nickel recovery from the rinse waters of plating baths
TRANSCRIPT
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
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
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
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
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
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
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
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.
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