SolidâLiquid Equilibrium and Process Design of CuSO4 + NaCl +

Article pubs.acs.org/IECR

Solid−Liquid Equilibrium and Process Design of CuSO4 + NaCl + (H2O or H2SO4/H2O) Systems at 298.15 K O. A. Rocha,† M. Claros,† T. A. Graber,†,‡ E. K. Flores,‡ and M. E. Taboada*,†,‡ †

Universidad de Antofagasta, Departamento de Ingeniería Química, Av. Angamos 601, Antofagasta, Chile Centro de Investigación Científico y Tecnológico para la Minería (CICITEM), Av. José Miguel Carrera 1701, 4° piso, Antofagasta, Chile

‡

ABSTRACT: The low availability of fresh water in Chilean deserts has encouraged research into new alternative water sources in the mining industry. One of alternative is the use of saline water; however, there are few data about this new approach. For the production of copper sulfate pentahydrated (CuSO4·5H2O) crystals, the effect of NaCl in the crystallization area must be known; therefore, the solid−liquid equilibrium of the CuSO4 + NaCl + (H2O or H2SO4/H2O) system at 298.15 K was experimentally determined by the wet residue method. The density and refractive index of saturated solutions were also measured. The addition of NaCl has a clear effect on the solid−liquid equilibrium, promoting the formation of solid phases. In the phase equilibrium diagram that includes sulfuric acid, the pH affects the solubility curve. Using the information of the new phase diagrams, six simulation cases, varying in NaCl content and the addition of sulfuric acid in the feed, were evaluated in terms of mass and energy balance. The addition of sulfuric acid increases the total fluxes but decreases the heat requirement. Contrary to the expected results, the yields for the cases that include sulfuric acid were lower. The best results were obtained using a 50% NaCl pulp, which produced the lowest total flux and a high yield of 75.6%.

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including a Na2SO4·CuSO4·2H2O·nNa2SO4 solid solution, in the reciprocal aqueous system that was studied. Several studies have investigated the solid−liquid equilibrium and crystallization of the copper sulfate pentahydrate (CuSO4·5H2O).5−7 Giulietti et al.8 presented a study of batch cooling experiments, from 70 to 30 °C, some of which included 2% free acid. They concluded that programmed cooling produced larger crystals than linear cooling. On the other hand, the free acid added did not affect the final results. Domic9 also presents a complete description of the industrial crystallization process, along with useful parameters. Taboada et al.10 described the process of obtaining lithium hydroxide monohydrate by drowning-out crystallization, including an example of the application of a simple heuristic for salt crystallization. The solubility is evaluated as a function of temperature using mass balance and a regression model. Ethanol is used as the principal solvent, and the phase equilibrium of the LiOH + C2H5OH + H2O ternary system at 298.15 K is experimentally determined. Crystallization and solvent extraction are presented for the conceptual process design. This drowning-out crystallization process was proposed as an alternative for the purification of LiOH·H2O. Considering the process design with METSIM, Hernández et al.11 presented a scorodite crystallization process based on an experimentally determined phase diagram. This software is valuable for simulating and evaluating industrial process designs using experimental data and knowledge of certain industrial parameters.

INTRODUCTION

The mining activity with respect to the main mineral products of the Antofagasta region, copper and saline systems, is growing. This highly arid region and its water scarcity present an opportunity to develop new approaches for mining processes. The use of seawater has become a viable alternative in the mining industry. Many large mining companies, such as Minera Esperanza, Minera Michilla S.A.,1 and Las Luces (from the Las Cenizas group2), have incorporated seawater into their processes with successful results. Moreno et al.2 have described the Las Luces mineral process with seawater in milling and flotation in detail. Their operational results show that after 15 years, the metallurgic results remained unaffected by the salinity of the seawater and that the increase in the concentration of dissolved salts is mainly due to evaporation and water recirculation. Considering the equilibrium data for the ions involved in this system, Christov3 presented a thermodynamic study of the quaternary Na−Cu−Cl−SO4−H2O system at 298.15 K, including solubility simulations. The crystallization of simple salts CuCl2·2H2O and CuSO4·5H2O was assessed. The Pitzer model is also used for the thermodynamic simulation of the (NaCl + CuCl2)(aq), (Na2SO4 + CuSO4)(aq), and (CuCl2 + CuSO4)(aq) systems, in good agreement with the experimental solubility isotherms. Druzhinin and Kosyakina4 also studied quaternary systems composed of the same ions, finding the crystallization zone of the double salt CuSO4·Na2SO4·2H2O in the ternary CuSO4 + Na2SO4 + H2O system at 298.15 K. The double salt CuCl2·NaCl·2H2O was present in the CuCl2 + NaCl + H2O ternary system. Nine crystallization fields were determined, © 2013 American Chemical Society

Received: Revised: Accepted: Published: 6803

November 8, 2012 April 16, 2013 April 19, 2013 April 19, 2013 dx.doi.org/10.1021/ie3030642 | Ind. Eng. Chem. Res. 2013, 52, 6803−6811

Industrial & Engineering Chemistry Research

Article

The pH values were measured in an Accumet pH meter 50 with a measurement range from −2 to 20 between 268.15 and 378.15 K and ±0.002 precision.

