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
  • C25C3/18—Electrolytes

Definitions

• sludge inhibits or interferes with continued access of electrolytic bath to the cathodes, with the result that upon depletion of the aluminum chloride in the sludge layer by electrolysis, bath solvent in the sludge electrolyzes, with attendantloss of current efficiency in the production of aluminum.
• the sludge layer also interferes with circulation of the electrolytic bath, with further resultant impairment of electrical efficiency.
• the bath contains alkali metal halide or alkaline earth metal halide as the solvent for the aluminum chloride
• carbonaceous cathodes of the cell are attacked by alkali metal or alkaline earth metal produced by electrolysis of such salts, causing spalling and disintegration of the cathodes, with attendant change in the anode-cathode distance and increase in maintenance expense, as well as introducing into the electrolyte particles of carbon which contribute to the formation of sludge at the cathode.
• a further disadvantage of metal oxides in the electrolytic bath is that dissolved metal oxides have a lower electrodecomposition potential than aluminum chloride, and upon electrolyzing release oxygen at the cell anodes.
• Carbon is the most practical material to use for anodes, but evolved oxygen. reacts with the carbon to form gaseous oxides.
• Such consumption of anode carbon affects the operating characteristics of the cell deleteriously by changing the anode-cathode distance, as well as adding to anode expense.
• aluminum chloride dissolved in molten salt of higher electrodecomposition potential than aluminum chloride such as alkali metal halide or alkaline earth metal halide is electrodecomposed continuously, the concentration of aluminum chloride in the electrolytic bath of salt and aluminum chloride being in the range of l to 15 percent by weight, and preferably 3 to 10 percent by weight, and being maintained in such ranges by adding aluminum chloride continuously or intermittently to the bath to replace aluminum chloride electrodecomposed.
• Molten aluminum produced settles out of the bathand can be withdrawn in any suitable way, such as by tapping or siphoning it from the It has been found that in carrying out the above process continuously (i.e., for periods of over 700 hours of operation) under conditions in which metal oxides are introduced into the electrolytic bath it is highly important that the concentration of metal oxides in the bath, expressed as oxygen, be kept below 0.25 percent by weight, and preferably below 0.1 percent by weight, and more preferably below 0.05 percent.
• Metal oxides such as alumina, silica, (silicon is considered herein as a metal, albeit it is a metalloid), iron oxide, titanium oxide, and lime are only slightly soluble in the electrolytic bath, and as mentioned previously, metal oxides are a primary cause of the formationof the above-mentioned undesirable sludge. Moreover, although metal oxides are only slightly soluble in the bath, electrolysis of the dissolved oxides releases oxygen at carbonaceous anodes of the cell, oxidizing the carbon with resultant increase in the anode-cathode distance in the cell and an attendant gradual increase in electrical resistance.
• the electrolysis of aluminum chloride can be carried out indefinitely without formation of sludge at the cathodes in amounts which significantly affect the process or equipment detrimentally.
• the process can be continued indefinitely with an anode-cathode distance of less than 1 inch, a cathode currentdensity of about 10 amperes, a voltage of less than 5 volts between anode and cathode, and a current efficiency of better than percent with respect to electrodecomposition of aluminum chloride.
• Metal oxides may enter the bath in various ways; for example, as impurities in bath components (i.e., aluminum chloride or solvent) fed to the cell. Also, moisture which leaks into the cell or is present in cell walls or bath components used in the process reacts with molten aluminum in the cell to form alumina. Likewise, contact of the bath with cell linings or other structural parts of the cell which contain metal oxides, such as refractories containing alumina or silica, can introduce such oxides into the bath.
• bath components i.e., aluminum chloride or solvent
• moisture which leaks into the cell or is present in cell walls or bath components used in the process reacts with molten aluminum in the cell to form alumina.
• contact of the bath with cell linings or other structural parts of the cell which contain metal oxides, such as refractories containing alumina or silica can introduce such oxides into the bath.
• introduction of metal oxides into the bath is controlled, as indicated above.
• introduction of metal oxides into the bath is controlled, as indicated above.
• the aluminum chloride fed to the bath contain a total of less than 0.25 percent by weight of metal oxides, preferably less than 0.1 percent, and even more preferably less than 0.05 percent by weight.
• references herein to metal oxides include oxygenated compounds containing additional ions besides metal and oxygen, e.g., oxyhalides and oxynitrides.
• the electrolyte employed consists essentially of one or more alkali metal halides or alkaline earth metal halides which have a higher electrodecomposition potential than aluminum chloride, the chlorides being preferred, and the process is carried out at a temperature below 730 C. but above the melting point of aluminum (660 C.).
• a mixture of equal parts by weight of sodium chloride and lithium chloride is particularly satisfactory as the ele ctrolytejlt will be understood that other components can also be added to the bath, if desired, to modify bath characteristics.
• Electrolytic cells of known types employing spaced monopolar electrodes, or spaced bipolar electrodes between anode and cathode terminals, can be used in producing aluminum in accordance with the invention.
• a particularly suitable type of cell is described in U.S. Pat. application Ser. No. 178,650, of Dell, I-Iaupin and Russell, filed Sept. 8, 1971.
• the cell be closed except for one or more outlets for such gaseous materials, and one or more inlets for feeding aluminum chloride into the cell.
• Undissolved metal oxide can be separated from the bath, as by filtration of undissolved oxides from the bath, to reduce the concentration of metal oxide in the bath to the desired level.
• Another alternative is to alter the conditions of operation of the electrolysis process temporarily so that undissolved metal oxide in the bath is dissolved and electrolyzed until the concentration of metal oxide in the bath returns to the desired level.
• undissolved oxide in the bath can be dissolved by temporarily increasing the capacity of the bath to dissolve metal oxide present by increasing the concentration of aluminum chloride in the bath, or decreasing the temperature of the bath sufficiently that enough additional metal oxide dissolves and electrolyzes for the concentration of metal oxide into the bath to return to the predetermined desired level, whereupon the prior concentration of aluminum chloride in the bath, or the prior bath temperature, can be restored.
• Another way of increasing the capacity of the bath to dissolve metal oxides so that metal oxide can be removed by electrodecomposition thereof is to add to the bath a component suitable for that purpose.
• a component suitable for that purpose for example, when the baths solvent for aluminum chloride is alkali metal chloride, a small concentration of a fluoride e.g., about 1 percent by weight, expressed as fluorine can be introduced into the bath for that purpose; magnesium fluoride, aluminum fluoride, sodium fluoride, calcium fluoride, or cryolite are examples of fluorides that can be used.
• a further alternative procedure is to reduce the current density employed in the electrolytic cell to a level at which the rate of electrolysis of dissolved metal oxide increases relative to the rate of electrolysis of aluminum chloride, and maintaining such reduced current density until sufficient metal oxide has been electrolyzed that the amount of metal oxide in the bath returns to the desired level. Thereafter the current density can be increased to return it to its original level.
• aluminum was produced by continuous electrolysis of aluminum chloride at 695700 C. in an electrolytic cell of the type described in the aforesaid U.S. Pat. application of Dell, Haupin and Russell consisting ofa metal shell having an electrolysis chamber lined with silicon nitride-bonded fused silica, and having a graphite anode in the upper portion thereof and a graphite between each of the opposed electrodes was about 1 inch.
• the cell was closed except for an inlet through the top for feeding aluminum chloride into the electrolytic bath, an outlet in the top for chlorine and aluminum chloride vapors generated, and an outlet for withdrawal of molten aluminum produced.
• electrolysis compartments were kept immersed in electrolytic bath consisting essentially of sodium chloride and lithium chloride, plus about 6-7 percent by weight of aluminum chloride.
• the cell was operated continuously for 120 days at about 3.3 volts per electrolysis compartment and an average cathode current density of 8.5 amperes per square inch, without noticeable formation of sludge in the bath.
• concentration of metal oxides (expressed as oxygen) in the electrolytic bath remained at less than 0.002 percent by weight of the bath throughout the operation.
• Molten aluminum produced collected in the lower part of the electrolytic chamber and was drawn off periodically. 5 and 2/10 kilowatt hours of electric power was consumed per pound of aluminum produced.
• a process in accordance with claim 9 wherein the removal of metal oxide from the bath is effected by increasing the capacity of the bath thru effected by increasing the capacity of the to dissolve metal oxide and thereafter electrolyzing sufficient dissolved metal oxide from the bath that the concentration of metal oxide in the bath returns to. the said predetermined desired level.
• a process in accordance with claim 10 wherein the said increasing of the capacity of the bath to dissolve metal oxide is effected by decreasing the temperature of the bath.
• a process in accordance with claim 9 wherein the said removal of metal oxide from the bath is effected by temporarily decreasing the current density sufficiently to increase the rate of electrolysis of metal oxide from the solution.
• a process in accordance with claim 9 wherein the said removal of metal oxide is effected by filtration of undissolved oxides from the bath.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)

