Note: Descriptions are shown in the official language in which they were submitted.
CA 02248474 1998-09-28
TITLE OF THE INVENTION
MAGNESIUM COMPOUNDS FROM MAGNESIUM SILICATES
FIELD OF THE INVENTION
The present invention relates to the production of magnesium
compounds, such as magnesium oxide or magnesium carbonate, from
naturally occuring magnesium silicates.
BACKGROUND OF THE INVENTION
Silicates of magnesium are minerals of very broad occurrence
in nature and appear in various forms such as talc, phlogopite, blotite,
chorite, sepiolite and the like.
By the examination of the chemistry of silica and carbon
dioxide, it appears obvious that, carbon dioxide in water being much more
acidic than silica in the same solvent, the naturally occuring silicates of
basic
oxides, such as magnesium oxides, should dissolve in water saturated with
carbon dioxide with the corresponding release of free silica.
MgO. Si02 + H2O.CO2-~MgCO3 + Si02.H20
In effect, from the data of the chemical litterature, the ionization
constants of carbonic acid and silicic acid are the following:
H2CO3: K,=4.31 x10-'
HzSiO4: K,=2x10-10
From these data, it can be seen that carbonic acid is roughly
one thousand time more acidic than silicic acid and should attack very
strongly the structure of naturally occurring magnesium silicates. In
practice,
this is not observed and minerals such as talc, soapstone and other
magnesium silicates can resist weathering (rain containing carbon dioxide)
over millions of years. Therefore silicic acid, while being an acid much
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weaker than carbonic acid, can proved to be inert to carbon dioxide in many
instances.
From these considerations, the use of magnesium silicate as a
substrate for lixiviation by carbon dioxide has not been very promising. One
of the area where the largest amounts of magnesium silicate (3MgO. 2SiO2.
2HZ0) is handled is close by asbestos mining operations. The fibrous
structure of asbestos ores represents only a few percents of the ore body
which is treated and very large amounts of magnesium silicate wastes is thus
rejected as tailings.
OBJECT AND STATEMENT OF THE INVENTION
It is an object of the present invention to produce magnesium
compounds of appropriate composition and structure by the digestion of
silicates in the presence of carbon dioxide in water at ambient temperature.
This is achieved by providing a process that involves the
leaching of magnesium compounds from calcined magnesium silicates
having initially a phyllosilicate structure, these magnesium silicates showing
a
Mg0 to Si02 ratio higher than 1.0 on a weight basis. This leaching is done
with carbon dioxide on a slurry of ore in water at substantially ambient
temperature and substantially atmospheric pressure.
More specifically, the present invention relates to a process for
the leaching of magnesium compounds from calcined magnesium silicates
having initially a phyllosilicate structure, the magnesium silicates showing
a Mg0 to Si02 ratio higher than 1.0 on a weight basis; the leaching being
done with carbon dioxide on a slurry of ore in water at substantially
ambient temperature and substantially atmospheric pressure.
The present invention also relates to a process for producing
magnesium compounds from calcined magnesium silicates having a
phyllosilicate structure and showing a Mg0 to Si02 ratio higher than 1.0 by
weight and the presence of free brucite; comprising the step of leaching
the silicates with carbon dioxide in water at ambient temperature and at
about atmospheric pressure.
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The present invention also relates to a process for producing
magnesium compounds comprising the steps of:
grinding mine tailings containing magnesium silicates to form
particles having a predetermined mesh size; the magnesium silicates
having a phyllosilicate structure and showing a MgO to Si02 ratio higher
then 1.0 on a weight basis;
calcining the particles at a temperature between 500 to
700 C to form calcined particles;
leaching the calcined particles in water at substantially
ambient temperature while stirring with carbon dioxide at a relative
pressure of about one atmosphere;
maintaining the leaching and stirring steps for a given period
and filtering to provide magnesium compounds.
DESCRIPTION OF THE INVENTION
Tailings of various asbestos mines in the province of Quebec,
in Canada, have been examined in order to evaluate their reactivity towards
leaching by carbon dioxide. In order to promote the reaction to optimal
yields,
the tailings were calcined at low temperature, such as 600 C in order to
open-up the structure by the elimination of water.
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It has been observed that, with tailings having a ratio of
magnesium oxide to silica close to one or less than one, in such mining sites
as JM Asbestos (0.97), King Beaver (0.95), British Canadian #1 (0.99),
Bolduc (0.97) or Bell (0.99), the leaching of the material under standard
conditions (ore ground to -200 mesh, calcined at 600 C, water saturated with
CO2 at one atmosphere, stirring at 20 C for 24 hrs) led to very poor yields of
leaching, about two percent of the available magnesium oxide in the starting
silicate being extracted. However, with other mineral sources where the ratio
of magnesium oxide to silica was above 1.0 in such mining sites as National
(1.15), Carey (1.12) or Boston (1.10), it was possible to extract about 50%
of the available magnesium oxide from the starting ore.
Following these observations, the mineralogy of the various
ores were examined in order to provide an understanding as to why a slight
variation of the magnesium oxide to silica ratio had such a striking effect on
the availability of magnesium oxide during the leaching with carbon dioxide.
