US3074861A - Metallic magnesium via electrolytic amalgamation - Google Patents

Description

Jan. 22, 1963 J. M. AVERY 3,07

METALLIC MAGNESIUM VIA ELECTROLYTIC AMALGAMATION Filed Feb'. 15, 1960 2 Sheets-Sheet 1 4 I I4 14 I z 3 T I 6 f [E f LIN-s nite Patented Jan. 22, 15. 263

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METAl-LMC MAGNESlUi /l VlA ELECTRGLYTKC All KALGAMATHEN Julian M. Avery, Greenwich, Conn, assignor to Ethyl Corporation, New York, N.Y., a corporation oi Virgrnia Filed Feb. 15, 196i), Ser. No. 8,718 l Claim. ass- 124 This invention relates to a process for the production of metallic magnesium by the electrolysis of an aqueous solution of magnesium chloride wherein magnesium metal is deposited on a flowing mercury cathode, thereby producing an alloy or amalgam of magnesium and mercury from which pure metallic magnesium is recovered.

Prior art processes concerned with the production of metallic magnesium rely principally upon methods for the electrolyticdecomposition of the fused salts of magnesium, particularly by electrolysis of molten magnesium chloride. These processes include all the disadvantages associated with high temperature e ectrolytic operations and high purity magnesium cannot be produced because of the high afiinity of this particular metal to occlude impurities, particularly chlorides. For these reasons, a new low temperature electrolytic process capable of producing high purity magnesium metal at low cost is much desired.

According it is an object of this invention to provide a new and improved process for the production of metallic magnesium by electrolysis of an aqueous magnesium chlo ride solution in a cell of the type characterized by a flowing mercury cathode. More specifically, it is an object of the present invention to provide a method for the production of metallic magnesium by electrolytically decomposing magnesium chloride and depositing the magnesium metal in a flowing mercury cathode, then recovering high purity metallic magnesium from the mercury by reworking the mercury-magnesium alloy in a separate zone. An especially preferred object is the provision of a new and practical process for the production of metallic magnesium of high purity by electrolytic amalgamation in combination with selected recovery ope-rations whereby the metallic magnesium is recovered by reworking the amalgam as defined hereinafter.

In order to facilitate an understanding of the present invention, reference is made to the accompanying drawings in which:

FIGURE 1 is a schematic diagram showing an elevation view of a magnesium-amalgam cell in combination with a distillation column, the latter apparatus providing for a one-step dissolution of the amalgam;

FIGURE 2 is a schematic elevation view of apparatus in combination with the electrolytic cell providing a two step technique for dissolution of the amalgam, i.e., a

settler-cooler, for separation by fractional crystallization,

is interposed between the electrolytic cell and a distillation column;

FIGURE 3 is a schematic elevation view showing the electrolytic cell in combination with apparatus providing for a two-step distillation; and

FIGURE 4 is a flow sheet demonstrating, by use of block diagrams, a highly preferred operational technique employing generally the apparatus referred to in FIG- URES 1 through '5.

Thus, the foregoing and other objects are achieved according to the present invention which comprises passing an electric current through an aqueous magnesium chloride solution, thereby decomposing the salt and depositing the magnesium metal within a flowing mercury cathode,

' then transporting the fiowable mercury cathode to a second zone for separation and recovery of the metallic magnesium, and recovery of mercury for recirculation.

After dissolving magnesium in the mercury cathode to 2 form an amalgam, the amalgam is withdrawn from the electrolysis compartment and sent to another zone, or zones, for separation of the mercury and magnesium. The selected techniques generally ermloyed for separation of these metals are as follows:

(1) The amalgam is cooled to a low temperature, i.e., 20 C. to about l0 C., whereupon a phase separation occurs. The upper stratum conforms generally with an alloy having the tformula Mg l'ig and is a solid or slurry under the temperature conditions of the operation. The lower stratum consists essentially of mercury containing Mg Hg in solution. The lower stratum of liquid is recycled to the electrolysis zone and the Mg r g alloy is then distilled to separate metallic mercury and metallic magnesium, as given in the following one-step distillation technique.

(2) The amalgam, whether taken directly from the electrolysis zone or whether first stratified by cooling and only the upper stratum treated, is subjected to a high temperature, low pressure one-step distillation (FIGURE 1) wherein mercury is boiled oil or removed from the amalgam leaving behind metallic magnesium which is drawn oil and recovered. The distillation is generally conducted at from about 656 C. to about Q C. and preferably at from about 720 C. to about 990 C. The pressure is generally maintained at from about 0.1 pound per square inch absolute to about 30 pounds per square inch absolute. Preferably the pressure is maintained at from about 0.3 pound per square inch absolute to about 2 pounds per square inch absolute.

