electrochemical treatment and reuse of nickel plating rinse waters
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Electrochemical Treatment and Reuse of Nickel Plating Rinse Waters Paul T. Bolger and David C. Szlag Bord na Mona Environmental Ltd., Main Street, Newbridge, County Kildare, Ireland*
*Research performed at the National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 26 W. Martin Luther King Drive, Cincinnati, OH 45268
The treatment and disposal of nickel-contaminated rinse waters places an additional cost-burden on a metal plating facility. Increasing the resource productivity within an elec- trolytic nickel plating process by creating a recycle loop for the waste components of the rinse water can reduce dispos- al and raw material costs. In this study, an electrowinning cel1,fitted with an anion exchange membrane, was used to recover valuable components from a simulated electrolytic nickel plating rinse water by a combination of electrodeposi- tion and electrodialysis. The anion exchange membrane in the cell permitted a dual electrode function. Nickel metal was recovered from the rinse water at the cathode and a sul- furichydrochloric acid mixture was generated at the anode (which can be used in thepickling baths to clean metal parts). The boric acid in the rinse water could not be recov- ered by anion exchange due to its high acid dissociation constant. The cell worked effectively in both a batch and a continuous feed mode. The research demonstrated that this type of electrochemical cell could feasibly replace the tradi- tional ion exchange process for treating rinse waters, and eliminate regeneration chemicaWwaste while producing useful process materiak;.
INTRODUCTION Nickel electroplating (the electrolytic deposition of
a layer of nickel on a substrate) is a versatile surface finishing process that has a wide range of decorative, engineering and electroforming applications. The nickel coatings are generally deposited from a Watts nickel solution that contains nickel sulfate, nickel chloride, and boric acid, along with organic brighten- ing agents in the case of decorative coatings. After coating, the plated parts are washed in a series of rinse water baths. These baths accumulate the dragout constituents of the Watts nickel bath over time and eventually require treatment. A range of technologies for treating Watts nickel rinse waters have been used, including ion exchange [ l l , elec- trowinning [Z, 31, evaporation [41, reverse osmosis [51, and electrodialysis [6, 71.
Many of these treatments are quite costly. For example, treatment by ion exchange requires expen- sive resins, large amounts of regenerant solution, and additional tanks, and can have a significant associated downtime if there are not dual ion exchange beds available. To reduce rinse water treatment costs, increased emphasis is being placed in metal plating shops on in-process recycling, low cost metal recov- ery, and pollution prevention [S-101. This research explores a simple, continuous rinse water treatment system that encompasses the aforementioned princi- ples. The technology is essentially a combined elec- trowinning/electrodialysis unit that recovers nickel metal while generating a mixed inorganic acid solu- tion that can be reused in plating shop pickling baths (See Figure 1).
The use of electrowinning to treat wastewaters contaminated with metal ions has been advanced by recent progress in electrolytic cell design, especially the development of high surface area cathodes, mak- ing it possible to recover metals to very low concen- trations 121. Nickel(I1) has been successfully recovered from rinse waters by electrowinning, but the elec- trodeposition process tends to become inhibited by production of hydrogen ions at the anode via water electrolysis. These hydrogen ions can be neutralized by passing the electrolysis solution through a weak base anion exchange column that replaces the sulfate and chloride anions in the rinse water with hydroxide ions [31, but ion exchange adds to the overall expense of the treatment.
The pH shift could be avoided by creating a sepa- rate cathode and anode compartment in the elec- trowinning cell using an anion exchange membrane. In this type of cell, nickel(I1) would be plated on the cathode as usual and the excess anions (counter-ions) in the catholyte would migrate across the anion exchange membrane to the anode compartment to maintain electroneutrality. This cell would permit dual electrode use, recovering nickel metal from the rinse
Environmental Progress (V01.21, No.3) October 2002 203
Table 1. Composition of the simulated Watts nickel rinse water used in treatment studies.
