Recovery of nitric acid from waste etching solution using solvent extraction

A process was developed to recover nitric acid from the waste stream of wafer industry using solvent extraction technique. Tributyl phosphate (TBP) was selected among several extractants because of its better selectivity towards HNO3, overall superiority in operation, favorable physical properties and economics. The waste solution containing 260 g/L CH3COOH, 460 g/L HNO3, 113 g/L HF and 19.6 g/L Si was used as feed solution for process optimization. In the pre-treatment stage >99% silicon and hydrofluoric acid was precipitated out as Na2SiF6. Equilibrium conditions for HNO3 recovery were optimized from the batch test results as: four stages of extraction at an organic:aqueous (O:A) ratio of 3, four stages of scrubbing at O:A ratio of 5 and five stages of stripping at an O:A ratio of 1.5. The extraction of HNO3 was suppressed by the presence of acetic acid (HAc) in the feed solution. To examine the feasibility of the extraction system a continuous operation was carried out for 200 h using a multistage mixer-settler. The concentration of pure HNO3 recovered was 235 g/L with a purity of 99.8%.