CA1141171A - Magnesium production - Google Patents

Abstract

MAGNESIUM PRODUCTION

Docket 6631-G

Robert M. Kibby ABSTRACT OF THE DISCLOSURE

Magnesium metal is produced in a magnesium reduction fur-nace by the reaction of aluminum metal with a calcium magnesium aluminate slag or with magnesium oxide in the presence of such slag, wherein aluminum is fed to the magnesium reduction furnace as an aluminum silicon alloy and wherein magnesium oxide is fed in less than stoichiometric amounts so that not all of the alumi-num is reduced, such additions producing magnesium vapor and two liquid layers: an aluminum silicon alloy having a reduced alumi-num content and a MgO?CaO?Al2O3 ? TiO2 slag. The aluminum silicon alloy layer is tapped and recovered from the furnace. The re-covered alloy may be used in the production of silicon alloy product by addition to aluminum.

Description

7~ -BACKGROUND OF THE INV:E:NTION
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This invention relates to the metallothermic preparation of metals and more particularly is concerned with a novel pro-cess for the production of magnesium metal by the metallothermic reduction of magnesium oxi.de at high temperatures in the presence of an aluminum silicon alloy reducing agent and a molten oxidic slag in an electric furnace and the condensation and recovery of vaporized magnesium in a condenser. ..

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Thermal reduction processes for the conversion of magnesium oxicle or of substances containing magnesium oxlde to metallic magnesium have evolved along two general lines: I
those which use carbon as a reducing agent (i . e ., carbo- ¦
thermic processes) and those which use free metals as a reducing agent (i.e., metallothermic processes). In both types of processes the necessary heat of reaction is usualJ~ ¦
supplied by an electric arc furnace in which an electric current ~ay be passed throu~h ~le feedstoc~ mixture ~nc~ usually is passed !
through the resulti7~g liquid or solid slag by-product.
It is known that aluminum is a very effective free-metal reducing agent for magnesium oxide. It is also known that aluminum can be obtained relatively cheaply in the form of an aluminum silicon alloy, for example, by carbo-thermic smelting of aluminum-silicate ores. Examples of known processes for the production of ma~nesium wherein an aluminum-silicon alloy is used as the metallic reducing agent are described below.
.. U.S. Patent No. 3,579,326 teaches a process for producing magnesium by reducing magnesium oxide from an oxidant containing a major proportion of magnesia (rather ~ !
than dolomitic lime) with a metallic aluminum-silicon alloy reductant having a ratio of silicon to aluminum of at least 0.8 to 1.0 (i.eO, of at least abou~ 40 percent Si) at a temperature of at least 1400C and at a pressure of about 1 atmosphere in the presence of a molten slag containing 15 to 35 percent alumina, less than 30 percent calcium oxide, 5 to 25 percent magnesium oxide, and ~5 to 50 percent silica.
I'he molecular r\tio o= magnesium oxide to calcium oxide in ..., . , ' ...
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the oxidant of the '326 process is at least 2 1. The ratio of aluminum and maynesium oxides to silicon dioxide in the slag is less than 1.6, the aluminum and magnesium oxides comprise less than 50 percent of the slag, and the ratio of calcium oxides to -the silicon dioxide of the slag is less than 1.6. Generally the patent teaches that:

The composition of slag is determined by ;
the ratio of aluminum to silicon fed as ,, the reducing agent, the degree of utiliza-tion of silicon as reductant, which for reasons of econom~ should be as high ~s i~
fea.sible; and the relative proportion of ~, magnesium oxide red as magnesia and as ., dolomitic lime. (Column ~, lines 38-43) , ~emphasis added) The examples summarized in Table I of said patent show production of a ferrosilicon alloy by-product containing from 56 to 75 percent Si when the metallic aluminum silicon alloy reductant also contains iron, but show no production by-product alloy when iron is not present in the reductant.
U.S. Patent No. 3,782,922 teaches a process for producing magnesium by reducing magnesium oxide from an oxidant containing a major proport;on of magnesium oxide (the weight ratio of MgO:CaO in the oxidant is between about 1.1 and 2.3) with a substantially pure aluminum reductant (i.e., the reductant contains at least 85 percent aluminum) ¦ ~, 'at a temperature between about 1300C and 1700C and at a pressure of about 1 atmosphere wherein the slag produced as a by-product of the reaction has a composition of about 35-65 percent alumina, 35-55 percent calcium oxide, 0-10 percent silica, and less than 5 percen~ magnesia (when the slag is removed from the system). The particular calcium aluminate slag produced by this process is said to be highly advantageous in that aluminn can readily be recovered by leaching the ~: ' . ' . ~ ~- ..... ,..... 1.,, 11~11'71 slag with Na2C03 solution. ~mong -the asserted advantayes of the'~22 process is the virtuall~ complete consumption of -the aluminum reducin~ agent in the primary reaction, which avoids the necessity of recycling or dlsposing o considerable quantities of spent metal (Column ~, lines 69-74). However I it is noted at the bottom of Table III appearing in Column 11 of the specification that "The magnesium producecl ~
contain up to 20 percent Al." Compare the following statements appearing at Column 3, lines 24-~5 of the same specification: ¦

