US3382162A - Method of operating an alumina reduction cell - Google Patents

Method of operating an alumina reduction cell Download PDF

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US3382162A
US3382162A US465113A US46511365A US3382162A US 3382162 A US3382162 A US 3382162A US 465113 A US465113 A US 465113A US 46511365 A US46511365 A US 46511365A US 3382162 A US3382162 A US 3382162A
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lithium
sodium
aluminum
weight
fluoride
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Trupiano Roberto
Gambaretto Gianpaolo
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Montedison SpA
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Montedison SpA
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    • 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

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  • Our present invention relates to a novel improved process for the production of aluminum by electrolysis and, more particularly, to a novel composition of lithium, sodium, fluorine and aluminum (i.e. a sodium lithium fluoaluminate) which may be used directly in electrolytic baths, and to process for making this composition.
  • a novel composition of lithium, sodium, fluorine and aluminum i.e. a sodium lithium fluoaluminate
  • Metallic aluminum is at present industrially obtained primarily by the electrolysis of a solution of alumina (A1 0 in a fused-fluoride bath, with the concentration of the aluminum oxide in the bath maintained at 2 to 6% by weight; the fluorides of the bath generally comprising sodium cryolite (Na AlF and aluminum fluoride (AlF).
  • Na AlF sodium cryolite
  • AlF aluminum fluoride
  • a much higher voltage i.e. 4.7 to 5 volts
  • This resistance alone requires 1.9 volts of the applied 4.7 volts.
  • About 18 kw.hr., of which about 40% is consumed to overcome the electrolyte resistance, is conventionally required for the electrolytic production of 1 kg. of aluminum.
  • the electrolyte resistance can be decreased by the additional of a lithium compound (eg. lithium carbonate, lithium hydroxide, lithium fluoride or lithium cryolite) to the electrolysis bath.
  • a lithium compound eg. lithium carbonate, lithium hydroxide, lithium fluoride or lithium cryolite
  • the power required for the production of a given quantity of aluminum is lowered, the current density and the aluminum output from each cell are raised, the bath-solidification temperature is reduced, and the cells are operated at lower temperatures. The latter results in smaller heat losses, higher current yields, and a decrease in the anode consumption.
  • the lithium is normally present in the electrolyte in an amount equal to that which would correspond to additions of lithium fluoride, ranging between 2 and 20% by weight, based on the weight of molten electrolyte, and preferably between 3 and 8%, i.e. in the range of 0.8 to 2.15% by weight when expressed as lithium.
  • An object of our invention is to provide a novel sodium-lithium fluoaluminate composition, comprising aluminum, fluorine, sodium and lithium in certain definite proportions, which is suitable for direct use in the electrolytic bath.
  • Another object of our invention is to obviate the drawbacks inherent in the use, in electrolysis baths of the character described, of the simple salts of lithium because of volatilization and segregation of the compounds, and the drawbacks resulting from the use of a mechanical mixture of sodium cryolite and salts of lithium.
  • Yet a further object is to provide a process which gives the desired composition of chemically bound sodium, lithium, fluorine and aluminum without waste of ingredients, and particularly of expensive lithium compounds, and wherein interaction goes to completion.
  • Still other objects of our invention include the provision of an improved method of operating an aluminumeiectrolysis plant, and the provision of a highly effective and unique process for making such compositions.
  • compositions of aluminum, sodium, fluorine and lithium containing the four elements in a definite proportion, which are homogeneous and stable, and have characteristics not unlike those of well-defined crystallic compounds in this respect.
  • the scope of the instant invention is not to be limited by speculative explanations, the available information indicates that the compositions according to the invention are double salts of lithium, sodium and aluminum fluoride.
  • a proper name for the compositi us of the invention is sodium-lithium fluoaluminate denoting a substance from which lithium fluoride, sodium fluoride, aluminum fluoride and cryolite cannot be mechanically separated or removed by aqueous extraction.
  • composition of the invention could also be a mixture of compounds Li Na Al F and LisNagAlgplg, or other combinations of these four elements.
  • the crux of the invention is that in the described compositions the starting materials (considered here in terms of the simple salts ie, sodium fluoride, lithium fluoride and aluminum fluoride) do not retain their identity and that the lithium is chemically bound to the other elements, namely sodium, fluorine and aluminum, and apparently is present not in the form of lithium cryolite but in a novel chemical combination.
  • the starting materials considered here in terms of the simple salts ie, sodium fluoride, lithium fluoride and aluminum fluoride
  • the lithium is chemically bound to the other elements, namely sodium, fluorine and aluminum, and apparently is present not in the form of lithium cryolite but in a novel chemical combination.
  • sodium-lithium fluoaluminate many compounds may be used starting materials such as hydrofluoric acid, aluminum hydroxide, aluminum oxide, aluminum fluoride, sodium aluminate, lithium aluminate, sodium fluoaluminate, lithium fluoaluminate, sodium carbonate, sodium hydroxide, lithium carbonate, lithium hydroxide, sodium fluoride, lithium fluoride.
