Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR THE PREPARATION OF A MAGNESIUM
CHLORIDE SOLUTION BY QUICK LIXIVIATION OF
ASBESTOS TAILINGS
FIELD OF THE INVENTION
The present invention is concerned with a process for the quick preparation of
a magnesium chloride (MgCl2) solution from asbestos tailings.
More particularly, the invention is concerned with a process wherein the
requested magnesium chloride solution is obtained by the lixiviation of
asbestos tailings
with an aqueous hydrochloric acid solution.
By "quick lixiviation", there is meant a lixiviation carried out for a period
of time
ranging from 5 to 15 minutes.
BACKGROUND OF THE INVENTION
As is known, there is presently a very high demand for magnesium in metal
form.
As is also known, magnesium can be produced by electrolysis of MgCl2
solutions.
As is further known, MgClz solutions can be obtained by subjecting magnesium
silicate-containing mineral such as serpentine to a lixiviation with
hydrochloric acid.
Such minerals are found, by way of example, as tailings in asbestos mines.
There is an excellent bibliographical review on the chemical transformation of
asbestos tailings. This review entitled "The activation of magnesium in
serpentine by
calcination and chemical utilisation of asbestos tailings - a review" was
published by
N. NAGAMORI, A.J. PLUMTON and R. LE HOUILLER in CIM Bulletin, December
1980, pp. 144 to 156.
In this article, it is mentioned on page 149 that:
"Both natural and calcined serpentine can be leached with hydrochloric acid...
A leaching with an excess of HCI is suggested to recover silica gel, which can
be obtained by the ftltration of the upper layer only of the leached slurry
after
a settling. Leaching with an excess of serpentine is proposed to minimize the
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dissolution of silica in the leached slurry so as to facilitate the subsequent
filtration".
It is therefore clearly suggested that the lixiviation must be carried out
with an
excess of tailings in order to reduce the risk of dissolution of silica and,
accordingly, the
formation of silica gel.
U.S. patent No. 2,398,493 (BUTT et al) assigned to INTERNATIONAL
MINERALS AND CHEMICAL CORP. discloses a process for the production of
magnesium using serpentine or any other asbestos tailings as starting
material. The
tailings used as starting material is subjected to a screening and to a
preliminary
treatment to remove the magnetic elements contained therein. The material is
then
subj ected to a lixiviation with an aqueous solution of a hydrochloric acid
solution having
a concentration of 20% by weight, obtained by recycling the chloride obtained
in the
electrolysis cell used for the production of pure magnesium. The salt solution
that is
produced is then subjected to a supplemental purification treatment by
precipitation,
which is achieved by addition of magnesia (Mg0), followed by a filtration.
T'he purified
salt solution that is obtained, is then concentrated and dried before
introduction into the
electrolysis cell.
In the BUTT patent, there is disclosed that the asbestos tailings must
preferably
have a granulometry of about 60 mesh. It is also suggested that the
lixiviation be carried
out at a temperature of 95 to 100°C or higher. Example 2 which is the
most relevant one,
makes reference to a reaction temperature of 110°C. In the same example
2, reference
is also made to a reaction time of about 15 min. and the stoichiometric ratio
tailings/HCL, as it can be calculated, is of about 1.09.
U.S. patent No. 2,549,798 (GEE et al) assigned to the UNITED STATES OF
AMERICA, discloses a process for preparing magnesium chloride from asbestos
tailings, comprising a lixiviation carried out in two steps with an
hydrochloric acid
solution having a concentration of 20% by weight, followed by a reaction of
the obtained
salt solution with a magnesia, and a final separation of the impurities.
In this patent, there is mentioned in column 3, lines 4 to 11 that the
tailings used
as starting materials must have a granulometry lower than 20 mesh in order to
obtain
good results. It is also indicated that a major portion of the ground tailings
should have
a granulometry lower than 200 mesh. In the only example given in this patent,
reference
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made to a lixiviation temperature of 109°C and the stoichiometric ratio
tailings/HC1 is
of about 1.15. This ratio is actually an average ratio, since the lixiviation
is carried out
in two successive steps, the first being one carned for about 10 min. with the
stoichiometric ratio tailings/HCl of about 0.27, the other one for about 30
min. with a
stoichiometric ratio tailings/HCl of about 6.15.
