gypsum is apparently the way ...

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gypsum is apparently the way to go: CaSO4 + 2NaOH = Na2SO4 + Ca(OH)2

CaSO4 + Na2CO3 = Na2SO4 + CaCO3

Ca(OH)2 + CO2 = CaCO3 + H2O

This might flocculate the bauxite reidues. However I am not mining residue or water specialist. I just know quite a bit of chemistry.