Caustic Soda Liquid | Leading Manufacturer of High-purity Caustic Soda Liquid (32%, 48%, 50% NaOH Solution)

Bauxite slurry entering a digestion train is blended with sodium aluminate liquor from evaporator recovery circuits and with 50% NaOH solution at controlled injection points to avoid localised precipitation of sodium aluminosilicate scale. In alumina refineries, 50% is preferred over 32% and 48% because lower water content reduces freight load and evaporative duty in the liquor cycle. The caustic demand is fixed not by total Na₂O but by the molar ratio of Na₂O to Al₂O₃ in the clarified pregnant liquor, typically maintained between 1.45 and 1.65 for gibbsitic bauxite processed at 145–155 °C, and above 1.70 for boehmitic bauxite run at 240–265 °C. In digestion, caustic concentration is normally held at 140–240 g/L Na₂O; the exact setpoint is selected according to reactive silica content because sodalite formation consumes sodium oxide and reduces the available caustic inventory. Rotary vacuum drum filters or pressure filters separate red mud from sodium aluminate liquor, and the filtrate passes through security filtration before seeded precipitation of gibbsite. The precipitation circuit operates with a solids charge of 400–800 g/L seed and a temperature ramp from 75 °C down to 55 °C over 24–48 h. Smelter-grade alumina is then washed, calcined, and shipped. Incoming 50% NaOH is assayed by automatic potentiometric titration following ASTM E291-18; the material is REACH-registered under 01-2119457892-27-0000 and supplied as CAS 1310-73-2.

Effective Alkali, Not Total NaOH, Governs Kraft Cooking Rate

In kraft cooking, the control variable is effective alkali, defined as NaOH + 0.5 Na₂S, expressed as Na₂O on oven-dry wood. For softwood linerboard, effective alkali charge in continuous digesters is typically 14–18% Na₂O, with sulfidity maintained at 25–35%. A 50% NaOH stream is diluted with weak black liquor to reach an effective alkali concentration of 90–120 g/L in white liquor before the blend is heated to 165–170 °C in the digester. Caustic soda liquid enters the chemical recovery cycle through the dissolving tank, where green liquor dregs are removed by gravity settling and candle filtration. Incoming caustic is specified with low sodium carbonate because carbonate accumulates in the liquor cycle and shifts the measured effective alkali; the certificate of analysis reports Na₂CO₃, NaCl, and Na₂SO₄. Brownstock washing efficiency is assessed by residual alkali titration, and kappa number is measured according to ISO 302. The final pulp is used for unbleached kraft linerboard, bag paper, and semi-bleached packaging grades. Caustic losses in the fiber line, mostly as sodium bound to dissolved lignin, are compensated by adding fresh 32% or 50% solution at the recausticising area after lime mud filtration.

A fat blend with a saponification value of 195 mg KOH/g requires 0.139 g NaOH per gram of oil for complete saponification assuming negligible free fatty acid neutralisation. In bar soap manufacturing, 50% NaOH is first diluted with process water to 28–33 wt% to prevent localised soap graining and reduce viscosity before metering into a jacketed crutcher. The reaction mass is held at 80–90 °C under planetary agitation; the aqueous caustic phase is added over 30–60 s per 500 kg batch, then mixed under high-shear dispersion until the batch becomes translucent and the free alkali drops below 0.1 wt%. Over-feeding or under-dilution produces a grainy curd with free alkali above 0.15 wt%, which creates skin irritation and accelerates rancidity in the finished bar. The saponified soap is then salted out with sodium chloride, washed, and vacuum-spray-dried to a moisture content of 10–14%. Liquid soap bases are finished by neutralising excess caustic with citric acid or fatty acid addition. Sodium soap must comply with the Detergent Regulation (EC) No 648/2004 for final product safety, while the caustic feedstock is governed by REACH and the manufacturer’s certificate of analysis under ASTM E291-18.

