US2387979A - Method of reducing magnesium oxide - Google Patents

Description

e a Patented on. so, 1945 a UNITED STATES PATENT OFFICE- I METHOD OF REDUCING MAGNESIUHOXIDE Hugh 8. Cooper, Cleveland, Ohio, assignor, by direst and mesne assignments, to Acme Aluminum Alloys, Inc., Dayton, Ohio, a corporation No Drawing. Application July 31, 1942,

Serial No; 453,051

7 Claims. (01. 75-67) This invention relates to metallurgy and more particularly to thermal reduction processes'and has for its object the provision of an improved thermal reduction process for the production voi metallic magnesium. v

Another object is the provision oi a relatively low temperature thermal reduction process for producing metallic magnesium.

form the intermetallic compound, calcium silicide Still another object is to provide a low temperature vacuum distillation method for the duction of metallic magnesium. I Other bbjects will be apparent as the invention pro- .is more fully hereinafter disclosed.

In accordance with these objects I have discovered that calcium silicide is an efiective thermal reducing agent for magnesium oxide at temperatures within the'range 900-1l50 C. and at reduced pressures approximating millimeter of mercury. v I

Heretoiore in the art, both calcium and silicon have been employed as thermal reducing agents for metal oxides at atmospheric and reduced pressures, and both reducing agents have been employed as thermal reducing agents for magnesium oxide. Each when used alone have certain defects which do not appear when the two agents have been reacted together to form the silicide intermetallic compound.

For example, silicon whenused alone as a thermal reducing agent requires a reaction temperature in the neighborhood of 1400-1500 C. to

reduce magnesium oxide at which temperature the silicon dioxide resulting, being strongly acidic reacts readily with unreduced magnesium oxide forming a thermally stable magnesium silicate. This reaction greatly lowers the eillciency oi the reducing reaction and interferes with the yield of magnesium obtainable by the process. The temperaturepi the reaction is in excess of tempera-'- tur'es easily obtained and requires the use of special high temperature equipment which at present is diillcultly obtainable.

On the other hand when calcium is employed as a reducing agent for magnesium oxide, while the temperature oi reduction is materially lower,

the diiiiculty in fragmenting the calcium into small sized particles required tor the reduction and the high chemical activity of the calcium when reduced to such small particle sizes with the atmospheric gases and water vapor, coupled with the high vapor pressure of the calcium at the thermally reducing temperature of magnesium oxide and the high cost of this reducing agent,

renders the use 01 this method economically'unpractical.

When calcium and silicon are first reacted to (CaSiz) all of the advantages of the low temperature reduction of magnesium oxide by calcium is retained with none of the attendant disadvantages, and the relatively low cost of the calcium silicide as compared to the cost of either calcium orsilicon render the process decidedly commercially practical.

In the reductionof magnesium oxide by calvessel, or retort and briquet charge therein is heated to a temperature within the range 900- 1150 0., preferably about ll00 0., and is maintained at this temperature for an extended time interval adapted to obtain the maximum yield of magnesium at minimum cost.

In the above process, many factors enter into the eiilciency oi the process. The most important factor is the relative proportions of magnea slum oxide (MgO) and calcium silicide (cash) in the mixture. The second most important iactor is the temperature of reduction. A third important factor is the length of the path of travel for the magnesium from the point of reduction and vaporization to the point of condensation. Other factors are the mode oi heating the charge and the retort apparatus employed.

With respect to the relative proportions of magnesium oxideto calcium silicide, the primary reduction reaction under the low pressure conditions invloved is between the calcium content of the silicide and the magnesium oxide.

Under the low pressure conditions present, the calcium content of the calcium silicide tends to be vaporized and the calcium vapors at the temperature of heating react with the magnesium oxide resulting in the formation of calcium oxide and magnesium metal vapors which vapors are distilled off from the reaction mixture.

However, the silicon when set free by the calcium, vaporization and its reaction with magnesium oxide, is in a highly reactive condition, and is reactive as a reducing agent towards both calcium and magnesium oxide. This circumstance results in the reduction of some of the magnesium oxide by the silicon but the predominant reaction is a reduction of the calcium oxide by the silicon for re-use in the reduction of more magnesium oxide.

The extent to which this silicon regeneration oi calcium vapors may go varies widely depending upon the fineness of subdivision of the magnesium oxide and the calcium silicide, upon the intimacy of the admixture of the two reacting constituents and upon the pressure employed in compacting the mixture together. With any given particle size of mixed oxide and silicide and intimacy of mixture there is an optimum I pressure to provide the right porosity for vacuum Mgo-l-Casiz Mg-l-Cao-i-siz 40.36 96.8-

Such a proportioning is roughly 2 parts calcium silicide to each part magnesium oxide. The calcium silicide content may be reduced to as low as 2 parts silicide to each part magnesium oxide without markedly effecting the efliciency the interior I of the reduction reaction or extending the time interval of reduction inordinately. By the use of an excess of calcium silicide over the 2 to 1 ratio above indicated the rate of reduction is accelerated and the time interval for complete reduction of the magnesium oxide is reduced.

