Westlake Caustic Soda

In alumina refining, Westlake Caustic Soda is dosed into spent Bayer liquor to replace sodium hydroxide consumed during bauxite digestion. The digestion circuit maintains a caustic concentration of 180–260 g/L Na₂O for gibbsitic bauxite at 140–155°C, while diasporic bauxite requires 220–280 g/L Na₂O and 240–270°C due to slower dissolution kinetics. The addition ratio is controlled through the caustic-to-alumina molar ratio, typically held between 1.40 and 1.65, and total caustic consumption is reported in the range of 60–120 kg NaOH per tonne of smelter-grade alumina on a 100% basis. Downstream process equipment includes agitated autoclaves or tube digesters with residence times of 20–60 min for gibbsitic bauxite, flash cooling trains, countercurrent red mud washers, security filtration, gibbsite precipitation at 60–75°C with fine seed recycle, and calcination at 950–1250°C in rotary kilns or gas suspension calciners. Terminal product types include smelter-grade alumina, chemical-grade alumina, aluminum hydroxide, and zeolite 4A feed. Compliance for the caustic soda supplier quality is typically anchored to GB/T 209-2018 for membrane-grade liquid caustic soda, with sodium carbonate limited to ≤0.2% by mass to reduce scale formation in heat exchangers, while plant environmental obligations under Directive 2010/75/EU and local bauxite residue management plans govern sodium-bearing process discharges. In cold climates, storage of 50% NaOH below 12°C risks crystallization, and carbon steel piping must be heat-traced above 15°C to avoid flow loss.

What Limits Effective Alkali Charge in Continuous Kraft Cooking?

The primary constraint on effective alkali charge in continuous softwood kraft cooking is the trade-off between delignification rate and pulp strength loss. Westlake Caustic Soda is used to causticize green liquor and to supplement white liquor, with the mill maintaining an effective alkali charge of 15–25% on oven-dry wood for softwood, sulfidity between 25–35%, and a liquor-to-wood ratio of 3.5–5.0 L/kg. White liquor active alkali concentrations are typically 85–120 g/L as Na₂O, and continuous digesters operate at 150–170°C with a pressure of 7–10 bar; the H-factor required for softwood to reach kappa 25–30 before oxygen delignification is commonly reported between 1200 and 1800. Downstream of the blow tank, brown stock washing is followed by oxygen delignification at 90–100°C, 6–8 bar oxygen partial pressure, and a NaOH charge of 2–4% on oven-dry pulp. Further bleaching in extraction-stage vessels uses 1.5–3.0% NaOH and 0.3–0.5% H₂O₂ at 70–85°C for 45–90 min, with terminal viscosity checked by ISO 5351:2010. The regulatory framework includes the EU BAT conclusions for pulp, paper and board under 2014/687/EU for chemical consumption and effluent load, and mill permits under Directive 2010/75/EU. Terminal product types include bleached softwood kraft pulp, bleached eucalyptus kraft pulp, and dissolving-grade pulp for viscose or cellulose derivatives. Operational boundaries: sulfidity above 35% reduces delignification selectivity, and white liquor oxidation must be controlled because polysulfide formation can raise viscosity but increase sulfur-to-wood variability; caustic dosing must be trimmed against on-line kappa analyzers rather than fixed flow, because chip moisture shifts of ±3% can alter effective alkali demand by more than 1% on wood.

At chain mercerizer speeds above 45 m/min, the control band for caustic soda concentration and fabric tension becomes the dominant variable for uniform luster development. Mercerizing liquor is maintained at 18–25% NaOH by weight with temperature held at 15–25°C, because this window maximizes cellulose II conversion while limiting hydrolytic degradation; residence time under alkali is 25–60 s, and the fabric is stretched to 2.5–5.0 cN/dtex during stabilization. Recovered wash liquor at 5–12% NaOH is routed to an evaporator train and reconcentrated to 25–30% before blending with fresh 50% Westlake Caustic Soda, which is controlled by density and conductivity metering at the make-up skid. The production sequence includes singeing, desizing, scouring, high-efficiency squeeze impregnation, clip-chain stabilization, hot water recovery at 70–90°C, and neutralization with acetic acid or formic acid at 0.5–1.0% in the final wash box. Terminal product types include high-luster cotton yarn, mercerized woven shirting, denim with reduced dye uptake variability, and lyocell/cotton blends where alkali swelling improves fibrillation resistance. Compliance references for the downstream textile mill include the ZDHC MRSL Version 3.0 for chemical inputs, OEKO-TEX Standard 100 for finished article residues, and the EU BAT reference for textile wet processing under Directive 2010/75/EU; effluent pH is neutralized before discharge and caustic recovery is integrated to avoid exceeding discharge limits for sodium salts. Operational boundaries: NaOH concentration below 16% w/w leads to incomplete mercerization and uneven dye uptake, while concentrations above 25% w/w increase fabric stiffness and raise recovery energy demand; the process is incompatible with direct contact to aluminium or zinc alloy drying cylinders upstream of neutralization.