(3) in another embodiment (FIGURE 3), the amalgam is subjected to a two step distillation technique. Mercury is separated by a first distillation step at comparatively low temperature and pressure. The temperature is thus maintained at ttrom about 200 C. to about 600 C., and pretrably from about 200 C. to about 350 C. The pressure is maintained at from about 0.5 pound per square inch absolute to about 2 pounds per square inch absolute. There is thus left, after said first distillation step, an alloy corresponding roughly to the formula Mg l-lg The free mercury from the first distillation is recycled to the electrolysis zone and the magnesium mercury alloy is passed to a second distillation zone. Within this zone the alloy is also subjected to low pressure, and also is subjected to an even higher temperature. The temperature employed is on the order of from about 65? C. to about 1200 C., and preferably from about 720 C. to about 900 C. The pressure is maintained on the order of from about 0.1 pound per square inch absolute to about 30 pounds per square inch absolute, and preferably @rom about 0.3 pound per square inch absolute to about 2 pounds per square inch absolute. The second distillation step substantially completes the dissolution of the alloy into its component parts. The mercury is distilled oh? and recycled to the electrolysis zone and the metallic magnesium is recovered.

The process will be better understood from the following more detailed description of the invention.

Referring to FIGURE 1, is shown a preferred amalgam cell it"! in combination with a distillation unit 3%. The cell 1b is composed of an enclosing vessel 9 through the top of which is projected a plurality of graphite anodes 14 14 14 and upon the bottom of which ilows a thin film of mercury forming the cathode 12. The bottom of the cell it: is generally sloped very slightly to avoid accumulation of the mercury therein. Between the cathode l2 and anodes 14 is maintained an electrolyte 11 consisting of an aqueous solution of magnesium chloride. Of course, during operation of the cell, the concentration of magnesium chloride is depleted by electrolysis. The magnesium chloride within the cell 10 is maintained fairly constant, however, by addition of fresh magnesium a?) chloride, in hydrated or dehydrated form, or as a concentrated solution. The more dilute or spent solution is discharged from the cell it through line 16 and into the holdup tank 5. Through the feed port 6 fresh magnesium chloride is added. Make-up quantities of water can be added to the tank 5 through line 7. The desired charge is thus introduced through line 15 and into the cell 1%). During operation of the cell lit the magnesium chloride is decomposed. The chlorine produced is removed from the cell through the chlorine duct 13 and the magnesium is deposited within the flowing mercury cathode 12. The concentration of the magnesium within the mercury is generally maintained at as high a concentration as possible at the existing temperature while yet providing for the amalgam 12 to remain sufiicientiy fluid to be removed from the cell It). The amalgam is discharged from the cell 10 through line 17 and into the distillation column 30 (source of heat for distillation not shown). Within the distillation column 31 free mercury is removed from the top of the column 3% and recycled through line 18 back into the cell 10. Makeup quantities of materials can be added to the cell it) through line 15. Molten metallic magnesium is removed from the bottom of column it through line 19 and sent to storage.

In all embodiments of the invention, the operation of the electrolytic cell 10 to produce magnesium mercury amalgam is substantially as described. Thus, the electrolysis step is generally conducted at a temperature of between about to 100 C. and preferably at a temperature of from about 80 to 85 C. At the preferred temperature mercury is cycled through the cell lit at such velocity with such surface area exposed to the aqueous solution as to provide a relatively high concentration of magnesium in mercury 12 While yet permitting it to remain sufliciently fluid. The concentration or" magnesium in mercury is generally from about 0.05 percent to about 1.5 percent.

in one embodiment of the invention, the electrolytic cell 10 is operated under a pressure of 100 pounds per square inch absolute. This permits operation of the cell up to temperatures as high as 170 C. thus permitting a concentration of magnesium in mercury of from about 2 to 4 weight percent.

The concentration of the magnesium chloride within the aqueous electrolyte 11 is preferably maintained at about a 30 weight percent concentration. The solution can of course be saturated at the existing temperature conditions. At 100 C. this usually corresponds to about 73 weight units of magnesium chloride per 100 Weight units of water, etc.

The cell voltage is usually maintained within a range of from about 4 to volts, preferably from about 4.3 to 4.7 volts.

The distillation is generally conducted at reduced pressure, from about 0.1 pound per square inch absolute to about 30 pounds per square inch absolute and from about 650 C. to about 1200 C. Preferred conditions are from about 0.3 pound per square inch absolute to about 2 pounds per square inch absolute, and from about 720 C. to about 909 C.

