Research On Ramoval Of Ca²⁺ In Spent Electroless Nickel Plating Bath

Electroless nickel plating developed rapidly as a widely used surface treatment technology meanwhile brought some serious problems such as environmental contamination and waste of resources. Many methods were studied to regenerate spent electroless nickel plating bath. Because of ingredients complex, a single method can't meet the requirements, and the synthesis method is popular. But Ca2+ is introduced into the bath during the synthesis method which cause adverse effects to the performance of plating bath and the coating. In this paper, we adopt ion-exchange, electrodialysis and electrodeionization to removal Ca2+ and regenerate H2PO2-.Two cation-exchange membranes are introduced into the ED and EDI stack which result in two chambers increased. The protection chamber avoids CaSO4 and Ca(OH)2 precipitation generated and anion concentration chamber prevents H2PO2- from oxidation by anode. All the H2PO2- are recovered in the whole process.Ion-exchange process compared the effect of geltyperesin and macroreticular resin to remove Ca2+. Meanwhile the influence of operating conditions such as the amount of resin, the pH value of the bath, time, temperature was studied. It showed that, when 40mL 001×7 resin was added to 400mL bath, under the condition of pH5,15min processing time,30℃, the removal rate of Ca2+ was 86.2%. When 60mL D001 resin was added to 400mL bath, under the condition of pH5,30min processing time,30℃, the removal rate of Ca2+ was 77.9%. Compared with D001 resin, the ability of 001×7 resin to adsorb Ca2+ was stranger and the time to reach adsorption equilibrium was shorter.001×7 gel-type resin was more suitable for the removal of Ca2+.In the process of electrodialysis, the influence of operating conditions such as applied current, time, temperature, pH value on the separation performance of electrodialysis were studied. The results showed that, when the applied current was small, the separation effect was not very well while too large, the polarization would occur and precipitation on the membrane surface would generate. The applied current must be lower than limiting current in order to achieve ideal result. Under the optimized conditions of applied current 180mA, temperature 30℃, pH7, processing time 8h, the removal rate of Ca2+ was 71% and the loss rate of H2PO2- was 58.7%. The membrane stack current of electrodeionization is an important factor in its desalination efficiency. The stack current of EDI must be greater than limiting current to generate H+ and OH- which made resin electrical regenerated. But too large applied current would reduce the current efficiency and increase energy consumption.The optimized parameters of electrodeionization process were applied current of 150mA, temperature of 30℃, pH of 4.8, flow rate in diluted chamber and concentrated chambers of 6L/h and 2.5L/h respectively, processing time of 8h. In that case, the Ca2+ removal rate came up to 99.5% and residual only 3mg/L while the loss rate of H2PO2- was 73.8%.Electroless nickel-plating took place in the three regenerated bath. The result showed that the deposition rate and coating thickness of regenerated bath in EDI process were better than the others. The results of the polarization curves of coating test showed that the coating corrosion resistance and the performance of regenerated bath would be significantly enhanced.