Ee

JD

h

••••

a

ARRAA

KPRER

1

gwurptfiw[c

j

0h

Journal of Hazardous Materials 262 (2013) 709– 716

Contents lists available at ScienceDirect

Journal of Hazardous Materials

jou rn al hom epage: www.elsev ier .com/ locate / jhazmat

lectrochemical reactor with rotating cylinder electrode for optimumlectrochemical recovery of nickel from plating rinsing effluents

.R. Hernández-Tapia, J. Vazquez-Arenas ∗, I. Gonzálezepartamento de Química, Universidad Autónoma Metropolitana, San Rafael Atlixco 186, C.P. 09340, México, D.F., Mexico

i g h l i g h t s

Rotating cylinder cathode enhanced mass transport rates of Ni(II) species.pH control around 4 is crucial to recover high purity nickel.Increasing cathodic currents increased energy consumptions for nickel recovery.Specific energy consumptions increase drastically at the end of electrolysis.

r t i c l e i n f o

rticle history:eceived 30 May 2013eceived in revised form 10 August 2013ccepted 12 September 2013vailable online 20 September 2013

eywords:lating

a b s t r a c t

This study is devoted to analyze the metallic electrochemical recovery of nickel from synthetic solutionssimulating plating rinsing discharges, in order to meet the water recycling policies implemented in theseindustries. These effluents present dilute Ni(II) concentrations (100 and 200 ppm) in chloride and sul-fate media without supporting electrolyte (397–4202 �S cm−1), which stems poor current distribution,limited mass transfer, ohmic drops and enhancement of parasitic reactions. An electrochemical reactorwith rotating cylinder electrode (RCE) and a pH controller were utilized to overcome these problems. ThepH control around 4 was crucial to yield high purity nickel, and thus prevent the precipitation of hydrox-

insing effluentslectrochemical recovery of nickelCE

ides and oxides. Macroelectrolysis experiments were systematically conducted to analyze the impactsof the applied current density in the recovery efficiency and energy consumption, particularly for verydiluted effluents (100 and 200 ppm Ni(II)), which present major recovery problems. Promising nickelrecoveries in the order of 90% were found in the former baths using a current density of −3.08 mA cm−2,and with overall profits of 9.64 and 14.69 USD kg−1, respectively. These estimations were based on the

e for

−1

international market pric

. Introduction

Major concerns have arisen as a result of environmental impactsenerated by the sustained over-exploitation of surface and groundater from industry. This situation has not only consumed nat-ral resources, but also promoted the contamination of naturaleservoirs. The treatment of multiple industrial effluents has beenarticularly promoted by the establishment of strict environmen-al laws, which regulate and control the level (i.e. concentration,raction) of these pollutants in the discharges. The electroplatingndustry has not been exempted from these regulations since it

astes large volumes of water, resulting from rinsing operations1–4]. Thus, it has been spurred to optimize its processes (e.g. dis-harge minimization, recycling), and develop new technologies for

∗ Corresponding author. Tel.: +55 5804 4600x2686.E-mail addresses: jgva@xanum.uam.mx,

orge gva@hotmail.com (J. Vazquez-Arenas).

304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2013.09.029

nickel ($18 USD kg )© 2013 Elsevier B.V. All rights reserved.

the treatment of these industrial effluents. Although these effluentscan be processed with conventional methods such as coagulation[5,6] and electrocoagulation [7–9], high costs of operation due tothe generation of large amounts of sludge detract from their pro-fitability.

As in any industrial process, the treatment of these effluentsrelies on the possibility of reutilization and costs demanded fortheir treatment. Electrodeposition using rotating cylinder electrodereactors (RCE) has been successful in the treatment of dilutedand concentrated effluents containing Cu, Zn, Ni, Cd from miningwastes, wastewater, plating and recycling operations [2,10–12].The RCE is comprised of an inner cylinder (cathode) which isused to deposit the metal (large surface area) and promote tur-bulence to enhance mass transfer (high mass-transfer coefficient),and multiple anodes concentrically attached to the reactor walls

and interconnected. This creates a uniform concentration profilein the inter-electrode gap, in order to attain satisfactory currentand potential distributions to selectively recover [12–17]. The RCEcan operate in continuous or batch mode, and can produce high

710 J.R. Hernández-Tapia et al. / Journal of Hazar

Nomenclature

RCE rotating cylinder electrodeHER hydrogen evolution reaction˚e

Ni current efficiency� density (g cm−3)� absolute viscosity of the solution (g cm−1 s−1)D diffusion coefficient (cm2 s−1)ω rotation rate of cylinder (rad s−1)z number of electronsF Faraday constant (C mol−1)CNi(II) initial concentration of Ni(II) (mol cm−3)Ct Ni(II) concentration at any moment of electrolysis

(mol cm−3)�C concentration gradient (mol cm−3)MNi molar mass of nickel (kg mol−1)Vr volume of solution in the electrochemical reactor

(cm3)Qt total charge passed during the electrolysis (C)I current (mA)j current density (mA cm−2)jL mass transport limited current density for an RCE

(mA cm−2)U cell potential (V)t time (s)

e −1

pgmcompctrHhchnettaafbatustaddestosc

wNi specific electrical energy consumption (kW h kg )dcyl diameter of the cylinder rotator (cm)

urity powders or hard deposits presenting different morpholo-ies [10,12,14]. In addition, this type of technologies can performetal removal at high current efficiencies under dilute or con-

entrated conditions (10–10,000 ppm), and with the appropriateperational variables can handle electrolytes polluted with otheretals or organic compounds, and counteract the occurrence of

