DE1044420B - Process for the liberation of chloride-fluoride baths for the melt-flow electrolysis of undesired oxides - Google Patents

Description

GERMAN

Chloride-fluoride baths are continuously used for melt flow electrolysis, as they are sufficient have high electrical conductivity, but these baths often have the disadvantage that they contain oxides do not dissolve, which then accumulate locally and interfere with electrolysis.

The electrolytic refining of certain metals is carried out using the three-layer process: A layer of a heavy alloy containing the impure metal to be refined serves as the Anode and lies in the liquid state at the bottom of the crucible of the electrolysis cell. She is covered by a layer of molten electrolyte on which floats the refined metal that makes up the cathode represents.

The electrolyte usually consists of a mixture of chlorides and fluorides, in which a notable proportion of a halogen compound of the metal to be refined is contained to a deposit to prevent alkali or alkaline earth metals at the cathode, their salts in general represent most of the electrolyte.

For example, when refining aluminum, a bath is used that contains 60% BaCl ₂ and 40% chiolite (2AlF ₃ -3NaF), with some sodium chloride being added to the mixture.

It has also been suggested a sodium-free, barium chloride, calcium fluoride and aluminum fluoride containing bath, with the proportion of the latter compound between 25 and 35% can fluctuate.

In the course of the electrolysis, salts must be added periodically to improve the composition of the bath to be kept within the limits in which refined metal can be obtained at the cathode. the Additives include aluminum fluoride with one that is always present due to the way the fluoride is made certain free clay content. The residual moisture in this salt leads to hydrolysis reactions of the following general type:

Procedure for exemption

of chloride-fluoride baths

for melt flow electrolysis

of unwanted oxides

Applicant:

Pechiney, Company de Produits

Chimiques et Electrometallurgiques,

Paris

Representative: Dr.-Ing. F. Wuesthoff, Dipl.-Ing. G. Pulse

and Dipl.-Chem. Dr. rer. nat. E. Frhr. v. Bad luck man,

Patent Attorneys, Munich 9, Schweigerstr. 2

Claimed priority:
France of March 30 and July 6, 1955

Emile Pruvot, Hermillon, Savoy (France),
has been named as the inventor

2 AlF ₃ + 3 H ₂ O = Al ₂ O ₃ + 6 HF.

In the course of the reaction, this alumina precipitates on the anode, giving rise to the ohmic resistance. the contact points of the two layers enlarged; the alumina also attaches itself to the electrolysis cell all around and forms very alumina-rich crusts in which up to 50% Al ₂ O _{3 was} found; cffes leads to a narrowing of the cell cross-section and thus to an increase in the current density, which is another cause of the increase in the voltage between the poles of the cell.

For all these reasons, free clay, whether it is present from the outset or formed secondary, is highly undesirable.

The method according to the invention helps avoid these disadvantages by using the electrolysis or correction tape free of oxygen compounds. The procedure is that you can take the bath with you treated with a substance that has a strong fluorinating effect on the oxidic compounds and convert them to fluoride. Gaseous hydrofluoric acid can be used for this purpose, however particularly good results can be achieved with the acidic fluorides of the alkali and alkaline earth metals.

According to a preferred embodiment of the invention, acidic ammonium fluoride is added to the bath components (Ammonium bifluoride), which disappears completely during the course of treatment.

The fluorinating effect of ammonium fluoride begins at 200 ° C, a temperature that is by far sufficient to dehydrate most of the halides that make up the bath. The effect of the sour Fluorides can be extended up to 600 ° C with advantage.

It was found that certain chlorides partially increased during the implementation of the process Fluorides can be converted, which is sometimes disadvantageous.

809 679/321

In this case one preferably excludes those chlorides which tend to be included in the electrolysis transferring unwanted fluorides from treatment with a fluorinating agent. Such Chlorides can be separated - if necessary in vacuo be dried before treating them with the compounds that have been subjected to fluorination were mixed.

The following example serves to explain the invention, without restricting them.

example

When aluminum is continuously refined using a bath of 60% BaCl ₂ and 40% chiolite (2 Al F ₃ · 3 NaF), compensating additives containing AlF ₃ , BaCl ₂ and NaCl must be added periodically. When fluorinating with NH ₄ F-HF, part of the barium chloride is converted into barium fluoride, the presence of which changes the density, melting point and selectivity of the bath and can make it unsuitable for aluminum refining. You therefore proceed as follows:

10 parts of aluminum fluoride are mixed with 3 parts of ammonium bifluoride NH ₄ F-HF. The mixture is heated to 400 to 500 ° C. with stirring, being careful initially to avoid drifting off. The end of the reaction is indicated by a decrease in foaming. The heating is preferably carried out in a graphite crucible, the mixture to be heated being added in successive partial quantities in order to avoid excessive forceful stripping. An aluminum stirrer can be used.

When the mass stops bubbling, the hot aluminum fluoride is poured into an aluminum container, in which it can cool down. As soon as it is cold, it is immediately mixed with the other corrective salts, BaCIg and NaCl, mixed previously at 120 ° C had been carefully dried.

In this way an additional, oxide-free bath is obtained, which leads to excellent results in aluminum refining.

The method according to the invention is not limited to aluminum refining; it can be in everyone Case in which the presence of an oxide compound is undesirable, namely just as in metal extraction by electrolysis as in the refining of metals.

The process is particularly useful, for example, for deoxidizing baths, such as those in the electrolytic preparation or refining of titanium and zirconium.

The refining bath generally consists of sodium chloride or a mixture of NaCl with other alkali or alkaline earth chlorides, e.g. B. KCl.

According to the invention, Na Cl or Na Cl and KCl are dry mixed with about 10'Λ NH ₄ F-HF; the mixture is heated to 300 to 500 ° C and the treatment is continued in the same way as for the refining of aluminum. A small amount of the chlorides are converted into the fluorides, which does not interfere with the refining in any way.

In this way titanium or zirconium are obtained in the form of crystals with a larger diameter and have a lower hardness than those obtained from baths which do not undergo treatment with ammonium bifluoride.

Claims (3)

PATENT CLAIMS: 1. Procedure for the liberation of the preparation or refreshment of chloride-fluoride baths for salt mixtures of undesired oxides used for melt-flow electrolysis, characterized in that, that one treats the mixtures or their oxidative constituents with a substance that on the oxides have a strong fluorinating effect or convert them into fluorides. 2. The method according to claim 1, characterized in that the fluorinating agent used is gaseous hydrofluoric acid, an alkali or alkaline earth bifluoride, but preferably ammonium bifluoride (NH ₄ F-HF), which latter is allowed to act on the salt mixture between 200 and 600 ° C. 3. The method according to claim 1 and 2, characterized in that the optionally in the Halides of which contain salt mixtures and tend to convert into undesired fluorides excludes the fluorination treatment. Considered publications:
KA Hofmann: Textbook of Inorganic Chemistry, 1924, p. 227. ίΟΪ 675/321 11.58