Nobian Caustic Soda

Why Do Boehmite Digestion Lines Demand Low-Carbonate 50% NaOH?

The Bayer circuit consumes 0.05–0.12 tonnes of sodium hydroxide per tonne of alumina depending on bauxite mineralogy, residue washing efficiency, and liquor purge constraints. Digestion of gibbsitic bauxite is conducted at 145–150 °C with free caustic concentration near 180–220 g/L Na₂O. Boehmite requires 230–245 °C and 240–270 g/L Na₂O, while high-temperature diaspore lines may operate at 250–275 °C. The alumina-to-caustic ratio leaving digestion typically ranges 0.60–0.75 for low-temperature plants and 0.65–0.80 for high-temperature plants. Pregnant liquor molar ratios below 0.55 reduce precipitation yield and increase scale risk in flash tanks. Membrane-grade 50% sodium hydroxide from Nobian carries a low sodium chloride mass fraction because diaphragm-grade chloride accumulates in closed Bayer liquor and accelerates pitting in stainless steel flash train segments. Sodium carbonate input matters because carbonate accumulation above 6 g/L Na₂O equivalent promotes scale in shell-and-tube heat exchangers and reduces alumina precipitation yield. Published operating data from low-temperature and high-temperature refineries confirm that every 1 g/L Na₂O increase in carbonate above a site-specific setpoint raises specific steam consumption by 0.2–0.5% in digestion. No universally valid value exists for all bauxite feeds because organic carbon content and causticisation lime quality shift the equilibrium. Low-carbonate caustic is therefore controlled as an incoming raw material specification rather than corrected only by lime causticisation.

Red mud settlers operate with flocculant dosing of 20–50 g/t dry solids. Caustic ionic strength has a direct effect on anionic polyacrylamide bridging performance. Liquor conductivity and anionic charge density must remain stable to avoid fines carryover and high overflow turbidity. Countercurrent residue washing uses 10–20% of the total process water inventory to recover entrained sodium hydroxide. Wash water pH control below 10.5 leads to sodium aluminate hydrolysis and aluminium hydroxide loss to red mud. In shell-and-tube digestion heaters, operational experience shows that tube-side caustic velocities below 1.5 m/s favour alkaline scale nucleation, while velocities above 2.5 m/s increase erosion-corrosion on carbon steel tube sheets. Digestion vessels are fabricated in accordance with ASME BPVC Section VIII Division 1 or equivalent pressure vessel codes. Caustic stress corrosion cracking of carbon steel is mitigated by post-weld heat treatment and by restricting operating temperature below the material-specific caustic concentration threshold. For high-silica boehmite, desilication of recycled liquor with lime is used before digestion. Refineries monitor sodium hydroxide quality using ISO 979 for total alkalinity, ISO 981 for chloride, and ISO 3196 for carbonate content. The chloride value matters because chloride may concentrate in the liquor loop by evaporation and purge underflow; a reduction in chloride input from membrane-grade lye lowers the purge volume needed to keep chloride below 3–5 g/L Na₂O equivalent in liquor.

Kraft mills use sodium hydroxide to alkalise medium-consistency oxygen delignification systems at 8–12% pulp consistency and 90–110 °C. Caustic charge is normally 1.5–3.0% on oven-dry pulp. The entering pH is held between 11.5 and 12.5, and the stage exit pH is controlled at 10.2–11.0. Mill operating data show that an exit pH below 10.0 is associated with reduced pulp viscosity and lower tear strength after bleaching. The reason is kinetic: hydroxide ion promotes both lignin degradation and cellulose alkaline hydrolysis, and selectivity worsens outside the pH window. In extraction stages of elemental chlorine-free sequences, additional sodium hydroxide is charged at 0.5–2.5% on pulp. The actual demand is not fixed by pH alone because dissolved carbon dioxide from upstream washer carryover consumes alkalinity. Low-carbonate fresh lye therefore reduces unnecessary caustic consumption through carbonic acid neutralisation. The mill recovery cycle uses make-up caustic to balance the sodium-to-sulphur ratio after liquor losses. Bleached kraft mills typically consume 20–60 kg NaOH per air-dried tonne of pulp, with the lower end associated with low-kappa cooking and the upper end associated with high oxygen delignification push and multiple alkaline extraction stages. Standard method TAPPI T 624 om-22 is used to determine sodium hydroxide in white liquor and includes direct titration of filtered liquor.