arasitic reactions and ohmic drop during electrolysis [10]. Theseharacteristics are of great economic benefit since they augmenthe profitability of the process. Effluents containing nickel rep-esent high economic interests due to its price ($18 USD kg−1).owever, nickel electrolysis from plating rinse waters has beenardly explored [3,4,18–21], and most of these studies have beenonducted under ideal conditions (e.g. substrate, [Ni(II)] ≥ 500 ppm,igh electrolyte conductivity, microelectrolysis), which do notecessarily resemble the real conditions and avoid some of thexisting problems arising in diluted discharges. Optimal condi-ions to electro-recover nickel from wastewater were proposed onhe basis of a Taguchi analysis, revealing that voltage, boric acidnd Ni(II) concentration affect differently both recovery efficiencynd energy consumption [1]. Nevertheless, electrolysis was inef-ectively utilized to decrease the bulk concentration (1000 ppm)elow 119 ppm (batch mode) and 350 ppm (continuous), presum-bly due to the configuration of the reactor. Likewise, statisticalools were employed to recover nickel from spent catalyst liquorssing a typical electrolytic cell [21]. Statistical regression demon-trated that initial bulk concentration and current density havehe most significant effects on current efficiency, while time hadn opposite behavior. Nickel recovery by electrodeposition fromilute industrial wastewater (i.e. galvanic nickel plating) was con-ucted employing a three-dimensional electrode in a gas diffusionlectrode packed bed electrode cell and a RCE [19]. This workhowed that solution composition and electrolysis variables affect

he formation of other nickel compounds (e.g. oxide, hydroxide,xyhydroxide) which can block the cathode surface, while nickelelectivity relies on the applied current density and Ni(II) con-entration. A similar study was conducted to reuse nickel plating

dous Materials 262 (2013) 709– 716

rinse waters through the use of an electrochemical cell dividedby an anion exchange membrane (ElectroMPcell) [4]. It was con-cluded that this technology can potentially replace the traditionalion exchange process for treating rinse waters, while yielding valu-able materials. 90% nickel removal and 74% current efficiency wereachieved from similar effluents containing 2000 ppm Ni(II) in arotating packed cell at 50 ◦C and 325 A m−2 [3]. While the currentefficiency dropped to 45% and the energy consumption drasticallyincreased when the nickel removal was nearby 100%.

Although, the nickel plating from rinse waters entails elec-trolysis as in electrowinning (high Ni(II) concentrations) orelectroforming (nickel anodes are utilized to maintain a high-concentrated Ni(II) solution), it involves diluted nickel solutions(100–500 ppm) with low electrolyte conductivity (i.e. absence ofsupporting electrolyte). This situation modifies considerably oper-ational variables such as current density, electrolyte conductivity,pH control and residence time; and the phenomena controllingthe behavior (mass transfer and kinetics) of the electrochemicalreactor. Particularly, the severe depletion of Ni(II) concentrationat long residence times stems poor current distribution, ohmicdrop, high electrode overpotentials and enhancement of parasiticreactions (i.e. hydrogen evolution) unlike what occurs in elec-trowinning or electroforming. Accordingly, the current efficiency,energy consumption and nickel purity are remarkably affected, andcannot be easily adjusted as in electrowinning operations. To theauthor’s knowledge, appropriate conditions have not been figuredout yet for the nickel recovery by electrodeposition from typicalindustrial effluents involving dilute nickel solutions (<500 ppm).Thus, this study aims to analyze and determine the variables thatcontrol the nickel recovery by electrodeposition process at lowNi(II) concentrations (100 and 200 ppm) without supporting elec-trolyte (variable electrolyte conductivity), in order to mimic rinsingsolutions of plating effluents. Additional efforts are oriented to min-imize the operating costs. The feasibility of this process is estimatedfrom the economic benefits obtained for these processes, based onthe international market price for nickel, $18 USD kg−1 [22].

2. Experimental

Typical synthetic solutions were prepared to simulate the con-ditions of the rinsing solutions generated from the nickel platingindustry, e.g. Watts-type baths. Different NiSO4 (99.9%, J.T. Baker),NiCl2 (98%, J.T. Baker) and B(OH)3 (99.8%, Merck) were preparedaccording to the conditions reported in Table 1. The nickel platingwas conducted using an electrochemical reactor (600 mL volume)with rotating cylinder electrode shown in Fig. 1. A cylinder (3.8 cmdiameter × 11 cm length) made of stainless steel (S316) was usedas cathode, and six RuO2/TiO2 dimensionally stables anodes (2 cmwidth × 0.3 cm thickness) were utilized as anodes. These anodescatalyze the oxygen evolution unlike conventional anodes (Pb–Sbor carbon); and decrease anode potentials and cell voltages. Amembrane-divided reactor was not used in the RCE reactor toavoid potential drops across the electrolyte (conductivity from 397to 635 �S cm−1) and keep the operational costs as low as possi-ble. In addition, the production of Cl2 or other chloride speciesaffecting the cathodic current efficiency is expected to be lowat the anode, mainly as a result of a low chloride concentra-tion in the effluents. The RCE was rotated at 300 rpm employinga variable revolution motor CaframoTM. A Hg/HgSO4(s)/K2SO4(sat)electrode (SSE) was used as reference. Prior to each experiment,the RCE was polished using SiC-type abrasive papers (600 and

1200 grades) and then Buehler alumina powder (final grain size≤0.05 �m) to a mirror finish. The electrode was then sonicated andrinsed with deionized water to remove remaining particles on itssurface.

J.R. Hernández-Tapia et al. / Journal of Hazardous Materials 262 (2013) 709– 716 711

Table 1Typical synthetic solutions simulating plating rinsing effluents (e.g. Watts-type baths for nickel deposition) at pH 4. Boric acid was added in each solution to attain a Ni(II)/BoricAcid ratio equal to 0.165. The cathodic limiting current density (jL) for an RCE was determined at 300 rpm using Eq. (1).