To the best of our knowledge, there are no systematic studies of the effect of additional sodium chloride from seawater on the solid−liquid equilibrium during the production of copper sulfate pentahydrate crystals. Within the scope of a broader project comprised of several studies on the effect of seawater in different mining processes,12 CuSO4·5H2O crystals are important products and the effect of the NaCl of seawater or saline water in the crystallization process is of great interest; for this reason in this work, we present the experimental phase diagrams of the CuSO4 + NaCl + H2O and CuSO4 + NaCl + H2SO4/H2O systems at 298.15 K obtained by the wet residue method. In the second system, an aqueous solution with a H2SO4/H2O mass ratio of 0.1 corresponding to pH 2 was used; also, physical properties of the saturated solutions are presented for both systems. Furthermore, using the experimental data, a crystallization process is conceptually designed, and six different cases with industrial application are studied using simulation with METSIM, including mass and energy balance, based on the solid−liquid diagram experimentally obtained for the two systems.

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RESULTS AND DISCUSSION Phase Diagrams. The experimental measurements, mass fraction composition of cations (wCu2+ and wNa+) in the equilibrated solution, solid phase, density, and refractive index, together with their respective solid phase corresponding to the CuSO4 + NaCl + H2O ternary system at 298.15 K are listed in Table 1. Bibliographic data were used for solubility and properties of the binary system.13 Table 1. Experimental Solubilities, Refractive Indexes, and Densities of the CuSO4 + NaCl + H2O Ternary System at 298.15 K composition of the equilibrated solution

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EXPERIMENTAL PROCEDURES Determination of the CuSO4 + NaCl + (H2O or H2SO4/ H2O) System Diagrams at 298.15 K. To obtain the solubility isotherm of the ternary system, data from the CuSO4−H2O and NaCl−H2O binary systems at 298.15 K are used.13 Copper sulfate pentahydrate (CuSO4·5H2O, ≥99% pure) and sulfuric acid (H2SO4, ≥99% pure) were obtained from Merck. Milli-Q water was used for all experiments. Solutions were prepared at a H2SO4/H2O mass ratio of 0.1 at pH 2 or using pure H2O. The salts were dissolved in excess to ensure saturation. The masses were recorded using a Mettler Toledo AX204 analytical balance with a precision of 0.07 mg. The equilibrium data were obtained using a thermostatic rotary water bath with a holder containing ten 60 mL tubes. The system temperature is controlled at 298.15 K with an accuracy of 0.1 K. Samples were vigorously agitated over 4 days to ensure equilibrium. The agitation time was previously determined preparing various solutions at a constant concentration; the density was measured every 24 h, until this property reached a constant value indicating that equilibrium was reached. After being stirred, the samples were decanted for 24 h at a constant temperature. The obtained liquid is separated from the solid by filtration with Whatman membrane filters (GD/X, 25 mm), with a nominal pore size of 0.45 μm. The salt concentrations in the liquid and the wet solid phase were determined by atomic absorption spectrometry (AA) using a Varian model 220 Spectra AA instrument for copper and sodium ions. The double salt solid phases also were analyzed by X-ray diffraction in an automatic diffractometer (Siemens model D50000). Physical and Chemical Properties. A Mettler Toledo DE-50 vibrating tube densimeter was used to measure the density of the solutions with an uncertainty of less than ±5 × 10−2 kg/m3. For temperature control, the densimeter has a selfcontained Peltier system with an uncertainty of ±0.01 K. The densimeter was previously calibrated at atmospheric pressure. The refractive indexes were measured with a Mettler Toledo RE-40 refractometer with an uncertainty of ±1 × 10−4 nD units. The densities and refractive indexes were measured in triplicate at 298.15 K.

a

properties

wCu2+

wNa+

density (g/cm3)

refractive index nD

0.0724 0.0681 0.0706a

0 0.0250 0.0325

1.21110 1.25093 1.25573

1.3686 1.3786 1.3823

0.0697 0.0551 0.0450 0.0359 0.0185 0

0.0396 0.0606 0.0812 0.0922 0.1006 0.1039

1.25947 1.25551 1.25905 1.27549 1.25053 1.19790

1.3854 1.3877 1.3899 1.3903 1.3879 1.3794

solid phase CuSO4·5H2O CuSO4·5H2O CuSO4·5H2O + CuSO4·Na2SO4·2H2O CuSO4·Na2SO4·2H2O CuSO4·Na2SO4·2H2O CuSO4·Na2SO4·2H2O solid solution NaCl NaCl

Invariant point.

