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Hydrometallurgy 89 (2007) 189 195

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Dissolution kinetics of galena in acetic acid solutions

with hydrogen peroxide
Salih Aydoan a,, Ali Aras a , Gkhan Uar a , Murat Erdemolu b
a
Seluk University, Department of Mining Engineering, 42075 Konya, Turkey
b
Inn University, Department of Mining Engineering, 44280 Malatya, Turkey
Received 23 May 2007; received in revised form 12 July 2007; accepted 13 July 2007
Available online 20 July 2007

Abstract

The kinetics of leaching lead from galena in acetic (ethanoic) acid solutions with hydrogen peroxide are investigated with
regard to stirring speed, temperature and concentration of HAc and H2O2 concentration. Oxidation of galena with H2O2 to produce
lead sulphate which dissolves by complexing Pb2+ with acetate anion (PbCH3COO+ and Pb(CH3COO)2). Results indicate that the
rate of galena dissolution is controlled by a surface chemical reaction with an apparent activation energy is 65.6 kJ mol 1 in the
temperature range 3070 C. Both HAc and H2O2 affect the rate of extraction of lead as an acetate complex. The order of reaction
was 0.79 and 0.31 for H2O2 and HAc concentrations, respectively.
2007 Elsevier B.V. All rights reserved.

Keywords: Galena; Leaching; Kinetics; Acetic acid; Hydrogen peroxide; Shrinking core model

1. Introduction In hydrometallurgical processes, carboxylic acids have

been proposed as an alternative, and probably less
Short-chain or low molecular weight carboxylic acids expensive, leaching agent (Panias et al., 1996; Ambikadevi
(LMWCA) having 15 C atoms per molecule are usually and Lalithambika, 2000). Organic acids such as oxalic,
referred to as an aliphatic mono-carboxylic acids. These citric, ascorbic, acetic, fumaric and tartaric acid, have
may affect mineral weathering rates by at least three been used for their ability to solubilize iron and other metal
mechanisms (Drever and Stillings, 1997): by changing oxides. Of the above organic acids, oxalic (Baumgartner
the dissolution rate far from equilibrium through et al., 1983; Cornell and Schindler, 1987; Blesa et al.,
decreasing solution pH or forming complexes with 1987; Taxiarchou et al., 1997; Ubaldini et al., 1996) citric
cations at the mineral surface; by affecting the saturation (Waite and Morel, 1984) and ascorbic acids (Afonso et al.,
state of the solution with respect to the mineral; and by 1990; Parida et al., 1997) are the most used carboxylic
affecting the speciation in the solution of ions. acids, due to their effectiveness as solvent reagents.
Generally, the studies carried out were mainly focused on
the dissolution of iron oxides and aluminum oxide; together
with those on the dissolution of non-oxide minerals
Corresponding author. such as feldspar (Drever and Stillings, 1997) hornblende
E-mail address: saydogan@selcuk.edu.tr (S. Aydoan). (Zhang and Bloom, 1999) and apatite (Goyne et al., 2006).
0304-386X/$ - see front matter 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.hydromet.2007.07.004
190 S. Aydoan et al. / Hydrometallurgy 89 (2007) 189195

