Befar Group Caustic Soda Pearls

Bayer Liquor Preparation and Bauxite Digestion Control

Befar Group caustic soda pearls are dissolved in process condensate to a stock concentration of 400–500 g/L NaOH before injection into the digestion liquor circuit. Digestion control is set by the molar ratio of Na₂O to Al₂O₃, maintained at 1.45–1.65 for low-temperature gibbsitic bauxite at 140–150 °C and at 1.50–1.75 for boehmitic bauxite at 220–260 °C. Make-up addition is not a fixed percentage of bauxite mass; it is calculated from reactive silica content. When reactive silica exceeds 8 % w/w, caustic consumption rises to 100–180 kg NaOH per tonne of alumina because sodalite-type desilication products immobilize Na₂O in red mud. Pre-desilication at 90–105 °C for 8–12 h in agitated holding tanks reduces downstream scale formation in shell-and-tube heat exchangers. The compliance boundary for incoming solid sodium hydroxide is GB/T 209-2018, with REACH-registered substance under Regulation (EC) No 1907/2006; refinery liquor and alumina quality plans are operated under ISO 9001:2015.

After digestion, slurry passes through flash-cooling trains, sand traps, high-rate thickeners, security filtration, gibbsite precipitation at 60–70 °C with seed charge, and fluidized-bed calcination at 950–1050 °C. In high-silica bauxite service, localized pearl dissolution can create caustic hot spots that trigger premature desilication precipitation on mixer impellers; dissolution tanks are therefore fitted with eductor circulation to limit local NaOH concentration to 200 g/L during make-up. The terminal product is smelter-grade alumina (SGA) with attrition index and particle size distribution controlled for Hall-Héroult smelting.

Fortification of white liquor with sodium hydroxide pearls in a kraft mill addresses alkali losses through green liquor dregs, bleach plant effluent, and pulp washing carry-over. The pearls are first dissolved in weak wash water at 50–60 °C to a stock concentration of 400–500 g/L NaOH; the solution is then metered into the white liquor line after causticizing, where effective alkali is maintained at 18–22 % as Na₂O on oven-dry softwood and 14–18 % on oven-dry hardwood. Process measurement follows ISO 302:2015 Kappa number for delignification control, and mill operating permits typically reference the EU BAT conclusions for kraft pulping under 2014/687/EU.

White liquor is fed to continuous digesters at 160–170 °C with H-factors of 1200–1800 for softwood and 800–1200 for hardwood; pulp then moves through oxygen delignification, two-stage washing, and an oxidative alkali extraction stage where NaOH addition of 0.5–1.5 % on pulp enhances dissolution of oxidized lignin fragments before chlorine dioxide bleaching. Direct addition of pearls into hot white liquor without pre-dissolution is avoided because the exothermic heat of solution can raise local temperature above 85 °C and accelerate carbonate scaling on transfer piping; metering skids with mass flow controllers are used instead. Terminal products are bleached softwood kraft pulp (BSKP), bleached hardwood kraft pulp (BHKP), and fluff pulp for tissue, packaging board, and absorbent hygiene.

Why Does Saponification Rate Decline Below 70 °C in Batch Soap Kettles?

Below 70 °C, the saponification rate is limited by the high viscosity of the soap mass and reduced contact between molten triglycerides and the aqueous caustic phase; industrial practice therefore holds the crutcher at 70–90 °C for the first saponification stage. For coconut oil with a saponification value of 250–264 mg KOH/g, the stoichiometric NaOH requirement is 17.8–18.8 kg per 100 kg oil; production batches are run with free caustic excess of 0.3–0.5 % by weight of oil, then trimmed with fatty acid before finishing. Compliance analysis uses ASTM D460-91(2005) for total alkali in soap and free caustic, with product specifications for toilet soap base requiring free NaOH below 0.05 %.

Manufacture is performed in jacketed agitated crutchers with scraped-wall heat transfer surfaces; high-shear dispersion of the caustic solution into the oil phase at agitator tip speeds below 3 m/s prevents air entrapment. Pearls are dissolved to 50 % w/w NaOH prior to oil contact to avoid localized gelation at the oil-water interface. After saponification, soap is dried in vacuum spray dryers to 11–13 % moisture and extruded as soap noodles or bars. Terminal product types are toilet soap base, laundry soap bars, and soap noodles for downstream cosmetic compaction.

