DE1250131B - - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

C22d

German KL: 40 c - 3/12

Number: 1 250 131

File number: J 23064 VI a / 40 c

Filing date: January 29, 1963

Opened on September 14, 1967

The invention relates to the use of a new product in the electrolysis baths for aluminum production.

The product consists of sodium lithium fluoroaluminate, in which sodium, lithium, aluminum and fluorine in appropriate predetermined percentages are included. It can be used directly in the electrolysis baths for aluminum extraction to improve the operating conditions of the baths and reduce their operating costs.

Today aluminum metal is obtained industrially by electrolysis of aluminum oxide (Al ₂ O ₃ ), which is dissolved in a bath of molten fluoride. These fluorides consist of cryolite (Na ₃ AlF ₆ ) and aluminum fluoride (AlF ₃ ). The aluminum oxide concentration in the bath is 2 to 6 percent by weight.

Theoretically i, 8 V are sufficient for electrolysis; practically however, a higher voltage (4.7 to 5 V) is required. The additional tension is used to overcome it the cell resistances caused by the anode effect, the electrode connections, the Busbars and the electrolyte resistance, which alone requires 1.9V. To get through Electrolysis to gain 1 kg of aluminum consumes around 18 kWh, of which around 40% are used Serve overcoming the electrolyte resistance.

It is known that the electrolyte resistance decreases when a lithium compound is added, such as, for. B. lithium caitonate, Lithium hydroxide, lithium fluoride or lithium cryolite, to the electrolysis bath. Through this the energy consumption for producing a certain amount of aluminum is reduced, so that gives a significant economic benefit. As · ■ further advantages can be the current density and thus the amount of aluminum produced per cell can be increased and the solidification temperature of the bath can be reduced. Due to the lower solidification temperature, the cells can be operated at lower temperatures, which means less thermal decomposition, higher currents, less anode consumption, etc. has the consequence.

The presence of lithium in electrolytic eleven also causes some disadvantages: The solubility of the animine oxide (A1 _S O ₃ ) decreases as the concentration of the lithium salt in the bath increases. When the sodium _{cryolite (Na 3 AlF 6} ) in an electrolysis bath is completely replaced by lithium cryolite, the solubility of the aluminum oxide is so low that electrolysis is hardly possible.

As a result, the electrolyte normally contains an "amount of lithium which can be obtained by adding 2 to 20 weight percent, preferably 3 to 8 weight use of sodium lithium fluoroaluminate as Bath component in aluminum electrolysis

Applicant:

Societä Edison, Milan (Italy)

Representative:

Dr.-Ing. H. Negendank, patent attorney, Hamburg 36, Neuer Wall 41

Named as inventor:

Roberto Trupiano,

Gianpaolo Gambaretto, Milan (Italy)

Claimed priority:
Italy of January 30, 1962 (737), of February 1, 1962 (1836)

percent lithium fluoride, i.e. 0.8 to 2.15 percent by weight lithium.

It is well known that the composition of an electrolysis bath changes over time, as components are lost due to dragging in the smoke and volatilization due to the high temperature (over 900 ° C). The losses are based on 4 kg of bath components for each ICO kg of aluminum produced. It is therefore necessary to maintain the initial bath composition by periodically adding the bath components. In normal electrolysis in a bath of sodium cryolite _{(Na 3 AlF 6} ) and aluminum fluoride (AlF ₃ ), these two components must be added to the cell periodically. If lithium salts are used in the presence, these must also be added periodically.

The periodic addition of a lithium salt to a predominantly sodium cryolite Bath leads to disadvantages because of the local changes in concentration at the points where the bath is added local and periodic reductions in alumina solubility occur in the lithium salt. With these solubility fluctuations there are periodic sudden changes in electrical conductivity and the density of the weld pool at this point. These phenomena disturb the electrolysis equilibrium until even conditions have returned, i.e. that

709 647/461

Lithium salt evenly distributed in the bath. Added to this is the further disadvantage that when one is added pure lithium salt to the bath, the volatilization and mechanical losses of lithium are very high lie. Because of the high cost of the lithium salts S, this is an aspect of the economics of the Procedure of great importance.

