nickel recovery from the rinse waters of plating baths

8
Nickel recovery from the rinse waters of plating baths G. Orhan a , C. Arslan a, * , H. Bombach b , M. Stelter b a Chemistry and Metallurgy Faculty, Metallurgical and Materials Engineering Department, Istanbul Technical University, 80626 Maslak, Istanbul, Turkey b TU Bergakademie Freiberg, Institut fu ¨r NE-Metallurgie und Reinsstoffe, Leipziger Str. 23, 09596 Freiburg, Germany Received 14 December 2001; received in revised form 11 April 2002; accepted 12 April 2002 Abstract An electrolytic process is applied to the de-metalization of rinse waters emerging from nickel-plating baths. Optimum conditions of nickel recovery from this type of solution have been investigated through a series of experiments carried out in a rotating packed cell (RollschichtzelleR). The effects of temperature and pH of electrolyte, current density, cell rotation speed, and diameter of the cathode granules were examined. Ninety percent of the nickel was recovered from a solution of 450 mL volume, with a pH of 5.5 F 0.05, 2 g/L initial nickel concentration, and by using cathode granules of 5 mm in diameter. At 50 jC electrolyte temperature and 325 A/m 2 current density, 74% current efficiency was attained with 4.2 kWh/kg Ni energy consumption. D 2002 Published by Elsevier Science B.V. Keywords: Nickel recovery; Reduction electrolysis; Rotating cathode; Rollschichtzelle; Plating industry; Rinse waters 1. Introduction Electrolytic nickel plating is used either for the protection of steel, brass, pressure cast zinc and plastic materials or simply for decorative purposes. Practical applications of nickel plating are carried out either in sulfate, chlorate, watts or sulfamate type of baths with various contents, details of which can be found else- where (Blatt and Schneider, 1996). A typical compo- sition of the industrially used sulfate bath is 300 – 350 g/L NiSO 4 7H 2 O and 25–38 g/L H 3 BO 3 (Brillas et al., 1999; www.surtec.de). Almost all the rinse waters emerging from metal plating plants materialize during the washing opera- tion of plated materials. These solutions with low metal ion contents either must be recycled within the process or be discarded to the receiving medium after decreasing their concentrations to acceptable levels determined by the environmental regulations and/or standards. Various industrial techniques and processes exist for the treatment of this type of solution. These processes can be classified under three different head- ings according to the fundamental principles they rely on; chemical (neutralization, chemical precipitation), physical (evaporation) and electrochemical (electrol- ysis). Either one or more than one of these techniques are utilized in combination for the de-metalization of these solutions. It is the aim of this present study to recover nickel from the rinse waters of nickel sulfate plating baths via an electrolytic process. The electrolysis cell used for the experiments had an increased cathode surface thus 0304-386X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII:S0304-386X(02)00038-5 * Corresponding author. E-mail address: [email protected] (C. Arslan). www.elsevier.com/locate/hydromet Hydrometallurgy 65 (2002) 1 – 8

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Page 1: Nickel recovery from the rinse waters of plating baths

Nickel recovery from the rinse waters of plating baths

G. Orhan a, C. Arslan a,*, H. Bombach b, M. Stelter b

aChemistry and Metallurgy Faculty, Metallurgical and Materials Engineering Department, Istanbul Technical University,

80626 Maslak, Istanbul, TurkeybTU Bergakademie Freiberg, Institut fur NE-Metallurgie und Reinsstoffe, Leipziger Str. 23, 09596 Freiburg, Germany

Received 14 December 2001; received in revised form 11 April 2002; accepted 12 April 2002

Abstract

An electrolytic process is applied to the de-metalization of rinse waters emerging from nickel-plating baths. Optimum

conditions of nickel recovery from this type of solution have been investigated through a series of experiments carried out in a

rotating packed cell (RollschichtzelleR). The effects of temperature and pH of electrolyte, current density, cell rotation speed,

and diameter of the cathode granules were examined. Ninety percent of the nickel was recovered from a solution of 450 mL

volume, with a pH of 5.5F 0.05, 2 g/L initial nickel concentration, and by using cathode granules of 5 mm in diameter. At 50

jC electrolyte temperature and 325 A/m2 current density, 74% current efficiency was attained with 4.2 kWh/kg Ni energy

consumption.

D 2002 Published by Elsevier Science B.V.

