US3630859A - Electrolytic cell bath composition for production of magnesium - Google Patents

Abstract

AN ELECTROYLTIC CELL BATH COMPOSITION FOR PRODUCTION OF MOLTEN MAGNESIUM CONTAINS ABOUT 6.36.5 PERCENT, BY WEIGHT, OF MAGNESIUM CHLORIDE WITH ESSENTIALLY THE REMAINDER BEING LITHIUM CHLORIDE AND BARIUM CHLORIDE IN A WEIGHT RATIO TO EACH OTHER SUFFICIENT TO PROVIDE THE COMPOSITION WITH IMPROVED ELECTRICAL CONDUCTIVITY AND A SPECIFIC GRAVITY HIGH ENOUGH TO CAUSE MOLTEN MAGNESIUM FORMED THEREIN DURING ELECTROLYSIS TO FLOAT THEREON. A WEIGHT RATIO OF BARIUM CHLORIDE TO LITHIUM CHLORIDE OF AT LEAST ABOUT 1:7.5 IS EMPLOYED.

Description

United States Patent 3,630,859 ELECTROLYTIC CELL BATH COMPOSITION FOR PRODUCTION OF MAGNESIUM James G. Macey, Salt Lake City, Utah, assignor to Pete Prestininzi, Long Beach, Calif., and Ruth G. Macey and Anne M. Macey, with rights of survivorship, fractional part interest to each No Drawing. Filed Feb. 16, 1970, Ser. No. 11,832

Int. Cl. C22d 3/08 US. Cl. 204-70 7 Claims ABSTRACT OF THE DISCLOSURE An electrolytic cell bath composition for production of molten magnesium contains about 636.5 percent, by weight, of magnesium chloride with essentially the remainder being lithium chloride and barium chloride in a weight ratio to each other sufficient to provide the composition with improved electrical conductivity and a specific gravity high enough to cause molten magnesium formed therein during electrolysis to float thereon. A weight ratio of barium chloride to lithium chloride of at least about 1:75 is employed.

BACKGROUND (1) Field of the invention This invention relates to electrolytic cell bath compositions, particularly those utilized for the production of molten magnesium from magnesium chloride.

,(2) Prior art Magnesium metal is produced most economically by the electrolysis of molten magnesium chloride that exists in an electrolytic cell as a portion of a molten mixed salt bath, the other salts in this molten bath mix or melt being, conventionally, chlorides of sodium, potassium, and/or calcium. Electrical conductivity of such a melt is so low as to interpose severe restrictions on the possible distance between the anode and the cathode of the cell. If a wider spacing could be used between the anode and cathode, a stronger and more rugged cell design having a much longer operating life could be employed.

It has recently been discovered that a high content of lithium chloride in the magnesium electrolytic cell bath, preferably upwards of 60 percent by weight, more than doubles cell conductivity, reducing power requirements, per unit of magnesium metal produced, by more than one third. Anode to cathode distance can be much greater, permitting a far stronger cell design with a much longer operating life before a complete rebuilding is necessary. However, lithium chloride in high concentrations in such cells lowers the specific gravity of the cell bath to such an extent that the molten magnesium produced sinks to the bottom, mixes with oxide sludge conventionally present and requires special means for recovery in purified form. It would be highly desirable to lower capital plant costs, production costs and plant maintenance costs by providing an improved way of obtaining magnesium electrolytically from magnesium chloride.

SUMMARY OF THE INVENTION Such an improved way has now been discovered. It employs the improved electrolytic cell bath composition described in the abstract above. The composition is readily usable in a cell of conventional design which keeps chlorine rising out of the cell melt at the anode from mixing with magnesium metal forming at the cathode. Such a feature is, conventionally, a refractory semi-wall of, for example, silica brick or the like, extending from the roof of the cell down into the melt, preferably by several inches.

Patented Dec. 28, 1971 A substantially thicker semi-wall than the ones now conventionally used can be employed, because of the greater cell bath composition conductivity (more than double the conventional value for cells containing the usual mix of chlorides of sodium, potassium and/ or calcium along with magnesium). Accordingly, such a cell has a longer operating life before it becomes necessary to shut down the cell and rebuild the wall. Both maintenance costs and cell downtime are thereby minimized.

Due to the doubled electrical conductivity of the cell, electric power requirements, per pound of magnesium metal produced, are reduced by more than one third.

Of considerable importance, a cell melt is employed which is heavy enough to insure that the molten magnesium metal being formed by the electrolytic action, floats upon the cell melt. This avoids intermingling the pure magnesium metal with high magnesium oxide sludge which may form on the cell bottom if moisture is present in the magnesium chloride fed to the cell, as is usually the case. Secondary separation of the metal from the sludge is thus avoided, saving equipment and processing costs.

The highly conductive cell bath composition of the invention facilitates substantially higher production of magnesium metal and chlorine from a cell of given dimen sions. Thus, more metal is produced from a total magnesium reduction plant of any given size, and capital requirements for a given capacity of magnesium metal per year will be substantially reduced.