U.S. patent No. 5,091,161 and its Canadian counterpart No. 1,303,327 (HARRIS
et al) both granted to METALLURGY NORANDA INC. disclose and claim a process
for preparing an aqueous solution of magnesium chloride (hereinafter called
"salt
solution") from siliceous magnesium minerals, such as serpentine which is one
of the
main components of asbestos tailings.
This process is devised to be carned out in a continuous manner. It comprises
a first step wherein the material to be treated is introduced into a reactor
containing a
hydrochloric acid solution. The starting products are reacted at a temperature
higher
than SO°C but lower than the ebullition temperature of the solution.
More particularly,
the reaction is carried out at a temperature preferably ranging between 80 and
90°C, in
such a manner that the pH remains under 1.5. Such permits to extract magnesium
from
the tailings while avoiding the formation of silica gel. In the examples given
in this
patent, the lixiviation reaction is carned out in the reactor with a
stoichiometric ratio
tailings/HCl ranging between 1.03 and 1.18 for a period of time of about 130
min.
The process disclosed and claimed in these patents to HARRIS et al also
comprises a second step wherein the salt solution that is obtained is
transferred into a
second reactor in which reactive magnesia, which can be calcinated magnesia or
magnesium oxide obtained by spray roasting, is introduced to react with the
salt solution
and to keep the pH of the same between 4 and 7. This particular treatment
carried out
in the second separate reactor at a pH different from the first one causes a
precipitation
of almost all the other impurities containing the salt solution while avoiding
again the
formation of silica gel.
Last of all, in a third step, the purified salt solution is transferred into a
concentration system in order to recover the requested magnesium chloride
solution.
Upon reading of the most relevant documents known to the inventors, including
those quoted hereinabove, it seems to be known for numerous years that the
main
difficulty of the existing process of preparation of magnesium chloride
solutions by
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lixiviation of asbestos tailings with hydrochloric acid lies in the
filtration, or rather in
the impossibility of filtration of the solution in the presence of silica gel.
The formation
of silica gel is, depending of the authors, attributed to the ratio
tailings/HCI, the reaction
temperature, the pH at the end of the reaction and the reaction time. It is
also suggested
that the granulometry of the tailings play a role.
Upon reading of the very same documents, it seems also that, to solve this
problem, it has already been suggested:
1- to use a stoichiometric ratio tailings/HCL higher than 1 (see in particular
the article of N. NAGAMORI et al mentioned hereinabove; see also U.S. patent
No.
4,289,736 (LALANCETTE) assigned to the UNIVERSITE DE SHERBROOKE or U.S.
patentNo. 4,944,928 (GRILL et al) assigned to VEITSCHER MAGNESTIWERKE AG;
furthermore, see the examples given in the various patents listed hereinabove;
2- to carry out the lixiviation at a high temperature, that is at a
temperature
higher than 80°C (see almost all the documents listed hereinabove);
3- to carry out the lixiviation at a pH lower than 1.5 and, at the end of the
reaction, to quickly increase the pH (see in particular the patents of HARRIS
et al); and
4- preferably, to proceed to the pH increase into successive steps instead of
one (see the patent of BUTT).
In practice, it seems obvious that there is an interaction between these
different
parameters regarding the formation of silicic acid during the lixiviation
reaction and the
resulting formation of silica gel.
