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

Description

May 7, 1968 Reservoir /Mo1'her Liquor R. TRUPIANO ETAL METHOD OF OPERATING AN ALUMINA REDUCTION CELL Filed June 18, 1965 Reacfor Reacfor /Mo1'her Liquor Fi/frafion F ilfra fion Aluminum Electrolysis Bath ' Fig.1

Calcinafion Aluminum Electrolysis Bath Roberfo Trupiano Gr'dnpfi 0/0 Gambareffo I N VEN TOR.

Attorney United States Patent 3,382,162 METHOD 0F OPERATING AN ALUMHNA REDUCTTON CELL Roberto Trupiano and Gianpaolo Gambaretto, Mestre,

Venezia, Italy, assignors to Montecatini Edison S.p.A.,

Milan, Italy Continuation-impart of application Ser. No. 253,915, Jan. 25, 1963. This application June 13, 1965, Ser. No. 465,113

Claims priority, application Italy, Jan. 30, 1962, 1,737/62, l'atent 665,295; Feb. 1, 1962, 1,836/ 62, Patent 683,596

Claims. (Cl. 284-67) ABSTRACT OF THE DISCLQSURE In the electrolytic production of aluminum wherein aluminum oxide is dissolved in a lithium-containing electrolysis bath subject to losses of lithium, 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 amminate, sodium fluoaluminate, lithium fluoaluminate, hydrofluoric acid and fluoaluminic acid; drying and calcining the coprecipitate thus obtained at a term perature between substantially 400 and 550 C; and adding the calcined coprecipitate to the electrolysis bath in an amount at least suflicient to replace the lithium lost.

This application is a continuation-in-part of our copending application Ser. No. 253,915, filed Jan. 25, 1963, now abandoned.

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.

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 Although a voltage of 1.8 volts is theoretically sufiicient to initiate the electrolysis, a much higher voltage (i.e. 4.7 to 5 volts) is generally required in practice to overcome the various electrical resistances and voltage drops that are encountered in the cell, such as those due to polarization and, above all, the electrical resistance of the electrolyte. 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.

In commonly owned US. Patent 3,128,151, a process is described for the production of a sodium fluoaluminate composition with a molar ratio of sodium fluoride to aluminum trifluoride between 1.67 and 2.65. The composition described in that patent is characterized by a chemical bond between sodium, aluminum and fluorine and is designed to eliminate, at least in part, the necessity of separately using cryolite and free aluminum fluoride. The process is, however, relatively expensive because of the large energy requirements due mainly to the electrolyte resistance in the cell.

It is known that 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. By the use of these additives 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.

However, a few drawbacks are also caused by the presence of lithium in the electrolysis bath. For example, the solubility of aluminum oxide in the bath is decreased with increasing concentrations of lithium salts. Thus, e.g. in an electrolytic bath wherein lithium cryolite (Li AlF is substituted for the entire amount of sodium cryolite (Na AlF the solubility of alumina is so low that electrolysis becomes very diificult if not impossible.

Consequently, 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.

It is also known that changes occur in the composition of an electrolysis bath, in the long run, because of entrainment of components of the bath by the fumes and by the volatilization of components. This volatilization is due, above all, to the very high temperature, above 900 C., at which the electrolysis is conducted. These losses are usually on the order of 4 kg. of bath components per kg. of aluminum produced.

It is thus essential that the initial composition of the bath be periodically restored by suitable replenishment of its constituents. Under normal conditions, i.e. when the electrolysis is carried out in a bath consisting merely of sodium cryolite (Na AlF and aluminum fluoride (AlF periodic additions of both these substances to the cell are required. When lithium salts are also initially present, a periodic addition of the lithium salts must also be made.

This periodic addition of a simple lithium salt, to a bath which for the aforestated reasons consists mainly of sodium cryolite, usually results in significant drawbacks, owing, for instance, to unavoidable local variations in the lithium-salt concentration of the bath and the consequent localized periodic decreases in the alumina solubility; moreover, the use of the simple lithium salts leads ,to abrupt periodic changes in the electric conductivity and density of the molten bath occurring at the very spot at which the lithium salt is introduced. All the above phenomena will alter the equilibrium of electrolysis for a longer or shorter time, i.e. until the conditions are restored and until a uniform distribution of lithium in the molten bath is attained.

Another disadvantage resides in the large losses of lithium due to volatilization, which takes place when a single lithium salt is introduced into the cells, as well as in the unavoidable mechanical losses mentioned above. These lithium losses add heavily to operating costs, owing to the very high prices of lithium compounds.

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.

A further object is to prepare a composition which may also be used for periodic additions, in the course of the electrolysis, for the purpose of compensating for losses of the starting materials. Still another object is to provide a composition comprising sodium, aluminum, fluorine and lithium which is characterized by low volatility.

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.