Saponification Reactor Causticity and High-Shear Neutralization

A 50% sodium hydroxide solution is metered into a high-shear neutralization loop at a ratio calculated from the feedstock saponification value, with a deliberate free-alkali excess of 0.2–0.5% by mass in the finished soap to ensure complete saponification and to prevent hydrolytic rancidity during storage. Fatty acids derived from coconut, palm kernel, and tallow are preheated to 75–90°C, and the exothermic neutralization is maintained at 110–120°C under atmospheric pressure; the heavy-duty mixer operates at tip speeds above 20 m/s to prevent localized over-alkalization and gel formation. The resulting soap melt is dried under vacuum at 40–50 mbar and 115–125°C, cooled in a plodder at 30–40°C, and extruded into billets for stamping. Terminal product types include toilet soap bars, laundry bars, and liquid soap bases where sodium hydroxide is used for pH adjustment after saponification. Free caustic alkalinity is measured by ISO 456:1983, and skin cleansing finished products must meet the safety requirements of Regulation EC 1223/2009; the manufacturing plant also applies REACH exposure scenarios for sodium hydroxide under its CLP classification as Skin Corr. 1A H314. Operational boundaries: free alkali above 0.5% can cause skin irritation and bar efflorescence, while free alkali below 0.2% risks residual free fatty acid and odor instability; caustic dosing must be interlocked with pH and conductivity analyzers because feedstock saponification value variation of ±5 mg KOH/g can shift the stoichiometric demand by more than 0.3% NaOH on batch weight.

When Caustic Scrubbing Confronts Sulfidic Spent Caustic in LPG Treatment

Caustic scrubbing of LPG and light naphtha operates with a recirculating caustic phase in which fresh 50% Westlake Caustic Soda is diluted to 8–12% NaOH by weight to extract hydrogen sulfide and low-molecular-weight mercaptans without excessive hydrocarbon emulsion carryover. The addition ratio in the prewash loop is maintained at 5–15 vol% of liquid hydrocarbon feed, with spent caustic blowdown managed when sodium sulfide concentration approaches 15–25 g/L and mercaptide concentration begins to suppress mass transfer; the contactor design is typically a jet mixer or packed tower followed by a settling drum with 20–40 min residence time. The downstream process includes water wash, salt dryer, and product stabilization, while the spent sulfidic caustic is sent to wet air oxidation at 190–220°C and 60–90 bar before biological treatment. Terminal product types include copper-strip 1a LPG, gasoline blendstock with mercaptan sulfur meeting ASTM D3227-20, and jet fuel components after further hydrotreating. Compliance references include ASTM D4952-19 for active sulfur qualitative testing, ASTM D130-19 for copper strip corrosion, and refinery effluent limits under 40 CFR Part 419 for petroleum refining discharge; storage tanks and piping must use post-weld heat-treated carbon steel or stainless steel because wet sulfidic caustic can cause sulfide stress cracking. Operational boundaries: caustic concentration below 6% NaOH sharply reduces mercaptan extraction efficiency, while concentration above 15% NaOH increases viscosity at winter temperature and raises emulsion stability; the system is incompatible with aluminium fittings and with acidic regenerant streams that can release hydrogen sulfide.

In high-volume potato peeling, lye concentration is held at 8–15% NaOH by weight and temperature at 80–90°C for a contact time of 1–5 min, followed by rotary steam peeling to loosen and remove the periderm. For tomato peeling, the addition ratio is lower, typically 1–3% NaOH at 85–95°C for 10–30 s, after which the fruit passes through a mechanical washer and a citric or phosphoric acid neutralization bath at 0.5–1.0% acid and 20–30°C. The downstream production sequence includes belt conveying, low-pressure steam injection, brake rolls and rotating rubber fingers to dislodge softened peel, final rinse, and continuous inspection. Terminal product types include frozen French fries, potato chips, canned whole tomatoes, and peeled tomato dice for sauces. The food-grade caustic soda must meet the monograph requirements of the Food Chemicals Codex and is permitted as a pH control agent under FDA 21 CFR 184.1763 and as additive E524 under Regulation EC 1333/2008; CIP applications use 1–2% NaOH at 70–80°C for protein and fat removal in stainless steel processing lines. Operational boundaries: sodium hydroxide solution must not contact galvanized steel, aluminium, or copper equipment, and storage systems must be segregated from acid CIP tanks to avoid exothermic neutralization; residual lye on peeled surfaces must be monitored by rinse-water pH below 8.5 before freezing or heat processing.