Under controlled tension and at caustic bath concentrations of 20–24 % w/w NaOH (28–30 °Bé) at 15–20 °C, cotton woven fabric is run through a chain mercerizer with dwell time 45–60 s. Longitudinal tension is held at 2–4 % extension to convert cellulose I to cellulose II and stabilize the mercerized state. Weak spent liquor at 4–6 % NaOH is recovered by countercurrent washing, vacuum extraction, and evaporative caustic recovery. For knitted cotton tubes, the concentration is reduced to 18–20 % NaOH to avoid fabric contraction.

Fabric performance is evaluated under ISO 5077:2007 for dimensional change after washing; effluent and chemical recovery are covered by the ZDHC Manufacturing Restricted Substances List and facility discharge permits. The input caustic soda pearls meet GB/T 209-2018; textiles for sensitive skin are additionally assessed against OEKO-TEX Standard 100. Below 15 °C, bath viscosity increases and diffusion into cellulose slows, producing streakiness; above 25 °C, swelling is less uniform and luster decreases. Automatic temperature control on the saturator chiller is therefore set with deadband ±2 °C. Terminal products are mercerized cotton woven fabric, sewing thread, dyed warp yarn, and consolidated knitted apparel blanks.

When Sodium Hypochlorite Generation Exceeds 15 % w/w Available Chlorine

Production above 15 % w/w available chlorine shifts the decomposition equilibrium toward chlorate and oxygen formation, so reaction cooling and excess alkalinity become the controlling variables. The chlorine absorption reactor is charged with 25–30 % w/w NaOH solution made from pearls and process water; chlorine gas is injected through a submerged sparger at 10–15 °C, maintaining final product at 12–15 % NaOCl and 0.5–1.5 % free NaOH. Stoichiometrically, 1.13 t of 100 % NaOH is consumed per tonne of chlorine absorbed; practical make-up is 1.2–1.3 t per tonne chlorine because of excess alkalinity retention. Liquid bleach produced under AWWA B300-18 specifications should retain excess alkalinity of 0.5–1.0 % as NaOH when shipped as 10–15 % available chlorine.

Continuous generation uses a packed absorption column with countercurrent caustic circulation and external plate heat exchangers in FRP/CPVC or titanium construction; nickel-bearing alloys and unprotected carbon steel are excluded because of stress corrosion cracking in hypochlorite service. Product is cooled to 5–10 °C before storage to minimize chlorate formation; storage tanks are vented and UV-shielded. Terminal product types are sodium hypochlorite solution for municipal disinfection, industrial bleach for textile and pulp, and swimming pool sanitizer.

In tomato, peach, and root vegetable peeling lines, food-grade sodium hydroxide pearls are dissolved to 1.0–2.5 % w/w NaOH in peeling baths; the bath is held at 90–95 °C for 15–60 s, then the loosened peel is removed by rotary drum washers with high-pressure water sprays. For lye treatment of green olives, the concentration is 1.5–3.0 % w/w NaOH at 15–25 °C for 8–12 h, with soaking terminated when the alkali front reaches 60–70 % of flesh thickness; subsequent washing and acidification in brine at pH 4.0–4.5 complete debittering.

Use is restricted to food-grade sodium hydroxide meeting GB/T 209-2018 food-grade requirements and FDA 21 CFR 184.1763 GRAS status; EU processing corresponds to food additive E 524 under Regulation (EC) No 1333/2008 Annex II, with heavy metals according to the JECFA specification. Overexposure of edible tissue to alkali above 2 % NaOH or extended bath time causes tissue sloughing and yield loss; continuous lye peelers therefore use recirculation pumps with inline conductivity meters to maintain concentration within ±0.1 %. Terminal products are canned peeled tomatoes, peach halves and dice, pitted green olives, and frozen root vegetable cuts.

Sodium hydroxide pearls are dissolved to 10–20 % w/w NaOH and injected into recirculation lines downstream of flow meters, using chemical metering pumps and injection quills. Addition ratio is determined by pH and alkalinity target rather than fixed water volume: for low-pressure boiler water, caustic alkalinity is maintained at 300–600 ppm as CaCO₃ with pH 10.0–11.0; high-pressure units on coordinated phosphate programs require free caustic below 2 ppm. Titration alkalinity is measured according to ASTM D1067-16.

pH correction ahead of reverse osmosis requires static mixer residence time of 10–20 s and degasifier operation if raw water bicarbonate exceeds 200 mg/L; elevated pH shifts carbonate equilibrium toward CaCO₃ scaling on membrane elements. Drinking water treatment uses caustic soda conforming to AWWA B501-19 and NSF/ANSI/CAN 60; the feed system is interlocked with pH analysers and fail-safe shutoff valves. Terminal product types are potable water, boiler feedwater, closed-loop heating/cooling water, and demineralization pretreatment.