Note: Descriptions are shown in the official language in which they were submitted.
lOflZ9Z`7
Background of the Invention
The present invention relates to the production of
magnesium metal by the reduction of magnesium oxide at
elevated temperatures in the presence of an aluminum metal
reducing agent and a molten oxidic slag, in an electric
furnace, and the condensation of vaporized magnesium in a
condenser.
An advantageous method of producing magnesium
lies in the chemical reduction of magnesium oxide with a
reducing agent in the presence of a molten oxide slag, in
an electric furnace. A variety of metallothermic processes,
as they are commonly called, for the production of magnesium
employing various reducing agents, various types of
reactants, and under various conditions of temperatures
and pressures have been proposed.
In general, the various metallothermic processes
are concerned with the production of metallic magnesium by
the reduction of magnesium oxide with a metal reductant at
elevated temperatures. Magnesium oxide, usually in the form
of calcined dolomite (dolime) or calcined magnesite or
mixtures thereof, is caused to react with a metallic reducing
agent, such as silicon, aluminum, calcium or mixtures or
alloys thereof, in the presence of a molten slag bath in a
furnace at temperatures in excess of 1300C, to release
. I .
bm:
~ .~
1082927
magnesium vapor which may be condensed and collected. Some
of the processes are carried out in the presence of an
inert gas.
An early process of this type, called the Pidgeon
process provides for the production of magnesium by the -~
reduction of magnesium oxide with ferrosilicon.
A more recent process is exemplified by U. S.
Patent 2,971,833 and is known as the Magnetherm process.
The process is operated under a very high vacuum (pressure
ranging between 5 and 20 millimeters of mercury) and at a
temperature of about 1500C utilizing an electric furnace.
Silicon is employed as the reductant, preferably in the form
of ferrosilicon containing 70-80 percent Si, silicon of a
purity in excess of 97%, or an aluminum-ferrosilicon.
Care is taken to avoid that the silicon content of the
residual ferrosilicon drops below 33.5%. A magnesia
containing substance is dissolved in a liquid slag
consisting essentially of lime, silica and alumina wherein
the ratio of calcium oxide to silicon dioxide and the ratio
of aluminum oxide to silicon dioxide are controlled.
Processes employing aluminum metal as the
reductant are exemplified by U. S. Patents 3,782,922,
2,527,722 and 2,527,724. The first patent discloses the
production of magnesium by the reduction of magnesium oxide
from a mixture of magnesium oxide and calcium oxide with
a reducing agent comprising at least 85%
,:..
-,
....
, . .
~ 2
. ,! ~
i,~,~ . bm
. A~ ,
: . ` ~ 10~32927
¦¦aluminum i ~e presence of a molte~ calcium-aluminate slag ba~h
at a temperature of about 1300-1700~C and a pressure of about
atmospheric. Slag is removed when the magnesium oxide content
thereof is less than five percent, with the slag comprising 35-
65 percent aluminum oxide, 35-55 percent calcium oxide and 0-10
percent silicon dioxide. The other two patents utilize magnesium
silicate in pure.or ore for~ and aluminum as reactants. Dolomite
is also used in the former and the magnesium silicate is of a
particular granular size in the latter.
U. S. Patent 3,658,509 also discloses the use of aluminum
as the reductant, as well as silicon and aluminum-silicon alloys
with the latter being preferred. In such process, an inert gzs
is used to obviate at least in part the need of a high vacuum,
and the slag contains 20-50 percent silicon dioxide.
The primary object of this invention is to provide a
process for production of magnesium metal wherein aluminum metal
; is reacted with a calcium aluminate magnesium slag, which
eliminates the disadvan~ages of the prior art processes, while
retaining the benefits thereof.
Another object of this invention is to provide a process
for production of magnesium metal at atmospheric pressure by
reaction of aluminum metal with a calcium magnesium aluminate
slag, or with magnesium oxide in the presence of such slag,
wherein the silicon dioxide content of the slag is maintained at
~` 25 a low level.