  • any compound of fluorine and aluminum is suitable provided that the two elements may interact to form aluminum fluoride. It is also possible to use sodium fluoaluminate with a molar ratio of sodium fluoride to aluminum trifluoride ranging between 1.67 and 2.65, as described in U.S. Patent 3,128,151.
  • Any compound of sodium and lithium may be used, provided that it may form an aqueous solution or dispersion, with the cost factor the only limitation.
  • the reactants designed to provide the four eleents in such a proportion that the respective amounts of each correspond to those to be attained in the sodiumlithium fluoaluminate, as will be described further below, are caused to react in a closed system.
  • FIGS. 1 and 2 are flow diagrams illustrating the process.
  • the reaction vessel 1 in FIG. 1 is a container provided with a cover and means (cg. a propeller-type stirrer) for mechanical agitation.
  • Vessel 1 is connected with a reservoir 2 and with a tank 3, which contains a neutralization iiixture.
  • a reservoir 4 serves to store the neutralization mixture.
  • the tank 3 and the reservoir 4 are also provided with stirring devices.
  • a filtering device is shown at 5.
  • T he reaction will now be described in detail, by refercnce to an example in which aluminum hydroxide, hydrofluoric acid, lithium carbonate and sodium carbonate are used as the starting materials.
  • Aluminum hydroxide and hydrofluoric acid are placed in vessel 1, so as to dissolve the aluminum hydroxide in the hydrofluoric acid and to obtain essentially an acidic solution of aluminum fluoride.
  • the exothermic reaction brings this solution to a temperature of to C.
  • Reservoir 2 contains a solution of lithium, sodium and fluoride in the same proportion as in the desired final product.
  • the lithium and sodium compounds are fed in predetermined amounts, in the form of an aqeuous suspension of preselected concentration, to vessel 3 from the reservoir 4.
  • reservoir 2 contains the mother liquor from the filtration, i.e. from unit 5, which is recycled.
  • the reaction in vessel 1, between aluminum hydroxide, hydrofluoric acid and the mother liquor from reservoir 2, is exothermic and maintains the temperature of the solution between about 80 and 120 C.
  • the neutralizing tank 3 is kept at a temperature between about 30 and 60 C., by means of suitable cooling means (not shown).
  • the acidic solution of aluminum fluoride in hydrofluoric acid, additionally containing sodium and lithium in solution is transferred from vessel 1 to container 3.
  • Lithium carbonate and sodium carbonate, which are continuously mixed in container 4 in a predetermined amount of water, so as to form an aqueous suspension, are continuously fed to container 3.
  • the lithium carbonate and sodium carbonate in container 3 continuously neutralize the free hydrofluoric acid from vessel 1. Simultaneous reaction with the aluminum fluoride in situ gives rise to the formation of the desired product, i.e. a coprecipitate containing aluminum, fluorine, sodium and lithium in chemical combination.
  • the precipitate of sodium-lithium fluoaluminate is filtered off from its mother liquor at the filtering device 5, dried, and then calcined at a temperature in the range of about 400 to 550 C.
  • the amount of each starting material must be selected within a narrow range.
  • the final composition must have the general formulation Na Li Al F wherein the Na weight percentage ranges from 16.4% to 32.4%, the Li percentage ranges from 0.26% to 5.14% by weight, the Al percentage ranges from 13% to 16.2% by weight, and the F percentage ranges from 54.5% to 62.3% by weight.
  • the formula of the product may be also written as: aLi AlF :bNa AlF :cAlF
  • a particularly advantageous coprecipitate corresponds to the formula Na Li Al F and has a lithium content of about 5% by weight, a melting point of about 710 C. and a density of 2.77. The melting points of the coprecipitates are, in all cases, less than the electrolysisbath temperatures (about 900 C.) but are usually in excess of 700 C.
  • the purpose of keeping the amount of water essentially constant is to prevent undue dilution of the mother liquor, which in this process is a necessary ingredient as a source of lithium, sodium and fluorine.
  • the amount of water in reservoir 2 which contains sodium fluorine and lithium, should not substantially exceed the proportion of 1000 parts of water per 3 parts of sodium, lithium, and fluorine ion combined.
  • a slightly more concentrated solution may be used, for instance up to a total of 6 parts of sodium, lithium and fluorine per 1000 parts of water.
  • the amount of water used in container 4 to suspend the lithium carbonate and the sodium carbonate is kept preferably low, i.e. less than the weight of lithium carbonate and sodium carbonate combined in the range of one part of water to 1.11.4 parts of combined solids.
  • the amount of time for adequate precipitation of the desired reaction product in vessel 3, is between about 1 and 3 hours; in practice, stirring for approximately 2 hours is satisfactory.
  • FIG. 2 shows diagrammatically another embodiment of the invention.
  • reservoir 4 is eliminated and the suspension of sodium carbonate and lithium carbonate is replaced with an aqueous suspension of lithium carbonate, which is continuously transferred from container 3' to reaction vessel 1'.
  • the reactant in vessel 1 is a sodium fluoaluminate composition, with a molar ratio of sodium fluoride to aluminum trifluoride between 1.67 and 2.65, prepared as described in US. Patent 3,128,151.
  • the mother liquor from a previous run is continuously recycled and fed from reservoir 2 to vessel 1'.
  • the temperature in vessel 1' is kept in a range between about 40 and 80 C.
  • the material from vessel 1 is filtered through the device 4'.
  • the lithium sodium fluoaluminate cake thus obtained is 6 dried and calcinated at substantially 400550 C., while the mother liquors are integrally reutilized by recycling them into the reaction vessel 1 through the storage reservoir 2'.
