Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention relates to the ex-traction of magnesium
from magnesium oxides. More particularly~ the in~ention relates
to a process for extracting magnesium from compounds containing
magnesium oxides.
One of the main sources of magnesium metal produced on
an industrial scale is dolomite which is a carbonate of calcium
and magnesium. The magnesium metal is produced using a reduction
process and in the past silicon or ferrosilicon has been the
reducing agent. This process is ~nown as the Pidgeon process. In
the case when the starting material is calcined dolomite the fol-
lowing reaction occurs:
(A) 2(CaO. MgO) ~ Si = 2 Mg + (CaO)2SiO2
According to this process -the magnesium metal is liberated from
the intimate mixture in the vapor state leaving a bi-calcium
silicate as a dross.
In -the past, this type of operation has been carried out
at reduced pressure and with furnace temperatures up to about
1500C., and at these temperatures the dross is in a mol-ten state
at the end of the reduction step. Thus, the starting material
must be contained in a crucible or other container to prevent the
liquid dross from spreading over the floor of the furnace. Further-
more, these high temperatures lead to high maintenance costs of
the furnaces due to the frequent need to change the refractory
linings which have only a limited life at these temperatures.
Examples of these processes are described by Van Embden in U.S.
Patent 2,252,052 Cooper in U.S~ Patent 2,429,66~ and Ar-tru and
Marchal in French Paten-t 762,671. The process defined ln this
French Patent requires the addition o~ a third component, alumina~
to act as a melting medium but does not -take any part in the
chemical action defined in formula tA).
Furthermore, with the high te~nperatures of the known
processes of producing magnesium metal, a high degree of contam~
ination occurs because as well as magnesium o-ther metals are
also vaporized and subsequently condensed in the reduction pro-
cess. These other metals are present as impurities in the mag-
nesium oxide and include for e~ample, calcium, lead, antimony,
manganese, nickel, silicon, tin, copper, iron and others.
The problem of high temperatures can be overcome in
part by making use of the silicothermic process in the solid
state. Unlike known methods this process does not require high
temperatures, thus the dross does not become molten and can be
extracted from the furnace in the solid state. In this process
an intimate mixture of fine powders is prepared and compacted or
compressed into blocks consisting of calcined dolomite and a re-
ducing agent and it is possible to achieve distillation of the
magnesium whilst limiting the tempera~ures of the reaction to
below the melting point of the blocks. By achieving this lower
temperature, ~he degree of contamination is considerably reduced.
It is one purpose o~ the presen-t invention to provide
a process which can be carried out at temperatures below the
melting point of the starting materials and this is achieved in
part by replacing the silicon either in whole or in part with
aluminum.
As well as -the reaction identified as formula (A),
other possible chemical reactions include the following formulas
invol~ing dolomite, magnesium oxide and aluminum:
(B) 3(CaO MgO) + 2Al = 3 Mg + (CaO)3 A1203
(C) 8tCaO. MgO) -~ 4MgO -~ 8Al =
12Mg -~ (CaO)3A1203 + (CaO)s(A1203)3
(D) 5(CaO MgO) + 4MgO + 6Al = 9Mg -~ (CaO)s(A1203)3
(E) 5(CaO. ~gO) + 2Al + Si =
5Mg + (CaO)3A1203 + (CaO)2 SiO2
(F) 17(CaO MgO) + 4MgO + 6Al + 6Si =
21Mg + (CaO~s(A1203)3 + 6(CaO)2SiO2
s~
The (E) and (F) formulas represent reactions when
dolomite, magnesium oxide and a mixture of two reducing agents,
silicon and aluminum are used~ The dolomite may be replaced by
a mixture o~ calcium oxide and magnesium oxide from any source
provided the proportions of the mixture of oxides correspond to
the stoichiometric ratio of dolomite.
The process of the present invention is carried out by
first taking a measured amount of powdered aluminum, and in some
cases the addition of ferrosilicon, magnesium oxide and calcium
oxide, mixing these powders together and compressing them into a
block for reduction by means of heating in a furnace un~er control-
led pressure. The heating need not exceed 1100C and is prefer-
ably in the range of 800 - 1100C. Under these temperature con-
ditions and at a controlled pressure, the magnesium metal vaporizes
and is recovered by condensation leaving behind an exhausted dross
in the solid state consisting of a particular calcium aluminate or
more generally a stoichiometric e~uivalent mixture of other alumi-
nates. If a mixture of two reducing agents is used, the dross
contains calcium silicate and calcium aluminate If the two
reducing agents are used under the working conditions according
to the present lnvention, the agents work together in a synegistic
manner.
