www.elsevier.com/locate/ijminpro
Int. J. Miner. Process
Dissolution kinetics of natural magnesite in acetic
acid solutions
Oral Lacin*, Bqnyamin Dfnmez, Fatih Demir
Department of Chemical Engineering, Ataturk University, 25240 Erzurum, Turkey
Received 16 August 2003; received in revised form 12 May 2004; accepted 13 May 2004
Abstract
Dissolution of magnesite in acetic acid solutions was investigated. The influence of various parameters such as reaction
temperature, particle size and acid concentration was studied in order to elucidate the kinetics of magnesium carbonate. The
leaching rate increased with decreasing particle size and with increasing temperature. Initially, the dissolution in terms of acid
concentration increased until a definite concentration and then fell with increasing concentration. A kinetic model was
researched to describe the dissolution and to analyse the kinetic data, basically. Dissolution curves were evaluated in order to
test shrinking core models for fluid–solid systems. Consequently, it was determined that the dissolution of natural magnesite
was controlled by chemical reaction, i.e., 1�(1�x)1/3=kt. The apparent activation energy of leaching process was found as
78.40 kJ mol�1.
D 2004 Elsevier B.V. All rights reserved.
Keywords: dissolution of magnesite; acetic acid; leaching kinetics
1. Introduction
Magnesite ore, still, is the basic raw material for
manufacturing of magnesium and its compounds.
Also, these products have rather wide usage fields
and their costs are high (Bengtson, 1999; Maniocha,
1997; Bukovisky, 1997). In order to recover magne-
sium and its compounds from magnesite ore, the
0301-7516/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.minpro.2004.05.002
* Corresponding author. Fax: +90 442 2360957.
E-mail address: [email protected] (O. Lacin).
hydrometallurgical methods are usually used (Kova-
cheva et al., 2001).
In this direction, as leaching agent, generally used
are the chemical compounds such as inorganic/organic
acids or bases and their salts. Although magnesite
dissolution has been examined with inorganic reagents
(Fredd and Fogler, 1998; Economou et al., 2002), the
studies concerning dissolution kinetics of magnesite in
organic acid have nearly been limited (Demir et al.,
2003).
Organic acids have high selectivity although their
dissolving abilities are weak. Therefore, it is
advantageous for particularly dissolution of carbona-
. 75 (2005) 91–99
Table 1
Chemical analysis of the natural magnesite
Component [wt.%]
MgO 45.95
CaO 1.40
Fe2O3 0.52
SiO2 1.98
Loss on ignition [at 850 8C] 50.15
O. Lacin et al. / Int. J. Miner. Process. 75 (2005) 91–9992
ceous compounds. Also, in scale-up studies con-
ducted with inorganic acid, high CO2 pressure and
froth forming owing to fast dissolution can lead to
some risks. Organic acids can be an attractive
extracting agent because the extraction is performed
at mildly acidic conditions (pH 3–5); their degrada-
tion is biologically easy (Veeken and Hamelers,
1999). Additionally, in the industrial processes,
organic acids can cause a little corrosion effect
(Bilgic, 2002). However, the organic acids could not
be generally used as leaching agent for hard
dissolving compounds. Also, use of organic acids
at high temperature may be limited because of low
boiling temperatures and their decomposition.
In aquatic solutions, acetic acid is weakly disso-
ciated (pKa=4.76) and at concentrations of 1.0, 0.1 and
0.01 M the resulting pH is 2.4, 2.9 and 3.4,
respectively. Many metals, as well as their oxides
and carbonates, dissolve in aqueous solutions of acetic
acid to give simple salts. The reactions are consid-
erably slower than those of hydrochloric acid or
sulfuric acid, but the rate is still higher 10–11 times
than with most other organic acids (Wagner, 1978).
Today, acetic acid is also widely used as a solvent in
the chemical industry and a raw material for many
organic syntheses such as the manufacture of vinyl
acetate and cellulose acetate. In addition, calcium
magnesium acetate can be used as an additive to coal-
fired combustion units, for example, boilers used by
electrical utilities.
