ISSN 0104-6632
Printed in Brazil
www.abeq.org.br/bjche
Vol. 32, No. 03, pp. 629 - 635, July - September, 2015
dx.doi.org/10.1590/0104-6632.20150323s00003393
*To whom correspondence should be addressed
This is an extended version of the manuscript presented at the
VII Brazilian Congress of Applied Thermodynamics – CBTermo 2013, Uberlândia, Brazil
Brazilian Journal
of Chemical
Engineering
SOLUBILITIES AND PHYSICAL PROPERTIES OF
SATURATED SOLUTIONS IN THE COPPER
SULFATE + SULFURIC ACID + SEAWATER
SYSTEM AT DIFFERENT TEMPERATURES
F. J. Justel, M. Claros and M. E. Taboada
*
Department of Chemical Engineering, University of Antofagasta, Angamos 601, Antofagasta, Chile.
E-mail: franciscajustel@gmail.com; mclaro[email protected]
*
E-mail: [email protected]
(Submitted: March 28, 2014 ; Revised: September 25, 2014 ; Accepted: September 26, 2014)
Abstract - In Chile, the most important economic activity is mining, concentrated in the north of the country.
This is a desert region with limited water resources; therefore, the mining sector requires research and
identification of alternative sources of water. One alternative is seawater, which can be a substitute of the
limited fresh water resources in the region. This work determines the influence of seawater on the solid−liquid
equilibrium for acid solutions of copper sulfate at different temperatures (293.15 to 318.15 K), and its effect
on physical properties (density, viscosity, and solubility). Knowledge of these properties and solubility data
are useful in the leaching process and in the design of copper sulfate pentahydrate crystallization plants from
the leaching process using seawater by means of the addition of sulfuric acid.
Keywords: Seawater; Copper sulfate; Sulfuric acid.
INTRODUCTION
The most important economic activity in Chile is
mining. Currently, there is a worldwide shortage of
available fresh water. Therefore, mining industries
are developing new methods to optimize water use
(Torres et al., 2013). In northern Chile, for example,
certain mining companies are using raw seawater in
their production processes (Rocha et al., 2013) and
purified seawater by reverse osmosis (Philippe et al.,
2010). In a mining process, the solid−liquid equilib-
rium and physical properties of solutions change
upon seawater incorporation, especially the density
and viscosity; which are used in pipe-sizing and
pumping calculations. These properties are related to
the cost of energy required to bring seawater to
mining operations, usually farther than 120 km
(Hernández et al., 2012). Copper sulfate pentahydrate
(Blue vitriol) is a copper salt with a wide range of
commercial applications: in agriculture as a pesti-
cide, fungicide, feed, and soil additive (Milligan and
Moyer, 1975); in mining, it is used as a floatation
reagent in recovery of zinc and lead (De Juan et al.,
1999); as a blue and green pigment in dyes, as a print
toner in photography, in the production of other cop-
per compounds, and in leather tanning (Richardson,
1997).
Actually, the production process of copper sulfate
pentahydrate includes the following steps: 1) Heap
leaching, where copper is obtained from oxidized
ores using a mixture of sulfuric acid and water; 2)
Solvent extraction, where copper is extracted from
the leaching solution by mixing with a product called
organic; 3) Crystallization, where the copper-loaded
organic is discharged using a concentrated acid solu-
tion; 4) Re-crystallization, where copper sulfate is
630 F. J. Justel, M. Claros and M. E. Tabeada
Brazilian Journal of Chemical Engineering
dissolved in fresh water at a temperature of 80 – 90 °C,
and then crystallized by cooling to 25 – 30 °C, in order
to remove the impurities (Tabilo, 2012).
Copper sulfate in distilled water solutions has
been investigated for crystallization, supersaturation,
solid-liquid equilibrium, and properties (De Juan et
al., 1999; Domic, 2001, and Milligan and Moyer,
1975). In these studies, crystallization conditions of
copper sulfate solutions were determined as a func-
tion of both temperature and sulfuric acid concentra-
tion. In order to optimize the water use, it is interest-
ing to investigate the influence of seawater on the
copper sulfate crystallization process. In the litera-
ture, there is a publication available of the behavior
of copper sulfate in a seawater system (Hernández et
al., 2012); this paper provides solubilities and physi-
cal properties data of CuSO
4
in seawater at pH 2.