Bath no. [Ni2+] (ppm) [SO42−] (ppm) [Cl−] (ppm) Conductivity (�S cm−1) jL (mA cm−2)

1 100 120 35.0 397 −0.6162 200 240 70.0 635 −1.233 500 600 175.0 1515 −3.084 750 900 262.5 2041 −4.625 1000 1200 350.0 2488 −6.166 1250 1500 437.5 2994 −7.707 1500 1800 525.0 4202 −9.24

Fig. 1. Sketch of the electrochemical reactor with rotating cylinder electrode uti-le

tstertpssrct

3

iTd1

Fig. 2. Plots of log Ni(II) concentration as a function of electrolysis time. The exper-iments were performed on a rotating cylinder electrode at 300 rpm, applying acurrent density of −6.16 mA cm−2, and controlling the pH around 4 during electroly-

tration drops with time as a result of the occurrence of the nickel

ized to perform the electrochemical recovery of nickel. (a) Rotating cylinderlectrode, (b) rotating motor, (c) pH control system.

The pH of the bath was controlled between 3.9 and 4.3 duringhe electrochemical recovery through an on-off pH system. Thisystem (Fig. 1) was equipped with a cell for outer flow to the elec-rochemical reactor, in order to monitor the pH at any instant. Thelectrolysis were conducted in galvanostatic mode, using four cur-ent density values: −6.16, −4.13, −3.08 and −1.85 mA cm−2. Thehree electrodes were connected to a Princeton Applied ResearchTM

otentiostat/galvanostat model VMP3, equipped with data acqui-ition EC-Lab version 9.98. A Princeton Applied ResearchTM powerupply was coupled with the potentiostat to generate output cur-ent and voltage of 20 A and 20 V, respectively. 0.5 mL samples wereollected from the baths to track the decay of Ni(II) bulk concen-ration during electrolysis.

. Results and discussion

This section describes the experimental results correspond-ng to the macroelectrolysis of nickel plating using a RCE reactor.

he cylinder rotation speed was fixed at 300 rpm, as previouslyetermined by our research group for Ni(II) concentrations around200 ppm [23]. Mass transport limited current densities (jL) were

sis. Synthetic solutions simulating plating rinsing discharges (Table 1) with differentinitial Ni(II) concentrations are shown: (a) 100, (b) 200, (c) 500, (d) 750, (e) 1000, (f)1250 and (g) 1500 ppm.

calculated for the RCE using the equation proposed in [24], and theNi(II) concentrations reported in Table 1:

jL = 0.0487zFCNi(II)d+0.4cyl

(�/�)−0.334D0.664ω0.7 (1)

where z is the number of electrons (2) transferred in theelectrochemical reaction, F is Faraday’s constant equals to96485.5 C mol−1, CNi(II) is the initial concentration (mol cm−3), dcylis the diameter the cylinder rotator (cm), � is the solution density(g cm−3), � is the absolute viscosity of the solution (g cm−1 s−1), D isthe diffusion coefficient (cm2 s−1), and ω rotation rate of the cylin-der (rad s−1). As observed in Table 1, a jL value of −6.16 mA cm−2

was determined at 1000 ppm Ni(II). This is the highest value con-sidered in this work to ensure the formation of nickel deposits onstainless steel under mass-transport control at any bath compo-sition below 1000 ppm Ni(II). Less negative experimental currentsof −4.13, −3.08 and −1.85 mA cm−2 were utilized to analyze theinfluence of applied current density on current efficiency andenergy consumption. Note that even the lowest experimental cur-rent used in this study (−1.85 mA cm−2) is more negative thanthe limiting current densities estimated with Eq. (1) for 100(−0.616 mA cm−2) and 200 ppm (−1.23 mA cm−2), whereby nickeldeposition is guaranteed to proceed under mass-transport con-trol at these concentrations. These baths present major recoveryproblems, whence they constitute the main interest of this study.

Fig. 2 shows plots of the log[Ni(II)] as a function of electrolysistime. A general trend is observed in this figure, the Ni(II) concen-

recovery by electrodeposition on the RCE. The rinsing solutionscontaining low Ni(II) concentrations present a more drastic decayin concentration with time (Fig. 2a–d) in comparison with higher

712 J.R. Hernández-Tapia et al. / Journal of Hazardous Materials 262 (2013) 709– 716

F ts were performed on a rotating cylinder electrode at 300 rpm, applying a current densityo simulating plating rinsing discharges (Table 1) with different initial Ni(II) concentrationsw

cNtNaiobt5ceahtftts2p(ga

tipewndsvswmsmb

te

˚

wrm

0

0.2

0.4

0.6

0.8

1

100806040200

curr

ent

effi

cien

cy

Nirecovered (%)

a)

b)

c)d)

e)

f)

g)

Fig. 4. Current efficiency as a function of the percentage of recovered Ni. The exper-iments were performed on a rotating cylinder electrode at 300 rpm, applying acurrent density of −6.16 mA cm−2, and controlling the pH (4) during electrolysis.

ig. 3. Images of the nickel deposits obtained after 2 h of electrolysis. The experimenf −6.16 mA cm−2, and controlling the pH (4) during electrolysis. Synthetic solutionsere used: (a) 100, (b) 200, (c) 500, (d) 750, (e) 1000, (f) 1250 and (g) 1500 ppm.

oncentrations (Fig. 2f and g). This leads to a faster depletion ofi(II) species in the reactor, which denotes a kinetic variation in

he nickel electrodeposition for the different bath compositions.ote in Fig 2a–e the formation of two distinct regions separated by

pronounced (Fig. 2c–e) or gradual (Fig. 2a and b) change in slopen each curve. This action suggests a kinetic modification in the firstrder depletion, as a result of the predominant Ni(II) species or H+

eing reduced [19]. The comparison of Fig. 2g with a similar elec-rolytic process [3] (initial [Ni(II)] = 2000 ppm, 32.5 mA cm−2, pH 4,0 ◦C and 4 rpm using a rotating packed cell) revealed that the pro-ess shown in Fig. 2g depletes the total [Ni(II)] in 200 min, while thexperiment reported in Ref. [3] presents a bulk [Ni(II)] ≈ 200 ppmt similar time. Although this finding could be associated with aigher initial bulk [Ni(II)], the experimental evidence indicates thathe lack of pH control and lower rotation could be the responsibleor such behavior, since baths with lower pH values need longerimes to remove the remaining bulk [Ni(II)] [3], and higher rota-ion would be more effective to enhance mass-transfer to cathodeurface. Other studies were less successful to drop a [Ni(II)] from50 ppm to approximately 59 ppm in a 24 h batch operation (6 V,H 4), most likely due to the use of an anion exchange membranei.e. potential drop) [4]; and from 1000 ppm to 116.66 ppm in a sin-le electrolyser cell during 8 h (4 V, pH 4), presumably as a result ofn inappropriate mixing in the cell at the end of electrolysis [1].