Keywords: Nickel recovery; Reduction electrolysis; Rotating cathode; Rollschichtzelle; Plating industry; Rinse waters

1. Introduction

Electrolytic nickel plating is used either for the

protection of steel, brass, pressure cast zinc and plastic

materials or simply for decorative purposes. Practical

applications of nickel plating are carried out either in

sulfate, chlorate, watts or sulfamate type of baths with

various contents, details of which can be found else-

where (Blatt and Schneider, 1996). A typical compo-

sition of the industrially used sulfate bath is 300–350

g/L NiSO4�7H2O and 25–38 g/L H3BO3 (Brillas et

al., 1999; www.surtec.de).

Almost all the rinse waters emerging from metal

plating plants materialize during the washing opera-

tion of plated materials. These solutions with low

metal ion contents either must be recycled within the

process or be discarded to the receiving medium after

decreasing their concentrations to acceptable levels

determined by the environmental regulations and/or

standards. Various industrial techniques and processes

exist for the treatment of this type of solution. These

processes can be classified under three different head-

ings according to the fundamental principles they rely

on; chemical (neutralization, chemical precipitation),

physical (evaporation) and electrochemical (electrol-

ysis). Either one or more than one of these techniques

are utilized in combination for the de-metalization of

these solutions.

It is the aim of this present study to recover nickel

from the rinse waters of nickel sulfate plating baths via

an electrolytic process. The electrolysis cell used for

the experiments had an increased cathode surface thus

0304-386X/02/$ - see front matter D 2002 Published by Elsevier Science B.V.

PII: S0304 -386X(02 )00038 -5

* Corresponding author.

E-mail address: [email protected] (C. Arslan).

www.elsevier.com/locate/hydromet

Hydrometallurgy 65 (2002) 1–8

Page 2: Nickel recovery from the rinse waters of plating baths

enhancing the ionic movements by convective diffu-

sion. It was therefore suitable for the electrolysis of

solutions with low concentrations.

2. Theoretical

The potential vs. current density curve for hydro-

gen has an important role in determining the electrol-

ysis conditions of metals, such as nickel, which are

below hydrogen in the EMF series (Fig. 1). The pre-

requisite in the electrolysis of these metals, which are

below hydrogen in the EMF series, is to carry out the

operation at high current densities (though limited by

high energy consumption) and at the lowest possible

temperature. The third critical parameter for attaining

high current efficiencies in nickel electrolysis is the

pH of the electrolyte (Blatt and Schneider, 1996).

Cathodic reduction of nickel is only possible within a

very narrow pH range, as compared to copper (Bev-

erskog and Puigdommenech, 1997). Cathodic nickel

reduction may seem to be thermodynamically possible

within the pH range of 4.9–8.3, but this is only valid

for the bulk acidity of the solution. The pH value of

the solution that exists within the Nernst diffusion

layer shows an exponential increase toward the inner

Helmholtz layer due to the cathodic reduction of

hydrogen. The practical meaning of this pH increase

is the formation of h-Ni(OH)2 on the cathode surface.

As is well known, there is a potential difference

between h-Ni(OH)2 and metallic nickel, since nickel

transforms into its oxy-hydroxides generating gal-

vanic current. Due to these unwanted side-reactions

and phases, electrolysis conditions become uncontrol-

lable and thus the operation is carried out at the

highest possible pH values.

Electrolysis of solutions with low metal ion con-

tents is quite different from classical metal electro-

winning operations. Conventional electrolysis opera-

tions are carried out at very high metal concentrations

(i.e. 65–80 g/L for nickel) and at current densities

ranging from 30% to 35% of the limiting current

densities, to obtain a smooth and compact cathodic

surface, to avoid the side reactions, and to be able to

work at high current efficiencies. Major reactions

taking place during the electrolysis are:

at the cathode : Ni2þ þ 2e� ! Nij

E ¼ �0:27þ 0:0591logC2þNi

at the anode : H2O ! 2Hþ þ 1=2O2 þ 2e�

E ¼ 1:229� 0:0591 pH:

On the other hand, in electrolysis operations where

metal ion concentrations are low, as in the case of de-

metalization electrolysis of rinse waters from plating

industries, the system is diffusion controlled. Thus,

the diffusion limiting current decreases as the concen-

tration diminishes. The difference between the bulk

solution concentration and that of the phase boundary

(Dc) is the driving force of diffusion and is very small

in dilute solutions. This concentration difference

depends on the thickness of the diffusion layer (dN).Therefore, cell design must be so tailored that the

smallest possible diffusion layer thickness can be

reached at high electrolysis rates. Moreover, electrol-

ysis cells with high volume-time efficiencies (n) must

be utilized for economical processing of this type of

solution (Kreysa, 1981; Friedrich and Raub, 1983).

