Hydrometallurgy 62 (2001) 175 – 183
www.elsevier.com/locate/hydromet
Preparation of a magnesium hydroxy carbonate
from magnesium hydroxide
A. Botha, C.A. Strydom*
Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa
Received 29 June 2001; received in revised form 14 September 2001; accepted 15 September 2001
Abstract
The preparation of a magnesium hydroxy carbonate from magnesium hydroxide and carbon dioxide is described. The procedure
involves the formation of a magnesium hydroxide slurry and sparging CO2 gas through it. Various experimental conditions are
evaluated in order to obtain the conditions that result in the formation of the magnesium hydroxy carbonate. Slurry pH, slurry
temperature, drying temperature, drying time and HCl addition are the conditions that are evaluated. The products that are obtained
are identified by XRD and its decomposition characteristics studied with TG/DTA. It is evident from the results obtained that the
experimental parameters have a significant influence on the products obtained. D 2001 Elsevier Science B.V. All rights reserved.
Keywords: Magnesium hydroxy carbonate; Magnesium hydroxide; CO2, preparation; Hydromagnesite; Dypingite
1. Introduction
Hydromagnesite, the most commonly available
magnesium hydroxy carbonate, has several useful
applications. Although it occurs naturally, it could
also be synthetically manufactured. The magnesium
hydroxy carbonates are hydrated basic magnesium
carbonates containing the equivalent of 40 –45% of
MgO (Reynolds et al., 1993). Two forms are essentially used — light and heavy — the difference being
the number of water molecules that are included in the
compound. Light magnesium carbonate has the
empirical formula (MgCO3)4Mg(OH)24H2O and
he a v y m a g n es i u m c a r b o n at e ( M g C O 3 ) 4 M Mg(OH)25H2O. It can be used in pharmaceuticals
as an inert vehicle and an adsorbent. Due to its fine
*
Corresponding author. Fax: +27-12-362-5297.
E-mail address: [email protected] (C.A. Strydom).
texture and high absorbency, it is used in cosmetic
manufacturing as a carrier and retainer of perfumes. It
is also used in the rubber industry as a reinforcing
agent and as an extender for titanium dioxide in paint,
lithographing inks and as a precursor for other magnesium-based chemicals. The aim of this study is
ultimately to study the prepared compound as a flame
retardant. Magnesium hydroxy carbonate decomposes
during an endothermic reaction and produces decomposition products, H2O and CO2, that are non-toxic.
These are just a few of the favourable characteristics
which has resulted in the use of magnesium hydroxide
and aluminium hydroxide as flame retardants.
Various procedures have been documented for the
synthetic preparation of magnesium hydroxy carbonate. Prakash and Gupta (1987) formed magnesium
carbonate trihydrate by the carbonation of Mg(OH)2
slurries and then, by boiling the pulp, formed the
magnesium hydroxy carbonate. The formation of a
0304-386X/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 3 8 6 X ( 0 1 ) 0 0 1 9 7 - 9
176
A. Botha, C.A. Strydom / Hydrometallurgy 62 (2001) 175–183
Mg(HCO3)2 solution by carbonation of a MgO-containing residue slurry and further precipitation of
hydromagnesite by addition of pure magnesium
oxide as the precipitating agent was described by
Fernández et al. (2000). Preparation by addition of a
precipitating agent to a magnesium salt solution
using basic reagents such as sodium or potassium
carbonates or bicarbonates has also been discussed in
the literature (Black and Bergmann, 1939). The
precipitation of magnesium hydroxy carbonates with
defined stoichiometry was not possible by the aforementioned approach, and it was shown by Choudhary et al. (1994) that the preparation conditions
strongly influenced the properties of the magnesium
oxide that was formed from the magnesium hydroxy
carbonate. While studying the separation of magnesium/calcium carbonates from dolomite, Cáceres and
Attiogbe (1997) recovered pure hydromagnesite from
the aqueous residue. Morie et al. (1986) manufactured magnesium hydroxy carbonate by mixing a
basic magnesium carbonate suspension and a magnesium oxide suspension while bubbling with gaseous CO2.
