Download Acetic acid leaching of magnesia from magnesite via calcination

Indian Journal of Chemical Technology
Vol. 13, March 2006, pp. 135-138
Acetic acid leaching of magnesia from magnesite via calcination
Ayhan Demirbas
Department of Chemical Engineering, Selcuk University, Campus, 42031 Konya, Turkey
Email: [email protected]
Received 2 November 2004; revised received 20 November 2005; accepted 16 December 2005
In this study, magnesite mineral was first calcinated between 800-1100 K for an hour and dissolved in acetic acid
solutions. The effects of temperature, solid-to-liquid ratio, reaction time, stirring speed and acid concentration on the
dissolution rate of magnesia in acetic acid were investigated. It was observed that the dissolution of magnesia increased with
increasing temperature, stirring speed, reaction time and acid concentration and decreased with increase in solid-to-liquid
ratio. The dissolution process is controlled by pseudo first-order reaction rate. Activation energy for the reaction was
calculated as 8.78 (kJ/mol).
Keywords: Magnesite, Magnesia, Magnesium acetate, Calcination, Dissolution
IPC Code: C04B2/00
Magnesium is the eighth most abundant element and
constitutes about 2 percent of the Earth’s crust. It is
the third most plentiful element dissolved in seawater,
with a concentration averaging 0.13 percent.
Although magnesium is found in over 60 minerals,
only dolomite, magnesite, brucite, carnallite, and
olivine are of commercial importance. Of these
minerals, magnesite and dolomite are the largest
sources of magnesium and magnesium compounds.
Magnesium and magnesium compounds are produced
from seawater, well and lake brines and bitterns, as
well as from the minerals. Magnesium alloys are used
to develope creep-resistant to produce components
such as cylinder blocks, engine covers, oil pans, and
transmission cases.
Magnesia (MgO) is mostly produced by
calcinations of magnesite (MgCO3). Magnesite is the
main source material for magnesia and magnesium
metal. Traditionally, these ores have been treated by
pyrometallurgical methods. Magnesite is separated
from impurity silica and iron by crushing, grinding,
scrubbing, screening and heavy media separation or
flotation. Caustic magnesia is a comparatively more
porous and reactive material obtained at relatively low
temperatures. Magnesite is basic raw material of
alkaline refracters and is used in iron-steel industry
and other industries such as cement, glass, sugar, lime
and paper. In addition, it is used in paint and ink
industry, medicine industry as antacid, and in the
production of many magnesium chemicals as raw
material1.
Depending on the thermal condition, caustic
magnesia or dead-burned magnesia can be obtained.
Dead-burned
magnesia,
produced
at
high
temperatures, is a basic refractory, whereas causticcalcined magnesia, formed from cryptocrystalline
magnesite is an active alkali used in water treatment,
mineral processing, and in specialized cement
products. Magnesia can be formed by calcinations of
magnesium chloride (a precursor of metal) produced
by direct acid leaching, filtration, impurity removal
and crystallization.
Information about the leaching of magnesia with
acetic acid is important for the development of
industrial process to produce Mg(CH3COO)2 and pure
MgO2. In this study, the production of magnesia via
calcination of magnesite and the dissolution kinetics
of magnesia in acetic acid solutions has been
investigated to produce magnesium acetate.
Experimental Procedure
The magnesite mineral samples used in the study
were supplied from Turkish mine sources. The
chemical analysis of ore sample was carried out by
standard gravimetric and volumetric methods3. The
analytical results are given in Table 1.
The average density of used magnesite ore was
determined as 2.978 gcm−3. In all experiments
magnesite minerals of the particle size, −0.31 +0.23
mm were used. The magnesite was calcinated
between 850-1125 K for an hour to obtain magnesia.
The calcinator consisted of a 100 mL stainless steel
INDIAN J. CHEM. TECHNOL., MARCH 2006
136
Table 1—Chemical composition of magnesite mineral
Component:
MgO
CaO
Fe2O3
SiO2
Ignition loss
Weight, % :
46.36
1.06
0.41
0.71
51.46
reactor, externally heated by an electric ring furnace.
A similar calcinatory apparatus was used in the earlier
study4. The maximum temperature and the heating
procedure including the heating rate, isothermal time,
were controlled automatically.
Thermogravimetric analysis (TGA) of the
magnesite was done by using a TG analyzer described
earlier5. The samples were heated, under a nitrogen
purge, at heating rate of 10 K/min to a final
calcination temperature of 1125 K. The TGA provides
the continue measurements of the sample weight as a
functions of time and temperature.
After the calcination, the magnesia was dissolved
in acetic acid solutions. The dissolution processes
were carried out in a 250 mL 3-necked roundbottomed glass reactor equipped with a mechanical
stirrer, at atmospheric pressure and constant
temperature.
At the beginning of each experiment, a 100 mL
acetic acid solution with a known concentration was
put into the reactor and heated to desired temperature.
Then, a given amount of magnesia was added to the
reactor while stirring the reaction mixture at a
constant rate. After the end of reaction, the reaction
mixture was filtered and the amount of Mg2+ was
determined complexometrically.
Results and Discussion
The magnesite mineral was calcinated for an
hour at 1125 K to obtain magnesia (MgO) according
to Eq. (1).
MgCO3 → MgO + CO2
…(1)
The thermal decomposition curve (TGA) of
magnesite to magnesia is shown in Fig. 1. The pure
MgCO3 is obtained by a two-step process viz. the
formation of magnesium bicarbonate solution and the
precipitation of magnesium carbonate6-8.