First insights about the influence of the parasitic reactions inhe experiments shown in Fig. 2 were obtained through visualnspection of the quality of the deposits. Fig. 3 shows the mor-hological features of these nickel deposits. Note that for all thexperimental conditions (including 100 and 200 ppm), a pure metalas recovered unlike what has been reported in the literature (i.e.ickel oxide, hydroxide and oxyhydroxide) [19]. This quality of theeposits was achieved on account of the pH control of the rinsingolutions during electrolysis, which buffer the depletion of H+ in theicinity of the RCE. Otherwise, the conjugated depletion with Ni(II)pecies generates the formation of hydroxide/oxide compounds,hich drop the current efficiency and decrease the quality of theetallic nickel. When the Ni(II) concentration was increased in

olution, the thickness of the deposits augmented and tended to beore brittle (Fig. 3f and g). Consequently, the nickel was released

y rotation effects of the RCE.In order to analyze the influence of the parasitic reactions in

he nickel electrolysis, current efficiencies were calculated from thexperiments shown in Fig. 2, using the following equation:

eNi = zF�CVr (2)

Qt

here �C = (CNi(II) − Ct) is the concentration gradient in mol cm−3

esulting from the initial Ni(II) concentration (CNi(II)) and at anyoment of electrolysis (Ct), Vr is the volume of solution in cm3,

Synthetic solutions simulating plating rinsing discharges (Table 1) with differentinitial Ni(II) concentrations are shown: (a) 100, (b) 200, (c) 500, (d) 750, (e) 1000, (f)1250 and (g) 1500 ppm.

and Qt is the total charge passed during the electrolysis at constantcurrent. Fig. 4 shows the current efficiencies vs. the percent-age of recovered nickel at different initial Ni(II) concentrations(100–1500 ppm). Note that the addition of NaOH during electrol-ysis affects favorably the current efficiency, since it controls theacidification produced in the anode (i.e. neutralizing H+) as a resultof water oxidation. Consequently, the transport of H+ to the cathodesurface is mitigated, as well as their reduction. The plots in Fig. 4reveal that the current efficiency depends significantly on Ni(II)concentration, low concentrations are characterized by low currentefficiencies (Fig. 4a and b) whereas high concentrations presentan opposite behavior (Fig. 4e–g). This is a consequence of thelimited mass-transfer of Ni(II) species and the ohmic drop (i.e. lowelectrolyte conductivity, Table 1) effects occurring in the reactor,which cause inhomogeneous current and potential distributions.This inhomogeneity results from the depletion of the electroactiveionic species (Ni(II), H+) and the absence of a supporting elec-trolyte in typical nickel plating rinsing discharges, which producea modification in the electrolyte conductivity (or resistance) acrossthe electrochemical reactor (inter-electrode gap). The dependenceof the current and potential distributions on the geometry anddimensions of the electrochemical reactor has been described inrefs. [25,26]. A similar decay in current efficiency as a function

of residence time (i.e. increase of % Ni recovered) was observedfor the electrochemical recovery of nickel in effluents containing a[Ni(II)] of 117386.8 ppm (62.5 mA cm−2, pH ∼ 2.5) [27]. Since in thepresent study, low Ni(II) concentrations (100 and 200 ppm) are the

J.R. Hernández-Tapia et al. / Journal of Hazardous Materials 262 (2013) 709– 716 713

Fig. 5. Normalized decay of Ni(II) concentration (log[Ni(II)]t/[Ni(II)]t=0) as a functionof electrolysis time obtained at initial Ni(II) concentration of 100 ppm. The exper-ip(

otTdea

(aedtos6rhyfrdtl−iccnirttcd

pabeee∼[

Fig. 6. Normalized decay of Ni(II) concentration (log[Ni(II)]t/[Ni(II)]t=0) as a functionof electrolysis time obtained at initial Ni(II) concentration of 200 ppm. The exper-iments were conducted utilizing a rotating cylinder electrode at 300 rpm, and thepH (4) was controlled during electrolysis. Different current densities were applied:(a) −6.16, (b) −4.31, (c) −3.08 and (d) −1.85 mA cm−2.

0

0.2

0.4

0.6

100806040200

curr

ent

effi

cien

cy

Nirecovered (%)

a)

b)

c)

d)

Fig. 7. Current efficiency as a function of the percentage of recovered Ni obtainedfrom solutions containing an initial Ni(II) concentration of 100 ppm. The experi-

ments were conducted utilizing a rotating cylinder electrode at 300 rpm, and theH (4) was controlled during electrolysis. Different current densities were applied:a) −6.16, (b) −4.31, (c) −3.08 and (d) −1.85 mA cm−2.

nly ones presenting low current efficiencies (Fig. 4a and b), fur-her studies are merely focused on these experimental conditions.hese experiments will involve the variation of the applied currentensity in order to determine the operating conditions where thelectrodeposition efficiency increases and the costs of the processre minimized.