The aim of this study is to investigate a procedure
for the preparation of a magnesium hydroxy carbonate from magnesium hydroxide. The procedure
developed by Pond and Heneghan (1965) will serve
as the foundation for the following study. It will be
shown that various experimental conditions influence
the product formed, and that by manipulating these
conditions, the required product can be obtained. The
formation of hydromagnesite and dypingite, two of
the magnesium hydroxy carbonates, are shown as
well as the formation of hydromagnesite from dypingite. These two compounds are very similar with
respect to their chemical composition, hydromagnesite (MgCO 3 ) 4 Mg(OH) 2 4H 2 O and dypingite
(MgCO3)4Mg(OH)25H2O (Raade, 1970). The process presents a non-polluting procedure for the preparation of a magnesium hydroxy carbonate.
2. Experimental procedures
2.1. Instrumentation
X-ray powder diffraction analyses were performed
on a Siemens D501 diffractometer using Cu Ka
radiation. The PDF-2 database from ICDD volumes
1– 45 were used to analyse the data.
Thermogravimetric and calorimetric analyses were
performed on a NETZSCH STA 409 simultaneous
TG/DTA instrument. Sample sizes varied between 12
and 14 mg. A heating rate of 10 C min 1 was used
in an air atmosphere. All data were obtained using
platinum crucibles.
2.2. Mg(OH)2 as reagent
In all instances a standard procedure, as stated
(Section 2.2.1), was followed except for one or two
variations depending on the experimental conditions
employed to achieve specific results. These deviations
will be discussed under the appropriate heading.
2.2.1. Standard procedure
A Mg(OH)2 slurry was obtained by suspending
approximately 5.0 g Mg(OH)2 (CP from UniLAB,
Saarchem) in 125 mL deionised water. The suspension was agitated vigorously with a magnetic stirrer
and the pH measured before sparging the CO2 (HP
compressed gas from Air Products, South Africa). The
CO2 was sparged through the solution at 190 mL
min 1 while stirring continuously. The final pH of
the slurry was obtained by stopping the CO2 flow at
the desired pH between 7.5 and 9.0. The slurry was
then filtered and the solid product washed with
deionised water. The solid product was then dried at
the desired temperature.
2.2.1.1. Variation of slurry pH. The influence of
slurry pH on the resulting product was investigated
by preparing three slurry solutions at 20 C with a final
pH of 9.3, 8.2 and 7.3, respectively. After filtration all
three solid products were dried at 60 C (Table 1.1A).
2.2.1.2. Variation of slurry temperature. The influence of temperature of the Mg(OH)2 slurry was investigated by preparing two slurries. The one was heated
to 40 C and the other to 65 C before CO2 was
sparged through the slurry. The addition of CO2 was
continued until the slurry pH stabilised despite the
continued addition of CO2. At this point, the CO2 flow
was stopped and the slurry cooled down to measure the
pH at ambient temperature. During the process of
cooling to ambient temperature, an increase in pH
A. Botha, C.A. Strydom / Hydrometallurgy 62 (2001) 175–183
177
Table 1.1
Summary of the experimental procedures followed in the various approaches of preparing a magnesium hydroxy carbonate
Reagents
Slurry temperature
(C)
Final pH of slurry
before filtration
Drying temperature
(C)
Product formed
(confirmed by XRD)
Mg(OH)2 + CO2
A
D
20
20
20
40
65
65 C + 0.1 M HCl
65 C + 1 M HCl
19
9.3
8.2
7.3
8.6
9.9
9.1
8.3
7.6
E
20
7.8
60
60
60
60
60
60
60
90
100
20
80
120
Mg(OH)2
Mg(OH)2 + MgCO33H2O
MgCO33H2O
MgCO33H2O
mainly Mg(OH)2+(hydromagnesite)
hydromagnesite
hydromagnesite
unidentified
hydromagnesite
MgCO33H2O
unidentified
hydromagnesite
B
C
was observed. This increase is consistent with the
temperature dependance of pH measurements. No
other physical changes were observed when cooling
the slurry to ambient temperature. After filtration, both
products were dried at 60 C (Table 1.1B).