Referring to FIGURE 2 is shown a preferred combination which permits an even more economical and cleaner method of separation than that shown with reference to FIGURE 1. In this embodiment (FIGURE 2), the electrolytic cell permits discharge of the amalgam directly into a fractional crystallization zone or into a cooler settler tank 49. To effect fractional crystallization, the temperature of the amalgam is generally lowered from about C. to about l C. whereupon a two-phase separation occurs. The lower phase or stratum 41 within the cooler settler tank 40 is a solution of Mg Hg in mercury. The upper stratum 42 is substantially the compound Mg i-Ig The lower stratum 41 is withdrawn from .the cooler settler 46 passed through line 43 and back into the electrolytic cell 10,. The upper stratum 42;, consisting essentially of Mg Hg is discharged from the cooler settler 49 through line 44 and into the distillation column 50. Metallic mercury is taken off the top of the distillation column 50 and passed through line 45 back into the electrolytic cell iii. Molten magnesium is removed from the bottom of distillation column 50 through line 46 and sent to storage. Make-up quantities of mercury can be added through line 47.

Referring to FIGURE 3, is shown another highly preferred embodiment for separation of metallic magnesium from the amalgam. This embodiment is characterized by a two-step distillation technique (sources of heat for distillations not shown) which permits substantially complete dissolution of the amalgam into mercury and magnesium. Thus, the magnesium amalgam is discharged from the electrolytic cell it through line 61 and into a first distillation column 60. This column 66 is maintained at reduced pressure, 0.05 pound per square inch absolute to 2 pounds per square inch absolute and at sufiicient temperature (200 0-359 C.) to distill off the preponderance of the metallic mercury. Thus, metallic mercury is removed from the overhead of the column 6%) through line 62 and recycled back into the electrolytic cell 10. From the bottom of this column 60 is removed an amalgam conforming substantially to a compound of magnesium mercury alloy having the formula Mg Hg This alloy is discharged through line 63 and into the second distillation column 70. Within this column 76 the Mg Hg alloy is separated into its component partsviz., metallic mercury and metallic magnesium. The mercury is removed from the overhead of the column 7% through line 64 and sent back to the electrolytic cell 10. Molten metallic magnesium is removed from the bottom of column 7 0 and discharged through line 65 to storage.

Referring to FIGURE 4 there is shown a more elaborate and even more highly preferred embodiment of the present invention. In this embodiment the amalgam is discharged from the electrolytic cell 10 through line 81 and into the settler cooler 36. Within the settler cooler fractional crystallization occurs and a two phase system is formed. The lower stratum, consisting of Mg Hg in mercury, is discharged through line 82 and into the column 98. The upper tratum, consisting essentially of the alloy Mg I-lg is discharged from the top of the settler cooler 80 through line 83. Within column a compressive distillation is conducted. Free mercury is removed from the top of the column 90 discharged through line 91 into the manifold line 1M. Before discharge to line 191 however, the vapors are compressed and used for heat to the column reboiler. Mg I-Ig is discharged from the bottom of the compressive distillation column 90 through line 92 into line 83. The Mg I-Ig from the bottoms of columns 89, 9% are thus discharged through line 83 and into a fractional distillation column 160. Mercury is removed from the overhead of column 100, passed through line 93 and discharged into the manifold line Hill. A magnesium-mercury mixture, or alloy conforming substantially to the formula MgI-Ig, is discharged from the bottom of the column through line 94 and into the distillation column Ht Within the column the magnesium-mercury mixture is separated into its component parts-viz., metallic mercury and metallic magnesium. The metallic mercury is taken from the top of the column 110 through line 95 and discharged into the manifold line 101. Molten metallic magnesium is removed from the bottom of column lit discharged through line 96 and sent to storage.

The following non-limiting example is illustrative of the invention and the procedural steps followed are illustrated primarily by reference to FIGURE 4 of the drawmg.

The electrolytic cell 1% is operated at a temperature of 85 C. and at a pressure of 5 inches of water. An electrolyte consisting of a 30 weight percent concentration of magnesium chloride in Water is maintained within the cell 10. A voltage of 4.5 is impressed between the anodes and the cathode. Substantially pure metallic mercury is passed into the cell 10 through line 79 and the amalgam discharged from the cell through line 81 is 0.5 weight percent magnesium in mercury. The amalgam is charged into the settler cooler 80 which is operated at a temperature of C. A two-phase separation occurs. The lower stratum is discharged through line 82 into the compressive distillation column 90. The vapors from the distillation column 90 are recompressed and used as reboiler heat. The magnesium mercury alloy (Mg Hg from the upper stratum of settler cooler 80 and the bottom of column 90 are manifolded within line 83 and discharged into the fractional distillation column 100 which is operated at a temperature of 650 C. and 2 pounds per square inch absolute. Substantially free metallic mercury is removed from the overhead of column 100, passed through line 93 and into the manifold line 101. The bottoms of the column 100 (MgI-Ig), corresponding to 11 weight percent magnesium in mercury, is discharged into the second distillation column 110. Mercury is removed from the overhead of this column 110 through line 95 to the manifold line 101. Substantially pure molten metallic magnesium is removed from the bottom of column 110 and passed through line 96 to storage.