Finally, tnere is the ~uestion of aluminum itself as an impurity in the magnesium product, since aluminum has, a vapor pressure of about 10 mm. ~Ig at 1500C. This means that magnesium produced by the present process will ine~ritably contain aluminum -- how much depends upon the operating temperature. At I 1~00C., for example, it would contain about 0.5 percent, at 1500C. about 1.2 percent and at 1600C., about 2 percent of alur,linum.
Ho~ever, this is not a serious problem and may-in fact be beneficial, because: (a) the principal use for magnesium today is to produce aluminum alloys for fabrication;
and (b) a major portion of the magnesium used for fabricated magnesium products contains a substantial proportion of aluminum -- generally . from 3 to 9 percent. Thus the presence of a small amount of aluminum in the magnesium produced by the present process is not det~i-mental, especially if the magnesium operation is associated with the production of aluminum, which is likely to be the case because of the advantage of recovering A12O3 from the slag produced, and the possibility of using captive scrap as the reducing agent.

Whatever the specific aluminum content of the magnesium produced by the '922 process may be, it is clear that the product ~ill contain a significant amount of aluminum.
Furthermore~ since nearly all of the magnesium oxide present in the oxidant charge is converted to metallic ma~nesium ~rapor in the reduction furnace ~the magnesia content .. : _ ~ ~ ... ......

11~1171 of the slag is less than 5 percent, preferably less than

2 percent), it is apparent that, accordin~ to the '922 teaching, the quantity of alumlnum reduc-tant ~ed to the furnace may be somewhat greater than the stoichiometric amount. This is expected ~ecause of relatively high vapor pressure of aluminum metal at the process temperature and the consequent carry-over o~ aluminum vapor with the volatile magnesium product. Nevertheless, the '922 process does not produce a by-product spent alloy reductant (as noted above). ¦
Figure l of the patent does show removal of a "metallic residue" from the furnace, but that "residue" refers to "impurities" such as copper or chromium which are present in certain alloys and scraps which may be employed as the "substantially pure aluminum" reductant in the process (see Column 8, lines 43-56).
U.S. Patent No. 4,033,758 teaches a process for producing magnesium by reducing magnesium in a calcium magneslum aluminum silicate slag or magnesium oxide in the presence of such a slag with a metallic aluminum silicon alloy reductant comprising from 15 to 75 percent by weight aluminum and Irom 20 to 80 percent by weight silicon at a temperature of about 1400 to 1650C and a pxessure of about 25 to 500 mm of ~Ig (about 0.03 to 0.66 atmosphere~ in the presence of a molten slag containing ll to 38 percent alumina, 42 to 65 percent calcium oxide, l to ll percent magnesium oxide, and 5 to 19 percent silica. The amount of magnesium oxide fed to the reaction zone is at least 101 percent of the amount theoretically required to react with the aluminum silicon alloy reductant.

119Lil71 U.S. Patent No. 4,033,759 teaches a process for producing magnesium by redllcing magnesium in a calcium magnesium aluminum silicate slag or maynesium oxide in the presence of such a slag with metallic aluminum reductant containing at least 80 weight percent aluminum at a temperature of about 1350 to 1700C and a pressure of about 0.5 to 2.0 atmospheres in the presence of a molten slag containing 28 to 64 percent alumina, 30 to 65 percent calcium oxide, 6 to 13 percent magnesium oxide, and less than S percent silica. The amount of magnesium oxide fed to the reaction zone is at least 110 percent oE the amount theoretically required to react with the alumlnum metal reductant. The patent suggests that lt is essential to keep the silica concentration in the furnace at a low value (less than 5 weight percent) when using aluminum metal as a reducing agent because of the undesirable side reaction of aluminum with silica to produce by-product silicon metal ~Column 3, lines 42 to 63). The combination of reactants taught by the '759 patent, particularly the high concentration of aluminum metal in the reductant and the stoichiometric excess of magnesium oxide, is said to provide a superior process in that the reaction between aluminum metal and magnesium readily occurs and a high utilization of expensive aluminum metal i5 obtained.

