Minerals Engineering 41 (2013) 1–8

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Minerals Engineering

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Electro-remediation of copper mine tailings. Comparing copper removalefficiencies for two tailings of different age

Henrik K. Hansen a,⇑, Victor Lamas a, Claudia Gutierrez a, Patricio Nuñez a, Adrian Rojo a,Claudio Cameselle b, Lisbeth M. Ottosen c

a Departamento de Ingeniería Química y Ambiental, Universidad Técnica Federico Santa Maria, Casilla 110V, Valparaíso, Chileb Departamento de Ingeniería Química, Universidad de Vigo, C.P. 36.310 Vigo, Spainc Department of Civil Engineering, Building 116, Technical University of Denmark, 2800 Lyngby, Denmark

a r t i c l e i n f o

Article history:Received 9 July 2012Accepted 19 October 2012Available online 4 December 2012

Keywords:Electric fieldIon exchange membranesMine tailing agingSoil remediation

0892-6875/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.mineng.2012.10.002

⇑ Corresponding author. Tel.: +56 322654030; fax:E-mail address: henrik.hansen@usm.cl (H.K. Hanse

a b s t r a c t

This work compares and evaluates the copper removal efficiency when applying electric fields to two minetailings originating from the same mine but of different age. Eight experiments were carried out – four ontailings deposited more than 20 years ago (old tailings) and four on tailings deposited less than 2 years ago(new tailings). Parameters analyzed were the applied voltage drop, acid concentration during pretreat-ment, and the use of either passive or ion exchange membranes in the experimental setup.

The comparison of the results confirms that there are differences in the electroremediation between thetwo tailings, even if the pH is similar and a mineralogical analysis showed similarities between the sam-ples with respect to composition. It was found that an electroremediation is more favorable on the old tail-ings. The results showed that the best experimental conditions for both tailings is a pretreatment withH2SO4 1 M followed by applying 40 V for 7 days, using ion exchange membranes. In this case 16.7% of cop-per was removed from the anode section for the old tailings, whereas only 11.2% was removed from thenew tailings. The current efficiencies with respect to copper for the old and new tailings were 1.7% and1.6%, respectively.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

At present, Chile is one of the World’s most important copperproducers (Government of Chile, 2009), and due to this activitylarge volumes of mining residues have been accumulated – in par-ticular mine tailings. These fine grained residues, generated duringthe concentration of the minerals, contain elevated amounts ofheavy metals such as copper, lead, and arsenic (Kelm et al.,2009), and are deposited in impoundments. These impoundmentscause concern to the communities due to dam failures or naturalleaching to groundwater and rivers. The need for remediation orcontrol of these wastes is obvious (Renner and Ponce, 2007).

During the last decades, a series of actions have been initiated –both with respect to legal aspects and to possible remediationtechnologies – in order to solve the problems caused by the depos-iting of mine tailings. From a technological point of view, a series ofsolutions is under development in order to restore the solid mate-rial and to recover the valuable metals. Among these methods bio-remediation and phytoremediation can be mentioned but due tothe texture of the tailings and lack of organic material and nutri-

ll rights reserved.

+56 322654278.n).

ents, the efficiency of these methods is limited. Another potentialmethod is related to soil washing, but again the large content ofvery fine material in the tailings limits the success of the method(Dermont et al., 2008).

One method that successfully has been applied to treat heavymetal polluted soil is electrokinetic remediation (Ottosen et al.,2001; Ricart et al., 2004). Some success of this method when ap-plied to copper mine tailings has been demonstrated in laboratoryscale experiments (Hansen and Rojo, 2007; Hansen et al., 2007).Electrokinetic remediation basically consists in the application ofa low intensity direct electric current though the contaminatedmatrix: soils or mine tailings. The electric current mobilizes andtransports metal ions according to their electrical charge towardsthe electrodes. This method is especially attractive when treatingfine solid materials, since metals are adsorbed typically in the finefraction due to the large surface area. When applying the electricfield, the current passes through the soil as cations that movetowards the cathode and anions that move towards the anode(Ottosen et al., 2001). The electric current also induces reactionsupon the electrodes and into the contaminated matrix (in this casemine tailings): adsorption/desorption, precipitation/dissolution,redox reactions, etc. Those reactions may help in the extractionand dissolution of metal ions, which can be removed by migration.

2 H.K. Hansen et al. / Minerals Engineering 41 (2013) 1–8

Of course, the operating conditions (current intensity, duration, pHcontrol, and addition of complexing agents) may be adjusted to fa-vor the dissolution and the transportation of the metals out of thetailings. The extraction and dissolution of metal ions also dependson the speciation of metals in the tailings, and it is well known thatthe heavy metal speciation changes in the tailings impoundmentswith time. Several investigators have shown the importance ofweathering, oxidation/reduction processes and reactions betweenminerals on element speciation, solubility and leaching capacityin tailing deposits (e.g. Licskó et al., 1999; Dold and Fondbote,2001; Ramos Arroyo and Siebe, 2007). Time has a crucial effecton these phenomena.