From the foregoing it is apparent that the process combinations taught herein are subject to a considerable number of variations without departing from the spirit and scope of this invention.

The composition of the aqueous magnesium chloride electrolyte can be maintained near saturated conditions at the existing temperature of operation. The concentration of magnesium chloride employed is determined only by principles of cell operation. As stated, generally a concentration of about 30 weight percent magnesium chloride in the electrolyte is preferred.

It is preferable to operate the electrolytic cell at a temperature of from about 0100 C. when atmospheric pressure is employed. The preferred temperature of operation is from about 80-85 C., however, because at this temperature there is a better overall balancing of the operative factors involved. When pressure is employed the cell can be operated at temperatures as high as 170 C. quite satisfactorily. Operation at this temperature makes it possible to obtain greater concentration of magnesium in mercury, viz., as high as from 24 Weight percent magnesium in mercury. When atmospheric pressure is employed at temperatures below 100 C., the concentration of magnesium in mercury is from about 0.2 to about 1.5 or higher weight percent magnesium in mercury. Operations even within the very lower limits of this range are in many instances highly preferred.

The thickness of the flowing cathode is not critical but is generally maintained at from about 3 to about 5 millimeters.

When fractional crystallization is employed as a preliminary step in separating the mercury and magnesium, the temperature is generally maintained on the order of about 20 C. to about C. Lower temperatures can of course be used but this is unessential, and is generally uneconomical.

Under conditions wherein it is desired to partially separate the mercury and magnesium in a preliminary distillation step prior to a more stringent distillation step, it is generally desirable to operate at a temperature of from about 200 C. to about 600 C. A temperature of from 200 C. to about 350 C. is preferred. Such operation is generally carried out in at least a partial vacuum, viz., on the order of from about 0.5 pound per square inch absolute to about 2 pounds per square inch absolute. In all embodiments, particular attention is required that the distillation be carried out in the absence of air. Any inert atmosphere may be used to blanket the reaction. For example, a blanketing layer of argon, krypton, etc., can be provided. In such preliminary distillation step, it is often generally desirable to remove sufiicient mercury such that the amalgam remaining substantially conforms to the formula MgHg.

When a second distillation step is employed, the temperature is generally maintained at from about 650 C. to about 1200 C. and at a pressure of from about 0.1 pound per square inch absolute to about 2 pounds per square inch absolute. Where especially high purity magnesium is desired a retarding step can be added to the process.

Having described the invention, what is claimed is:

A process for the production of metallic magnesium comprising passing an electric current through an aqueous magnesium chloride solution which is in contact with a flowable mercury cathode in an electrolysis zone thereby decomposing the magnesium chloride and causing deposition of the magnesium and formation of a magnesium amalgam within the flowable mercury cathode, and transporting the magnesium amalgam from the electrolysis zone into a reduced temperature zone wherein a twophase separation occurs producing an upper stratum, consisting essentially of the magnesium-mercury alloy Mg Hg and a lower stratum, consisting essentially of a solution of Mg I-Ig in mercury, passing the lower stratum from the low temperature zone into a first distillation Zone and therein recovering .a magnesium-mercury alloy which is essentially Mg Hg5, then transporting the upper stratum from the reduced temperature zone and combining it with the magnesium-mercury alloy Mg Hg obtained from the said first distillation zone, and feeding these combined portions into a second distillation zone, recovering from said second distillation Zone .a mag nesium-mercury alloy consisting essentially of MgHg, then'feeding the monomagnesium-mercury alloy, MgHg, into a third distillation zone and recovering metallic magnesium therefrom.

References Cited in the file of this patent UNITED STATES PATENTS 678,816 Shinn July 16, 1901 739,139 Baker et a1 Sept. 15, 1903 789,721 Decker May 16, 1905 1,961,160 Moulton June 5, 1934 2,224,814 Gilbert Dec. 10, 1940 OTHER REFERENCES Hansen: Constitution of Binary Alloys, 1958, pages 822-824.

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