SOLUBILITIES AND PHYSICAL PROPERTIES OF SATURATED SOLUTIONS IN THE COPPER SULFATE + SULFURIC ACID + SEAWATER SYSTEM AT DIFFERENT TEMPERATURES

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 equilibrium 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 pesticide, 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 copper 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 solution; 4) Re-crystallization, where copper sulfate is dissolved in fresh water at a temperature of 80 - 90 ºC, and then crystallized by cooling to 25 - 30 ºC, 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 function of both temperature and sulfuric acid concentration. In order to optimize the water use, it is interesting to investigate the influence of seawater on the copper sulfate crystallization process. In the literature, there is a publication available of the behavior of copper sulfate in a seawater system (Hernández et al., 2012); this paper provides solubilities and physical properties data of CuSO₄ 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 temperatures (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 addition, 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 sulfate 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 obtained 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	Cu2+	CI-	SO4-2	HCO3-	NO3-
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 densimeter 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 viscosities 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 excess of copper (II) sulfate pentahydrate was added to ensure saturation of the solution. Several saturated solutions (CuSO₄ + 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 density was measured. The equilibrium time was determined 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 solutions (CuSO₄ + acid seawater) at different acid concentrations 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, maintaining the work temperature. Then, in the thermostatic bath, and using a syringe filter at a slightly 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 CuSO₄ solubility was obtained by stoichiometry.

All measurements of the physical properties and solubilities were performed at four different temperatures: 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 temperatures and acid concentrations.

Table 2
Solubility (wCuSO₄), density (ρ), and viscosity (η) for saturated solutions of copper sulfate in seawater at various acid concentrations and temperatures.

wH2SO4	wCuSO4	ρ/g·cm3	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

Solubilities

Table 2 shows the solubility results, expressed as mass fraction of copper sulfate (wCuSO₄) for different acid mass fractions (wH₂SO₄). A significant decrease 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 SO₄^2- 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 processes design of copper sulfate pentahydrate.

The solubility results of the saturated solution may be correlated with the sulfuric acid composition by the following equation:

where s is the solubility in mass fraction, w₂ represents H₂SO₄ 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.

The values for density and viscosity were correlated as a function of copper sulfate and sulfuric acid composition following Equations (2) and (3), respectively:

where, w₁ represents the CuSO₄ mass fraction, w₂ represents sulfuric acid mass fraction, and A, B, C, 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 deviations (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

• AAD = |(s^exp - s^cal)/n|^, where n is the number of experimental points.

The results show that these equations fit satisfactorily the density, viscosity, and solubility experimental 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.

Figure 1
Solubility for the saturated solutions (CuSO₄ + acid seawater): ■, 293.15; ♦, 298.15 K; ▲, 308.15 K; ●, 318.15 K; ─, correlations with Eq. (1).

It is possible to note that, for all the temperatures, the solubility decreases with increasing acid concentration. 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 solutions of copper sulfate in acid seawater at four different temperatures (293.15, 298.15, 308.15, and 318.15 K).

Figure 2
Density for the saturated solutions (CuSO₄ + 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 temperatures (at low temperatures this decrease is not clear).

This phenomenon could be attributed, at low acid concentrations, to the copper sulfate solubility decrease, and therefore, the density; however, as the acid concentration increases, the solution density begins to increase, due to the high density of the sulfuric acid.

Figure 2 shows that the density values increased slightly with temperature.

Figure 3 compares the viscosity of saturated solutions of copper sulfate in acid seawater at four different temperatures (293.15, 298.15, 308.15, and 318.15 K).

Figure 3
Viscosity for the saturated solutions (CuSO₄ + acid seawater): ■, 293.15 K; ♦, 298.15 K; ▲, 308.15 K; ●, 318.15 K; ─, correlations with Eq. (3).

It is possible to note that, for all the temperatures, viscosity values decrease with increasing acid concentration. Also, the figure shows that viscosity levels decrease slightly with increasing temperature. This behavior is expected, as observed in the work of Hernández, Hotlos and Price (Hotlos and Jaskula, 1988; Price and Davenport, 1980; Hernández et al., 2012).

These results confirmed the good fit between experimental 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 CuSO₄ - H₂SO₄ -H₂O at different temperatures (Equations (4) and (5)). The parameters of these equations were adjusted in acid seawater; for Y₀, solubility values of CuSO₄·5H₂O in fresh water from the literature (Linke and Seidell, 1965) were utilized.