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ID=22710521

Family Applications (1)

Country Status (20)

Cited By (13)

Citations (5)

Family Cites Families (1)

• 1971
  • 1971-10-26 US US00192653A patent/US3725222A/en not_active Expired - Lifetime
• 1972
  • 1972-09-01 AU AU46232/72A patent/AU453929B2/en not_active Expired
  • 1972-09-11 CA CA151,416A patent/CA981209A/en not_active Expired
  • 1972-09-12 GB GB4234172A patent/GB1403893A/en not_active Expired
  • 1972-09-12 SE SE7211732A patent/SE396776B/xx unknown
  • 1972-09-15 PH PH13909A patent/PH9821A/en unknown
  • 1972-10-03 ZA ZA727061A patent/ZA727061B/xx unknown
  • 1972-10-04 YU YU02492/72A patent/YU249272A/xx unknown
  • 1972-10-11 CS CS726860A patent/CS202530B2/cs unknown
  • 1972-10-12 NL NL7213843.A patent/NL155891B/xx not_active IP Right Cessation
  • 1972-10-13 DE DE2251262A patent/DE2251262C2/de not_active Expired
  • 1972-10-16 IT IT53404/72A patent/IT966362B/it active
  • 1972-10-18 CH CH1521572A patent/CH555410A/fr not_active IP Right Cessation
  • 1972-10-19 DD DD166348A patent/DD99610A5/xx unknown
  • 1972-10-19 PL PL1972158371A patent/PL82400B1/pl unknown
  • 1972-10-19 AT AT894372A patent/AT327578B/de active
  • 1972-10-19 BR BR7305/72A patent/BR7207305D0/pt unknown
  • 1972-10-19 FR FR7237157A patent/FR2158238B1/fr not_active Expired
  • 1972-10-19 RO RO72565A patent/RO60672A/ro unknown
  • 1972-10-19 JP JP72104090A patent/JPS5215043B2/ja not_active Expired

Patent Citations (5)

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