Caustic Concentration

Caustic concentration refers to the measured amount of sodium hydroxide (NaOH), commonly known as caustic soda, dissolved in the process liquor circulating within an alumina refinery's Bayer process circuit, typically expressed in grams per litre (g/L) of sodium hydroxide or as equivalent sodium carbonate (Na₂CO₃). Caustic concentration is one of the most critical control parameters in alumina refining, as it directly governs the rate and efficiency of bauxite dissolution (digestion), the quality of aluminum hydroxide precipitation, the performance of the liquor evaporation circuit, and the overall efficiency and economics of the refinery. In the digestion stage of the Bayer process, bauxite is mixed with hot caustic liquor (sodium hydroxide solution) under elevated temperature and pressure in autoclave digesters. The caustic NaOH dissolves the aluminum-bearing minerals (gibbsite, boehmite, and diaspore) to form sodium aluminate (NaAlO₂) in solution, while most impurities (silica, iron oxide, titanium oxide) remain undissolved in the residue, known as red mud or bauxite residue. The caustic concentration in the digester must be optimized for each specific bauxite type: too low and extraction of alumina is incomplete; too high and silica dissolution increases, contaminating the liquor. Spent liquor — depleted in alumina content after precipitation of aluminum hydroxide — is recycled through evaporators to restore the caustic concentration to the required digestion strength (typically 120-280 g/L NaOH, depending on bauxite mineralogy). Precise measurement and control of caustic concentration at multiple points in the Bayer circuit is performed continuously using automated conductivity sensors, online titration systems, and manual laboratory analysis under strict QA/QC protocols.