Minerals Engineering 55 (2014) 52–56

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Minerals Engineering
journal homepage: www.elsevier.com/locate/mineng

Electrokinetic remediation of mine tailings by applying a pulsed variable

electric field
Adrián Rojo ⇑, Henrik K. Hansen, Omara Monárdez
Departamento de Ingeniería Química y Ambiental, Universidad Técnica Federico Santa María, Casilla 110-V, Valparaíso, Chile

a r t i c l e i n f o a b s t r a c t

Article history: Following the flotation step of copper sulphide mineral processing, a considerable amount of ground ore
Received 9 May 2013 in the form of a pulp containing heavy metals and other polluting compounds is discarded as waste,
Accepted 8 September 2013 known as ‘‘mine tailings’’. This waste is deposited behind dams, and unless it is treated, it represents a
Available online 5 October 2013
danger to the environment because the natural oxidation of heavy metals makes the waste chemically
unstable.
Keywords: Electrokinetic remediation (EKR) is a technology used to remove contaminants from soils. In recent
Electrokinetic remediation
years, the technology has been of research interest for stabilising mine tailings from the copper industry.
Heavy metals
Copper mine tailings
Nine EKR experiments with pulses of sinusoidal electric fields (by applying simultaneously DC and AC
Pulsed electric field voltages) and an AC voltage frequency of the order of kHz were performed to improve conventional EKR
and stabilise synthetic tailings samples. The synthetic tailings were prepared based on representative
data for tailings from a combined Cu–Mo concentrator plant.
It was found that, in general, the use of a pulsed sinusoidal electric field favoured overall copper
removal in the EKR cell, and particularly good results were observed when this type of electric field pro-
duced periodical polarity reversal in the electrodes.
The best results in terms of overall cell removal and specific energy consumption were obtained under
the following conditions: (i) effective voltage of 14.6 V (VDC = 10 V and VAC = 15 V), (ii) AC voltage fre-
quency 1000 Hz, (iii) electrical field applied in pulses with a time ratio of 25.
Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction the ions. The rate of electro-migration depends on the values of

the effective ionic mobility of contaminants, considering soil
Electrokinetic remediation (EKR) is an in situ treatment tech- porosity and tortuosity. Electro-osmosis uses an electric field to
nology for restoration of contaminated hazardous waste sites (Acar move contaminants from one location to another within flowing
and Alshawabkeh, 1993). The conventional application of this water, typically from the anode toward the cathode. Depending
alternative treatment uses direct current (DC) applied across elec- on the flow direction, the migration of some ions may be enhanced,
trodes inserted in the soil to generate an electric field for the mobi- while others with the opposite charge may be slowed.
lisation and extraction of contaminants (Probstein and Hicks, The relative contribution of electro-migration and electro-
1993). In the case of waste from copper mining, previous work osmosis to overall transport in an electric field varies for different
(Rojo et al., 2006) has shown that the conventional DC system soil types, water content, types of ions, pore fluid chemistry, and
was limited with regard to metal removal efficiency and had a very boundary conditions. For all of the above, EKR refers to coupled
high electrical energy consumption. Under this scenario sinusoidal charge and fluid transport.
EKR was applied by an electric field through the simultaneous Water is electrolysed by the electric field electrodes in a remedi-
application of DC and AC voltages. ation cell. Oxidation at the anode and reduction at the cathode gen-
The driving mechanisms for EKR are transport phenomena in an erate, respectively, an acid and a base front in the remediation cell.
electric field, particularly electro-migration (ionic migration) and The acid generated at the anode moves through the soil towards
electro-osmosis, coupled with electrolysis and geochemical reac- the cathode by electro-migration and electro-osmosis, while the
tions (Alshawabkeh and Bricka, 2000). base generated at the cathode moves towards the anode by elec-
Electro-migration is the transport of charged ions in the pore tro-migration and diffusion. Acid production generally enhances
fluid liquid toward the electrode that is opposite in polarity to the process, and a high pH zone adversely affects the extraction of
heavy metals from soils. In relative terms, the acid front dominates
⇑ Corresponding author. Tel.: +56 322654463; fax: +56 322654278. the base front due to the greater mobility of H+ ions and backflow
E-mail address: adrian.rojo@usm.cl (A. Rojo). due to electro-osmosis that retards the base front.

