What methods are available for treating spent electroless nickel plating solution?
Frank AltmayerFirst, treating a spent electroless nickel plating solution can be dangerous because it involves decomposition (plateout) of the solution, which, if not done very carefully, can result in an eruption due to the release of hydrogen gas. You must proceed with the utmost caution and exercise a high degree of chemical safety, wearing full protective gear at all times when near the process.
I’ll be borrowing rather heavily from an AESF Illustrated Lecture written by Dr. Robert Wing in 1980, so I’ll start by giving Dr. Wing credit for a job well done in working on the treatment of complex wastes.
Assuming your electroless nickel plating solution is proprietary, it is difficult to know which complexing agents are part of the formulation, so the following treatment schemes must be bench-tested to determine which one or combination works best.
Note: This was originally published in 1997; please check the current regulations.
The following are treatment schemes with which I am familiar:
Scheme #1
Most EN platers start by carbon-filtering the spent solution to remove some of the organics that tend to complex with nickel metal. With carbon filtration, the final treated solution contains about half as much dissolved nickel as an untreated solution. Heat the solution to 180–200°F, and raise the pH to 8–10 with sodium hydroxide. Add 8–10 g/L of sodium hypophosphite. If the solution does not decompose, add 0.5 mL (12%) sodium borohydride for each gallon of solution. Allow three hours for the reaction to complete, then let the waste sit undisturbed for two or three days.
Scheme #2
Load the spent solution (at room temperature) with a high-surface-area material (e.g., steel wool) to cause plateout of the bulk of the nickel (usually ~1 oz/gal in concentration). As the plateout subsides, the temperature of the spent solution is slowly increased to keep the plating going until it reaches the solution's boiling point. The addition of 8–10 g/L of sodium hypophosphite during the reaction sometimes helps drive the process to completion.
Scheme #3
Raise the pH to 12 with calcium oxide. Allow to settle 16 hr. The whole process may take several hours, after which the treated solution will still contain ~ 25–50 ppm of dissolved nickel. This is usually the case, no matter which of the treatment schemes you follow. Assuming one of the schemes has been successful, the spent solution is now at the point at which Dr. Wing’s work comes in handy.
There are several treatment schemes that Dr. Wing developed or evaluated to treat and remove the last 50 ppm or so of dissolved nickel.
Simple pH adjustment won’t work because the nickel is complexed with organics. You need to find a way to destroy the organic or precipitate the nickel, despite the presence of the complexing agent.
Sulfide Precipitation
In sulfide precipitation, the metal ions are converted to metal sulfides, which generally are lower in solubility than metal hydroxides (except for trivalent chromium). The sulfide can be introduced in several ways. Still, because sulfide-bearing chemicals can be dangerous to handle, the most common forms of sulfide addition are: Ferrous sulfide, which has low solubility, but enough to precipitate metal hydroxides without introducing dangerous levels of free sulfide; and thiocarbamates, which are too expensive to use for bulk treatment and are almost always used as a “polishing” operation.
Ferrous sulfide is available as a proprietary process (U.S. Filter/Permutit Sulfex) for waste streams that do not require the removal of hexavalent chromium. The process can even reduce hexavalent chromium to trivalent chromium and precipitate metals simultaneously (the chromium is precipitated as a hydroxide). The use of sulfides other than ferrous sulfide or thiocarbamates is discouraged for safety reasons.
The system for precipitating insoluble metal sulfides from ferrous sulfide comprises three stages. A two-stage lime neutralization system is used to adjust the pH of raw wastewater containing dissolved metals to 8.5–9.5. The wastewater is then routed to a mixing tank, where ferrous sulfide slurry and polymer are added. The wastewater is then routed through a clarification system.
Because of the instability of ferrous sulfide, it must be generated onsite from sodium sulfide and ferrous sulfate. The sulfide is released from ferrous sulfide only when other heavy metals with lower equilibrium constants for their sulfide form are present in solution.
When the pH is maintained between 8.5 and 9, the liberated iron will form a hydroxide and precipitate. The unreacted ferrous sulfide is filtered or settled out with the metal sulfide precipitate, while the effluent is practically sulfide-free. Anionic polymers aid in the settling of metal sulfide precipitates. Conventional techniques easily dewater the sludge.
The table compares the solubilities of metals precipitated as hydroxides versus sulfides. Note that nickel, as the sulfide, has about 20 orders of magnitude less solubility than the hydroxide.
Removing Dissolved Metals With Insoluble Starch Xanthate
Insoluble starch xanthate (ISX) is an ion exchange material (1.5–1.8 meq metal ion/g capacity) that instantaneously removes heavy metal ions by exchange with sodium or magnesium. It differs from most ion-exchange systems in that it is selective for heavy metal ions. Sodium chloride concentrations up to 10 percent do not hinder metal removal. It is effective for removing noncomplexed metals over the pH range of 3–11. For chelated metals, optimum removal occurs at pH 3–5. ISX can be slurried and metered in for large-volume, continuous-flow operations, or precoated onto diatomaceous earth-type filters to enable metal removal.
Dr. Wing has written many papers on the manufacture and use of ISX.