Transition metal management is a process conflict when caustic soda specifications are set only by total alkalinity. Iron and manganese carried with wood chips catalyse hydrogen peroxide decomposition in subsequent alkaline peroxide stages. A bleach plant may add DTPA chelant at 0.2–0.5% on pulp and magnesium sulphate at 0.05–0.1% as magnesium ion to protect peroxide. A high transition metal content in make-up caustic adds redox variability that is difficult to distinguish from chip supply variation. For this reason, Nobian membrane-grade caustic soda is transported in dedicated alkali-resistant tanks and lines to avoid iron contamination from shared chemical unloading systems. In peroxide-stabilised bleaching, caustic activates hydrogen peroxide at 60–80 °C and pH 10.8–11.4. Caustic dosing is trimmed by residual peroxide, not by pH alone. Residual peroxide below 0.1 g/L after 60 minutes indicates either excessive catalyst carryover or insufficient alkali buffering. Caustic mixing must be completed before peroxide addition because local alkali spikes above pH 12.5 generate chromophores from extractives. Static mixers and medium-consistency pumps with high-shear mixing are used to ensure alkali distribution before oxygen gas injection. Mechanical seals on oxygen stage feed pumps are specified with caustic-compatible elastomers, typically EPDM or PTFE, because standard nitrile seals embrittle at high pH and service temperature.

The Free Alkali Window in Soap Vacuum Finishing Is Narrow

During batch saponification, fatty acid blends are processed with 50% sodium hydroxide at 70–90 °C in jacketed stainless steel crutchers equipped with contra-rotating agitators. The caustic dose is calculated from the saponification value of the fat charge. Palm kernel oil has a saponification value of 245–255 mg KOH/g, coconut oil 248–268 mg KOH/g, and palm stearin 193–205 mg KOH/g. A slight excess of 0.5–1.0% total alkali as Na₂O is maintained to push saponification to completion without leaving free fat above 0.1%. Lower excess alkali leads to rancidity-prone free fatty acid in finished bars. Higher excess alkali above 1.5% causes skin irritation and increases soap viscosity during vacuum drying. In continuous saponification lines, stoichiometric control is automated through near-infrared moisture and free alkali analysers; batch plants still verify total free alkali by ISO 684:2008. The terminal products are sodium laurate, sodium stearate, sodium palmitate, and sodium oleate. These salts are then dried into soap noodles at 12–14% moisture and extruded into laundry bars or toilet soap. Sodium chloride from diaphragm-grade caustic above 1% of soap mass shifts the soap phase boundary and can cause salting-out, graining, and lower plodder throughput. Membrane-grade low-chloride caustic is therefore used where high-speed vacuum finishing lines operate above 1,500 kg/h.

A production bottleneck in batch soap plants is temperature control during the gel phase. Saponification is exothermic and the reaction mass passes through a high-viscosity gel between 50 °C and 70 °C for tallow-rich blends. If cooling water is applied too early, the batch stalls and caustic droplets remain undispersed. If the jacket temperature exceeds 100 °C, water boils at atmospheric pressure and foam carries fatty acid salt into the vacuum duct. Crutcher operations therefore hold temperature at 80–85 °C during the final stage of saponification and begin vacuum drying only when free alkali is within specification. Glycerine released from triglycerides remains in the soap mass; in soft-oil soaps, glycerine above 1.5% increases stickiness during milling and reduces bar hardness. Soap plants with low salt and low glycerine specifications can operate twin-screw vacuum plodders at 30–40 bar die pressure. The operational boundary of caustic selection is that micropearl or liquid 50% sodium hydroxide must be free of insoluble particulate above 50 µm because screen packs and spinnerets in soap finishing lines clog at elevated sieving residue. Sodium hydroxide solution is also used to produce sodium cocoate and sodium tallowate for cold-process soap. In that route, lye concentration is reduced to 20–25% before addition to oil at 40–50 °C; trace caustic droplets cause local saponification spots that appear as white specks in translucent soap. The pH of finished soap solution at 1% is typically 9.5–10.5, governed by residual free alkali and soap hydrolysis.

