Note: Descriptions are shown in the official language in which they were submitted.
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F0147
METHOD OF PRODUCTION OF MAGNESIUM AND
CHLORINE FROM MAGNESIUM CHLORIDE SOLUTIONS
CONTAINING AMMONIUM CHLORIDE
Field of the Invention
The invention relates to the production of magnesium from magnesium
chloride solutions, produced by leaching oxygen-containing raw material, such
as
solutions of lake water, and from magnesium chloride solutions resulting from
the
underground leaching of chlorine-magnesium salts.
Background of the Invention
The method of processing the magnesium chloride solution by evaporation
and crystallization of bishofite MgCl2 ~ 6 H20 is well known. In present
practices,
the solution is dehydrated in multistage dryers, equipped with rakes, and
then, the
resultant granular product containing MgCl2~(1.25 to 1.5) H20, is fed into
electrolytic cells. From the electrolytic chamber anode gases containing
chlorine,
hydrogen chloride, carbon monoxide and carbon dioxide, are delivered to the
regeneration furnace for the production of hydrochloric acid. This process by-
product provides an essential component used in the neutralization of
magnesium
hydroxide (see O. A. I,ebedev, "Production of magnesium by electrolysis."
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Metallurav, Moscow, 1988; pages 38-39). The major drawbacks of this method are
as follows: substantial losses of magnesium occur by the accumulation thereof
in
the sludge, which is discarded from the electrolytic cells, and that the
electrolytic
cells experience a short service life.
Another drawback of this prior art method is that a considerable amount of
magnesium oxide is present in the dehydrated magnesium chloride (from 1.5 to
3%). These factors considerably reduce economic feasibility of magnesium
chloride production by dehydrating its crystalline hydrates.
The production of (anhydrous) dehydrated magnesium chloride, an essential
component in the electrolytic production of magnesium metal by means of
dehydration of ammonium carnallite (NHdCI ~ MgC12~6H20), is also known in the
art. In this method ammonium carnallite is consecutively heated in multistage
dryers in dry air up to 360 ° C. The complete dehydration and
decomposition of
the ammonia carnallite results in MgCl2 and NH4Cl. This method does not result
in hydrolysis (see, M. A. Aydenzon, "Magnesium". MetallurQV, Moscow, 1969;
.°
pages 111-112).
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Also, in the prior art, there is the method of production of practically
anhydrous magnesium chloride as disclosed by the international patent
application
No. 95/ 11859. According to this document, the dehydration of MgCl2 solutions
is
provided by mixing such solutions with ethylene glycol, removing water
therefrom
in rectification columns, and subsequently precipitating magnesium ammoniate
crystals (MgCIZ ~ nNH3) from the resulant mixture. The anhydrous ammoniate
crystals produced are then washed with methanol, and further processed by
drying
in the fluidized bed unit. As a result, ammonia and anhydrous magnesium
chloride
are produced.
This method too has the same essential drawbacks. Some of the organic
solvents, such as ethylene glycol used in the dehydration process is removed
along
with the water from the fractionating columns. This waste water contaminated
with ethylene glycol needs to be treated in the fractionating columns. Also,
after
washing magnesium ammoniate crystals, the used methyl alcohol solvent includes
some ethylene glycol from the distillation columns. Here, again, the ethylene
glycol needs to be separated from the methyl alcohol in the fractionating
columns.
Construction of the organic solvent recovery systems, such as fractionating
distillation columns, require considerable capital investments. Furthermore,
the
fractional distillation required for separating the ethylene glycol from the
water
and the methyl alcohol requires additional energy consumption.
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Russian patent RU 2136786, granted to Russian National Aluminium
Magnesium Institute, discloses the method utilizing ammonium chloride for the
production of magnesium from oxygen-containing raw material or from chlorine-
magnesium solutions. According to this patent, the solution of magnesium
chloride and ammonium chloride is vaporized, so that during the process,
ammonium carnallite crystals are formed. The crystals of ammonium carnallite
in
turn, are dehydrated in the fluidized bed dryer and is further reduced
(decomposed) in the melt of recycled electrolyte of the electrolytic cell.
Dehydrated magnesium chloride is fed to an electrolytic chamber, and the
gaseous
ammonium chloride released by the process is fed into absorption by the
magnesium chloride solutions.
In the process, the exhaust gases resulting from the carnallite dehydration
containing hydrogen chloride and ammonium chloride are formed. These gases
are treated with condensate resulted from evaporation of magnesium chloride
and
ammonium chloride solutions. The gasses are then used in the leaching of the
initial raw material. In this invention the processes of evaporation and
crystallization of ammonium carnallite take place. Additionally, the method of
Russian patent RU 2136786 discloses the following steps: using ammonium
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chloride; thermally decomposing anhydrous ammonium carnallite; and currently
therewith isolating anhydrous magnesium chloride.