U.S. Patent No. 3,441,402 claims a metallothermic method for the production of magllesium in a submerged arc furnace wherein an oxidant mixture of calcined dolomite and a second magnesium ore selected from the grou~ consistiny of calcined magnesite and dried serpentine is reduced with ..

.. . ............ ...... Jl 1171 1l a reducin~ agen~ aL t~mper~turcs below abou~ 1500C and E~r~ssurcs o ~bout 1 at:mo~ph~rc. The }~atcnt tca.ch~ the use of aluminum-silicon alloys containin~ frorn about 30 to 100 weight percent: aluminum as a reducing a~ent in the process ~Column 3, lines 32-35). The Examples su~gest the addition of aluminum-silicon alloy in approximately stoichion~etric amounts: the operations of Exam?le 1 show a slight (6 - 13~) excess of silicon-aluminum alloy (which alloy contains 66.34 weight percent alu~inum) and Example 2 also suggests an excess o~ silicon-alumin~Q - , alloy. The Examples further suggest that ~he reduction of -the mixt~re of magnesium or~s occurs in the presence of a molten slag containin~ about 20 to 30 percent alumina, 30 to 50 percent calcium oxide, less than 10 percent ma~nesium oxide, and 15 to ~0 pexcent silica.
: Of th~ five foreqoing ~atents describing metallo-thermic processes for the conversion of magnesium oxide or of substances containin~ magnesium oxide to me~allic magnesium -¦
using an ~iuminu~-silicon alloy as reductant, three of t'n~
pate~ts t~ach or suggest the addition o~ less than the stoichiometric amount of magnesium oxide theoretically reguired to react with the meta11ic reductant: ~.S. Patent Nos. 3,~79,326; 3/782J922; and 3,~41,402. }lowever, none o these thre~ patents teach or suggest the addition of less than the stoicl.liometric amount o~ ma~nesium o~ide ` .
theoretically re~uired to react wi~h the aluminum component of an alumi~un~-silicon alloy reductant -- with ~he possible exccption o~ U.S. Patent No. 3,7R2,922 (which, it shoùld be noted, cmploys a "substantially pure aluminum reduc~ant"~.

q~e cxcess addition o~ reductall~ in the '~22 process is neccsaary to compcnsatc for r~ductant losses caus~d by the ;' -', ,'~. I
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relatively hiyh vapor pressure of the reductant at process temperatures. ~ccordingly, none of the patents specifically teach or suggest the addition of less than the stoichiometric amount of magnesium oxide theoretically required to react with the aluminum component of an aluminurn-silicon alloy rcduct~lt present in a magnesium reduction zone. Furthermore, none o~ the patents specifically teach or suggest the addition of less khan the stoichiometric amount of magnesium oxide theoretically required to react with aluminum combined with the retention of unreacted aluminum in the by-product alloy; none of the patents teach or suggest the addition of sufficiently less than stoichio-metric amounts of magnesium oxide so as to have aluminum remaining in a by-product alloy, It may be noted, though, that U.S. Patent No. 2,847,295 does teach the addition of a surplus of metallic reductant -- an amount exceeding the theoretically necessary amount to completel~
reduce magnesium oxide contained in an oxidant feed -- and more ,i particularly teaches that when the reductant component actin~
as the reduction material is added with accompanying metals, the surplus of the reductant component should be such that compounds or alloys o the component with the accompanying metals will remain after the reaction is completed. rrhe object of the surplus addition is to minimize the deleterious ef"ect of the compounds or alloys on the reaction capability of the reductant component acting as the reduction material and to maintain an '~almost metallic conductivity" in shaped bodies comprising the reaction medium until the end of the reaction.
Unlike the hereinbeforc described process, the '295 process does not reducc magnesium oxide in the presence of a molten oxidic slag, Rather, the reactant and residual materials are maintained . ~1 ~. , , , 1. ~ r -in the solid state withollt the appea~ance of a liquid phase (see Column ~, lines 11~15 and Column ~, lines 7-~3).
~ nother process of interest (althou~h it does not teach the use of an alum:inum s~licon alloy reductant) is disclosed in sritish Patent No. 922,300 whi.ch is a modified aluminothermlc process called the "MC process". The process described therein is a two-stage cyclic process for the production of magnesium. In the first stage, a material con-taining ma~nesium oxide and lime is reduced by means of aluminum to form magnesium metal vapor and a calcium alumina~e slag. In the second stage, the calcium aluminate slag is reacted with carbon in the presence of an auxilliary metal selected from the group consisting of iron and copper to form calcium carbide, a residual slag, and an alloy of aluminum metal with the auxilliary metal which alloy is returned to the first stage for the production of further quantities of magnesium metal. The auxilliary metal thus serves as a carrier metal for the aluminum metal reductant. Like the teachings previousl~ discussed, this patent neither shows nor suggests the addition of less than the stoichiometric amount of magnesium oxide theoretically required to react with the aluminum component of an aluminum alloy reductant;
the Example shows the addition of an excess of about 14 percent magnesium oxide.

