Abstract:
An ion exchanger system consisting of a strong acid cation exchanger column and a strong base anion exchanger column was used as the demineralizing unit. Another strong acid cation exchanger column was used as a recovery column, converting Na₂Cr₂O₇ (resulting from the regeneration of the strong base anion exchanger column) into H₂Cr₂O₇. All the column used, were made of 6.35 cm. diameter PVC pipe with the height of 120 cm. and filled with 2 cu.dm. of exchanger resins. The demineralization flow rate was 16 BV/hour. From the experiments, with the synthetic waste concentration of 100 mg./cu.dm Cr VI, the effluent at 200 BV were found to be from as low as trace to 0.18 mg./cu.dm. Cr VI. Consequently, it could be disposed with minimum harmful effect to the environment or reused as the process rinsing water. The exhausted strong base anion exchanger column was regenerates with sodium hydroxide solutions of different concentrations ranging from 3.5 to 15 percent by weight. It was found that, with counter current flow regeneration, the optimum result was obtained at 7.5 percent by weigh NaOH concentration (162.5 gm. NaOH/cu.dm. of resin) with 4.5 BV/hour regeneration flow rate. The peak concentration in eluted Na₂Cr₂O₇ was as high as 22,422 mg./cu.dm. Cr VI. It was also evident that if only the concentrated portions of the eluted Na₂Cr₂O₇ solution were separate and passed through the recovery column, the H₂Cr₂O₇ solution of 7,688 mg./cu.dm. Cr VI concentrated Na₂Cr₂O₇ solution. This chromic acid solution can be further evaporated in order to achieve a solution of 130,000 to 200,000 mg./cu.dm. Cr VI at less expense, then the evaporated solution can be directly reused for the electroplating process.