The magnesia obtained from calcination process on
dissolution in acetic acid yields magnesium acetate
according to Eq. (2).
MgO+2CH3COOH→Mg(CH3COO)2+H2O
…(2)
Fig. 1—Thermal decomposition curve of magnesite (MgCO3) to
magnesia (MgO).
Particle size, 0.15-0.23 mm; Heating rate, 10 K/min
Fig. 2—Effect of temperature on the dissolution.
X = Degree of dissolution
Effect of temperature
The effect of the temperature on the dissolution
process was determined in the temperature range of
285-325 K, maintaining the solid-to-liquid ratio at
1/100 (g/mL), stirring speed at 500 min−1, and acid
concentration at 0.366 M. As evident from the plot of
the experimental results shown in Fig. 2, the degree of
dissolution increases regularly as the temperature
increases.
Effect of solid-to-liquid ratio
The effect of the solid-to-liquid ratio on the
dissolution process was studied using different ratios
in the range of 0.5-10/100 (g/mL). For these
experiments the value of other parameters, maintained
constant during the reaction were: leaching
temperature, 293 K; stirring speed, 500 min−1 and
reaction time, 10 min. Figure 3 shows the
DEMIRBAS: ACETIC ACID LEACHING OF MAGNESIA FROM MAGNESITE VIA CALCINATION
Fig. 3—Effect of solid-to-liquid ratio on the dissolution
X=Degree of dissolution; Acid conc., 0.366 M
Stirring speed, 500 rpm
137
Fig. 4—Effect of acid concentration on the dissolution
X=Degree of dissolution
experimental results. The dissolution rate decreases
substantially with increasing solid-to-liquid ratio. This
can be attributed to the decrease in the fluid reactant
per unit weight of solid.
Effect of acid concentration
The effect of acid concentration on dissolution
process was studied using the acid concentration in
the range of 0.336-3.666 M. For these experiments,
the values of the other parameters viz. leaching
temperature, 283 K; reaction period, 10 min; stirring
speed, 500 min−1 and solid-to-liquid ratio of 1/100
(g/mL) were maintained constant during the reaction.
The results obtained by these experiments are shown
in Fig. 4.
Effect of stirring speed
The effect of stirring speed on the dissolution
process was investigated in the range of 100-650 rpm
at 283 K with solid-to-liquid ratio of 1/100 (g/mL),
and reaction period of 10 min. Increase in stirring
speed causes a decrease in thickness of film layer,
therefore, dissolution rate increases (Fig. 5).
Dissolution kinetics
Since the fluid-solid heterogeneous reaction
systems have significant applications and importance
in chemical and metallurgical processes, a successful
reactor design for such process depends essentially on
kinetic data. The reactions occurring in the fluid-solid
system generally have the following steps: (i)
Diffusion of fluid reactant through the fluid layer to
the surface of the solid, (ii) Reaction of the fluid
reactant and solid on the surface of the solid, and (iii)
Diffusion of the products through the film layer to the
Fig. 5—Effect of stirring speed on the dissolution
X=Degree of dissolution
bulk fluid. The slowest of these sequenced steps is
rate-determining step. Because the products are not
solid, as seen in Eq. (2), there is no ash film
resistance. The experimental data have been evaluated
according to shrinking-core model.
The experimental results were used to determine
the kinetic parameters of pre-exponential factor
(frequency factor) A and the activation energy E. The
rate of mass loss for a decomposition reaction is
generally described by the following equations,
(dX/dt) = k f (W)
…(3)
X = (Wo – W)/Wo at any time ‘t’
…(4)
where t is time, k is the reaction rate constant, X is the
degree of dissolution or conversion, Wo is the initial
weight and W is the weight of residue.
138
INDIAN J. CHEM. TECHNOL., MARCH 2006
Fig. 6—Plots of −ln(1−X) versus t at different temperatures
k = A e−E/RT
…(6)
Lnk = lnA (−E/RT)
...(7)
where A is the pre-exponential factor, E is apparent
activation energy, T is the absolute temperature and R
is universal gas constant. From a plot of lnk versus
1/T a straight line of (−E/RT) slope was obtained for
different temperatures. The Arrhenius plot for the
temperature range of 283-323 K is given in Fig. 7.
The continuous recordings of the TGA of weight loss
with time and temperature enables the determination
of (dX/dt) and k, and consequently the activation
energy E and the pre-exponential factor A are
calculated. The value of activation energy has been
found to be 8.78 in kJ.mol−1, as obtained from the
slope of this line (Fig. 7).
Conclusion
The magnesite can be calcinated between 850-1125
K for an hour to obtain magnesia. The magnesia can
be dissolved in acetic acid solutions. The leaching of
magnesia with acetic acid produce pure
Mg(CH3COO)2. It was observed that the dissolution
of magnesia increased with increasing temperature,
stirring speed, reaction time and acid concentration
and decreasing with solid-to-liquid ratio.
Fig. 7—Arrhenius plot for the temperature range of 283-323 K
An evaluation was done bearing in mind the
equilibrium for first-order reactions:
t = (−1/k) [ln (1−X)]
…(5)
The plots drawn between [-ln (1-X)] and t, in the
range of 1-20 min for reaction period gave straight
lines (Fig. 6). In addition, the application of Eq. (6)
gave a value of regression coefficient as 0.9998
(approximately). The rate constant, k, has been
calculated from slope of these lines for different
temperatures.
The reaction rate constant k is also defined by the
Arrhenius equation (Eq. 6),
Acknowledgement
This study has been supported by Scientific
Research Project of Selcuk University.
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