There are hardly any studies aimed at leaching 3. Results and discussion
sulphide minerals using organic acids. Evans and Masters
(1976) developed a method for leaching finely divided 3.1. Dissolution of galena
galena bearing material to convert lead to soluble lead
acetate with concurrent conversion of sulphur to elemen- The oxidation and dissolution mechanisms of galena
tal state. The method involves forming slurry consisting still remain uncertain with a wide variety of possible
of the material dispersed in an aqueous medium contain- mechanisms having being proposed in the literature.
ing free acetate ions and having a pH below 5.1, at a Surface species found on galena during oxidation and
temperature of 60 to 120 C under oxygen pressure. dissolution have been studied using many techniques
Geisler and Puddington (1996) is described a method for (Gerson and O'Dea, 2003). It has been assumed that
dissolving galena by in-situ leaching with acetic acid and complexation takes place at the surface of hydrous
acetate solution in the presence of an oxidant such as galena sulphides. The adsorption of H+ ions onto a
oxygen gas. Accordingly, the experiments conducted by surface of S atom in the aqueous phase is found to be
Greet and Smart (2002) demonstrated that EDTA readily favorable whereas the adsorption onto a surface Pb atom
dissolves lead hydroxide, lead carbonate, lead sulphate, is not favorable. On exposure to air and aqueous
and lead hydroxy-carbonate all typical galena solution, hydroxides form on galena surface, often with
oxidation products. a lead deficient sulphide beneath the hydroxides. On
This present study aims to examine the effectiveness oxidation of galena in air, a metal deficient sulphide is
of acetic acid to leach galena in aqueous oxidizing formed, covered with a layer of lead hydroxide, oxide
conditions and dissolve lead. For this purpose, galena and carbonate. On further exposure to air, basic sulphate
was leached with acetic acid in the presence of hydrogen and lead sulphate are also observed. An ion exchange
peroxide, and effects of some parameters such as stirring process between protons and lead ions is assumed to
speed, temperature, acetic acid and hydrogen peroxide occur at the surface resulting in the formation of a lead
concentrations on the kinetic parameters of galena deficient surface.
dissolution were investigated. It has been reported by Hsieh and Huang (1989) that as
the dissolution rate is much higher under acidic conditions
2. Materials and methods as compared to alkaline conditions, a protonated surface,
PbSH22+, initiates the dissolution process: In the presence
A galena concentrate sample obtained from the of oxygen,
Koyulhisar complex sulphide ore concentrator in Sivas
province of Turkey was used. The sample was sieved
2solid 2O2 PbS2 d 2O2solid Pb
PbSH2 2 2
into three size fractions but experiments were performed

1
only with the 4575 m fraction which contained SO2
4 2H
79.0% Pb, 14.61% S, 2.97% Fe, 1.90% Zn and 0.50%
Cu.
The dissolution experiments were carried out in a Pb2 SO2
4 PbSO4solid 2
glass reactor of 1 L, equipped with a Teflon coated
mechanical stirrer, maintained in a constant temperature Accordingly, Jennings et al. (2000) have stated that the
bath. The ranges of dissolution parameters chosen were formation of PbSO4 prevents Pb2+ from undergoing
0600 min 1 for stirring speed, 303343 K for hydrolysis and generating acid,
temperature, 0.12.0 mol/L for hydrogen peroxide
concentration and 0.55.0 mol/L for acetic acid Pb2 2H2 OPbOH2solid 2H 3
concentration. For each experiment, 1 g of galena was
stirred in 500 mL of H2O2/HAc mixture for up to 90 min Instead, the overall reaction for galena weathering by
and sampled at various time intervals. Lead in the H2O2 is presented as,
solution was determined using flame atomic absorption
spectrophotometer (GBC Scientific Equipment, SensAA PbS 4H2 O2 PbSO4solid 4H2 O 4
Model, Australia). For calculation of the fraction of lead
extracted, the equation incorporating correction factors Hydrogen peroxide, H2O2, is a strong oxidant and
to account for volume losses during sampling was used. environmentally safe reagent as apart from water no
Distilled water and reagent grade chemicals were used to other reaction products generated during the dissolution
prepare all the solutions. of sulphide minerals. The oxidative action of hydrogen
 S. Aydoan et al. / Hydrometallurgy 89 (2007) 189195 191

