Caustic Soda

Caustic soda is the common industrial name for sodium hydroxide (NaOH), a highly alkaline, strongly corrosive inorganic compound that exists as white solid pellets, flakes, or prills when pure but is most commonly used in industrial applications as an aqueous solution of varying concentrations (typically 30-50% by weight). In the alumina refining industry, which processes bauxite to produce alumina for aluminum smelting, caustic soda is by far the most important and costly chemical reagent, as it is the solvent medium of the Bayer process — the universal industrial method for refining bauxite into alumina. In the Bayer process, caustic soda solution (at concentrations of 120-280 g/L NaOH, depending on bauxite type and digestion conditions) is heated under pressure and mixed with crushed bauxite in autoclave digesters. The hot caustic selectively dissolves the aluminum-bearing minerals (gibbsite, boehmite, and diaspore) to form a sodium aluminate solution, while most impurities (iron oxide, titanium dioxide, and most silica as desilication products) remain undissolved and are separated as bauxite residue (red mud). The caustic soda is subsequently regenerated through evaporation of the spent liquor after alumina precipitation and recycled through the circuit. Global alumina production consumes enormous quantities of caustic soda: a modern refinery producing three million tonnes of alumina per year may consume 80,000 to 100,000 tonnes of NaOH annually as make-up additions to replace circuit losses. Caustic soda is also used in gold mining for pH adjustment in leach circuits and cyanide management; in mineral processing as a pH modifier in flotation and hydrometallurgy; and in numerous mining-adjacent applications including effluent treatment, boiler water treatment, and degreasing of equipment. Caustic soda is manufactured globally by the chlor-alkali electrolysis process.