The volume-time efficiency is defined as

n ¼ q� k � c ð1Þ

where c is the metal ion concentration, k the charge

transfer coefficient, and q the surface area of the

cathode. Since metal ion concentrations are low in

rinse waters, charge transfer coefficient and relatedFig. 1. Schematic illustration of current density–polarization curve

for nickel (Charlot et al., 1962; Pletcher and Walsh, 1993).

G. Orhan et al. / Hydrometallurgy 65 (2002) 1–82

Page 3: Nickel recovery from the rinse waters of plating baths

current density must be high. The cathode electrode’s

surface area must also be as large as technologically

possible for obtaining high volume-time efficiencies.

In an earlier study, where electrolytic treatment of

nickel-containing rinse waters was investigated, For-

nari and Abbruzzese (1999) obtained 99.5% nickel

recovery from a solution initially containing 10 g/L

Ni2 + , in their 180 mL laboratory-scale electrolysis

cell with a 3.05-kWh/kg Ni specific energy consump-

tion. Puippe (1995) reported 80% nickel recovery at

the end of a 24-h electrolysis operation where solu-

tions with 1.8–2.0 mg/L initial compositions were

employed. Danneels (1990) treated a nickel solution

initially containing 5.52 g/L, for more than 30 h, and

reached a final concentration of 0.04 g/L, with less

than 1% current efficiency. Hartinger (1990) pro-

cessed 3.5 g/L Ni2 + solutions at 200–250 A/m2

current densities and with 30% current efficiencies

decreasing the final concentration of nickel to 5 mg/L.

Blatt and Schneider (1996) claimed 60% current

efficiency in a system where solutions with 5.5 g/L

initial concentrations were treated and 200 mg/L final

concentrations were achieved. In a work by Ripperger

(1984), a final nickel concentration of less than 0.1

mg/L was attained when solutions with 200 and 400

mg/L initial nickel concentrations were treated by a

microfiltration process.

3. Experimental

Rotating action of an electrolysis cell on a hori-

zontal axis enables metal granules (serving as cath-

Fig. 2. Schematic of the rotating cell.

Fig. 3. Experimental apparatus.

G. Orhan et al. / Hydrometallurgy 65 (2002) 1–8 3

Page 4: Nickel recovery from the rinse waters of plating baths

ode) and electrolyte to circulate (Fig. 2). Because of

the simultaneous motion of the electrolyte and cath-

ode granules, mass transport within the electrolyte/

cathode boundary layer is superior to that of classical

electrolysis cells. Using metal granules as cathode

increases the effective surface area for plating. The

diameter and the width of the rotating cell are 38.5

and 6 cm, respectively. Activated titanium is used as

anode material. The power supply utilized in the

experimental work was Ruhrstrat 44T-20R10 and a

shunt with 20 A capacity (0–30 mV linear) was used

to control the passing current.

Electrolyte temperature was thermostatically con-

trolled by a Haake D8 heater (F 0.1 jC) while its

circulation was carried out by Multifix MC 2000 PEC

and Ismatec IPN dosage pumps. A WTW brand pH-

meter was used for measuring the pH values of

electrolyte while a Keithley multimeter was used for

measuring the other electrolysis quantities. Chemical

analyses were carried out with an atomic absorption

spectrophotometer (Perkin-Elmer). X-ray analysis of

the nickel deposited on the cathode was made by a

diffractometer (Philips PW 3020). A scanning elec-

tron microscope (JEOL 5410) was used to take micro-

graphs of the nickel powders obtained. A schematic of

the experimental apparatus is given in Fig. 3.

4. Results and discussions

The technological applicability of nickel recovery

and its ability to meet economical criteria were

investigated by simulating rinse waters emerging from

various plants; solutions containing 2 g/L Ni2 + and 1

g/L H3BO3 were prepared. Total electrolyte volume

used in the experiments was 3.25 L while the effective

cell volume was 2 L. The variable parameters, the

effect of which are investigated by the repeated ex-

periments, are listed as follows:

Electrolyte pH (2.5–6.0), Cathodic current density (130–325 A/m2),

Fig. 4. Effect of pH on nickel recovery (2 g/L Ni2 + , 1 g/L H3BO3, 5 mm cathode granule diameter, 325 A/m2).