SUMMARY OF THE I~IVENTION
.,, ` Generally, the present invention may be characterized as a metallothermic or modified aluminothermic process for the product:ion of magnesium operable at atmospheric pressure .
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11~1171 wherein an aluminum silicon alloy reductant and a macJnesi~
oxide o idant charyed principally as dolomite in an amount corresponding to less than the stoichiometric a~ount of m~gnesiu~
oxide theoretically required to consume the alu~inum componen-t o~
the aluminum-silicon alloy reductant are reacted in a magnesium reduction furnace containing two li~uid layers -- an aluminum~
silicon alloy of increased silicon content (relative to the reductant charged) and a molten calcium magnesium aluminate slag, evolving magnesium vapor from the reaction zone, condensing and recovering the magnesium as a product, tapping the aluminum-silicon alloy of increased silicon content from the reaction zone as a by-product, and rejecting the calcium magnesium aluminate slag from the system. The alloy by-product may be used to produce a silicon alloy product by addition to aluminum.
Because of the addition of less than stoichiometric amounts of magnesium oxide theoretically required to react with the aluminum component of the alioy reductant, two immiscible liquid phases are present in the reduction furnace -- a lower layer comprising slag and an upper layer of allo~. By controllin~
the ratio of Al:Si in the alloy reductant, the charae rate of the alloy reductant, the charge rate of the oxidant,and the weight ratio of ~lgO to aluminum in the charge, the stoichiometric excess of the aluminum reductant component can be maintained. In the process of the present invention, the presence of the silicon alloy in the reduction furnace has the further advantage of reducing the amount of aluminum vapors evolved from the reactant mass, allowing the production of a relativelv pure magnesium product as compared to the magnesium product produced b~ the process of V. S. Patent 3,782,922. ~owever, . ~. ..... ~..... ,.... ~-~117~1 this is accomplished without sacrificing the highly clesir~ble reactivity qualities of an aluminum reducin~ a~ent. Similar to the auxilliary metal of the modified aluminothermic process of British Patent No. 922,300, the silicon component of the alloy reductant passes through the system with minimal or no "losses" to the molten slag in the form of silica. The process of the present invention is thus a desirable alternative to and improvement over known metallothermic processes for the production of magnesium using highly effective aluminum in the form of relatively inexpensive aluminum silicon alloy as reductant BRIEF DESCRIPTION O~ TH~ DRAI^lING

_ _ . . . .. _ _ Figure 1 shows an apparatus suitable or carryina out the process of this invention. -' Figure 2 shows the relationship of pressure to reaction temperature in the furnace as a function of the percent aluminum in the by-product alloy for typical charge ComPOSitiOnS.

DETAILED DESCRIPTI~N OF TH~ I~lVE~lTIO~

. . .._._ ~ he aluminum silicon alloy reductant of the present invention may have a Si:Al ratio between about 0.4:1 to 4:1. However, since the spent alloy reductant withdrawn -from the reduction furnace has a Si:Al ratio ~etl~een about 2:1 to 6:1, the Si:Al ratio of the alloy charged to the reduction furnace is desirably within the ranqe from about 0.4:1 to 2:1.