This work focuses on the application of the electro-remediationtechnology to remove copper from two different mine mailingoriginally from the same Chilean copper mine – one depositedmore than 20 years ago and the other deposited recently. The gen-eral objective is to analyze how the age of the tailings affects theremediation results. In order to achieve this objective, a series ofexperiments is carried out taking into account the effect of the ap-plied voltage drop, the mineral acid concentration during pretreat-ment of tailings, and the use of either passive or ion exchangemembranes in the process. Copper removal and electric currentefficiencies are compared in order to determine the most favorableremediation conditions.

2. Background

Electrokinetic remediation (EKR) is a physical–chemical tech-nique based on an electrochemical process, where an electric fieldis generated by applying a direct current or a voltage drop acrosstwo inert electrodes located on both ends of an electrochemicalcell. The application of an electric field generates physical andchemical changes in the soil, inducing the transport of contami-nant species according to their charge towards the anode or cath-ode (Ottosen et al., 1997; Ottosen et al., 2005; Acar andAlshawabkeh, 1993). The application of an electric field leads to aseries of reactions such as electrolysis on the electrodes, sorption(adsorption/desorption), precipitation/dissolution and redoxreactions.

In EKR, the electrodes (anode and cathode) often are applieddirectly in the ground installing them in wells around the contam-inated area. An improvement to electro-remediation is electrodia-lytic remediation (EDR) that incorporates the use of ion exchangemembranes to prevent the ions generated at the electrodes to beincorporated to the soil during the treatment. Thus, the objectiveof the membranes is to isolate the treated soil and the species gen-erated in the electrode reaction product of electrolysis, but allowthe passage of the pollutant species from the soil to the catholytesand anolytes (Hansen et al., 2005). EDR typically is carried out atconstant voltage or constant current. Fig. 1 shows a generalizedview of EDR.

Several parameters can be calculated to evaluate the efficiencyof the electrokinetic/electrodialytic process: normalized copperconcentration, copper removal efficiency and electric currentefficiency.

2.1. Normalized copper concentration

The normalized copper concentration is calculated as reportedin the following equation:

Cr;i ¼Ci

Cinitialð1Þ

where Cr,i is the normalized copper concentration in zone i, i repre-sents the anodic, central or cathodic zone, Ci is the final copper con-

centration in zone i, and Cinitial is the initial copper concentration inmine tailing specimen used in each experiment.

The copper concentration after the experiment is calculated asdescribed in the following equation:

Ci ¼mj;i � Cj;i þmjþ1;i � Cjþ1;i

mj;i þmjþ1;ið2Þ

where j is each slide of the mine tailing specimen used in eachexperiment. The mine tailing specimen was divided in six slicesfrom the cathode to the anode as reported in Fig. 2. Thus, j repre-sents parts 1 and 2 for the cathodic zone; 3 and 4 for the centralzone; and 5 and 6 for the anodic zone. mj,i is the mass of part j inzone i, and Cj,i is the copper concentration of part j in zone i.

The normalized concentration after the experiment may belower or higher than 1. In the case that the value of Cr,i was higherthan 1, it means that there was an accumulation of copper in thissection of solid as a consequence of the electrokinetic/electrodia-lytic treatment. If the normalized copper concentration is less than1, it means that there was a removal of copper.

2.2. Copper removal efficiency

One of the most important parameters to quantify the effective-ness of the EKR/EDR is the copper removal efficiency. It is esti-mated based on the initial and final copper concentration in thetailing specimen, and it is calculated as described in the followingequation:

gremoval;i ¼mI;i �mF;i

mI;i

� �� 100 ð3Þ

where gremoval,i is the removal efficiency in zone i (expressed as %),mI,i is the initial copper mass in zone i (expressed in g), and mF,i isthe final copper mass in zone i (expressed in g).

The calculation of the removal efficiencies will be made only inthe anodic zone because it is the only area of the cell where copperis removed without receiving inputs from adjacent compartments,and therefore is an area to visualize the movement of copper in thecell.

2.3. Current efficiency

In practice, not all of the electric current that flows through theelectrokinetic cell is used for the transport and removal of copper,especially due to several phenomena that cause the loss of currentefficiency, such as current leakage, current short cut, and parasiticreactions among others. The current efficiency permits to quantifythe amount of power actually being used to remove the pollutantspecies of interest, in this case: copper.