SUMMARY OF THE INVENTION
The present invention is based on the discovery made by the inventors that, in
a process for the preparation of a magnesium chloride solution by lixiviation
of asbestos
tailings, one may convert almost entirely the hydrochloric acid used as
starting material
into magnesium chloride or other metal chlorides provided that:
- the asbestos tailings used as starting material have a granulometry equal to
a
lower of -62 mesh;
- the lixiviation is carried out at a temperature close to the temperature of
ebullition of the mixture, this temperature of ebullition ranging between 115
and 120°C;
and
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- the lixiviation is carned out with asbestos tailings and hydrochloric acid
in such
amounts that the stoichiometric ratio tailings/HCl ranges between 1.2 and
2.25.
Thus, it has been found in a very surprising manner that, in the above
condition
of operation (1) the time required to obtain almost entire transformation of
the
5 hydrochloric acid into magnesium chloride and other chlorides is of about 5
to 15 min.
and (2) there is no filtration problem because of the presence of silica gel.
This time of reaction is very short as compared to the time of reaction
exemplified in the document discussed herein above, which are about one to
several
hours. It is obvious that a short time of reaction favorizes continuous
conditions.
Thus, the invention lies in the selection of a plurality of very specific
parameters
which, when combined together, gives a result which is extremely interesting
from an
industrial standpoint and unexpected in view of what was already known in this
field.
The process according to the invention as claimed hereinafter is thus
essentially
a process for the preparation of a magnesium chloride solution wherein
asbestos tailings
previously demagnetized if such is needed, are subjected to lixiviation with
an aqueous
hydrochloric acid solution having a concentration ranging between 18 and 25%
by
weight, preferably about 20% by weight. This process is characterized in that:
before the lixiviation, the asbestos tailings are treated in such a manner as
to have
a granulometry equal to or lower than -62 mesh;
use is made of asbestos tailings and hydrochloric acid (HCl) in such amounts
that
a suspension is obtained which, at the beginning of the lixiviation, has a
stoichiometric
ratio tailings/HCl ranging between 1.2 and 2.25, preferably 1.6 and 2, the
most preferred
range between about 1.8; and
the lixiviation is carried out at a temperature ranging between 100°C
and a
temperature of ebullition of the suspension, the lixiviation temperature being
preferably
of about 110°C.
By producing as disclosed herein above, one may complete the lixiviation in a
period of time ranging from 5 to 15 min., the preferred period of time being
between 5
and 7.5 min.
Preferably, the lixiviation is carned out under stirring and comprises a
preheating
of the hydrochloric acid solution to a temperature of about 60°C
followed by an addition
of the asbestos tailings, this addition causing a fast increase of a
temperature which is
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then kept at the requested value.
Once the lixiviation is completed, the pH of the suspension can be quickly
increased to a value ranging between 4 and 5 and the suspension can be
subjected to a
separation in order to recover the requested magnesium chloride solution in
the form of
a first salt solution. The pH of the first salt solution that is recovered,
can then be
increased into a value ranging between 6 and 7 and the first salt solution can
then be
subjected to another separation in order to recover a second salt solution.
Alternatively, once the lixiviation is completed, the pH of the suspension can
be
quickly increased directly to a value ranging from 6 and 7 and the suspension
can be
subjected to a separation.
In all cases, the requested increase in pH is preferably obtained by addition
of
magnesium oxide (Mg0) in the suspension and/or the first salt solution.
The invention will be better understood upon reading the following detailed
description and non-restrictive examples, made with reference to the
accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a curve giving the reaction kinetics expressed as the total amount
of
hydrochloric acid consumed as a function of the time during a lixiviation
reaction
carried out with stoichiometric ratios tailings/HCl equal to 1,20; 1,50; 1,80
and 2,25,
respectively, using, as a stating material, asbestos tailings having a
granulometry lower
than 62 mesh; and
Fig. 2 is curve similar to the one shown in Fig. 1, except that the reactions
were
carried out with stoichiometric ratios tailings/HCl equal to 1.20 and 1.80,
respectively,
using, as a starting material, asbestos tailings having a granulometry lower
than 20 mesh
only.
DETAILED DESCRIPTION OF THE INVENTION
As previously explained, the process according to the invention is intended to
be
used for the preparation of a magnesium chloride solution like those used for
the
production of magnesium in the metal form by electrolysis.
