Continuous multistage electrodialytic treatment of copper smelter wastewater

Minerals Engineering 100 (2017) 187–190

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Continuous multistage electrodialytic treatment of copper smelter wastewater Henrik K. Hansen a,⇑, Andrea Lazo a, Claudia Gutierrez a, Manuel Durán a, Pamela Lazo b, Adrián Rojo a a b

Departamento de Ingeniería Química y Ambiental, Universidad Técnica Federico Santa María, Valparaíso, Chile Instituto de Química y Bioquímica, Facultad de Ciencias, Universidad de Valparaíso, Chile

a r t i c l e

i n f o

Article history: Received 9 August 2016 Revised 17 October 2016 Accepted 7 November 2016 Available online 16 November 2016 Keywords: Copper recovery Arsenic removal Electrodialysis

a b s t r a c t In this work, a continuous multistage electrodialytic technique is evaluated as a possible treatment of copper smelter wastewater. Six experiments with different values of feed flow rate, current density and treatment time were carried out. It was possible to concentrate copper during experiments and a maximum recovery of 10% was achieved. The experiment with a feed flow rate of 30 mL min 1, current density of 300 A m 2 and a treatment time of 2 h was the most efficient, with a copper recovery of 5.1%. In sections where copper was concentrated, a solution with mCu/mAs ratio around 2 was obtained, this value is substantially higher than 0.1, which is the value for a mining wastewater. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Chile is one of the main copper manufacturers in the world. During the processing and concentration of the minerals considerable amounts of residues and wastewaters are produced. During the smelting of copper sulphides and the subsequent gas treatment, considerable volumes of wastewater are generated containing mainly copper, arsenic and other inorganic species in concentrations that exceed the local legal threshold values (Gutiérrez et al., 2010). Different treatment alternatives exist such as chemical precipitation, electrodialysis, ion exchange, and adsorption (Hansen et al., 2010). At present the wastewater is processed by chemical precipitation to meet the environmental regulations (Chilean Republic, 2000). Wastewaters are treated with Ca (OH)2 to increase the pH to approximately 10, which favors the precipitation of heavy metals as hydroxides but also precipitates calcium sulphate. The Ca(OH)2 addition produces a great volume of sludge, owing to the fact that initial pH of the wastewater is very acidic (pH < 1) Núñez et al., 2011. The sludge contains a mix of inorganic elements including Cu and As. This methodology has the disadvantage that great quantity of sludge is produced and requires a subsequent treatment (Gutiérrez et al., 2015). Other alternatives or complementary techniques for treatment of acid effluents from smelting as electrodialysis are used. Electrodialysis is utilized to transport, separate and concentrate ions according to their electrical charge, by means of ion exchange membranes, ⇑ Corresponding author. E-mail address: [email protected] (H.K. Hansen). http://dx.doi.org/10.1016/j.mineng.2016.11.006 0892-6875/Ó 2016 Elsevier Ltd. All rights reserved.

under the action of electrical field (Sadrzadeh and Mohammadi, 2009). The advantage of this technique is the recovery and separation of the copper present in the arsenic-rich wastewater (Hansen et al., 2015; Caprarescu et al., 2011, 2014). The overall aim of this work is to study copper and arsenic separation, both elements contained in mining wastewater, by means of continuous electrodialytic technique, in order to achieve this (i) a electrodialytic cell in laboratory scale will be designed, (ii) the influence of feed flow rate, current density and treatment time will be analyzed and (iii) the energy consumption and its relation with copper recovery will be studied. 2. Experimental The main equipment for the wastewater treatment is the electrodialytic cell. It is designed to clean the wastewater as a continuous process mainly and is showed on Fig. 1. The solution to be treated is first pumped into section 3. After leaving this section, the solution is pumped into section 7 and then, it will be passed through section 11, and finally the solution abandons this last section with a lower charge of ions. The application of an electrical field in the cell produces the migration of ionic species present in the wastewater to the electrodes with opposite charge. The ions with positive charge cross the cationic membrane to enter to the section where cations are concentrated (cation sections 4, 8 and 12) and become trapped due to the anionic membrane which impedes their pass to the following section. The same applies to negative ions, which migrate to the opposite direction (to anion sections 2, 6 and 10), crossing the anionic membrane and

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Fig. 1. Electrodialytic cell.

