US2431723A - Electrolytic method for producing magnesium alloys - Google Patents

Description

Dec. 2, 1947. A. YERKES ELECTROLYTIC METHOD FOR PRODUCiNG MAGNESIUM ALLOYS Filed Jan. 17, 1942 ELECTROLYTIC cgu.

PURlFICATION CELL Fig. 3

INVENTQR BY Leland A. Yerkes ATTORNEY Patented Dec. 2, 1947 UNITED STATES PATENT OFFICE ELECTROLYTIC METHOD FOR PRODUCING MAGNESIUM ALLOYS Claims.

amended April 30, 1928; 370 0. G. 757) This invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to electrolytic deposition of magnesium from a molten bath and more particularly to increasing the solubility of magnesia and the like in a molten bath containing magnesium halides.

Numerous attempts have been made to electrolyze magnesia in molten fluoride-oxide mixtures to obtain magnesium, e. g., U. S. 1,408,141; 1,408,142; 1,310,449; 1,584,689; and 1,533,911. These processes have not been fully successful,

since magnesia has but low solubility in suchbaths (about 0.1% MgO; Thompson. M. de K.. and Kaye, A. L. Trans. Electrochem. Soc. vol. 67, 22 pp., preprint 1935, The Decomposition Potentials and conductivities of MgO, and Alkaline Earth Oxides in Fused Fluorides).

The principal object of my invention is the provision of an improved process for producing magnesium.

A further object is to increase the magnesium oxide content of a molten electrolytic bath, whereby to facilitate electrolysis and recoveries of magnesium.

While the invention may be practiced in various specific Ways, and by the use of many types of apparatus, I show in the drawing one practical embodiment wherein Fig. 1 is a sectional view through a suitable type of cell,

Fig. 2 is a diagrammatic view showing a cycle which may be used, and

Fig. 3 is an illustrative electrical installation.

Referring now generally to the invention, I have discovered that boron compounds, preferably when utilized within certain limits in the magnesium halides containing bath, will materially increase the solubility of magnesia therein, and that this increased solubility of magnesia makes more practical the electrolysis of this magnesia-containing bath to form magnesium metal. This increased solubility of the magnesia makes MgO more nearly the product reduced in practice, possibly since magnesia, (according to Thompson and Kaye cited previously) has a lower decomposition voltage than has the magnesium halide MgClz. The amount-of boron compound,

calculated as B203, to give increased solubility of the magnesia may vary from traces to an upper limit of about 5% of the bath. At high concentrations of boron, the electrolytic production of magnesium boride obscures or prevents the reduction of magnesium metal. I have found that a satisfactory practical percentage is about 0.5%, although, as stated, up to about 5% maybe used to advantage. Even 0.1% to 0.3% may be-used to advantage.

The bath may contain any alkaline earth or alkali metal halide in addition to the magnesium halides. For example, a bath of NaCl, MgClz, B203 and MgO may be utilized. The use of salt and similar halides in the bath is usually for the purpose of rendering the bath more fluid or lowering its melting point.

Magnesium oxyhalides, such as magnesium oxy chloride, magnesium chlorate, magnesium'hypochlorite, magnesite, brucite, etc., may be substituted for magnesia. This bath may be used for electrolytic purposes, either to produce magnesium directly by usual practice or to plate into other metals, e. g. lead, either to form a useful alloy or one which can be subsequently refined to give magnesium metal.

The halide of magnesium preferably employed is magnesium chloride, as the basis of the bath. Into this is placed a boron compound, suchas B203, and such MgO as can be dissolved therein. The electrolytic production of magnesium is then carried out by methods old in the art.

The process employed is preferably a continuous one, although some of the features of my invention may be employed with equal advantage if the electrolysis is carried out on a batch basis. Using the continuous process, purified MgO is fed to the reduction cell along with such MgClz and B203 (or other boron compound) as may be required, while electrolysis continues. Chlorine or oxygen or both are liberated at the surface and metallic magnesium is formed at the cathode. When a lead cathode is used, the magnesium mixes with the molten lead. The lead-magnesium mixture is circulated from the reduction cell to a separation furnace. The magnesium may-be removed from the lead by electrolysis of the molten lead-magnesium alloy to yield metallic magnesium, as more fully described by Jessup "in British Patent No. 268,737. -The lead product resulting from this separation, which still contains some magnesium, is returned to the reduction cell. Continued operation may result in building up impurities in the electrolyte or lead cathode in which event operations may be discontinued to renew or purify the constituents of the reduction cell.

Referring now to the drawings, I show in Fig. 1 the cell I with a refractory lining 2. An anode 3 projects into the electrolyte layer 4 which floats on top otthe molten cathode 5; e. g. lead. A lne 6 comprises an outlet for withdrawing the leadmagnesium mixture for separation, and the lead with reduced magnesium content is returned through the return line 1 comprising aninlet A suitable purification or separation cell oriurnace 8 receives the lead-magnesium mixture, reduces the magnesium content, and returns thelead to the reduction cell, a pump 9 of suitabledesign being employed to circulate the'load. The cell 9 and general arrangement of Fig. 2 are described by Jessup, in the patent referred to hereinabove.