0892-6875/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.mineng.2013.09.004
 A. Rojo et al. / Minerals Engineering 55 (2014) 52–56 53

Geochemical reactions in the soil pores can enhance or retard in all three zones of the cell, the differences in the total copper con-
the process; precipitation/dissolution, sorption, redox and com- tent are concluded to represent the soluble copper removed.
plexation reactions are all dependent on the ph. In this case,
the predominant acid front assists in desorption and dissolution 2.3. Experimental EKR cell
of metals, while the base front assists in immobilisation and pre-
cipitation of metal hydroxides. Innovative methods are required A schematic description of the remediation cell is given in Fig. 1.
to enhance the EKR technique to avoid the immobilisation of Experiments were carried out in an opened acrylic
metals and to reduce the high electrical energy consumption. 30 cm  10 cm  10 cm high rectangular box divided into three
Due to the large amount of accumulated tailings from copper compartments. The length of the central compartment was
mining in Chile, the energy expenditure could become a major 20 cm, with two electrode compartments with lengths of 5 cm
issue restricting widespread field application of the EKR each. The power supply, which combined the AC–DC voltages,
technology. was connected to the cell by two titanium rods immersed in the
The main goal of this work is to evaluate the technique of applying electrode compartments. Both nylon mesh (149 lm) and filter pa-
a pulsed sinusoidal electric field to enhance EKR of mining waste per (grade 131) were used to separate the central compartment
from the copper industry. This investigation is part of the search from the electrode compartments. The pre-treated synthetic mine
for a remediation technique to environmentally stabilise the large tailings were placed in the central compartment. The initial
amount of solid waste generated by the Chilean copper industry electrolyte in the anode compartment was distilled water, while
(Government of Chile, 2011; Minería Chilena Magazine, 2010). the initial electrolyte in the cathode compartment was distilled
water with pH adjusted to between 2 and 4 using a dilute (1 M)
sulphuric acid solution. To control pH in the cathode compartment,
2. Experimental details
a sample was taken each day for pH monitoring, and a continuous
drop addition of concentrated acid to maintain pH below 4
2.1. Experimental synthetic mine tailings
was supplied.
After the experiments were carried out (Hansen et al., 2005),
The synthetic mine tailings used for the EKR experiments were
the synthetic mine tailing sample was segmented into three slices
prepared using dry sand (<200 lm), copper concentrate (chalcopy-
of equal size, and the copper concentration was measured. Because
rite) and copper sulphate pentahydrate. Based on data from Minera
of the sand in the synthetic sample, percolation of the pore
Los Pelambres, the synthetic sample was adjusted to 820 mg/kg of
solution occurred; therefore, each slice was split into a top and
total copper in the tailings, with 45% soluble copper (Antofagasta
bottom part to better assess copper removal. In this work, the
Minerals, 2012).
anode slice was defined as the zone closest to the anode, the centre
slice was defined as the zone in the middle, and the cathode slice
2.2. Analytical methods was defined as the zone closest to the cathode.