''' .
lO~
Another object of the invention is to provide a process
for producing magnesium metal utilizing aluminum metal as the
reductant, wherein magnesium vapor is produced at a pressure of
one atmosphere thereby eliminating the need for vacuum equipment
and permitting a continuous operation at such normal pressure.
An important object of the present invention is to
provide a process of the foregoing type which is economical and
which can utilize aluminum metal scrap as the metal reductant.
'
Other objects and advantages of the present invention
will become more readily apparent from a consideration of the
description hereinafter.
Summarv of the Invention
:
¦ The instant invention relates to a process for the
I production of magnesium metal by reaction of a calcium magnesium
15 ¦ aluminate slag or of magnesium oxide in the presence of such
¦ slag, with aluminum metal of about 80 percent or greater purity
¦ at reasonable pressures, preferably about atmospheric pressure,
; ¦ and at elevated temperatures of about 1350-1700C, preferably
~ I about 1500C. Magnesium evolves as a vapor and is condensed
;j 20 ¦ and collected in suitable apparatus. The concentration of
¦ silicon dioxide in the slag is kept at a low value, about five
¦ weight percent or less, preferably two weight percent or less.
,~ ¦ Description of the Preferred Embodiment
In the process of the present invention, aluminum metal
¦¦ is reacted th a calcium magnesium aluminate slag, or with
<, . ' ' '~
.~ - 4 -
'~.'' I
10829Z7
magnesium oxide in the presence of such slag to produce
magnesium metal. The process is illustrated by the following
equations:
2 Al + 3 MgO ~ Slag ~ 3 Mg~ Slag
.
2 Al ~ 3 CaO-MgO-Al2O3 (slag) ~ 3 Mg ~ ~ Slag
The reduction reaction is car~ied out in an internally
heated electric furnace at a temperature of about 1350-1700C,
with 1450-1600C being preferred and about 1500C being most
preferred and at a pressure of about 0.5-2.0 atmospheres,
preferably about atmospheric pressure. The concentration of
the various slag components is controlled. Silicon dioxide is
kept at a low level, no greater than five weight percent and
preferably about two weight percent or less. ~s the reaction
continues, MgO in the slag is consumed by reaction with the
; 15 aluminum metal. Additional MgO or dolirne (T CaO-MgO, where
0.5 ~ T < ~.0) is added as necessary to maintain the desired slag
composition as will be more fully explained hereinafter. Other
oxides such as Al2O3 may also be added to maintain the slag
composition. It is necessary however that the silicon dioxide
content of the slag be kept at five weight percent or less.
Materials added to the slag shou~d aiso contain no more than five
weight percent SiO2. ;.
It has been found to be advantageous to operate with a
reducing agent containing 80 weight perc~nt or more aluminum
~ 25 metal, to restrict silicon dioxide content of feed materials and
!., slag to 5 weight percent or less, and to use an excess of
magnesium oxide such that slag in the reactor and slag tapped or
; 5
~1 lO~9Z7 1
withdrawn from the reactor contain 6-13 weight percent magnesium
oxide. Thls cor~ination of reactants provides a superior
process, one in which the reaction between aluminum metal and
magnesium readily occurs, and one in which a high utilization of
5 aluminum metal is obtained. The advantages of using these
conditions will become more apparent from the discussion in the
following paragraphs.
,~
Aluminum is an active metal, and reacts at room
temperature with a variety of acids, bases, and other chemical
reagents. Its chemical reactivity extends also to the higher
temperatures required for the production of magnesium metal, and
in high temperature systems it is a superior reducing agent in
comparison with less active metallic reducing agents, such as
ferrosilicon or silicon metal.
, 15 Al~inum metal is a more active reducing agent, in that
it produces a higher vapor pressure of the desired magnesium
; product at a lower ter,lperature than other known reducing agents.
