Okay I will explain you friend...
To get pure alumina, you should follow these below steps.
Step (1):- Impurity alumina is treated with conc. NaOH solution to form sodium aluminate.
Step (2):- sodium aluminate is treated with carbon dioxide gas to form precipitated Al2O3
Step (3):- Precipitated Al2O3 separated by filtration and heated at 1470K to give pure alumina.