This process which is essentially a lixiviation process, uses, as starting
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materials, of:
- an aqueous solution of hydrochloric acid; and
- asbestos tailings.
The hydrochloric acid solution has a concentration ranging between 18 and 25%
by weight. Preferably, this concentration is of about 20% by weight, since
this
percentage is the one which corresponds to an azeotropic mixture. The acidic
solution
may advantageously be obtained by recycling the hydrochloric acid generated
during the
production of magnesium by electrolysis.
The asbestos tailings used as starting material can be of any known type. In
the
tests that were carried out by the inventor, the asbestos tailings were from a
mine of JM
ASBESTOS INC. (Quebec). These tailings also called "raw residues" are obtained
from
a grinding system used for the extraction of chrysotile asbestos fibers. Table
I
hereinafter gives the granulometric distribution of these tailings.
TABLE I
Distribution of the granulometric fractions
of the raw residues
Granulometric fraction I Distribution
25
In order to eliminate an important part of the magnetic fraction which
may consume hydrochloric acid, it is very useful to demagnetize the tailings
before using
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them. This demagnetization can be carried out with an apparatus of the type
Eriez. By
way of example, Table II hereinafter gives the percentage of magnetic and non-
magnetic
fractions obtained for different granulometric fractions.
TABLE II
Distribution of the magnetic and non-magnetic
fractions in the tailings
Distribution (%)
Granulometric fraction raw residues
(mesh) _4+l0 ~ -10+24 ~ -24+62 ~ -62
magnetic ~ 20 ~ 38 I 40 I 30
non-magnetic ~ 80 I 62 I 60 I 70
Table III hereinafter gives a full chemical analysis of the residues.
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TABLE III
Chemical analysis of the raw residues
Chemical compounds Percentage (%)
(oxides) Raw residues Demagnetised
residues
Mg0 37.6 38.0
Si02 38.9 ~ 41.3
Fe203 4.87 2.05
Fe0 2.35 1.76
A1203 1.37 1.66
Ca0 0.27 0.64
Ni0 0.33 0.33
Cr203 0.34 0.32
Na20 0.18 0.26
K2p 0.22 0.17
Mn0 0.09 0.10
Ti02 0.04 0.04
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Once the residues are screened and demagnetised, the lixiviation reaction can
be
carried out by using the process disclosed hereinabove, which is the one that
was used
for carrying out in labs the following lixiviation tests with hydrochloric
acid on asbestos
tailings.
5
EXAMPLE 1
2.16 kg of an aqueous solution of hydrochloric acid having a concentration of
20% by weight of acid was poured in a plate container of 4 liters. The acidic
solution
was heated to a temperature of 60°C by keeping it under stirnng with a
stirrer of
10 trademark Lightin model serial No. 30, provided with an acid-resistant
blade. When the
acidic solution reached the temperature of 60°C, the requested amount
of asbestos
tailings previously demagnetized was then added. The temperature of the
suspension
under stirnng increases to about 110°C within less than 5 minutes and
was kept at this
value for the duration of the experiment by means of a heating plate of
trademark
Corning PC 351.
During the tests that were carried out, samples of about 50 g of solution were
picked up every 2.5 minutes and filtered on a Buckner funnel. Samples of l Og
of the
filtered solution were then poured into 100 ml of cold water. The samples were
then
analysed with a spectrophotometer of trademark Varian 400. The magnesium
dissolution
kinetics was then established.
Table IV gives the results obtained with different multiplication factors of
the
tailings/HCl stoichiometric ratio, using, as a starting material, screened
and/or ground
tailings having a granulometry lower than 62 mesh.
