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Chemical Industry & Chemical Engineering Quarterly 19 (2) 263−271 (2013)
CI&CEQ
SOUHEIL BEHIJ
HALIM HAMMI
AHMED HICHEM HAMZAOUI
ADEL M’NIF
MAGNESIUM SALTS AS COMPOUNDS OF
THE PREPARATION OF MAGNESIUM OXIDE
FROM TUNISIAN NATURAL BRINES
Technological park of Borj Cedria,
National Center of Research in
Materials Sciences, Valorization
Laboratory of Useful Materials,
Tourist road of Soliman, Soliman,
Tunisia
Magnesium oxide is one of the most important magnesium compounds used in
the industry. The production of MgO is often done from calcined magnesium
carbonate or from natural magnesium saline solutions (sea water and brines).
In the case of these solutions, magnesium oxide is precipitated after the addition of a strong base (e.g., ammonia). Magnesium hydroxide is calcined after
its separation from the excess resulting from the strong base through filtration.
Thus, magnesia qualities may differ depending on several physical parameters
and particularly on the nature of the compound. Consequently, two different
compounds were selected: magnesium chloride and magnesium sulphate which
can be recovered from Tunisian natural brines. Three physical factors were
considered: calcination temperatures, precipitation temperatures and calcination time of Mg(OH)2. The decomposition of Mg(OH)2 was investigated by
DTA/TGA. Mass losses varied in the range 23.0–29.9%. The starting decomposition temperatures were between 362 and 385 °C. The MgO produced from
MgSO4 under 1000 °C within 48 h of calcination time and with 40 °C as the
reaction temperature for Mg(OH)2 showed good crystallinity and was of a crystallite size of 86.3 nm with a specific surface area equal to 16.87 m2g-1. Finally,
morphological differences between MgO agglomerates at different temperatures were observed by SEM. Consequently, magnesium sulphate was selected
as the precursor for preparing MgO.
SCIENTIFIC PAPER
UDC 546.46-31:550.4(611)
DOI 10.2298/CICEQ111207060B
Keywords: cristallinity; crystallite size; Mg(OH)2; MgO or periclase; specific surface area.
Magnesium oxide (MgO or periclase) is among
the most important industrial magnesium compounds.
Approximately 20% of the world’s production comes
from seawater, brines and desalination of reject brine
[1]. Magnesium oxide is used as an exceptionally important material in catalysis [2,3], toxic waste remediation [4], or as additives in refractory products,
paint, manufacture of fertilizers, animal foodstuff,
building materials (Sorel cement, lightweight building
panels) and superconductor products [5-7]. A panel of
fundamental and applied studies exists in literature
reviews [7-11]. It particularly shows that magnesium
Correspondence: H. Hammi, Technological park of Borj Cedria,
National Center of Research in Materials Sciences, Valorization
Laboratory of Useful Materials, Tourist road of Soliman B. P. 73
- Soliman 8027, Tunisia.
E-mail: [email protected]
Paper received: 7 December, 2011
Paper revised: 8 June, 2012
Paper accepted: 8 June, 2012
hydroxide production from seawater or brines is precipitated by the addition of a strong base (e.g. ammonia). Later on, separation is calcined to produce MgO.
It is important to note that magnesia qualities may differ depending on the physico-chemical conditions of
preparation and on the compound type, which is why
two compounds were selected to be tested: magnesium chloride and magnesium sulphate. The respective final products are compared on the basis of their
X-ray diffractograms, specific surface area and microstructural differences.
To accomplish this task, we first performed a
DTA/DTG decomposition study of Mg(OH)2 (brucite).
After that, XRD results were used to study the impact
of calcination temperature, the impact of calcination
time and finally the impact of Mg(OH)2 reaction temperature. Additionally, X-ray diffractograms were used
to calculate MgO crystallite size depending on some
physical parameters such as calcination temperatures
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CI&CEQ 19 (2) 263−271 (2013)
and reaction times. At a later stage, the specific surface area was determined in order to confirm the particles’ cristallyte size variations. Finally, MgSO4⋅7H2O
was selected as the compound used for MgO preparation.