The proposed equations are shown below:

where:

Y = mass percentage of CuSO₄·5H₂O in saturated solution

ρ= density of saturated solution in g·cm^-3

X = mass percentage of H₂SO₄ in solution

T = temperature in ºC

Y₀ =20.37e^0.01316T = mass percentage of CuSO₄·5H₂O

in saturated solution with no acid content

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 solubility for saturated copper sulfate in acidic seawater system.

Property	Parameters		Temperature	AAD
Solubility	a1 b1	0.1165 0.0943	293.15 K	0.2222
	c1 d1	21.7293 0.0931	298.15 K	0.3287
	e1 f1	30.6388 16.7409	308.15 K	0.2861
	g1 h1	2.0939 15.0081	318.15 K	0.4501
Density	a2 b2	0.0094 1.187	293.15 K	0.0008
	C2 d2	0.00017 1.5986	298.15 K	0.0005
	e2 f2	0.9897 0.00162	308.15 K	0.0011
	g2 h2 i2	15.51 0.00038 34.16	318.15 K	0.0014

• AAD = |(s^exp - s^ca1)/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 seawater 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.

Figure 4
Density for the saturated solutions (CuSO₄ + 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 (CuSO₄ + acid seawater): ■, 293.15; ♦, 298.15 K; ▲, 308.15 K; ●, 318.15 K; ─, correlations with Eq. (5) (Milligan and Moyer, 1975).

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 function of acid concentration at four different temperatures 293.15 K, 298.15 K, 308.15 K, and 318.15 K is performed and the results are shown in Figure 6.

Figure 6
Solubility for the saturated solutions (CuSO₄ + 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 phenomenon is due to the presence of salts in seawater, which contribute to decrease the solubility of copper sulfate. 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 SO₄^2- 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 concentration, 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 temperature, there is a decrease in the solution viscosity.

With increasing acid concentration, there is a decrease 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 adequately 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, respectively, at 308.15 K; and 0.0014, and 0.4501, respectively, 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 contribute to decrease the solubility of copper sulfate.

ACKNOWLEDGMENT

This work was supported by Fondecyt Project 1140169. Francisca Justel gratefully acknowledges the CONICYT grant.

REFERENCES

• Cisternas, L., Diagramas de fases y su aplicación. Reverte (2009). (In Spanish).
• De Juan, D., Meseguer, V. and Lozano, L., Una contribución al estudio de la solubilidad del CuSO₄·5H₂O en medio acuoso. Revista de Metalurgia, 35, 4752 (1999). (In Spanish).
• Domic, E., Hidrometalurgia: Fundamentos, Procesos y Aplicaciones. Chile, Andros Impresos (2001).
• Hernández, P., Galleguillos, H., Graber, T., Flores, E. and Taboada, M., Effect of seawater on the solubility and physicochemical properties of acidic copper sulfate solutions. Journal of Chemical & Engineering Data, 57(9), 2430-2436 (2012).
• Hotlos, J. and Jaskuła, M., Densities and viscosities of CuSO₄ - H₂SO₄ - H₂O solutions. Hydrometallurgy, 21(1), 1-7 (1988).
• Linke, W. and Seidell, A., Solubilities of Inorganic and Metal Organic Compounds. American Chemical Society, Washington DC (1965).
• Milligan, D. and Moyer, H., Crystallization in the copper sulphate - sulphuric acid - water system. Engineering and Mining Journal, 176(10), 85-89 (1975).
• Philippe, R., Dixon, R. and DalPozzo, S., Seawater supply options for the mining industry. WIM2010 Proceeding of the Second International Congress on Water Management in the Mining Industry, Santiago, Chile, 13-20 (2010).
• Price, D. and Davenport, W., Densities, electrical conductivities and viscosities of CuSO₄/H₂SO₄ solutions in the range of modern electrorefining and electrowinning electrolytes. Metallurgical Transactions, B, 11(1), 159-163 (1980).
• Richardson, H., Handbook of Copper Compounds and Applications. CRC Press (1997).
• Rocha, O., Claros, M., Graber, T., Flores, E. and Taboada, M., Solid-liquid equilibrium and process design of CuSO₄ + NaCl + (H₂O or H₂SO₄/H₂O) systems at 298.15 K. Industrial & Engineering Chemistry Research, 52(20), 6803-6811 (2013).
• Tabilo, F., Proyecto Anico. Thesis, Universidad de Chile (2012). (In Spanish).
• Torres, M., Meruane, G., Graber, T., Gutiérrez, P. and Taboada, M., Recovery of nitrates from leaching solutions using seawater. Hydrometallurgy, 133, 100-105 (2013).

Publication Dates

• Publication in this collection
  Sept 2015

History

• Received
  28 Mar 2014
• Reviewed
  25 Sept 2014
• Accepted
  26 Sept 2014