ISSN 0040-5795, Theoretical Foundations of Chemical Engineering, 2007, Vol. 41, No. 5, pp. 572–576. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © D.F. Kondakov, V.P. Danilov, 2007, published in Khimicheskaya Tekhnologiya, 2007, Vol. 8, No. 1, pp. 2–6.
INORGANIC TECHNOLOGY
Manufacturing of Magnesium Hydroxide from Natural
Magnesium Chloride Sources
D. F. Kondakov and V. P. Danilov
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991
Russia
e-mail: [email protected]
Received September 28, 2006
Abstract—Various processes for manufacturing magnesium hydroxide have been considered and tested in
order to decide whether they can be employed to the full-scale production of this product from natural sources
(bischofite and magnesium chloride brines). The process for the synthesis of magnesium hydroxide from magnesium chloride using sodium hydroxide has been improved.
DOI: 10.1134/S0040579507050193
INTRODUCTION
A process for preparing magnesium hydroxide from
magnesium chloride is needed for the following reasons: Mg(OH)2 is a valuable and widely used chemical
product [1]; its natural deposits are rare (e.g., a brucite
deposit on the Far East on the Russian Federation), as
distinct from sodium magnesium, whose large deposits
exist on the territory of the Russian Federation and the
ancient Soviet Union (Volgograd bischofite, magnesium chloride brines in Kara-Bogaz-Gol, and others).
The following processes are now known for preparing magnesium hydroxide [1]: the thermal decomposition of salts to oxide and subsequent hydration of the
latter; direct precipitation of magnesium hydroxide
with alkali from solutions of magnesium salts; precipitation of basic magnesium carbonates from salt solutions, followed by their thermal decomposition to magnesium oxide and the hydration of the latter; and magnesium hydroxide precipitation from solution with lime
milk.
Our goal in this work was to experimentally study
the utility of the specified processes for preparing magnesium hydroxide from bischofite (MgCl2 · 6H2O) and
magnesium chloride brines.
Inasmuch as the precipitation of magnesium
hydroxide from solution with lime milk is used in the
processing of dilute magnesium-containing solutions
(such as seawater, native brines, and some others) [1],
this process is not considered in this work.
and Eriochrome Black indicator [2]; carbonates were
determined as carbon dioxide by chromatographing
the calcination products at 1000°C on Carlo Erba Elemental Analyzer EA1108. Thermogravimetric experiments were carried out on a Q-1500D Paulik-PaulikErdey thermal analyzer. The phase composition was
monitored by X-ray powder diffraction on a DRON-1
diffractometer using CuKα radiation.
The starting reagents were sodium hydroxide and
sodium carbonate of reagent grade, distilled water, and
crystalline magnesium chloride flakes from BischofiteAvangard. The formula of the latter was MgCl2 · 6H2O;
the specified composition, in wt %: MgCl2, 46–47;
CaCl2, 0.4; NaCl, 0.7; KCl, 0.3; Br, 0.6; and sulfates, 0.06.
Chemical analysis determined 46.8% MgCl2 in the
starting reagent, which corresponds to 99.94% MgCl2 ·
6H2O. The DTA weight loss during the heating of the
reagent was 80.12% (Fig. 1), which also matches the
calculated concentration of 99.94% MgCl2 · 6H2O in
the reagent.
RESULTS AND DISCUSSION
Thermal Decomposition of Magnesium Chloride
When magnesium chloride hexahydrate is heated, it
first melts in its own water of crystallization; then, basic
magnesium chloride is formed as a result of hydrolysis
[1]; during subsequent heating, the latter decomposes
to magnesium chloride:
MgCl 2 ⋅ 6H 2 O
EXPERIMENTAL
In this work, magnesium in samples was determined
chelatometrically by a standard procedure using EDTA
572
MgCl 2 + 6H 2 O
Mg ( OH ) m Cl 2 – m + mHCl
MgO + ( 2 – m )HCl.
