US2382450A - Electrolytic production of magnesium - Google Patents

Description

Patented Aug. 14, 1945 ELECTROLYTIC PRODUCTION OF MAGNESIUM Cyril M. Slansky, Midland, Mich assignor to The Dow Chemical Company, Midland, -Mich., a corporation of Michigan No Drawing. Application March 2, 1942, Serial No. 433,007

6 Claims.

This invention relates to an improvement in the production of metallic magnesium by the electrolysis of fused magnesium chloride.

In the conventional electrochemical preparation of metallic magnesium, a. fused salt bath consisting of alkali and alkaline earth metal halides and essentially comprising magnesium chloride is electrolyzed in a ferrous metal vessel, a portion of which also serves as the cathode for the electrolysis. During the process the molten magnesium liberated at the cathode rises to the surface of the bath and forms a pool from which the metal is withdrawn from time to time as it is produced.

Occasionally, however, in the electrolysis of magnesium chloride derived from certain natural sources, much of the molten magnesium liberated at the cathode fails to rise to the surface of the bath and form a pool of. metal. Instead, the electrodeposited 'metal in part detaches itself from the cathode in the form of discrete fine globules which bl come dispersed in the cell bath as a relatively stable emulsion and which do not coalesce to a pool of metal even after along period of time. As electrolysis proceeds under these conditions a. substantial part of the dispersed globules eventually reaches the anode area of the cell and is there reoxidized. Much more of the dispersed metal together with some of the cell bath settles to the bottom of the vessel as a. heavy sludge, which must be withdrawn, and from which the dispersed metal cannot readily be recovered. As a result of these phenomena in which much of the electrodeposited metal never becomesgcoalesced in usable form but remains dispersed in the bath and is eventually reoxidized, the current efliciency of the magnesium-producing cell may fall to such a lowvalue that economical production of magnesium from the particular magnesium chloride involved is not feasible.

The occurrence of the result just described has long been acknowledged in the published art, but the causes have remained obscure. It has recently been observed, however. that the dispersion of electrodeposite'd magnesium in the electrolysis of fused magnesium chloride is associated, at least in large part, with the presence of traces of inorganic boron-compound impurities, probably alkalior alkaline-earth metal borates, in the magnesium chloride. In particular, it has been found that when the magnesium chloride being electrolyzed is substantially boron-free, the metallic magnesium liberated collects satisfac torily as a p]. However, when the magnesium chloride being fed to the cell contains as impurity a boron-compound in a proportion equivalent to as little as 0.0015 per cent by weight of boron, the electrodeposited magnesium tends to form as discrete globules dispersed in the cell bath, and when the boron content of the magnesium chloride is as high as 0.03 to 0.06 per cent, the quan-' v tity of dispersion formed becomes so great, and the current eiliciency of the cell falls to so low a value, that operation becomes entirely unsatisfactory.

The formation of the undesired globular dispersion of metallic magnesium can, of course, be avoided by utilizing in the electrolysis only magnesium chloride which is substantially free of inorganic boron compounds. However, magnesium chloride of such purity is readily available from natural sources in only a, few localities. The magnesium chloride commonly produced at most other localities contains as impurities boron compounds in a quantity more than suflicient to cause during electrolysis the dispersions of the metallic magnesium, and these boron compounds often can be removed only by rather extended and expensive chemical processing, with consequent increase in cost of the magnesium ultimately produced.

An object of the present invention, then, is to provide a method of producing metallic magneslum by the electrolysis of magnesium chloride containing as impurities traces of boron compounds wherein the disadvantages occasioned by the boron content of the magnesium chloride are substantially avoided. Another object is to provide a method of treating dispersions of fine globules of metallic magnesium in molten alkaliand alkaline-earth metal 'halide baths to recove the magnesium content thereof.

According to the invention, in the electrolysis of molten salt b'aths essentially comprising magnesium chloride containing as an impurity a small proportion of an inorganic Iboron compound, the dispersing action of the boron compound on the electrodeposited magnesium may be substantially eliminated by maintaining dissolved in the electrolysis bath a small proportion of manganese or a reducible inorganic manganese compound. The presence of the manganese compound insures rapid coalescence of the electrodeposited magnesium to a single pool, and

minimizes .to a great degree or even eliminates v altogether any dispersing of magnesium in theform of discrete fine globules in the cell bath. While no adequate theoretical explanation of the action of the manganese or manganese compound has been developed, the efiect is very marked, and permits satisfactory production of metallic magnesium even from magnesium chloride containin such quantities of boron com-- pound impurities that operation would be otherwise virtually impossible.