The total and soluble copper contents were determined 2.4. Experimental EKR plan
according to the following methods. All analyses were performed
in triplicate and the results averaged. Nine EKR experiments, each with a remediation time of 7 days,
The total copper content of the synthetic tailings was deter- were carried out with the conditions given in Table 1. In all
mined by adding 20 mL 1:1 HNO3 to 1.0 g of dry material and treat- experiments, a sample of approximately 1.5 kg (solid dry weight)
ing the sample in an autoclave according to the Danish Standard DS synthetic mine tailings was adjusted to an initial humidity of
259:2003 (30 min at 200 kPa (120 °C)). The liquid was separated 20% using sulphuric acid solution. The addition of acid is needed
from the solid particles by vacuum through a 0.45 lm filter and di- to ensure dissolution of the copper and its subsequent removal
luted to 100.0 mL. The metal content was determined by atomic by EKR process. This process is possible if the pH of the tailings
absorption spectrometry (AAS) in flame. This determination was is below 4.
performed for the original and final tailings, before and after EKR According to previous work (Hansen and Rojo, 2007; Rojo et al.,
treatment. 2010, 2011, 2012), a pulsed sinusoidal electric field produces good
The soluble copper content of the synthetic tailings was deter- results if a polarity reversal of the electrodes occurs. In the case of
mined by adding 50 mL H2SO4 5% (v/v) to 5.0 g of dry material and pulses (Sun et al., 2012), the ratio of the times for which the
stirring the sample in a 250 mL Erlenmeyer flask for 30 min. The li- electric field is ON and OFF is 25 (tON/tOFF = 2500/100). As shown
quid was separated from the solid particles by vacuum through a in Table 1, in the sinusoidal EKR experiments with a negative Vmini-
0.45 lm filter and diluted to 100.0 mL by adding 10 mL concen- mun, a polarity reversal of the electrodes occurs periodically.
trated HCl and distilled water. The metal content was determined The experimental plan considered a conventional reference EKR
by Flame Atomic Absorption Spectrometry (FAAS). This determina- configuration with 20 V DC, and eight sinusoidal pulsed electric
tion was performed for the original tailings, before EKR treatment, field (DC + AC) EKR configurations with effective voltages of
because the soluble copper in the final tailings was determined by approximately 20 V (14.6–26.7 V). Experiments were defined
mass balance assuming that only the soluble copper was removed. based on the partial results that were initially obtained under fixed
The effectiveness of the EKR process applied to soils contami- conditions with an effective voltage of 14.6 V (10 V DC, 15 V AC)
nated with heavy metals such as copper requires a prior geochem- and a pulse time ratio of 25 (tON/tOFF = 2500/100, t in second). In
ical dissolution of the metal in the pore solution. In the case of experiments 2, 3, 4 and 7, the effect of an AC frequency in the range
tailings from copper mining mineral processing, the copper of 50–2000 Hz was analysed. Then, to confirm the effect of AC
content at the time of disposal is present as soluble or insoluble frequency, experiments 5 and 8 were performed with an effective
species in the acidic environment (pore solution) where the reme- voltage of 26.7 V, pulse time ratio 25 and frequencies of 1000 and
diation occurs. Therefore, only soluble copper is involved in the 2000 Hz, respectively.
remedial action or removal process. During remediation under The effect of electrode polarity reversal and the effective voltage
acidic conditions with an electric field, only dissolution and was verified in experiment 6 with an effective voltage of 22.6 V and
transport of soluble copper occur, while the insoluble copper re- a pulse ratio 25 was performed. Finally, because the results of
mains fixed in the tailings. Because insoluble copper remains fixed experiment 4 were the most attractive, a similar experiment 9
54 A. Rojo et al. / Minerals Engineering 55 (2014) 52–56

Fig. 1. Schematic description of EKR cell.