(1)
MANUFACTURING OF MAGNESIUM HYDROXIDE FROM NATURAL
The DTA traces for a commercial magnesium chloride hexahydrate sample (Fig. 1) display a series of
endotherms, which signify a multistage process. The
match between the observed weight loss and the value
calculated from the equation of reaction (1) (the difference is 0.06%) validates the high quality of the starting
magnesium chloride, on one hand, and the completion
of reaction (1), on the other.
This process for manufacturing magnesium hydroxide from magnesium chloride hexahydrate has the following strength: additional reactants are not required,
except for water added during the hydration of magnesium oxide to hydroxide; as a result, the embodiment of
this process requires simpler equipment compared to
the other processes considered here. In particular, a precipitation reactor is not required. Another strength of
this process is the feasibility of obtaining one more
product, namely, hydrogen chloride. However, the
extremely high corrosiveness of hydrogen chloride at
elevated temperatures, a significant volume of hydrogen chloride (1250 kg HCl per ton of final Mg(OH)2),
and the necessity for cooling and absorbing it hamper
the embodiment of the process. In addition, the process
is extremely hazardous for operators.
Precipitation of Magnesium Hydroxide from Sodium
Hydroxide Solution
The reaction of magnesium chloride with a sodium
hydroxide solution by the equation
MgCl 2 + 2NaOH
Mg ( OH ) 2 + 2NaCl
(2)
produces a jelly, slowly filtration magnesium hydroxide
precipitate, which hampers filtration and washing
stages.
Therefore, we studied whether changing concentrations of the reacting MgCl2 and NaOH solutions can
help improve the filtration properties of magnesium
hydroxide precipitates. For this purpose, two versions
of the reaction between magnesium chloride and
sodium hydroxide were studied: the reaction between
solid MgCl2 · 6H2O and solid NaOH without water
added and the reaction between dilute solutions of the
reagents. In both cases, the filtration rate of the resulting magnesium hydroxide precipitate was extremely
low. Likely, the reagent concentrations in the range
specified do not noticeably influence the particle size or
properties of magnesium hydroxide.
The duration of reaction (2) was also varied via
varying the rate of dispensing of aqueous alkali to magnesium chloride solution. Within periods of 20 s to 12
min, the reaction time did not noticeably influence the
filtration and washing rates of magnesium hydroxide
precipitates.
Freezing can sometimes help agglomerate jelly precipitates [3]: particles coarsen during freezing due to
their compression by internal pressure, which is created
by an increase in the volume upon the phase transition
573
TG
DTA
712
536
977
132
314
–59.45%
314
536
259
–80.12%
820
220
Fig. 1. Thermoanalytical curves for the decomposition of
magnesium chloride hexahydrate.
from liquid water to ice. Our studies showed that the
freezing of a wet magnesium hydroxide precipitate via
dipping it into liquid nitrogen did not coarsen gel particles, nor it influenced the filtration rate.
We also studied whether magnesium hydroxide gel
particles can coarsen during heat treatment at 100–
700°C. Mixtures to be heat-treated consisted of the
products of the reaction between solid NaOH and solid
MgCl2 · 6H2O, which contained a jelly magnesium
hydroxide precipitate, sodium chloride, and a minor
amount of a saturated sodium chloride solution. The
experiment was intended to exclude the filtration and
washing of jelly magnesium hydroxide from the flowsheet.
Heat treatment was carried out in a muffle furnace;
the temperature was monitored with a thermocouple. To
interpret the processes that occurred during heat treatment, heating traces were recorded (Figs. 2, 3). The
heating traces show that water first vaporized. Then, at
322–439°C, magnesium hydroxide was dehydrated; the
weight loss fully correlated with the reaction
Mg ( OH ) 2
MgO + H 2 O.
(3)
The endotherm at 814°C was due to the melting of
sodium chloride.
