use al80 as feed for bayer or hall process?

Fact: 1
Dehydration of the aluminium hydroxide produced by Bayer process converts it to aluminium oxide (Al2O3).

Fact: 2
Al2O3 is the feedstock for the Hall (electrolysis) process to produce aluminium metal,

Query
As I understand Al80 is Al2O3, why isn’t that material suitable for feedstock for Hall process?

Is it too impure – contain silica etc?

If it is too impure, why can’t it be used as feedstock for Bayer process - i.e. purify it via redissolution with caustic soda etc.

Is crystal structure of Al80 such that it is difficult to redissolve in ammonia or caustic?

Appears a no brainer but plainly there’s a problem with doing the bleed’n obvious.

Any one out there know?