Membrane-grade 50% NaOH is diluted to 15–20% by mass before entering the packed absorption column of a sodium hypochlorite synthesis tower. Chlorine gas is introduced at 0.3–1.5 bar gauge countercurrently to the caustic recirculation stream. The product specification is typically 13–15% w/w available chlorine with excess sodium hydroxide held between 0.5% and 1.0% by mass to maintain pH 12.5–13.0. This excess alkali suppresses sodium chlorate formation and prevents hypochlorous acid accumulation. If excess caustic falls below 0.2%, the liquor pH drops below 11.5 and the disproportionation rate to chlorate increases sharply. The tower residence time is set at 10–20 minutes, and product storage is kept below 20 °C to limit decomposition. Each tonne of chlorine gas consumed yields approximately 1.05 tonnes of sodium hypochlorite on a pure NaOCl basis; the commercial product is produced or diluted directly at 13–15% w/w available chlorine. A common failure mode in bleach plants is sodium carbonate scaling on random packing. Carbon dioxide from vent gas reacts with excess caustic to form sodium carbonate, which precipitates when carbonate solubility is exceeded at high ionic strength. Low-carbonate membrane-grade lye reduces packing scale frequency and extends service intervals beyond 6 months in chlorine absorbers. In chloralkali facilities, the same sodium hydroxide is also used to scrub chlorine tail gas. Final chlorine emission after caustic scrubbing can be held below 5 mg/Nm³ when pH and oxidising potential are controlled, but exhaustion of caustic below 0.5% causes rapid breakthrough of chlorine gas.

Sodium hydroxide is further consumed in the synthesis of sodium phenolate from phenol at 40–60 °C under a nitrogen pad to prevent quinone colour bodies. The reaction is conducted at roughly 1:1 molar ratio; unreacted phenol above 0.1% must be steam-stripped before downstream salicylate chemistry. In propylene oxide production by the chlorohydrin route, saponification of propylene chlorohydrin with sodium hydroxide consumes on the order of 1.2–1.5 tonnes of NaOH per tonne of propylene oxide, although modern hydrogen peroxide propylene oxide routes have reduced this caustic intensity. Published data for the exact split by production route is plant-specific and limited in open literature. Another downstream is the neutralisation of dodecylbenzene sulfonic acid after SO₃ sulfonation. The acid is neutralised with 20–25% NaOH at 40–50 °C to produce sodium dodecylbenzene sulfonate, a main anionic surfactant in laundry powders and liquid dishwash. In this neutralisation, local overdosing of caustic above pH 9.5 discolours the paste and raises free caustic above the 0.1% specification. Continuous loop neutralisers are preferred because they provide rapid isothermal mixing and avoid batch-to-batch colour variation. Equipment in these chemical synthesis plants includes glass-lined reactors for phenol chemistry, fluoropolymer-lined columns for hypochlorite, and Alloy 904L or titanium plate heat exchangers in recycle loops where chlorinated brine and hot caustic alternate. Caustic service design must avoid dead legs because stagnant sodium hydroxide absorbs carbon dioxide and forms crystalline sodium carbonate plugs in instrument impulse lines.