Numerous studies for the dissolution of magne-
site were found in the literature (Jordan et al., 2001;
Ekmekyapar et al., 1993) and some of them shall be
briefly discussed at this point. The dissolution
kinetics of magnesite in water saturated by chlorine
gas was studied by Ozbek et al. (1998) and it was
found that the dissolution process was also con-
trolled by surface reaction. Also, Demir et al. (2003)
found that the leaching kinetics of magnesite in
citric acid solutions was controlled by chemical
reaction in developing semi-empirical model. The
activation energy of the process was determined as
61.35 kJ mol�1.
Abali et al. (1992) examined the reaction kinetics
of magnesite with SO2 gas in aqueous medium. The
results obtained from experiments showed that the
dissolution rate was controlled by surface reaction and
the activation energy for process was 81 kJ mol�1.
Also, KurtbaY et al. (1992) investigated the dissolution
kinetics of magnesite in HCl solution and it was
determined that the dissolution rate was controlled by
surface reaction. Another interesting search discussed
here was the comparative study by Chou et al. (1989),
who investigated the dissolution of various carbonates
(including calcite, magnesite and dolomite) in HCl
solutions at 25 8C by using a continuous fluidized bed
reactor and samples of relatively coarse particle size.
A chlorination study of magnesium carbonate by
Kennedy and Harris (2000) was performed in a stirred
tank reactor. Chlorination rates were measured over a
range of temperatures from 740 to 910 8C. Activationenergy of process was calculated as 80 kJ mol�1 over
the temperature range from 740 to 825 8C and the
fastest chlorination was reached at a temperature of
860 8C.The present research aimed to study the leaching
kinetics of natural magnesite in the acetic acid
solutions.
2. Methods and materials
The magnesium carbonate ore used in the work was
provided from Erzurum-Oltu in Turkey. After crushing
and washing, the sample was ground, and its chemical
composition was analyzed by standard gravimetric and
volumetric methods (Furmann, 1963). The results
were given in Table 1. An X-ray diffractogram
illustrating the contents of the sample was given in
Fig. 1. The ore was sieved using ASTM standard
sieves, giving particle size fractions of 138, 215, 478
and 855 Am.
The acetic acid, CH3COOH, used as leachate was
of reagent grade. The main production routes for
acetic acid were liquid-phase oxidation of n-butane
and methanol carbonylation, both with feedstocks
derived from natural gas or petroleum (Busche, 1990;
Fig. 1. X-ray diffractogram of the magnesite ore.
O. Lacin et al. / Int. J. Miner. Process. 75 (2005) 91–99 93
Wagner, 1978). Alternative processes included the
destructive distillation of wood and the fermentation
of ethanol or sugars. It was used as a food acidulant
and preservative for thousands of years.
2.1. Experimental procedure
Dissolution experiments were carried out in a well-
mixed spherical glass batch reactor (500 mL) heated
by a constant temperature bath and equipped with a
mechanical stirrer having a digital controller unit, a
thermometer and a back cooler. After adding 250 mL
of acetic acid solution to the reaction vessel and
setting the temperature at the desired value, a charge
of 2.0 g of magnesite was approximately added to the
reactor while stirring the content of the reactor at a
certain speed. After each test, an amount of sample
taken from the leach slurry was filtered immediately,
and the Mg2+ content in the leach solution was
determined complexometrically by EDTA at the
medium of buffer solution (about pH=10) (Gulensoy,
1974).
Dissolution behavior for samples of natural mag-
nesite was tested under reaction conditions which
were as follows: temperature from 40 to 70 8C,concentration of acetic acid from 1 to 10 M and
particle size from 138 to 855 Am.
3. Results and analysis
When magnesium carbonate is added into the acetic
acid solution, the reactions taking place in the medium
can be written as follows:
2CH3COOHðaqÞX 2Hþ ðaqÞ þ 2CH3COO�ðaqÞ ð1Þ
MgCO3ðsÞþ2HþðaqÞYMg2þðaqÞþ CO2ðgÞþH2OðIÞð2Þ
The overall reaction can be written as follows:
MgCO3ðsÞ þ 2CH3COOHðaqÞYMg2þðaqÞ
þ2CH3COO�ðaqÞ þ CO2ðgÞ þ H2OðIÞ ð3Þ
3.1. Effect of particle size
The experiments were performed for four different
particle sizes (138, 215, 478 and 855 Am) in solutions
containing 3.0 M acetic acid at stirring speed of 500
rpm. Because a little dissolution occurred at 40 8C,the effect of particle size was studied at 60 8C. FromFig. 2, the smaller the particle size, the faster was
magnesite dissolution. The results showed that the
particle size had a little effect on the dissolution of
magnesite.