Figure 1 contains the phase diagram of the free acid system, where numeral I corresponds to the Na+ ion fraction in the

Figure 1. Phase diagram for the CuSO4 + NaCl + H2O system at 298.15 K: (●) experimental equilibrium data, (▲) composition of mixture points, and (■) wet solid composition.

sodium chloride salt, numeral II corresponds to the Cu2+ and Na+ ion fraction of the double salt found by the wet residue method CuSO4·Na2SO4·2H2O, which was corroborated by Xray diffraction shown in Figure 2, and numeral III is the Cu2+ ion of the CuSO4·5H2O salt. In the diagram, three crystalsaturated solution areas can be distinguished: the crystallization of CuSO4·5H2O, the crystallization of the CuSO4·Na2SO4·2H2O double salt (also found by Druzhini and Kosyakina4 in the Na2SO4 + CuSO4 + H2O ternary system, 6804

dx.doi.org/10.1021/ie3030642 | Ind. Eng. Chem. Res. 2013, 52, 6803−6811


Article

Figure 2. Diffractograms of the CuSO4·Na2SO4·2H2O (Kroehnkita salt) system (a) without acid and (b) with acid.

In the equilibrated solution, the pH was measured and gives as a result of 3 at all points. For a Cu2+ fraction of 0.0359, the saturated solution is in equilibrium with a solid solution. The invariant points, and their respective properties, were determined to intersect the solubility and property curves of the neighbor systems. The cation fractions obtained by AA are included in Table 1; the invariant point corresponds to the CuSO4·5H2O + CuSO4·Na2SO4·2H2O solid phase. Table 2 contains the corresponding data for the CuSO4 + NaCl + H2SO4/H2O system at 298.15 K. The complete phase diagram is given in Figure 3 (also given as mass fractions of copper and sodium ions). As one can see, the solubility of the salts, sodium chloride and copper sulfate, is slightly lower in the system containing sulfuric acid than in the binary system of water and the salt; for

which include the same ions as our system), and the crystallization of NaCl. A double salt crystallization zone corresponding to CuSO4·Na2SO4·2H2O was found; by mixing the CuSO4 + NaCl + H2O system, we formed the saline pair, following CuSO4 + 2NaCl ↔ CuCl 2 + Na 2SO4

This reciprocal saline pair reaction explains the formation of the double salt CuSO4·Na2SO4·2H2O; this salt is part of a larger system studied by Christov,3 where a quaternary diagram is presented of the CuCl2 + CuSO4 + NaCl + Na2SO4 aqueous system. The ternary phase diagram determined experimentally represents a diagonal cross section. 6805



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Na+, verifying the effect of this cation on the phase equilibria of the system. As observed in Figure 3, the first solid solution area features an abrupt change in the saturation curve that could be related to the pH variation in this zone. As shown in Figure 4, the pH increases as the Na+ concentration decreases.

Table 2. Experimental Solubilities, Refractive Indexes, and Densities of the CuSO4 + NaCl + H2SO4/H2O Ternary System at 298.15 K composition of the equilibrated solution

properties

wCu2+

wNa+

density (g/cm3)

refractive index nD

solid phase

0.0596 0.0564 0.0581 0.0586 0.0619 0.0741 0.0827 0.0722 0.0639 0.0586 0.0524 0.0438 0.0349 0.0182 0

0 0.0109 0.0209 0.0299 0.0328 0.0319 0.0304 0.0466 0.0641 0.0733 0.0819 0.0825 0.0871 0.0872 0.0929

1.21274 1.22667 1.25461 1.27224 1.28590 1.30315 1.32066 1.31530 1.32669 1.33420 1.37780 1.36650 1.36020 1.30710 1.24450

1.3685 1.3724 1.3775 1.3813 1.3848 1.3909 1.3966 1.3980 1.4010 1.4020 1.4009 1.3979 1.3952 1.3884 1.3803

CuSO4·5H2O CuSO4·5H2O CuSO4·5H2O CuSO4·5H2O solid solution solid solution CuSO4·Na2SO4·2H2O CuSO4·Na2SO4·2H2O CuSO4·Na2SO4·2H2O solid solution solid solution solid solution NaCl NaCl NaCl

Figure 4. Sodium chloride concentration as a function of pH.