The current efficiency is the ratio between the amounts of cop-per transported in each experiment and copper that theoreticallyshould be transported if all the current was transported by copperions in the following equation:

gcurrent ¼MCu; exp

MCu; theo� 100 ð4Þ

where gcurrent is the current efficiency (expressed as%), MCu,exp is themass of copper removed (expressed in g), and MCu,theo is the mass ofcopper theoretically removed by a given electric current (expressedin g).

The mass of copper removed experimentally is estimated underthe assumption that the treated soil is perfectly homogenized, andtherefore the entire volume of soil has the same concentration. Onthe other hand, the mass of copper theoretically removed by agiven current is estimated under the assumption that all copperis present as Cu2+. Therefore, the mass of copper theoretically re-moved (MCU,theo) can be calculated with the following equation:

+ -+ -

Anion exchange membrane

-+

H+ OH-

E

I

Cation exchange membrane

Anode Cathode

Fig. 1. Electrodialytic soil remediation. Basic principle.

3 4 5 6

Cathodic zone

1

Anodic zoneCentral zone

2

Fig. 2. Mine tailing zones in the electrokinetic/electrodialytic cell.

H.K. Hansen et al. / Minerals Engineering 41 (2013) 1–8 3

MCu;theo ¼I � Dt2 � F �mwðCuÞ ð5Þ

where I is the electric current (expressed in A), Dt is the time periodwith applied current (expressed in s), F is Faradays constant(F = 96485.34 C/mol), and mw(Cu) is the molecular weight of copper(63.5 g/mol).

3. Experimental

3.1. Mine tailings samples

Two different mine tailings samples were used both from Com-pañía Minera San Esteban Primera S.A. situated in Copiapó, III regionof Chile. One tailing sample was taken from the impoundment no. 3of the company. This impoundment was closed more than 20 yearsago (MT 20). A second sample was taken from impoundment no. 4D,which is still in operation. The actual sample from impoundment no.4D was considered to be less than 2 years old (MT 2). The mineralprocessing operation has not been changed significantly duringthe last decades, and the original copper ore is similar for both cases(Compañía Minera San Esteban Primera S.A., 2012). Therefore it canbe considered that the two samples in the moment of depositing the

tailings in the ponds, where similar too. A series of characteristics ofthe two tailing samples are listed in Table 1. From the table it can beseen that the main differences in composition are calcite content(higher in MT 2), free chalcopyrite content (higher in MT 2), andmagnetite content (higher in MT 20). The last is an indication ofthe ongoing oxidation of the tailings.

3.2. Reagents

Distilled water, sulfuric acid (95%, analytical grade) and nitricacid (65%, analytical grade) were the only reagents used in theexperiments for the removal of copper from mine tailings in EKR/EDR experiments.

3.3. Experimental setup: electrokinetic/electrodialytic setup

The ERK/EDR experiments were carried out in an acrylic cell,whose sketch is shown in Fig. 3. Mine tailings were placed in thecompartment between the electrode chambers, which had thedimensions: length: 15 cm, internal diameter: 8 cm and wall thick-ness: 1 cm. The electrode chambers are 4 cm length. Mine tailingswere separated from the electrolyte solutions in electrode cham-bers either by ion exchange membranes (Ultrex CMI-7000 cationexchange membrane or Ultrex AMI-7001 anion exchange mem-brane) or by filter paper grade no. 131, depending on the experi-ment. The electrodes were made of titanium, and electrolyteswere initially dilute nitric acid. During EKR experiments, the elec-trodes were connected to a power supply Extech model 382285and a Unit-T model UT60A multimeter was used to record the elec-tric data (voltage drop and electric current intensity).

3.4. Analysis

3.4.1. pH measurementpH of the mine tailing was determined using the US-EPA Stan-

dard SW-846 method 9045.20 g of dry solid was added to 20 mLdistilled water. After 30 min shaking, pH was measured in the

Table 1Mineralogical properties of the mine tailings.

Sample code MT 20 MT 2Mine tailing age >20 years <2 yearsSampling impoundment Tranque de Relave no. 3 Tranque de Relave no. 4D

Ganguespeciation Quantitya Quantitya

Volcanic rock: Andesited Abundant –Calcite, CaCO3 Abundant AbundantQuartz, SiO2 Low LowLimonite, FeO(OH) � n H2O Very low TracesSericite, KAl(AlSi)3O10(OH)2 Traces TracesPyrite, FeS2 Isolated grains GrainsFeldspar, KAlSi3O8 – AbundantHornblende, NaCa2(Mg, Fe, Al)5(Si, Al)8O22(OH)2 – LowBiotite, K(Mg, Fe)3AlSi3O10(OH)2 – Very low

