DE1047450B - Electrolyte for the electrolytic deposition of aluminum - Google Patents

Description

The problem of the electrodeposition of aluminum under mild temperature conditions and with little power consumption is not yet solved satisfactorily.

Technical aluminum production uses smelting electrolysis exclusively at 800 to 900 ° C in an electrolyte consisting largely of molten cryolite. The power consumption is about 24 kWh / kg aluminum.

In principle, the same process is also used for electrolytic refining in the so-called "three-layer process" applied. The power consumption is slightly lower, but is still 2OkWh per kg.

A number of electrolytes have been proposed for electrolysis at lower temperatures, But none of them really caught on in practice.

In addition to certain complex compounds of aluminum chloride with organic bases, such as pyridine, certain purely inorganic electrolytes are mentioned in the literature, such as. B. sodium aluminum chloride in the melt or, more recently, a complex aluminum compound made up of lithium hydride and forms aluminum chloride in ethereal solution (D. E. Couch and A. Brenner, Journal Electrochemical Society, 99 [1952], p. 234).

So far nothing has become known about the use of such electrolytes on an industrial scale. It has now been found that, under certain conditions, electrolytes are excellent for deposition of aluminum under mild temperature conditions from real organic aluminum compounds can be built, d. H. from those compounds containing at least 1 carbon atom contained in direct bond to aluminum.

Aluminum trialkyls, aluminum dialkyl hydrides - AlR ₂ H -, aluminum alkyl halides such as AlR ₂ Cl, AlRCl ₂ and similar substances in which R is an alkyl radical are all not conductors of electrical current. But they can be converted into electrolytes by converting them into complex compounds, e.g. B. forms sodium ethyl with aluminum triethyl sodium aluminum tetraethyl - NaAl (C ₂ H ₅ ) ₄ . This actually shows very good conductivity in molten form. This has already been shown by F. Hein (Zeitschrift für inorganic Chemie, 141, pp. 161 to 226 [1924]), who obtained an oily product from sodium ethyl + aluminum triethyl and examined it for its conductivity, which he did as a solution of sodium ethyl in aluminum triethyl referred to, which, however, as we know today for sure, the just mentioned complex

Electrolyte for electrolytic
Deposition of aluminum

Applicant:

Dr. Dr. eh Karl Ziegler,
Mülheim / Ruhr, Kais er-Wilhelm-Platz 1

Dr. Dr. e. H. Karl Ziegler

and Dr. Hans-Herbert Lehmkuhl, Mülheim / Ruhr,
have been named as inventors

connection has been. F. Hein did not specify which element is deposited on the cathode during electrolysis. If one examines this, one finds that mainly sodium is formed at the cathode. In addition, some aluminum can also deposit, and the formation of spongy mixtures of sodium and aluminum on the cathode is often observed. The ratio of the amount of sodium and aluminum formed depends on the current density. At very low current densities, aluminum appears to be preferred. With technically usable current densities, however, you never get exclusively aluminum. This electrolyte cannot therefore be used for the deposition of aluminum. The same applies to a number of other such organic complex compounds, for example KAI (C ₂ H ₅ ) ₂ C1 ₂ ; NaAl (C ₂ H ₅ ) ₃ F; among others

To excellent electrolytes of high specific conductivity for the deposition of metallic However, aluminum is always used when using electrolytes, in addition to such complex compounds still contain excess organic aluminum compounds. Such electrolytes or electrolyte mixtures can by no means be created arbitrarily, for example by adding aluminum triethyl with any of the above complex compounds, because in many cases there are two in principle possible components of such a mixture are actually not homogeneous with one another miscible. For example, from the above sodium aluminum tetraethyl + aluminum triethyl do not create a homogeneous mixture or solution, as the two substances can reach high temperatures are practically completely insoluble in each other. In all the combinations, however, in which there is sufficient mutual miscibility or solubility, can also be observed in the

809 700/431

Electrolysis the separation of pure aluminum of excellent quality.

Particularly suitable electrolytes have been found to be homogeneous melts of true organic aluminum _{compounds of the general formula Al R (R ') 2} , in which R is alkyl, R' is alkyl, hydrogen or halogen, and their complex compounds with alkali compounds of the general formula MeR ', in which Me an alkali metal means, proven,

The electrolytes can be homogeneously melting compounds from the real organic ones Aluminum compounds of the given general formula with alkali metal compounds of the given act general formula.