peroxide in acidic solutions is based on its reduction of activity. When conditions are oxidizing, anglesite is
according to, the most stable in an acid environment and cerussite in
most alkaline conditions.
H2 O2 2H 2e 2H2 O E 0 1:77V 5 Regardless of very low solubility of PbSO 4
(Ksp = 1.82 10 8), acetic acid (CH3COOH) and acetate
Hydrogen peroxide can also act as a reducing agent, (CH3COO) rapidly form complexes with lead ions. The
undergoing oxidation: lead acetate complexes formed have stoichiometries which
can be described by the following generalized reaction;
H2 O2 O2 2H 2 6
Pb2 nAc PbAcn 2n 8
Overall reaction can be written:
where Ac is the acetate ion (CH3COO) and n is the
2H2 O2 O2 2H2 O 7 number of acetate ligands in the nth lead species. The
degree of association of lead is defined in terms of average
By decomposition of hydrogen peroxide, oxygen is ligand number, nav, calculated from the following
adsorbed on the mineral surface whereby electron expressions:
transfer takes places in the solution. The potential Pn h i
2n
value of 1.77 V is adequate to oxidize almost all of the mHAc  HAc  Ac   n Pb Ac n
metal sulphides (Aydoan, 2006). nav 1 9
mPb2 mPb2
The net results of the oxidation of sulphides, despite
the consequences of details of the dissolution mechanism where mHAc represents the stoichiometric concentration of
for galena, are (1) to get the metal ion into solution or into acetic acid, [HAc] and [Ac] are the calculated acetic acid
the form of an insoluble compound stable under surface and free acetate concentrations, respectively, and mPb2+
conditions, (2) to convert the sulphur to sulphate ion, and represents the total stoichiometric metal concentration
(3) to produce relatively acid solutions. Discussion of (Bnzeth and Palmer, 2000).
such reaction for lead minerals can be refined by the use The leaching solutions after removal of un-dissolved
of EhpH diagrams (Fig. 1), which serves only to solids were analysed for SO42 ion by adding 0.1 mol/L Ba
express the relationships quantitatively (Krauskopf and (NO3)2 solution instead of BaCl2 which is generally used
Bird, 1994). Of the four lead minerals shown in Fig. 1, for sulphate precipitation. Nitrate salt was employed to
galena is the most stable at low values of Eh, regardless avoid precipitation of PbCl2 (Ksp = 1.78 10 5). It was
observed that BaSO4 (Ksp = 1.08 10 10), as identified by
chemical analysis, precipitated without delay.
In the literature, there are some studies both on the
oxidation of galena (Wittstock et al., 1996) and the
dissolution of lead sulphate in acetate medium (Giordano,
1989). The thermodynamic stability constants at 298 K
for the complexes PbCH3COO+ and Pb(CH3COO)2 are
log K1 = 2.4 and log K2 = 3.4, respectively, as calculated
by Giordano (1989). He found that lead hydrolysis is
significant in those experiments conducted at 343 and
353 K; however hydrolyzed lead is not detected at 298, 313
and 328 K. The author concludes that lead acetate
complexes predominate over Pb2+ and PbOH+ at tempera-
tures between 298 and 358 K in solutions which are slightly
acid and have free acetate concentrations N 0.01 m.

3.2. Effects of parameters

The effect of stirring on the dissolution rate was

Fig. 1. EhpH diagram for lead minerals at 298 K and 1 bar. Total
determined at 323 K in solutions containing 3 mol/L
carbonate = 10 3 mol/L and total dissolved sulphur = 10 2 mol/L, HAc and 0.5 mol/L H2O2. Interestingly, for this reactive
(Krauskopf and Bird, 1994). sulphide at low pulp density, there was no significant
192 S. Aydoan et al. / Hydrometallurgy 89 (2007) 189195

Fig. 2. Effect of temperature on galena dissolution (no stirring; HAc:

3 mol/L; H2O2: 0.5 mol/L). Fig. 4. Effect of HAc concentration on galena dissolution (no stirring;
H2O2: 0.5 mol/L; temperature: 323 K).