Principal speciesb Size (lm) % w/wc Size (lm) % w/wc

Chalcopyrite CuFeS2 2–200 0.62 1–300 0.41Pyrite FeS2 20–250 1.06 10–300 0.76Magnetite Fe3O4 2–250 7.25 Max 150 0.29Calcite CaCO3 10–500 25.59 10–450 31.72Volcanic rock 10–500 65.48 – –Non-metallics (feldspar–quartz–hornblende) – – 4–500 66.82

Copper liberation degree % w/w % w/w

Free chalcopyrite particles 13 25Chalcopyrite particles in rock 87 –Chalcopyrite with non-metallics – 75

a Qualitative microscopic analysis of loose grains.b Microscopic analysis of briquette.c Semiquantitative microscopic analysis of briquette.d Species similar to Andesite.

Mine tailing CathodeAnode

4 [cm] 4 [cm]15 [cm]

8 [cm]

+ -

8 [cm]

Anion exchange membrane

1 [c

m]

Cation exchange membrane

Fig. 3. Electrodialytic cell setup.

4 H.K. Hansen et al. / Minerals Engineering 41 (2013) 1–8

supernatant solution. pH was measured with a Orion 370 pH meterwith a combined pH electrode.

3.4.2. Determination of total copperTotal copper content of tailing samples was determined by add-

ing 20 mL 1:1 HNO3 to 1.0 g of dry solid and treating the sample inautoclave, according to the Danish Standard DS 259:2003 (30 minat 200 kPa (120 �C)). The liquid was separated from the solid parti-cles by vacuum filtration through a 0.45 lm filter and diluted to100.0 mL. The metal content was determined by AAS in flame.The analysis procedure was done in triplicate, and the results re-ported are the average value. En general the standard deviationsfor the triplicates were below 3%.

3.4.3. Determination of soluble copperThe soluble copper content of the tailings was determined by

adding 50 mL H2SO4 5% (v/v) to 5.0 g of dry material, and stirringthe sample in a 250 mL Erlenmeyer flask for 30 min. The liquidwas separated from the solid particles by vacuum through a

0.45 m filter and diluted to 100.0 mL by adding 10 mL concen-trated HCl and distilled water. The metal content was determinedby AAS in flame. The analysis procedure was done in triplicate, andthe results reported are the average value.

3.5. Preparation and final characterization of the sample

Mine tailing specimens were stove-dried for 2 days at 70 �C.Once dried, the material was pulverized in a mortar and sievedwith meshes #4 and #20. A homogeneous sample was obtained.Either 1 M or 5 M sulfuric acid was added to the tailings until anaverage humidity of 20% was reached. With this humidity, the tail-ings were water saturated compact matrices. The mine tailingstreated like this were used for the EKR/EDR experiments.

Once the experiments were completed, mine tailing sampleswere segmented into six slices of equal size. The slices were driedat 105 �C for 24 h and copper content and pH were measured. Inthis work, anodic zone is defined as the two slices closest to the

H.K. Hansen et al. / Minerals Engineering 41 (2013) 1–8 5

anode, central zone the two slices in the middle, and cathodic zonethe two slices closest to the cathode (Fig. 2).

3.6. Experimental plan

Eight remediation experiments were carried out with the condi-tions given in Table 2. The experiments were carried out in pairsfor the two tailing specimens (MT 20 and MT 2), under the sameoperating conditions.

4. Results and discussion

4.1. pH

The pH is a critical parameter in electro-remediation, since thisparameter determines the extent of adsorption/desorption of ionsin the soil particle surface. Variation in pH can affect the solubility,mobility, availability, and ionic speciation of heavy metal contam-inants and other soil constituents. Theoretically, low pH values re-sults in greater solubility of metal ions, which in turn increasestheir mobility under the effect of an electric field.

Table 3 shows the pH values of the mine tailings before andafter the electrokinetic experiments. It includes the pH in each sec-tion of the specimen into the electrokinetic cell. As can be seen inTable 3, there is a variation of soil pH during the electro-remedia-tion, although both the initial and final values of pH are slightlyalkaline (except for exp. 4, which yielded a pH less than seven inmore than one section). On the other hand, acidification was initi-ated in the anodic and central sections for both tailings. This is be-cause the acid front generated at the anode. The use of anionicmembrane as separator for anode-mine tailing sample does notprevent the movement of H+ ions into the sample. It is supposedthat water near the anionic membrane (in anode section) dissoci-ates into H+ and OH� (Hansen et al., 1999). The OH� ions migrateback through the membrane to the anode, while H+ penetrate thesample towards the cathode, acidifying the tailing sample.

Table 2Experimental plan.