Mg2+ solution. After that, an excess of ammonium
hydroxide was added and vigorously stirred at 50 °C
with pH 10. A white precipitate of Mg(OH)2 was
obtained which was thoroughly washed with distilled
water and dried at 100 °C during 2 h. At this stage,
the following reaction takes place:
EXPERIMENTAL
Mg2+ + 2OH-  Mg(OH)2
Methodology
In the second step, magnesium hydroxide powders were calcined to produce MgO. The heating
treatment was carried out in an electric furnace by
raising the temperature 10 °C/min. Therefore, the following thermal reaction takes place:
The industrial exploitation of the abundant natural brines in the Sebkhas and Chotts of the Tunisian
South is conditioned by the development of specific
processes, the technique and economic viabilities of
which have to be demonstrated beforehand. For this
purpose, we agreed to study this kind of saline solutions previously studied by many authors [12,13],
assimilated to a quinary system represented by Na+,
K+, Mg2+/Cl-, SO2-//H2O.
Natural Brine is a highly concentrated complex
aqueous solution. In the case of the brine called
Sebkha El Melah, magnesium, chloride and sulphate
ions seem to be sufficiently high to recover magnesium salts like epsomite (MgSO4⋅7H2O) and bischofite
(MgCl2⋅6H2O) useful in a wide range of applications
(such as fertilizers, Sorel cement, and refractory compounds).
Epsomite is obtained by polythermal crystallization conducted in two steps. The first one is completed at 35 °C, temperature during which the brine is
evaporated to reach a density of 1.29 g.cm-3 and the
maximum concentrations of magnesium and sulphate
ions. The second step consists in cooling the above
mentioned pretreated solution at 0 °C in order to crystallize MgSO4⋅7H2O.
Purification of the product, realized by washing it
with a saturated solution of Epsom salts, provides
epsomite of 99% purity [14].
For bischofite synthesis, the concluded process
is mainly composed of six stages based on two main
unit operations, firstly isothermal evaporation and
crystallization and then chemical precipitation.
The resulting brine, composed mainly of magnesium chloride, is concentrated by isothermal evaporation at 35 °C to precipitate magnesium chloride. The
recovered product, collected from the final stage, is
mainly composed of bischofite the purity of which
exceeds 90% [15].
MgO powders preparation is done in two steps
starting from two different precursors: bishofite
(MgCl2⋅6H2O) and epsomite (MgSO4⋅7H2O) successfully produced from Tunisian natrual brines.
At first, each reactant was dissolved in deionized water at room temperature to produce a 0.8 M
264
Δt
→ MgO + H2O
Mg(OH)2 ⎯⎯
(1)
(2)
Materials
An X-Ray diffractometer with a Philips PW 3040
generator, goniometer PW 3050/60 θ/2θ and a
cathode of copper PW 3373/00 was used to distinguish MgO from two different precursors. Microstructural differences between MgO agglomerates were
examined using SEM FEI Quanta 200. The decomposition of precipitated Mg(OH)2 was analysed by
DTA/TGA (DSC) Setaram Setsys Evolution. The specific surface area was determined by a QuantaChrome Autosorb-1 apparatus.
RESULTS AND DISCUSSION
Starting from the two compounds prepared from
the natural brine Sebkha El Melah as described
above (MgCl2⋅6H2O and MgSO4⋅7H2O), the two successsive steps of brucite and magnesia preparation
were performed as previously indicated. The thermogravimetric decomposition (DTA/DTG) of Mg(OH)2
was studied. The recovered products were characterized by XRD and SEM.
Mg(OH)2 characterization
Mg(OH)2 decomposition (DTA/DTG) study
The start of Mg(OH)2 decomposition was studied
using DTA/DTG experiments. Mg(OH)2 experimental
diagrams related to the compounds magnesium chloride and magnesium sulphate are respectively presented in Figures 1a and 1b. It becomes possible to
compare the mass losses of the two obtained powders to the theoretical ones from these two diagrams.
The two DTA-DTG analyses show two endothermic peaks respectively related to dehydration process and to magnesium hydroxide decomposition.