effect of stirring on the rate of dissolution. In previous immediately precipitates at the un-reacted galena
studies, it was reported that oxygen derived from the surface, inhibiting further attack of hydrogen peroxide.
decomposition of H2O2 caused self mixing with mineral In order to determine the effect of hydrogen peroxide
particles (Aydoan, 2006; Adebayo et al., 2006; concentration, the experiments were performed by varying
Antonijevi et al., 1997, 2004). But in this study, no the initial H2O2 concentration in the range of 0.12.0 mol/
oxygen gas emission was observed yet the dissolution L at 323 K in solutions containing 3 mol/L HAc. Only 40%
rate was high without stirring except for simply shaking of lead was extracted using 0.1 mol/L H2O2 within 90 min,
to homogenize the solution prior to the sampling. while the extraction was almost completed by leaching
The effect of temperature on dissolution was carried with 0.5 mol/L H2O2. It was observed that as the H2O2
out in the 303343 K temperature range again in concentration increased, oxidation of galena together with
solutions containing 3 mol/L HAc and 0.5 mol/L H2O2. dissolution of lead sulphate significantly accelerated
It was observed that fast leaching rates occurred within (Fig. 3). However, the rate drastically decreased when
the first 20 min. (Fig. 2). The fraction of lead extraction using 1 and 2 mol/L H2O2 after 20 min of leaching,
after 20 min was 0.69, 0.77 and 0.85 at 323, 333 and suggesting the same phenomenon observed during leach-
343 K, respectively. After that time, the leaching slows ing at higher temperatures, e.g., fast precipitation of lead
down or stops suggesting that the lead sulphate produced sulphate inhibits further oxidation of unreacted galena.

Fig. 3. Effect of H2O2 concentration on galena dissolution (no stirring; Fig. 5. Variation in surface chemical reaction model equation with time
HAc: 3 mol/L; temperature: 323 K). at various temperatures (symbols as in Fig. 2).
 S. Aydoan et al. / Hydrometallurgy 89 (2007) 189195 193

reaction products are crucial for a complete understand-

ing of the system. In order to elucidate the reaction
mechanism of galena dissolution with hydrogen perox-
ide in acetic acid solutions, the shrinking core model was
utilized (Habashi, 1999). According to the shrinking core
model, if the reaction of galena can be represented as,

Afluid bBsolid Fluid products Solid products 10

and the rate of the reaction is controlled by surface

chemical reaction, then the integral rate equation is
constituted as follows:

kc MB CA
1  1  X1=3 t kr t 11
qB ar0
Fig. 6. Arrhenius plot of the data presented in Fig. 5. where X is the fraction reacted, kc is the kinetic constant,
MB is the molecular weight of the solid, CA is the
To establish the effect of acetic acid, four different concentration of the dissolved lixiviant A in the bulk of
HAc concentrations were examined in the range of 0.5 the solution, a is the stoichiometric coefficient of the
5.0 mol/L at 323 K in solutions containing 0.5 mol/L reagent in the leaching reaction, r0 is the initial radius of
H2O2. Increasing the HAc concentration up to 3 mol/L the solid particle, t is the reaction time, B density of the
increased the rate of sulphate dissolution (Fig. 4). But a solid and kr is the rate constant calculated from Eq. (11).
further increase in HAc concentration caused a decrease Eq. (11) was applied to the data obtained from each
in the dissolution rate. The slower kinetics with 5 mol/L temperature which gives a straight line (Fig. 5). The rate
acetic acid is likely due to the increased viscosity and constants were calculated as slopes of the straight lines.
lower mass transport of H2O2 through the solution and By using these values, the Arrhenius equation k = AeE/RT,
surface layer. This effect could also rationalize why the (plotted in Fig. 6), gives an activation energy of 65.6 kJ
reaction order of HAc is unusually low. The change in mol 1 that supports the proposed surface chemical
the rate at higher reagent concentrations has been reaction as the rate controlling step. The activation
claimed to be due to the larger quantity of solid lead energies of galena dissolution reported by various authors
sulphate produced. are summarized in Table 1 and compares favourably.

3.3. Dissolution kinetics

Understanding the mechanism of a leaching system is

the main consideration, while a knowledge of the
kinetics of the rate controlling processes and solid

Table 1
Reported activation energies for leaching of galena with different
leaching media
Leaching reagent Activation energy Reference
(kJ mol 1)
Ferric fluosilicate 62.1 Chen and Dreisinger
(1994)
Ferric chloride 4045 Dutrizac (1986)
Cupric chloride 33 Dutrizac (1989)
Ferric nitrate 47 Fuerstenau et al.
(1987)
HCl 64.4 Nunez et al. (1988)
HClO4 71.5
Fig. 7. Determination of reaction order for galena dissolution with
HBr 66.5
respect to H2O2 and HAc concentrations.
194 S. Aydoan et al. / Hydrometallurgy 89 (2007) 189195

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