Nickel may be present in plating wastes or in the waste from nonferrous metallurgical plants.
Nickel plating is carried out at a pH range of 1.5 to 6.0, with the majority of solutions operative between 2.0 and 4.5. The concentration of nickel in rinse waters following nickel plating varies widely, depending on the method to minimize drag-out and whether flowthrough or countercurrent rinsing is used. As a general rule, a three-stage counterflow rinsing operation reduces rinse water consumption by 90-95%, making it more economical to recover nickel for reuse.
Nickel may be reclaimed from the rinse tank by evaporation, and the concentrated solution returned to plating. The condensate is recovered and reused as makeup to the rinse system. Ion exchangers are also used for the recovery of nickel and water. The rinse water is passed through cation and anion exchangers in series, with deionized water recycled into the rinse tanks. The cation exchanger, which removes the nickel ions, is periodically regenerated with acid. The regenerating solution containing the concentrated nickel salts can then be treated and reused in plating operations.
If it is uneconomical to segregate the nickel rinse water for recovery, or if waste contains other metals that would interfere with recovery operations, nickel can be completely removed by precipitation with lime at a pH of 8.0 or higher. Settling characteristics depend on the technique used for precipitation, flocculation, and settling. Designs should achieve a separation rate of less than 0.5 gpm/sq ft, with the clarifier effluent filtered to remove nickel hydroxide present as suspended solids in the clarifier overflow.
Precipitated hydroxide can only be concentrated to 1.0-2.0% by plain gravity settling. Sludge dewatering requires precoat filtration or plate and frame filtration.
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