In the past this process was not feasible due to the
scarcity of the required raw materials Eowever, in recent years
there has been a demand for a magnesium oxide and calcium oxide
of a high purity and these have been obtained using calcination
of the respective carbonates or making use of precipitation pro-
cesses followed by subsequent drying o~ the corresponding oxides.
Furthermore, aluminum in granular form or chip form is now avail-
able in large quantities from a machining process or from scrap.
This alumino-ther~c process carried out in the solid
state has a number of advantages over known processes of recover-
5~
ing magnesium metal. One advantage i5 that, due to the lowerreaction temperature, it is possible to produce a much purer
magnesium metal because the lower temperatures avoid the dis-
tillation of other metals in the magnesium metal. Furthermore,
it has been found that a greater magnesium content may be used in
the starting mix, thus resulting in a higher output of magnesium
from a single furnace. Still further, by keeping the temperature
below 1100C there is less wear in the reaction furnace which
tends to keep operating and maintenance costs down.
The present invention provides a process of extracting
magnesium from compounds containing magnesium oxides comprising
the steps of mixing magnesium oxides and calcium oxide with alumi-
num, and in some cases ferrosilicon, compressing the mixture, heat-
ing the compressed mixture uniformally to a tamperature in the
range of 800 - 1100C~, under a controlled pressure to vaporize
magnesium, and condensing the vaporized magnesium on a cooled
sur-faceO
In a preferred embodiment the proportion by weight of
the various components of the mixture should correspond to the
stoichiometric ratio and preferably the mixture contains 2 0 -
2.5 parts by weight magnesium oxide, 1 5 - 2.0 parts by weight
calcium oxide and 1.0 - 1.5 parts by weight aluminum. The com-
pounds should preferably be in powder or granular form before
mixing. In one embodiment the mixture is compacted into a block
which may then be heated uniformally so that the temperature
difference across the block does not exceed 100C. In still
another embodiment the block is formed integral with an electrical
heating conductor and in yet a further embodiment tbe block is
formed as a hollow column with the heating conductor formed as a
helical coil within the blockO
In a preferred embodiment the process is carried out
in a lower portion of an electric furnace, the lower portion
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having an insulated lining and the block being spaced from the
lining. An upper portion forms the cooled surface for condensing
the magnesium metal thereon and is joined to the lower portion to
form an enclosed sealed chamber. The pressure may be controlled
by an exhaust pump and a filtex prevents vapori~ed magnesium from
passing through to the pump. Separate condensation chambers,
which are removable, may be provided, each condensation chamber
having a cooled surface therein.
In drawings which illustrate embodiments of the
invention,
Fig. 1 is a diagrammatic cross sectional view through
one embodiment of an electric furnace suitable for the process of
the present invention.
Fig. 2 is a side view, partly in cross section, of an
electrical hea-ting conductor together with the compounds in a
solid compressed form shaped as a hollow column structure for
insertion-in the electric furnace shown in Fig. 1.
~ ig. 3 is a diagrammatic cross sectional view through
another embodiment of an electric furnace having at least one
separate condensation chamber.
Fig. 4 is a top plan view of the electric furnace shown
in Fig. 3.
Fig. 1 and 2 illustrate an electric furnace suitable
~or the process of this in~ention. The furnace has a top bell-
shaped portion 10 and a lower cylindrical portion 11 which define
a substæntially cylindrical chamber. The top and lower portions
10 and 11 are joined together in a detachable manner at flange 12
to form a sealed chamber, the joint 12 being provided with a
suitable seal in order to maintain the sealed chamber. The lower
portion 11 is lined internally with refrac-tory material 13 and
constitutes the heating chamber or reduction chamber of the fur-
nace. A block or charge 14 of the compounds to be heated are
-- 5 --
36~
placed in the lower portion 11 spaced apart from the ref:ractory
lining 13 so that as the metal in the charge vaporizes due to
heat the vapor can escape from the sides as well as the top of
the charge 14.
The top portion lO is provided wi-th a suitable cooling
means (not shown). The top portion 10 constitutes the condensa-
tiOIl chamber for the magnesium vapors which are set free during
the course of the reduction from the charge 14. The top portion
10 and the lower portion ll are both made from suitable material
preferably metal which has suitable s-trength to withstand the
mechanical and thermal stresses to which the ~urnace is subjected,
A controlled pressure is maintained within the sealed chamber by
means of a suitable suction through a conduit 15 preferably con
nected to an exhaust pump. A pressure sensing device may be
included in the sealed chamber, and in a preferred embodiment a
filter is supplied on this exhaust conduit 15 to prevent metallic
vapors passing to the exhaust pump.