The present work studies the effect of seawater
(3.5% salinity) on the solid-liquid equilibrium of
copper sulfate in acid solutions at different tempera-
tures (from 293.15 K to 318.15 K). This temperature
range was chosen because is within the range in
which the crystallization process operates. In addi-
tion, the physical properties, density, and viscosity of
the saturated solution are experimentally measured
and correlated with empirical equations, finding a
good agreement.
From the results obtained in this investigation,
and in order to minimize the use of fresh water, the
next step of this work is to perform the copper sul-
fate crystallization process from leaching solutions
using seawater to study the effect of the ions present
in seawater on the habit and size of copper sulfate
pentahydrate crystals.
MATERIALS AND METHODS
Reagents
Analytical grade reagents were used (copper (II)
sulfate pentahydrate, Merck, 99%; absolute sulfuric
acid, Merck, 95 to 97%, absolute). The experiments
were performed using filtered natural seawater ob-
tained from San Jorge Bay, Antofagasta, Chile. Table 1
shows the composition of the seawater, obtained by
chemical analysis, used in this work (Hernández et
al., 2012).
Table 1: Individual ions in seawater from Bahía San
Jorge, Chile (mg·L
−1
).
Na
+
Mg
+2
Ca
+2
K
+
B
+3
Cu
2+
Cl
-
SO
4
-2
HCO
3
-
NO
3
-
9480 1190 386 374 4.6 0.072 18765 2771 142 2.05
Apparatus
The solutions were prepared using an analytical
balance (Mettler Toledo Co. model AX204, with
0.07 mg precision). To obtain the phase equilibrium
data at different temperatures, a rotary thermostatic
bath (to ± 0.1 K, 50 rpm) with a capacity of ten 90 mL
glass flasks was used. The densities were measured
using a Mettler Toledo DE-50 vibrating tube den-
simeter with ± 5·10
−2
kg·m
−3
precision.
The kinematic viscosities were obtained using a
calibrated micro-Ostwald viscometer with a Schott-
Gerate automatic measuring unit (model AVS 310),
equipped with a thermostat (Schott-Gerate, model
CT 52) for temperature regulation. The absolute vis-
cosities were calculated by multiplying the kinematic
viscosity and the respective density.
Procedures
Equilibrium Time Determination
The equilibrium time was determined at 298.15 K.
Acidic seawater was prepared by adding sulfuric
acid to seawater and stirring the solution until it
reached pH 2; this pH was used because it is similar
to the pH levels in copper mining operations. The
masses of copper (II) sulfate pentahydrate in the
solution (seawater at pH 2) were measured. An ex-
cess of copper (II) sulfate pentahydrate was added to
ensure saturation of the solution. Several saturated
solutions (CuSO
4
+ acid seawater) were placed in
closed glass flasks and immersed in a rotary water
bath at 298.15 K, these solutions were mechanically
shaken. Every hour, the rotation was stopped, one
flask was removed from the bath and, maintaining
the work temperature (298.15 K) and using a syringe
filter (to ensure that no copper sulfate pentahydrate
solid was present in the solution), the solution den-
sity was measured. The equilibrium time was deter-
mined when the solutions that were taken at different
times (every one hour), reached constant densities.
Measurement of Physical Properties in Different
Conditions
After the equilibrium time was determined, ten so-
lutions (CuSO
4
+ acid seawater) at different acid con-
centrations were prepared.
These solutions were stirred in a rotatory water
bath for 8 hours (equilibrium time). The rotation was
then stopped and the solutions were decanted, main-
taining the work temperature. Then, in the thermo-
static bath, and using a syringe filter at a slightly
Solubilities and Physical Properties of Saturated Solutions in the Copper Sulfate + Sulfuric Acid + Seawater System at Different Temperatures 631
Brazilian Journal of Chemical Engineering Vol. 32, No. 03, pp. 629 - 635, July - September, 2015
elevated temperature (to prevent salt precipitation at
lower temperatures), the solutions (without solid)
were obtained for each equilibrium point.
