Selective extraction of nitric and acetic acids from etching waste acid using N235 and MIBK mixtures

• A method of almost complete extraction of HNO3 and HAC from etching waste acid.
• The MIBK + N235 mixtures extraction of HNO3 and HAC were very effective.
• Extraction mechanism of the etching waste acid was established.

The selective extraction of nitric and acetic acids from a simulated etching waste acid was investigated using a mixture of the extractants Alamine 336 (N235) and methyl isobutyl ketone (MIBK). The effects of dilution of the etching waste acid, N235 and MIBK concentrations, organic/aqueous (O/A) phase ratio, extraction temperature, and contact time were systematically studied. The results demonstrate that with an extractant mixture of the composition 12.5 vol% N235 and 87.5 vol% MIBK, 75% acetic acid and 85% nitric acid were extracted. However, only 3% phosphoric acid was co-extracted in a single-stage contact under the following conditions: phosphoric acid concentration of 385 g/L in the diluted acid, O/A ratio of 1.5:1, reaction temperature of 25–35 °C and reaction time of 10 min. The McCabe-Thiele analysis predicts that over 98% acetic acid and 99% nitric acid can be extracted, with co-extraction of only 5% phosphoric acid through a three-stage counter-current operation at an O/A ratio of 3:2. Further, ∼99% HNO3, 99% HAC, and 99% H3PO4 loaded in the organic phase were stripped using 0.25 mol/L NH3·H2O as the stripping agent at an O/A ratio of 1:2. Therefore, it is feasible to achieve almost complete separation of nitric acid and acetic acid from the waste acid to obtain purified phosphoric acid and a binary compound fertilizer containing the nutritional elements nitrogen and phosphorus. The compositions of the extraction complexes of different acids were determined to be H3PO4·2N235, HAC·1.5N235 and HNO3·N235 for the N235 extraction and HAC·2MIBK for the MIBK extraction of HAC. The results can explain the extraction tendencies of the three acids obtained above. Analyses of the FT-IR spectra of the loaded organic phases indicate that the N235 extraction of HNO3 occurred by the ion pair association mechanism, while the acetic acid extraction using N235 or MIBK was realized through hydrogen bonding.