Fig. 5 shows the normalized decay of Ni(II) concentrationlog[Ni(II)]t/[Ni(II)]t=0) as a function of time, in a bath containingn initial concentration of 100 ppm during electrolysis at differ-nt current densities. As described before in Fig. 2, a first orderepletion is suggested to occur under these experimental condi-ions due to the linear profile. As observed, there is an independencef the concentration plots regardless of the value of the current den-ity. Additionally, 94% nickel recovery is reached in approximately0 min for all the experiments. A similar high percentage of nickelecovery has not been reported in this period by any other study,owever, other works have observed that the first hour of electrol-sis also proceeds thermodynamically (i.e. linear decay of [Ni] as aunction of time) regardless of the current density imposed in theeactor [1,3]. Longer residence times are characterized by a slowecay of the concentration subject to mass-transport control. Whenhe nickel concentration is increased to 200 ppm (Fig. 6), a simi-ar behavior is observed for most of the current densities, except1.85 mA cm−2 where a slower concentration drop is observed

n comparison with the experiments conducted at more negativeurrents. This finding indicates that at 100 ppm, most of the appliedurrent is consumed in the parasitic reactions (HER), whereby theickel reduction is strongly affected by this reaction, without being

nfluenced by the imposed current. On the other hand, the Ni(II)eduction competes effectively with the HER at 200 ppm, whencehe application of a less negative current (−1.85 mA cm−2) shrinkshe recovery rate. Therefore, the Ni(II) consumption in the electro-hemical reactor relies on the current density at 200 ppm, whichiffer at 100 ppm.

Current efficiencies are plotted in Fig. 7 as a function of theercentage of recovered nickel for the experiments performedt 100 ppm Ni(II) (Fig. 5). Note that the current efficiencies areelow 0.6 as predicted from Fig. 4, and less negative currentsntail larger efficiencies (Fig. 7d). A typical behavior for metal

lectrodeposition is observed from this plot, the first stages oflectrolysis are characterized by current efficiencies from ∼0.55 to0.21, which are lower than those values obtained for high initial

Ni(II)] (Fig. 4c–f). As electrolysis progresses, this concentration is

ments were conducted utilizing a rotating cylinder electrode at 300 rpm and the pH(4) was controlled during electrolysis. Different current densities were applied: (a)−6.16, (b) −4.31, (c) −3.08 and (d) −1.85 mA cm−2.

consumed and consequently the current efficiency starts decayingat a rate proportional to the percentage of nickel remaining in solu-tion (mass-transfer control to the RCE), and H+ concentration sincethe depletion of these ions decreases the electrolyte conductivity,leading to increased ohmic drop. It is worth mentioning that para-sitic reactions occur even at the start of electrolysis. At this point,the magnitude of these reactions in comparison with the nickelelectrolysis relies on the applied current density since this valuedefines the reduction rate imposed on the RCE, and the conductivityof the solution (see Table 1). These undesirable reactions are laterenhanced in the system as observed in the current efficiency decaysat larger percentages of recovery. As stated in Ref. [3], electrolysiscarried out in low concentrated electrolytes differs from typicalelectrowinning operations, whereby at high [Ni(II)] current den-sities ranging from 30 to 35% of the limiting current densities areimplemented, whereas at low concentrations (i.e. rinsing effluents)the current densities need to be more moderated since the processis mass-transport controlled, and need to avoid more important

contributions from the side-reactions.

Current efficiencies as a function of the recovered nickel areshown in Fig. 8 for initial [Ni(II)] concentrations equals to 200 ppmand different current densities. A similar trend as obtained in

714 J.R. Hernández-Tapia et al. / Journal of Hazardous Materials 262 (2013) 709– 716

0

0.2

0.4

0.6

0.8

1

100806040200

curr

ent

effi

cien

cy

Nirecovered (%)

a)

b)

c)

d)

Fig. 8. Current efficiency as a function of the percentage of recovered Ni obtainedfrom solutions containing an initial Ni(II) concentration of 200 ppm. The experi-mp(

Feeadcdoeaednetddois2

fiaradtawirrt

uTtdptai

Fig. 9. Images of the nickel deposits obtained after 2 h of electrolysis in a solu-tion containing an initial Ni(II) concentration of 100 ppm. The experiments wereconducted utilizing a rotating cylinder electrode at 300 rpm, and the pH (4) was con-trolled during electrolysis. Different current densities were imposed on the cathode:(a) −6.16, (b) −4.31, (c) −3.08 and (d) −1.85 mA cm−2.

Fig. 10. Images of the nickel deposits obtained after 2 h of electrolysis in a solu-tion containing an initial Ni(II) concentration of 200 ppm. The experiments were

ents were conducted utilizing a rotating cylinder electrode at 300 rpm, and theH (4) was controlled during electrolysis. Different current densities were applied:a) −6.16, (b) −4.31, (c) −3.08 and (d) −1.85 mA cm−2.

ig. 7 (100 ppm) was observed; the current efficiency drops as thelectroactive species were consumed due to the aforementionedffects. However, the efficiencies present a characteristic behaviort the start of electrolysis. Curves (Fig. 8a–d) obtained at currentensities of −6.16, −4.31, −3.08 and −1.85 mA cm−2 present effi-iencies of 0.75, 0.86, 0.63 and 0.73, respectively. This behavior isifferent from the systematic shift observed in Fig. 7 at early stagesf recovery, and could be associated with the influence of differ-nt predominant Ni(II) species in the imposed current density, as

result of a higher concentration. Similar limitations in currentfficiency were reported for a nickel recovery from rinse waterischarges where a 74% current efficiency was obtained with 90%ickel removal ([Ni(II)] = 2000 ppm, 50 ◦C, 32.5 mA cm−2, pH 5.5,lectrolysis time >6 h). The current efficiency dropped to 45% whenhe nickel recovery was close to 100%, or the bulk pH was loweredown [3]. A similar current efficiency value (67%) was reported toeplete 250 ppm Ni(II) to approximately 59 ppm in a 24 h batchperation (6 V, pH 4) [4]. To the authors state of knowledge, a sim-lar current efficiency (Fig. 8b) has not been achieved by any othertudy under similar experimental conditions ([Ni(II)] = 200 ppm,5 ◦C, 4.31 mA cm−2, pH 4, electrolysis time = 2 h).

