Caustic Soda

Caustic soda is sodium hydroxide (NaOH, CAS 1310-73-2, EINECS 215-185-5), supplied commercially as anhydrous micropearls, flakes, prills, and as 32 wt%, 50 wt%, and 73 wt% aqueous solutions. Packaging configurations include 25 kg HDPE bags for solids, 200 L HDPE drums, 1,100 kg intermediate bulk containers, and 20–30 t tank trucks for liquid. The 50 wt% membrane-grade liquid assays at 49.5–50.5 wt% NaOH by ISO 979:1974 or ASTM E291-18, has a density of 1.52–1.53 g/cm³ at 20 °C, a viscosity near 80 mPa·s at 20 °C, and freezes near 12 °C. Anhydrous solid melts at 318 °C and has an enthalpy of solution near 44.5 kJ/mol. As an alkali source, sodium hydroxide delivers free hydroxide directly; it differs from potassium hydroxide in equivalent mass (40.00 g/mol versus 56.11 g/mol), from soda ash in water solubility (1090 g/L versus 215 g/L at 20 °C) and absence of carbonate buffering, and from hydrated lime in complete dissociation and absence of acid-neutralisation sludge. Commercial model designations are supplier-specific; procurement specifications therefore state form, concentration, sodium chloride, sodium chlorate, iron, and mercury ceilings by certificate of analysis.

What separates membrane-grade sodium hydroxide from diaphragm and mercury-cell material?

The chlor-alkali production route controls halide and metal residues. Membrane cells use perfluorinated sulfonic acid/carboxylic acid bilayer membranes; diaphragm cells use polymer-modified asbestos substitutes. Membrane cells generate 32 wt% catholyte, which is concentrated in nickel-based evaporators to 50 wt%; diaphragm cells generate 11–12 wt% liquor containing sodium chloride, requiring salt-separation evaporators; mercury cells yield 50 wt% directly but have been largely retired because of mercury regulation. Membrane-cell 50 wt% product commonly carries sodium chloride below 50 mg/kg, sodium chlorate below 10 mg/kg, and iron below 2 mg/kg in producer certificates of analysis. Diaphragm-cell liquid of similar strength typically contains 0.8–1.2 wt% sodium chloride and 0.1–0.2 wt% sodium chlorate. The chloride difference becomes a process constraint in closed-loop systems: substitution of 1000 kg of 50 wt% diaphragm-grade caustic introduces approximately 8–12 kg NaCl, which is retained in reuse water and can raise chloride-assisted corrosion risk in stainless steel. Mercury-cell material may contain trace mercury; published data for active mercury-cell supply is limited because many regions have phased out the process. Grade selection therefore follows the destination process: membrane-grade for foods, pharmaceuticals, and high-purity neutralisation; diaphragm-grade for pulp, alumina, and water treatment where downstream washing or purge streams tolerate chloride.

The values are representative ranges from commercial certificates of analysis and supply contracts; actual specifications vary by producer and region.

In alumina refining, caustic soda maintains Bayer digestion liquor at caustic concentrations of 150–280 g/L expressed as Na₂O. Diasporic bauxite digestion in 230–260 °C autoclaves or double-pipe heaters with residence times of 10–60 min uses 50 wt% membrane-grade NaOH injected into spent liquor to hold the caustic-to-alumina ratio near 1.2–1.5. The membrane grade is preferred in closed circuits because substituting 1000 kg of 50 wt% diaphragm-grade caustic introduces about 8–12 kg NaCl, which concentrates in evaporator liquors and can raise titanium pitting risk. Red mud washing in counter-current decantation trains uses wash streams with residual NaOH of 20–80 g/L; evaporation returns wash water to digestion. Calcium hydroxide cannot replace sodium hydroxide here because it precipitates calcium carbonate and calcium aluminate scale; sodium carbonate would add carbonate alkalinity that requires causticization, raising lime demand and carbon dioxide release.

When caustic soda replaces sodium carbonate in pulp bleaching and kraft extraction

Kraft pulping uses caustic soda in white liquor preparation and in oxidative extraction bleaching. In the bleach plant, a high-pH extraction stage receives 50 wt% NaOH to raise stock pH to 10.5–11.5 at 60–90 °C with 10–15 wt% consistency in a pressurised reactor. Direct caustic addition avoids the carbon dioxide evolution and buffering lag of sodium carbonate; pH rises rapidly, allowing oxygen or peroxide to remove chromophores from fibre without carbonate scaling of medium-consistency pumps. The substitution ratio is not fixed because stock kappa number, carry-over acid, and washing efficiency alter demand. In comparison with calcium hydroxide, caustic soda generates no solid carbonate sludge and maintains solubility at higher pH. The operational boundary is chloride: diaphragm-grade caustic should not be used in closed bleach filtrate loops without chloride purge because accumulation above 150–200 mg/L can induce stress-corrosion cracking in 316L stainless steel equipment.