_{It is already known to produce a cryolite ionite of the formula Li 3} Na ₃ (AlFg) ₂ with a stoichiometric composition from aluminum hydrogen sulfite, hydrogen fluoride, lithium and sodium chloride obtained by digesting silica minerals with sulphurous acid. The use of this product in electrolysis baths for aluminum production has not yet been proposed, and its lithium content is uneconomically high for this purpose.

The object of the invention is to create a bath component in electrolysis baths for Aluminum extraction, the aforementioned disadvantages of bath operation through their direct use be eliminated.

According to the invention, the problem is solved thereby that as a bath component in the aluminum electrolysis baths Sodium lithium fluoroaluminate in a composition of 16.4 to 32.4 percent by weight Na, 0.26 to 5.13 weight percent Li, 13.0 to 16.2 weight percent Al, and 54.5 to 62.3 weight percent F is used.

This sodium lithium fluoroaluminate is not a mechanical one Mixture of the corresponding simple fluoroaluminates or the corresponding cryolites, but a chemical compound with special properties, especially special lattice properties. This finding is of crucial importance as the invention is carried out by mechanically mixed sodium and lithium fluoroaluminate is not dissolved. Rather, it comes on using a Lithiumnatnumfluoroaluminat, which by co-precipitation of the starting materials was obtained under the conditions described below. If this product is radiographically is examined, the spectrum yields lines that appear in the spectra of the compounds Sodium fluoride, lithium fluoride, aluminum fluoride, sodium cryolite and lithium cryolite not found are.

The product to be used according to the invention corresponds to the general formula

Na ^ Lij / AlsFK

The composition can also be indicated in the following way:

0Li ₃ AlF ₆ -ANa ₃ AlF ₆ -CAlF ₃

A product in which a = 1, b = 4.37 and c - 0 contains 27.9% Na, 1.93% Li, 13.4% Al and 56.8% F and corresponds to a product with 15 % Li ₃ AlF ₆ .

A product in which a = 1, b = 14.6 and c = 0 contains 31.2% Na, 0.64% Li, 13% A1 and 55.2% F and corresponds to a product made from 5% Li ₃ AlF ₆ and 95% Na ₃ AlF ₆ .

An article in which a - \, ό = 6.18 and c = 1.92 contains 26.3% Na, 1.28% Li, 15.2% Al and 57.2% F and corresponds to an article from 10% Li ₃ AlF ₆ , 80% Na ₃ AlF ₆ and 10% AlF ₃ .

55

60 The advantages of using these fluoroaluminates in an electrolytic cell operating in the presence of lithium are numerous and obvious.

The product can have the same composition as the melt, so that when it is used no local changes in the bath composition to supplement the volatilization losses occur and therefore avoid all the disadvantages that occur when using pure lithium salts.

The management of the cell is simplified because the bath returns to its initial state with a single treatment is brought without having to dose different components separately. The mechanical Losses, especially of the expensive lithium component, are greatly reduced, since in the product used according to the invention the percentage of lithium is always lower than that of pure salts. As a result, the weight will be the same of the lost product, the loss of lithium will be less. To further reduce the mechanical Losses on introduction into the cells are contributed by the fact that the product used is usually is in granular form, while pure lithium salts are generally powdery. The lithium losses due to thermal volatilization are also lower than when used separately Sodium cryolite and lithium salt, since the lithium in the lithium sodium fluoroaluminate to be used according to the invention is chemically bound to the other elements of the product is bound, so that the product has its own melting point, rapidity and has solution uniformity in the weld pool.

For the production of lithium sodium fluoroaluminate the following compounds can be used as starting materials: hydrogen fluoride, aluminum hydroxide, Aluminum oxide, aluminum fluoride, sodium aluminate, lithium aluminate, sodium fluoroaluminate, lithium fluoroaluminate, Sodium carbonate, sodium hydroxide, lithium carbonate, lithium hydroxide, sodium fluoride and lithium fluoride.

