question_answer

In aluminum extraction by the Bayer's process, alumina is extracted from bauxite by sodium hydroxide at high temperature and pressures. \[A{{l}_{2}}{{O}_{3}}(s)+2O{{H}^{-}}(aq)\to 2A{{l}^{-}}_{2}(aq)+{{H}_{2}}O(\ell )\] Solid impurities such as \[F{{e}_{2}}{{O}_{3}}\] and \[Si{{O}_{2}}\] are removed and then Al\[{{(OH)}^{-}}_{4}\] is reprecipitated. \[Al{{(OH)}^{-}}_{4}\to A{{l}_{2}}{{O}_{3}}\cdot 2{{H}_{2}}O(s)+2O{{H}^{-}}(aq)\] In the industrial world

A) carbon dioxide is added to precipitate the alumina

B) Temperature and pressure are dropped and the super- saturated solution is seeded

C) both [a] and [b] are practiced

D) the water is evaporated.