Table 1
the overall synthetic sample in the cell. In this case, the reported
Summary of the experimental conditions.
values were weighted from the top and bottom concentrations of
Exp. Applied potential DV (V) Frequency fVAC (Hz) Pulses ON/OFF (s) each slice to consider the percolation effect of the pore solution.
DC AC Veffective Vmaximun Vminimun For this reason, the displacement of the copper to estimate metal
1 20 – 20 20 20 – – removal was the result of the combined effects of electrokinetic
2 10 15 14.6 25 5 50 25 (2500/100) phenomena and percolation. On the other hand, the measured
3 10 15 14.6 25 5 500 25 (2500/100) pH of the tailings in the three slices, including the top and bottom
4 10 15 14.6 25 5 1000 25 (2500/100) divisions, after the EKR experiments was practically uniform,
5 20 25 26.7 45 5 1000 25 (2500/100)
6 20 15 22.6 35 5 1000 25 (2500/100)
registering an average value of 2.8, similar to the initial pH of the
7 10 15 14.6 25 5 2000 25 (2500/100) synthetic tailings. The measured value of 2.8 ensures the dissolu-
8 20 25 26.7 45 5 2000 25 (2500/100) tion of soluble copper for subsequent removal by electrokinetic
9 10 15 14.6 25 5 1000 20 (2000/100) phenomena.

3.1. Copper removal

was implemented with a pulse ratio of 20 (tON/tOFF = 2000/100).
Consequently, the last two experiments (6 and 9) were imple-
The overall removal results in Table 3 for EKR with a sinusoidal
mented to verify the recommendations from previous work.
electric field using pulses and an AC voltage frequency between 0.5
and 1 kHz, as observed in experiments 3, 4 and 5, show that those
3. Results and discussion conditions can lead to better copper removal than conventional
EKR with a DC voltage (experiment 1). Table 4 shows the extended
Table 2 shows the total and soluble copper concentrations in effect of an AC voltage frequency between 0.05 and 2 kHz for EKR
the anode, centre and cathode slices of the synthetic tailings in with a pulse ratio of 25 and an effective voltage of 14.6 V.
the cell both before and after the EKR experiments, where c0 and It can be observed from Table 4 and Fig. 2 that there was an in-
cF correspond to initial and final experimental concentrations. In crease in copper removal when the AC voltage frequency increases
these experiments, because the pore solution accumulated at the to 1 kHz. However, when 2 kHz is selected, copper removal be-
bottom of the cell, it was necessary to determine the final experi- comes negligible. Because the same result was observed when
mental concentrations cF in the top (T) and bottom (B) of each of increasing from 1 to 2 kHz for a similar EKR with an effective volt-
the above-mentioned slices. The final total and soluble concentra- age of 26.4 V, Table 5 elaborated to highlight this specific phenom-
tions were consistent with the differences in the humidity of the enon. In these cases, for the effective voltages of 14.6 and 26.4 V
synthetic tailings observed by percolation of the pore solution. In obtained with VDC/VAC ratios of 10/15 and 20/25, it is estimated
general, the observed concentrations at the bottom of the cell were that, at the high frequency of 2 kHz, the DC voltage was inadequate
higher than those at the top. to produce a net electrokinetic phenomenon in the sinusoidal elec-
Table 3 includes a summary with general EKR results in terms of tric field obtained by simultaneously applying AC and DC voltages.
the removal (positive values) and/or accumulation (negative val- Consequently, the results obtained correspond to an EKR with AC
ues) of total and soluble copper from the three slices and from voltage. Furthermore, if the DC voltage was increased to avoid this
 A. Rojo et al. / Minerals Engineering 55 (2014) 52–56 55

Table 2
General remediation results: c0 and cF initial and final copper concentration in (mg/kg), and synthetic tailings mass in (kg).