Saponification of fats and oils with 50 wt% NaOH proceeds according to the saponification value. An oil with a saponification value of 200 mg KOH/g consumes approximately 142 mg NaOH/g oil. In batch crutchers of 2,500 L capacity with anchor-stirrer agitation at 70–80 °C, caustic soda is added below 50 °C to limit gel-phase viscosity before saponification raises temperature. Sodium hydroxide yields firmer sodium soaps than potassium hydroxide, which is used for soft soaps; soda ash is not a drop-in because it generates carbon dioxide and leaves carbonate in the neat soap phase. Exothermic neutralisation of free fatty acid is handled by adding caustic to the oil phase under agitation, not by adding water to solid caustic.

CIP caustic strength control and stainless steel corrosion boundaries

Cleaning-in-place circuits use 1.5–3.0 wt% NaOH at 75–85 °C for 15–30 min to saponify fat residues and hydrolyse protein deposits. Conductivity sensors with temperature compensation hold return conductivity at a setpoint equivalent to 20–60 mS/cm at 25 °C; dosing pumps inject 50 wt% membrane-grade NaOH into the recirculation line upstream of a static mixer. Membrane-grade material is preferred because chloride introduced with diaphragm-grade caustic concentrates in rinse water and can promote pitting or chloride stress-corrosion cracking in 316L stainless steel at temperatures above 60 °C. For alkaline cleaning at 80 °C or above, materials of construction shift to nickel 200/201 or PTFE-lined carbon steel. Sodium metasilicate or sodium carbonate detergents are sometimes used for mild buffered washing, but they require higher concentrations to reach equivalent free hydroxide chemical potential.

Demineralised water pH correction following strong-acid cation exchange uses 10–15 wt% NaOH metered through static mixers at line velocities of 0.5–2.0 m/s. The caustic raises pH to 8.5–9.0 and shifts the Langelier saturation index into the range +0.2 to +0.5 to protect downstream distribution piping without forming excess carbonate scale. In cold-lime softening, sodium hydroxide precipitates magnesium hydroxide at pH 10.5–11.0, while soda ash or lime provides carbonate or calcium. Compared with lime, caustic soda reduces sludge volume and eliminates slurry handling; compared with soda ash, it does not increase total carbonate alkalinity.

Wet flue gas desulfurisation uses sodium hydroxide as an alternative to limestone or lime slurry when absorber pH must respond rapidly to sulfur dioxide load changes. A 20–25 wt% NaOH solution is metered into the recirculation loop to hold pH at 5.5–6.5. The reaction produces sodium sulfite and sodium sulfate; purge water sent to effluent treatment carries sulfate load. Sodium carbonate dissolution is slower, and calcium hydroxide requires slurry handling. Scrubber loops of fiberglass-reinforced plastic or lined carbon steel are common; substitution of caustic soda for lime eliminates the ball mill and limestone slurry system but raises raw-material cost per equivalent of alkalinity.

Cotton mercerization bath chemistry and sodium hydroxide recovery at 20–30°Bé

Mercerization of cotton uses sodium hydroxide at 20–30°Bé (14–25 wt% NaOH) and 15–25 °C with dwell times of 30–60 s under controlled tension. The caustic swells the cellulose crystallites; concentration and temperature control determine the extent of conversion to cellulose II and the final tensile modulus. Wash water from the saturator is sent through counter-current extraction and evaporation at 75–85 °C, recovering 90–95% of NaOH. Sodium carbonate and hydrated lime cannot generate the same fibre swelling; potassium hydroxide is less commonly used because the equivalent mass is higher and the hydrated cation is larger. The process boundary is viscosity control in the caustic saturator, where concentration must be maintained against evaporative losses.

Sodium hypochlorite is generated by chlorinating 15–20 wt% NaOH at 20–30 °C; the pH is held at 12.5–13.0 to keep free chlorine as hypochlorite. In the sulfation of lauryl alcohol, chlorosulfonic acid ester is neutralised with 50 wt% NaOH at pH 7.5–8.5 and 45–50 °C to form sodium lauryl sulfate. The addition rate is controlled by jacket cooling and pH setpoint because the neutralisation exotherm can exceed 44.5 kJ/mol and temperatures above 50 °C promote ether by-products. Caustic soda is used rather than sodium carbonate in these exothermic neutralisations because carbonate buffering would release carbon dioxide and reduce pH-control accuracy. Potassium hydroxide can be substituted for liquid salts but changes the cation, the physical form, and raw-material cost.

Materials of construction for liquid caustic service impose sharp boundaries. 50 wt% NaOH at ambient temperature can be stored in welded carbon steel; above 40 °C, stress-corrosion cracking risk increases and preferred materials are nickel 200/201 or PTFE-lined steel. 316L stainless steel is restricted to low-stress intermittent contact below 60 °C; 304L is generally not recommended for hot caustic. Solid anhydrous caustic must be stored with barrier packaging under relative humidity below 30% to limit caking and carbon dioxide absorption. Dilution must add caustic to water under agitation; water added to solid or concentrated caustic can create a local enthalpy spike of approximately 44.5 kJ/mol. Sodium hydroxide is incompatible with aluminium, zinc, tin, galvanized steel, acids, chlorinated solvents, ammonium salts, and certain aldehydes; reaction with amphoteric metals releases hydrogen. Transport classification for solid caustic is UN 1823 and for solutions is UN 1824.