Conceptual Process Design. For the CuSO4·5H2O crystallization process design, the experimental phase equilibrium diagrams determined at 298.15 K and the crystallization zone of this salt were used. For practical purposes, it is assumed that the Cu2+ and Na+ ions found in the phase equilibrium diagrams correspond to CuSO4 and NaCl salts, in the zone close to the binary and pseudobinary systems, respectively. The crystallization zone used can be seen in Figure 5. All the processes designed were simulated using METSIM. In Figure 5, the lines represent main currents of the process flow sheet shown in Figure 6. Line 1 is the saturated solution feed, which is mixed with line 2, and they become stream 3, which enters the evaporator giving line 4 of pure water and line 5 of crystals and a saturated solution. Line 5 crystallizes until point 7, and the supernatant liquid is point 12, which is recycled to the feed. Crystallization of CuSO4·5H2O from the CuSO4 + NaCl + H2O System. The starting point is a CuSO4 solution near saturation at 298.15 K. Six different processes with different drowning-out agents are compared. In cases 1 and 4, solid sodium chloride is used. However, it is known that the control feed is more manageable and common with pulps than solids, especially in continuous processes. In addition, the information about the process using aqueous saline solutions, such as seawater, is of great importance. Therefore, the crystallization process has been evaluated with 90 and 50% sodium chloride in the systems without and with acid in cases 2, 3, 5, and 6. Case 1: Initial Flux of Pure NaCl as the Drowning-Out Agent. In the CuSO4·5H2O and saturated solution area, a process has been designed (flow sheet shown in Figure 6) in which a nearly saturated solution of copper sulfate in water (stream 1) is mixed with a recirculation current (streams 8 and 12) and pure sodium chloride stream 2. All solutions are at 298.15 K. Here, the sodium chloride undergoes a change in phase from solid to liquid along with a change from aqueous CuSO4 to solid CuSO4·5H2O.

Figure 3. Phase diagram for the CuSO4 + NaCl + H2SO4/H2O pseudoternary system at 298.15 K: (●) experimental equilibrium data, (▲) composition of mixture points, and (■) wet solid composition.

NaCl, the solubility in water is 26.43 wt % and in acidic water is 23.62 wt %, and for CuSO4, the solubilities are 18.49 and 15.45 wt % for pure and acidic water, respectively. Therefore, we can conclude that the sulfuric acid must be competing for the water in the system, acting as an electrolyte. The densities of the saturated solutions containing acid are higher than those of the acid-free solutions, which was expected because sulfuric acid is denser than water. For the system including the sulfuric acid, the diagram is also composed of three crystallization areas corresponding to CuSO4·5H2O, NaCl, and double salt CuSO4·Na2SO4·2H2O, the same as found in the previous case without acid. Both equilibrium diagrams feature solid solution areas, which is in accordance with other studies (Christov3 and Druzhinin and Kosyakina4). The first solid solution area is between areas III and II. The second, a wide solid solution area, is located between crystallization zones II and I, which is similar regardless of the presence of acid. As observed from Figure 3, the wider solid solution area is close to higher concentrations of 6806



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are included: one washing stage with an efficiency of 0.8, a solid fraction of 0.95, and an approximate volume of 0.6. The obtained crystals (stream 9) are directed to a dryer at 353.15 K, where the product is obtained with a copper sulfate recovery yield of 56.9% (stream 10). All the parameters included in the simulation were suggested by Xstrata Copper staff based on real data from industrial plants. A feedback-type controller is used to limit the work area. The copper sulfate mass fraction was limited to 0.1775−0.1820 in the copper sulfate pentahydrate crystallization area. The main equations to write in APL programming are as follows. The solubility curve, given by the equation wCuSO4 = 0.1809 − 0.0845wNaCl

(1)

A straight line connecting the binary invariant point (no composition of NaCl) with the CuSO4·5H2O crystal point:

wCuSO4 = 1.00 − wH2O

(2)

A straight line connecting the ternary invariant point, which circumscribes the CuSO4·5H2O crystallization area: wCuSO4 = 1.078 − 1.218wH2O Figure 5. CuSO4·5H2O crystallization conceptual process design.