It is well known that when mixing various simply dissolved reagents, such as lithium, sodium, Aluminum and fluorine contain a precipitate, considering its composition The extent of the possible reaction equilibria cannot be foreseen. These balances can by changes in the reaction temperature, the reagent concentration in the solution, the Types of treatment, etc., can be influenced considerably and in an unpredictable manner. The below The method described eliminates these disadvantages in a simple and immediate manner and ensures that the composition and stability of the product determined in advance are achieved without any uncertainty or dependence on the reaction equilibria. According to this process, the reagents are added in the desired proportions with each other for the implementation, the mutual relationships between those contained in the reagents Elements lithium, sodium, aluminum and fluorine are the same as those in the present invention to use Lithiumnatnumfluoroaluminat wants to achieve. The implementation is preferably carried out between 30 and 60 ° C in a »closed system«, whereby only that which emerges from the system is Forms lithium sodium fluoroaluminate.

5 6

For the purposes of explanation, a possible single leaking product, the lithium sodium

Procedural scheme described. It is made from aluminum fluoroaluminate. The lithium sodium fluoroaluminate

hydroxyd, anhydrous hydrogen fluoride, lithium is so forced to use the four components lithium,

carbonate and sodium carbonate. Sodium, aluminum and fluorine in the same

According to Fig. 1, proportions are to be contained in the agitator, such as those in the one

see reactor 1 continuously aluminum hydroxide, fed raw materials occur

anhydrous hydrogen fluoride and those of the In order to emphasize the validity of the above,

Lye container 2 incoming mother liquor fed. Another method is shown in Fig. 2,

The aluminum hydroxide dissolves in the fluorine - the same using other raw materials

hydrogen and forms an acidic solution of io conditions is sufficient.

Aluminum fluoride. The reaction is exothermic; In the reactor 1 are sodium fluoroaluminate

The temperature of the solution obtained is between 80 and has a predetermined composition (Her-

and 120 ^° C. From reactor 1, the fluorine position according to Italian patent 630 569)

hydrogen acidic aluminum fluoride solution in the and coming from the tub 2 mother

Neutralization conditioning system 3, into which a 15 lye is continuously fed. In the same

Aqueous suspension of a reactor prepared in container 4 becomes an aqueous suspension prepared in container 3

mechanical mixture of lithium carbonate and lithium fluoride suspension introduced continuously.

Sodium carbonate is introduced continuously. The temperature of the reactor is between 40 and

Neutralizing system is maintained by a convenient 8O ⁰ C. After a sufficient condition

System that removes heat from the solution for a period of time, the slurry on the filter 4 is filtered.

Temperature between 30 and 6O ⁰ C kept. The one in the cake made from lithium sodium fluoroaluminate is made

the herkommenden from the reactor 1 contained calcined solution and dried at 400 to 550 ⁰ C. the

Hydrogen fluoride is returned to the neutralization plant. Mother liquor is returned via the tub 2

with lithium carbonate and sodium carbonate new- transported into the reactor and reused,

neutralized, and by simultaneous reaction with the 25 The process for producing the according to the invention

present aluminum fluoride form the carbonates to be used in the following product

the precipitate of the desired lithium sodium examples explains,
fluoroaluminate. This precipitate is on the

Filter 5 separated from the mother liquor, then dry- Example 1

net and at a temperature between 400 and 55O ⁰ C 30 It should lithium and sodium fluoroaluminate with

calcined. The mother liquor still contains a certain amount of the following composition:
Amount of dissolved lithium, sodium and fluorine.