-Insofar as is known, manganese or any reducible compound of manganese may beemployed in the invention. As a practical matter, however, finely divided metallic manganese, manganese halides, manganese oxides, especially manganese dioxide, and the alkali metal permanganates, all preferably in the anhydrous state, are commonly used. The proportion of manganese or manganese compound required depends largely upon the concentration of inorganic boron compound in the magnesium chloride, and in general should be sufllcient to provide at least one part by weight of manganese (Mn) per part of boron (B) present as boron compound impurity in the magnesium chloride. Optimum results are obtained when the manganese or manganese compound is added in a quantity such that the ratio Mn:B is from 1.5 i

to 15, although other ratios are operable. It is preferable, however, that the total quantity of manganese or manganese compound added represent less than about 0.1 per cent by weight of manganese relative to the magnesiumchloride, since at higher concentrations the added manganese may appear in the electrodeposited magnesium to an undesired degree. In practice, this upper limit is rarely even approached, since the boron content of magnesium chloride from most sources is ordinarily not high enough to necessitate the use of such a quantity of manganese or manganese compound.

In offsetting the undesirable eilect of boron impurities in magnesium chloride, the manganese material may be added to the electrolysis bath in any desired manner. In general, however, best results are obtained when the manganese material is introduced into the cell simultaneously with the boron-containing magnesium chloride, conveniently by incorporating the material in the magnesium chloride before introduction thereof into the cell.

Aside from the addition of manganese or manganese compound to the cell, or to the boron-containing magnesium chloride, the production of metallic magnesium according to the invention is carried out entirely in conventional manner. Bath compositions, temperatures, current densities, etc. are those used in commercial practice. Due to the presence of the manganese material the electrodeposited magnesium rapidly coalesces to a single pool, and maybe removed from the cell from time to time as more magnesium is liberated. The boron impurities, rendered largely harmless by the manganese material, appear in part to escape from the cell with the anode gases and in part to be converted to insoluble compounds, possibly also containing manganese, which settle from the cell bath and are removed at infrequent intervals.

The process of the invention is particularly advantageous in the production of magnesium from magnesium chloride derived from sea water or i from natural inland .brines which contain, in addition to boron compound impurities, a small proportion of manganese salts. In this case, the process for separating magnesium chloride from the source material, which is ordinarily operated to remove manganese salts, may be controlled so that a manganese salt is present in the magnesium chloride product in a proportion sufficient to minimize the adverse effect of the small Proper the production of magnesium by the electrolysis of magnesium chloride containing boron impurities in a proportion corresponding to from 0.0015 to 0.06 per cent or more of boron. In such instances, it may be desirable to withdraw the dispersion from the electrolytic cell and recover the magnesium therefrom. To this end, the molten bath containing the dispersed molten magnesium is agitated and manganese or a reducible manganese compound is added thereto in a proportion sufllcient to effect coalescence of the molten magnesium globules, after which the coalesced magnesium is ladled ofl. Best results are obtained when the manganese material, preferably either metallic manganese, a manganese halide, or manganese dioxide, is added in a proportion such that the manganese (Mn) content of the bath is at least 1.5 times by weight the boron content thereof.

Other modes of applying the principle of the invention may be utilized instead of those explained, change being made as regards the details disclosed, provided the steps recited in any oi. the following claims, or the equivalent thereof, are employed.

I claim:

1. In a process for the production of molten metallic magnesium wherein magnesium chloride containing as impurity an inorganic boron-containing substance in a proportion corresponding to between about 0.0015 and about 0.06 per cent of boron is electrolyzed in a fused salt bath above the melting point of magnesium, the method of minimizing the dispersing action of the boron substance on the electrodeposited molten magnesium and of insuring coalescence thereof which comprises incorporating in the magnesium chloride being fed to the electrolysis bath a substance selected from the class consisting of manganese and reducible manganese compounds in a proportion such that the manganese content of the magnesium chloride is at least 1.5 times by weight the boron content thereof but does not exceed about 0.1 per cent by weight.

2. A process according to claim 1 wherein metallic manganese is added to the magnesium chloride.

3. A process according to claim 1 wherein manganese dioxide is added-to the magnesium chloride.

4. In a method of recovering molten magnesium from a dispersion thereof in the form of discrete fine globules in a fused salt bath above the melting point of magnesium essentially comprising magnesium chloride and containing as impurity an inorganic boron-containing substance in a proportion representing between about 0.0015 and about 0.06 per cent by weight of boron in the bath, the steps which comprise adding to the bath a substance selected from the class consisting of manganese and reducible manganese compounds in a proportion such that the manganese content of the bath is at least 1.5 times by weight the boron content thereof, agitating the bath to efl'ect coalescence of the dispersed magnesium, and recovering the coalesced magnesium.

5. A process according to claim 4 wherein the substance addedto the bath is metallic manganese.

6. A process according to claim 4 wherein the substance added to the bath is manganese dioxide.

CYRIL M. SLANSKY.