3.2. Effect of reaction temperature
Experiments were carried out at the 40–70 8Ctemperature range in 3.0 M acetic acid at stirring
speed of 500 rpm for 215 Am. Typical rate curves
were shown in Fig. 3. From this figure, it was
Fig. 2. Effect of particle size on the leaching of magnesite.
O. Lacin et al. / Int. J. Miner. Process. 75 (2005) 91–9994
observed that the dissolution rate was very sensitive to
reaction temperature.
3.3. Effect of acid concentration
The effect of acid concentration, in the range of
1.0–10.0 M, was performed at 40 8C with an
agitation speed of 500 rpm for 215 Am. From Fig.
4(a) and (b), for the concentration range of 1.0–3.0
M, the increase in acid concentration increased the
Fig. 3. Effect of temperature on
dissolution rate of magnesite, but for 3.0–10.0 M,
the increase in concentration decreased the dissolu-
tion rate. For both situations, the concentration
effect was clearly shown in Fig. 5 at two different
time. It could be attributed that the intensity of
negative effect of water (the solvent) decrease, after
a certain value of acid concentration, was more
dominant than that of positive effect of increase of
acid concentration. Again, when the acid concen-
tration exceeded a definite value, the number of
the leaching of magnesite.
Fig. 4. (a) Effect of acid concentration on the leaching of magnesite (for 1.0–3.0 M). (b) Effect of acid concentration on the leaching of
magnesite (for 3.0–10.0 M).
O. Lacin et al. / Int. J. Miner. Process. 75 (2005) 91–99 95
hydrogen ions in the medium might decrease due
to decrease of water amount more and more
(Marinovic and Despic, 1997). In addition, this
behavior could be explained by the fact that as
the acid concentration in the medium increased,
the appearance rate of product increased and as the
product reached the saturation value near the solid
particle, it forms a difficult soluble solid film layer
around the particle. Consequently, the dissolu-
tion process slowed down (Ozmetin et al.,
1996).
4. Kinetic analysis
Fluid–solid heterogeneous reaction systems have
applications in chemical and hydrometallurgical
processes. A successful reactor design for these
processes depends basically on kinetic data. In the
fluid–solid systems, the reaction rate may be gen-
erally controlled by one of the following steps:
diffusion through the fluid film, diffusion through
the ash or the chemical reaction at the surface of the
core of unreacted materials (Levenspiel, 1972). The
Fig. 5. Effect of acid concentration on the leaching of magnesite (for 30 and 60 min).
O. Lacin et al. / Int. J. Miner. Process. 75 (2005) 91–9996
rate of the process is controlled by the slowest of
these sequential steps.
In order to determine the kinetic parameters and
rate-controlling step about leaching of magnesite in
acetic acid solutions, the experimental data are
analysed on the basis of the shrinking core model.
This reaction model between a fluid and a solid may
be represented by:
A fluidð Þ þ bB solidð ÞY Products ð4Þ
If no ash layer covers the unreacted core as the
reaction proceeds, there could be only two controlling
Fig. 6. 1�(1�x)1/3 vs. time at va
steps, namely, fluid film diffusion or chemical
reaction.
If the process is controlled by resistance of fluid
layer, the Eq. (5) is used.
t ¼ RqB
3bk1CA
XB ð5Þ
If this is controlled by resistance of chemical
surface reactions, the Eq. (6) is used.
t ¼ RqB
bkSCA
1� 1� XBð Þ1=3h i
ð6Þ
The fit of all the experimental data into the integral
rate was tested by using a computer program, and the
rious reaction temperatures.