Accordingly, there is well-established need for a method of production of
magnesium and chlorine from magnesium chloride solutions which is capable of
reducing the costs of processing of the solutions of magnesium chloride and
ammonium chloride. This is accomplished by means of minimizing the process
areas where vapor losses occur and where ammonium carnallite crystallizes.
SUMMARY OF THE INVENTION
The invention is used in the production of magnesium from magnesium
chloride solutions, which are produced by leaching oxygen-containing raw
material into surface water, such as lakes, and from magnesium chloride
solutions
which are produced by the underground leaching of chlorine-magnesium salts
into
the water table.
One of the main objects of the invention is to reduce costs during the
processing of the solutions of magnesium chloride and ammonium chloride. This
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is accomplished by minimizing the process areas where vapor losses occur and
where ammonium carnallite crystallizes.
The method of producing magnesium and chlorine of the invention includes
the following steps: processing of chlorine-magnesium solutions utilizing
dehydrated compounds of ammonia; separating anhydrous magnesium chloride
and electrolytic decomposition thereof into magnesium and chlorine products;
after decontamination, an initial chlorine-magnesium solution is saturated
with
ammonium chloride in a spray tower and the resultant solution is fed into the
fluidized bed dryer for the direct production of solid anhydrous ammonium
carnallite. In preparing the initial chlorine-magnesium solution, the
decontamination step entails the removal of certain impurities therefrom,
which
impurities are known to interfere with further processing. The subsequent
decomposition of the anhydrous ammonium carnallite produces anhydrous
magnesium chloride, ammonia, and hydrogen chloride. The ammonia and
hydrogen chloride are supplied together with exhaust gases to a spray tower;
and
the anhydrous magnesium chloride is then fed to the electrolytic cells for the
'
production of magnesium and chlorine.
Furthermore, in the method of the invention, the solution of salts is supplied
through a dispersing device into the bed of fluidized material.
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By circulating the effluent gasses as described heretofore, the solution of
salts in the spray towers is heated to a temperature of between 85 to 120
° C and
the water content thereof is reduced to 55 to 65 percent.
According to the invention optimal drying and dehydration of the salt
solution with minimal hydrolysis of magnesium chloride occurs when the content
of ammonium chloride in the salt solution is maintained between 40 and 55
percent of the magnesium chloride content in the solution.
There are two embodiments of the invention disclosing decomposition of
dehydrated ammonium carnallite. According to the first embodiment, the
decomposition occurs in the fluidized bed dryer within the stream of flue
gases,
that are produced in the special burner where the fuel is combusted together
with
chlorine gas.
As to the second embodiment, decomposition of the anhydrous ammonium
carnallite takes place in the melting unit within the recycled electrolyte.
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The decontaminated chlorine-magnesium solutions are initially supplied to
the absorber of fluidized bed dryer. Then the solutions are directed to the
spray
tower associated with the device capable of carrying out decomposition of the
anhydrous ammonium carnallite.
In order to compensate the losses of ammonia during the production
process, this ingredient is introduced into the salt solution before supplying
of the
salts to the fluidized bed dryer.
During the decomposition of carnallite in the fluidized bed dryer, the gases
remaining in the process effluent after ammonium chloride is absorbed, contain
a
significant proportion of hydrogen chloride gas, which, in turn, is dissolved
in
water. The resultant byproduct is an 18 to 20 percent hydrochloric acid.
Brief Description of the Drawings
FIG. 1 shows the process diagram of the method of the present invention.
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Descriution of the Preferred Embodiment
A system for the method of production of magnesium and chlorine of the
invention typically consists of spray towers 1, 2, and 3; fluidized bed dryers
6
and 7; a melting unit 5 and electrolytic cells 4.
The initial raw material in the form of a decontaminated solution of
magnesium chloride is supplied for spraying in the spray tower 1. Exhaust
gases
from the fluidized bed dryer 7 containing NH4Cl and HC 1 are also fed into the
spray tower 1. As the exhaust gases transfer considerable thermal energy, the
aqueous portion of the solution is partially evaporated. Additionally,
preliminary
saturation of the solution occurs as HC 1 and NH3 are absorbed thereby. This
is
accompanied by the formation of the dissolved ammonium chloride. The solution
from the spray tower 1 is fed for the spray nozzle in the spray tower or
absorber 2.
Furthermore, effluent gases from the devices for decomposition of anhydrous
ammonium carnallite, namely the melting unit 5 and/or the fluidized bed dryer
6
are fed into the spray tower 2. The solution of salts is heated in the spray
towers
up to the temperature of 85 to 120° C. Through utilization of the heat
of the
exhaust gasses from the fluidized bed dryer 6 and melting unit 5, the
solutions of
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salts are evaporated in the spray towers to obtain a water content between 55
and
65 percent.