Experimentno./tailingsample

Remediationtime (days)

Appliedvoltage(V)

Use ofmembranes orfilter paper

ConcentrationH2SO4 (M)

1/MT 20 7 20 Membranes 12/MT 20 7 40 Membranes 13/MT 20 7 20 Membranes 54/MT 20 7 20 Filter paper 51/MT 2 7 20 Membranes 12/MT 2 7 40 Membranes 13/MT 2 7 20 Membranes 54/MT 2 7 20 Filter paper 5

Table 3pH measurements in the different experiments.

Experiment no./tailing sample pH0 Part of tailing volume

1 2 3 4 5 6

1/MT 20 8.6 9.3 8.1 8.0 8.1 8.0 8.02/MT 20 8.5 9.1 9.1 8.1 7.9 7.9 8.03/MT 20 8.1 9.6 7.5 7.2 7.2 7.2 7.24/MT 20 8.2 8.1 7.9 7.3 6.9 6.3 6.5

1/MT 2 8.2 8.2 7.9 7.7 7.8 7.6 7.62/MT 2 8.5 8.7 8.2 8.1 8.1 8.1 8.13/MT 2 8.3 8.7 8.1 8.0 7.6 7.9 7.74/MT 2 8.5 7.7 7.5 7.2 7.2 7.1 6.6

pH0: initial pH.

It can be standed out that in both tailings the initial pH in theelectro-remediation experiments, after pretreatment, is around8.3, the remaining alkalinity is mainly due to the high content ofcalcite in tailings (25.6% in MT 20 and 31.7% in MT 2, see Table 1).In the deposits, calcite is able to neutralize the acid water forma-tion, which is usually formed by the oxidation of pyrite and canmobilize heavy metals from the tailings. So, it can be stated thatthe two tailing samples are similar in terms of mineralogical com-position and pH, despite the different age of the deposit. Thus, sim-ilar remediation efficiencies between the two tailings could beexpected. However, the strongest pretreatment of mine tailingswith 5 M H2SO4 in the exp. 3 and 4 resulted in a slightly lower pHfor the sample MT 20. It can be explained by its lower calcite con-tent. High contents of calcite in the solid matrix normally slowthe heavy metal removal down during EKR (Paz-García et al.,2011; García-Delgado et al., 1996).

4.2. Copper concentration

Table 4 shows the normalized copper concentration after theexperiments, the removal efficiency of copper and the anodic cur-rent efficiency. It can be noted that the initial copper concentrationof tailings MT 20 is lower than MT 2. The MT 20 tailings weredeposited in an impoundment for 20 years, and the progress ofgeochemical phenomena occurring within the tailing may explainthe lower copper concentration. Natural acidification of the tailingsin the impoundments causes the leaching of oxidized metals. Thisprocess slowly occurring for 20 years decreased the concentrationin MT 20 tailings. A normalized copper concentration greater thanone in some of the sections of the tailings during or after EKRmeans accumulation of copper has occurred. This accumulationis due to precipitation/co-precipitation with calcite or on mineralssurfaces.

Based on the results reported in Table 4, it can be stated that alltests show an increasing trend of copper concentration in thedirection towards the cathode. As a consequence, in the anodicarea, in all experiments a significant removal of copper wasachieved, while in the cathodic area an accumulation of coppercoming from the central and anode sections was builded up. Thehighest copper removal in the anode section was found for MT20 tailings, in exp. 4 with a value of 25.5%. The highest removalin the anodic section for MT 2 was observed in exp. 1 and 2. Theelectric current efficiencies are quite low. Between 1% and 2% ofthe current is used for the mobilization of Cu+2 ions, the rest ofthe current registered in the experiment was due to the H+ gener-ated by hydrolysis on the anode and to the transportation other io-nic species present in the tailings that were not measured orregistered in this study.

4.2.1. Effect of the voltageThe influences of voltage drop in the electro-remediation exper-

iments can be discussed comparing the results in exp. 1 and 2 forboth tailings, where the voltage varies from 20 to 40 V. both exper-iments were carried out with the same pretreatment and anionand cation exchange membranes were used to separate the tailingsample from the electrodes. Fig. 4 shows the results of copper re-moval for each type of tailings. As can be seen, there is a relation-ship between increasing the applied voltage and copper removalefficiency, since the higher voltage increases the removal efficiencyfor both tailings. However, there are some differences in the behav-ior of both tailings. First, MT 20 shows an increase of more thandouble in the removal efficiency due to increased voltage. On theother hand, MT 2 only increased the removal efficiency of approx-imately 21%, which is much smaller than that presented by the oldtailings. This suggests that there is a greater electrical resistance inMT 2 as can be seen in Table 4 when comparing the total charge

Table 4Experimental EKR/EDR results.