2.2.1.3. Addition of HCl at elevated temperatures.
Two Mg(OH)2 slurries were prepared and heated to 65
C before the addition of CO2. The CO2 was sparged
through the suspensions until the pH stabilised despite
the continued addition thereof. At this point, 0.1 M
HCl was added to the one suspension to lower the pH
further while continuing sparging CO2. After the pH
was lowered satisfactory, the CO2 flow was stopped
and the suspension cooled down.
The pH of the other suspension was lowered
similarly by the addition of 1 M HCl so that a lower
final pH than the aforementioned suspension could be
obtained. The CO2 flow was then stopped and the
suspension cooled down. The pH of both suspensions,
9.1 (0.1 M HCl) and 8.3 (1 M HCl), were measured at
ambient temperature before filtration. The solid products were washed thoroughly with deionised water to
ensure that all the chloride ions were removed. The
solids were then dried at 60 C (Table 1.1C).
2.
3.
MgCO33H2O was prepared as described in
Table 1.1A (pH 7.8). The product obtained
was then divided into three parts and dried at 80,
100 and 120 C (Table 1.2F).
The influence of drying temperature was also
evaluated on the product obtained for 1 M HCl in
Table 1.1C. The product was divided into three
parts and dried at 20, 60 and 120 C (Table 1.2G).
2.2.1.5. Variation of drying time. The drying time
was evaluated for a Mg(OH)2 slurry prepared at 20 C
with a final pH between 7.5 and 9.0. After filtration,
the product was dried at 120 C. Samples were
removed after 2, 3, 6, 9, 12, 15 and 24 h of drying
and analysed by XRD (Table 1.2H).
2.3. MgCO33H2O as reagent
MgCO33H2O was prepared as described in Table
1.1A (Section 2.2.1.1). The product was then suspended in deionised water and heated to 90 C. The
pulp was then cooled down and the pH measured at
ambient temperature, after which the solid was filtered
and dried at 65 C (Table 1.2I).
2.4. MgO as reagent
2.2.1.4. Variation of drying temperature.
1.
Five Mg(OH)2 slurries were prepared at 20 C
with a final pH between 7.5 and 9.0, after
which the solid products were dried at 20, 80,
90, 100 and 120 C (Table 1.1D and E).
Exactly the same procedure was followed as discussed for Mg(OH)2 in the standard procedure (Section 2.2.1), except that MgO instead of Mg(OH)2 was
used. The MgO was prepared by heating Mg(OH)2
(CP from UniLAB) for 2.5 h at 800 C. A MgO slurry
178
A. Botha, C.A. Strydom / Hydrometallurgy 62 (2001) 175–183
Table 1.2
Summary of the experimental procedures followed in the various approaches of preparing a magnesium hydroxy carbonate (continued)
Reagents
Slurry temperature
(C)
Final pH of slurry
before filtration
Drying temperature
(C)
Product formed
(confirmed by XRD)
F
MgCO33H2O
20
7.8
G
65 C + 1 M HCl
8.3
H
20
8.0
I
MgCO33H2O
MgO + CO2
J
19
90
20
7.9
9.7
8.4
65
80
100
120
20
60
120
120 — 3 h
120 — 6 h
120 — 24 h
65
65
20
80
120
MgCO33H2O
unidentified
unidentified
unidentified
dypingite
hydromagnesite
hydromagnesite
dypingite
hydromagnesite
hydromagnesite
MgCO33H2O
hydromagnesite
MgCO33H2O
unidentified
unidentified
was prepared at 20 C with a final pH between 7.5 and
9.0, after which the solid product was divided into
three parts and dried at 20, 80 and 120 C (Table 1.2J).
