METHOD FOR RECOVERY OF SILICA FROM MAGNESIUM SILICATES

METHODE POUR LA RECUPERATION DE SILICE DES SILICATES DE MAGNESIUM

English Abstract

A method for the recovery of silica from magnesium silicate by digestion with
hydrochloric acid followed
by physical and chemical purification. The magnesium silicate is selected as a
serpentinic mineral or
waste resulting from treatments of a magnesium silicate.

Claims

5
Claims
1. A method for recovery of silica from magnesium silicates, comprising
leaching
the magnesium silicates with hydrochloric acid, separating a resulting silica
from dense material and
purifying with nitric acid.
2. The method of claim 1, comprising selecting the magnesium silicates as
ones
of: i) serpentinic minerals and ii) wastes from previously treated magnesium
silicates.
3. The method of any claim 1, wherein the magnesium silicates are
serpentinic
magnesium silicates deprived from 20% to 80% of a magnesium content thereof by
leaching by one of:
sulfuric, hydrochloric and carbonic acids.
4. The method of any one of claims 1 to 3, wherein the hydrochloric acid
leaching
of the magnesium silicates is done by counter current leaching in at least two
steps, at a temperature in
a range between 80 C and 90 C.
5. The method of any one of claims 1 to 3, wherein the hydrochloric acid
leaching
of the magnesium silicates is done by counter current leaching in at least two
steps, at a temperature in
a range between 80 C and 90 C; and said hydrochloric acid leaching is followed
by gravimetric physical
separation of impurities of density above 3.5.
6. The method of any one of claims 1 to 3, wherein the hydrochloric acid
leaching
of the magnesium silicates is done by counter current leaching in at least two
steps, at a temperature in
a range between 80 C and 90 C; and said hydrochloric acid leaching is followed
by gravimetric physical
separation of Ni3Fe.
7. The method of any one of claims 1 to 3, wherein the hydrochloric acid
leaching
of the magnesium silicates is done by counter current leaching in at least two
steps, at a temperature in
Date Recue/Date Received 2023-06-20

6
a range between 80 C and 90 C; said hydrochloric acid leaching being followed
by gravimetric physical
separation of impurities and a final acid leaching with an acid other than
hydrochloric acid.
8. The method of any one of claims 1 to 3, wherein the hydrochloric acid
leaching
of the magnesium silicates is done by counter current leaching in at least two
steps, at a temperature in
a range between 80 C and 90 C; said hydrochloric acid leaching is followed by
gravimetric physical
separation of impurities and a final acid leaching with nitric acid.
9. The method of any one of claims 1 to 8, wherein the hydrochloric acid
leaching
of the magnesium silicates is done by counter current leaching in at least two
steps, at a temperature in
a range between 80 C and 90 C; the method further comprising raising a pH of a
magnesium chloride
solution resulting from the hydrochloric acid leaching to a range between 4
and 5 by addition of one of:
Mg(OH)2, Mg0 and serpentinic tailings.
10. The method of any one of claims 1 to 8, wherein the hydrochloric acid
leaching
of the magnesium silicates is done by counter current leaching in at least two
steps, at a temperature in
a range between 80 C and 90 C during between one and two hours; the method
further comprising
raising a pH of a magnesium chloride solution resulting from the hydrochloric
acid leaching to a range
between 4 and 5 by addition of one of: Mg(OH)2, Mg0 and serpentinic tailings.
11. The method of any one of claims 1 to 8, wherein the hydrochloric acid
leaching
of the magnesium silicates is done by counter current leaching in at least two
steps, at a temperature in
a range between 80 C and 90 C; the method further comprising raising a pH of a
magnesium chloride
solution resulting from the hydrochloric acid leaching to a range between 4
and 5 by addition of one of:
Mg(OH)2, Mg0 and serpentinic tailings; and treating a resulting purified
solution of magnesium chloride
with a stoichiometric amount of calcium hydroxide using attritional mixing,
yielding magnesium hydroxide
and calcium chloride.
12. A system for recovery of silica from magnesium silicates, comprising at
least a
first and a second hydrochloric acid leaching reactors; a gravimetric
separator; a final acid leaching
Date Recue/Date Received 2023-06-20