When Lime Softening Sludge Requires Post-precipitation Caustic pH Trim

In municipal drinking water plants, sodium hydroxide is used after lime softening to trim pH upward without adding calcium hardness. Lime softening removes carbonate hardness but leaves residual calcium and magnesium that must remain soluble through final pH control. Sodium hydroxide dosing is preferred over sodium carbonate because it raises pH without contributing to sludge volume. In low-alkalinity water with 10 mg/L CaCO₃ alkalinity, addition of 2–4 mg/L as 100% NaOH may raise pH from 6.8 to 7.2, but the exact dose depends on dissolved CO₂, temperature, and total inorganic carbon. Product used in drinking water treatment must comply with EN 896:2013 in the European Union or NSF/ANSI/CAN 60 in North America. Sodium hydroxide for municipal use is often obtained as 25% or 50% solution and post-diluted to 0.5–2.0% before injection through quill-style diffusers. Undiluted 50% lye injection into low-flow lines causes local precipitation of calcium carbonate on the pipe wall and rapid scaling of spring-loaded check valves. A membrane or peristaltic metering pump with stroke length control is used for 0.5–10 mL/min dose rates; pump head materials are typically EPDM and PTFE because PVC becomes brittle after prolonged caustic contact at concentrations above 20%.

Ion exchange demineralisation uses sodium hydroxide for strong-base anion resin regeneration. A typical regeneration sequence for a mixed-bed or two-bed plant includes a 4% NaOH solution at 40–50 °C, applied at 60–100 g/L resin at 2–4 bed volumes per hour. Silica elution from anion resin is incomplete below 40 °C, and resin bead expansion above 60 °C risks osmotic shock. Wastewater neutralisation is a separate shallow application. Sodium hydroxide is added to batch equalisation tanks to bring acidic industrial effluent into the pH range 6.0–9.0 before discharge. The critical boundary is that caustic should not be added to wastewater containing aluminium or zinc where hydroxide precipitation will form voluminous gelatinous solids. In such streams, caustic is replaced by sodium carbonate or lime slurry unless the metal hydroxide sludge is the intended removal mechanism. Sodium hydroxide is also used to regenerate degasifier media and to control pH in cooling water systems, but these are low-volume ancillary uses compared with municipal pH adjustment and anion regeneration. A compliance checklist for water treatment would normally cite AWWA B501-19 for caustic soda quality, EN 896:2013 for human drinking water treatment, and ISO 979 for total alkalinity assay. The maximum use rate is determined by treated water pH, not by total alkalinity; overdosing to pH above 9.0 causes taste complaints and may mobilise lead from older plumbing.

Caustic Peeling and Lye Reduction Circuits in Vegetable Canning Lines

For fruit and vegetable peeling lines, sodium hydroxide concentrations of 8–18% by mass are applied at 90–95 °C for 20–90 seconds, depending on cultivar and cuticle thickness. Tomato peeling often uses 15–18% lye at 95 °C for 20–40 seconds, while peach peeling may use 2–5% lye at 88 °C for 30–60 seconds. Caustic concentration is maintained by conductivity because soluble peel solids change density and hide true alkali strength. In shell-and-tube lye heaters, accumulated peel solids above 10–12% dry matter reduce heat transfer and force a weekly caustic change-out. The spent lye is screened, then neutralised with acid before biological treatment. Sodium hydroxide is authorised as food additive E 524 in the EU under Regulation (EC) No 1333/2008 for food surface treatment and as a processing aid where residues are removed. In the United States, food-grade sodium hydroxide must meet the Food Chemicals Codex monograph for sodium hydroxide. Terminal products from this route include canned whole tomatoes, diced potatoes, citrus segments, and peeled pears. The lye peeling process imposes a process conflict: extended caustic contact at high temperature weakens fruit texture and raises drained weight loss. Canning lines therefore use fast caustic treatment followed by high-pressure water sprays at 7–10 bar. The water spray stage determines final peel removal, not the caustic alone. Lye concentration and spray pressure are controlled as paired variables; increasing caustic above 20% without improving spray coverage produces surface saponification of cuticular wax and off-flavour rather than faster peeling.