The second phenomenon occurs at 362 and 385 °C,
respectively, for the compounds magnesium chloride
and magnesium sulphate. However, in terms of bru-
S. BEHIJ et al.: MAGNESIUM SALTS AS COMPOUNDS OF THE PREPARATION…
CI&CEQ 19 (2) 263−271 (2013)
(a)
(b)
Figure 1. DTA/DTG of Mg(OH)2 obtained from two different precursors : a) MgO from chloride and b) MgO from sulphate.
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S. BEHIJ et al.: MAGNESIUM SALTS AS COMPOUNDS OF THE PREPARATION…
cite (Mg(OH)2) theoretical decomposition, 30.864%
mass loss should be achieved, which is slightly larger
than the observed one 29.883 and 23.022%, respectively. This is due to the incompletion of the decomposition reaction within this temperature range. These
results are in good agreement with others presented
elsewhere [16-18].
As a preliminary conclusion, it comes into view
that Mg(OH)2 prepared from the two studied compounds is of good chemical purity, but its morphology
varies from one compound to another. Furthermore,
the starting decomposition temperature differs depending on the compound’s nature. In order to clarify the
compound’s impact on the final product, we studied
MgO prepared under different physical conditions.
Mg(OH)2 X ray diffraction description
Magnesium hydroxide prepared from the two
above-mentioned compounds was characterized by
X-rays, it was found that the produced compounds
correspond to Mg(OH)2 regardless of the considered
compound. Figure 2 shows a marked diffractogram of
the compound obtained from MgSO4.7H2O; an identical one is produced when MgCl2 is used as the precursor.
Mg(OH)2 SEM description
The morphology of Mg(OH)2 from different precursors is explored by SEM analysis. Brucite was
dried accordingly at 100 °C and observed after cooling. Microstructural differences were detected for the
two compounds (Figure 3). This observation indicates
that magnesium oxide products may present different
physical properties related to the compound.
Magnesium hydroxide derived from magnesium
sulphate shows a plate-like shape, the particles were
joined to each other forming spherical particles around
15 µm, which tend to form large agglomerates (30 µm).
MgO characterization
MgO X-ray diffraction study
X-ray diffraction allows, in the first step, a qualitative comparison of the crystallinity of the obtained
materials; such a parameter is related to the peaks’
intensity and their affinities. In the second step, the
diffractograms are used to calculate the average crystallite size of the prepared MgO in a more quantitative
term. Regarding these two parameters, the following
effects were investigated: calcination temperature,
calcination time and reaction temperature.
Calcination temperature effect. As previously
concluded, Mg(OH)2 decomposition starts at the range
varying from 362 to 385 °C, thus a calcination temperature ranging from 500 to 1000 °C for a duration of
2 h was operated in order to study the effect of such a
factor on the different obtained products from the two
studied compounds. Figures 4a and 4b illustrate
Figure 2. Diffractogram of Mg (OH)2 prepared from MgSO4⋅7H2O.
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S. BEHIJ et al.: MAGNESIUM SALTS AS COMPOUNDS OF THE PREPARATION…
Mg(OH)2 sulfate
CI&CEQ 19 (2) 263−271 (2013)
Mg(OH)2 chloride
Figure 3. Microstructure of Mg(OH)2 powders obtained from different precursors (2500×).
Figure 4. Calcination temperature effect on MgO obtained from different compounds: a) magnesium chloride, b) magnesium sulphate.
X-ray diffractograms for the precusors magnesium
chloride and magnesium sulphate, respectively.
Figure 5 shows that all samples were magnesium oxide formed with CFC structure (NaCl type).
The best crystallinity corresponds to the calcination
temperature of 1000 °C with regard to the two compounds. Nevertheless, it is important to note that
MgO, obtained from MgSO4⋅7H2O, shows the highest
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CI&CEQ 19 (2) 263−271 (2013)
Figure 5. Diffractogram showing the calcination time on MgO obtained from magnesium sulphate precursor.
intensity and the sharpest peak at the same temperature. In addition, unlike the MgCl 2 compound,
MgSO4⋅7H2O shows a gradual increase of intensity
parallel to that of temperature. These remarks clearly
indicate the influence of the compound on the properties of the final product (MgO).
