Section Seven
INDUSTRIAL WASTES
G. PLATING WASTES
77    REMOVAL OF HYPOPHOSPHITE AND PHOSPHITE FROM
ELECTROLESS NICKEL PLATING BATHS
Wei-chi Ying, Scientist
Robert R. Bonk, Technical Service Engineer
Michael E. Tucker, Staff Technician
Occidental Chemical Corporation
Grand Island, New York 14072
INTRODUCTION
Electroless nickel plating (EN) is a popular commercial technique for depositing nickel coating on a
suitably treated surface by controlled chemical reduction of nickel ions. The nickel coating catalyzes
the reduction reaction, and the deposition of nickel continues as long as the substrate remains in
contact with the EN solution. Typical EN applications include parts which are difficult to plate, such
as valves and machine tools, as well as larger objects, such as caustic railcars and barges.1-2 The
physical properties of the coated surface such as coating uniformity, corrosion and wear resistance,
lubricity and ductility compare favorably with electroplated nickel surfaces.3
EN baths are mixtures of several chemicals each performing specific functions: a source of nickel
ions, such as nickel chloride or sulfate, reducing agents to supply electrons for the reduction of free
nickel ions, complexing agents (organic acids) to control the amount of free nickel ions in solution,
buffering agents to resist the pH changes associated with the nickel reduction reaction, accelerators to
enhance the speed of the reaction, and inhibitors to moderate the deposition process. The EN bath is
normally heated to about 85 to 95°C.
The major types of wastewater resulting from EN process are spent EN baths, stripping solutions,
and rinse waters. The spent baths contain large amounts of total soluble nickel species Ni(II), reducing, and complexing agents. Ni(II) can be removed either by NaOH or Ca(OH)2 as Ni(OH)2 or NaBH4
as Ni0.4'5 The major reducing agents are residual hypophosphite and phosphite species resulting from
the oxidation of hypophosphite when Ni(II) is reduced to Ni° in the EN process. Hypophosphite and
phosphite species have to be oxidized to phosphate species which can then be removed by precipitation
using lime, ferric chloride, or alum as illustrated in Figure 1. Oxidation is also essential to minimize
adverse effects of organic acids on removal of nickel and/or phosphorus species. Lime is preferred
precipitation agent since it is capable of removing phosphate at the same basic pH range ideal for
nickel removal and that it is a cheap source of the hydroxide ions required for maintaining the high
pH and for forming nickel hydroxide.
Direct one-step methods for removing hypophosphite and phosphite are either ineffective, i.e., lime
precipitation,6 or too costly, i.e., ion exchange.7 Therefore, initial chemical oxidation, for converting
hypophosphite to phosphite and then to phosphate, followed by lime precipitation, for removing both
phosphate and nickel, was investigated in this study for treatment of spent EN baths. Laboratory
evaluation is essential in order to select the best treatment process and to define the optimum
operating conditions capable of producing an effluent which meets the discharge limits at a reasonable cost for equipment, labor, power, chemicals, and sludge disposal. In this chapter, experimental
results for removing hypophosphite and phosphite species from various solutions, including three
spent EN baths are presented. Effects of complexing agents and reducing agents on removing Ni(II)
and phosphorus are demonstrated. Data for a wide variety of reaction conditions, i.e., pH, reagent
type and dose, reaction time and temperature, were obtained to illustrate chemical principles governing the precipitation and oxidation reactions and to allow estimation of the chemical cost for treating
typical spent EN baths.
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
699