After heat treatment was over, in order to separate
the sodium chloride from the product magnesium oxide
and to convert the latter to hydroxide, water was added
to the mixture of magnesium oxide and sodium chloride (1.80 g MgO + 3.63 g NaCl) to bring the volume to
20 ml; sodium chloride dissolved, while magnesium
oxide converted to hydroxide. Part of the MgO had not
enough time to convert to Mg(OH)2 at this stage; therefore, a mixture of magnesium oxide and magnesium
hydroxide entered the filtration stage. The precipitate
THEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING
Vol. 41
No. 5
2007
574
KONDAKOV, DANILOV
wet precipitate was allowed to stand for ~1 h at 20°C to
fully convert magnesium oxide to hydroxide; the latter
was then dried.
The flowsheet for magnesium hydroxide preparation from magnesium chloride is shown below.
was filtered from an NaCl solution and washed with
water. Filtration was carried out in vacuo on a Buchner
funnel using two White Band filters 55 mm in diameter.
The volume of the MgO + Mg(OH)2 + NaCl slurry was
20 ml; the wash-water volume was 20 ml. The washed,
Precipitation
of magnesium
hydroxide
frommagnesium
chloride and
sodium hydroxide
Heat treatment
of an Mg(OH)2 +
+ NaCl + H2O
mixture
NaCl
dissolution
and MgO
hydration
The results of the set of experiments are listed in the
table.
The table makes it clear that, after the precipitate
was heated to 650–700°C, its filtration and washing
rates became several times the filtration rate of magnesium hydroxide prepared at 20°C. Calcination at 700°C
for 20 min gave the best results. The filtered and waterwashed MgO + Mg(OH)2 mixture fully converted to
magnesium hydroxide after the wet precipitate was
allowed to stand at 100°C for 20–30 min.
Although the flowsheet seems illogical (first, magnesium hydroxide is dehydrated, then the dehydration
product (magnesium oxide) is converted back to
hydroxide), this process can be positioned as techno-
Filtration
and washing
of an Mg(OH)2 +
+ MgO mixture
from NaCl
solution
Complete
MgO
hydration
Drying
of Mg(OH)2
logically acceptable because of the significant efficiency of filtration and the noticeable improvement of
the washing of magnesium hydroxide from sodium
chloride.
Magnesium Precipitation from Solution with Sodium
Carbonate
It is known that the reaction of sodium carbonate
with water-soluble magnesium salts yields a mixture of
basic magnesium carbonates [1]:
Na 2 CO 3 + MgCl 2
(4)
Mg ( OH ) 2 – x ( CO 3 ) 0.5 – x + NaCl.
The mixture produced by the reaction of solutions of
magnesium chloride and sodium carbonate was carefully washed from sodium chloride and dried to con-
TG
DTA
TG
322
982
DTA
439
401
346
174
408
792
Fig. 2. Thermoanalytical curves for the decomposition of a
mixture of Mg(OH)2, NaCl, and water.
819
Fig. 3. Thermal curves for the decomposition of a mixture
of Mg(OH)2 and NaCl.
THEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING
Vol. 41
No. 5
2007
MANUFACTURING OF MAGNESIUM HYDROXIDE FROM NATURAL
575
Results of experiments intended to prepare magnesium hydroxide via precipitation with sodium hydroxide followed by heat
treatment and hydration
Synthesis
temperature, °C
Calcining time,
min
Filtering time, s
Washing time, s
<–50*
–
590
1293
–
–
471
1323
85.11
100
500
20
250
767
88.98
100
600
20
202
589
91.63
88.10
650
20
174
530
93.91
82.84
700
20
50
56
97.27
37.73
700
1
67
85
97.56
44.32
700
90
65
102
96.94
22.24
750
1
–
–
99.37
5.78
750
1
–
–
95.91
89.65**
20**
Degree of washing Degree of hydration
MgO from NaCl, % at 20°C for 10 min, %
–
*Freezing temperature in liquid nitrogen.
**Magnesium hydroxide synthesis temperature.
***Hydration at 100°C for 80 min.
stant weight at 120°C, and its composition was determined.
X-ray powder diffraction showed a phase with the
parameters similar to those of basic magnesium carbonates and did not show Mg(OH)2 and MgCO3
phases.