The effects of the applied current density in the morphologicaleatures of the nickel deposits recovered from the bath containingnitial [Ni(II)] of 100 ppm (Table 1) are presented in Fig. 9. Note thatll the deposits were metallic and present different morphologiesegardless the recovery kinetics were virtually the same (Fig. 4). Forll the deposits, 99% nickel recovery was achieved in 2 h. The nickeleposit obtained at the most negative current (−6.16 mA cm−2) washe most homogeneous (Fig. 9a), whereas those deposited at −4.31nd −3.08 mA cm−2 (Fig. 9b and c) were duller since the depositsere more dispersed due to less negative currents imposed dur-

ng electrolysis. At −1.85 mA cm−2, (Fig. 9d) the nickel was mainlyecovered at the bottom of the cylinder, probably due to a poor cur-ent distribution and wall effects. The deposit in the upper part ofhe cylinder was also scattered.

The nickel deposits obtained for the same current densities butsing an initial [Ni(II)] of 200 ppm (Table 1) are shown in Fig. 10.hese deposits are more homogeneous and dense compared tohose produced in Fig. 9 (100 ppm). This suggests that the currentensity imposed on the RCE plays a very important role in the mor-

hology and quality of the electrochemical recovery of nickel. Onhe other hand, the variation of the current density at 200 ppmlso presented some differences as compared to those producedn 100 ppm, but these were reflected at more negative currents

conducted utilizing a rotating cylinder electrode at 300 rpm, and the pH (4) was con-trolled during electrolysis. Different current densities were imposed on the cathode:(a) −6.16, (b) −4.31, (c) −3.08 and (d) −1.85 mA cm−2.

(Fig. 10a). As observed in the image obtained at −6.16 mA cm−2

(Fig. 10a), the deposit present some dark spots dispersed along theRCE, which were related to the scattering of metallic nuclei due toinstantaneous electrodeposition at high current densities [28]. Forthe other current densities, this contribution was not so important,whereby it was not reflected in the quality of the deposits and 98%nickel can be recovered electrochemically in 80 min. The experi-ments shown in Fig. 10 indicate that the nickel plating becomes animportant influence in the morphology and quality of the deposits,when the initial [Ni(II)] is increased in solution and the appropriatecurrent density is selected for this process.

In order to evaluate the profitability of the electrochemicalrecovery of nickel at dilute conditions (Figs. 5 and 6), its specificenergy consumption (kW h kg−1) was calculated as follows [25]:

weNi = zFUcel

˚eNiMNi

(1

3.6 × 106

)(3)

where Ucel is the cell voltage (V), ˚eNi is the current efficiency and

MNi is the molar mass of nickel (0.0586934 kg mol−1). Fig. 11 showsthe effect of Ni conversion on specific electrical energy conver-sions for an initial [Ni(II)] of 100 ppm (Fig. 4), and utilizing four

current densities (−6.16, −4.13, −3.08 and −1.85 mA cm−2). Thesecurves present a typical increase in the energy consumption as thenickel concentration depletes in solution, and the slope becomessteeper in the last portion of the plots as a result of the decrease in

J.R. Hernández-Tapia et al. / Journal of Hazardous Materials 262 (2013) 709– 716 715

Fig. 11. Energy consumptions as a function of the percentage of recovered Ni fromsolutions containing an initial Ni(II) concentration of 100 ppm. The experimentswere conducted utilizing a rotating cylinder electrode at 300 rpm, controlling thep−

cwaTtdNfcToirartm(cilae

Fig. 12. Energy consumptions as a function of the percentage of Ni recovered fromsolutions containing an initial Ni(II) concentration of 200 ppm. The experimentswere conducted utilizing a rotating cylinder electrode at 300 rpm, controlling the

TEE

a

H (4) during electrolysis and applying different current densities: (a) −6.16, (b)4.31, (c) −3.08 and (d) −1.85 mA cm−2.

urrent efficiency (see Fig. 7). This feature can be associatedith the enhancement of parasitic reactions, and ohmic drops

nd limited mass-transfer increase in the electrochemical reactor.hese effects are mainly related to the consumption of electroac-ive species (e.g. Ni(II), H+) nearby the surface of the RCE, whichecrease the electrolyte conductivity and suppresses the flux ofi(II) species at long residence times. A similar finding has been

ormerly described [3], and a bigger difference is obtained in energyonsumption when the current density is varied from 65 to 50 ◦C.able 2 shows a comparison of the energetic demands (kWh kg−1)f nickel recoveries extracted from Figs. 11 and 12 at differentnitial [Ni (II)] of 100 and 200 ppm (third and sixth columns),espectively. The operating costs per kilogram of nickel (fourthnd seventh columns) produced as a function of the applied cur-ent density, and the overall profits calculated from subtractinghe operating costs per kilogram of nickel to the international

arket price for nickel (i.e. 18 USD kg−1) [22] are also includedfifth and eight column). The values tabulated in Table 2 werealculated for two different scenarios: 90% Ni recovery and forndustrial disposals of plating rinsing discharges below permissible

evels (4 ppm Ni(II)) [29]. Note that for both scenarios the over-ll profits increase when the initial [Ni (II)] concentration in theffluent was higher (e.g. 200 ppm). As observed, all the processes

able 2nergy consumptions calculated for a 90% electrochemical recovery of nickel, and for industimated costs for different applied current densities are also provided.

Initial [Ni (II)] (ppm) jappl (mA cm−2) wconsumption

(kWh kg−1)bOperating cost(US$ kg−1)b,d

100 −6.16 102.30 14.94

−4.13 71.50 10.44

−3.08 57.29 8.36

−1.85 25.00 3.65

200 −6.16 51.15 7.47

−4.13 27.70 4.04

−3.08 20.85 3.04

−1.85 15.80 2.31

1000 −6.16 11.59 1.52

1500 −6.16 9.80 1.29

Residence time was not sufficient to deplete all the [Ni(II)] in the electrochemical reactob Energy consumption, operating cost or overall profit estimated for the electrochemicc Energy consumption, operating cost or overall profit estimated for the electrochemicd These calculations were based on the energetic consumption rate in Mexico, 0.1314 Ue Overall profit for the electrochemical recovery of nickel estimated from the internatio

pH (4) during electrolysis and applying different current densities: (a) −6.16, (b)−4.31, (c) −3.08 and (d) −1.85 mA.

