Regeneration of spent alkali from aluminum washing

Results obtained revealed that CaO was more efficient in the removal of aluminum from the spent liquor with higher hydrolyzing rate of Na2AlO2 than Ca(OH)2, H2O2, or their mixture. The removal efficiency value of aluminum was 96.1% with 100 g/L of CaO at 60 °C after 30 min treatment, while the efficiency values were 83.2 and 90.1% with Ca(OH)2 and H2O2, respectively after 2 h treatment. The presence of Ca(OH)2 in the H2O2/Ca(OH)2 mixture has no great effect on the Al removing efficiency comparing to that using H2O2 alone, however, it greatly facilitated the separation of the Al precipitate from the solution as calcium aluminate hydrate. Caustic soda concentration of 102 g/L, which is applicable for reusing as etching solution, was obtained by heating the solution for 1 h at 95 °C.