Under any given conditions it is necessary to determine the precise ratio of reacting constituents that produces the best results in the most economically practical manner.

In the thermal reduction of magnesium oxide in accordance with the present invention, 1 have found that the most economically practical apparatus to be employed is that which is described and claimed in my co-pending application Serial No. 453,984, filed August 7, 1942, which application is assigned to the same party and in the same manner as is the present application.

In the apparatus ofthis said co-pending application the charge of briquets is disposed in a graphite-clay crucible which is located centrally within a. vertically sustained tubular metal chamber which is heated externally and uniformly to I the desired reaction temperature.

The upper and lower ends of the tubular chamber extend beyond the heated central portion enclosing the crucible and are provided with removable end closure members provided with vacu- I um seal joints securing the same in closure posievacuate the interior of the chamber to the de sired low pressure. The location of the condensing surface in relatively closed spaced relation to the open end of the crucible provides a means for maintaining the condensed magnesium at a temperature approximating but below the solidification temperature or melting point whereby grain growth is promoted with the formation of large sized crystals and aggregates which are resistant to oxidation except superficially on exposure to the air during the transfer of the condensed metal from the reducing chamber to a bath of molten salt for fusion and casting into ingot form.

The exterior surface of the tubular metal chamber is protected from oxidation at the temperature of heating by means of an enclosing refractory sleeve disposed in close space relation to the exterior surface of the chamber, and by providing in the said space gap a protective or non-oxidizing atmosphere such as H2, N2. cracked ammonia gases, illuminating gas, coke oven gas, or the like.

For more specific description of the preferred apparatus for the practice of the present invention, reference should be made to this description of said co-pending application.

It is believed apparent from the above dis-.

closure that the method of thermally'reducing magnesium oxide by the reducing agent calcium silicide at temperatures within the range 900- 1150 C. and at reduced pressures of the order of less than one millimeter of mercury, may be varied widely without essential departure therefrom, and all such modifications and adaptations thereof are contemplated as may fall within the scope of the following claims:

What I claim is:

1. In the reduction of magnesium oxide with a metallic reducing agent at elevated temperatures and under reduced pressures, the improvement which comprises forming porous aggregates consisting of an intimate mixture of magnesium oxide and calcium silicide, each in finely divided state and in the relative proportions of one part oxide to about 2 parts silicide and heating the said aggregates to a temperature within the range 900-1150 C. in a container closed tothe atmosphere and evacuated to a pressure of less than one millimeter of mercury.

2. The method of producing metallic magnesium which comprises forming an intimate mixture of magnesium oxide and calcium silicide, each in finely divided condition, and in the relative proportions of one'part oxide to from 2 to 3 parts silicide, compacting the said mixture to porous aggregates, heating the porous aggregates to a temperature within the range 900-1150 C. in a'container closed to the atmosphere and evacuated to a pressurebelow about one millimeter of mercury and collecting the magnesium vapors within said container upon a condenser surface maintained at a temperature silicide with finely divided magnesium oxide in the relative proportion of 2 to 3 parts silicide to each one part oxide, compacting the said mixture into porous aggregates, heating the said aggregates to a temperature within the range 900- denser disposed inrelatively close spaced relationship to the said aggregates, the said surface being maintained at a temperature approximating but below the melting point 01' magnesium.

4. The method of producing magnesium which comprises forming a mixture or substantially pure magnesium oxide and calcium silicide each in finely divided condition. the relative proportions 01' oxide to silicide being within the range 1 to 2 and 1 to 3, compacting the mixture to porous aggregates and heating the said aggregates to a temperature within the range 1150 C. in a container closed to the atmosphere and evacuated to a relatively low pressure, thereby to effect vaporization of the calcium from silicide combination and the reduction of the magnesium oxide by the vaporized calcium and the reduction of the calcium oxide product by the silicon residue of the said silicide.

5. The method of claim 4. wherein the magnesium vapor evolved from said heated aggregates is collected upon a condenser surface mainthe melting point of magnesium.

tained at a temperature approximating but below 6. The method oi claim 4, wherein the magnesium vapor evolved from said heated aggro gates is collected upon a condenser surface sustained in relatively close spaced relationship to the said ag regates and maintained at a temperature approximating but below the melting nesium vapors evolved from the said Cheated aggregates upon a condenser surface maintained at a temperature approximating but below the melting point 01' magnesium disposed in a position providing a relatively short path oi travel for said vapors.

HUGH 8. COOPER.