becoming trapped here because the cationic membrane blocks their passage to the following section. In consequence, there is a constant flow of ions from the wastewater to the cationic and anionic sections. Copper would be present as cations and would end in sections 4, 8 and 12, whereas arsenic would either by negatively charged or neutral at the actual pH, and therefore remain in the wastewater (neutral) or enter sections 2, 6 and 10 (negatively charged). Therefore, a separation of Cu and As is feasible. During the continuous passage of the wastewater through the three sections of the cell, ions are recovered from the wastewater batch wise in this design; but sections 2, 6 and 10 could be combined via a continuous separate flow where copper is recovered without mixing with arsenic. The same could be the case with sections 4, 8 and 12. To avoid the mixing between copper and arsenic recovered in two consecutive stages, an additional section between anionic and cationic section of two consecutive stages will be considered placed between an anion and a cation exchange membrane (sections 5 and 9). In this way these act as bipolar membranes, and ions cannot pass these sections. In summary, the electrodialytic cell consists of thirteen sections; between each section an ion exchange membrane was placed. The electrodes were installed at each end of the cell. All sections of the cell have dimensions of 6 cm (W)  3 cm (D)  8 cm (H), and a total volume of 157.5 mL for sections 2–12 was used. The electrodes were made of stainless steel with dimensions of 6 cm height, 3.8 cm width and 0.3 cm thickness. The ion exchange membranes used were AMI-7001 and CMI-7000 from ‘‘Membrane International INC”. The mining wastewater was provided by CODELCO, Ventanas Smelter and Refinery, located in V Region of Chile. The pH of the wastewater was adjusted to 2, using a solution of calcium hydroxide (Ca(OH)2 17.5%). Sections 1, 2, 4, 5, 6, 8, 9, 10, 12 and 13 were filled with 0.5 M sulphuric acid (H2SO4) and sections 3, 7 and 11 were filled with the wastewater. The analysis of arsenic and copper was based on Chilean Official Standard NCh 2313/10 Of. 96. Copper and arsenic in sections 5 and 9 were not analyzed, because, in a previous experiment, arsenic was not detected and the copper concentration was lower than

0.5 mg L 1 in these sections. All experiments were carried out in duplicate and the error was always lower than 5%. 3. Results and discussion Experimental conditions and results are shown in Table 1. Copper was recovered in sections 4, 8 and 12, whereas arsenic was detected in sections 2–12. When working with different feed flow rates in concentrate sections, the overall copper recovery is lower when higher flow rates are used, due to the lower residence time in the cell. Moreover, when the current density increases, so does the copper recovery too. This is because with the increase of current density, a higher amount of energy is available to move copper ions to the concentrate section. In dilute sections the overall recovered mass of copper is negligible (<1 mg) independently of the increase in the flow rate and current density, due to the effectiveness of anion membrane, which avoids the passage of copper ions to dilute sections. The overall recovery of copper in dilute sections is lower than 1%. Additionally, the presence of arsenic in the concentrate and dilute sections is primarily due to the existence of this element in a neutral molecule as H3AsO3 and H3AsO4 at the pH level used here and for that reason arsenic cannot be retained in a specific way by membranes. Therefore, mass transfer is taking place due to the permeability of membranes and diffusion. On the other hand, in the three concentrate solutions, mCu/mAs ratio was increased to around 2 from 0.1 initially – indicating the potential of electrodialysis to separate copper from arsenic. No clear trend was observed for the energy consumption with the change in current densities, due to the ratio between the recovered mass of copper and energy consumption was very close for the studied values of current density. A higher efficiency in copper removal for 300 A m 2 compared to 500 A m 2 with a flow rate of 30 mL min 1 was observed. However, when the flow rate was 60 mL min 1 this trend was reversed with values close to each other. Moreover, the energy can be better used with lower feed flow rates, i.e. 30 mL min 1. When comparing experiments 1, 5 and 6, in sections 4, 8 and 12 (corresponding to the concentrate sections), the optimum copper

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H.K. Hansen et al. / Minerals Engineering 100 (2017) 187–190 Table 1 Experimental conditions and results. Experimental conditions Current density, Am 2 Flow rate, mL min Time, min pH of wastewater