These quantities depend on the precipitation temperature - ^ i "» "^ / 0

and the other reaction conditions. The - ^ a 31.20 / ₀

Mother liquor is again 35 ^ l 13.03 / _{0 through the tub 2}

conveyed into the reactor 1 so that the contained ? 55.13 / ₀

Amounts of lithium, sodium and fluorine are prepared at the same time.

weight to form the necessary buffer and everything to do with. For this purpose, with reference to Fig. 1

The lithium, sodium and aluminum used were placed in a 5 m ^{3 reactor lined with hard rubber}

and fluorine quantitatively as lithium sodium fluorine- 40 content hourly 20001 from storage container 2 of

aluminate can be precipitated. the filtration of the pulp in the apparatus 5 originating

After this, the only one from the system is fed from mother liquor. The composition of the

The resulting product is lithium sodium fluoroaluminate, mother liquor is the following:
which consequently introduced the whole into the system

Lithium, sodium, aluminum and fluorine contain 45 r! ? 1 ς / 1

got to. Subsystem in this case all the pros ^ ^a 2.15 g / l

Understand the course of the procedure described. Did- ' &'

neutrally the mother liquor with all in them will be in the reactor, also continuously,

dissolved components kept in circulation. Around 376 kg of aluminum hydroxide and 593 kg per hour

Volume of the circulating mother liquor / fed constantly to 50 98 ° _cent, hydrofluoric acid. As a result of

hold, it is sufficient that the new exothermic effect introduced into the system arises the reaction

Amount of water (which is used to suspend the lithium temperature in the reactor to a value of about

and sodium carbonate is necessary) so to regu- 90 to 95 ⁰ C a. In this way one is formed

lieren that this amount of water plus the by solution of fluoroaluminic acid, which by a

Reactions, the amount of water produced is the same and 55 overflows into the reactor 3, which is lined with hard rubber

never larger than that from the system in the form of 5 m ³ , into which the prepared in apparatus 4

Moisture is the amount of water applied to the suspension of lithium and sodium carbonate

the cake of lithium sodium fluoroaluminate is dehydrated.

hold is. In this way, all losses will also be the amount of the suspension supplied as mentioned

Excluded from reagents, the lithium 60 alkali carbonate is the following:

as a result of the high cost of this element, very much after- _. _, _,. "",. . -..-, "

would be part. Li ₂ CO ₃ (98.5 ° _/? Ig) 34.7 kg / h

By appropriately changing the order of S ^a A ^C ° ³ ^ ⁹⁸ ° ^ ° ^ig ^ Ill

All compounds ^H 2 ° ⁵⁸³

of lithium, sodium, fluorine or comparable aluminum 65 The temperature is 4O ⁰ C, and the Konditiowendet be, which allow the reaction and / or nierungszeit the suspension is about 2 hours. The sequence of different reactions in the pulp, which bring about the mixture of the fluorine-closed circuit thus formed, so that the aluminate contains, is nitrated in the filter 5.

In this way, every hour 2000 kg of moist cake (humidity = 50 ° / ₀ ) and 2000 kg of mother liquor are separated, which has the same composition as that of the feed mother liquor, ie

Li. 0.6 g / l

Na 2.15 g / l

F 2.55 g / l

10

The filtered cake is dried and calcined at 55O ⁰ C; after calcination, 1000 kg of product per hour with the desired composition are obtained, namely:

Li 0.64%

Na 31.20%

Al 13.03%

F 55.13%

Example 2

It is said to have lithium and sodium fluoroaluminate with the following composition:

Li 3.22%

Na 24.63%

Al 13.81%

F 58.34%

be prepared.

The procedure described in Example 1 is followed, using the same apparatus. Every hour 2000 kg mother liquor is fed into reactor 2, which has the following composition:

Li 0.60 g / l

Na 1.55 g / l

F 1.65 g / l

together with 399 kg of aluminum hydroxide and 626 kg of 98% hydrofluoric acid per hour. Due to the exothermic effect of the reaction the temperature is adjusted to a value of about 90 to 95 ⁰ C. The fluoroaluminic acid formed by the reaction passes through the overflow into reactor 3, into which 174.5 kg of Li ₂ CO ₃ (98.5%) and 578 kg of Na ₂ CO ₃ (98%) are fed per hour, suspended in 560 kg of H ₂ O. This suspension comes from the apparatus 4, from which it is drawn off through the overflow. The conditioning time of the slurry in the reactor is about 2 hours, and the temperature is 4O ⁰ C.