In a preferrecl embodiment of the process of this invention, the Si:Al. ratio o the alloy reductant charged to the reduction ¦furnace is bout 0.7:1 and tbe si:~l ratio of the spen~ alloy . . , ,,,, . . .

reductant withdraWn f~om the reduction furnace is about ~
Various processes are known which produce aluminum-silicon alloys having the foregoing composltion. For example, startlng from kaolin (or quartzite) and alumina it is well known tha-t alloys containing 60 or even 70 percent aluminum may be produced in an arc furnace. In particular, the processes disclosed and elaimed in U.S. Patent Nos. 3,25~,988 and 3,665,362 produce alloys suitable for use as the reductant of the invention.
The magnesium oxide reactant comprises dolime (CaO.
XMgO, where 0,5 ~ X ~ 2,0) or other minerals or mixtures of ~inerals consisting of magnesium oxide and ealeium oxide having a molar ratio of MgO:CaO less than about 4:1. Magnesia may be used to supply MgO and lime may be used to supply CaO. Preferabl~, the oxide charge eontains principally Cabout 50 weight percent or more) dolomite. The presence of impurities such as titania should not exceed about 5 weight percent. In other words! the oxide charge to the process of this invention comprises magnesium oxide and calcium oxide in a molar ratio from about 1.0:1 to 4.0~ referably, the oxide charge comprises magnesium oxide and calcium oxide in a molar ratio of from about 1.3:1 to 2.1:1.
The amount of magnesium oxide charged is less than the stolchiometric amount theoretically required to consume the aluminum component of the alloy reductant. The amount of magnesium oxide in the oxidant charge is between about 80 to 9~ and preferably, 88 to g~ percent by weight of the stoichi.ometric amount, An in~ortant consideration in the o~eration of the reduction furnace is that the m~terial in the furnace be molten so that it can be tapped. ~e furnace temperature should be high enough to fo~la molten slag, but higher t~mperatures are not preferred. The necessary sla~ temperature is kn~l to ~ '7~

depend on the ratio of A12O3:CaO:MgO in the s1ag and particu]arly on the alumina and calcium oxide con-tent of the slay. Preferah.1.y the alumina present in the slag of the process of this invention is derived totally from the reaction products of the reduction of the oxide charge to metallic magnesium. The slags ~eneratecl by the process of this invention have l.iquidus temperatures in the range of 1500C to about 1900C. The magnesium pressure produced will depend upon the thermodynamic activity of the aluminum in the aluminum silicon alloy which is periodically tapped, as a by-product, and upon the activity of magnesium oxide in the molten slag in contact with the by-product alloy layer in the rurnace.
The activity of aluminum in the by-product alloy layer in the furnace is determined principally.by the percent a].uminum in the alloy, which is controlled by increasing or decreas.ing the heat release in the furnace. .
The activity of magnesium in the slag is determined ~y .
the temperature of the slag and its composition, which in turn is controlled by the weight ratio of MgO to CaO and the weight ratio of MgO to aluminum in the furnace feed.
Figure 2 may be used to illustrate the inherent stability of the system with respect to the control of percent .
aluminum in the by-product alloy. Consider,. for e~ample, that a steady state has existed correspondlng to point ~, where the furnace cha e is 40~ Si:60~ Al by weight, the red~ctant cha~qe . ' '', ' ''`

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is 1.5 parts by weight M~O to 1 part CaO, the weight ratio o~ ~go to aluminum in the charye is 2:1, the by-product alloy contains 20% aluminum and the temperature is 1790C. I~ or some reason there should occur a temporary diminution of the rate of maanesium production, then the aluminum concentration in the by-product alloy would tend to increase; for example; to polnt B. The activity of the aluminum would therefore increase, the equilibrium pressure of magnesium would tend to increase and the reaction to produce magnesium would proceed faster, tending to counteract the effects of the disturbance.
If the furnace has been operating steadily at a point of A of Figure 2, and it is desired to increase the percentage of aluminum on the by-product alloy, one of two control actions can be taken: (a) the heat input to the furnac~ can be decreased to achieve a reaction temperature of, say, 1650C, correspondiny to point C, or (b) the weight ratio of MgO to aluminum in the furnace charge can be decreased.
.. The relationships of these operatin~ parameters are further described in the Examples, infra.
Qualitatively, the sla~ composition of the process of the present invention may be expressed as McJO.CaO.A12O3.TiO2.
The titania content is an impurity of the aluminum-silicon alloy and will be present in the alloy in amounts less than about

3 weight percent. Silica may also be present in the .~lag hut in amounts less than about 5 weigbt percent.