O. Lacin et al. / Int. J. Miner. Process. 75 (2005) 91–99 97
multiple regression coefficients obtained for the
integral rate expression were calculated. In the
calculations, it was seen that the best value of
regression coefficient correcting the rate expression
was for surface reaction control. The coefficient
value was calculated as 0.9972. To confirm the
results of these statistical analyses, the experimental
data for each parameter were analysed by graphical
methods.
From the results of the statistical analysis, it was
found that the leaching of magnesite in acetic acid
solutions was controlled by chemical reaction. Also, it
was determined that the integral rate expression
obeyed the following equation:
1� 1� xð Þ1=3 ¼ kt ð7Þ
Eq. (7) yields the best straight lines in comparison
with other equations tested. From Arrhenius equation,
k term was known as:
k ¼ k0e�E=RT ð8Þ
For the reaction temperature, particle size and the
concentration, the plots of 1�(1�x)1/3 vs. t were
shown in Figs. 6–8. From the slopes of the straight
Fig. 7. 1�(1�x)1/3 vs. time a
lines in Fig. 6, the apparent rate constants were
evaluated.
As shown in Fig. 9, the plot of lnk vs. ln(1/T) was
obtained for each value of the temperature, and the
following values were calculated:
E ¼ 78:40 kJ mol�1; ko ¼ 9430
Such a value for the activation energy indicated
that the leaching of magnesite with acetic acid
solutions was controlled by chemical reaction. Thus,
Eq. (7) could be given as
1� 1� xð Þ1=3 ¼ 9430e�78:40=RT t ð9Þ
5. Discussion and conclusions
The kinetics of the liquid–solid reaction between
natural magnesite and acetic acid, an organic acid,
solutions is studied. Based on the results obtained in
this research, the following conclusions can be
drawn:
! In the dissolution process, it is observed that the
reaction rate is very sensitive to temperature in
the range of 40–70 8C. Thereby, the solubility
t various particle sizes.
Fig. 8. 1�(1�x)1/3 vs. time at various concentrations.
O. Lacin et al. / Int. J. Miner. Process. 75 (2005) 91–9998
increases with increasing reaction temperature
and reaction period, and with decreasing particle
size.
! In terms of acid concentration, it is shown that
the dissolution rate increases for the acid concen-
tration range 1.0–3.0 M and decreases for range
3.0–10.0 M.
! The dissolution kinetics follows a shrinking core
model with the surface chemical reaction as the
rate-controlling step. The apparent activation
energy of leaching is found to be 78.40 kJ
Fig. 9. Arrhenius plot for the le
mol�1. Such a value demonstrates that the
process is a chemically controlled reaction and
agrees with the values of similar works concern-
ing dissolution of magnesite (Demir et al., 2003;
Ozbek et al., 1998) for liquid–solid reaction
systems.
! During dissolution, when it is studied at large
scale, the abundant amount of CO2 can be
produced.
! In the leaching of magnesite, it is found that a little
calcium and iron also dissolve.
aching of magnesite ore.
Explanation of symbols
Symbol Meaning Unit
XB=X converted fraction [–]
T temperature [K]
E activation energy [kJ mol�1]
t reaction time [s]
k reaction rate constant [s�1]
ks rate constant for surface
reaction
[cm s�1]
R universal gas constant [kJ mol�1 K�1]
qB molar density of B in
the solid
[g mL�3]
R average radius of solid particles [cm]
b stoichiometric coefficient [–]
CA bulk concentration [mol cm�3]3 �2 �1
O. Lacin et al. / Int. J. Miner. Process. 75 (2005) 91–99 99
k l mass transfer coefficient
(in Eq. (5))
[cm cm s ]
References
Abali, Y., Colak, S., Ekmekyapar, A., 1992. Magnezit mineralinin
sulu ortamda SO2 gazi ile cfzqnme kinetigi. Doga. Turkish
Journal 16, 319–324.
Bengtson, K.B., 1999. Dunite (olivine) as a source of magnesium
chloride for the manufacture of magnesium metal, light metals.
Proceedings of Sessions, TMS Annual Meeting (Warrendale,
Pennsylvania), pp. 1151–1154.
Bilgic, S., 2002. The inhibition effects of benzoic acid salicylic acid
on the corrosion of steel in sulfuric acid medium. Material
Chemistry and Physics 76, 52–58.
