
• Consumable anodes are made of carbon and react to form carbon dioxide and carbon monoxide, the relative amounts, as is known, being indicative of the current efficiency.
• An example of inert anode is set forth in U.S. Patent No. 4,620,905.
• a cermet is provided in which nickel is present as a continuous phase of relatively high conductivity as compared to the ceramic phase.
• the characteristic feature of inert anodes is that they are not consumed during the electrolysis, so that, in the electrolysis of alumina (AI2O3) , oxygen is evolved as the anode product, rather than carbon oxides as is the case when using carbon anodes.
• Suitable molten salt compositions are those which have a limited solubility for alumina. Examples include: about 5 to about 100% metal chlorides (i.e. alkali and alkaline earth metal chlorides, and Group III metal chlorides, e.g., sodium and potassium chlorides, magnesium and calcium chlorides, aluminum chloride, etc.), and about 2 to about 100% metal fluorides (i.e. alkali and alkaline earth metal fluorides, and Group III metal fluorides, e.g., sodium and potassium fluorides, magnesium and calcium fluorides, aluminum fluoride, etc.).
• the chlorides are, in general, less chemically aggressive than the fluorides.
• An example of a chloride-based molten salt comprises about .5 to about 15 wt.% aluminum chloride, from about 3 to about 40 wt.% of an alkaline earth metal chloride selected from the group consisting of barium, calcium, magnesium, and strontium chloride, from about 10 to 90% lithium chloride and about 10...to 80 wt.% sodium chloride and has a NaCl/LiCl ratio of. about 2.33. It is molten at less than about 650°C.
• This bath is the subject of U.S. Patent No. 4,440,610.
• Suitable fluorides are cryolite (N 3 lFg) , gF , AIF3, potassium fluoride and calcium fluoride.
• An example of a fluoride-based molten salt is formed from about 35 wt.% lithium fluoride, about 45 wt.% magnesium fluoride and about 20 wt.% calcium fluoride.
• the low temperature operation made possible by this molten salt is indicated by the fact that it has a solidus temperature of approximately 680°C.
• the weight ratio of NaCl/LiCl or NaCl/KCl is preferably between about .25 or 4.
• the reactant, e.g. alumina, in the molten salt can be present at a concentration of about 0.1 to about 2%, part of which can be present as undissolved, solid suspension.
• chlorides and fluorides may be advantageous, in order to achieve desires physical properties (density, viscosity, etc.) and chemical reactivity.
• Example 1 The apparatus used in this example is shown in Fig. 1, except that an anode of rectangular cross section was used.
• the apparatus was heated by electrical resistance to bring the chloride-base electrolyte in a quartz crucible to a temperature of 740°C.
• the nominal electrolyte composition was 64 wt.% NaCl, 27 wt.% LiCl, 4 wt.% AICI3, 5 wt.% A1F 3 , and 2 wt.% AI2O3.
• the alumina in this electrolyte had an estimated solubility less than .2 wt.%.
• the sides of the anode were shielded with boron nitride to eliminate the sides of the anode as electrolysis regions.
• a superficial anode current density of 2.0 amperes/sq. in. was achieved with no measurable chlorine generation ( ⁇ 1.0 ppm) .
• a current efficiency of aluminum production of greater than 80% was measured by CO2 and CO evolution.
• the carbon anode was prepared by drilling fifty-two, .375 in. diameter holes through the six inch length of the anode.
• the bottom cross section of the anode was 4.5 in. by 3 in., or 13.5 sq. in.
• the extended area of the anode generated by the holes through the entire length was 360 sq. in. A portion of this extended area will be available for electrolysis depending on the magnitude of the overpotential associated with the electrochemical reaction.
• a measure of the change in hole size after electrolysis demonstrated that the hole diameter had increased three inches into the depth of the electrode illustrating electrochemical activity to this depth.
• Example 2 Using an apparatus as described in Example 1, an electrolysis was carried out in an electrolyte composed of 43 wt.% NaCl, 43 wt.% KC1, 12.5 wt.% cryolite and 1.5 wt.% alumina. The estimated alumina solubility was less than .6 wt.%.
