US3518172A - Process for the electrolysis of aluminum chloride - Google Patents

Description

United States Patent O 3,518,172 PROCESS FOR THE ELECTROLYSIS OF ALUMINUM CHLORIDE Gilbert S. Layne and James O. Huml, Midland, Mich., and Richard Dale Smith, Madison, Conn., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Feb. 24, 1967, Ser. No. 618,335 Int. Cl. C22d 3/12 US. Cl. 204-67 8 Claims ABSTRACT OF THE DISCLOSURE -The molten salt electrolysis of aluminum chloride to produce aluminum and chlorine is improved by employing as at least one bath component an alkali or alkaline earth fluoride as a vapor pressure control agent. The elfective volatilization temperature of aluminum chloride is thereby significantly increased.

BACKGROUND OF THE INVENTION Aluminum is commercially produced by the Hall-Heroult process in which A1 dissolved in a molten c-ryolite bath is electrolyzed. In addition to the electrical energy required, the process requires the consumption of about 0.5 pound of carbon or graphite electrode per pound of aluminum produced. Oxides of carbon are consequently produced as valueless by-products.

Since the early days of aluminum technology, inventors have been seeking ways to electrolyze AlCl Although slightly more theoretical electrical energy is required, the electrolysis of AlCl does not require the consumption of carbon electrodes and, indeed, produces a valuable byproduct, chlorine gas. However, the practical use of AlCl to produce aluminum and chlorine has been thwarted by a number of problems. For instance, the vapor pressure of AlCl gas is high over melts at temperatures of greater than 660 C., the melting point of aluminum, such melts tend to creep and the electrical conductivity of melts containing A101 is usually poor.

SUMMARY OF THE INVENTION This invention relates to an improved process for the electrolysis of aluminum chloride and more particularly relates to a process \for the electrolysis of aluminum chloride in a fused salt bath with improved physical and electrochemical properties.

It is an object of this invention to provide an improved method whereby aluminum chloride may be retained in a fused salt electrolytic bath. It is a further ob ject of this invention to provide an improved process for the electrolysis of aluminum chloride. These and other objects and advantages of the process will be readily appreciated and become better understood by reference to the following detailed description.

It has now been discovered that a molten halide salt bath containing aluminum chloride may be electrolyzed above the normal volatilization temperature of aluminum chloride by the addition to such molten salt bath of certain alkali or alkaline earth metal fluorides as vapor pressure control agents. Mixtures containing these vapor pressure control agents must be molten or produce a molten mixture when contacted with aluminum chloride. The vapor pressure control agents are added in an amount suflicient to decrease the vapor pressure of the resultant molten salt bath to the desired level at electrolysis temperature. As used herein, the term halide refers to chloride or fluoride.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS As vapor pressure control agents, lithium fluoride, sodium fluoride, potassium fluoride, magnesium fluoride, calcium fluoride, strontium fluoride, and barium fluoride have been found to be effective but particular advantage has been found in the use of calcium fluoride. These alkali and alkaline earth metal fluorides may be used as vapor pressure control agents alone, mixed with one another, or as mixed salts with aluminum fluoride. Likewise, such fluorides may be used in combination with other standard fused salt bath components such as sodium chloride or calcium chloride. The proportion of such fluoride to be added to a fused salt bath of aluminum chloride will depend on the desired vapor pressure of the molten salt mixture at the selected temperature of electrolysis. In general, however, a concentration of A101 in the fused salt bath of up to about 20 percent is preferred. The proportion of fluoride ion required as a vapor control agent is at least 2 fluoride ions to 1 Al ion, preferably an atom ratio of fluoride ion to aluminum ion of from about 2:1 to about 10:1.

There are other factors to be considered in the optimization of the bath composition for AlCl electrolysis. It is usually desirable, for instance, to maximize the electrical conductivity of the bath and to minimize the fluorine content of the anode chlorine gas. It has been found beneficial to maintain a chloride to fluoride ion ratio in the bath of at least 2:1, preferably from about 2:1 to about 10:1. This insures that the fluorine content of the chlorine gas is at a low level. The selection of the cations required to fulfill the above conditions plus any further dilution of these baths with metal halides should be made on the basis of improvement in electrical conductivity and other physical properties of the molten salt bath. I

To illustrate, a favorable composition for A101 electrolysis is 8 mole percent AlCl 25 mole percent CaF and 67 mole percent CaCI This system, which melts below 700 C. has, at 975 C., a vapor pressure of about 7 mm. Hg and an electrical conductivity of about 2.6 reciprocal ohm-cm. These values compare favorably with those found in the -Hall-Heroult Al O -cryolite cell and are superior to those melts previously proposed for AlCl electrolysis.