Our invention is based on the discovery that it is possible to prepare 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. Although 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. The possibility cannot be excluded, however, that a true chemical compound is formed under the experimental conditions, described hereinuelow, of a composition having the formula of, say Li Na Al F Li Na Al F or multiples thereof. Although less likely, the composition of the invention could also be a mixture of compounds Li Na Al F and LisNagAlgplg, or other combinations of these four elements. Apart from the actual composition present, 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.

When the four elements sodium, aluminum, fluorine and lithium are in an ionized state together in solution with proper choice of the respective composition, the product which precipitates will be of the predetermined composition, whereas accidental achievement of a product in which the four elements are in the proportion desired for the electrolysis of aluminum, is not reproducible, and cannot constitute a process suitable for largescale industrial operations.

We have now found that it is possible to react the four elements under selected conditions, as to proportions and environmental parameters, so as to obtain a composition exactly 5 needed for the electrolytic production of aluminum.

F r the production of the sodium-lithium fluoaluminate according to this invention, 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.

In general, 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.

In accordance with the process according to our instant invention, 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.

For the purpose of better describing the invention, reference is made to the accompanying drawing in which 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. After the reaction has begun, during continuous operation, 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.

Given amounts of lithium, sodium and fluorine will remain dissolved in the mother liquor, depending on the temperature at which the precipitation occurs, and in general on all other reaction conditi 's. The mother liquors are all recycled to reaction vessel 1, by passing through the storage reservoir 2.

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 As an example, a product in which a=l, [2:437 and c= will contain 27.9% by weight Na, 1.93% by weight Li, 13.4% by weight Al and 56.8% by weight F, being essentially identical with a product with an Li AlF content of 15% and a Na AlF content of 85%.

A product in which a=1, 11:14.6, and 0:0 will contain Na=31.2%, Li:0.64%, Al=13% and F=55.2%, corresponding to a product containing 5% of Li AlF and 95% of Na AlF A product in which a: 1, b=6.l8 and 0:1.92 will contain Na=26.3%, Li=1.28%, Al=15.2% and F=57.2%, thereby conforming to a product with a Li AlF content of 10%, a Na AlF content of 80% and an AlF content of 10%. 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.

It is advantageous for the successful operation of the process to keep a balance between the amount of water, which is added to keep the lithium carbonate and the sodium carbonate in suspension, in container 4, and the amount of water which is formed during the reaction in vessel 1 from the interaction between aluminum hydroxide and hydrofluoric and, so that the total amount of water added is substantially the same as the amount which is removed when the precipitate of the desired product is filtered off. 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. Specifically, 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. Here 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'. Instead of aluminum hydroxide and hydrofluoric acid, 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.

After allowing suflicient time for precipitation of the desired product, i.e. sodium lithium fluoalurninate, 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'.

It is manifest that, in accordance with the process of our invention, all the starting ingredients are utilized and there is no waste, because the mother liquors are recycled and the expensive lithium compounds are reused.

For the purpose of further illustration, the following examples are set forth below:

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.

Owing to the exothermic nature of the reaction, the temperature in reaction vessel 1 reached about to C. The solution of fluoaluminic acid, i.e. aluminum fluoride in hydrofluoric acid, was allowed to flow into vessel 3 as in FIG. 1. 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.

The resulting slurry, containing the mixture of fluoaluminates thus formed, was filtered in the device 5.

Thus, 2000 kg. per hour of moist cake, with a moisture content of 5 0%, and 2000 kg. per hour of a mother liquor having a composition similar to that of the introduced mother liquor, i.e. Li=0.6 g./liter, Na=2.15 g./liter and F=2.55 g./liter, were recovered.

The filtered-off cake was dried and calcinated at 550 0, thereby yielding 1000 kg. per hour of a product which had the composition (by weight): Li=0.64%, Na=31.20%, Al=13.03%, F=55.l3%.

Example II The apparatus and the procedure were essentially as described in Example I.

2000 kg. per hour of a solution, having the following composition:

Li=0.60 g. per liter, Na=1.55 g. per liter, F=1.65 g. per liter, were continuously fed into the reaction vessel 1 from the reservoir 2. 399 kg. per hour of aluminum hydroxide and 626 kg. per hour of 98% hydrofluoric acid were also placed in vessel 1.

By virtue of the exothermic nature of the reaction, the temperature reached about 9095 C. The fluoaluminic acid resulting from the reaction was transferred into the reaction vessel 3. At the same time, 174.5 kg. per hour of 98.5% Li CO and 578 kg. per hour of 98% Na CO suspended in 560 kg. of water were continuously fed into the same reaction vessel from the tank 4.

A period .of two hours was satisfactory for the conditioning of the slurry in the reaction vessel 3, during which time the temperature was kept at about 40 C.

2000 kg. per hour of moist cake, of moisture content 50%, were filtered off from 2000 kg. per hour of a mother liquor having a composition essentially equal to that of the mother liquor which had been recycled. This cake, after drying and calcination at 510 C., gave 1000 kg. per hour of the required product, having a composition (by weight) as follows: Li=3'.22%; Al=13.81%; Na:24.63%; F=58.34%.