Component Nickel Sulfate Chloride Boric Acid Concentration/mM 4.2 (250 ppm) 3.4 1.6 2.4
Anolyte out Watts nickel rinsewater out
f 4 -F AEM
I I I
I I Nil*
cr + ci- Nio I 4
I & B O 3
so,% < I ' so.= I
I Wntb nickel rinsewater
in (eptholyte) f
Figure 1. Treatment of Watts nickel rinse water using an electrowinning/electrodialysis cell.
water at the cathode and generating an acid mixture at anode that may be used in the pickling baths to clean metal parts before plating. A flow diagram of a Watts nickel plating line is shown in Figure 2, includ- ing the likely location of an ion exchange and elec- trowinning treatment unit within the rinse water treat- ment system. Although the ion exchange unit is placed at the end of the line, to avoid overloading the resin, the electrowinning unit is placed earlier in the rinse water line, to take advantage of the higher con- centrations of nickel in the first save rinse water as electrowinning operates more efficiently at higher metal concentrations. The anolyte from the electro- chemical cell is occasionally bled off to the pickling bath, and the electroplated nickel is recycled back to the plating bath.
EXPERIMENTAL METHODS A simulated Watts nickel rinse water was prepared
from nickel sulfate, nickel chloride, and boric acid in deionized water. The concentrations of the different components in the rinse water were based on the typ- ical concentrations of nickel, sulfate, chloride, and boric acid found in a first save rinse (See Table 1). The solution pH was approximately 4. The Watts nick- el rinse water was the catholyte in the electrolysis cell and the anolyte was deionized water adjusted to pH 1.5 with concentrated sulfuric acid.
The electrolysis/electrodialysis cell was a bench- scale ElectroMPcell (Electrocell AB) with a nickel cath- ode and a platinized titanium anode. The high surface area cathode necessary for efficient nickel(I1) removal at low concentrations was designed in-house b inserting a segment of carbon felt (Renovare), 10 cm in area and 1.2 cm thick, into a compartment frame in which the center plastic grid had been removed.
When the cell was assembled the carbon felt was in direct contact with the nickel cathode plate and the Watts nickel rinse water flowed through, and across the surface of, the carbon felt. The anion exchange membrane was a low resistance Tokuyama Neosepta AM-1 membrane. The potential was controlled by a 10 volt/l5 amp power supply (Kocour Company) and the potential d rop and current across the cell was measured using a multimeter. The catholyte and anolyte solutions were recirculated around the cell by quasi-diaphragm pumps, and the flow rates were carefully regulated (1-2 1 min-l) to maintain equal hydraulic pressure on either side of the membrane and prevent water transport across the membrane. The electrolyte flow rates were based on the optimum flow velocity of a sheet-flow ED frame [111. A lower volume of anolyte (1O:l) than catholyte (30:l) was used in order to produce a more concentrated acid solution for reuse in a pickling bath.
The electrolysis treatment trials were carried out over a 12 to 24 hour period and samples were removed at selected time intervals from the anolyte and catholyte, and diluted for ion analysis. The chlo- ride and sulfate anions were quantified on a Dionex 500 Ion Chromatograph with an ED50 conductivity detector, and a 4 mm Dionex AS11-HC anion exchange column using 15 mM NaOH as eluent under an isocratic gradient. Nickel and borate (as boron) were quantified by inductively coupled plasma spec- troscopy on a Perkin Elmer 3300 DV spectrometer.
The rinse water treatment studies were carried out in a batch and a continuous feed mode. The batch mode was utilized to determine the quantity of nick- el(I1) (and anions) that could be removed from the Watts nickel rinse water by the electrochemical cell in a 24 hour period. In the continuous feed mode there was constant addition of a concentrated Watts nickel solution to the catholyte (via peristaltic pump) to sim- ulate the conditions under which an electrowinning cell would have to operate in a metal finishing line. The desired objective of operating the electrowinning cell in a continuous feed mode was to maintain the nickel(I1) ions at a constant concentration, that is, the rate of nickel(I1) removal by electrowinning equiva- lent to the rate of nickel(I1) addition.
RESULTS AND DISCUSSION The electrochemical thermodynamics of nickel
deposition from an aqueous solution indicate that the process is not an efficient one, as the reduction of water is thermodynamically more favorable as shown in Equations 1 and 2.
Ni2+ + 2e- + Nio I@= -0.25 V H30i + 2e- -+ H2 + OH l? = 0.00 V (2)
204 October 2002 Environmental Progress (Vo1.21, No.3)
Table 2. The mmoles of nickel(II), sulfate, chloride and boric acid in the catholyte and anolyte before and after the 24 hour electrodeposition trial at a potential of 6 V under batch conditions.
Nickel(II) Sulfate chloride Boric Acid (mmoles) (mmoles) (moles) (mmoles)
Catholyte/O hours 126 105 51 67 An