Physical properties (density and viscosity) were
measured in triplicate for each solution. On the other
hand, copper (II) concentration was measured in
duplicate by atomic absorption and the CuSO
4
solu-
bility was obtained by stoichiometry.
All measurements of the physical properties and
solubilities were performed at four different tempera-
tures: 293.15, 298.15, 308.15, and 318.15 K.
RESULTS AND DISCUSSION
Experimental Results
The solubilities, densities, and viscosities are shown
in Table 2, for the system studied at different tem-
peratures and acid concentrations.
Solubilities
Table 2 shows the solubility results, expressed as
mass fraction of copper sulfate (wCuSO
4
) for differ-
ent acid mass fractions (wH
2
SO
4
). A significant de-
crease in solubility was clearly observed with the
increase of sulfuric acid in solution; this behavior
was observed for all the temperatures. This behavior
of the solubility is due to the common ion effect,
because copper sulfate and sulfuric acid share the
same SO
4
2-
ion (Cisternas, 2009).
Solubility, expressed as a mass fraction, decreases
from approximately 0.1684 to 0.0813 at 293.15 K;
0.1763 to 0.0956 at 298.15 K; 0.2020 to 0.1330 at
308.15 K; and 0.2335 to 0.1526 at 318.15 K. These
results show that sulfuric acid might be used as an
advantageous co-solvent in the crystallization proc-
esses design of copper sulfate pentahydrate.
The solubility results of the saturated solution
may be correlated with the sulfuric acid composition
by the following equation:
0.5
2
=+×
s
ABw (1)
where s is the solubility in mass fraction,
2
w
repre-
sents H
2
SO
4
mass fraction, and A and B are fitting
parameters.
Physical Properties
Table 2 presents the densities and viscosities of the
saturated solutions for the copper sulfate + seawater +
sulfuric acid system.
Table 2: Solubility (wCuSO
4
), density (ρ), and
viscosity (η) for saturated solutions of copper sul-
fate in seawater at various acid concentrations
and temperatures.
wH
2
SO
4
wCuSO
4
ρ/g·cm
−3
η/mPa·s
293.15 K
0.0036 0.1684 1.21779 2.549
0.0075 0.1669 1.21679 2.514
0.0152 0.1620 1.21807 2.485
0.0235 0.1570 1.21861 2.448
0.0396 0.1506 1.21897 2.379
0.0608 0.1369 1.22326 2.370
0.0828 0.1260 1.22722 2.353
0.1056 0.1179 1.22998 2.342
0.1298 0.1043 1.23603 2.361
0.1810 0.0813 1.25036 2.432
298.15 K
0.0035 0.1763 1.22742 2.368
0.0071 0.1745 1.22705 2.345
0.0143 0.1705 1.22707 2.293
0.0221 0.1661 1.22731 2.261
0.0375 0.1572 1.22838 2.213
0.0571 0.1477 1.23131 2.175
0.0775 0.1388 1.23369 2.164
0.0994 0.1284 1.23776 2.160
0.1214 0.1182 1.24130 2.165
0.1691 0.0956 1.25438 2.204
308.15 K
0.0034 0.2020 1.25381 2.113
0.0070 0.1995 1.25379 2.114
0.0142 0.1952 1.25285 2.069
0.0219 0.1898 1.25355 2.032
0.0369 0.1832 1.25575 1.982
0.0561 0.1754 1.25452 1.949
0.0761 0.1659 1.25710 1.927
0.0981 0.1558 1.25794 1.910
0.1183 0.1498 1.26527 1.910
0.1640 0.1330 1.27592 1.937
318.15 K
0.0032 0.2335 1.28672 2.014
0.0065 0.2316 1.28680 2.004
0.0132 0.2252 1.28478 1.966
0.0203 0.2228 1.28468 1.934
0.0350 0.2076 1.28364 1.866
0.0527 0.2039 1.28466 1.840
0.0711 0.1973 1.28634 1.801
0.0940 0.1758 1.28865 1.777
0.1134 0.1703 1.29342 1.797
0.1579 0.1526 1.30287 1.811
The values for density and viscosity were correlated
as a function of copper sulfate and sulfuric acid com-
position following Equations (2) and (3), respectively:
(
)
0.5 2.5
11 2
exp ln=+× +×ABw w Cw
ρ
(2)
(
)
2.5 2.5 3.5
12 21
exp =+×+×+××ABw C wDww
η
(3)
where,
1
w represents the CuSO
4
mass fraction,
2
w
represents sulfuric acid mass fraction, and
A, B, C,
632 F. J. Justel, M. Claros and M. E. Tabeada
Brazilian Journal of Chemical Engineering
and D are fitting parameters. The units for density
and viscosity used in these equations are g·cm
−3
and
mPa·s, respectively.
