Removing Al and regenerating caustic soda from the spent washing liquor of Al etching

Spent liquor from washing of aluminum section materials after etching with caustic soda (NaOH) has been treated. Aluminum was removed from the liquor and caustic soda was regenerated by adding precipitating agents to hydrolyze sodium aluminate (Na₂AlO₂), separating the aluminumprecipitate, and concentrating free NaOH in the resulting solution for reuse in the etching process. Four systems were investigated: hydrated lime [Ca(OH)₂], hydrogen peroxide (H₂O₂), H₂O₂/Ca(OH)₂ mixture, and dry lime (CaO). Results revealed that CaO was more efficient in the removal of aluminum from the spent liquor with a higher hydrolyzing rate of Na₂AlO₂ than Ca(OH)₂, H₂O₂, or their mixture. For more information, contact M.A. Barakat, Central Metallurgical R&D Institute, P.O. Box 87 Helwan 11421, Cairo, Egypt; fax: +20-2-501-0639; e-mail: mabarakat@gmail.com.