Exp. Total copper (mg/kg) Soluble copper (mg/kg) Synthetic tailings mass (kg)
a
c0 cF c0 cF
Anode Centre Cathode Anode Centre Cathode Anode Centre Cathode Sub Total
1 T 853 658 673 760 376 181 196 283 0.254 0.241 0.266 0.762 1.5
1 B 804 921 857 327 444 380 0.233 0.235 0.270 0.738
2 T 869 825 764 816 462 418 357 409 0.214 0.247 0.241 0.703 1.5
2 B 855 880 904 448 473 497 0.285 0.242 0.270 0.797
3 T 803 606 609 588 454 257 260 239 0.234 0.256 0.243 0.732 1.5
3 B 702 740 699 353 391 350 0.240 0.246 0.281 0.768
4 T 787 543 552 527 403 159 168 143 0.284 0.250 0.249 0.783 1.5
4 B 695 699 563 311 315 179 0.229 0.248 0.241 0.717
5 T 832 570 551 606 323 61 42 97 0.215 0.247 0.285 0.748 1.5
5 B 707 720 763 198 211 254 0.264 0.258 0.231 0.752
6 T 835 681 567 662 382 228 114 209 0.238 0.243 0.247 0.728 1.5
6 B 855 875 965 402 422 512 0.250 0.270 0.252 0.772
7 T 822 799 634 705 309 286 121 192 0.235 0.243 0.231 0.709 1.5
7 B 875 938 925 362 425 412 0.234 0.276 0.281 0.791
8 T 823 800 635 706 305 282 117 188 0.229 0.157 0.283 0.669 1.5
8 B 876 935 926 358 417 408 0.274 0.233 0.324 0.831
9 T 784 644 663 571 359 219 238 146 0.191 0.237 0.222 0.650 1.5
9 B 795 781 822 370 356 397 0.292 0.261 0.297 0.850
a
Determined by mass balance.

Table 3 60 Total copper

Removal (and/or accumulation) of total and soluble copper in percentage.

Exp. Anode Centre Cathode Overall Soluble copper

50
Removal = ((c0 cF)/c0)  100 (%)
Total Soluble Total Soluble Total Soluble Total Soluble
Global removal (%)

1 14.7 33.3 6.7 15.3 5.2 11.8 8.8 19.9 40

2 3.1 5.8 5.5 10.3 0.7 1.4 3.1 5.8
3 18.5 32.7 16.2 28.6 19.4 34.2 18.0 31.9
4 22.4 43.7 20.6 40.2 30.8 60.1 24.5 47.9 30
5 22.4 57.8 23.4 60.3 18.7 48.2 21.5 55.3
6 7.8 17.0 12.7 27.8 2.4 5.2 7.7 16.7
7 1.8 4.8 3.2 8.5 0.5 1.2 1.4 1.0
20
8 2.2 6.0 1.1 3.30 0.0 0.1 0.5 1.3
9 6.2 13.6 7.5 16.5 8.8 19.3 7.8 17.1

Removal: positive values 10

Accumulation: negative values

0
Table 4 0.05 0.5 1 2
Overall removal of total and soluble copper, frequency AC voltage effect. Effective
Frequency AC in kHZ
voltage 14.6 V, pulses time ratio 25.

Exp. Frequency fVAC (kHz) Overall removal (%) Fig. 2. Histogram of global removal, total and soluble copper, according to AC
frequency in kHz. Effective voltage 14.6 V, pulses time ratio 25.
Total copper Soluble copper
2 0.05 3.1 5.8
3 0.5 18.0 31.9 Table 5
4 1 24.5 47.9 Overall removal of total and soluble copper, frequency AC voltage from 1 to 2 kHz
7 2 0.4 1.0 effect. Pulses time ratio 25.

Exp. Frequency fVAC (kHz) Veffective (V) Overall removal (%)