Homogeneously melting mixtures of real organic aluminum compounds can also be used as electrolytes of the given general formula or homogeneously melting solutions of complex compounds of the general formula given with alkali alkylene, hydrides or halides and real organic aluminum compounds of the given general formula can be used.

For example, by dissolving 1 mol

Sodium aluminum diisobutyl dihydride

- Na [Al- (ISO-C ₄ Hg) ₈ H ₈ ] -

in aluminum triethyl a suitable electrolyte. This is the underlying complex compound very easily by adding sodium hydride to diisobutylaluminum hydride, according to the process of the patent 942 026 produced, won. Of course, you can also do the same in reverse Electrolytes made from sodium aluminum triethyl hydride and diisobutyl aluminum hydride or more simply by dissolving sodium hydride in a mixture of aluminum triethyl + diisobutylalumiuiumhydrid produce.

Fluorine-containing electrolytes have proven to be particularly suitable for mixing together electrolytes of the type mentioned proven organic aluminum complex compounds, e.g. B. forms that from sodium fluoride and 1 mol of aluminum triethyl formed

Sodium fluoride in a mixture of aluminum trimethyl + Manufacture aluminum triethyl. This is also a complex compound of 1 mole of sodium fluoride with 2 moles (here different from one another) aluminum trialkylene.

For the electrolyte mixtures described below, it is currently not possible to state which complex individual compounds are actually in the liquid electrolyte.

An excellent electrolyte has e.g. B. in the melt exactly or approximately the following composition:

NaF-2Al (C ₂ H ₅ ) ₃ -3 Al (C ₂ Hg) ₂ F.

It can also be done taking into account the course of the following reactions:

₃ + 2A1 (C ₂ H ₅ ) ₃ = 3A1 (C ₂ H ₅ ) ₂ F and
NaF + Al (C ₂ Hg) ₂ F = NaAl (C ₂ Hg) ₂ F ₂

Sodium aluminum triethyl fluoride - NaAl (C ₂ H ₅ ) ₃ F -

45

with a second mole of aluminum triethyl a new and well-defined second complex compound of the composition NaF-2Al (C ₂ Hj) ₃ , which has a very low melting point. The melt shows the specific conductivity of

3.3 · iO- ^s Ohm- ¹ cm- ¹ at 24 ° C and 15.0 · IO- ³ Ohm- ¹ cm- ¹ at 80 ° C and 42.5 · IO- ³ Ohm- ¹ cm- ¹ at 160 ° C.

and produces a dense, firmly adhering material during electrolysis Excellent quality aluminum. Similarly, an electrolyte can be dissolved by dissolving

KF-2A1 (C ₂ H ₅ ) ₃ NFAl (CH)

e ₈ y ₈ KF-1.5 A1 (C ₂ H ₅ ) ₃ NaF-2Al (C ₄ Hg) ₂ H "KF-2Al (C ₄ H ₉ ) ₂ H NaF-2 Al (C ₄ H ₉ ) _S KF-2Al (C ₄ H _S ) ₃ Na F • Al (C ₄ H ₉ ) ₂ H • Al (CH ₃ ) ₃ KF • Al (C ₄ H ₉ ) 2 H • Al (CH ₃ ) ₈ NaF-Al (C ₄ H ₉ ) ₃ -Al (CH ₃ ), KF-A1 (C ₄ H ₉ ) ₈ H-A1 (CH ₃ ) ₃ LiH-Al (C ₄ He) ₈ H-Al (C ₈ H ₅ ) , to write :

NaAl (C ₂ H ₅ ) ₂ F ₂ -2Al (C ₂ Hg) ₂ F-2A1 (C ₂ H ₅ ) ₃ or
NaAlF ₄ -4Al (C ₂ H ₅ ) ₃ ,

as can be easily seen by decomposing into

NaF + AlF ₃ + 4A1 (C ₂ H ₅ ) ₃

recognizes.