(3)

Table 3 summarizes the streams and the mass balance according the designed crystallization process. Stream 3 has the greater flux, which indicates that the evaporator must be larger. The rate of crystal production is 1408 kg/h with a yield of 60%, corresponding to a yearly production of 10137.6 tons of copper sulfate pentahydrate. Case 2: Initial Flux of 90% NaCl as the Drowning-Out Agent. The flow sheet for this process is the same as that in Figure 6. Table 4 provides the mass balance for the 90% NaCl pulp. Stream 2 is now an aqueous feed instead of a solid feed, as it was in the previous case. Stream 10, which corresponds to the CuSO4·5H2O solid product, exhibits an increase in product yield, with a rate of production of solid CuSO4·5H2O of 1480 kg/h and a yield of 63.1%, corresponding to an annual production of 10656 tons. Case 3: Initial Flux of 50% NaCl as the Drowning-Out Agent. Following the flow sheet shown in Figure 6, stream 2 is pulp with 50% NaCl in this case. The quantity of product obtained is greater than that in the other two cases, i.e., 1775 kg

The solution with a NaCl mass fraction of 0.0378, a H2O mass fraction of 0.8029, and a CuSO4 mass fraction of 0.1592 (stream 3) passes to the evaporator at 373.15 K and atmospheric pressure, where CuSO4 is concentrated into a mass fraction of 0.2140. Next, the concentrated solution enters a DTB crystallizer, where the solution is cooled from 373.15 to 298.15 K. In this unit, crystallization of CuSO4·5H2O occurs, yielding the main product. The weight fraction of the solids in the pulp stream is set equal to 0.85. This crystallizer originates two streams after the clarified solution (stream 6) is carried to a storage tank: a purge stream (stream 13) and a stream that is recirculated to the mixer (stream 12). Stream 7, containing the copper sulfate pentahydrate crystals, enters the filter, where the liquid solution (stream 8) containing a mass fraction of copper sulfate of 0.1724 is recirculated to the mixer. The following parameters

Figure 6. Design flow sheet for the crystallization of CuSO4·5H2O. 6807



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Table 3. Mass Balance (in kilograms per hour) for the Compounds by Stream in the Process with NaCl Solid Feed, without Acid stream

total flux

H2O(aq)

CuSO4(aq)

NaCl(aq)

NaCl(s)

CuSO4·5H2O(s)

1 2 3 4 5 6 7 8 9 10 11 12 13

10000 200 19626 5314 14312 12456 1855 432 1422 1412 10.4 8993 3462

8500 0 15757 0 10390 9591 344 333 10 0.5 0 6925 2666

1500 0 3125 0 3032 2148 77 74 2 2.4 0 1551 597

0 0 742 0 742 716 25 25 0.8 0.8 0 517 199

0 200 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 146 0 1408 0 1408 1408 0 0 0

H2O(g)

5314

10.4

Table 4. Mass Balance (in kilograms per hour) for the Compounds by Stream in the Process with 90% NaCl, without Acid stream

total flux

H2O(aq)

CuSO4(aq)

NaCl(aq)

CuSO4·5H2O(s)

1 2 3 4 5 6 7 8 9 10 11 12 13

10000 200 18598 5530 13068 11459 1609 113 1495 1484 10.9 8285 3174

8500 20 14987 0 9407 8823 99 87 11 0.5 0 6379 2444

1500 0 2957 0 2869 1988 22 19 2 2.5 0 1437 550

0 180 654 0 654 646 7 6 0.8 0.8 0 467 179

0 0 0 0 138 0 1480 0 1480 1480 0 0 0

H2O(g)

5530

10.9

Table 5. Mass Balance (in kilograms per hour) for the Compounds by Stream in the Process with 50% NaCl, without Acid stream

total flux

H2O(aq)

CuSO4(aq)

NaCl(aq)

CuSO4·5H2O(s)

1 2 3 4 5 6 7 8 9 10 11 12 13

10000 200 15685 6403 9282 7308 1973 179 1793 1780 13 5306 2002

8500 100 12823 0 6406 5627 151 138 1.3 0.6 0 4085 1542

1500 0 2490 0 2465 1319 35 32 3 3 0 957 361

0 100 371 0 371 361 9 8 0.8 0.8 0 262 99

0 0 0 0 38 0 1775 0 1775 1775 0 0 0

of solid CuSO4·5H2O per hour with a yield of 75.6%, corresponding to an annual production of 12780 tons. Energy Balance Using METSIM. The heats required for the CuSO4 + NaCl + H2O systems, cases 1−3, are listed in Table 6. As observed, case 3 requires the most total energy because of the larger quantity of water added with the NaCl pulp; however, this process also produces a higher yield. In contrast, less heat is required with the addition of the salting-out agent (NaCl) in solid form. The total mass flux for each case is as follows: 79474 kg/h for case 1, 75015 kg/h for case 2, and 61911 kg/h for case 3. On the basis of these total masses, larger equipment is necessary for

H2O(g)

6403

13

Table 6. Heats (in megacalories per hour) Required for the Equipment in the Process equipment evaporator crystallizer dryer total heat total heat/kg of product

case 1 (100% NaCl

case 2 (90% NaCl)

case 3 (50% NaCl)

