Liquor clarification is a mineral processing and hydrometallurgical unit operation that involves the removal of suspended solid particles — including fine mineral particles, colloidal material, and precipitates — from a liquid process stream (commonly referred to as a "liquor") to produce a clear or clarified solution that can be effectively processed in downstream recovery stages. It is a critical step in the Bayer Process for alumina production from bauxite ore, where the clarification of sodium aluminate liquor from the insoluble red mud slurry produced during digestion is essential for the production of high-purity alumina.
In the Bayer Process, after the digestion of bauxite in hot caustic soda, the resulting slurry contains a pregnant sodium aluminate liquor loaded with dissolved alumina, along with an insoluble residue (red mud) consisting of iron oxides, silica compounds, titanium oxides, and unreacted minerals. Liquor clarification involves a staged separation process including gravity settling in large primary settlers or thickeners (often called mud settlers), followed by secondary settling or filtration to reduce the suspended solids content of the overflow liquor to acceptable levels before it proceeds to the precipitation (crystallization) stage.
Flocculating agents (synthetic polyacrylamide-based flocculants) are typically added to the slurry feed to promote the aggregation of fine red mud particles into larger, faster-settling flocs. Starch has historically also been used as a flocculant in some refineries. The clarified liquor (known as the green liquor or clarified sodium aluminate solution) must meet strict suspended solids specifications, as solid contamination in the pregnant liquor would result in impurities in the final alumina product.
The performance of the liquor clarification circuit is measured in terms of the underflow density of the red mud, the suspended solids content of the overflow liquor, and the wash efficiency of the red mud counter-current decantation (CCD) wash train used to recover residual caustic from the red mud before its disposal.