Once the desired quantity of alkali or alkaline earth metal fluoride has been added to the fused salt bath and electrolysis started, aluminum chloride may be continuously added to produce aluminum and gaseous chlorine. While the mechanism of this process is not known with certainty, it is thought to proceed according to the following equations wherein M is an alkali or alkaline earth metal fluoride as defined above and X is the valence of such alkali or alkaline earth metal fluoride.

(2) electrolyses XAlFa 3MC1 XAl +2C C12 1 BMF The net effect of the reaction therefore is AlCl +electrolysis Al+ Cl 'f process can be batch or preferably continuous as taught in the patent art.

- 3 The following examples are provided as a more detailed description of the various embodiments of the invention but are not to be construed as limiting the scope thereof.

Example 1 A 1309 gm. portion of Na AlF was heated in a graphite crucible to 1000 C. in an argon atmosphere. Gaseous aluminum chloride was then added below the surface of the melt by passing it through a graphite tube immersed in the molten Na AlF During the addition of AlCl to the molten salt bath, no AlCl fumes were noticeable above the melt. After a total of 209 gm. of AlCl had been added, the bath was cooled and analyzed by Xray diffraction. Analysis showed about 40 weight percent AIF 40 weight percent NaCl and about 20 weight percent Na Al F Example 2 The following experiment was conducted to illustrate the exchange of halide ions:

To a graphite crucible in an inert atmosphere was added 56 gm. of Alland 58.5 gm. of NaCl. The salt mixture was then melted by heating to 900 C. Electrolysis was conducted at a potential of 3.0 volts using a centrally located graphite rod as the anode and the crucible as the cathode. Chlorine gas and aluminum metal were produced but no AlCl vapors were observed over the bath.

Example 3 In an inert atmosphere, gaseous AlCl was added to a pure CaF melt contained in a graphite crucible by introduction of the gas beneath the melt surface. Additions of AlCl began at a temperature of 1450 C. and were completed at 900 C. The final melt contained 25 mole percent AlC1 and 75 mole percent CaF The melt had a freezing point of about 675 C. and a vapor pressure of about 11 mm. Hg at 900 C. A decomposition potential of 2.1 volts was observed during a brief period of electrolysis and the products of electrolysis were identified as aluminum and chlorine.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that we limit ourselves only as defined in the appended claims.

We claim:

1. In a process for the electrolysis of aluminum chloride in a fused halide salt bath at a temperature above the normal volatilization temperature of said aluminum chloride, the improvement which comprisese employing, as at least one molten component of said fused salt bath, an alkali or alkaline earth metal fluoride vapor pressure control agent in an amount sufficient to increases the effective voltalization temperature of the aluminum chloride to above the temperature of electrolysis of the melt and maintaining in said fused salt bath an ion ratio of fluoride ion to aluminum ion of at least 2 to 1.

2. The process of claim 1 wherein the vapor pressure control agent is at least one member selected from the group consisting of LiF, NaF, KF, MgF CaF SrF and B3132.

3. The process of claim 1 wherein the vapor pressure control agent is calcium fluoride.

4. The process of claim 1 wherein the Vapor pressure control agent is Na AlF 5. The process of claim 1 wherein the ion ratio of chloride ion to fluoride ion is at least 2 to 1.

6. The process of claim 1 wherein the ion ratio of fluoride ion to aluminum ion is from about 2:1 to about 10:1 and wherein the ion ratio of chloride ion to fluoride ion is from about 2:1 to about 10:1.

7. The process of claim 1 wherein the fused salt bath consists essentially of from about 2 to about 12 mole percent A1Cl from about 19 to about 29 mole percent CaF and the remainder being CaCl 8. The process of claim 1 wherein the fused salt bath consists essentially of about 8 mole percent A1Cl about 25 mole percent CaF and about 67 mole percent CaCl References Cited UNITED STATES PATENTS 2,919,234 12/1959 Slatin 204-67 3,103,472 9/ 1963 Slatin 204-67 FOREIGN PATENTS 648,337 1/1951 Great Britain. 687,758 2/ 1953 Great Britain. 757,908 9/1956 Great Britain.

JOHN H. MACK, Primary Examiner DONALD R. VALENTINE, Assistant Examiner