3942.77 −692.97 5.80 3255.60 2.31

4030.90 −625.04 6.00 3411.86 2.31

4358.21 −414.31 6.80 3950.69 2.23

case 1, where solid NaCl is added to the feed stream. However, this case required less energy in the evaporator. The best 6808



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Table 7. Mass Balance (in kilograms per hour) for Compounds by Stream in the Process with Solid NaCl and Acid stream

total flux

H2O(aq)

CuSO4(aq)

NaCl(aq)

NaCl(s)

H2SO4(aq)

CuSO4·5H2O(s)

1 2 3 4 5 6 7 8 9 10 11 12 13

10000 200 22539 4117 18422 17033 1388 126 1262 1253 9 12212 4820

7727 0 16981 0 13311 12775 104 94 9 0.4 0 9159 3615

1500 0 3289 0 3257 2470 20 18 1 1.8 0 1771 699

0 0 710 0 710 704 5.7 5 0.5 0.5 0 505 199

0 200 0 0 0 0 0 0 0 0 0 0 0

772 0 1556 0 1091 1082 8 8 0.8 0.8 0 776 306

0 0 0 0 51 0 1249 0 1249 1249 0 0 0

H2O(g)

H2SO4(g)

3651

465

9

Table 8. Mass Balance (in kilograms per hour) for Compounds by Stream in the Process with 90% NaCl and Acid stream

total flux

H2O(aq)

CuSO4(aq)

NaCl(aq)

H2SO4(aq)

CuSO4·5H2O(s)

1 2 3 4 5 6 7 8 9 10 11 12 13

10000 200 22102 4253 17849 16418 1430 130 1300 1291 9.3 11772 4646

7727 18 16672 0 12867 12313 107 97 69 0.4 0 8829 3484

1500 0 3229 0 3197 2385 20 18 1 1 0 1710 675

0 180 639 0 639 634 5 5 0.5 0.5 0 455 179

772 1.8 1560 0 1093 1084 9 8 0.8 0.8 0 777 307

0 0 0 0 50 0 1287 0 1287 1287 0 0 0

H2O(g)

3787

H2SO4(g)

466

9.3

(stream 7) contains copper sulfate pentahydrate crystals; this solution enters the filter, where the clarified liquid solution (stream 8) containing a mass fraction of copper sulfate of 0.1450 is recirculated to the feed entrance. The parameters included in the process are as follows: a washing stage with an efficiency of 0.8, a solid fraction of 0.95, and an approximate volume of 0.6. In this unit, the formation of CuSO4·5H2O produces the main product. The obtained crystals (stream 9) are sent to a dryer at 353.15 K, where the product is obtained with a recovery of 53.2% (stream 10), corresponding to an annual production of 8993 tons. To limit the working area, a feedback-type controller is used in the copper sulfate pentahydrate crystallization area, using the solubility equation and the mass fraction of sodium chloride (wNaCl), a straight line connecting the invariant point with no composition of NaCl and as solvent the H2SO4 + H2O, with the CuSO4·5H2O crystal point and a straight line connecting the ternary invariant point (eqs 4−6, respectively). The copper sulfate mass fraction was limited to 0.1497−0.1472. The following empirical equations were used.

conditions correspond to case 3 because the energy requirement per kilogram of product is less and the equipment is smaller. CuSO4 + NaCl + H2SO4/H2O System. Following the process design in Figure 6, a new pseudoternary system is studied. In this system, sulfuric acid is added in stream 1 to resemble a real industrial process, in which copper sulfate solutions are available in acid media. The system was studied to understand the effect of the sodium chloride in the CuSO4·5H2O crystallization process. To this end, the previous experimental diagram observed in Figure 3 is used with a sulfuric acid:water ratio of 0.1. Case 4: Initial Flux of Pure NaCl. Following the diagram in Figure 6, a solution near saturation of copper sulfate in water and H2SO4 in a 0.1 ratio (stream 1) and a recirculation current (streams 8 and 12) are mixed with pure sodium chloride added to stream 2. The obtained solution in stream 3 has a mass fraction of NaCl of 0.0315, a mass fraction of H2O of 0.7534, a mass fraction of H2SO4 of 0.069, and a mass fraction of CuSO4 of 0.1459. This solution passes to the evaporator at 373.15 K and atmospheric pressure, yielding a mass fraction of CuSO4 of 0.1772. Next, this concentrated solution enters to a DTB crystallizer, where the solution is cooled from 373.15 to 298.15 K. The equipment parameters are the same as those used in the previous case. This crystallizer originates two streams after the clarified solution (stream 6) is carried to a storage tank: a purge stream (stream 13) and a stream that recirculates to the feed entrance (stream 12). The second stream from the crystallizer

wCuSO4 = 0.1491 − 0.3070wNaCl − 3.861wNaCl 2

(4)

wCuSO4 = 1.067 − 1.188wH2O

(5)

wCuSO4 = 1.154 − 1.426wH2O

(6)