When Chlorine Gas Is Fed Below a 32% NaOH Surface at pH 11.8

Chlorine is sparged into a continuously recirculated working solution prepared from 32% membrane-grade NaOH and softened water to an initial caustic strength of 15–20 wt%. The reaction liquor is cooled through a plate-and-frame heat exchanger to hold the bulk temperature at 18–25 °C, because sodium hypochlorite decomposition to chlorate accelerates sharply above 30 °C and becomes difficult to correct downstream. Excess caustic at the recirculation discharge is maintained at 0.2–0.5 wt% as NaOH, corresponding to a product pH of 11.5–12.5. The Cl₂ feed rate is interlocked with oxidation-reduction potential and pH sensors; if pH drops below 11.0, chlorine absorption efficiency falls and chlorate by-product increases. Materials of construction in the reaction loop are typically titanium or PVDF-lined steel for wetted parts, and storage tanks are vented to a caustic scrubber because of hypochlorite off-gas. The finished sodium hypochlorite is sold at 12.5–15% available chlorine and must meet EN 901 for use in drinking water disinfection. Production-scale systems track sodium chlorate by ion chromatography against the buyer’s specification rather than relying only on final pH.

What Sets the Lye Peeling Window for High-Density Tomato Lines?

Caustic concentration, bath temperature, and immersion time are interdependent; for tomatoes, the working bath is typically 8–15 wt% NaOH at 90–100 °C with dwell times of 15–45 s. Peaches require a lower concentration of 2–4 wt% NaOH at 60–70 °C for 30–60 s to avoid flesh softening and excessive yield loss. The lye peeler is operated as a continuous rotary tunnel or immersion trough with a recirculated caustic bath, and the spent peel slurry is continuously neutralised with food-grade acid before discharge to meet municipal pH limits. Bath strength is checked by titration at 15–20 min intervals because the organic load from peel carbohydrates rapidly reduces free NaOH and shifts the peeling window. Food-grade sodium hydroxide used in peeling and pretzel dip operations must meet FDA 21 CFR 184.1763 and Commission Regulation (EU) No 231/2012 for E524; the latter specifies purity and heavy metal limits that are documented on the certificate of analysis. Peeled tomato and peach lines then pass through water sprays, acidified rinse, and steam injection to remove residual lye from the product surface. Finished products include canned diced tomatoes, tomato paste, and clingstone peach halves packed under thermal process conditions.

Sodium hydroxide is metered upstream of rapid sand filtration in municipal plants when raw water alkalinity falls below 20 mg/L as CaCO₃ and lime softening would overshoot the desired finished-water pH. A 25% or 50% solution is injected by positive displacement diaphragm pumps into a static mixer at a dosage typically ranging from 0.5 mg/L to 5.0 mg/L as dry NaOH; the target pH after filtration is usually 7.5–8.5 to support chloramine stability and reduce lead and copper corrosion in distribution piping. The chemical must be certified to NSF/ANSI 60 and meet AWWA B501-19 assay limits. In industrial wastewater neutralisation, 50% NaOH is preferred over hydrated lime when sludge reduction and rapid pH correction are required; the caustic is dosed into a turbulent neutralisation tank with a retention time of 5–10 min and a pH controller set at 6.5–9.0 depending on the discharge permit. Overdosing above 10 mg/L in soft finished water may exceed the action level for pH, but published data for the exact alkalinity response of individual distribution systems is often limited, so jar testing with site-specific raw water is used to establish the final dose.

Caustic Concentration Alone Does Not Guarantee Mercerized Cotton Properties

Fabric is framed under tension and run through an 18–25 wt% NaOH bath at 15–20 °C; the key control is caustic activity, not the refractive-index-derived concentration reading alone. Mercerization swelling converts the cellulose I lattice to cellulose II after washing and neutralization, and the process removes soluble short-fiber material from the cotton surface. The bath is often supplemented with a wetting agent because high-strength NaOH wets cotton slowly, but the wetting agent must be stable under 50% alkali storage and must not lower the apparent caustic activity. Retention of width and length is controlled by the stenter tension, and the residual alkali after neutralization is checked on the fabric by titration or by pH of water-extracted wet-processed textiles following AATCC TM 81. Lines producing high-wet-modulus staple fiber use similar alkali concentrations but apply different stretch ratios during coagulation. The resulting fabrics and yarns include mercerized cotton yarn, high-wet-modulus staple, and uniform-dyeing woven fabric. Barium activity number measurements are used in some mills as an indirect process check because direct NaOH concentration alone is not a reliable predictor of swelling under high machine speed.