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Quantitatively, the slag composition may be eXpressed as follows:

Percent (~t.) I-lighly Component Broad Ranye Preferred RangePreferred _ _ _ R_n~e A123 ~1-63 41-58 49-52 CaO 27~54 40-54 43 4a MgO <10 ~7 <5 SiO2 0-5 0-5 0-5 The molten slags of the process of this invention having a composition within the less preferred ranges may be referred to as basic slags. Basic s]ags have a calcium oxide content of at least 40 percent anZ
usually about 50 percent. Such slags are characterized by a relatively sharp melting point and form a fluid slag of low viscosity with little superheat. To be contrasted-with basic slags are acidic slags which have a somewhat vague melting point and form rather viscous, "glassy" slags which require considerable superheat to achieve lower viscosity.
In the process of thls invention the more fluid acid slags are desired because of mass transport considerations, and such more fluid slags occur at charge weight ratios of MgO/
CaO below about 1.50 to 1, corresponding to molar ratios below ¦about 2.0 t 1.

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In the process of this invention the lower the ra-tio of MgO to CaO in the charge, the higher the unit consumption of oxidant raw materials, but t;he lower the ratio of magnesia to dolomite in the charge. Thus for practical and economlc reasons, the preferred ratio of MgO to CaO is around 1.5 to 1, and the ratio of MgO to aluminum in the charcJe composite is around 2.0 to 1, corresponding to a by-product alloy having 22 aluminum, a slag containiny about 44~ CaO, 50~ A1203 and 6%
MgO, and USillg a reductant feed of 40~ silicon 60~8 aluminum.
In carrying out the invention, a slag of a composition within the foregoing ranges is prepared and melted in a magnesium production furnace. The various slag ingredients may bè mixed together or a slag of a suitable composition from a previous operation may be used. Heat is supplied for melting either by striking an arc between electrodes suitably located inside the magnesium production furnace or, preferably, by suitably locating one or more carbor e]ectrodes so as ~o pass a current t~rough the slag (i.e.,in a direct arc furnace) or by any other suitable means. After the desired temperature o the molten slag is achieved, aluminum silicon alloy is charged to float as a liquid layer upon the molten slag. At the same time an oxide feed having the above-described composition is added to the slag or, alternatively, the oxide feed is inten~ingled with the alloy and the muxture is charged to the furnace. The charge rates 11'71 of alloy and oxide are adjusted so that less than the stoichiometric amount of magnesium oxide theoretically reyuired to consume the aluminum component of the alloy charge is added to the system ancl so that the slag eomposition remains relatively¦
eonstant. Magnesium vapor is evolved, conducted to a suitable condenser, and is condensed at a pressure of about one atmosphere.
As the reaction proeeeds, the levels o~ slag and spent alloy in the furnace rise. Periodically, portions of the slag layer and of the spent alloy layer are removed through suitable tap holes in the furnace ~lall.
Referring now to Figure 1, which shows an apparatus suitable for carrying out the proeess of this invention, an example of a preferred embodiment of this proeess will be described. A molten aluminum silieon alloy from a standard eleetric arc furnace adapted to produce aluminum silicon alloy is introduced to magnesium production furnace 10 through line 1. The alloy contains about 60 weight percent aluminlm ancl 40 weight percent silicon. Simultaneously with the introduction of alloy, dolomite eontaining ealcium oxide¦
and magnesium oxide in a molar ratio of about 1:1 is introdueed through line ~. The molar ratio of MgO:Al eharged in ~he oxidant and reductant respectively ls about 1.4:1. The alloy charged to the furnace floats as a liquid layer 13 upon a slag layer 14 eontain 3 about 44 percent CaO 50 percent ~1203~ and 6 I ' ' ,,. ,.. !,",~

1~ '7:1 percent MgO. Heat is applied in the furnace 10 by conducting electric current between electrodes 15 an~ 16 through the liculd slag to maintain the liquid at about 1700C and to cause the aluminum in the alloy layer 13 to react with /~aO in the slag 14 to produce Mg(V). The reaction occurring in the furnace may be expressed as follows: .
3 MgO + 2 Al __ > 3 Mg~.V) + A12O3 Heat transferred from the arc is sufficient to vaporize magnesium from the metal layer at about 1 atmosphere furnace pressure. Magnesium vapor evolved in the furnace 10 is removed through line 17 to condenser 20. The heat transfer rate in condenser 20 is adjusted to condense magnesium at a pressure of about 1 atmosphere and the condensed magnesium is cooled to about 350C before leaving the condenser ~ischarge circuit through line 21. Approximately 90 percent of the ma~nesiu~. .
charged as dolomite and magnesia is recovered as magnesium metal. .
The spent aluminum silicon alloy in layer 13 is at least periodically tapped through tap hole 19 to be used in production of a silicon alloy by addition to aluminum and the slag in layer 14 is tapped through line 22 at a rate to maintain slag level and ~s re tec -rom ehe netal produclng system.