30
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TABLE IV
Reaction kinetics with tailings of less of 62 mesh
Reac- Multiplication factor of the stoichiometric ratio (tailingsIHCI)
tion
time 1.20 1.50 1.80 2.25
(min)
1 2 3 1 2 3 1 2 3 1 2 3
0.0 0 0 0 0 0 0 0 0 0 0 0 0
2.5 29.030.3 32.736.8 48.051.6 40.363.1 67.340.7 79.785.2
5.0 46.648.3 51.460.3 78.783.4 60.194.2 99.448.6 95.199.4
7.5 56.358.1 61.569.7 91.096.0 60.895.2 99.949.6 97.199.9
10.0 69.471.5 75.571.9 93.898.6 60.394.2 98.848.2 94.398.7
12.5 75.877.9 82.172.5 94.799.4 58.691.7 99.148.7 95.399.8
15.0 78.882.3 86.671.8 93.798.3 60.494.5 98.949.2 96.398.6
The results that are reported hereinabove are illustrated in Fig. 1.
From a practical standpoint, the stoichiometric ratio tailings/HCl is defined
as
the ratio between the amount of tailings and the amount of HCl (100%) required
to
transform all the metal oxides of Table III, except Si02, in their
corresponding metal
chlorides. Under these conditions, the multiplication factor of the
stoichiometric
tailings/HC ratio represents the multiplication factor of the amount of
tailings used in
the test as compared to the amount that is required for the stoichiometric
ratio.
In Table IV hereinabove, column 1 of each test carried out with a same factor
gives the percentage of Mg2+ lixiviated in the solution, expressed as a
function of the
amount of Mg2+ initially present in the tailings. Column 2 gives the
percentage of HCl
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used for dissolving Mg2+ with respect to the initial amount of HC1. Last of
all, column
3 gives the percentage of HCl used for dissolving Mg2+ and the other metal
impurities
with respect to the initial amounts of HCI. This last percentage is the one
that is given
in Fig. 1.
As is clearly shown in Table IV and Fig. 1, the time required for 98 to 100%
of
HCl to be used for dissolving Mg2+ and the other metallic impurities are
respectively of
minutes, 5 minutes and 5 minutes for multiplication factors of the
stoichiometric
ratio equal to 1.50, 1.80 and 2.25. When the multiplication factor of the
stoichiometric
ratio is 1.20, only 87% of HCl is used after a reaction time of 15 min.
10 By of comparison, Table V and Fig. 2 give the results obtained under the
same
experimental conditions on asbestos tailings having an higher granulometry
fraction of
about 20 mesh, with multiplication factors of the stoichiometric ratio of 1.20
and 1.80.
20
30
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TABLE V
Reaction kinetic with tailings of -20 mesh
Reaction Multiplication
factor
of the
stoichiometric
ratio
time (Tailings/HCl)
(minutes) 1.20 1.80
1 2 3 1 2 3
0.0 0 0 0 0 0 0
2.5 17.2 17.0 19.7 31.8 49.7 53.7
S.0 27.2 28.4 30.9 46.2 72.3 77.4
7.5 38.1 39.7 42.7 52.7 82.6 88.1
10.0 53.2 55.6 59.3 58.1 91.0 96.7
12.5 64.5 67.3 71.8 57.5 90.1 95.5
15.0 69:1 72.2 76.9 58.1 91.0 96.5
As can be seen, with this granulometry and for a multiplication factor of the
stroechiometric ratio equal to 1.20, the percentage of HCl used to dissolve
Mg2+ and the
other metal impurities was equal to 77% after a reaction time of 1 S minutes.
For a
multiplication factor of the stoichiometric of 1.80, this percentage was equal
to 97%
after a reaction time of 10 minutes.
In all the tests reported in Table IV and Table V and illustrated in figures 1
and
2, separation of the liquid phase from the residual solid phase has never been
a problem.
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Decantation occurred and the liquid phase was separated by conventional
techniques of
filtration by gravity or under vacuum. No silica gel was produced or no other
gelatinization of the reaction products was detected.
Thus, the process according to the invention which uses an excess of asbestos
tailings over the basic stoichiometric amount required for the reaction
permits a quick
and almost complete use of the hydrochloric acid. As a matter of fact,
hydrochloric acid
is the only starting material which is expensive in the lixiviation process.