The same electrolyte can also be made from sodium fluoride, aluminum triethyl and aluminum diethyl fluoride be melted together. Another excellent electrolyte of this group is formed, if you boil finely powdered, dry cryolite with more than 6 moles of aluminum triethyl. The cryolite goes into solution. Two layers are formed, the upper one consisting of aluminum triethyl, the can be easily separated while the lower layer has the composition:

3 NaAlF ₂ (C ₂ Hg) ₂ + 4Al (C ₂ Hg) ₃

Similar electrolytes are listed in the table below. The compositions specified here do not need to be fulfilled precisely, but only approximately. The formulas in the table are written in such a way that you can see from which basic substances the electrolytes are mixed. If one wanted to express all formulas correctly in the chemical sense, one would have to combine 1 mole of the alkali halide or hydride and the like present in the electrolyte with 1 mole of the organic aluminum compound to form a complex of the type Me (AlR ₃ X). It can be seen from the formulas of the electrolytes listed below that in all cases more than 1 mol of the organic aluminum compound is available per mole of alkali halide hydride and the like, as corresponds to the essence of the process according to the invention:

NaH-Al (C ₄ H ₉ ), H • Al (C ₂ H ₅ ) ₈
AF ^ Al (CH)

CH ₃

ArF ^ Al (C ₂ Hg) ₃
3KCl-4Ai (CLH _s ), Cl

C4 JHg ^ = - C .tLj ~ * ~ "C H

CH _a

I 047 450

5 6

Many of the electrolytes described above can have aluminum facings on wires, at the expense The electrical energy to be used cannot be mixed with solvents at will or to a limited extent. Only such solvents come in.

Question which the organic aluminum compounds the new electrolytes can use in all the cases

do not decompose, e.g. B. Hydrocarbons, in particular 5 advantage, where it is applied to the absolute aromatic nature. Ether and tertiary separation of a very pure aluminum also matter, Amines, such as tetrahydrofuran, dimethylaniline, di- d. H. you can use them both for the Erzeuoxan, Pyridine and the like application of aluminum coatings, for example, on others

It is not advisable to dilute the elec- trometals, such as copper, or even on a basic trolyte by such solvents too far too io material of less pure aluminum or also drift, since the specific conductivity then drops for aluminum refining, in all cases electrolytic and it is therefore expedient to use a greater amount of elec- tricity in the presence of aluminum trischer Anode energy to separate the aluminum.

needed. In this case, provided that the current

Such a dilution may, however, be indicated, the density ^{remains below about 2 A / dm 2} , and the electrolytes, which are very sensitive to air, practically do not absorb losses of electrolyte. Work against it are borrowed to protect. Also affect type and z. B. with anodes made of copper, iron or platinum, the amount of solvent used on the gas evolution occurs at the anodes. The gas properties of the deposited aluminum, in particular, in the case of electrolytes containing ethyl, consist of a mixture of impermeability and hardness of the coatings. 20: gen of ethane and ethylene, and it will then be possible to gradually exhaust electrolyte combinations by using suitable solvents during the electrolysis, which occur without aluminum trialkyl because of the electrolyte in the limited miscibility and solubility added in excess. You can of course not use solvents such. B. compensate for the combination that from time to time the nation sodium aluminum tetraethyl + aluminum 25 necessary amounts of organic aluminum triethyl in the presence of a little toluene. connection again admits. When using.

Since the conductivity of the electrolyte with the tem- aluminum anode makes itself a similar gas temperature increases sharply, the electrolysis temperature development is only noticeable at a high current density, Temperatures of over 100 ° C up to a maximum of 200 ° C Otherwise, you will regularly see exactly that on the anode It is usually useful, if not necessary, to have an amount of aluminum that was found on the cathode. As diluents are divided into such.

In the case of high-boiling aromatics, such as methylnaphthalene, ordinary primary aluminum is used

Diphenyl ether, tetralin and the like are suitable. as the anode, the impurities in the aluminum

If one works, which is often advantageous, without miniums in the form of a black sludge inconspicuous diluent, then the dissolved can be returned. The sludge initially remains loosely adhering to the heated melts of the electrolytes against the anode, but dissolves very easily in the course of the electrolyte layered. But with paraffin oil \ something ielfach suspended in the electrolyte, they are all immiscible. But even if this precautionary measure is adhered to, it is also advisable to carry out the electrolysis in closed vessels and to interfere with the separation of the free space above the oil with an indiff - You can solve this difficulty in a very easy way

retentive protective gas, such as nitrogen, to fill. Deal wise by putting the anodes with one

It should also be mentioned that you can of course get close-fitting sachets out of an ordinary also surrounds several of the above-mentioned electrolytes in cotton fabric, making the anode more suitable Way can mix with each other, e.g. B. sludge can be completely retained. Such one also in the electrolyte protective devices for the anode are strange -