7
reactor; the magnesium silicates being leached in counter current in the first
reactor with residual acid
from the second reactor, at a temperature in a range between 80 C and 90 C;
impurities being removed
from a resulting hydrated silica in said gravimetric separator before an acid
leaching with an acid other
than hydrochloric acid in said final add leaching reactor.
13. The system of any one of claims 11 and 12, wherein said final acid
leaching
reactor is a nitric acid leaching reactor.
14. The system of any one of claims 11 to 13, wherein said first and a
second
hydrochloric add leaching reactors are configured as a counter current system,
the magnesium silicates
being digested twice in the counter current system, a first digestion being
done mainly in said first reactor
with the residual acid from said second reactor which is fed with concentrated
HCI (37%).
15. The system of any one of claims 11 to 14, wherein said first, second
and final
reactors have a temperature in a range between 80 C and 90 C.
16. The system of any one of claims 11 to 15, wherein said gravimetric
separator
removes impurities of density above 3.5.
17. The system of any one of claims 11 to 16, wherein said gravimetric
separator
removes Ni3Fe.
18. The system of any one of claims 11 to 17, comprising a first vessel
receiving a
magnesium chloride solution resulting from acid leaching in said first and
second reactors and one of:
Mg(OH)2,Mg0 and serpentinic tailings, said first vessel yielding a magnesium
chloride solution with a pH
to a range between 4 and 5 .
Date Recue/Date Received 2023-06-20

8
19. The system of claim 18, comprising a an attritional mixing vessel
receiving a
resulting purified solution of magnesium chloride and a stoichiometric amount
of calcium hydroxide, said
attritional mixing vessel yielding magnesium hydroxide and calcium chloride.
20. The system of any one of claims 11 to 19, wherein the magnesium
silicates are
serpenfinic magnesium silicates deprived from 20% to 80% of a magnesium
content thereof by leaching
with an acid.
Date Recue/Date Received 2023-06-20

Description

TITLE OF THE INVENTION
Method for recovery of silica from magnesium silicates
FIELD OF THE INVENTION
[0001] The present invention relates to recovery of silica from
magnesium silicates.
More specifically, the present invention relates to recovery of silica from
magnesium silicates such as
chrysotile tailings (3Mg0.2Si02.2H20).
BACKGROUND OF THE INVENTION
[0002] The extraction of magnesium from magnesium silicates such as
serpentinic
tailings leaves a silica of variable composition, according on the acid used
and the operational
conditions. Methods using nitric acid (US patent 1,454,583), sulfuric acid (US
patent 4,277,449),
hydrochloric acid (US patent 7,780,941) or even carbonic acid (CA patent
2,378,721) have been
presented. Typically, the obtained silica remains tainted with insoluble
elements such as awaruite
(Ni3Fe); the magnesium extraction is far from complete and the physical
properties of silica such as
color, specific surface or level of hydration is not optimal. Moreover, the
resulting salt of magnesium
has no significant market (ex: Mg (NO3)2) or poor commercial value (ex: MgSO4,
MgCl2), being
accessible as naturally occurring species (Epsom salt, bischofite) in brines.
[0003] Therefore, there is a need in the art for a method for
recovery of silica from
magnesium silicates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] In the appended drawings:
[0005] FIG. 1 is a flowchart of a method according to an embodiment
of an aspect of
the present disclosure.
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2
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0006] The present invention is illustrated in further details by the
following non-limiting
examples.
[0007] A method according to an embodiment of an aspect of the
present disclosure
as illustrated in the flowchart of FIG. 1 for example, comprises using a
magnesium silicate derived from
chrysotile tailings (3Mg0.2Si02.2H20) as a source of silica. By using residual
magnesium silicates
resulting from partial extraction of magnesium by carbonation or sulfation,
the ratio of magnesium to
be extracted per unit weight of silica obtained is thus reduced from 3
Mg0.2SiO to 1.5 Mg0.2Si02.
Extraction of magnesium chloride (MgCl2) is performed using hydrochloric acid
(HCl) as an extraction
reagent in a countercurrent extraction (steps 1- 8). After separation of dense
impurities, by cycloning
through vortex separation or gravity extraction leveraging the different
specific weighs (step 9), the
extraction is completed in a polishing circuit using nitric acid (HNO3) (step
10). Pure silica (98-99%
SiO2) is obtained (step 12) by calcination of the hydrated product at 1000 C
(step 11).
[0008] The magnesium chloride (MgC12) resulting from the extraction
with hydrochloric
acid (HCI) (steps 1-8) is purified by pH adjustment with a base such as
chrysotile tailings or magnesium
oxide (MgO) or magnesium hydroxide (Mg(OH)2) (step 6) to precipitate the
traces of iron, nickel,
chromium, manganese, and aluminium dissolved along with magnesium. This
cleaned solution (step
7) is then contacted at room temperature with equimolar amounts of calcium
hydroxide (Ca(OH)2)
under attritional conditions selected to renew reactive surfaces, under ball
milling or grinding for
example (step 13). A spontaneous reaction yields magnesium hydroxide (Mg(OH)2)
(step 14) and
calcium chloride (CaCl2) (step 15), a commodity of broad uses for dust control
and de-icing of roads
for instance.
[0009] In a system leading to silica (SiO2) and magnesium hydroxide
(Mg(OH)2), the
starting magnesium silicate is the residual material Y2 Mg0.2Si02 left by the
formation of schoenite
from potassium bisulfate (KHSO4) and serpentinic tailings as follows:
Date Recue/Date Received 2023-06-20