Table 7 summarizes the mass balance for each stream according to the presented crystallization process design. 6809



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Table 9. Mass Balance (in kilograms per hour) for the Compounds by Stream in the Process with 50% NaCl and Acid stream

total flux

H2O(aq)

CuSO4(aq)

NaCl(aq)

H2SO4(aq)

CuSO4·5H2O(s)

1 2 3 4 5 6 7 8 9 10 11 12 13

10000 200 20356 4796 15559 13962 1596 145 1451 1441 10.3 10011 3951

7727 90 15435 0 11093 10472 119 108 10 0.5 0 7508 2963

1500 0 2987 0 2958 2045 23 21 2 2 0 1466 578

0 100 356 0 356 352 4 3 0.3 0.3 0 252 99

772 9 1576 0 1105 1092 12 11 1 1 0 783 309

0 0 0 0 46 0 1437 0 1437 1437 0 0 0

Case 5: Initial Flux of 90% NaCl. This process is represented, as are all cases, by Figure 6. Table 8 summarizes the mass balance for the 90% NaCl pulp. In this process, stream 2 is an aqueous feed instead of a solid NaCl. As observed in stream 10, which correspond to the CuSO4·5H2O solid product, a lower product yield is obtained (1287 kg/h) than in the corresponding case without sulfuric acid (1480 kg/h); this behavior could be related to the difference in pH. The annual production in this case is 9266 tons. Case 6: Initial Flux of 50% NaCl. Following the flow sheet in Figure 6, stream 2 is a pulp with 50% NaCl in this case. This process produces 1437 kg of solid CuSO4·5H2O per hour with a yield of 61.2%, corresponding to an annual production of 10346 tons. The total fluxes are 93372, 91411, and 83468 kg/h for cases 4−6, respectively, all of which include sulfuric acid. In all cases, the total fluxes are higher than those for the free acid systems described above. Energy Balance with METSIM. The heats required for the equipment in the process including sulfuric acid are listed in Table 10.

evaporator crystallizer dryer total heat total heat/kg of product

case 4 (100% NaCl)

case 5 (90% NaCl)

case 6 (50% NaCl)

3279.34 −934.71 5.10 2349.73 1.88

3332.99 −904.64 5.20 2433.55 1.89

3545.20 −784.61 5.70 2766.30 1.93

H2SO4(g)

4325

471

10.3

Table 11. Heats (in megacalories per hour) Required per Piece of Equipment for All Cases 100% NaCl equipment

case 1, without acid

evaporator crystallizer dryer equipment

case 4, with acid

3942.77 −692.972 5.8 90% NaCl

3279.34 −934.71 5.1


evaporator crystallizer dryer

case 5, with acid

4030.898 −625.036 6 50% NaCl

3332.99 −904.64 5.2

equipment


case 6, with acid

evaporator crystallizer dryer

4358.205 −414.314 6.8

3545.2 −784.61 5.7

A heat exchanger placed before the evaporation stage can reduce the amount of heat required in the evaporator using the water vapor generated in the evaporation as a heating medium. Production. Table 12 shows the total flows and the yields for every process case for a constant feed of CuSO4 of 1500 kg/ h.

Table 10. Heats (in megacalories per hour) Required for Each Piece of Equipment for the Acid-Containing Systems equipment

H2O(g)

Table 12. Total Fluxes, Heats, and Yields for All Study Cases

without acid

with acid

Among these cases, the first case with solid NaCl in the feed entrance requires the least heat in the evaporator, which is logical because there is less water in the feed. The first case also requires the least heat in the crystallizer and the dryer. Table 11 compares the heats required for each piece of equipment for all cases. As observed, the heat requirements for the evaporators are higher in the acid-free systems for all cases, whereas the total heats by kilogram of product are very similar. The temperatures of the streams for the studied cases are the same; crystallization occurs at 25 °C, and evaporation and drying occur at 100 °C.

case

total flux (kg/h)

total heat/kg of product (Mcal h−1 kg−1)

yield (%)

1

79474

2.31

60.0

2 3 4 5 6

75015 61911 93372 91411 83468

2.31 2.23 1.88 1.89 1.93

63.1 75.6 53.2 54.9 61.2

The addition of acid contributes to the crystallization via a cosolvent effect, producing a drowning-out crystallization. However, the yields are better for the cases without acid. Additionally, major fluxes are needed when acid is added to the system. With regard to the CuSO4·5H2O crystals, the highest yield was obtained with the CuSO4 + H2O + 50% NaCl system, with an annual production of 12780 tons. 6810