EXPLANATION AND EXAMPLES The achievement of these substantial operating and capital investment economies requires that lithium chloride be used to optimum concentrations in a magnesium reduction cell bath wherein the molten magnesium metal being formed will float. Of course, magnesium chloride, preferably in anhydrous form, should be present in a sufiicient concentration in the composition so that economical production of magnesium occurs. It has been determined that magnesium chloride should be present in a concentration of at least about 6 percent by weight of the composition in order for the process to be economical. Loading of the composition with magnesium chloride up to about 36.5 percent by weight of the composition can be tolerated. Accordingly, the useful range for magnesium chloride concentration in the composition is about 6-365 percent, by weight of the composition. In order to prevent the thus formed pure magnesium metal from sinking in the cell hath made lighter in specific gravity by high concentrations of lithium chloride, a cell bath composition is employed which makes adequate but minimal use of the heavy inorganic chloride, barium chloride, to raise the specific gravity of the cell melt to a level well above that of the molten magnesium while permitting use of lithium chloride in percentages by weight that range upwards from 56 percent to assure the highest possible electrical conductivity. A specific gravity sufiiciently higher (at least about 0.04) than that of molten magnesium metal (1.557) at a suitable operating temperature, for example, approximately 800 C. is achieved by using lithium chlo ride in a concentration not in excess of about seven and a half (7.5) times that of barium chloride in the composition. Such a ratio assures the required spread between the specific gravity of the melt and the molten magnesium for all conventional cell operating temperatures, for example 700 C.-950 C. A practical operating range is usually about 750-850 0, preferably about 800 C.

It will be understood that any conventional type of barium chloride, magnesium chloride and lithium chloride can be used in the composition, preferably pure and anhydrous.

The following specific example further illustrates certain features of the invention.

3 EXAMPLE An electrolytic cell is employed to convert magnesium chloride to magnesium for recovery thereof in purified form. The cell is of conventional design such as is currently employed in electrolytic magnesium production in Norway. It employs steel cathodes and 4-inch thick graphite anodes separated by acid brick divider walls disposed within a hollow brick chamber having a ceramic top and chlorine exit conduits adjacent the upper end of each anode. A cell melt composition is employed which, however, drastically differs from that conventionally employed in such cell. Instead of a conventional mix of magnesium chloride and sodium chloride and/or potassium chloride and/or calcium chloride, the cell employs the composition of this invention, the ratio of constituents of which range between the extremes shown in the table set forth below, decreasing during operation of the cell from the highest value shown for the magnesium chloride (composition V) to its lowest value (composition I) before additional magnesium chloride is used to replenish the dwindling supply in the melt.

The cell is operated at about 800 C. with a power requirement less than that that of conventional cells employing conventional cell melts. Anhydrous magnesium chloride is periodically added to the melt in the conven tional manner and pure molten magnesium is periodically passed from the top of the melt and out of the cell. Chlorine gas is kept separated from the molten magnesium and passes through the exit conduits from the cell. This cell operates continuously for more than a year without repair and in an economical manner.

In a second run, employing the same cell and bath composition, except for an operating range of about 750-850" C. and the substitution of hydrous magnesium chloride for anhydrous magnesium chloride, some magnesium oxide sludge forms on the bottom of the cell and is periodically withdrawn, but does not mix with the pure molten magnesium at the top of the melt, so that contamination of the thus formed magnesium does not occur.

When bath compositions outside the range demonstrated by the table above are employed, the advantages of the invention are not obtained, specifically reduced power consumption and flotation of the molten magnesium. Lithium chloride concentrations below about 56 percent result in sharp increases in power consumption. The cell is operated most efficiently with lithium chloride levels above about 60 percent. When the magnesium chloride content drops below about 6 percent, the cell operates inefficiently and when the magnesium chloride content exceeds about 36.5 percent, suflicient barium chloride must be added to the melt to keep the molten magnesium from sinking in the melt so that the lithium chloride content again falls below that level which affords improved electrical conductivity and resulting reduced power consumption. When the lithium chloride concentration in the melt exceeds about 83 percent while retaining at least about 6 percent magnesium chloride, the barium chloride content is insufficient to increase the specific gravity enough to assure proper flotation of molten magnesium. Accordingly, the barium chloride weight ratio to lithium chloride should be maintained at at least about 1:75 with a barium chloride range of about 7.5-11 weight percent, and a lithium chloride range of about 5683 weight percent, the magnesium chloride proportionately ranging in concentration between about 6 and 36.5 weight percent, as previously described and as set forth in the table above.

Various changes, modifications and alterations can be made in the present composition. All such changes, modifications and alterations as are within the scope of the appended claims form part of the present invention.

What is claimed is:

1. An electrolytic cell bath composition for production of molten magnesium which composition contains about 636.5 percent, by weight of said composition, of magnesium chloride, the balance of said composition consisting essentially of lithium chloride and barium chloride in a weight ratio to each other sufiicient to provide said composition with improved electrical conductivity and a specific gravity sufiiciently high to cause molten magnesium formed therein during electrolysis to float thereon.

2. The composition of claim 1 wherein said barium chloride and lithium chloride are present in a weight ratio to each other of at least about 1:7.5.

3. The composition of claim 2 wherein said lithium chloride is present in a weight concentration in excess of about 56 percent and wherein said molten magnesium chloride floats on said surface at bath temperatures in excess of about 800 C.

4. The composition of claim 2 wherein said lithium chloride is present in a weight concentration in excess of about 60 percent and wherein the specific gravity of said composition at 800 C. is at least about 1.62.

5. The composition of claim 2 wherein said lithium chloride is present in a concentration of about 83 percent, by weight of said composition, wherein said barium chloride is present in a concentration of about 11 percent, by Weight of said composition, and wherein said magnesium chloride is present in a concentration of about 6 percent, by weight of said composition, and wherein the specific gravity at 800 C. of said composition exceeds that of molten magnesium at that temperature by about 004+.

6. The composition of claim 1 wherein only magnesium chloride, lithium chloride and barium chloride are present, except for trace impurities.

7. The composition of claim 2 wherein only magnesium chloride, barium chloride and lithium chloride are present, except for trace impurities and wherein the specific gravity of said composition at 800 C. is at least about 1.6.

References Cited UNITED STATES PATENTS 3,317,414 5/1967 Foulgner 204- 3,389,062 6/1968 Love 20470 3,418,223 12/1968 Love 20470 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner