US2351488A - Method of producing magnesium - Google Patents

Description

June 13,1944,

-H. s.- cooPER 2,351,488-

METHOD OF PRODUCING MAGNESIUM Filed March 10, 1942 VACUUM PUMP WATER OUT WATER IN INERT g :11]; GAS VACUUM SEAL REFRACTORY c 9 VACUUM SEAL MgO+Al -$i ALLOY H +l5%H o REFRACTORY HEATING ELEMENT ELECTRIC GENERATOR INVENTOR.

Holy/1 5. Coo 9e BY Q l Patented June 13, 1044- ass sts Fries METHOD OF PRODUCING MAGNESIUM Hugh S. Cooper, Cleveland, Ohio, assignor of onehalt to Frank H. Wilson, Cleveland, Ohio Application March 10, 1942, Serial No. 434,078

8 Claims.

This invention relates to metallurgy and more particularly to a distillation process for the production of vaporizable metals, such as magnesium.

Her'etofore in the art, many attempt have been made to provide a commercially practical process for preparing magnesium by the reduction of magnesium oxide in an inert atmosphere at a temperature substantially above the boiling point of magnesium by a reducing agent that'is substantially non-volatile at the temperatureof reduction and pressure of inert atmosphere employed. A reducing agent well suited for this purpose is silicon, which reacts with magnesium at temperatures approximating 1600 C.

at atmospheric pressures in accordance with the following equation:

2MgO+Si=2Mg+SiOa-92.92 kg. Cal.

At this temperature of reaction the magnesium which has a boiling point of about 1120" C., is vaporized and may be collected by condensation at a pointremote from the zone of reaction. At

' reducing agent for use in this general process.

A further object is to provide an economically practical distillation method for the production of magnesium by the reduction of its oxide at temperatures above its boiling point with a reducing agent which is non-volatile at the temperature of reduction and pressure employed and the oxide compounds of which are substantially non-reactive with magnesium oxide.

Other objects and advantages will be apparent as the invention is more fully hereinafter disclosed. I

In accordance with these objects, I have discovered that by, employing an aluminum-silicon alloy-as a reducing agent, particularly an alloy containing from about 20% S1 to about 50%. Si. the reduction reaction results in the formation of aluminum silicates in accordance with the foliowing equation (Where the 50% Si alloy .is employed):

As aluminum silicate has a melting point approximating 1800 C., the reduction reaction may proceed to completion without substantial loss of. magnesium as heretofore experienced in the use of silicon alone and without substantial fusing or sintering of the reacting mixture, even at temperatures as high as 1600 C.

I have further found that by conducting the reduction reaction of the present invention at reduced pressures the temperature of reduction may be markedly decreased to temperatures within the range l200-1400 C. and the rate of reaction greatly accelerated over that obtainable heretofore. I

In the practice of the present invention, the aluminum-silicon alloy reducing agent of the present invention preferably-contains substantially equal amounts of aluminum and silicon (as the atomic weights are approximately equal). Aluminum-silicon alloys containing about 50% Si have a melting point approximating 1100 C. Aluminum-silicon alloys containing about 20% Si have a melting point of about C. The melting points of alloys of intermediate Si content are at intermediate temperatures (30% Si at 825 C.; 40% Si at 950 C.) and all of the alloys are freely molten at temperatures within the range 1100-1400" C. Each of these alloys are extremely brittle and minuted to fine particle size.

As an example of the present invention, the magnesium oxide and the Al-Si. (20-50% Si) alloy are first reduced to fine particle size, preferably passing 100 mesh, and are formed into an intimate dry mixture using from 10 to 25% excess of the alloy over that empirically required to completely reduce the magnesium oxide present.

This mixture is then compacted into porous pellets or briquets of any desired size, using pressures of the order of 5,000-25000 pounds .per square inch. These pellets are charged into a suitable retort or crucible enclosed from the at' mosphere, and are heated in any convenient manner in an inert atmosphere, such as hydrogen or helium, substantially free of oxygen and nitrogen and at pressures'ranging from atmospheric pressures to a high vacuum to elevated temperatures substantially above the vaporization temperature of magnesium at the pressure are readily comemployed. The heating of the pellets is continuedforanextendedtimeintervalatleast suflicimt to obtain substantially complete reduction and vatlon ofthe content of the same.

The um vapors obtained may be collected in any convenient manner in a suitable condenserat a point remote from the reduction zoneasheretoforepracticedintheartof distilling metals and fused to coherent metal.

As a specific improvement of this process, however, I have discovered that by collecting the magnesium vapors upon a condenser surface maintained ata temperature within the range 500 to 600 C., the deposited magnesium forms in relatively large crystals or aggregations of crystals which are resistant to spontaneous com bustion on subsequent exposure of the same to surface oxidation.

In the practice of the above method, I prefer toemployreducedpressuresasbytheuseof reduced the boiling temperature of the magnesium is materially lowered. This permits me to employ as a reducing agent an aluminum-silicon alloy containing less than 50% Si and reducing temperatures materially lower than 1400 C. At extremely low pressures, reduction temperatures as low as 1100 to 1200 C. may be employed with advantage even with alloys containing 50% silicon, although alloys containing from 30 to 40% silicon are more effective reducing agents at these low pressures.

Where low pressures are employed, I have found that the rate of magnesium vaporization at the lower temperatures of reduction is markedly increased by providing a relatively short path of travel for the magnesium vapor from the reaction zone to the condenser. This may be most conveniently effected by disposing the condenser surface in relatively close proximity to the reaction mixture substantially as shown in the schematic drawing of Fig. 1.

In the drawing, the furnace is shown as consisting of two separable halves, hermetically joined together, the lower half being provided with a shallow'substantially dish-shaped hearth surfaced interiorly withea refractory silicate lining, in the bottom of which hearth is located a plurality of electrical resistor heating elements, such as graphite rods, and in which is disposed the charge of pellets or briquets to be reacted in direct thermal contact with the heating elements. The pellets as hereinabove disclosed consist of a mixture of MgO and an Al-Si alloy containing from about 20 to about 50% Si, the MgO in the mixture being sumcient in amount to provide three atomic weights of MgO for each two atomic weights of Al present in the alloy, and two atomic weights of MgO for each atomic weight of silicon present in the alloy, and the alloy being present in an amount suilicient to provide for an excess of fromto 25% of the amount required to reduce the MgO present.

The upper half of the furnace is provided with a centrally located depending condenser C which is interiorly water cooled substantially as shown, the bottom surface of the condenser C being disposed in relatively close spaced relation to the hearth and reacting pellets disposed thereon, as-compared to the side walls of the upper half, but spaced interiorly from the side walls and from thehearth a suflicient distance to provide for the heating of the side walls of the upper half of the furnace to temperatures substantially in excess of the volatilization temperature of the magnesium at the gas pressure employed. The water cooling of the condenser C is designed to maintain the magnesium condensate on the condenser surface at atemperature within the range 500-600 C.

Means are provided to evacuate the interior ofthefumacetothedesiredlowpressureas indicated in the drawing and preferably also such means should include means to repeatedly flush the furnace interior with an inert gas, such as hydrogen or helium, so as to remove substantially all atmospheric gases from the furnace interior. Preferably evacuation of the furnace is continued during the preliminary stages of heating until the adsorbed atmospheric gases are substantially removed from the furnace and then the furnace pressure is adjusted to the desired reduced pressure of theinert gas at which the reduction reaction is to be. performed.

In the practice of the present invention, a reduced pressure of less than one millimeter of mercury, preferably from .10 to .001 millimeter of mercury of the inert gas helium is employed, although any pressure up to atmospheric pressures of an inert gas may be utilized in the practice of present invention.

After the reduction reaction is completed the furnace interior is cooled to atmospheric temperatures before breaking the vacuum or restoring atmospheric pressures to the furnace interior. Restoration of atmospheric pressures to the furnace interior is preferably accomplished by means of an inert gas, such as CO2, helium or hydrogen, which contains a relatively small percentage of oxygen in the form of free oxygen or water vapor. The pressure is allowed to rise gradually to atmospheric pressure by slowly bleeding the inert gas to the furnace interior. In this manner, surface oxidation of the condensed magnesium is accomplished at a relatively slow rate with consequent limitation of heating of the individual magnesium grains or crystals avoiding thereby a heating up of the same to a temperature at which spontaneous combustion will occur.