Article

(8) Giulietti, M.; Derenzo, S.; Nývlt, J.; Ishida, L. Crystallization of copper sulphate. Cryst. Res. Technol. 1995, 30, 177−183. (9) Hidrometalurgia: Fundamentos, procesos y aplicaciones; Domic, E. M., Ed.; Andros Impresos: Santiago, Chile, 2001. (10) Taboada, M.; Graber, T.; Cisternas, L.; Cheng, Y.; Ng, K. Process design for drowning-out crystallization of lithium hydroxide monohydrate. Chem. Eng. Res. Des. 2007, 85, 1325−1330. (11) Hernández, P. C.; Taboada, M. E.; Graber, T. A.; Galleguillos, H. R. Crystallization of Hydrated Ferric Arsenate. Process Design Using METSIM. Ind. Eng. Chem. Res. 2009, 48, 10522−10531. (12) Taboada, M. E.; Hernández, P. C.; Galleguillos, H. R.; Flores, E. K.; Graber, T. A. Behavior of sodium nitrate and caliche mineral in seawater: Solubility and physicochemical properties at different temperatures and concentrations. Hydrometallurgy 2012, 113−114, 160−166. (13) Solubilities; Linke, W. F., Seidell, A., Eds.; American Chemical Society: Washington, DC, 1965.

High yields are desirable for valuable products; thus, case 3, which features a 50% NaCl solution, should be preferred. However, if the energy cost is too high, then the best option would be the addition of a 50% NaCl solution in acidic media, case 6.

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CONCLUSIONS The phase equilibrium experimental data obtained with two salts, NaCl and CuSO4, and the effect of sulfuric acid at 298.15 K are presented. The addition of NaCl has a clear effect on the solid−liquid equilibrium, promoting the formation of solid phases in both systems. In the phase equilibrium diagram including sulfuric acid, the pH affects the solubility curve. A double salt containing CuSO4 and Na2SO4 obtained by the wet residue method and corroborated via X-ray diffraction was found; this was a result of a reciprocal saline pair formed. The mass and energy balance were evaluated for six simulation cases with different NaCl concentrations with and without sulfuric acid in the feed. The best yields of CuSO4·5H2O production were obtained in cases without acid, which had lower total mass fluxes but higher heat requirements per kilogram of product compared to the cases with acid. The addition of sulfuric acid increases the total fluxes but decreases the heat requirement. Contrary to what was expected, the yields in these cases were lower. The best results were obtained with the addition of a pulp of 50% NaCl, which produced the lowest total flux and a high yield of 75.6%.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for funding provided by CONICYT through Fondecyt Project N° 1100685 and CICITEM R04I1001.

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REFERENCES

(1) Parrague, L.; Bernal, L. Chemical study for selectivity and recovery of metals sulphides by flotation using seawater. In Proceedings of PROCEMIN; 2009. (2) Moreno, P. A.; Aral, H.; Cuevas, J.; Monardes, A.; Adaro, M.; Norgate, T.; Bruckard, W. The use of seawater as process water at Las Luces copper−molybdenum beneficiation plant in Taltal (Chile). Miner. Eng. 2011, 24, 852−858. (3) Christov, C. Thermodynamic study of the Na-Cu-Cl-SO4-H2O system at the temperature 298.15 K. J. Chem. Thermodyn. 2000, 32, 285−295. (4) Druzhinin, I. G.; Kosyakina, O. A. Solubility and solid phases in the CuCl2+Na2SO4 ↔ CuSO4+Na2Cl2 aqueous reciprocal system at 25°C. Russ. J. Inorg. Chem. 1961, 6, 868−873. (5) Lyall, E.; Mougin, P.; Wilkinson, D.; Roberts, K. J. In Situ Ultrasonic Spectroscopy Study of the Nucleation and Growth of Copper Sulfate Pentahydrate Batch Crystallized from Supersaturated Aqueous Solutions. Ind. Eng. Chem. Res. 2004, 43, 4947−4956. (6) De Juan, D.; Meseguer, V.; Lozano, L. Una contribución al estudio de la solubilidad del CuSO4·5H2O en medio acuoso. Rev. Metal. (Madrid, Spain) 1999, 35, 47−52. (7) Giulietti, M.; Seckler, M. M.; Derenzo, S.; Schiavon, L. H.; Valarelli, J. V.; Nyvlt, J. Effect of Selected Parameters on Crystallization of Copper Sulphate Pentahydrate. Cryst. Res. Technol. 1999, 34, 959− 967. 6811


SolidâLiquid Equilibrium and Process Design of CuSO4 + NaCl +

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