The parameter values were obtained by means of
the least squares method, for all experimental data,
and are shown in Table 3.The absolute average de-
viations (AAD) for the fitted parameters are also
presented.
Table 3: Parameters values for density, viscosity
and solubility for saturated copper sulfate in acidic
seawater system.
Property
A B C D
AAD
293.15 K
ρ/g·cm
-3
-0.2322 -0.5861 2.6112 0.0005
η/mPa·s 0.8566 0.5137 1.6012 -1198.5 0.0056
w 0.1905 -0.2353 0.0043
298.15 K
ρ/g·cm
−3
-0.0399 -0.3351 1.9916 0.0004
η/mPa·s 0.9453 -0.4092 -4.2035 -1455.1 0.0029
w 0.1966 -0.2238 0.0038
308.15 K
ρ/g·cm
−3
0.2287 0.0037 1.6397 0.0007
η/mPa·s 0.6205 0.6883 3.2718 -607.6 0.0048
w 0.2181 -0.1980 0.0023
318.15 K
ρ/g·cm
−3
0.3255 0.1054 1.7079 0.0007
η/mPa·s 0.3784 1.4117 7.1461 -311.5 0.0055
w 0.2530 -0.2401 0.0037
exp
()/=−
cal
AAD s s n
,
where
n is the number of experimental
points.
The results show that these equations fit satisfac-
torily the density, viscosity, and solubility experi-
mental data.
The solubility of copper sulfate in acidic seawater
with different concentrations of sulfuric acid and
physical properties of the saturated solutions, at four
different temperatures (293.15, 298.15, 308.15, and
318.15 K) are shown in Figures 1 to 3, along with
correlated data.
It is possible to note that, for all the temperatures,
the solubility decreases with increasing acid concen-
tration. Also, the figure shows that solubility levels
increased with temperature; this is because, as the
solution temperature increases, the average kinetic
energy of the molecules that make up the solution
also increases. This increase allows the solvent
molecules to break apart the solute molecules more
effectively that are held together by intermolecular
attractions.
Figure 2 compares the density of saturated so-
lutions of copper sulfate in acid seawater at four
different temperatures (293.15, 298.15, 308.15, and
318.15 K).
Figure 1: Solubility for the saturated solutions
(CuSO
4
+ acid seawater): ■, 293.15; ♦, 298.15 K; ▲,
308.15 K; ●, 318.15 K; ─, correlations with Eq. (1).
Figure 2: Density for the saturated solutions (CuSO
4
+ acid seawater): ■, 293.15 K; ♦, 298.15 K;
▲,
308.15 K; ●, 318.15 K; ─, correlations with Eq. (2).
As can be seen, there is a slight decrease in the
density of the solutions at low acid concentrations.
However, at a certain point, it begins to increase.
This behavior is better observed at higher tempera-
tures (at low temperatures this decrease is not clear).
This phenomenon could be attributed, at low acid
concentrations, to the copper sulfate solubility de-
crease, and therefore, the density; however, as the
acid concentration increases, the solution density be-
gins to increase, due to the high density of the sulfu-
ric acid.