performed at 90% Ni recovery produced economic benefits, whichare increased when the applied current density was made less neg-ative in the reactor. This provides an excellent feasibility to carryout this process at industrial levels, and simultaneously complyingwith the strict environmental regulations. A slightly different situ-ation presents for the electrochemical recoveries of nickel reachinga final [Ni(II)] of 4 ppm. Not surprisingly, this process demanded ahigher energy consumption (refer to the last portion of the plotsshown in Figs. 11 and 12) to recover the last 10% Ni; hence theoverall profits decreased. However, there were only losses at themost negative current imposed in the reactor (−6.16 mA cm−2), andall the other current densities represent economic benefits to theprocess. Thus, if the solution is recycled to the process stream or dis-posed (i.e. 4 ppm Ni(II)), less negative currents would yield higherprofits in both cases. The specific energy consumptions reportedfor 100 and 200 ppm Ni(II) were high compared to other stud-ies, since these were carried out at higher [Ni(II)] concentrationsabove 1000 ppm [1,3]. As observed between the comparison ofFigs. 11 and 12, a lower [Ni(II)] would entail a remarkable increaseof Ucel (lower electrolyte conductivity), and decrease of ˚e . This

Nisituation can easily augment the energy consumptions (refer toEq. (3)). Thus, a fair comparison would be to compare we

Ni valuesdetermined under similar [Ni(II)]. An electrolytic process carried

strial disposals of plating rinsing discharges below permissible levels (4 ppm Ni(II)).

Overall profit(US$ kg−1)b,e

wconsumption

(kWh kg−1)cOperating cost(US$ kg−1)c,d

Overall profit(US$ kg−1)c,e

3.06 191.70 26.24 −8.247.56 107.30 14.69 3.319.64 85.90 11.76 6.24

14.35 37.50 5.13 12.8710.53 153.00 20.51 −2.5113.96 51.95 6.97 11.0314.96 37.50 5.03 12.9715.69 a a a

16.48 17.63 2.32 15.6816.71 a a a

r.al recovery of nickel of 90 wt.% Ni.al recovery of nickel, reaching a final [Ni(II)] equals to 4 ppm [29].S$ per kWh [30].nal market price for nickel, 18 US$ kg−1 [22].

7 Hazar

o3cvtOietf

4

fdii(ppe

acvetrcca−cetccta2dvkawt(e(a1cettciett

A

c

[

[

[

[

[

[

[

[

[

[

[

[

[[

[

[

[

[

[

16 J.R. Hernández-Tapia et al. / Journal of

ut under the following experimental conditions: 2000 ppm Ni(II),2.5 mA cm−2, pH 5.5, 50 ◦C, electrolysis time >6 h, 74% current effi-iency, demanded an energy consumption of 4.2 kWh kg−1 [3]. Thisalue is close to the 9.8 kWh kg−1 reported in Table 2, consideringhat the current study was performed at 1500 ppm Ni(II) and 25 ◦C.n the other hand, nickel removal from 1000 ppm to 116.66 ppm

n a single electrolyser cell during 8 h (4 V, pH 4) demanded annergy consumption of approximately 25.7 kWh kg−1 [1], wherebyhis value is significantly larger than the 11.59 kWh kg−1 reportedor 1000 ppm Ni(II) (Table 2).

. Conclusions

A systematic analysis for the electrochemical recovery of nickelrom synthetic plating rinsing solutions (e.g. low electrolyte con-uctivity) was presented, in order to comply the requirements

mposed by environmental recycling policies imposed for platingndustries. These discharges contained dilute Ni(II) concentrations100 and 200 ppm) in chloride and sulfate media without sup-orting electrolyte (397–4202 �S cm−1), which typically originatedoor current distribution, limited mass transfer, ohmic drops andnhancement of parasitic reactions.

An electrochemical reactor with rotating cylinder electrode and pH controller were utilized to overcome these anomalies. The pHontrol around 4 is decisive to yield high purity nickel, and thus pre-ent the precipitation of hydroxides and oxides. Macroelectrolysisxperiments were systematically conducted in two stages, wherehe first one comprises the kinetic evaluation of the process in theange of Ni(II) concentrations from 100 to 1500 ppm applying aurrent density of −6.16 mA cm−2 (i.e. no kinetic limitations asso-iated with the input signal). The second stage was dedicated tonalyze the impacts of the applied current density (−6.16, −4.13,3.08 and −1.85 mA cm−2) in the recovery efficiency and energy

onsumption, for those baths presenting major recovery problems,.g. 100 and 200 ppm Ni(II). Results from the second stage showedhat low current efficiencies (<0.6) are obtained at these low Ni(II)oncentrations, regardless of the current imposed in the rotatingylinder electrode. Nickel recoveries around 90% were found in thewo former baths using a current density equals to −3.08 mA cm−2,nd with specific energy consumptions of 57.29 (100 ppm) and0.85 kWh kg−1 (200 ppm). These consumptions are drasticallyecreased as the initial [Ni(II)] is increased in the reactor, or the celloltage and current efficiency are increased. The operating costs perilogram of nickel estimated under these experimental conditionsre 8.36 (100 ppm) and 3.04 (200 ppm) USD kg−1, respectively,hich provide an excellent feasibility to the process considering

he overall profits of the electrochemical recovery of nickel: 9.64100 ppm) and 14.69 USD kg−1 produced nickel (200 ppm). Thesestimations were based on the international market price for nickel$18 USD kg−1). On the other hand, energy consumptions of 85.90nd 37.50 kWh kg−1, respectively, with overall profits of 6.24 and2.97 US$ kg−1 produced nickel, were calculated when the nickeloncentration was consumed below international permissible lev-ls for industrial disposals (4 ppm Ni(II)). These costs are still withinhe profitable limits of the electrochemical recovery of nickel. Sincehe cell voltage remains constant during electrolysis, current effi-iency decreases irretrievably when the electrochemical recoverys continued beyond 90%. This results in significantly higher specificnergy consumptions, whereby the use of other technique suitableo remove the residual Ni(II) concentration could be a wise choiceo minimize the operational costs.

cknowledgment

The authors are indebted to CONACyT (Mexico) for their finan-ial support to carry out this work.

