Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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
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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
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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,
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~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
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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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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.
~ '' . - ].~ - ,,, , ............................. , ... ., 1~'`.
:::
11~
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.
. . .
- 20 -
: : : ., . .. ....
~ ...... ,.. "~ " ~ , !,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.
. - 21 -
.' ~ ', '" ,,...................... ,.. ....