Spanish-style green olive processing uses sodium hydroxide at 2.0–3.5% for 6–12 hours at 15–25 °C to hydrolyse oleuropein and remove bitterness. Sodium hydroxide penetrates the skin and outer flesh; the lye treatment is stopped when the alkaline front reaches 2/3 to 3/4 of the flesh thickness. Overly deep lye penetration causes soft olives and excessive loss of fermentable sugars. Subsequent washing with water removes caustic before brine fermentation at 5–7% sodium chloride. In modified starch production, sodium hydroxide is used to prepare starch in etherification and cross-linking reactions. Caustic level is expressed as 0.5–2.0% on dry starch, and the reaction temperature is held at 40–50 °C for most food-grade hydroxypropylation. Residual caustic is neutralised after the reaction to pH 5.0–6.5 with hydrochloric acid or citric acid. Product washing must reduce residual sodium to meet end-product limits; this is a wash-water intensive process and a known production bottleneck. Food-grade caustic soda also cleans equipment where acid-based cleaners cannot remove polymerised fat and carbonised sugars. Circulation cleaning in dairy and beverage plants uses 2–5% NaOH at 70–85 °C for 20–30 minutes. The terminal influence on food quality is indirect but critical: incomplete caustic rinsing leaves sodium hydroxide residues that raise product pH and cause soapy off-taste. Cleaning-in-place sensors verify rinse water conductivity below 50 µS/cm before sanitation starts.

Mercerization range data from cotton finishing show that sodium hydroxide penetrates the fibre at 18–25% by mass and 15–20 °C. The cotton yarn or fabric is passed through a caustic pad, stretched under tension, and held for 30–60 seconds before countercurrent washing. Mercerisation produces fibre swelling, increases dye uptake, and improves tensile strength. The alkali disrupts the hydrogen-bonded cellulose I lattice and converts it to cellulose II on regeneration. Caustic recovery from wash water uses multiple evaporation stages to reconcentrate lye to 30–40% before purification. Impurities such as cotton wax, starch size, and pectin accumulate in the recovered lye; these impurities increase colour and reduce wetting speed. In causticising of cotton knit fabric, a lower caustic concentration of 10–15% is used without tension to reduce shrinkage, but the process is not a substitute for full mercerisation. Sodium hydroxide also acts as a scouring agent for cotton at 8–12% solution with wetting agents at 95–100 °C. The operating boundary in textile pretreatment is that scouring with hard water forms calcium and magnesium soap deposits if caustic is added before water softening; therefore, knitted goods plants often use a sequestering auxiliary and softened water at 70–80 °C. A mill failure mode is yellowing of cotton after resin finishing when residual alkali is not neutralised. Residual fabric pH above 8.5 before finishing causes hydrogen peroxide residues to react with optical brighteners. Fabric after caustic scouring is neutralised with acetic acid and checked by a cold-water extract pH method. Laboratory test methods include ISO 3071:2020 for pH of aqueous extracts of textiles. A continuous mercerising range can recover 80–90% of input caustic through countercurrent wash and evaporation, reducing wastewater neutralisation load and chemical cost. Because 50% sodium hydroxide solution freezes near 12 °C, mills in cold climates heat caustic storage tanks with low-pressure steam or dilute to 20% lye to reduce crystallisation risk.

Caustic soda is also used in denim finishing to create a smooth surface and reduce indigo redeposition in wash cycles. This use is ancillary and process-specific, with published data limited to individual machine settings. The main technical requirement remains low transition metal content, because iron contamination from storage tanks or piping can produce rust spots on bleached cotton. Nobian membrane-grade sodium hydroxide is stored in unlined carbon steel tanks at ambient temperature and 50% concentration; at this concentration carbon steel is acceptable, but titanium or nickel alloys are required for heating elements above 80 °C. Sodium hydroxide is incompatible with aluminium, zinc, and galvanised steel in textile plant wetted parts. Wool is not processed with strong caustic because NaOH attacks disulfide bonds in keratin and causes fibre dissolution. Synthetic fibres such as polyester are scoured with mild alkaline systems near 1–2% caustic and 60–70 °C, above which polyester hydrolysis causes surface pitting and loss of tenacity. Polyamide is more sensitive; caustic scouring above pH 10.5 at boil discolours nylon and reduces tensile strength. These constraints define the application window for caustic soda in textile wet processing.