By filtration, every hour 2000 kg mother liquor, which has the same composition as the circulating mother liquor, is separated from 2000 kg moist cake (moisture = 50%), which after drying and calcination at 510 ^° C. is reduced to 1000 kg / h of the desired product , which has the following composition:

Li 3.22%

Al 13.81%

Na 24.63%

F 58.34%

The mother liquor, which comes from the filtration, is put back into circulation in the storage container 2.

Example 3

It is said to be lithium and sodium fluoroaluminate with the following element composition:

Li 1.28%

Na 26.31%

Al 15.17%

F 57.24%

be prepared.

The procedure described in Example 1 is followed and the same equipment is used.

Every hour 2000 kg mother liquor is fed into reactor 2, which has the following composition:

Li 0.55 g / l

Na 1.50 g / l

F 1.60 g / l

Furthermore, 438 kg of Al (OH) ₃ and ao 616 kg of 98% strength hydrofluoric acid are added per hour. The temperature adjusts to 95 ° C. The fluoroaluminic acid that forms as a result of the reaction passes through the overflow into reactor 3, which is continuously fed: 69.2 kg of Li ₂ CO ₃ (98.5%) and 618 kg of Na ₂ CO ₃ (98%) per hour , suspended in 550 kg of water. This suspension comes from the apparatus 4, from which it is drawn off through overflow. The conditioning time of the pulp in the reactor is about 2 hours, and the temperature adjusts to 40 to 45 ^° C.

Filtration gives 2,000 kg of mother liquor per hour with the same composition as Feed mother liquor, namely:

35

Li 0.55 g / l

Na 1.50 g / l

F- 1.60 g / l

The above-mentioned mother liquor is put into circulation again in the storage container 2. By filtration hour 2000 kg wet cake are (humidity = 50%) produced, which after preliminary calcination at 48O ⁰ C to 1000 kg / h, the desired product of the following composition:

Li 1.28%

Na 26.31%

Al 15.17%

F 57.24%

to reduce.

Example 4 (see Fig. 2)

Every hour 1000 kg of lithium and sodium fluoroaluminate with the following elemental composition should be used:

Li 1.93%

Na 27.89%

Al 13.40%

F 56.87%

be prepared.

For this purpose, 2,000 liters of mother liquor per hour, which come from the filtration of the pulp in the apparatus 4, are fed into a hard rubber reactor with a capacity of 7 m ^{3 from the storage container 2.}

The composition of the mother liquor is as follows:

Li 0.55%

Na 1.5%

F 1.6%

A suspension of sodium fluoroaluminate is also fed into the reactor. The services and the titles are as follows:

NaF: AlF molar ratio of sodium fluoroaluminate 2.46

Sodium fluoroaluminate feed rate 928 kg / h

Feed rate of water 800 kg / h

The slurry that flows continuously from the reactor 1 is filtered in the filter 3; In this case, 2000 l mother liquor per hour, which are recirculated, are separated from 2000 kg of moist cake (moisture = 50%), the cake after drying and calcination at 500 ^° C. to 1000 kg / h of the desired product of the following composition:

Li 1.93%

Na 27.89%

Al 13.40%

F 56.78%

reduced.

Claims (1)

Claim: The analysis of the sodium fluoroaluminate is as follows: Na 30.10% Al 14.45% F 55.45% 73.9 kg of lithium fluoride, suspended in 200 kg of water, are also fed into the reactor every hour. The conditioning time of the pulp in the reactor is 2 hours, the temperature is 40 ^° C. Use of sodium lithium fluoroaluminate in a composition of 16.4 to 32.4 percent by weight Na, 0.26 to 5.13 weight percent Li, 13.0 to 16.2 weight percent Al, and 54.5 up to 62.3 percent by weight F as a bath component in electrolytic baths for aluminum extraction. Considered publications:
German patent specification No. 649 818. 1 sheet of drawings 709 647/461 9. 67 © Bundesdruckerei Berlin