Electrolysis of the magnesium oxide to-magnesium requires direct current of controlledvoltage; A motor generator set U may be employed, operatingfromcommercial linevoltage, and delivering power at suitable voltage to-the cell-through the-anode 3; andcathode, The-cell may be heatedby resistance, I2-i through transformer [3, although otherheating; means; may-be employed-.-

Atl timeslt may-be necessary tocoolthe cell, in which case any suitable: cooling: means. (notmakingfrequentadditions of-MgO.-: The voltage averaged 4.5 with the current at150 amperes during-most: of the. period, but was. later increased to: 6.'5 volts .at. 135 amperes. Thecurrent density" was-.20 amperes per square inch-at -50 amperes and .35.:amperes per square inch at 135'amperes. The. gas givenoff atrthe. anode .was analyzed. The following table is.illustrative. of, the results:

Amperes Volts Temp. MgQrIn Cell 4.5 1.9L 814' 10%": 59 l h y s a e he 0% 4., 82 57 856 Excess MgQ. 4-.5 11.88- 840 Excess-not all dissolved that would dissolve; 6.5 63.00 816. Excess.

I have already-pointed out thatcertain features; of cell construction-and operation heretofore suggested by others may be employed in the practioe of my invention, Instead of a lead cathode; other-molten: metals may be used such as tin, zinc-and bismuth, from which magnesium may besuitably-separated. Uther-typcs of cells-may b'e used: With the particular design of cell shown in -the-drawing,- it --is desirable that a carbonaceous-anode be used; such as. azgraphite an srgases evolved are particularlycorrosive tometal 1 anodes it directly exposed to them. fiinee I agraphiteanode isburned by the gases e olved; toCO and CO2, it-isdesirable also that it be readily-replaceable.-

irrthe-practice of my invention, many commundsofberon may be used to secure thee- 1* vantageous results described. Examples other than B203, are borax, borax glass, ulexite, boracite, colemanite. kernite. boric acid and the like, Boron oxide, B203, however, is the preferred boron compound to use, so far as I have observed from results heretofore obtained. This com-- pound increases markedly the solubility of MgO in a MgClz bath, and the rate of solution is such that MgO can be continuously added. Other magnesium compounds are more readily introduced into the; solution as well; such as :magnesium oxyhalides, which by-heat arereduced to oxides and chlorides, and subsequently electro- Iyzed in the same manner as is MgO.

It shouldbe understood that my invention is not limited by the specific details of construction and operation of a cell discussed above.

Any suitable electrolyte may be used, such as MgCl2: towhichother electrolyte salts have been added. By my invention MgO may be made, at least for the most; part, the ultimate material reduced. The ratio of oxygen to chlorine in the table shows that much-more oxide is reduced than chloride, and "I-believe' that it is merely a question of determining optimum conditions to substantially reduce only MgO and-practically no lVIgClz;

I: wish topoint out also, that-instead of, ern-- ploying a molten metal; such aslea-d forthecathode, molten magnesium itself may be usedasthe;

cathode providing asuitable cell construction is used'so that MgOmay-beaddedand the evolvedgas, 02 and C12, canbe-JceDt outer-contact with the metallic magnesium.

What I claim as new and desire to protectby Letters Patent of theUnited States'is:

1; A process which consists of subjecting to electrolysis a fused mixture of'magnesium-oxide, magnesium chloride and from 0.1 to 5 .0%- calculated as B 03 of at least oneboron com-- pound, as a solubility promoter for M550, selectedfrom the group consisting ofgboro-n trioxide, boric acid, borax; borax glass, ulexite; beracite, colemanite andkernite, said-electrolysis lee-- ing carried out While employing asa molten me tallic cathode at leastone-metal=capable ofalloying with-magnesium, saidmetal being selected from the group consisting-of lead, tin, zinc and bismuth, to produce an alloy of magnesium with said molten cathode metal.

2. A process which-consists of subjecting to electrolysis a fused mixture of magnesium oxide,-

magnesium chloride and from 0.1 to 5.0%;o f -bo-.

ron trioxide, as. a solubility promoter-for MgO, said electrolysisbeing carried out while employing molten lead as a cathode to producean alloy of magnesium with lead;

3. A process which consists of subjecting to electrolysis a fused mixture of magnesium oxide, magnesium chloride and about 0.5% boron trioxide, as a solubility promoter for MgO, said electrolysis being carried out While. employing as a molten metallic cathode at least one metal capae ble of alloying with magnesium, said metal being selected from the group consisting of lead, tin, zinc, andbismuth, to produce an alloy of magnesiurn with said m'olten cathode metal.

ep o ess which QnSist fb ec n to lec ly is a fu edix r of ma nesium ox de, magnesiumchloride andabout 0.5% calculated a S' B303 of atleast one boron; compound, asa solubilitypromoter for MgO, selected fromthe, group consistin of boronv trioxide, boric acid, borax,- borax glassrulexite, boracite, colemanite and-'ksrnit said e Q trQ -Ydshe -carried out LELAND A. YERKES.

REFERENCES CITED The following references are of record in the file of this patent:

Number Number UNITED STATES PATENTS Name Date Kugelgen et a1 Feb, 25, 1908 Ashcraft Aug. 16, 1921 Cottringer et a1 Dec. 29, 1925 Andrieux Mar. 10, 1936 Schambra et a1 Feb. 12, 1945 Slansky Aug. 14, 1945 FOREIGN PATENTS Country Date France Apr. 8, 1935 OTHER REFERENCES Journal of the Chemical Society (London), 1891, page 1321.

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