• the anode had a 5-5/8 in. diameter, was 6 in. long and made of carbon.
• the perimeter area was electrically insulated with boron nitride as in the previous example.
• the anode was drilled through its length with one hundred two, .375 in. diameter holes. This provided an extended surface area of 721 sq. in. This is approximately 29 times the superficial area of this anode. A superficial current density of 4.2 amperes/sq. in. was achieved with no measurable chlorine, generation.
• Aluminum production current efficiency of greater than 80% was measured by C0 2 -CO evolution.
• Example 3 The apparatus described in Example 1 was employed and the electrolysis of MgO was carried out in an electrolyte composed- of 69.25 wt.% LiCl, 25 wt.% KC1, 5 wt.% LiF and .25% MgO.
• the electrolysis of MgO was carried out with no measurable chlorine generation at a superficial current density of 2.2 amperes/sq. in.
• An aluminum pool was used as the cathode to keep the magnesium from floating. Analysis of the aluminum pool at the completion of this experiment resulted in a content of 2.69 wt.% magnesium.
• An extended surface area anode is used in this process the basic design of which is shown in Figures 1 and 2.
• the concept as applied to commercial aluminum production is shown in Figure 3, where, however, an anode of rectangular cross section is used.
• a thermal balance is achieved based on the electrical energy used, the aluminum produced, heat losses to the environment, heat of reaction, and an operating temperature of 750°C.
• the nominal electrolyte composition is 42.75 % NaCl, 42.75% KC1, 12.5% Na 3 AlF 6 , and 2% I2O .
• the alumina in this electrolyte has an estimated solubility less than 0.5%.
• a superficial anode current density of 5.875 amperes/sq. in. at a cell voltage of about 3.22 V is estimated with no expected chlorine generation due to the extended surface area of the anode.
• the anode-cathode distance is 1.00 in.
• a current efficiency of aluminum production of about 92% or more is expected.
• the carbon anode is prepared by drilling .375 in. diameter holes through the 15 in height of the anode to achieve a porosity of 30-40%.
• the bottom cross section of the anode is 21 in. by 39.375 in., or 827 sq. in.
• the extended area of the anode generated by the holes through the entire height is 46,312 sq. in.
• a portion of this extended area will be available for electrolysis depending on the magnitude of overpotential associated with the electrochemical reaction.
• the anode top is submerged below the upper surface of the electrolyte to aid in electrolyte circulation.
• the connection of the anode rod to the carbon is protected from the electrolyte by a sleeve to protect the salt from getting to the junction between the anode ' collector bar and the carbon anode.
• the cell design shown in Figure 3 takes advantage of the following attributes of the invention: lower temperatures, low reactant-solubility, lower corrosion, greater density difference metal to salt, lower alkali metal activity and higher electrical conductivity.
• Expected energy required to produce aluminum based on an energy balance calculation using the 3.22 volts and 92% current efficiency cited above, is only about 4.7 Kwh/lb (kilowatt-hours per pound of aluminum) compared to conventional rates of about 6 or more.
• the extended surface area of the electrode is at least about 2 times and more preferably, at least about 15 times that of the superficial area of the electrode.
• the electrode can be consumable or inert.

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• 1988
  • 1988-07-19 EP EP19880907512 patent/EP0349601A4/en not_active Withdrawn
  • 1988-07-19 AU AU23192/88A patent/AU616430B2/en not_active Ceased
  • 1988-07-19 JP JP63506475A patent/JPH02503695A/ja active Pending
  • 1988-12-19 BR BR888807392A patent/BR8807392A/pt unknown
  • 1988-12-19 WO PCT/US1988/004565 patent/WO1989006289A1/en not_active Application Discontinuation
  • 1988-12-19 EP EP19890901180 patent/EP0370075A4/en not_active Withdrawn
  • 1988-12-19 AU AU29129/89A patent/AU2912989A/en not_active Abandoned
• 1989
  • 1989-08-25 FI FI894004A patent/FI894004A0/fi not_active Application Discontinuation
• 1992
  • 1992-01-23 AU AU10446/92A patent/AU632259B2/en not_active Ceased

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