3. Results and discussion
The products that were formed by varying the
experimental conditions during the preparation of a
magnesium hydroxy carbonate are summarised in
Tables 1.1 and 1.2. The products were identified by
XRD and their decomposition characteristics evaluated with TG/DTA. In certain instances, it was not
possible to identify the product formed since it could
not be matched with any of the compounds in the
XRD database.
3.1. Mg(OH)2 as reagent
3.1.1. Influence of slurry pH
The method followed by Pond and Heneghan
(1965) did not specify whether the slurry was to be
heated or not. It was initially decided to perform the
procedure at ambient temperature. Three slurries were
prepared with different slurry pH’s and dried below 75
C as suggested (Pond and Heneghan, 1965). From the
results obtained (Table 1.1A) it is evident that this
procedure did not deliver the required product. At a pH
of 9.3, it seemed that the reaction was incomplete due
to the presence of only Mg(OH)2 in the product. A pH
of 7.3 also seemed to be insufficient in forming the
required product. Only MgCO33H20 (nesquehonite)
was obtained by the aforementioned approach. The
formation of nesquehonite at ambient temperature is
well known (Langmuir, 1965). The transition between
nesquehonite and hydromagnesite occurs at 55 – 65 C,
with nesquehonite the stable phase below 55 C. A
mixture of the products just mentioned was obtained at
pH 8.2. It is evident when these experimental results
are viewed that pH alone was not sufficient to form the
magnesium hydroxy carbonate. Either the slurry had to
be heated or a different drying temperature had to be
used in order to obtain the required product.
3.1.2. Influence of slurry temperature
The following attempt involved heating the slurry.
An inherent problem that occurs at elevated temperatures is a decrease in the solubility of CO2 gas in water
(Pierantozzi, 1993). This resulted in difficulty in obtaining a slurry pH between 7.5 and 9.0. This is the pH range
which is suitable for the formation of the magnesium
hydroxy carbonate (Pond and Heneghan, 1965). It is
clear from the results in Table 1.1B that no significant
reaction took place between Mg(OH)2 and CO2 at a
slurry temperature of 65 C and that the Mg(OH)2
remained unchanged. This is probably due to the high
temperature of the slurry that resulted in very little of the
CO2 dissolving in the slurry and being available for
reaction with the Mg(OH)2. The formation of hydromagnesite at 65 C should be successful since its
A. Botha, C.A. Strydom / Hydrometallurgy 62 (2001) 175–183
formation above 55 C is favoured above that of nesquehonite (Langmuir, 1965). The XRD results showed
the presence of hydromagnesite, though it was evident
that reaction of the Mg(OH)2 was not completed.
At a slurry temperature of 40 C, the Mg(OH)2
reacted to form MgCO33H2O. If the reaction is to be
performed above 55 C, where the successful preparation of hydromagnesite is to be expected, an alternative approach will be required.
3.1.3. Influence of HCl addition at elevated temperatures
An attempt was made to lower the slurry pH through
the addition of HCl at a slurry temperature of 65 C.
Two different slurry pH’s were obtained through the
addition of 0.1 and 1 M HCl (Table 1.1C). The product
that was obtained in both instances is hydromagnesite.
It would seem that the addition of the acid had a
significant influence on the reaction mechanism
between Mg(OH)2 and CO2. A possible explanation
is that the acid contributed in an increased dissolution
of the Mg(OH)2, which subsequently resulted in a more
complete reaction with the CO2. Since a more complete
reaction could be obtained, it was possible to form
hydromagnesite above 55 C as expected.
179
3.1.4. Influence of drying temperature
The next approach was to evaluate the influence of
drying temperature. Since an increase in slurry temperature resulted in a decrease in solubility of CO2, it was
decided to perform the reaction at ambient temperature,
although the formation of nesquehonite is more likely at
ambient temperature. A drying temperature of 90 and
100 C was first evaluated and since the 100 C drying
temperature delivered hydromagnesite, three additional
temperatures (20, 80 and 120 C) were studied. It is
evident from Table 1.1D and E that 100 and 120 C
resulted in the formation of hydromagnesite, while 80
and 90 C resulted in the formation of an unidentified
crystalline structure. At 20 C, MgCO33H2O was
formed. The XRD pattern of this unidentified structure
(Fig. 1) is very similar to that obtained at 30 C by
Fernández et al. (2000).