Indeed, the
asbestos tailings are of very low cost.
The process according to the invention is therefore particularly interesting
if it
is used on the premises of mines where the supply of asbestos tailings of the
requested
granulometry is easily available. Moreover, the disposal of the solid residues
after
reaction, even in excess, does not present any major problem in this
particular case.
EXAMPLE 2
An amount of 2.16 kg of HCl 20% was introduced into a Becher of 4 liters. This
solution was stirred and heated to 60°C. An amount of 0.96 kg of
asbestos tailing
having a granulometric fraction equal to or lower than 62 mesh was
demagnetized and
added thereto. Under these conditions, the multiplication factor of the
stoichiometric
ratio tailings/HCl was equal to 1.80. After 3 to 4 minutes, the reaction
temperature
reached 105 to 110°C. After a reaction time of 7.5 minutes, the pH of
the suspension
was equal to about 0.9 to 1Ø In this test, HCl was almost entirely used.
At the end of the reaction, the pH of the suspension was adjusted to 4.2-4.3
by
addition of 15 g of MgO. The pH adjustment was almost instantaneous and the
suspension was centrifugated in order to separate the liquid from the solids.
The
centrifugal machine that was used was a laboratory centrifugal machine of
trademark
Soiltest model AP-174, having a capacity of 2 liters. The centrifugation speed
was
progressively raised up to 5000 rpm. The centrifugation time was about 10
minutes. The
amount of salt solution that was recovered was equal to about 88% of the
original
amount of salt solution. The amount of salt solution retains in the solid
after
centrifugation was equal to 466 g per kg (384 cm3/kg).
For comparison purpose, the reaction residues were washed with water. This
washing was repeated three times within the same centrifugation machine. The
amount
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of water held in the solids after centrifugation was equal to 391 g per kg
(391 cm3/kg),
that is an amount slidely higher than the amount of salt solution retained
during the
centrifugation after lixiviation. These data indicate that the liquid
retention is essentially
due to the granulometry of the residues and not to the generation of a
gelification system
5 during the lixiviation of the asbestos tailings with hydrochloric acid.
The pH of the salt solution recovered after the first centrifugation of the
suspension obtained during the lixiviation was then adjusted a second time to
6.2-6.3 by
addition of 52 g of Mg0 in order to transform the impurities present in the
form of
soluble metal chlorides into insoluble metal hydroxides. The suspension was
again
10 filtered and Table VI hereinafter gives the composition of the salt
solution that was so
obtained.
TABLEAU VI
Composition of the salt solution
20
The process of purification of the salt solution by successive adjustment of
the
pH to 4.2-4.3 and 6.2-6.3 is called "two-stage purification process" and this
process is
the one used in the industrial MgClz process called Riithner-HCl-Route. This
is also a
purification process disclosed in U.S. patent No. 5,091,161 (HARRIS et al).
The quick lixiviation process according to the invention can thus be used in
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16
combination with this conventional two-stage purification process.
EXAMPLE 3
A lixiviation process as disclosed in example 2 was carned out. After the time
of reaction of 7.5 minutes, the pH of the suspension was adjusted to 6.2-6.3
by addition
of 67 g of MgO. The adjustment of the pH was practically instantaneous and the
suspension was centrifugated in order to separate the liquid from the solids.
The
centrifugation conditions were identical to those described in example 2. The
amount
of salt solution that was recovered was equal to about 83% of the total amount
of salt
solution originally present. The amount of salt solution retained in the
solids after
centrifugation was equal 663 g per kg (551 cm3/kg). This amount is higher than
the one
reported in example 2, probably because of the presence of metal hydroxide of
high
specific surface area.
The composition of the salt solution that was so obtained is given in Table
VII.
TABLE VII
Composition of the salt solution
25
As can be seen, the quick lixiviation process according to the invention can
also
be used with a purification process carried out in a single stage.
