Recovery of phosphoric acid from mixed waste acids of semiconductor industry by diffusion dialysis and vacuum distillation

Etching processes during the production of semiconductors generate mixed waste acids containing acetic acid (HAc), nitric acid (HNO3) and phosphoric acid (H3PO4) with aluminum (Al) and/or molybdenum (Mo) as impurities. Diffusion dialysis with newly developed anion exchange membrane method followed by vacuum distillation was carried out to recover phosphoric acid from mixed waste acids. Experiments examined dialyzer performance under different operating conditions of various inlet acid and water flow rates, phosphoric acid and Al concentrations in the feed solution, and operational temperature. The recovered H3PO4 in this process was found to be 85 wt.% whereas the concentration of Al leakage was of 3.68 mg/kg. Although, diffusion dialysis was very effective for this purpose, it failed to achieve the multi-target of more than 80% H3PO4 recovery yield with 50% acid concentration and less than 1 mg Al/kg of 85% H3PO4 simultaneously. Complete separation of HNO3 and HAc from H3PO4 was achieved using vacuum distillation at 730 mm Hg vacuum pressure and 125 °C temperature.

► Phosphoric acid (H3PO4) is recovered from mixed waste acids by diffusion dialysis.
► Recovery of H3PO4 is affected by several parameters such as acid feed, aluminum feed, temperature and flow rates.
► Recovered acid contained Aluminum leakage of 3.68 mg/kg.
► About 80% of H3PO4 recovered along with the other acids such as nitric acid and acetic acid.
► Distillation at 730 mm Hg vacuum pressure and 125 °C temperature helped in obtaining 85% of pure H3PO4.