Product

In the context of the Bayer Process for alumina production, pregnant liquor is the clarified, aluminium-rich sodium aluminate solution obtained after the digestion and clarification stages, ready to proceed to precipitation.

Definition

Pregnant liquor is the supersaturated sodium aluminate solution (NaAl(OH)₄) produced when crushed bauxite is digested in hot caustic soda (NaOH) under elevated temperature and pressure, and the resulting slurry has been clarified to remove insoluble residues (red mud) by settling and filtration. The term "pregnant" reflects that the liquor is "carrying" dissolved aluminium in solution, ready to give birth to aluminium hydroxide crystals in the subsequent precipitation stage.

Key Characteristics

Role in the Bayer Process

Pregnant liquor sits at the critical mid-point of the Bayer Process sequence:

1. Digestion — bauxite dissolved in hot NaOH → sodium aluminate slurry
2. Clarification — insoluble red mud separated by settling → pregnant liquor produced
3. Precipitation — pregnant liquor cooled and seeded with aluminium hydroxide crystals → Al(OH)₃ precipitates out
4. Calcination — Al(OH)₃ heated to ~1,000°C → alumina (Al₂O₃) produced

Distinction from Spent Liquor

Once the aluminium hydroxide has been precipitated out, the remaining solution is called spent liquor — it is depleted of aluminium and is recycled back to the digestion stage after reconcentration with fresh caustic soda.​

References

1. Science Direct. Bayer Process (Accessed Feb 25, 2026)
2. Fortin, S., & Breault, R. (2003). Bayer process heat exchangers cleaning efficiency: Optimizing the acid dissolution of sodalite scale. In Heat Exchanger Fouling and Cleaning: Fundamentals and Applications. ECI Digital Archives
3. FluidFlow. The Bayer Process: Fluid Flow in Alumina Plants (Mar 15, 2022)