After atmospheric pressure is restored, the deposited magnesium is recovered from condenser C in any convenient manner such as, for example, immersing the condenser in a bath of molten salts having a temperature above about 650 C. to melt the magnesium crystals therefrom. a

A marked economic advantage of the present invention is that by reason of the use of an aluminum-silicon alloy as a reducing agent the process may be practiced in combination with an electric furnace process for producing the aluminum-silicon alloy directly from clay or aluminum-silicate. In this combination of processes, the residue from the magnesium oxide reduction which consists of substantially pure aluminum silicate can be returned to the electric furnace for reduction to an aluminum-silicon alloy for re-use in the process, with resultant great economy, as one skilled in the art will perceive.

Whereas, in the above description, the use of an aluminum-silicon alloy in the reduction of magnesium oxide is disclosed, it is believed apparent that the alloy is of utility in the reduction of other metal oxides, particularly those metal oxides wherein the metal constituent has a boiling point below about 1400 C. at atmospheric or reduced pressures, such as, Ca, Ba, Sr, Zn, Cd and Having hereinabove disclosed the present invention generically and specifically, it is believed apparent that the same may be widely varied without essential departure therefrom and all such modifications and adaptations of the same are contemplated as may fall within the scope of the following claims.

What I claim is:

1. The method of reducing substantially pure magnesium oxide which comprises mixing the oxide in its finely divided state with a finely divided reducing agent consisting of an alloy of aluminum and silicon containing silicon 20 to 50%, balance aluminum, the said oxide and reducing agent being proportioned to provide 3 molecular weights of the oxide for each 2 atomic weights of Al present in the alloy and 2 molecular Weights of the oxide for each atomic weight of silicon in the alloy-together with a slight excess of the reducing agent in the mixture, compacting the mixture into porous aggregates and heating the said aggregates in an inert atmosphere maintained at sub-atmospheric pressures to a temperature above the boiling point of magnesium but below about 1400 C. for a time interval adapted to obtain the substantially complete reduction of the MgO content of the mixture by the said reducing agent with volatilizatlon of the magneized magnesium is condensed upon a condenser surface maintained at a temperature within the range 500-600 C. thereby to obtain relatively large sized crystals of magnesium metal.

3. The method of claim 1, wherein a relatively short path of travel is provided for the magnesium vapors between the heated aggregates and a condenser surface maintained at a temperature approximating 500-600 C. thereby to accelerate the rate of vaporization of said magnesium from the said heated aggregates and to obtain relatively large sized crystals of magnesium metal.

4. The method of claim 1, wherein said inert gas consists of one of the gases hydrogen and helium, and the said sub-atmospheric pressure approximates one millimeter pressure, and wherei111o the temperature of heating approximates 5. In the reduction of substantially pure magnesium oxide with a metallic reducing agent, the improvement which comprises forming a mixture consisting of finely divided magnesium oxide and finely divided metallic reducing agent intimately mixed together, said reducing agent consisting of an alloy of aluminum and silicon containing 20 to silicon and the relative proportions of said oxide and reducing agent being such as to provide in the mixture 3 molecules of oxide for each 2 Al present and 2 molecules of oxide for each Si present in the alloy with a slight excess of the said alloy above this ratio, compacting the mixture into porous aggregates and enclosing the said aggregates in a container closed to the atmosphere but containing an inert gas at a pressure below atmospheric pressure, heating the said aggregates to a temperature above the boiling point of magnesium but not above about 1400 C., and collecting the magnesium vapor arising from the heated aggregates upon a condensing surface located in relatively close spaced relation to the heated aggregates maintained at a condensing temperature of 500-600 C. l

6. In the reduction of substantially pure magnesium oxide with a metallic reducing agent at sub-atmospheric pressures, the improvement which comprises a metallic reducing agent consisting of a finely divided alloy of aluminum and silicon containing 20-50% silicon balance substantially all aluminum. the amount of said agent being in excess of that theoretically required to obtain substantially complete reduction of the said magnesium oxide.

7. The method of treating surface unoxidized condensed magnesium in the form of crystals and aggregates to condition the same for exposure to the atmosphere which comprises controlling the rate of surface oxidation of said crystals to a relatively slow rate until the entire surface of the said crystals have been covered with a protective oxide film.

8. The method of claim 7, wherein an inert gas containing a relatively small percentage of oxygen is contacted with the surface of said crystals until the entire surface of the crystals are covered with a protective oxide film.

HUGH S. COOPER.

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