1

Results for copper Sample [Cu+2]Initial minput Section 2 (S2) Section 3 (S3) Section 4 (S4) Section 6 (S6) Section 7 (S7) Section 8 (S8) Section 10 (S10) Section 11(S11) Section 12 (S12) 20 min out 40 min out 60 min out 80 min out 100 min out 120 min out 140 min out 160 min out 180 min out mtotal Cu+2 mg S4 + S8 + S12 Cu+2 recovery % S4 + S8 + S12 mtotal Cu+2 mg S2 + S6 + S10 Cu+2 recovery % S2 + S6 + S10 Results for arsenic Sample [As]Initial minput Section 2 (S2) Section 4 (S4) Section 6 (S6) Section 8 (S8) Section 10 (S10) Section 12 (S12) mtotal As mg S4 + S8 + S12 As recovery % S4 + S8 + S12 mtotal As mg S2 + S6 + S10 As recovery % S2 + S6 + S10

Exp. 1 300

Exp. 2 500

Exp. 3 300

Exp. 4 500

Exp. 5 300

Exp. 6 300

30 60 2

30 60

60 60

60 60

30 120

30 180

[Cu+2] mg L 1

mCu+2 mg

362 1.6 328 47 <1 228 77 <1 266 47 248 324 294

[Cu+2] mg L 1 407

652 <1

[Cu+2] mg L 1

mCu+2 mg

414 733 <1

mCu+2 mg

392 1490 <1

mCu+2 mg

396 1411 <1

64.0

72.7

4.1

10.0

1.5

4.5

5.1

4.6

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

[As] mg L 1

mAs mg

5574 67 36 46 40 44 25 15.9

[As] mg L 1

mAs mg

4071 10,033 10.6 5.7 7.2 6.3 6.9 3.9

72 34 64 66 91 92 30.2

[As] mg L 1

9.6

mAs mg

4897 7328 11.3 5.4 10.1 10.4 14.3 14.5

55 18 36 25 36 49 14.4

[As] mg L 1

17.6 <1 24.7

mAs mg

5083 17,629 8.7 2.8 5.7 3.9 5.7 7.7

51 36 16 46 81 56 21.7

[As] mg L 1

16 <1 32.3 <1 24.4

mAs mg

3944 18,299 8.0 5.7 2.5 7.2 12.8 8.8

101 104 81 38 74 55 31.1

[As] mg L 1

2198 <1 36.4 <1 34.0 <1 30.7

mAs mg

4497 14,198 15.9 16.4 12.8 6.0 11.7 8.7

145 105 122 93 151 102 47.2

<1

<1

<1

<1

<1

<1

24.7

35.7

20.1

23.3

40.4

65.8

<1

<1

<1

<1

<1

<1

0.1 1.6

0.1 2.9

0.1 2.3

0.1 2.1

0.7 19.0 13.3 0.013 2.1

1.2 32.2 38.6 0.039 1.9

0.7 22.4 15.7 0.016 1.4

1.2 34.4 41.3 0.041 1.6

0.7 19.4 13.6 0.028 2.6

0.7 19.9 13.9 0.042 2.4

3.2

5.1

3.2

4.5

4.4

4.1

3.03 3.71 3.21

0.31 0.57 0.64

1.54 1.24 1.75

1.12 2.27 1.71

1.67 1.55 1.40

Copper recovery by stages % Stage 1 1.13 Stage 2 1.86 Stage 3 1.13

mCu+2 mg

407 713 <1

23.0

23.5

8.5 <1

21.7 <1

<1 328 102 <1 322 205 <1 326 155 307 325 315 317 314 314

[Cu+2] mg L 1

72.9

7.4

27.2 <1

4.6 <1

<1 337 138 <1 326 112 <1 323 157 322 304 322

[Cu+2] mg L 1

26.9

12.1 <1

22.2 <1

<1 385 29 <1 390 54 <1 394 61 386 386 385

[Cu+2] mg L 1

1.2 355 231 <1 331 216 <1 340 195 314 315 314 306 312 312 315 308 309 101.1

7.4 <1

<1 326 141 <1 298 173 <1 292 149 295 284 282

Comparison between arsenic and copper, initial and final mCu/mAs input 0.1 0.1 mCu/mAs (S4 + S8 + S12) 1.7 2.4 Energy consumption Applied current, A Average voltage, V Power, W Energy, kW h kgCu(S4 + S8 + S12)/ kW h Current efficiency %