NaF-CAl (CH)) wise exceptionally long shelf life despite the relatively

^{2 5 3 2} high electrolyte temperatures of mostly 60

certain amounts of sodium aluminum tetraethyl to -50 to 150 ° C and even up to 200 ° C. solve without affecting the usefulness of the electrolyte. The clamping voltage is during electrolysis

suffer from. The still existing between parallel plates of the same size easily An excess of aluminum triethyl is sufficient. Keep the between 0.3 and 1 volts at a distance enumerated in detail above, for example. Elek- the plates of about 3 cm. The energy requirement per kg trolytes contain aluminum, which is excreted organically bound to aluminum, is among these niches Residues methyl, ethyl and 1-butyl. It stands between 0.9 and 3 kWh at 150 ° C. It goes without saying that you can get a multitude of similar The aluminum coatings are initially tight and

Usable electrolytes with other alkyl compacts, but with increasing thickness of the residue can build up, even with those of higher separated stratum gradually grainy and warty. Carbon number. This can offer certain advantages, since the aluminum can, however, be used up to one point Sensitivity to air, especially self-igniting thickness without disturbance of the cells through bridging, in the case of larger alkyl residues, the formation is reduced. Within the layer thicknesses that but understandable that with the enlargement of the surface finishing in question is the organic part of the electrolyte systems the special coating completely tight and smooth and firmly adhering, fish conductivities have to drop, as in the 65 provided that the surface of the base material is completely Unit of volume, there are fewer ions in total. has cleaned. The deposited aluminum is ge-Therefore the transition to such a higher degree offers a spectroscopic investigation of minde-alkyl Connections usually no special at least 99.999% even if one of habitual advantages. But there were exceptions to the rule that primary aluminum was used as the anode. given if there is such. B. in the production of thinner 70 for the production of aluminum coatings

I 047450

For metal surfaces, it may be advisable to use anodes made of ultra-pure aluminum. In this case it is it is unnecessary to protect the electrolyte against contamination by the anode sludge in the above, described manner. The purity of the aluminum produced in accordance with the specification is beyond the spectroscopic examination still emerges from the following experiments:

An aluminum coating formed on a bare steel sheet as a base was peeled off as a foil and cut into pieces about 2 square centimeters. Such a piece was poured over with 5 cm of 20% hydrochloric acid. For comparison it was the same Amount of aluminum sheet with 99.8% aluminum subjected to the same sample. In a comparison experiment the temperature in the acid rose to 72 ° C in 12 minutes and the aluminum had settled then completely dissolved with vigorous evolution of hydrogen. The electrodeposited aluminum did not result in any increase in temperature and remained largely undissolved in the hydrochloric acid for hours.

In another similar pair of experiments, the aluminum obtained by the new process was used compared to the best refined pure aluminum (99.99%). There was no increase in temperature in any of the two cases had to be observed, however The 99.99% aluminum is completely dissolved in 12 hours. The aluminum obtained according to the invention only showed a weight loss of about a third.

Copper sheets or copper wires, according to the invention Procedures were provided with an aluminum coating a few μ thick, can be kept in nitric acid for hours without the nitric acid attacking the Copper can be determined.

The particularly high purity of the aluminum obtained is apparently a result of the fact that all electrolytes made from distillable and also distilled organic aluminum compounds were manufactured. Apparently this is an extremely effective separation of normal impurities of aluminum is possible in a very simple way.

example 1

42 g of sodium fluoride and 260 g of aluminum triethyl are brought together under nitrogen and heated with stirring to 100 to 120 ° C. The solid salt dissolves, and two layers of liquid are obtained. The upper layer consists of almost pure aluminum triethyl, the lower one is an excellent electrolyte for the electrolytic deposition of aluminum and has the exact composition

NaF-2 Al (C ₂ Hg) ₃ .

The electrolysis is carried out at a current density of 0.7 A / dm ² at a terminal voltage of 1.0 volts and a bath temperature of 80 ° C. Firmly adhering aluminum coatings several tenths of a millimeter thick are obtained.

Example 2

58 g potassium fluoride and 284 g aluminum diisobutyl hydride

HAIl CH ₂ -C

are heated to 100 ° C. under nitrogen and with stirring. The salt dissolves and you get one homogeneous liquid that solidifies in crystalline form at around 80 ° C. During the melt electrolysis of this reaction product on the composition

KF-2HAl (iC ₄ H ₉ ) ₂

at a current density of 0.4 A / dm ² , which requires a clamping voltage of 1.4 Volts, ίο at bath temperatures of 100 ° C firmly adhering aluminum coatings are obtained.

