How to Treat Spent Electroless Nickel Baths
What options do today’s platers have for dealing with the disposal of EN baths? Metal Chem’s Michael Aleksinas outlines the technology that’s currently available.
Q: The waste treatment from our spent electroless nickel has become a source of growing concern. We anticipate the EN area of our shop will grow significantly and hence volumes of EN waste will become a problem. Can you outline what options platers have today in dealing with the disposal of EN baths?
A: Waste treatment of EN baths has been an area of great concern for platers ever since the result of the Clean Water Act of 1977 (yes, more than 40 years ago). Fortunately, there have been great strides in this area, and many options exist. Depending on the size of your company, certain methods may make more sense than others. Here a list of options presently available:
Hydroxide precipitation. The precipitation of nickel hydroxide (Ni(OH)2) at an elevated pH is one typical method for treatment for nickel in EN baths. However, depending on the type of chelation present, the EN chemistry will dictate how successful this treatment will be for lowering the nickel-ion concentration. In certain cases, the additional use of lime can reduce nickel concentrations to less than 5.0 ppm. Treatment cost is considered relatively high, however, due to the large amount of sludge that is generated.
Catalytic decomposition. Another method is decomposition. This is a low-cost waste-treatment procedure generating a relatively small volume of sludge in a separate treatment tank. Typically, the spent EN solution is transferred to a cone-shaped tank equipped with a heat source and some form of agitation. The temperature is maintained at 160-165°F at a pH 8.0-8.5, and the decomposition is started by adding a dilute palladium solution or nickel particles/fines. The sludge is very dense and easy to handle, and contains 80-85 percent solids, which can easily be recycled.
Electrolytic plate-out. EN solutions also can be effectively treated using electrolytic methods. Large anode-cathode ratios are necessary to optimize the electrolytic recovery process. Depending on the type of EN chemistry, varying process times may be necessary. Nickel metal recovered here can be recycled and the supernatant passed through an ion-exchange process for a final polishing. Cost here would be considered mid-range due to the time and equipment required.
Controlled plate-out. This process simply plates out the nickel phosphorus alloy onto steel wool or other high-surface-area substitutes. It is economical, does not generate any sludge and can be done in the plater’s existing tank.
Generally, the chemistry’s pH is adjusted to a pH of 6.5-9.0, the solution is heated to 180-195°F, and a proprietary stabilized sodium hypophosphite solution is added to the bath. Reaction is completed in three to six hours, depending on the chemistry and age of solution.
Ion exchange. The newer versions of chelating resins can be useful for spent EN baths as well as rinse waters. The selectivity of an ion-exchange resin depends on the concentration of nickel metal available, the presence of other ionic species, flow rate and operating pH. Due to these factors, laboratory testing is essential to determine which type of resin is selected. Overall cost can be high due to the cost of the resin and the column itself. Usually the resins have been found to be suitable for rinse waters as a polishing step after other waste-treatment procedures.
Emerging technologies. It should be noted that the selection of EN chemistry can also have a significant impact on your overall EN waste-treatment costs. Newer available EN chemistries that have significantly longer bath lives can reduce overall waste costs significantly. This can be especially true if plating on aluminum is part of your process.
Finally, the removal of phosphorus and chelates may become necessary in the future. Recent research has shown effective ways of removing these constituents as well. Some of these include the oxidation of the phosphites to phosphates and the use of potassium permanganate to break down carboxylic acids into carbon dioxide and water. Work continues in these areas.
In summary, waste treatment of EN can be achieved in many different ways. Which procedure is most convenient for each plater depends on the amount of waste to be treated, the capability to handle sludge, availability of plant space and cost. All play critical roles in determining which procedure may be most suitable for your particular needs.
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