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Nickel is found in
plating, metal pickling and metal cleaning wastewater. The nickel from the
plating wastewater can be chelated from as nickel sulfamate from sulfamate
nickel plating, or nickel lactate from electroless nickel plating. Nickel
cleaning and stripping solutions are often have nickel cyanide and nickel EDTA
complex.
Nickel that isn�t complexed or chelated can be precipitated as nickel hydroxide
by adjusting the pH to 10.5 or higher. Complexed nickel must be treated with
sulfide or other strong reducing agents that can maintain a negative ORP of -700
mv or less. Even then the treatment may be difficult to reach acceptable limits.
Precipitating the nickel as a carbonate is not effective due to the high
solubility of nickel carbonate. The precipitation of nickel phosphate is
effective for lightly complexed nickel using a two-step process described in the
copper section.
To precipitate the nickel as the metal using ORP, a strong reducing agent, such
as ferrous ion or sulfide ion, must be used at a pH greater than 10.5.
Electroless nickel solutions should be pretreated by raising the pH to 11.5 and
allowing the nickel to plate out using the residual reducing agent, sodium
hypophosphite, in the solution before continuing the treatment. The plating out
or dropping out of electroless nickel solutions releases significant quantities
of hydrogen so the treatment vessel must be exhausted to keep the hydrogen below
the explosion limits.
Nickel can be removed with cation resins and chelated anion resins.
Low concentrations can be attained by using a reducing agent and sand filter or
other media column to precipitate the nickel metal on the surface of the media.
If nickel concentrations below 1 ug/l are required, a three-step process of
precipitation, oxidation and then final precipitation or adsorption will be
required.
The treatment equipment for removing nickel is similar to that used for copper
adding forced exhaust to prevent hydrogen build up during the treatment of
electroless nickel baths.
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