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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: olacin@atauni.edu.tr (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 ]

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