[

[

dous Materials 262 (2013) 709– 716

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013.09.029.

References

[1] R. Jin, C. Peng, A. Abou-Shady, K. Zhang, Recovery of precious metal material Ni

from nickel containing wastewater using electrolysis, Appl. Mech. Mater. 164(2012) 263–267.

[2] V. Coman, B. Robotin, P. Ilea, Nickel recovery/removal from industrial wastes:a review, Resour. Conserv. Recyl. 73 (2013) 229–238.

[3] G. Orhan, C. Arslan, H. Bombach, M. Stelter, Nickel recovery from the rinsewaters of plating baths, Hydrometallurgy 65 (2002) 1–8.

[4] P.T. Bolger, D.C. Szlag, Electrochemical treatment and reuse of nickel platingrinse waters, Environ. Prog. 21 (2002) 203–208.

[5] J.F. Blais, Z. Djedidi, R.B. Cheikh, R.D. Tyagi, G. Mercier, Metals precipitationfrom effluents: review, J. Hazard. Mater. Toxic Radioact. Waste 12 (2008)135–149.

[6] A.Lj. Bojic, D. Bojic, T. Andjelkovic, Removal of Cu2+ and Zn2+ from modelwastewaters by spontaneous reduction-coagulation process in flow conditions,J. Hazard. Mater. 168 (2009) 813–819.

[7] P.D.N. Meunier, C. Montané, R. Hausler, G. Mercier, J.-F. Blais, Com-parison between electrocoagulation and chemical precipitation formetals removal from acidic soil leachate, J. Hazard. Mater. 137 (2006)581–590.

[8] W.C.I. Heidmann, Removal of Zn(II), Cu(II), Ni(II), Ag(I) and Cr(VI) presentin aqueous solutions by aluminium electrocoagulation, J. Hazard. Mater. 152(2008) 934–941.

[9] M.Y. Mollah, P. Morkovsky, J.A. Gomes, M. Kesmez, J. Parga, D.L. Cocke, Funda-mentals, present and future perspectives of electrocoagulation, J. Hazar. Mater.114 (2004) 199–210.

10] J. St-Pierre, N. Massé, É. Fréchette, M. Bergeron, Zinc removal from dilute solu-tions using a rotating cylinder electrode reactor, J. Appl. Electrochem. 26 (1996)369–377.

11] D.R. Gabe, The rotating cylinder electrode, J. Appl. Electrochem. 4 (1974)91–108.

12] C.T.J. Low, C.P. De Leon, F.C. Walsh, The rotating cylinder electrode (RCE) andits application to the electrodeposition of metals, Aust. J. Chem. 58 (2005)246–262.

13] F.C. Walsh, A First Course in Electrochemical Engineering, Electrochemical Con-sultancy, New York, 1993.

14] D.R. Gabe, F.C. Walsh, The rotating cylinder electrode: a review of development,J. Appl. Electrochem. 13 (1983) 3–21.

15] D.R. Gabe, G.D. Wilcox, J. Gonzalez-Garcia, F.C. Walsh, The rotating cylinderelectrode: its continued development and application, J. Appl. Electrochem. 28(1998) 759–780.

16] J.M. Grau, J.M. Bisang, Removal of cadmium from dilute aqueous solutions witha rotating cylinder electrode of expanded metal, J. Chem. Technol. Biotechnol.78 (2003) 1032–1037.

17] F.C. Walsh, The role of the rotating cylinder electrode reactor in metal ionremoval, in: J.D. Genders, N.L. Weinberg (Eds.), Electrochemistry for a CleanerEnvironment, The Electrosynthesis Company Inc., New York, 1992.

18] P. Fornari, C. Abbruzzese, Copper and nickel selective recovery by electrowin-ning from electronic and galvanic industrial solutions, Hydrometallurgy 52(1999) 209–222.

19] K.N. Njau, M.Vd. Woude, G.J. Visser, L.J.J. Janssen, Electrochemical removalof nickel ions from industrial wastewater, Chem. Eng. J. 79 (2000)187–195.

20] R. Idhayachander, K. Palanivelu, Electrolytic recovery of nickel from spent elec-troless nickel bath solution, J. Chem. 7 (2010) 1412–1420.

21] M.E. Ossman, W. Sheta, Y. Eltaweel, Linear genetic programming for predictionof nickel recovery from spent nickel catalyst, Am. J. Eng. Appl. Sci. 3 (2010)482–488.

22] www.lme.com (accessed June 2013).23] F.J. Almazán-Ruiz, P.I. Benítez-Ramos, M.R. Cruz-Díaz, F.F. Rivera, I. González,

E.P. Rivero, Proceedings of the XXV Meeting of the Mexican Society of Electro-chemistry, 2010.

24] M. Eisenberg, C.W. Tobias, C.R. Wilke, Ionic mass transfer and concentrationpolarization at rotating electrodes, J. Electrochem. Soc. 101 (1954) 306–320.

25] D. Pletcher, F.C. Walsh, Industrial Electrochemisty, second ed., Blackie Aca-demic and Professional Publishers, Glasgow, UK, 1990.

26] J.R. Hernández-Tapia, J. Vazquez-Arenas, I. González, Electrochim. Acta 103(2013) 266–274.

27] A. Hankin, G.H. Kelsall, Electrochemical recovery of nickel from nickel sulfamateplating effluents, J. Appl. Electrochem. 42 (2012) 629–643.

28] A. Magdy, M. Ibrahim, Black nickel electrodeposition from a modified Watts

bath, J. Appl. Electrochem. 36 (2006) 295–301.

29] Mexican Standard NOM-001-ECOL-1996 (Diario Oficial de la Federation06/01/97 with corrections published 30/04/97) governing wastewater reuse,www.semarnat.gob.mx

30] www.cfe.gob.mx (accessed June 2013).