From chemical, X-ray powder diffraction, and differential thermal analyses, we can infer that the precipitate was a mixture of basic magnesium carbonates with
the bulk formula Mg7.45(CO3)6.45(OH) · 5.68H2O,
which contained a minor water-insoluble residue
(1.25%).
This precipitate of basic magnesium carbonates was
rapidly filtered and washed at room temperature; the filtration and washing rates were virtually unaffected by
the concentration of the starting reagent solutions.
Under the conditions comparable with those employed
to precipitate magnesium hydroxide from solution with
sodium hydroxide, the slurry-filtration time and the
precipitate-washing time were 60 and 120 s, respectively. The concentrations of the starting sodium carbonate and magnesium chloride solutions should be
such that reaction (4) produce an unsaturated sodium
chloride solution; this would ease precipitate washing.
Precipitation of basic
magnesium carbonates
from magnesium
chloride using
sodium carbonate
Filtration of basic
magnesium
carbonates
and washing
off NaCl
The filtered and washed precipitate after drying was a
free-flowing fine powder.
From DTA data (Fig. 4), the decomposition of basic
magnesium carbonates ended at 520°C. In this connection, basic magnesium carbonates were calcined at
550°C for 1 h in order to provide their complete decomposition to MgO. X-ray powder diffraction showed that
pure magnesium oxide was produced under these conditions. The difference between the DTA weight loss on
calcining and the calculated value was 0.42%. Thus, the
temperature chosen (550°C) provided the complete
decomposition of basic carbonates and the production
of pure magnesium oxide.
The following stage—conversion of magnesium
oxide to hydroxide—met no difficulties: magnesium
oxide that formed at ~550°C was easily converted to
hydroxide when brought in contact with water (table).
After magnesium oxide prepared at 550°C was moistened and allowed to stand for 50 min at room temperature, the magnesium hydroxide yield was 100%; after
boiling a magnesium oxide suspension with water for
30 min, the yield was also 100%.
The flowsheet of magnesium hydroxide preparation
from magnesium chloride via the precipitation and
thermal decomposition of basic magnesium carbonates
is shown below.
Thermal
decomposition
of basic
magnesium
carbonates
THEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING
Vol. 41
Hydration
of magnesium
oxide
No. 5
2007
Drying
of Mg(OH)2
576
KONDAKOV, DANILOV
TG
DTA
304
514
–56.42%
520
–1.5%
1000
this process, magnesium hydroxide was precipitated
with aqueous alkali, the product being obtained as a
precipitate with good filtration properties.
A process for preparing magnesium hydroxide from
magnesium chloride, based on the precipitation of basic
magnesium carbonates, their thermal decomposition,
and the hydration of the resulting magnesium oxide,
has been tested.
Parameter data have been obtained for both processes. These data were necessary for performance calculations, choosing equipment, and experimental
design for pilot studies.
Fig. 4. Thermoanalytical curves for the decomposition of a
mixture of basic magnesium carbonates.
REFERENCES
CONCLUSIONS
Various processes for manufacturing magnesium
hydroxide have been considered and tested in order to
decide whether they can be employed in the full-scale
production of this product from natural sources (bischofite and magnesium chloride brines).
A process has been proposed for the synthesis of
magnesium hydroxide from magnesium chloride. In
1. Pozin, M.E., Tekhnologiya mineral’nykh solei (udobrenii, pestitsidov, promyshlennykh solei, okislov i kislot)
(Technology of Mineral Salts: Fertilizers, Pesticides,
Industrial Salts, Oxides, and Acids), Leningrad:
Khimiya, 1974, part 1.
2. Unifitsirovannye metody analiza vod (General Water
Analyses), Moscow: Khimiya, 1973.
3. Khimicheskaya entsiklopediya (Encyclopedia of Chemistry), Moscow: Sovetskaya Entsiklopediya, 1990,
vol. 2.
THEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING
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No. 5
2007