Product

Spent caustic liquor is the residual process solution generated in the Bayer Process after aluminium trihydroxide (Al(OH)₃) has been crystallised and separated from the supersaturated pregnant sodium aluminate liquor. In this specific context, the term "spent" refers exclusively to the depletion of dissolved sodium aluminate (NaAl(OH)₄), whose concentration drops significantly as aluminium hydroxide precipitates out during the seeding and crystallisation stage — not to the depletion of caustic soda itself. The liquor therefore remains rich in free sodium hydroxide (NaOH), which constitutes its primary value as a recyclable reagent.

Before the spent liquor can be returned to the digestion stage, it undergoes reconditioning to restore its operational caustic strength: it is concentrated by multi-effect evaporation to remove excess water, supplemented with fresh NaOH to compensate for caustic losses, and subjected to causticisation with lime (CaO) to convert accumulated sodium carbonate (Na₂CO₃) — formed by reaction of NaOH with CO₂ and organic degradation products — back to usable NaOH.

A key operational challenge is the progressive build-up of sodium oxalate (Na₂C₂O₄), derived from humic acids in the bauxite, which reduces precipitation efficiency and must be periodically purged from the liquor circuit. Over successive recycling cycles, trace elements such as gallium and vanadium accumulate in the spent liquor to concentrations sufficient for commercial recovery as valuable by-products.

References

1. U.S. Environmental Protection Agency (EPA). TENORM: Bauxite and Alumina Production Wastes (Apr 22, 2015)
2. Wikipedia. Bayer process (Page version Feb 13, 2026)
3. Rosenberg S.P., Wilson D.J., & Health C.A. European patent EP1124758B1 — Process for the causticisation of Bayer liquors.  Sep 13, 1999: Application filed by Worsley Alumina Pty Ltd
4. Rosenberg S.P., Wilson D.J., & Health C.A. United States patent US6676910B1 — Processes for the causticisation of Bayer liquors in an alumina refinery.  Sep 13, 1999: Application filed by Worsley Alumina Pty Ltd
5. Pareek V., Brungs M.P., & Adesina A. A. (Jan 2003). Photocausticization of spent Bayer liquor: A pilot-scale study. Advances in Environmental Research, 7(2), 411–420. DOI: 10.1016/S1093-0191(02)00013-8
6. Metrohm. (Jul 2022). Analysis of Bayer aluminate liquors using online thermometric titration