  • Example I Two thousand liters per hour of a solution were fed into vessel 1 from reservoir 2. The solution contained 0.6 g. lithium ion, 2.15 g. sodium ion and 2.55 g. fluorine ion per liter. 376 kg. per hour of aluminum hydroxide and 593 kg. per hour of 98% hydrofluoric acid were also continuously fed to reaction vessel 1.
  • reaction vessel 1 Owing to the exothermic nature of the reaction, the temperature in reaction vessel 1 reached about to C.
  • a suspension of sodium carbonate and lithium carbonate was prepared in tank 4.
  • the flow rate of the suspension from tank 4 to vessel 3 corresponded to 34.7 kg. per hour of lithium carbonate and 732 kg. per hour of sodium carbonate in a total of 5 83 kg. of Water.
  • the temperature in vessel 1 was maintained at 40 C. About two hours were required for the conditioning of the suspension and complete precipitation.
  • Example II The apparatus and the procedure were essentially as described in Example I.
  • Example III 2000 kg. per hour of a solution of 0.55 g. of lithium, 1.50 g. of sodium and 1.60 of fluorine per liter were fed into the reaction vessel 2, together with 438 kg. per hour of Al(OH) and 616 kg. per hour of 98% hydrofluoric acid. The temperature reached about 95 C. in reaction vessel 1.
  • reaction vessel 3 Concurrently 69.2 kg. per hour of LiCO of 98% purity, and 618 kg. per hour of 98% Na CO suspended in 550 kg. of water, were continuously fed into reaction vessel 3 from the reservoir 4.
  • Example IV A system as shown in FIG. 2 was used. 2000 liters of a solution containing 0.55% of lithium, 1.5% of sodium and 1.6% of fluorine were prepared in reservoir 2 and continuously transferred to vessel 1.
  • a suspension of sodium fluoaluminate was also fed into the reaction vessel 1.
  • the flow rates and titers were as follows:
  • the slurry which was continuously discharged from the reaction vessel 1, was filtered in the device 3, thereby yielding 2000 liters per hour of mother liquor, which was recycled, and 2000 kg. per hour of a slurry of moisture content 50%.
  • the apparatus was a Seifert spectograph with a Debye- Scherrer cylindrical chamber, having a diameter of 114.6 mm.
  • Conventional Seifert X-ray tubes were employed, using FeK, radiation, with an acceleration field of 30 kv. and a current of 16 ma.
  • Each specimen was prepared by finely grinding it to powder, in an agate mortar, and retaining the fraction passing through a 10,000-mesh/cm. sieve (Tyler N. 250).
  • Specimen A is lithium fluoaluminate
  • specimen B is the sample prepared according to Example 11
  • specimen C represents cryolite.
  • specimen B The diffraction pattern (see Table I) of specimen B is seen to be entirely different from the diffraction patterns of both specimen A, lithium fluoaluminate, and specimen C, sodium fluoaluminate (cryolite).
  • Specimen A yields the characteristic bright lines 4.13; 2.19; 2.13, and specimen C shows the bright lines 2.73; 2.22; 1.94, Whereas specimen B displays the characteristic bright lines 1.96; 4.28; and 2.21, not present in the other spectrographs, whose optically determined relative intensities are 100, and 80, respectively.
  • the spectrograph of specimen B conclusively shows that a different chemical composition is present, which is not a simple mixture of specimens A and C. Not only the relative intensities are different, but the remaining line spacings differ appreciably.
  • the mixture of specimens A and C gives lines approximating those of the table for the individual components after mixture and free from the characteristic bright lines of specimen B described above.
  • Example V TABLE II Percent by weight Sample A Sample B 50 g. of sample A (prepared in accordance with Example III) and 50 g. of sample B are weighed into calibrated platinum crucibles which are placed in a mufi le furnace preheated to a temperature of 400 C. The temv perature is raised to 700 C. in a period of 30 minutes and this temperature is maintained for 1 /2 hours, whereupon the product is cooled in a. dryer (e.g. desiccator) for 1 /2 hours and weighed. The losses due to calcination and other thermal losses are illustrated in Table III.
  • a. dryer e.g. desiccator
  • sample A prepared in accordance wtih the present invention, is far superior with respect to thermal stability against lithium losses and changes in the composition of the product.
  • the loss of lithium in the coprecipitate of this invention is about one quarter that of the mechanical mixture.
  • compositions from sodium, lithium, fluorine and aluminum which comprise these elements in the ratio desired in the electrolytic production of aluminum which are suitable for direct introduction into the electrolytic apparatus, and which can be introduced at subsequent stages during the process to replace losses.
  • the improvement which comprises the steps of coprecipitating a polycationic sodium, lithium, fluoaluminate composition of chemically bound sodium, lithium, aluminum and fluorine with a sodium content ranging between 16.4% and 32.4% by weight, a lithium content ranging between 0.26% and 5.14% by weight, an aluminum content ranging between 13% and 16.2% by weight, and a fluorine content ranging between 54.5% and 62.3% by weight of the coprecipitate from an aqueous medium containing at least one sodium salt, at least one lithium salt, at least one aluminum salt and at least one fluoride salt selected from the group which consists of lithium carbonate, lithium fluoride and lithium hydroxide, sodium carbonate, sodium hydroxide and sodium fluoride, aluminum oxide and aluminum fluoride, sodium aluminate, lithium aluminate, sodium fluoaluminate,
  • coprecipitate consists essentially of 24.63% by weight sodium, 3.22% by weight lithium, 13.81% by weight aluminum and 58.34% by weight fluorine.
  • coprecipitate consists essentially of 26.31% by weight sodium, 1.28% by weight lithium, 15.17% by weight aluminum and 57.24% by weight fluorine.
  • coprecipitatc consists essentially of 27.89% by Weight sodium, 1.93% by weight lithium, 13.40% by weight aluminum and 56.87% by weight fluorine.