Experimentno./tailingsample

Initial copperconcentration(mg kg�1)

Average copperconcentrations in each zone(mg kg�1)

Relative concentrations ineach zone (Cfinal/Cinitial)

Copper removalefficiencies from theanodic zone (%)

Totalcharge(C)

Copper currentefficiency in the anodiczone (%)

Cathodic Central Anodic Cathodic Central Anodic

1/MT 20 713.8 743.4 719.2 659.4 1.04 1.01 0.92 7.62 7623 1.012/MT 20 735.8 729.1 647.5 612.6 0.99 0.88 0.83 16.74 10,291 1.693/MT 20 757.9 837.3 674.1 614.7 1.10 0.89 0.81 18.89 21,221 1.084/MT 20 776.3 864.5 715.8 578.4 1.11 0.92 0.75 25.50 54,499 0.33

1/MT 2 1011.8 1150.4 1027.2 918.5 1.14 1.02 0.91 9.22 6740 2.042/MT 2 1019.1 1212.5 1017.1 904.9 1.19 1.00 0.89 11.20 9945 1.663/MT 2 1004.4 1173.5 1091.3 958.9 1.17 1.09 0.95 4.53 18,603 0.264/MT 2 997.1 1172.5 1093.6 939.9 1.18 1.10 0.94 5.73 4947 1.01

Fig. 4. Copper removal efficiency as a function of the voltage drop for MT 20 andMT 2 mine tailings.

Fig. 5. Copper removal efficiency as a function of sulfuric acid concentration in themine tailing pretreatment.

6 H.K. Hansen et al. / Minerals Engineering 41 (2013) 1–8

passed in all experiments. It is clearly seen that more charge ispassed in the experiments with MT 20. Therefore, this could indi-cate that the limiting current density in MT 2 is lower than inMT 20. When passing the limiting current density, the electricalresistance increases (Ottosen et al., 2000b). Furthermore, it couldbe expected that the concentration polarization in MT 2 wouldbe more pronounced than in MT 20, since in MT20 there are moremobile ions and less amount of calcite.

The analysis of the pH profile for exp. 1 and 2 reveals that thereare not significant differences between the tailings (see Table 3).Therefore, it can be stated that the pH is not affected by the varia-tion of the applied voltage.

4.2.2. Effect of the acid pretreatmentThe influence of acid pretreatment in the electro-remediation of

mine tailings can be done comparing the results of exp. 1 and 3 forboth tailings. Acid pretreatment of tailings is general very impor-tant and necessary (Rojo et al., 2006). On the other hand, chalcopy-rite is well known for its high resistance to acid leaching. Therefore,the pretreatment of tailings with H2SO4 at room temperature couldbe considered as a mild treatment only. The applied voltage dropand the type of membranes is the same in these experiments.The high content of calcite in both tailings justifies the acid pre-treatment. With no acid pretreatment, the mobilization of copperwould have to be done only with the H+ ions electrogenerated atthe anode. It results in a much longer treatment time and higherelectric energy expenditure. On the other hand, Ottosen et al.(2000a) suggested ammonia addition for copper removal in soilswith high carbonate content but ammonia treatment was not con-sidered in this work. Fig. 5 shows the comparative results consid-ering the acid pretreatment.

It is important to mention that during the pretreatment with5 M H2SO4, an increase in volume of the tailings was observed. This

can be explained by the reaction that occurs between the acid andcalcite (CaCO3) represented in Eq. (6), where CO2 is produced.

CaCO3 þ 2Hþ ! Caþ2 þH2Oþ CO2 ðgÞ ð6Þ

The produced CO2 formed small bubbles of gas that remains inthe solid trapped among the solid particles. Those bubbles increasethe electrical resistance to the current flow, making it necessary toremove all the gas before EKR/EDR treatment. Furthermore, thedissolved Ca2+ precipitate with sulfate – forming gypsum that alsoadds to the volume increase.

It can be observed in Fig. 5 that the removal of copper in the an-ode section for MT 20 in exp. 1 is more than increased to more thantwice in exp. 3, while for MT 2 there has been a decrease in copperremoval from 9.2% to 4.5% between exp. 1 and 3. Therefore, for theolder tailings the stronger of the tested acidic pretreatments isenhancing the copper removal, whereas for the new tailings the ef-fect seems to be not very important for the copper removal. Thiscould be explained by the higher content of calcite of MT 2 andthe high pH values even after the acid treatment. The stronger acidpretreatment in exp. 3 could mobilize other ionic species than cop-per decreasing the possibilities of copper to migrate. This is alsoconfirmed by the copper current efficiency, which is lower inexp. 3 than exp. 1 (see Table 4). The acid demand and thus costsfor pretreatment is therefore lowest for the old mine tailings.