Figure 2 shows that the density values increased
slightly with temperature.
Figure 3 compares the viscosity of saturated so-
lutions of copper sulfate in acid seawater at four
different temperatures (293.15, 298.15, 308.15, and
318.15 K).
It is possible to note that, for all the temperatures,
viscosity values decrease with increasing acid con-
centration. Also, the figure shows that viscosity levels
decrease slightly with increasing temperature. This
behavior is expected, as observed in the work of
Solubilities and Physical Properties of Saturated Solutions in the Copper Sulfate + Sulfuric Acid + Seawater System at Different Temperatures 633
Brazilian Journal of Chemical Engineering Vol. 32, No. 03, pp. 629 - 635, July - September, 2015
Hernández, Hotlos and Price (Hotlos and Jaskula,
1988; Price and Davenport, 1980; Hernández
et al.,
2012).
Figure 3: Viscosity for the saturated solutions
(CuSO
4
+ acid seawater): ■, 293.15 K; ♦, 298.15 K;
▲, 308.15 K; ●, 318.15 K; ─, correlations with Eq. (3).
These results confirmed the good fit between ex-
perimental values for concentrations of the salt and
the physical properties of the saturated solutions at
four temperature levels, in a broad range of acid
concentrations.
On the other hand, looking for a single equation
that includes the effect of different temperatures, we
used the empirical models proposed in the work of
Milligan (Milligan and Moyer, 1975); to estimate the
density and solubility of the system CuSO
4
- H
2
SO
4
-
H
2
O at different temperatures (Equations (4) and
(5)). The parameters of these equations were ad-
justed in acid seawater; for
Y
0
, solubility values of
CuSO
4
·5H
2
O in fresh water from the literature
(Linke and Seidell, 1965) were utilized.
The proposed equations are shown below:
22 2 2 2
(( ) )
2
1
ln
+
⎡⎤
=+
⎣⎦
CA C a X B
ee
C
ρ
(4)
10
11 1 11
()
() )
1
1
1
ln
⋅+
⋅+
⎡⎤
=−+
⎣⎦
CAXY
CAXB CB
Yeee
C
(5)
where:
Y = mass percentage of CuSO
4
·5H
2
O in saturated
solution
ρ
= density of saturated solution in g·cm
-3
X
= mass percentage of H
2
SO
4
in solution
T = temperature in °C
0.01316
0
20.37=
T
Ye
= mass percentage of CuSO
4
·5H
2
O
in saturated solution with no acid content
1
()
11
=−
bT
Aae
(6)
11
1
()( )
11
1
−
−−
⎡
⎤
=+
⎣
⎦
dTe
BC e
(7)
1
2
11 1 1
1( )
−
⎡
⎤
=+ −
⎣
⎦
Cf gTh
(8)
(2)
2
()
22
=
d
cT
Abe (9)
222
=+
T
Bef (10)
1
2
22 2 2
1( )
−
⎡
⎤
=+−
⎣
⎦
Cg hTi
(11)
The parameter values are shown in Table 4. The
absolute average deviations (AAD) for the fitted
parameters are also presented.
Table 4: Parameter values for density and solu-
bility for saturated copper sulfate in acidic sea-
water system.
Property Parameters Temperature AAD
a
1
0.1165
b
1
0.0943
293.15 K 0.2222
c
1
21.7293
d
1
0.0931
298.15 K 0.3287
e
1
30.6388
f
1
16.7409
308.15 K 0.2861
g
1
2.0939
Solubility
h
1
15.0081
318.15 K 0.4501
a
2
0.0094
b
2
1.187
293.15 K 0.0008
c
2
0.00017
d
2
1.5986
298.15 K 0.0005
e
2
0.9897
f
2
0.00162
308.15 K 0.0011
g
2
15.51
h
2
0.00038
Density
i
2
34.16
318.15 K 0.0014
exp
()/=−
cal
AAD s s n
,
where n is the number of experimental points.
In Figures 4 and 5, the density and solubility values
of saturated solutions of copper sulfate in acid sea-
water at four different temperatures (293.15, 298.15,
308.15, and 318.15 K), and the correlations with the
Equations (4) and (5) can be seen.