Bukovisky, V., 1997. Yellowing of newspaper after deacidification
with methyl magnesium carbonate. International Journal for The
Preservation of Library and Archival Material 18 (1), 25–38.
Busche, R.M., 1990. Acetic acid from corn|economic trade or
between yield of Clostridium and concentration of Acetobacter.
Corn Utilization Conference III Proceedings. National Corn
Growers Association.
Chou, L., Garrels, R.M., Wollast, R., 1989. Comparative study of
the kinetics and mechanisms of dissolution of carbonate
minerals. Chemical Geology 78, 269–282.
Demir, F., Dfnmez, B., Colak, S., 2003. Leaching kinetics of
magnesite in citric acid solutions. Journal of Chemical Engineer-
ing of Japan 36 (6), 683–688.
Economou, E.D., Vaimakis, T.C., Papamichael, E.M., 2002. The
Kinetics of dissolution of the carbonate minerals of phosphate
ores using dilute acetic acid solutions: the Case of pH Range
from 3.96 to 6.40. Journal of Colloid and Interface Science 245
(1), 133–141.
Ekmekyapar, A., ErYahan, H., Dfnmez, B., 1993. Calcination of
Magnesite and Leaching Kinetics of Magnesia in Aqueous
Carbon Dioxide. Doga. Turkish Journal 17, 197–204.
Fredd, C.N., Fogler, H.S., 1998. The kinetics of calcite dissolution
in acetic acid solutions. Chemical Engineering Science 53 (22),
3863–3874.
Furmann, N.H., 1963. Standard Methods of Chemical Analysis, 6th
ed. D. Van Nastrand, New Jersey.
Gqlensoy, H., 1974. Kompleksometrik Titrasyonlar ve Komplekso-
metrinin Temelleri. Fatih YayVnevi, Istanbul, Turkey.
Jordan, G., Higgins, S.R., Eggleston, C.M., Knauss, K.G., Schmahl,
W.W., 2001. Dissolution kinetics of magnesite in acidic aqueous
solution, a hydrothermal atomic force microscopy (HAFM)
study: step orientation and kink dynamics. Geochimica et
Cosmochimica Acta 23 (65), 4257–4266.
Kennedy, M., Harris, R., 2000. Chlorination of magnesium
carbonate in a stirred tank reactor. Canadian Metallurgical
Quarterly 39 (3), 269–279.
Kovacheva, P., Arishtirova, K., Vassilev, S., 2001. MgO/NaX
zeolite as basic catalyst for oxidative methylation of toluene
with methane. Applied Catalysis 210 (1–2), 391–395.
KurtbaY, A., Ekmekyapar, A., Colak, S., 1992. HCl CfzeltilerindeManyezitin Heterojen Reaksiyon Kinetigi. Doga. Turkish
Journal 16, 90–94.
Levenspiel, O., 1972. Chemical Reaction Engineering, 2nd ed. John
Wiley and Sons Inc., New York.
Maniocha, M.L., 1997. Magnesia: moving beyond refractories.
Mining Engineering 49 (2), 26–29.
Marinovic, V., Despic, A.R., 1997. Hydrogen evolution from
solutions of citric acids. Journal of Electroanalytical Chemistry
431, 127–132.
Ozbek, H., Abali, Y., Colak, S., Ceyhun, I., Karagflge, Z., 1998.Dissolution kinetics of magnesite mineral in water saturated by
chlorine gas. Hydrometallurgy 00, 1–13.
Ozmetin, C., Kocakerim, M.M., YapVcV, S., YartaYV, A., 1996. A
semiempirical kinetic model for dissolution of colemanite in
aqueous CH3COOH solutions. Industrial & Engineering Chem-
istry Research 35, 2355–2359.
Veeken, A.H.M., Hamelers, H.V.M., 1999. Removal of heavy
metals from sewage sludge by extraction with organic acid.
Water Science and Technology 400 (1), 129–136.
Wagner, F.S., 1978. Acetic acid. Kirk-Othmer Encyclopedia of
chemical technology, vol. 1. Wiley, New York.
Top Related