Table 1

Changes observed in some electrolysis parameters when pH is

altered (2 g/L Ni2 + , 1 g/L H3BO3, 50 jC, 325 A/m2, f 5 mm)

pH Specific energy

(kWh/kg)

Current

efficiency (%)

Average cell

voltage (V)

no control 26–34 23 3.51–4.15

2.25–2.50 14 39 3.25

3.55–4.00 10.5 44 4.25

4.85–5.00 6.04 60 4.35

5.45–5.55 4.2 74 4.00

5.90–6.00 7.5 42 4.30

G. Orhan et al. / Hydrometallurgy 65 (2002) 1–84

Page 5: Nickel recovery from the rinse waters of plating baths

Electrolyte temperature (20–65 jC), Various cathode granule diameters (5, 7, 9 mm and

their mixtures).

4.1. Effect of pH

The following parameters were kept constant in the

first part of the experimental study where the effect of

pH on nickel recovery was investigated: 325 A/m2

cathodic current density, 50 jC electrolyte temperature,

4min � 1cell rotation, titaniumanode,and5mmcathode

granule diameter. The volume of cathode granules used

was 450 mL, when the anode–cathode spacing was 3

cm. Electrolyte pH was controlled by an automatic

dosage pump and by feeding 2MNaOH solution to the

electrolyte (except where pH was not controlled).

In the experiment where pH was not controlled

(without neutralizer addition), concentration of the free

acid increases in the cell causing a drop in pH, which

eventually impedes the electrolytic reduction; a decel-

Fig. 5. Effect of temperature and current density on nickel recovery (2 g/L Ni2 + , 1 g/L H3BO3, f 5 mm, pH= 5.45–5.55).

Fig. 6. Effects of temperature and current density on nickel recovery (2 g/L Ni2 + , 1 g/L H3BO3, f 5 mm, pH= 5.5F 0.05, . 130 A/m2, 5 325

A/m2, � theoretical line).

G. Orhan et al. / Hydrometallurgy 65 (2002) 1–8 5

Page 6: Nickel recovery from the rinse waters of plating baths

erating effect is seen in the first 2 h, terminating

thereafter (Fig. 4). Current efficiency in the experiment

without pH control remained at 29% for the first hour

of electrolysis whereas this value was 50% for the

experiment run at pH 2.50, and 85% at pH 5.55.

The maximum nickel recoveries are obtained

within the pH range of 5.45–5.55. The amount of

nickel that remains in the solution starts to increase at

higher pH values (5.9–6.0). Table 1 lists the variations

observed in some economically important parameters

when electrolyte pH is changed. As seen, specific

energy consumption and cell voltage show increasing

trend for pH values both below and above the optimum

(5.5F 0.05).

4.2. Effect of current density

The effect of current density on nickel recovery

was investigated by carrying out experiments at var-

ious cathodic current densities, while keeping pH at

Fig. 7. Effect of temperature on nickel recovery (2 g/L Ni2 + , 1 g/L H3BO3, f 5 mm, pH= 5.45–5.55, 325 A/m2).

Fig. 8. Effect of cathode granule diameter on nickel recovery (2 g/L Ni2 + , 1 g/L H3BO3, pH= 5.5F 0.05, 325 A/m2).

G. Orhan et al. / Hydrometallurgy 65 (2002) 1–86

Page 7: Nickel recovery from the rinse waters of plating baths

5.5 and other parameters constant. Fig. 5 displays

the results of experiments run at different current

densities and temperatures. By examining the first

hour of electrolysis from the viewpoint of reaction

rate, that is, the nickel concentration that remains in

the solution, it is clearly visible that the cathodic

reaction proceeds as thermodynamically expected for

both current densities. However, a second trend

appears for the low current density run (130 A/m2),

between the first and third hour of the experiment,

where the nickel concentration curve starts to deviate

from the theoretical line. There is a third zone on the

recovery curve that extends from the third to sixth

hour of the experiment which cannot be compared to

the theoretical line, since it crosses the x-axis by the

third hour.

In the experiment conducted at high current density

(325 A/m2), the nickel concentration curve follows the

theoretical line very closely for the first hour, but it

shows a different trend between the first and the third

hours, and by then, the nickel concentration in the

electrolyte is practically negligible.

Effects of current density and temperature on

nickel recovery are given in Fig. 6. The variation

between specific energy consumption values of the

experiments run at 50 jC and at different current

densities (130 and 325 A/m2) is about 16% (Fig. 6a),

while those run at 65 jC deviate by 3% (Fig. 6b).

4.3. Effect of temperature

Electrolyte temperature has been varied between 20

and 65 jC, in the experiments where effect of temper-

ature was investigated while keeping the current den-

sity (325 A/m2), pH (5.5F 0.05), and granule diameter

(5 mm) constant. Nickel recovery visibly rises as the

temperature increases, as shown in Fig. 7, confirming

that effective nickel recovery is only possible at tem-

peratures higher than 50 jC.