The usual method of removing magnesium from such high
temperature reaction systems is as a vapor, and it is
~ 20 subsequently condensed to a liquid or solid for recovery.
i~ Aluminum metal is superior as a reducing agent to less active
metals such as ferrosilicon or silicon.
' .
In a~dition to using aluminum metal as a reducing agent,
it is also essential to keep the concentration of silicon dioxide
in the reaction at a low value, at least less than or equal to
five weight percent. Because of its high reactivity, aluminum
~metal at hi temperatures reacts not only with magnesium oxide,
':
6 -
.~
~ 1~ lO~
but also with silicon dioxide and other siliceous materials.
Silicon metal is the product from reduction of silicon dioxide
or siliceous materials, rather than the desired magnesium
metal. Thus when silicon dioxide or other siliceous materials
are present in the reactor, part of the expensive aluminum
reducing agent will be converted to a by-product, namely silicon
metal, rather than the desired magnesium metal product. Although
it is possible to force the co-produced silicon metal to react
with magnesium oxide in the reactor to yield magnesium metal, to
do so requires the application of higher temperatures and lower
pressures than when aluminum metal is reacted directly with
magnesium oxide in the absence of siliceous materials. Silicon
dioxide concentration in the reactor and of incoming feed
materials must be restricted to five weight percent or less.
: 15 Additionally, an excess of magnesium oxide over theamount required to react with the aluminum reducing agent is
; added to keep the magnesium oxide content in the reactor and in
slag tapped from the reactor in the range of about 6-13 weight
percent. The rate and completeness of a chemical reaction are
dependent on the concentrations of the reactants. Thus, in the
reaction:
2 Al + 3 MgO ~ Al203 + 3 Mg ~
the rate of magnesium production and the completeness of the
reaction are dependent on the concentrations of aluminum metal
. 25 and magnesium oxide in the reactor. The high concentration of
aluminum metal employed in this invPntion facilitates reaction.
Further, an excess of magnesium oxide over the amount required
- : 108292'~
to react with the aluminum reduciny agent is added. The excess
of magnesium oxide increases the reaction rate, and drives the
reaction toward complete utilization of aluminum metal. This is
highly desirable because of the high cost of the aluminum
reducing agent. On the other hand, the concentration of
magnesium oxide in the reactor and in the tapped slag must be
limited to avoid increasing the melting point of the slag to too
; high a value and to avoid excessive loss of magnesium oxide in
the tapped slag. The magnesium oxide concentration range of 6-
10 13 weight percent inclusive, in the reactor, and in the tapped
slag, balances the need to increase rate and completeness of
reaction while avoiding excessive concentrations of magnesium
oxide in the slag.
Although aluminum metal used in the process should have a
1 ¦ purity equal to or greater than 80 weight percent and preferably
greater than or equal to 90 weight percent, most aluminum scrap
is suitable. ~letallic impurities in the aluminum, such as iron,
manganese and silicon can be tolerated. Such impurities are
inert under the process conditions and can be withdrawn from the
2 ¦ furnace or reactor from time to time, for example, when the slag
is tapped. Non-metallic impurities in the aluminum, for example,
~; aluminum oxide can also be tolerated. The total of these non-
r metallic impurities other than Al2O3, MgO or CaO should not
exceed five weight percent. The aluminum metal should also be
dried before use and should contain no more than 0.25 weight
percent water.
'.' ,
- 8 -
The oxides and other compounds making up the slag
composition are as follows by weight percent:
.'