Asbestos tailings are phyllosilicates with a structure of an
indefinitely extended silicon-oxygen sheet, the so-called "siloxane sheet
array". The hydroxyl groups are coordinated by magnesium ions above and
below siloxane sheets. If their giant molecules are seen as stack of such
alternates layers of Si-O planes and Mg(OH)2 planes, it will be readily
understood that the access to the Mg(OH)2 can be very limited if there is an
excess of Si-O planes. But, on the other hand, if there is an excess of
Mg(OH)2, even if this excess is slight, there might be a rather improved
access to the Mg(OH)2 by an opening of the lamellar structure. This
explanation is corroborated by the fact that with materials where the leaching
by carbon dioxide is efficient, it is noted that the ore contains some
brucite,
the tetrahedral arrangement of Mg3(OH)6.
Therefore, without going into limitative theoritical
considerations, it is noted that the main condition required for a magnesium
silicate to be leachable by carbon dioxide is to have a silicate structure
which
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is opened up by an excess of MgO over Si02, on a percent of weight basis.
This is confirmed by the fact that, with the asbestos fiber which has a MgO
to Si02 ratio of 1.0, the MgO can not be leached by carbon dioxide to a
significant amount. This opening of the phyllosilicate structure by the
presence of brucite in excess over silica is a basic requirement for leaching.
Additional opening is obtained by relatively low temperature calcination of
the
starting ore so as to disrupt the phyllosilicate planar structure by removal
of
water. This calcination must be sufficient to eliminate water from the
hydroxyl planes but must not exceed 700 C since, above that temperature,
there is a sintering of the material which takes place and the gains obtained
by removing water are lost due to the closure of the structure.
Hence, the implementation of the present invention calls for a
source of phyllosilicate of magnesium, relatively rich in MgO, with a ratio of
MgO to Si0z above 1.0 on a weigh basis. This material, if obtained from
tailings of asbestos mines, is already finely divided but can be submitted to
further grinding so as to obtain a granulometry of -200 mesh and then
calcinated at 600 C to constant weight. A slurry of such material is made
with water, the solid content being in the 1-10% range. This slurry is
saturated with carbon dioxide, a slight pressure being maintained over the
surface of liquid so as to insure a one atmosphere pressure (absolute) of
CO2 in the system, or thereabout. A slow stirring insures the homogeneous
consistency of the slurry.
The leaching process can thus be described by the following
equation:
3MgO.2SiO2.2H20 600 c 3MgO.2SiO2 + 2H20
3MgO.2Si02 + 3CO2 + 1.5H20 20 c 1.5Mg (HCO3)2 + 1.5MgO.2SiO2
After about twelve hours, the unreacted magnesium silicate is
filtered off and the solution of magnesium bicarbonate is heated to 100 C
inducing the formation of hydromagnesia (basic magnesium carbonate).
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4Mg (HCO3)2 loo c 3Mg C03.Mg(OH)2.3H20 + 5CO2
This basic magnesium carbonate is the commercial form of
magnesium carbonate and can be sold as such or transformed into
magnesium oxide by calcination at 600 C.
3MgCO3.Mg (OH)2.3H20 6oo c 4MgO + 3CO2 + 4H20
Carbon dioxide liberated on the course of the precipitation of hydromagnesia
or during the calcination of hydromagnesia can be recycled in order to
maintain a closed loop as far as the circuit of CO2 is concerned. The purity
of the magnesium oxide thus obtained is better than 99% MgO, the main
impurity being CaO.
While proceeding with the leaching of the starting ore, a circuit
of the slurry circulating over a magnet can remove a large portion of the
magnetic fraction which is rich in iron oxides, with some nickel.
Example
The ores were obtained from tailings at mining sites and were
ground to -200 mesh by ball milling. Calcination was done in an electrical
furnace at various temperatures, from 400 to 800 C, 600 C giving optimal
yields of leaching. The leaching operation was performed on 50g samples
in 1500m1 of tap water at 20 C. Stirring was maintained with a magnetic
stirrer and carbon dioxide was admitted to the system at a relative pressure
of 70cm of water, giving an absolute pressure close to one atmosphere.
Yields were determined against various durations of contact and
temperatures of calcination. Contact of about 12 hours gave near maximum
yields. The magnet used to achieve stirring collected the magnetic fraction
in ores which had a Ni/Fe ratio in the range of 0.02.
After the contact with C02, the slurry was filtered and the filtrate
heated at 1 00 C for half an hour. The resulting precipitated hydromagnesia
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was filtered and dried at 110 C overnight. Results thus obtained are
presented in Table 1.
The calcination of the hydromagnesia, at 600 C gave the
corresponding magnesium oxide with a purity above 99%.
Table I
Lixiviation of silicates
Sources MgO Si02 Ratio Yield of
(%) (%) MgO/SiOZ extraction. #
J.M. Asbestos 38.1 39.2 0.97 2.1
King Beaver 36.7 38.6 0.95 2.7
Br. Can.# 1 39.2 39.4 0.99 2.6
Bolduc Can. 36.3 37.5 0.97 1.9
Bell 38.2 38.6 0.99 2.3
National 38.9 33.9 1.15 51.5
Carey 41.0 36.5 1.12 49.2
Boston 38.5 34.9 1.00 50.1
Fiber #7 38.5 38.5 1.00 1.8
$ The yield is the percentage of the leached magnesium calculated as MgO
related to the available MgO in the starting material.
