Separation of NaOH and NaAl(OH)4 in alumina alkaline solution through diffusion dialysis and electrodialysis

Alkaline solution containing NaOH and NaAl(OH)4 is produced largely during extracting aluminum from aluminum ore. Conventionally alkali and alumina-bearing components in the solution are separated by purification, filtration and precipitation, so that gibbsite precipitation is obtained and the alkaline liquor can be returned to the circuit. The feasibility of membrane separation, including diffusion dialysis (DD) and electrodialysis (ED), is investigated by batch DD and ED running with total 6.15×10-4 m2 and 2.865×10-3 m2 membranes, respectively. The DD running shows that the dialysis coefficient of OH− (UOH−) is at a low level (0.0028-0.0031 m/h), mainly because of the hydrophobic nature of the membrane matrix and absence of a pressure difference or an electrical field as the driving force. The OH− recovery ratio is 6.3%, the Al(OH)4− leakage ratio is 0.6%, and the caustic ratio (αk) can reach up to 17.8 after 4 h. As for ED running, the OH− recovery ratio is substantially enhanced, while the Al(OH)4− leakage and αk value can still meet the industrial requirements. The OH− recovery ratio, Al(OH)4− leakage ratio and αk are 48.0%, 12.6% and 7.9, respectively, when OH− concentration is ~1.55 mol/L in the feed chamber, and the ED current density is 350.0 mA/cm2. The current efficiency is 62.5% and the energy consumption is 12.43 kW h/kg, relatively high because of use of a 0.01 m plexiglas spacer between two neighboring membranes, the increase of the solution temperature which is caused by the high current density. Overall, the ED process is a more effective approach to treat the alumina alkaline solution. Much higher alkali concentration and recovery ratio can be achieved in a short time because of the higher permeability, leading to higher treatment capacity for ED device. The DD process, on the other hand, has advantages of low energy consumption, low membrane fouling and environmental friendliness.