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... - 18 -11~L1171 XAII .S I~III

The following table describes three examples o~ the process of this invention generally carriecl out in conformallce to the foregoing description of Figure 1.

I XI III
Reductant Charge (lb~./hr.)100 100 100 Al 60 60 50 Si ~ 40 50 Oxidant Charge (lbs./hr.) 240 200 166.6 Dolomite 206 ~0 1]4.5 MgO 3~ 92 52,1 MgO/CaO weight ratio 1.0 1.5 1.5 ~IgO/Al weight ra~io 2.0 2.0 2.0 Slag Discharge (lb~./hr.)222.5 183.0 152.5 CaO 120.0 8~.~ 66.7 A123 94.5 g~.o 76.4 MgO ~.0 11.0 4.7 By-product Alloy (lbs./hr.) 50 51.2 59.5 A1 10 11.2 9.5 Si 40 40 ~ 50 Magnesium Product (lbs.hr.)67.5 65.8 5~.6 Weight Oxidant charges/Mg Prod. 3.56 3.0~ 3.05 Weight Reductant Alloy/Mg Prod. 1.48 1.51 1.6 Stoichiometric ratio of MgOB9 . 3~ 89 . 3%89 . 3 charged rela-tive to the amount reauired to react ., with avaiiable Al.

Reduction Temperature (C) 17~0 1660 1740 Mg Furnace Pressure (atm.) 1.0 1.0 1.0 Condensation Temperature (C)1000 1000 1000 ;~ ' .
:.
The system pressure shown in the Examples is controlled by adjusting the cooling rate of the condenser to maintain the recited condensate temperature. Mote that the temperatures shown do take into account the pressure drop caused by mass transport of ~lg(V) from the reaction zone (i.a., the magnesium reduction furnace) to the condenser. The lower limit of condensate temperature is the melting point of magne.sium or : ~ ph~sical violence in the furnace, whichever occurs first.

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The slags produced in Examples I, II and III are entirely liquid at the operatlng temperat-lres cited in the examples. The presence of aluminum in the by-product allov assures that the amount oE si:Llcon reacting to produce ma~nesiurn is insignificant, and the silicon an~ calcium oxide comnonents or the furnace feed pass on through to the slag discharge.
The process can be operated below 1.0 atm. (for example 0.8 atm or above) by application of control principles familiar to those normally skilled in the art in view of the principles that have been disclosed here n.
However, it is preferred to operate at 1 atm or more (for examvle, 1.5 atm.) to avoid leaking of air into the furnace.
In the Examples I, II and III, representing pre,erred embodiments of this invention, the amount of MgO contained in the feed is about 89~ of the `stoichiometric amount rec~uired to react completely with the aluminum provided by the reductant charge. The reason that the MgO is not entirely reacted is that the mass rate of aluminu~ withdrawn from the furnace with the by-product alloy is great enough to preclude complete reaction by the MgO, as dictated by steady state mas.s balance considera-tions. I~ the mass rate of aluminum withdrawn as a component of the by-product alloy were to be reduced to the rate ~llowing complete reaction of the MgO, the percentage and hence the actlvity of aluminum in the alloy would be so low that excessively high temperatures would be required to achieve the pressures near l atmosphere, which are preferred.

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~ ...... ,.. "~ " ~ , !,r The conditions of Example II are superior to tho.se of Example I, in that the operatlng temperature is lower, the consumption of raw materials is less, the by-produet slag produetion is less and the by~ roduct alloy has more aluminum.
in it. However, the conditions of Example II reauire a higher rate of magnesia and lower rate o.f dolomite which ma,y af~ect the economics of the proeess in some loeations.
~ he conditions or Example III illustra-te the point that a feed alloy leaner in aluminum can be used without sianificant penalty in oxidant material costs, but will result in higher reductant consu~ption than shown for Examples I and II.

