question_answer

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

A)  carbon dioxide is added to precipitate the alumina

B)  temperature and pressure are dropped and the super saturated solution seeded

C)  both  and  are practiced

D)  the water is evaporated