3
5.5H20 + 3Mg0. 2Si02. 2H20 + 3KHSO4
1 1
¨> 1 ¨2 (K2SO4. M gSO4. 6H20) + 1 ¨2 Mg0. 2Si02 (relation 1)
[0010] Using residual 1/2Mg0.2Si02 instead of the tailings 3Mg0.2Si02
reduces by half
the amount of magnesium to be digested with hydrochloric acid (HCI). Referring
to FIG. 1, the residual
magnesium silicate 1/2 Mg0.2Si02 is digested twice in the counter current
system, the first digestion
being done mainly in a first reactor (1) with the residual acid from a second
reactor (8) which is fed with
concentrated HCI (37%). The resulting magnesium chloride (MgCl2) is filtered
and a neutralizing agent,
such as of one of: Mg (OH)2, MgO and serpentinic tailings, is added to raise
the pH to a range between
4 and 5 (step 4) in order to precipitate the base metals Fe, Ni, Co, Mn, Cr,
Al extracted by the
hydrochloric acid (HCI) (step 5). A second filtration leads to a clean
solution of MgCl2 (7) which has
been concentrated by evaporation (6).
[0011] The hydrated silica from the second digestion (8) after
recycling the residual
acid and rinsings to the first reactor (1) (step 2) is freed by gravity or
cyclone (step 9) from dense
products such as awaruite (Ni3Fe). A final treatment of the hydrated silica is
applied in a closed loop
with nitric acid (HNO3) (10) to remove the last traces of impurities,
particularly those that need
oxidations, such as Fe, Mn, Cr. The obtained purified hydrated silica is then
transformed into anhydrous
silica by a thermal treatment at 10000C (step 11) that reduces the water
content from 6% to 1% or less.
[0012] Experiments were done using the residual material left after
the carbonatation
of calcined tailings, with a composition of 2.2Mg0.2Si02. The obtained silica
had a composition of
97.9% SiO2. With residual material from sulfation of tailings, the purity of
the obtained SiO2 was 98.3%;
the removal of magnesium and traces of base metals was done by between 90 and
95% in the first
reactor (1), by between 4 and 8% in the second reactor (8) and by less than 1%
in the nitric acid loop
(step 10). These leachings were done at a temperature of 85 C for 90 minutes.
The recovery of SiO2
from the starting residual magnesium was 96% and the chemical analysis
indicated a SiO2 at 98.3%.
The specific surface (BET) was 423 m2/g.
[0013] The magnesium chloride solution (7) can be treated by an
equimolar amount of
calcium hydroxide (Ca(OH)2) at 25 C in a reactor (13) with a ball milling
capability for the renewal of
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surfaces. After two hours, filtration gave 96% of the expected Mg(OH)2 (14)
and the solution of calcium
chloride (15).
[0014] Thus, essentially all the magnesium and silicium in the
starting residual silicate
was transformed into useful products.
[0015] There is thus presented a method for the recovery of silica
from magnesium
silicate by digestion with hydrochloric acid followed by physical and chemical
purification. The
magnesium silicate is selected as one of: a serpentinic minerals and waste
resulting from treatments
of a magnesium silicate. Such residual serpentinic silicates can be deprived
serpentinic magnesium
silicate can be deprived of between 20% and 80% of its magnesium content by
leaching either by
sulfuric, hydrochloric, or carbonic acids.
[0016] The hydrochloric acid leaching is done in a counter current
system with at least
two reactors, at a temperature in a range between about 80 oC and about 900C,
for example at about
85 C. The hydrochloric acid leaching is followed by a gravimetric physical
separation of impurities of
density above about 3, including products of the awaruite family Ni3Fe, then
followed by a final acid
leaching with an acid different than the hydrochloric acid, such as nitric
acid for example.
[0017] The magnesium chloride solution resulting from the
hydrochloric acid leaching
of the magnesium silicate is deprived of the traces of base metals by raising
the pH of this solution to
a pH in a range between 4 and 5 by addition of one of: Mg(OH)2, MgO and
serpentinic tailings. The
reaction is done at a temperature in a range between about 80 C and about 90
C, lasting between
about one hour and about two hours, for example at about 85 C during about 90
minutes. The purified
solution of magnesium chloride is treated with a stoichiometric amount of
calcium hydroxide using
attritional mixing, yielding magnesium hydroxide and calcium chloride.
[0018] The scope of the claims should not be limited by the
embodiments set forth in
the examples but should be given the broadest interpretation consistent with
the description as a whole.
Date Recue/Date Received 2023-06-20