Sodium Dithiocarbamate
The use of dithiocarbamates (DTC) as metal scavengers has gained acceptance among metal finishers who must meet very low heavy-metal discharge standards. DTC has a fairly low toxicity and is reported to degrade rapidly upon discharge into the environment. It is often added as a “polishing” operation after dissolved metals have been converted to metal hydroxides. Stoichiometric amounts of DTC will lower metal concentrations to 1 mg/L, while a 10percent excess of DTC will reduce metal concentrations to 0.2 mg/L or less. Effective metal removal is obtained over the pH range of 3–10. Cationic polymer (1–2.5 mg/L) aids in flocculation and sludge settling.
The major disadvantage only applies to stream dischargers. DTC is usually supplied commercially as a 40percent solution of the dimethyl or diethyl derivative. One gallon would contain 3.33 lb of DTC, which would remove from 0.5 to approximately 4.5 lb. of dissolved heavy metal, depending on the type of metal and the wastewater matrix.
Treatment with Sodium Hydrosulfite
Sodium hydrosulfite is a strong reducing agent effective for removing metals from chelated systems in batch. The chelated waste is adjusted to a pH < 5.0, and an automatic temperature controller maintains a temperature range of 60–71 C (140–160 °F). Hydrosulfite is metered to maintain an excess concentration of 200–500 ppm sodium hydrosulfite. A five percent lime slurry is then added to maintain a pH above 9. The waste is then clarified.
Because chelating agents remain in the clarifier overflow, the treated effluent goes directly to the final pH adjustment sump, bypassing the general rinsewater system entirely and avoiding reactions between the chelates and other heavy metals.
Many complex metals respond well to hydrosulfite at room temperature. Increasing the temperature greatly improves the sludge's settling and dewatering ability. In certain cases, the sludge must be removed quickly from chelate-containing supernatant water; otherwise, redissolution of the metal will result. The above procedure can be modified to accommodate flow-through treatment systems.
Sodium hydrosulfite treatment of chelated wastes offers the advantages of good removal/treatment of chelated metals at relatively low chemical cost. The sludge formed upon neutralization is easily dewatered, and a high-temperature metals reclaimer can often reclaim the heavy metals.
The major disadvantages of the process are the energy cost and the practicality of heating a waste stream to the required high temperature.
Sodium Borohydride
Sodium borohydride is a water-soluble reducing agent that will reduce metal ions to a lower valence state or to the free metal. A distinct advantage of borohydride is its high chemical efficiency. One pound of sodium borohydride can reduce six pounds of nickel from waste.
For the most efficient reduction of metallic ions, the pH must be maintained between 8 and 11. Below pH 8, hydrolysis of the borohydride produces hydrogen gas; above pH 11, the reduction decreases. Because hydroxide ions are consumed during reduction, the pH must be monitored and maintained at a higher level.
Borohydride may react with other compounds (e.g., organics) in wastewater, thereby reducing its availability to metal ions. Most users, therefore, find that a 100 percent excess of borohydride generally ensures rapid and complete metal reduction. The borohydride is added as a 12 percent solution in 43 percent caustic and should be handled as a neutralization with 50 percent caustic. The wastewater must first be adjusted to pH 8–11 before the addition of borohydride. This adjustment will precipitate some of the metals as hydroxides; borohydride, however, is a strong enough reducing agent to slowly convert hydroxides into “solid” metal.
After pH adjustment, the calculated amount of sodium borohydride is metered into the effluent to reduce the metallic contaminant to the elemental state. Because the normally low levels of dissolved metal ions in wastewaters to be treated make efficient mixing of the borohydride essential. The resulting mixture is held for at least five minutes to ensure complete reaction. Temperature is not critical. The precipitated metal is then recovered by clarification, followed by filtration. Flocculants, such as magnesium salts or commercial organic flocculants, may be used if required to facilitate settling of the precipitated metal. If necessary, the residual metal and borate levels may be further reduced by treatment with activated carbon and/or a suitable ion-exchange resin.
The precipitated metal must be removed from the treated wastewater quickly (<1 hr) because redissolution can occur.
Frank Altmayer is a Master Surface Finisher, an AESF Fellow, and the technical education director of the AESF Foundation and NASF. He owned Scientific Control Laboratories from 1986 to 2007 and has over 50 years of experience in the metal finishing industry. He received the AESF Past Presidents Award, the NAMF Award of Special Recognition, the AESF Leadership Award, the AESF Fellowship Award, the Chicago Branch AESF Geldzahler Service Award, and the NASF Award of Special Recognition.
References
1. R.E. Wing, L.L. Navickis, B.J. Jasberg, and W.E. Rayford, “Removal of Heavy Metals from Industrial Wastewaters Using Insoluble Starch Xanthate,” EPA600/278085 (May 1978).
2. R.E. Wing, W.E. Rayford, and W.M. Doane, Plat. and Surf. Fin., 65, 52 (December 1978).
3. R.E. Wing, “Case History Reports on Heavy Metal Removal Processes,” Proceedings, 66th Annual A.E.S. Technical Conference, Atlanta, GA (June 1979).
4. R.E. Wing, “Electroless Nickel Rinse Waters Are Treatable,” Electroless Nickel Conference, Cincinnati, OH (November 1979).
5. Konrad Parker, “Waste Treatment of Spent Electroless Nickel Baths,” Plat. and Surf. Fin., 70 (February 1983).