The experimental values for density, and solubility
in the saturated solutions were correlated adequately
using Equations (4) and (5) shown previously.
Also, the comparison between experimental values
of solubility for saturated solutions of copper sulfate
in seawater, with data of copper sulfate in fresh water
presented by Milligan and Moyer (1975) as a func-
tion of acid concentration at four different tempera-
tures 293.15 K, 298.15 K, 308.15 K, and 318.15 K is
performed and the results are shown in Figure 6.
634 F. J. Justel, M. Claros and M. E. Tabeada
Brazilian Journal of Chemical Engineering
Figure 4: Density for the saturated solutions (CuSO
4
+ acid seawater): ■, 293.15 K; ♦, 298.15 K;
▲,
308.15 K; ●, 318.15 K; ─, correlations with Eq. (4)
(Milligan and Moyer, 1975).
Figure 5: Solubility for the saturated solutions
(CuSO
4
+ acid seawater): ■, 293.15; ♦, 298.15 K; ▲,
308.15 K; ●, 318.15 K; ─, correlations with Eq. (5)
(Milligan and Moyer, 1975).
Figure 6: Solubility for the saturated solutions
(CuSO
4
+ acid seawater): ■, 293.15; ♦, 298.15 K; ▲,
308.15 K; ●, 318.15 K. Black lines show fresh water
data at different temperatures (Milligan and Moyer,
1975).
It is possible to note that the solubility of copper
sulfate pentahydrate in seawater is lower than the
solubility of this salt in fresh water. This phenome-
non is due to the presence of salts in seawater, which
contribute to decrease the solubility of copper sul-
fate. This is because the water activity of seawater is
lower than the water activity of fresh water and
therefore the solubility is lower. Furthermore, as
mentioned in Table 1, seawater composition presents
2771 mg·L
-1
of SO
4
2-
ion, which could be responsible
of the decrease in the copper sulfate solubility in this
medium, due to the common ion effect. This can be
the reason why the average deviation is higher with
this equation with respect to the equation proposed in
this work.
CONCLUSIONS
With increasing temperature and acid concentra-
tion, an increase is observed in the density of the
solutions, and there is a slight decrease in the density
of the solutions at low acid concentrations.
With increasing acid concentration and tempera-
ture, there is a decrease in the solution viscosity.
With increasing acid concentration, there is a de-
crease in the solubility; on the other hand, when the
temperature increases, the solubility increases.
The experimental values for density, viscosity,
and solubility in the saturated solutions, were ade-
quately correlated using Equations (1) to (3) proposed
in this work, with absolute average deviations for
density, viscosity, and solubility of 0.0005, 0.0056,
and, 0.0043, respectively, at 293.15 K; 0.0004, 0.0029
and, 0.0038, respectively, at 298.15 K; 0.0007, 0.0048,
and 0.0023, respectively, at 308.15 K; and 0.0007,
0.0055, and 0.0037, respectively, at 318.15 K.
The experimental values for density, and solubility
in the saturated solutions were correlated adequately
using Equations (4) and (5), with absolute average
deviations for density, and solubility of 0.0008, and
0.2222, respectively, at 293.15 K; 0.0005, and 0.3287,
respectively, at 298.15 K; 0.0011, and 0.2861, re-
spectively, at 308.15 K; and 0.0014, and 0.4501, re-
spectively, at 318.15 K.
The solubility of copper sulfate pentahydrate in
seawater is lower than the solubility in fresh water
due to the presence of salts in seawater, which con-
tribute to decrease the solubility of copper sulfate.
ACKNOWLEDGMENT
This work was supported by Fondecyt Project
1140169. Francisca Justel gratefully acknowledges
the CONICYT grant.
Solubilities and Physical Properties of Saturated Solutions in the Copper Sulfate + Sulfuric Acid + Seawater System at Different Temperatures 635
Brazilian Journal of Chemical Engineering Vol. 32, No. 03, pp. 629 - 635, July - September, 2015
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