4.2.3. Effect of use of membranes or filter paperThe effect of the use of either selective membrane or filter paper

can be determined when comparing exp. 3 and 4 for both tailings.Fig. 6 shows the copper removal for exp. 3 and 4. As it was reported

Fig. 6. Copper removal efficiency as a function of the use of ion exchangemembranes or filter paper.

Table 5Soluble copper concentration after EKR experiments.

Experimentno./tailingsample

Soluble copper concentrationsin each zone (mg kg�1)

Soluble copper/totalcopper in anodiczone (%)

Initial Cathodic Central Anodic

1/MT 20 138 153 144 55 8.32/MT 20 144 155 138 44 7.23/MT 20 145 160 150 31 5.04/MT 20 145 150 142 80 13.81/MT 2 118 135 124 85 9.32/MT 2 123 142 138 91 9.93/MT 2 123 135 140 90 9.44/MT 2 121 145 142 109 11.7

H.K. Hansen et al. / Minerals Engineering 41 (2013) 1–8 7

by Rojo et al. (2006), ion exchange membranes improve the re-moval of copper preventing the formation of an alkali front andtherefore the premature precipitation of mobile copper ions intothe solid material. Filter paper promotes the charge flow throughthe mine tailing specimen because the acid front can penetrate intothe specimen, acidifying the solid and releasing more ions, copperand others, attached to the solid particle surfaces or forming pre-cipitates. Furthermore, the ions released from the solid can freelymigrate towards the opposite charged electrode with no restric-tions of the ionic exchange membranes. Also, ions generated inthe electrodes can freely penetrate into the mine tailing specimen.It results in more charge transport during the experiments with fil-ter paper as separators.

As can be seen in Fig. 6, the change of ionic exchange mem-branes to filter paper as a separator between the electrode com-partment and the solid specimen favors the removal of copper inboth tailings, but in two different extents. The removal of copperin MT 20 increases by approximately 35%, whereas the copper re-moval is increased by around 26% for MT 2. This in accordance withthe earlier results since both tailings were pretreated with 5 MH2SO4, and the stronger acid pretreatment clearly favors the cop-per removal for the old tailings – in contrast to the new tailings.

Considering the following factors: (a) percentage of calcite inthe tailings, (b) sulfuric acid concentration used in pretreatment,and (c) the use of either ion exchange membranes or filter paper,it can be stated that the removal efficiency of copper increases withthe concentration of acid in the pretreatment for the MT 20 minetailing. This is probably due to the lower pH observed in the solidsample after the experiment. Moreover, current efficiency remainspractically constant comparing exp. 1 and 3. However, when thefilter paper is used, despite an increase in removal efficiency ofcopper, the current efficiency decreases (comparing exp. 3 and4). A reason for this is that the filter paper does not present a bar-rier for ions, and more electric charge is transported by a greateramount of ions (not only copper ions) than when using themembranes.

On the other hand, the behavior of MT 2 mine tailing is a littledifferent. When the acid concentration in the pretreatment is in-creased (comparing exp. 1 and 3), there was a clear decrease incopper removal efficiency, so that in exp. 3, the transport of otherions than copper ions was favored with the stronger acid pretreat-ment. This is concluded since the electric charge in exp. 3 was oneof the highest, but only one of the lowest current efficiencies wasobtained. Moreover, if the membranes are changed to filter paper(comparing exp. 3 and 4) an increase in copper removal efficiencycan be observed with a decrease in the electric charge. This sug-gests that during exp. 4 gas bubbles must have been present inthe tailings, resulting in slower movement of ions in the tailings.

Despite this, a higher current efficiency was obtained when usingfilter paper instead of membranes.

Thus, the assumption that the filter paper supports the removalof copper, in order to improve the charge flow, is valid for the oldtailings, while for the new tailings this is not the case. The electriccharge in exp. 4 for MT 2 was considerably lower with filter paper.

4.2.4. Effect of the age of the tailingsThe analysis and comparison of copper removal and current

efficiencies for both mine tailing used in this study, generallyshows that it is more advisable to apply EKR/EDR to the old minetailings. Although MT 20 had a lower amount of copper that MT2 and therefore less metal to be removed, the level of progress ofweathering and thereby generation of acidic waters in the old tail-ings allows greater amounts of heavy metals to be in solution thanin the new tailings.

When copper is present as sulfides, the remediation will be verylow since copper is not dissolved, which is crucial for the EK phe-nomena in electrochemical remediation. Therefore, during theaging of the tailings in the impoundments, natural oxidation ofthe sulfides is occurring, which is necessary for desorption and re-moval of copper using electric fields. In the case of the finer particlesize (MT 20), effectively the oxidation of the smaller particles is fa-vored during aging.