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Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved metallothermic process for the manu-facture of magnesium wherein an aluminum-silicon alloy reductant and an oxidant comprising an oxide mixture containing MgO and CaO are charged to the reaction zone of a reduction furnace in the presence of a molten calcium-aluminate slag and magnesium vapor is evolved from the reaction zone and recovered in a condensing means, the improvement which comprises charging less than the stoichiometric amount of magnesium oxide in the oxidant required to consume the aluminum component of the aluminum-silicon alloy reductant to form two liquid layers in the reaction zone, a lower layer comprising calcium-magnesium-aluminate slag and an upper layer comprising spent aluminum-silicon alloy reductant, and at least periodically tapping the spent aluminum-silicon alloy from the reaction zone as a by-product.

2. The process of claim 1 wherein the aluminum-silicon alloy charged to the reaction zone has a Si:Al weight ratio within the range from about 0.4:1 to 4:1.

3. The process of claim 1 wherein the aluminum-silicon alloy charged to the reaction zone has a Si:Al weight ratio within the range from about 0.4:1 to 2:1 and the spent aluminum-silicon alloy tapped from the reaction zone has a Si:Al ratio within the range from about 2:1 to 6:1.

4. The process of claim 1 wherein the aluminum-silicon alloy charged to the reaction zone has a Si:Al ratio of about 0.7:1 and the spent aluminum-silicon alloy tapped from the reaction zone has a Si:Al ratio of about 4:1.

5. The process of claim 4 wherein the reaction zone is maintained at A temperature of about 1700° C and a pressure of about 1 atmosphere.

6. The process of claim 1. wherein the amount of magnesium oxide in the oxidant charge is between about 80 to 98 percent by weight of said stoichiometric amount.

7. A metallothermic process for the production of magnesium which comprises;
(a) charging an aluminum-silicon alloy having a Si:Al weight ratio within the range from about 0.4:1 to 2:1 and an oxidant comprising magnesium oxide and calcium oxide having a molar ratio of MgO:CaO within the range of from about 4.0:1 to 1.0:1 to the reaction zone of a reduction furnace maintained at a temperature within the range of from about 1500 to 1900°C and a pressure of from about 1 to 2 atmospheres, the amount of magnesium oxide charged to the reaction zone being less than 100 percent of the amount theoretically required to consume the aluminum component of the alloy charged, and the alloy being charged to float as a liquid layer upon a molten slag which comprises, on a weight basis exclusive of other components, about 40 to 60 percent alumina, about 40 to 55 percent calcium oxide,less than 10 percent magnesium oxide, and about 0 to 7 percent silica;
(b) evolving magnesium vapor from the reaction zone;
(c) recovering the magnesium product in a condensing means; and (d) at least periodically tapping from the reaction zone an aluminum silicon alloy having a Si:Al weight ratio within the range from about 2:1 to 6.1 from the separate liquid alloy layer present in the reaction zone.

8. The process of claim 7 wherein the oxidant comprising magnesium oxide and calcium oxide contains principally dolomite.

9. The process of claim 7 wherein the alloy charged to the reaction zone has a Si:Al weight ratio of about 0.7:1;
the molten slag present in the reaction zone comprises about 50 to 55 percent alumina, 42 to 45 percent calcium oxide, less than 7 percent magnesium oxide, and 0 to 5 percent silica;
and the alloy tapped from the reaction zone has a Si:Al weight ratio of about 4:1.

10. The process of claim 9 wherein heat is supplied in the reduction furnace by conducting electric current through the slag to maintain the liquids within the temperature range of about 1500 to 1900°C.

11. The process of claim 7 wherein the amount of magnesium oxide charged to the reaction zone of said reduction furnace is 80 to 98 weight percent of the amount theoretically required to consume the aluminum component of the aluminum-silicon alloy charged to the reaction zone.

12. The process of claim 7 wherein the molar ratio of MgO:CaO in the oxidant charge is within the range from about 1.3:1 to 2.1:1.

13. The process of claim 12 wherein the amount of magnesium oxide charged to the reaction zone of said reduction furnace is about 88 to 92 weight percent of the amount theoretically required to consume the aluminum component of the aluminum-silicon alloy charged to the reaction zone.

14. The Process of claim 13 wherein the temperature of the molten materials present in the reaction zone is maintained about 1700°C.

15. The process of claim 13 wherein the pressure in the reaction zone is maintained at about 1 atmosphere.