After the saturation of the solution with ammonium chloride and in order
to obtain the required content, this solution containing MgCl2 + NHdCI is
supplied
to the fluidized bed dryer 7 for evaporation, so as to obtain anhydrous or
dehydrated ammonium carnallite.
In the above process, irretrievable losses of ammonia are compensated by
adding ammonia or ammoniated solutions into the solution containing magnesium
chloride solution, supplied into the fluidized bed dryer 7.
For the purposes of this application, the term spray tower is broadly defined
as a chemical processing unit in which a gaseous effluent is absorbed by a
sprayed
or sprinkled solute. The unit is alternately referred to as an absorber.
In the fluidized bed dryer 7 the feedstock is dehydrated and ammonium
carnallite is formed. The fluidized bed dryer 7 contains several chambers.
Initially, through a dispersing device, the solutions are introduced into the
first
chamber of the fluidized bed dryer for drying, so as to form
MgCl2 ~ NH4Cl ~ 6 H20
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After ammonium carnallite material moves through the subsequent
chambers of the dryer 7, water is removed and the material becomes more
substantially dehydrated in the following manner:
MgCI2~NH4Cl ~6 H20 -~ MgCl2. NH4C1 + 6 H20 + NH3 + HC 1
Thus, at the final stage, in the dehydrated or anhydrous ammonium
carnallite the water content does not exceed 2 to 6 percent and Mg0 content
does
not exceed 1.2 to 1.6 percent. In the last chamber the bed temperature is
between
180 and 230° C. Compared to the dehydration of bishofite (MgC12~6H20)
which
is formed when only chlorine-magnesium solution is subjected to the step of
drying; the dehydration of the ammonium carnallite is accompanied by a several
times lower rate of highly undesirable hydrolysis.
The anhydrous or dehydrated ammonium carnallite is subjected to the
thermal decomposition in the fluidized bed dryer 6 or in the melting unit or
electrolytic cell 5. This step is accompanied by the formation of anhydrous
magnesium chloride with the content of water not exceeding 0.2 percent and
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gaseous hydrogen chloride and ammonia. Hydrogen chloride and ammonia form
ammonium chloride that is collected in the spray tower 2.
According to one embodiment of the invention, the thermal decomposition
of dehydrated or anhydrous ammonium carnallite occurs in the fluidized bed
dryer
6 as follows:
MgCl2 ~ NH4C1 (solid)-~ MgCl2 (solid) + NH3 (gas) + HC 1(gas)
In this condition the bed temperature range is between 340 and
380° C,
whereas the stream of gaseous heat transfer medium contains less than 10 g/m3
of
water. Substantially solid anhydrous MgCl2 formed in the fluidized bed dryer 6
is
delivered to the electrolytic cells 4 for subsequent electrolysis.
In the present invention, the burners of the fluidized bed dryer 6 burn
hydrocarbon fuel and chlorine from the electrolytic cells 4 provided for the
production of magnesium. During the combustion process hydrogen from the fuel
combines with chlorine to form hydrogen chloride. With the removal of the
°
hydrogen chloride from the process effluent, the flue gases contain only water
vapor which is supplied to the burner of the fluidized bed dryer 6 along with
air.
This eliminates any special drying of the heat transfer medium and replaces
the
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heating thereof by electrical air heaters or any other heat exchange units.
Since
the formed hydrogen chloride is more hygroscopic in nature, (see above) upon
entering the fluidized bed it minimizes highly undesirable hydrolysis of the
anhydrous magnesium chloride. The content of magnesium oxide in anhydrous
magnesium chloride produced by the method of this invention generally does not
typically exceed 1.2 percent.
As to the second embodiment of the invention, the decomposition of
anhydrous ammonium carnallite (produced in the fluidized bed dryer 7) takes
place in the melting unit or electrolytic cell 5 at the temperature of about
660 ° C.
This unit is adapted to receive the melt of recycled electrolyte having a low
content of MgCl2 from the electrolytic cells 4. According to this method the
heat
of recycled electrolyte, having a temperature of about 700 ° C, is
mainly used to
decompose ammonium carnallite in the following manner:
MgCl2 ~ NH4C1 (solid) -~ MgCl2 (melt) + NH3 (gas) + HC 1 (gas)
Thus, formed dehydrated or anhydrous MgCl2 is dissolved in the recycled
electrolyte and in the form of the melt delivered for electrolysis in the
electrolytic
cells 4. However, concurrently with the decomposition of carnallite in the
melt,
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Mg0 is chlorinated in the process by hydrogen chloride. In this step, the
final
content of MgCl2 in the melt reaches 30 percent (max) and the Mg0 content is
less
than 0.02 percent. After the step of decomposition, the effluent gases from
the
melting unit 5 are supplied to the spray tower 2, so as to be absorbed by the
MgCl2
solution. The enhanced composition of the melt obtained by the method hereof
increases the service life of the graphite anodes of electrolytic cells
utilized in the
production of magnesium and thereby significantly reduces the operating costs
of
the electrolytic units.