Fig. 9. SEM micrograph of electrowon nickel.

Fig. 10. X-ray diffraction patterns of electrowon nickel.

G. Orhan et al. / Hydrometallurgy 65 (2002) 1–8 7

Page 8: Nickel recovery from the rinse waters of plating baths

4.4. Effect of granule diameter

Fig. 8 reveals the results of the experiments carried

out with different cathode granule diameters while

keeping the other parameters constant at previously

determined optimum values. A mixture of granules (5,

7 and 9 mm diameters, each having a volume fraction

of 1/3) gave the highest nickel recovery, since it

creates the most uniform bulk structure in connection

with the anode. Electrolyte pH is the most important

parameter affecting the electrolysis efficiency. A

homogeneous distribution of pH (5.5F 0.05, opti-

mized for a maximum nickel recovery at the interface

between the electrolyte and electrode) within the

electrolyte eventually depends on the granule diame-

ter utilized. Thus, the dimensions of the granules must

not create dead zones either in the electrolyte or

between the cathode granules.

SEM micrograph and X-ray diffraction patterns of

nickel, electrowon under the optimum conditions, are

given in Figs. 9 and 10, respectively. They both

indicate that the nickel obtained was in a pure state,

a fact that was confirmed by chemical analysis giving

99.97% Ni.

5. Conclusions

As a result of the experimental work, 90% of the

nickel was recovered from the solutions initially

containing 2 g/L, with a 74% current efficiency

and 4.2 kWh/kg Ni energy consumption, when the

electrolysis parameters were: pH 5.5F 0.05, 50 jCtemperature, 325 A/m2 current density, 450 mL

solution volume, and 5 mm cathode granule diame-

ter. Current efficiency decreases to 45% and specific

energy consumption increases to 14 kWh/kg Ni,

when nickel recovery approaches 100% under the

same conditions. Current efficiency reaches 80% by

simply increasing the temperature alone to 65 jCfrom 50 jC, though the rest of the parameters were

kept constant. Specific energy consumption drops to

3.44 kWh/kg Ni under the conditions when 90%

metal recovery is obtained. On the other hand, 80%

current efficiency was attained at 50 jC with a 4.2

kWh/kg Ni specific energy consumption, when a

mixture of 5, 7, and 9 mm granules were used as

cathode.

At 130 A/m2, specific energy consumption values

differ about 8%, depending on the temperature, that is,

3.61 kWh/kg at 50 jC and 3.33 kWh/kg at 65 jC,while this difference is about 20% at 325 A/m2 for the

same temperature range, namely: 4.2 kWh/kg at 50

jC and 3.44 kWh/kg at 65 jC.Current efficiency values decrease intolerably when

the nickel recovery operation is continued until the

nickel concentration is less than 10 mg/L in the

solution, which results in higher specific energy con-

sumptions as unavoidable side reactions occur. There-

fore, the limit of concentration at which the electrolysis

operation must stop should be determined from an

economic viewpoint when waste waters contaminated

by nickel are treated for de-metalization. The rest of

the nickel could easily be eliminated by the application

of other techniques such as ion exchange resins, etc.

References

Beverskog, B., Puigdommenech, I., 1997. Revised Pourbaix dia-

grams for nickel at 25–300 jC. Corros. Sci. 39, 969–980.Blatt, W., Schneider, L., 1996. Elektrolytische Ruckgewinnung von

Nickel aus konzentrierten galvanotechnischen Prozeßwassern

bzw. Aufkonzentrierten Eluaten. Galvanotechnik 87, 1118–

1124.

Brillas, E., Rambla, J., Casado, J., 1999. Nickel electrowinning

using a Pt catalysed hydrogen-diffusion anode: Part I. Effect

of chloride and sulfate ions and a magnetic field. J. Appl. Elec-

trochem. 29, 1367–1376.

Charlot, G., Badoz, J., Tremillon, B., 1962. Electrochemical Re-

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in der Galvanotechnik. Metalloberflache 44, 465–474.

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Friedrich, F., Raub, J., 1983. Die galvanische Metallabscheidung bei

hohen Elektrolysegeschwindigkeiten (Teil 1). Metalloberflache

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Hartinger, L., 1990. Handbuch der Abwasser-und Recyclingtechnik.

Carl Hanser Verlag, Munchen.

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mischen Abwasserreinigung. Metalloberflache 35, 211–217.

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Puippe, J.C., 1995. TurbocelR-eine hochleistungsfahige Metallruck-

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notechnik 75, 566–569.

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