In general, it should be noted that in the new mine tailings, cop-per could be expected to be found as residual copper sulfide, whichwas not liberated in the grinding process prior to flotation. The lowcopper removals obtained for these tailings were due to the lowsoluble copper content in the tailings, which can be seen in Table5 that shows the soluble copper content initially and in the differ-ent zones in the tailings after each experiment. In the old tailingsthe content of soluble copper is higher – despite the lower totalcopper content. This content, of soluble copper, is variable due tothe heterogeneous origin of the mine tailings in the ponds, amongothers reasons: copper grades depend on the original characteris-tics of the tails disposed, aging of the tailings in the ponds as con-sequence of physical–chemical changes due to weathering andbacterial actions in time (Rojo and Cubillos, 2009). On the otherhand, the fresh tailings have not yet been aged sufficiently – there-fore the lower soluble copper content.

From Table 5 it can be noted that in all experiments, in the an-ode section, the soluble copper content is lowered considerably. Inthe middle and cathode sections larger amounts of soluble copperare present. This is explained by the fact that the copper movedfrom the anode section is passing the middle and cathode sectionson its path to the cathode due to the electric field. For the old tail-ings, in exp. 3/MT 20, only 5% of the copper remaining in the anodezone is soluble. Therefore it can be concluded that EKR could be atool to stabilize this solid residue with respect to leaching risks.The use of this remediation technology will imply the periodicapplication of the method in order to remove the additional soluble

8 H.K. Hansen et al. / Minerals Engineering 41 (2013) 1–8

copper that will be generated with time. Therefore, the remedia-tion action for this heterogeneous solid waste is to remove thesoluble copper in the tailings and in this way making the finalresidue more stable.

Moreover, considering that in Chile a total of 867 deposits areregistered (Cartagena, 2007), 86% of the dams are inactive or aban-doned. Even if the exact age of these tailings is not known, it is as-sumed that most of these dams are more than 10 years old, andtherefore EKR/EDR could be a feasible technology to remediate –or stabilize – the abandoned tailing deposits rather than the newtailings coming directly from the flotation processes. On the otherhand, the construction of new tailing dams is governed by ChileanSupreme Decree No. 248, and therefore must provide advancedtechnological methods to prevent or minimize the leakage of con-taminated water outside the impoundment. One of these methodsincludes calcite addition to the tailings, which has been the case forthe mine of Compañía Minera San Esteban Primera S.A. Thus, thephenomenon of acid water has to be solved retaining the water in-side the impoundment, and after some years the tailings have beensufficiently aged for the EKR process, without being a severe riskfor the environment.

5. Conclusions

From the results obtained in this work, it can be seen that cop-per is mobilized and moved within the mine tailing sample for allexperimental conditions studied. With respect to total copper re-moval around 20% could be removed with the experimental condi-tions given. On the other hand, the remaining copper in the tailingsis basically all insoluble copper – meaning that the risk of leachingcopper to the environment would be low after the EKR treatment.

When comparing experimental EKR results on copper mine tail-ings of different age, differences in copper removal were found de-spite similarities in mineralogical composition and pH. The bestcopper removal from a mine tailing sample of more than 20 yearsof age was when applying a pretreatment of strong acidic acid andusing the classical EKR setup. On the other hand, for the tailingssample less than 2 years old, best removal was found with slightlyless acid pretreatment and using ion exchange membranes in thesetup.

When analyzing the effect of applied voltage, it was observedthat as expected when increasing the applied voltage from 20 to40 V, the copper removal also increases. Nevertheless, the amountof increase differs between the old and the new tailings – with theold tailings showing the largest increase. Applying 40 V – for bothtailings – the copper current efficiency is higher than with 20 V.

When analyzing the effect of acid concentration during pre-treatment, it can be concluded that a stronger acid pretreatmentfavors the calcite dissolution and thereby solubilize liberates somecopper that is removed by EKR – especially from the old tailings,whereas the opposite is the case for the new tailings. For thenew tailings, this is explained by the high amount of calcite, whichdissolves in sulfuric acid solution and afterwards CaSO4 is precipi-tated. The copper present as sulfides would probably need strongeracids and oxidizing agents in order to solubilize more copper – ifthe purpose is to remove all copper.

When analyzing the effect of using either ion exchange mem-branes or filter paper to separate the tailings from the electrolytesbetter copper removal is observed when using filter paper. This isexplained by the acidic front moving fast from the anode into themine tailings, when using filter paper. The presence of protons iscrucial in order to mobilize copper from the tailings – especially

in the new tailings, where large amount of insoluble copper stillis present. On the other hand, when using ion exchange mem-branes, the copper current efficiency is higher than using filterpaper.

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