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  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
US465113A 1962-01-30 1965-06-18 Method of operating an alumina reduction cell Expired - Lifetime US3382162A (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4417958A (en) * 1980-09-09 1983-11-29 Swiss Aluminium Ltd. Process for extinguishing the anode effect in the aluminum electrolysis process

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2075370A (en) * 1935-07-06 1937-03-30 Ig Farbenindustrie Ag Production of sodium aluminium fluoride
US2182510A (en) * 1938-07-29 1939-12-05 Aluminum Co Of America Production of double fluorides of alkali metals and aluminum
US2186433A (en) * 1936-11-17 1940-01-09 Firm Rutgerswerke Ag Process for the recovery of aluminum and fluorine compounds from the worn-out linings of the electric furnaces employed for the production of aluminum
US2196077A (en) * 1937-06-25 1940-04-02 Aluminum Co Of America Method of producing sodium aluminum fluoride
US2305921A (en) * 1938-03-30 1942-12-22 Eringer Josef Preparation of artificial cryolite
US2996355A (en) * 1958-09-12 1961-08-15 Reynolds Metals Co Process for the manufacture of sodium aluminum fluorides
US3128151A (en) * 1959-10-30 1964-04-07 I C P M Ind Chimiche Porto Mar Process for producing a sodium fluoaluminate composition having predetermined naf/alf3 ratio

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2075370A (en) * 1935-07-06 1937-03-30 Ig Farbenindustrie Ag Production of sodium aluminium fluoride
US2186433A (en) * 1936-11-17 1940-01-09 Firm Rutgerswerke Ag Process for the recovery of aluminum and fluorine compounds from the worn-out linings of the electric furnaces employed for the production of aluminum
US2196077A (en) * 1937-06-25 1940-04-02 Aluminum Co Of America Method of producing sodium aluminum fluoride
US2305921A (en) * 1938-03-30 1942-12-22 Eringer Josef Preparation of artificial cryolite
US2182510A (en) * 1938-07-29 1939-12-05 Aluminum Co Of America Production of double fluorides of alkali metals and aluminum
US2996355A (en) * 1958-09-12 1961-08-15 Reynolds Metals Co Process for the manufacture of sodium aluminum fluorides
US3128151A (en) * 1959-10-30 1964-04-07 I C P M Ind Chimiche Porto Mar Process for producing a sodium fluoaluminate composition having predetermined naf/alf3 ratio

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4417958A (en) * 1980-09-09 1983-11-29 Swiss Aluminium Ltd. Process for extinguishing the anode effect in the aluminum electrolysis process

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