Digestion is the core chemical process in alumina production via the Bayer Process, whereby crushed and ground bauxite ore is mixed with hot, concentrated caustic soda (sodium hydroxide) solution and subjected to elevated temperature and pressure to dissolve the aluminium-bearing minerals — gibbsite, boehmite, and diaspore — into a sodium aluminate solution. This process is performed in large pressurised digestion vessels and is the key step that determines how much aluminium can be extracted from the bauxite feed. During digestion, the caustic solution reacts with aluminium hydroxide minerals to produce soluble sodium aluminate (NaAlO₂), while silicate minerals may also react to form desilication products such as sodalite or cancrinite. Non-aluminium-bearing minerals such as iron oxides, titanium minerals, and residual silicates remain undissolved and form the solid residue known as bauxite residue or red mud. The completeness and efficiency of digestion are controlled by temperature, caustic concentration, digestion time, and particle size of the bauxite. Incomplete digestion results in alumina losses to the residue, reducing overall plant yield. The digestion liquor — sodium aluminate solution — is subsequently processed through clarification, precipitation, and calcination to produce smelter-grade alumina.