Calcination time effect. MgO calcination time is
studied in the range from 2 to 48 h. Figure 5 illustrates
XR diffractogramms for MgO obtained from magnesium sulphate at 1000 °C (a similar result is concluded for the other compound). Therefore, increasing
the calcination time improves the crystallinity. As a
result, 48 h of calcination time gave the best results.
CFC magnesium oxide was always obtained.
Reaction temperature effect. Mg(OH)2 reaction
temperature parameter was studied in the range 25–
–75 °C. Figure 6 illustrates only XRD results for MgO
obtained from the compound magnesium sulphate at
1000 °C and with 48 h of calcination time. The variations of the peak intensities, which play the function
of temperature, are irregular. However, in terms of
crystallinity, the below diffractograms enable us to deduce that 40 °C is the best reaction temperature for
the two compounds.
Average crystallite size. The above diffractogramms were handled to determine the average crystallite size, L, of the prepared powders. For this purpose, we applied the Scherrer formula:
L=
0.9λ
β cos θ
where: λ is the wavelength of the X-ray, θ is the
diffraction angle
associated with a Bragg peak,
1
2
2 2
β = ( βm − β s ) is the corrected full widh at half maxi-
Figure 6. MgO obtained from magnesium sulphate diffractograms vs reaction temperature.
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(3)
S. BEHIJ et al.: MAGNESIUM SALTS AS COMPOUNDS OF THE PREPARATION…
mum (FWHM), βm being the FWHM of the well defined (200) Bragg peak and βs being that of a standard
crystallized sample of MgO (across sample).
Considering the two tested reactants, the crystallite sizes with calcination temperature, calcination
time and reaction temperatures are shown respectively in Figures 7–9.
Figure 7 shows that for the two precursors, the
crystallite size increased in accordance with the
increase of calcination temperature. Crystallite sizes
were within the ranges: (64.7 to 115.4 nm) and (18.1
to 86.3 nm) for magnesium chloride and magnesium
sulphate, respectively. Therefore, the MgO obtained
from magnesium sulphate had the smallest particle
size. This result enhances the relation between the
full width at half maximum of Bragg peaks and crystallite size. In fact, the FWHM is the result of the convolution of an instrumental contribution and a size
effect.
For the compound magnesium sulphate, the
crystallite size increased with calcination time (Figure
8). For the other one (magnesium chloride), the evo-
CI&CEQ 19 (2) 263−271 (2013)
lution of crystallite was irregular. The final sizes were
lower than those obtained from magnesium sulphate
after 48 h of calcination time. The calcination time
effect on crystallite size seems to be more important
than the temperature effect (150 nm in 48 h and 80
nm at 1000 °C).
Figure 9 shows that MgO produced from magnesium chloride has an irregular evolution of crystallite sizes. It is therefore difficult to confirm the previous conclusion, but for energy considerations 40 °C
was retained as the reaction temperature.
BET Specific surface. In order to confirm the
above results, the specific surface areas by BET for
MgO powders burnt-off at 1000 °C during 24 h were
set up. The obtained results were 9.31 and 16.87 m2g-1
for magnesium chloride and magnesium sulphate,
respectively. These results confirm that the sample
obtained from the magnesium sulphate has the highest specific surface area which is in concordance with
the data [19-22] indicating that small particles have
the highest surface areas.
Figure 7. Development of crystallite size with temperature on the crystallisation of MgO.
Figure 8. Development of crystallite size with calcination time on the crystallisation of MgO.
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CI&CEQ 19 (2) 263−271 (2013)
Figure 9. Development of crystallite size with reaction temperature on the crystallisation of MgO.
SEM study
The morphology of MgO powders obtained from
different compounds is explored by SEM analysis.
Periclase powders (MgO) obtained at 1000 °C during
48 h from the two compounds after cooling and SEM
examination show the presence of fine particles, less
than 1 µm, forming agglomerates (less than 5 µm)
with homogeneous distribution (Figure 10).