When the TG curves (Fig. 2) were evaluated, there
seemed to be some similarities between hydromagnesite and the unidentified product. The thermal decomposition of hydromagnesite is expected to proceed via
dehydration (removal of water of crystallisation)
below 250 C, dehydroxylation (decomposition of
magnesium hydroxide to MgO) between approximately 250 and 350 C, and decarbonation (decom-
Fig. 1. XRD patterns for hydromagnesite (A), unidentified product (B) and dypingite (C).
180
A. Botha, C.A. Strydom / Hydrometallurgy 62 (2001) 175–183
Fig. 2. TG curves for dypingite (A), unidentified product (B) and hydromagnesite (C). ( y-axis: 7.5% per division).
position of magnesium carbonate to MgO) above 350
C (Choudhary et al., 1994).
4MgCO3 MgðOHÞ2 4H2 O
! 4MgCO3 MgðOHÞ2 þ 4H2 O ð< 250 CÞ
4MgCO3 MgðOHÞ2
! 4MgCO3 þ MgO
þ H2 O ð250 350 CÞ
4MgCO3
! 4MgO þ 4CO2
ð> 350 CÞ:
The total theoretical mass loss expected for hydromagnesite is 56.9%. This corresponds to a theoretical
mass loss of 19.3% (below 350 C) and 37.6% (above
350 C). The experimental results obtained were
17.4% (below 350) and 39.5% (above 350 C),
which corresponded to a total mass loss of 56.9%.
Dypingite gave a total mass loss of 57.5%, which
compared well with the expected theoretical value of
58.5%. The experimental mass loss for dypingite
below 350 C corresponded to 24.2%, and above
350 C to 33.3%, compared with a theoretical mass
loss of 22.3% and 36.2% respectively. It is evident
from these results that the experimental mass losses
were in close approximation to the expected theoretical values.
The unidentified product had a total mass loss of
64.6%. This consisted of a 25.6% mass loss below
350 C, and 39.0% above 350 C. The mass loss
above 350 C was close to that of hydromagnesite. It
is evident that the mass loss below 350 C was larger
than that of hydromagnesite. This could be attributed
to a relatively larger amount of crystallisation water in
the unidentified product’s composition, compared to
hydromagnesite, since this mass loss was experienced
largely below 250 C.
The difference between hydromagnesite and the
unidentified product was more significant when viewing the XRD (Fig. 1) and DTA (Fig. 3) results. The
endothermic reactions above 350 C in the DTA
A. Botha, C.A. Strydom / Hydrometallurgy 62 (2001) 175–183
181
Fig. 3. DTA curves for unidentified product (A), hydromagnesite (B) and dypingite (C).
curves seemed to be very similar but those below this
temperature differed significantly. The endothermic
peaks for hydromagnesite below 350 C were at 59
and 259 C, while those for the unidentified product
were at 56, 174 and 223 C. Above 350 C, the
endothermic peaks exhibited were at 424 and 517 C
for hydromagnesite, and at 430 and 519 C for the
unidentified product.
The similarities between hydromagnesite and the
unidentified product suggest a probable resemblance
between their compositions. It has been suggested
(Davies and Bubela, 1973) that an intermediate phase
exists between nesquehonite and hydromagnesite. The
unidentified product might be an intermediate phase
since it is evident that the formation of nesquehonite
at 60 C (Table 1.1A), is followed by the formation of
the unidentified product at 80 C, and finally hydromagnesite at 100 C (Table 1.1D and E).
By heating MgCO33H2O at 80, 100 and 120 C
(Table 1.2F), it was only possible to obtain the
unidentified product. These results supported the
expected conversion of nesquehonite to the unidenti-
fied product but not ultimately the conversion to
hydromagnesite as expected. An alternative mechanism possibly existed by which nesquehonite is converted to hydromagnesite.