Example 3

In a nitrogen atmosphere, 58 g of potassium fluoride are added in 198 g of aluminum triisobutyl

All CH ₂ -C

dissolved at about 100 ° C. 85 g of trimethyl aluminum are added to this reaction mixture with stirring and with heating to 80 to 90 ° C. Two layers of liquid form. The top layer has that composition

KF-Al (iC ₄ H ₉ ) ₃ -A1 (CH ₃ ) ₃

and is well suited for the electrical deposition of aluminum. At a bath temperature of 90 ° C., a terminal voltage of 1.2 volts is required to maintain a current density of 0.4 A / dm ^2.

Example 4

24 g sodium hydride, 142 g aluminum diisobutyl hydride

HAI CH ₉ -CC

'CH,

CH,

and 114 g of aluminum triethyl are combined in a nitrogen atmosphere and with stirring heated to 110 to 120 ° C. The solid sodium hydride dissolves. The resulting liquid reaction mixture has the composition

NaH • HAl (iC ₄ H ₉ ) ₂ • A1 (C ₂ H ₅ ) ₃ .

If one electrolyzes in this electrolyte at 80 ° C. and a current density of 0.5 A / dm ² , for which a terminal voltage of 2.6 volts is required, then firmly adhering, shiny aluminum deposits are obtained.

Example 5

^s / i Gfammol (= 55 g) potassium chloride and 1 gramol (= 120.5 g) aluminum diethyl monochloride - ClAl (C ₂ Hg) ₂ - ^; - ^: are brought together under nitrogen. The reaction mixture is heated

100 ° C, the potassium chloride dissolves smoothly. The result is a homogeneous liquid that cools down stiffens. During the melt electrolysis, this reaction product is deposited on the cathode solid aluminum precipitate. In the melt electrolysis of this reaction product from the composition

KCl-1.25

At a bath temperature of 140 ° C and a current density of 0.3 A / dm ² (clamping voltage 0.5VoIt), a solid aluminum deposit is deposited on the cathode.

Claims (12)

Patent claims: 1. Electrolyte for the electrolytic deposition of aluminum, characterized in that it ao from homogeneous liquid phases from real organic compounds of the general formula AlR (RO ₂ in which R is alkyl and R 'is alkyl, hydrogen or halogen, and their complex compounds with alkali compounds of the general formula MeR ' where Me is an alkali metal. 2. Electrolyte according to claim 1, characterized in that it consists of homogeneously tasting compounds consists. 3. Electrolyte according to claim 1, characterized in that it consists of homogeneously melting Mixtures consists. 4. Electrolyte according to claim 1, characterized in that it consists of homogeneously melting There is solutions obtained by adding solvents. 5. Electrolyte according to claim 1, characterized in that it contains fluorine-containing complex compounds, z. B. of the type NaF-
or NaF -Al (CH ₃ ) ,, - Al (C ₂ H ₅ ), contains. 6. Electrolyte according to claims 4 and 5, characterized by an addition of organic Solvents that do not decompose the organic aluminum compounds, such as hydrocarbons, especially aromatic, ether, e.g. B. tetrahydrofuran and dioxane, or tertiary Amines, e.g. B. dimethylaniline or pyridine. 7. Electrolyte according to Claims 1, 5 and 6, characterized in that it is mixed with paraffin oil is overlaid. 8. The method using an electrolyte according to claims 1, 5 to 7, characterized in that that one electrolyzes in an atmosphere of inert protective gases. 9. The method using an electrolyte according to claims 1, 5 to 7 and 8, characterized in that at temperatures between about 60 ¹ and 200 ° C, preferably about 100 to 150 ° C, is electrolyzed. 10. The method using an electrolyte according to claims 1 and 5 to 7 and according to claims 8 and 9, characterized in that one works with aluminum anodes. 11. Anode for performing the method according to claim 10, characterized in that they with a tightly fitting bag made of tissue, z. B. cotton, is surrounded. 12. The method using an electrolyte according to claims 1 and 5 to 7 one Anode according to claim 11 and according to claims 8 to 10, characterized in that the aluminum is deposited as a coating on other, optionally shaped metals. © 809 700/481 12.