Range Preferred Range
CaO 30-65 33-62
Al203 28-64 -32-62
MgO 6-13 6-10
; SiO2 ~ 5 < 2
Other oxides,
halides, sulfides ~ 5 < 2
Dolime, CaO or MgO may be added to the slag separately
or in combination as needed to maintain the desired slag
composition. As stated before, the dolime need not contain
equal amounts of CaO and MgO, but may contain T moles of CaO per
mole MgO, e.g., T CaO ~ 1 MgO, where 0.5 < T < 2Ø These oxides
; 15 should also contain not more than 5 weight percent SiO2 and
preferably not more than 2 weight percent. Such oxides should
also be substantially free of H2O and CO2 before use and should
contain not more than 0.5 weight percent H2O and 0.5 weight
percent CO2. The amount of MgO in the feed should be 101-150
percent of the amount required to react with the aluminum metal
in the feed, as determined by the following equation:
3 MgO + 2 Al ~ 3 Mg ~ + Al2O 3 ;,
. Additional MgO is added as necessary to maintain 6-13
weight percent MgO in the slag, but in no case should the MgO
added fall below 110% of the amount required to react with the
aluminum metal.
11 1082927
Other oxide additives, e.g., Al203, should contain not
more than five weight percent SiO2 and preferably more than
two weight percent. These oxide additives should also contain
- not more than 0.5 weight H20 and 0.5 weight percent CO2. The
S reaction should be carried out at a temperature of about 1350-
1700C, preferably about 1450-1600~C and at a pressure range of
about 0.5-2.0 atmospheres, preferably about 1.0 atmosphere.
; In carrying out the invention, a slag of a composition
within the foregoing ranges is prepared and melted. The various
slag ingredients, CaO, MgO, dolime, and Al203 may be mixed
.. together or a slag of a suitable composition from a previous
~ operation may be used. An atmosphere of inert gas, such as argon
i or hydrogen is provided during melting. Heat is supplied for
melting either by striking an arc between electrodes suitably
located inside the slag holding vessel or reactor, or by
suitably locating electrodes so as to pass a current through the
slag, or by any other suitable means. After the desired
temperature of the molten slag is achieved, a stream of aluminum
, particles is charged into the slag. Simultaneously therewith,` 20 an oxide feed stream containing oxides such as CaO, MgO, dolime
and/or Al203, is added to the slag to keep its composition from
changing. Alternatively, the aluminum metal and oxide feed may
be intermingled, and added to the reactor as a mixture.
~; Magnesium vapor is evolved from the surface of the slag, is
conducted to a suitable condenser, and is condensed at a pressure
of one atmosphere. An inert gas such as argon or hydrogen is
used to prevent air from contacting the magnesium. }~owever, the
inert gas does not flow continuously from reactor to condenser,
.
''
,,; - 10 -
:: .
lO~Z9~7 1
and hence does not transport magnesium metal vapor from reactor
to condenser. As the reaction proceeds, the slag level in the
reactor increases. From time to time, a portion of the slag and
any unreacted inert metals such as iron, silicon, etc. are
removed through a suitable tap hole in the reactor.
Examples
The examples described in the follo~7ing table are
illustrative of the process, and are carried out as described
hereinbefore. The reactor is kept at a temperature of 1500-1550C
and at about atmospheric pressure. Reactants and other oxides
are added to the reactor either separately, or as mixtures.
CaO and MgO are added either as the individual oxi~es or as
dolime. The reaction which occurs in the reactor yields
magnesium metal vapor and slag. The magnesium metal vapor flows
to a condenser, and is condensed to a liquid or solid. Slag
remains in the reactor and is removed from time to time.
.
- j 108;~
,; . ~ l o
., ~ C~
3 ~ r N
V ~ r o
~"~ h E3 a ~ o ) co ~ _
j~ U~ O I N a~
~ N N N N N N
~ ~ I ~¦ ol o N o
io ol O
~ oj~
c ¦ o I cO o N ; .
,, ' ' P;~
~,~`, I 11~ ' o
.,' .
~:l ~ ~
i~ - 12 -
108Z9Z~ ~
The foregoing disclosure and description of the
invention is illustrative and explanatory thereof and various
changes may be made within the scope of the appended claims
without departing from the spirit of the invention.