The results obtained for the different drying
temperatures that were evaluated for the addition of
1 M HCl are given in Table 1.2G. At ambient
temperature, dypingite was formed while hydromagnesite formed at 60 and 120 C. These results are in
contrast with those obtained in Table 1.1D and E
where the product consisted entirely of hydromagnesite at 100 and 120 C, as well as the results at 60
C where the product consisted entirely of nesquehonite (Table 1.1A). This difference was ascribed to
a slurry temperature of 65 C (Table 1.2G) at which
hydromagnesite is formed preferentially compared
with an ambient slurry temperature (Table 1.1D
and E). The formation of dypingite at ambient
temperature (Table 1.2G) supported the theory that
hydromagnesite is formed from dypingite. It was
evident that the product was not yet dry enough to
favour the conversion to hydromagnesite.
182
A. Botha, C.A. Strydom / Hydrometallurgy 62 (2001) 175–183
The DTA curve that was obtained for dypingite is
given in Fig. 3. The endothermic peaks at 258, 426
and 516 C are very similar to those obtained for
hydromagnesite. A very distinct difference between
the DTA curves of dypingite and hydromagnesite is
that dypingite contains loosely bound water that was
lost at comparatively low temperatures (50 –130 C).
The peaks around 100 C are very similar though less
pronounced than those observed by Raade (1970).
3.1.5. Influence of drying time
Up to the previous investigations, the products
obtained were dried for 24 h on average. It was
decided to investigate the influence of shorter drying
times (Table 1.2H). The XRD results indicated that
the formation of hydromagnesite at 120 C was
completed after 6 h. An interesting observation was
the formation of dypingite at 3 h drying time, which
was then followed by the conversion to hydromagnesite between 3 and 6 h of drying. Since hydromagnesite and dypingite only differ with respect to
dypingite having one water molecule more than
hydromagnesite, it is clear that the formation of
hydromagnesite at a longer drying time than dypingite would be feasible. No further changes were
observed between 6 and 24 h.
magnesite at 120 C compared to Mg(OH)2 in Table
1.1D and E.
4. Conclusions
The preparation of a magnesium hydroxy carbonate from magnesium hydroxide was shown to be
possible only if the experimental conditions were
chosen carefully. The resources that are available will
also determine the procedure that will be followed. In
many instances, it may not be viable to make use of
elevated temperatures as in Tables 1.1C and 1.2G in
an attempt to obtain the required product. The procedure in Table 1.1D and E that describes sparging CO2
through a magnesium hydroxide slurry at ambient
temperature and drying the solid above 100 C,
delivers a simple approach in preparing the magnesium hydroxy carbonate if magnesium hydroxide is
readily available. It is also important to note that the
magnesium hydroxy carbonate formed, hydromagnesite or dypingite, is influenced by the drying time
allowed. This procedure eliminates the unnecessary
use of foreign ions that could result in water effluents
that have an environmental impact.
Acknowledgements
3.2. MgCO33H2O as reagent
By boiling MgCO33H2O, which was formed as
described in Table 1.1A, in water and drying the solid
product at 65 C after filtration, it was possible to
form hydromagnesite (Table 1.2I). This approach
again confirms the conversion of nesquehonite to
hydromagnesite above 55 C. These findings correspond to those of Prakash and Gupta (1987).
3.3. MgO as reagent
This procedure was evaluated in order to determine
whether similar results could be obtained as those that
were obtained for Mg(OH)2 in Table 1.1E. The results
obtained for MgO (Table 1.2J) were different from
Mg(OH)2 when the solid product was dried at 80 and
120 C. In both instances, the unidentified product
was formed from MgO. It is possible that the reaction
has not gone to completion with respect to MgO. This
could explain the absence in the formation of hydro-
The authors wish to thank Dr. Sabine Verryn from
the Department of Earth Sciences from the University
of Pretoria, South Africa for doing all the XRD
analyses.
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