The mother liquor, recovered from the filtration at 5, was recycled into the tank 2, thereby consuming said solution.

7 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.

The fluoaluminic acid resulting from the reaction was transferred into the 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.

About two hours were required for the conditioning of the slurry in vessel 3. The temperature in vessel 3 was maintained at 40-45 C. The precipitate was then filtered through the device 5.

2000 kg. per hour of a mother liquor having a composition similar to that of the recycled mother liquor, i.e. Li=0.55 g. per liter, Na=1.50 g. per liter, F=1.60 g. per liter were obtained by filtration and were recycled into tank 2.

2000 kg. per hour of moist cake, of moisture content 50%, were filtered off. This cake yielded after calcination at 480 C., 1000 kg. per hour of the required dry product having the following composition: Li=1.28%, Na=26.31%, Al=15.17%, F=57.24%.

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:

Molar ratio NaF to AlF in the sodium fluoaluminate Flow rate of sodium fluoaluminate k-g. per hour Water flow rate do 800 The elemental analysis of sodium fluoaluminate gave the following results (by weight): Na=30.10%, Al=14.4,5%, F=55.45%. 73.9 kg. per hour of lithium fluoride, suspended in 200 kg. of water, were also continuously fed to reaction vessel 1. Two hours were required for the conditioning of turbid liquid in the reaction vessel. The temperature was kept at about 40 C.

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 latter, after drying and calcination at 500 C., gave 1000 kg. per hour of the required product, having the following elemental composition (by weight): Li=1.93%, Na=27.89%, Al=13.40%, F=56.78%.

For the purpose of determining the nature of the products prepared in accordance with the instant invention, X-ray crystallographic analysis was conducted and compared with samples of pure lithium fluoaluminate, sodium fluoaluminate, and a sample prepared according to Example II, which contained Li=0.6 g. per liter, Na=2.15 g. per liter, F=2.55 g. per liter.

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).

The powder was placed in a Lindemann glass tube having a diameter of 0.5 mm. The spectrograph was recorded on Ilford film with an exposure period of 1.5 hours. Specimen A is lithium fluoaluminate, specimen B is the sample prepared according to Example 11 and specimen C represents cryolite.

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.

TABLE I.-CRYSTALLO GRAPHIC DIFFRACTION PATTERNS [Lattice distances or constants of characteristic lines, Angstrom Units] Specimen "A Specimen B Specimen C 1 =Very bright. 2 =Brig 1 =Mod or atcly bright.

The compositions prepared, according to the other examples given earlier, which as indicated are not identical with the product of Example 11, show on spectrographic analysis the same characteristic lines as specimen B, while the characteristic lines of the lithium fluoaluminate and cryolite are absent in every instance.

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.

TABLE III Sample A Sample 13 Losses during calcinatiou, percent 2. 5 Total lithium content in the residue of the calcinated product, percent 1.252 1. 18 Loss of lithium, percent 2.20 8. 53

From the foregoing it will be apparent that 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. In fact, the loss of lithium in the coprecipitate of this invention (sample A) is about one quarter that of the mechanical mixture.

It will be manifest from the foregoing disclosure that this invention makes possible the preparation of 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.

We claim:

1. In a method of operating a plant for the electrolytic production of aluminum wherein aluminum oxide is dissolved in a lithium-containing electrolysis bath subject to losses of lithium therefrom, the improvement which comprises the step of: forming by precipitation from aqueous solution and drying a coprecipitate of chemically bound sodium, lithium, aluminum and fiorine, the sodium content ranging between 16.4% and 32.4% by weight, the lithium content ranging between 0.26% and 5.14% by weight, the aluminum content ranging between 13% and 16.2% by weight, and the fluorine content ranging between 54.5% and 62.3% by weight of the coprecipitate; and adding said coprecipitatc to said bath as the exclusive replacement of lithium.

2. In a method of operating a plant for the electrolytic production of aluminum wherein aluminum oxide is dissolved in a lithium-containing electrolysis bath subject to losses of lithium therefrom, 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, lithium fluoalurninate, hydrofluoric acid and fluoaluminic acid; drying and calcining the coprecipitate thus obtained at a temperature between substantially 400 and 550 C.; and adding the calcined coprecipitate to the electrolysis bath in an amount at least sufficient to replace lithium lost therefrom.

3. The method defined in claim 2 wherein said 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.

4. The method defined in claim 2 wherein said 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.

5. The method defined in claim 2 wherein said 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.

References (Zited UNITED STATES PATENTS OTHER REFERENCES lMellor, I. W.--A Comprehensive Treatise on Inorganic and Theoretical Chemistry, pp. 300-307, 1924.

HOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

G. KAPLAN, Assistant Examiner.

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