MgO sulfate
At 1000 °C, a well-defined plate-like morphology
was observed which can be related to periclase phase,
a similar morphology has been reported elsewhere
[23].
CONCLUSION
The results of this study demonstrate that the
MgO obtained from the two compounds has different
physical proprieties such as the crystallite size, shape,
and structure. A set of parameters like calcination
temperature, calcination time and temperature of brucite precipitation has to be controlled to produce a
high quality product.
Intermediate and final products were characterized by X-ray diffraction, DTA/DTG analysis, BET
and SEM.
This study shows that 40 °C is an appropriate
temperature to obtain Mg(OH)2 from the compounds
used in the experiments.
The calcination temperature for MgO production
with high crystallinity is set to be within the range 8001000 °C. Within this interval, the maximum brucite
decomposition into periclase is achieved. Calcination
time of 48 h confirm this result with a very pure product. Finally, magnesium oxide obtained from magnesium sulphate is the best approach to obtain the ideal
product, due to its higher surface area, and eventually, to its smallest primary particle size at the limits
of calcination temperature. Besides, the specific area
value can be related to the microstructure of the sections disposed around a central nucleus. According to
these results, we select magnesium sulphate as the
precursor to obtain magnesium oxide.
Acknowledgments
MgO chloride
Figure 10. Microstructure of MgO powders obtained from
different precursors (5000×).
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This work was supported by the National Centre
of Material Science Research. The authors would like
S. BEHIJ et al.: MAGNESIUM SALTS AS COMPOUNDS OF THE PREPARATION…
to thank the technical team for helpful assistance with
the XRD, SEM and DTA/DTG studies.
CI&CEQ 19 (2) 263−271 (2013)
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SOUHEIL BEHIJ
HALIM HAMMI
AHMED HICHEM HAMZAOUI
ADEL M’NIF
Technological park of Borj Cedria,
National Center of Research in
Materials Sciences, Valorization
Laboratory of Useful Materials, Tourist
road of Soliman, Soliman, Tunisia
NAUČNI RAD
T.
MAGNEZIJUMOVE SOLI KAO JEDINJENJA ZA
DOBIJANJE MAGNEZIJUM OKSIDA IZ TUNIŠKIH
PRIRODNIH SLANIH VODA
Magnezijum-oksid je jedno od najznačajnijih magnezijumovih jedinjenja koja se koriste
u industriji. Magnezijum-oksid se često dobija kalcinisanjem magnezijum karbonata ili iz
prirodnih slanih rastvora (morska voda i slane vode). U slučaju dobijanja iz slanih rastvora, magnezijum-hidroksid se najpre taloži dodavanjem baze (npr. amonijaka), odvaja
filtracijom i kalciniše. Kvalitet proizvoda se može razlikovati u zavisnosti od nekoliko fizičkih parametara, a naročito od prirode jedinjenja. Zbog toga su odabrana dva jedinjenja – magnezijum-hlorid i magnezijum-sulfat, koja se mogu dobiti iz tuniških prirodnih
slanih voda. Razmotrena su tri fizička faktora: temperature kalcinacije, temperature taloženja i vreme trajanja kalcinacije magnezijum-hidroksida. Dekompozicija magnezijumhidroksida je ispitivana metodom DTA/TGA. Gubici mase su varirali u opsegu od 23,0
do 29,9%. Početna temperature dekompozicije je bila između 362 i 385 °C. Magnezijum-oksid dobijen iz magnezijum sulfata kalcinacijom od 48 h na 1000 °C i reakcinom
temperaturom za magnezijum-hidroksid od 40 °C pokazao je dobru kristalnost, sa kristalima veličine 86,3 nm i specifičnom površinom 16,87 m2/g. Takođe, SEM metodom su
uočene morfološke razlike između aglomerata magnezijum-oksida na različitim temperaturama. Kao rezultat ovih ispitivanja, magnezijum sulfat je izabran kao precursor za
dobijanje magnezijum oksida.
Ključne reči: kristalnost; veličina kristala; Mg(OH)2; MgO; specifina površina.
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