specific phenomenon for both effective voltages, the positive effect Total copper Soluble copper
of polarity reversal of the cell was not produced. 4 1 14.6 24.5 47.9
Table 6 shows the effective voltage effect on overall copper re- 7 2 0.4 1.0
moval for EKR experiments with an AC frequency of 1 kHz and a 5 1 26.4 21.5 55.3
pulse ratio of 25. For comparison, the result with the conventional 8 2 0.5 1.3
reference EKR with 20 V DC, experiment 1, was included.
Table 6 shows that the greatest removal values were observed in
experiments 4 and 5, with effective voltages 14.6 and 26.7 V and 5 are comparable, the lower effective voltage in experiment 4 is
periodic reversal of the cell polarity. However, if the reversal undoubtedly more attractive due to the lower expected energy
phenomenon did not occur, as in experiment 6, the removal was consumption for removal.
similar to that of a conventional EKR. Then, as demonstrated in Table 7 shows the effect of the pulse ratio on overall copper
previous work (Rojo et al., 2012) the polarity reversal and pulses removal for EKR experiments with an effective voltage of 14.6 V
are favourable to reducing the polarisation of the cell and the and an AC frequency of 1 kHz. To compare the result of the conven-
displacement of copper. Although the results of experiments 4 and tional system, the reference EKR with 20 V DC was included.
56 A. Rojo et al. / Minerals Engineering 55 (2014) 52–56

Table 6 Comparing the specific energy consumption between experi-

Overall removal of total and soluble copper, effective voltage effect. AC frequency ments 4, 6 and 5, along with conventional EKR, as seen in Table 6,
1 kHz, pulses time ratio 25.
the consumption increased consistently with increasing effective
Exp. Veffective (V) Overall removal (%) voltage, and—as expected—the consumption increased in the con-
Total copper Soluble copper ventional EKR.
4 14.6 24.5 47.9
Because the energy expenditure of the EKR could become a ma-
1a 20.0 8.8 19.9 jor issue, restricting its applications, energy use was compared be-
6 22.6 7.7 16.7 tween a conventional and a sinusoidal (1 kHz of AC voltage) EKR.
5 26.7 21.5 55.3 The results in Table 8 show that energy use could be approximately
a
EKR with DC voltage. 10 times greater for the conventional EKR.

4. Conclusions
Table 7
Overall removal of total and soluble copper, pulses ratio effect. Effective voltage
14.6 V, AC frequency 1 kHz. For the conditions studied in this investigation, the conclusions
are as follows:
Exp. Pulses ON/OFF (s) Overall removal (%)
Total copper Soluble copper  Using a pulsed sinusoidal electric field with an AC voltage
1a – 8.8 19.9 frequency on the order of 1 kHz improved the EKR process in
9 20 (2000/100) 7.8 17.1 terms of increasing the displacement of total and soluble copper
4 25 (2500/100) 24.5 47.9 removed and reducing the specific energy consumption.
a
EKR with DC voltage.  A singular phenomenon was observed when increasing the
frequency to 2 kHz: for two effective voltages, the copper
removal was negligible, indicating that the DC voltage was
Table 8 inadequate to produce net electrokinetic phenomena during
General electric results. Effective voltage, charge, energy: total and specific.
the experiment.
Exp. Veffective QTotal Energy Mass of copper Energy consumption  As in previous work, EKR experiments with a pulsed sinusoidal
(V) (C) (kJ) removed (mg) (kWh/kg of total Cu) electric field and periodic reversal of polarity showed a sus-
1 20 18,325 367 112.1 909 tained improvement in removal of copper, specific energy con-
2 14.6 800 12 40.0 81 sumption and current efficiency.
3 14.6 1341 20 216.9 25
 For the tested conditions, an application of a pulsed sinusoidal
4 14.6 770 11 289.5 11
5 26.7 2683 72 268.0 74 electric field with an effective voltage of approximately 14.6 V
6 22.6 638 14 995.9 42 VDC = 10 V, VAC = 15 V), a frequency of 1 kHz for AC voltage,
7 14.6 1667 24 0 – and a pulse ratio of 25 (2500/100 (s)) was promising.
8 26.7 1138 30 0 –
9 14.6 2677 39 80.8 134

Acknowledgements

The results in Table 7 confirm that a pulse ratio of 25 is most This work was supported by the FONDECYT Project 1110057,
suitable for the conditions studied here. For a pulse ratio of 20 FONDECYT Project 1120111 and UTFSM Project 27.11.41.
(experiment 9), the removal was similar to a conventional EKR.
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