The charge 14 is shown in more detail in Fig. 2 and
is constructed in the form of a hollow column. The struc-ture
2a of electrical conducting elements 16 are formed in a helix and
the mass of compounds are compacted or compressed :in spaces 17
between the helical elements 16 which are suitably connected at
their terminals 1~ to electrical connections provided within the
furnace. The charge 14 is then heated by elec-trical resistance
to the desired temperature, and the conductor elements 16 maintain
a uniform temperaturc throughout the charge 1~. I'he preparation
of the charge 14 takes place outside the furnace wherein the mix-
ture of compounds is first prepared and then compacted between
these elements 16 prior to installation in the furnace. In the
embodiment shown the charge 14 has electrical conductor elements
16 in the form of a helical coil. It will be apparent to those
skilled in the art that other configurations of electrical
5~
conductor elements could also be employed in the charge, such
conductor elements could take almost any shape provided the mass
of the charge was evenly heated For example, conductor elements
may be installed within the hollow column as shown in ~ig. 2.
In operation powders of magnesium oxide, calcium oxide
and aluminum and in some cases ferrosilicon in powder or granule
form are mixed together according to the fol~owing weight ratios,
Magnesium o~ide 2.0 - 2.5 parts by weight
Calcium oxide 1.5 - 2.0 parts by weight
Aluminum 1.0 - 1.5 parts by weight
After mixing, these compounds are compacted or compressed to a
charge 14 such as that shown in Fig. 2. The mixture is compressed
within the conductor elements 16 so that the compounds are evenly
compressed and form a column or sel~ sustaining mass that retains
its shape when left. In this form the charge 14 is placed within
the furnace, the terminals 18 connected to the terminals within
the furnace, the top portion 10 of the furnace located on and
sealed to the flange 12, and gases are drawn through conduit 15
inside the sealed chamber. The conductor elements 16 commence
heating the block evenly and the temperature o the block in-
creases to within the range of 800 - 1100C. At this temperature,
the temperature range through the block does not exceed 100C.
At this temperature the magnesium vaporizes and commences to be
set free from the charge 14, these vapors exit from the outside
and the inside of the column structure and also on top o~ the
column, the vapors pass upwards from -the hot lower portion of
the furnace into the top portion 10 where they condense against
the cooled surface of the top portion 10. The condensed ~ag-
nesium metal forms crystals on these cooled surfaces, and this
process continues while the temperature of the block is main-
tained at between 8Q0 and 1100C until all the magnesium metal
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- 7 -
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has been released from the block and condensed in the cooled
surface. After a pre~set time the power to the conductor ele~
ments 16 is turned off to allow the furnace to commence cooling,
and the exhaust pump is stoppedO Before the furnace is cooled,
the top portion 10 is removed and the hot charge 14, which has
still retained its structure because the highest temperature
reached was below the melting temperature, is removed from the
furnace and replaced with a new charge 14~ At the same time, the
magnesium crystals are scraped off the inside surface of the top
portion 10 whlch is then replaced on top o~ the lower portion 11,
the flange 12 connected and the process recommenced~
Fig. 3 and 4 show another embodiment o~E the furnace in
which a top portion 20 has four condensation chambers 21 spaced
around the periphery of the top portion 20. These condensation
chambers 21 are joined to the top portion 20 by flanges 22 and
e~ch chamber 21 has an exhaust conduit 23 at the far end to suck
the metallic vapors into the condensation chambers 21 Each of
the condensation chambers ~1 has cooling means surrounding it so
that the magnesium metal condenses on the walls oE the chambers
210 After the chambers 21 have become full, then it is merely
necessary to uncouple the flanges 22, take off the chambers 21,
and replace them with new chambers 21. In this way less down time
is needed, and the scraping may be carried out when the condensa-
tion chambers 21 are separated from the furnaceO
Tests carried out using -the apparatus illustrated in
Figs. 1 and 2 have shown that as much as 25% by weight of the
charge mixture of magnesium has been recovered Erom each batchO
The following table summarizes the results of tests
carried out on the extraction of magnesium from diEferent compo-
nents. The results confirm the recovery percentages achi0ved
with this process~ The mixtures used in these tests correspond
to the proportions to comply with the formulas A to F.
-- 8 --
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Charges as high as one metric ton have been processed
and furthermore it has been ound that the energy required is in
the order of 1,800 kilowatts ~or the complete reduction of one
metric ton based on the recovery o~ 25% by weight of ~agnesium
from each charge,
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