Abstract
Electronic parts such as magnetic disks, tapes and thin film resistors are produced by electroless nickel plating method. The plating baths are usually discarded before the performance of these products deteriorates due to overload of the baths. The plating wastewater contains large quantities of phosphinate and phosphite which are hard to remove by conventional methods such as the precipitation method. However, when these contaminants are converted to the phosphate form, they can easily be removed by the precipitation method as calcium phosphate. Thus, the conversion of phosphinate and phosphite to phosphate by wet oxidation was studied. The influence of initial oxygen partial pressure, total pressure, pH and presence of co-ion nickel on the oxidation of phosphate was investigated.An aqueous solution of 0.1 mol/dm3 phosphinic acid or phosphonic acid was oxidized by oxygen (0-5 MPa) and/or nitrogen at pH range of 1-12.5 at 453 K. Complete oxidation of phosphinic acid to phosphonic acid was achieved in 1 hour in a nitrogen atmosphere. However, oxidation of phosphonic acid did not proceed in the anoxic condition. In the presence of oxygen, the oxidation rate of phosphonic acid increased with an increase in oxygen pressure. However, the rate of oxidation was lower than that of phosphinic acid; 88 % of phosphonic acid was oxidized by a 5 MPa oxygen gas in 12 hours. The oxidation of phosphonic acid was strongly affected by initial pH of solution. At pH less than 3, it was enhanced with a decrease in pH though it did not proceed at pH range of 3-6. In an alkaline condition, it was enhanced with an increase in pH. At pH 11, more than 99 % of phosphonic acid was oxidized in 3 hours. In addition, the presence of nickel in solution enhanced the oxidation of phosphonic acid.