mCu+2 mg

24,284 22.8 16.5 19.2 14.6 23.8 16.1

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recovery, mCu/mAs ratio and recovered mass per energy unit were obtained in experiment 5 after 2 h treatment. Voltage tends to remain constant, due mainly to the equilibrium between two phenomena that occur simultaneously: (1) the increase of voltage generated by electrical resistance produced by the displacement of ions from wastewater to the sections and (2) the decrease of voltage due to the continuous feeding of wastewater with a new charge of ions, that helps to conduct an electrical current inside the system. Generally, the current efficiency percentages with respect to copper were quite low; due mainly to the large amount of species migrating together with copper to the different sections but that subject was not covered specifically in this study. An increase in current efficiency with increasing current density was observed. Also, the efficiency improves with increasing the treatment time to 2 h. Therefore, the results of current efficiency corroborate the previous analyses, where the best process is experience 5. Copper recovery was analyzed by stages, the recovery of a single stage was lower than the global recovery, (i.e., the recovery of three stages), this proves the larger efficiency to carry out the electrodialysis in several stages to increase the removal. On the other hand, the recovery efficiency of copper is comparable between each stage of the same experience, which verifies that the amount of copper recovered from each stage is the same, independently of the section position with relation to the position of the electrode. These results allow estimating the number of necessary stages to removal, for example, 98% of the copper contained in the wastewater. Using the parameters of experience 5, with an average recovery by stage of 1.7%, 56 stages will be necessary. Therefore, the increase of the number of stages in the electrodialytic cell would be appropriate for future researches. 4. Conclusions Considering that the arsenic content in the wastewater entering the process is roughly ten times higher than the copper content, it is remarkable to obtain a liquid solution with a copper content that

doubles the arsenic content, thereby achieving a wastewater in the concentrate sections (4, 8 y 12) with much lower content of arsenic than the original mining wastewater. The increase in the feed flow rate produces a decrease in the copper recovery. On the other hand, an increase in current density produces an increase in copper recovery. An optimum time of 2 h was found, for feed flow rate and current density of 30 mL min 1 and 300 A m 2, respectively. These parameters maximize the copper recovery and minimize the energy consumption and associated costs. The electrodialytic treatment in multistage should be considered as a previous stage of conventional treatment of mining wastewater, as a way to recover a large portion of the copper contained in the wastewater, thus avoiding its precipitation and disposal together with another metals. References Caprarescu, S., Vaireanu, D.I., Cojocaru, A., Maior, I., Purcar, V., 2011. A 3-cell electrodialysis system for the removal of copper ions from electroplating wastewater. Optoelectron. Adv. Mater. 5 (12), 1346–1351. Caprarescu, S., Radu, A.L., Purcar, V., Sarbu, A., Vaireanu, D.I., Ianchis, R., 2014. Removal of copper ions from simulated wastewaters using different bicomponent polymer membranes. Water Air Soil Pollut., 225–2079 Chilean Republic, 2000. Supreme Decree 90: Emissions Standard for the Regulation of Pollutants Associated with Discharges of Liquid Waste to Surface Marine Waters and Continents. Chilean Republic/Ministry General Secretariat of the Presidency of the Republic. Gutiérrez, C., Hansen, H.K., Nuñez, P., Jensen, P.E., Ottosen, L.M., 2010. Electrochemical peroxidation as a tool to remove arsenic and copper from smelter wastewater. J. Appl. Electrochem. 40, 1031–1038. Gutiérrez, C., Hansen, H.K., Nuñez, P., Valdez, E., 2015. Electrochemical peroxidation using iron nanoparticles to remove arsenic from copper smelter wastewater. Electrochim. Acta 181, 228–232. Hansen, H., Arancibia, F., Gutiérrez, C., 2010. Adsorption of copper onto agriculture waste materials. J. Hazard. Mater. 180, 442–448. Hansen, H.K., Gutierrez, C., Ferreiro, J., Rojo, A., 2015. Batch electrodialytic treatment of copper smelter wastewater. Miner. Eng. 74, 60–63. Núñez, P., Hansen, H.K., Aguirre, S., Maureira, C., 2011. Electrocoagulation of arsenic using iron nanoparticles to treat copper mineral processing wastewater. Sep. Purif. Technol. 79, 285–290. Sadrzadeh, M., Mohammadi, T., 2009. Treatment of sea water using electrodialysis: current efficiency evaluation. Desalination 249, 279–285.