Because of the presence in the spray tower 2 of the solution saturated with
magnesium chloride and ammonium at temperatures exceeding 60 ° C, it is
difficult to collect hydrogen chloride. Therefore, hydrogen chloride is
released to
spray tower 3 irrigated with water at a temperature lower than 40 ° C.
There,
water saturated with hydrogen chloride forms hydrochloric acid (HCI) with a
concentration from 18 to 20 percent. This hydrochloric acid is available for
use in
leaching magnesium-oxide containing materials utilized in obtaining of
magnesium
chloride solutions or for use in producing calcium chloride. '
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Example 1. Initially; a decontaminated solution of magnesium chloride
and ammonium chloride, containing MgCl2 - 18.8 percent and NH4C1- 10 percent
and HCl-4 percent, is supplied through an injector nozzle into the fluidized
bed
dryer at the rate of 1200 kg/hour. The diameter of the furnace grid was 1.50
m.
'The final temperature in the fluidized bed was 190 and 200 ° C. As a
result, 1030
kg of granulated dehydrated product, containing MgCl2 - 60 percent, NH4Cl -
31.6
percent, Mg0 - 1.4 percent and HZO - 5.8 percent was produced. In the spray
tower, exhaust gases are released into a sprayed magnesium chloride solution.
The
liquor of following composition is produced: MgCl2 - 33.0 percent, NH~CI - 2.1
percent HCl - 3.7 percent and H20 - 60.5 percent.
Dehydrated granulated ammonium carnallite is loaded into the fluidized bed
dryer with grid diameter of 300mm. The weight of carnallite was 150 kg. The
temperature in the bed at the end of the run is 300° C (approx.). The
heat transfer
medium which was formed by the combustion of 7.5 kg of fuel oil and 26 kg of
chlorine, was supplied for a 2.5 hour period to the burner below the furnace
grid.
The combustion products under the grid are diluted with free air to raise the
heat
transfer medium temperature to 650 ° C. Anhydrous magnesium chloride in
quantity of 85 kg is produced, having the following composition: MgCl2 - 98.3
percent, Mg0 - 1.0 percent; in which there was no NH4C1 present.
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After the decomposition of ammonium carnallite, the gases from the
fluidized bed dryer are supplied to the spray tower which is sprayed with the
solution from the spray tower of the fluidized bed dryer for the dehydration
of
chlorine-magnesium solutions with ammonium chloride. The solution, containing
MgCl2 - 18.8 percent, NH4C1- 10.9 percent and HCl-4 percent was produced in
the spray tower operating at the temperature of 70 ° C (approx. ).
From this unit gases were fed to another absorber, which is operated with
water sprayed having a temperature of 40 ° C. From the absorption of
simultaneously, the hydrogen chloride gas was absorbed by water, so as to
produce hydrochloric acid containing 20 percent of HCI.
The processing of anhydrous magnesium chloride, produced by electrolytic
decomposition of anhydrous ammonium carnallite is conducted in an electrolytic
cell operated at an amperage of 2000 amps and an electrolytic temperature of
700 °
C. The gaps) between the electrodes are 70 mm (approx.), and the average MgCl2
content in the bath is 16 percent. In a pilot test over a 48 day period, the
current
efficiency was 91 percent with other main characteristics of the cell being
similar
to those when this cell was used for processing of anhydrous magnesium
chloride
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from the titanium production. This magnesium chloride used in the pilot run
was
obtained during the reduction of titanium tetrachloride. The quality of the
magnesium produced met the highest industry standard for crude magnesium.
Example 2. In this example, the method of production of anhydrous
ammonium carnallite is similar to that of Example 1. The anhydrous ammonium
carnallite produced hereby is supplied to an electrolytic cell. The bath
thereof has
a square 300 x 300 mm top and a depth of 500 mm. Into the bath, 30 kg of
molten electrolyte, at the temperature of 700 ° C is introduced.
Anhydrous
ammonium carnallite is also introduced into the bath at the rate of 30 to 35
kg/hour. After the loading of 18 kg of anhydrous carnallite, 40 kg of the melt
is
received having a temperature of 660 ° C. The resultant product has a
MgCl2
content of 30 percent, with Mg0 being absent therefrom. The melt meets all
major standards applicable to the raw materials utilized in electrolysis.
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