Ineos Inovyn Caustic Soda

The Bayer circuit at a refinery producing smelter-grade alumina receives Ineos Inovyn 50 wt% membrane-grade sodium hydroxide into raw caustic storage, where it is diluted with process condensate to the target spent-liquor concentration before bauxite slurry charging. The digestion unit operates with a free Na₂O concentration of 120–250 g/L, depending on whether the feed is gibbsitic or boehmitic bauxite. Gibbsite dissolves at 140–160 °C in a series of agitated autoclaves, while boehmite requires 200–240 °C and a corresponding steam pressure of 5–35 bar. The molar A/C ratio in the pregnant liquor is maintained between 0.65 and 0.75; below this window aluminium trihydrate precipitates prematurely in the flash tanks, and above it the caustic inventory is consumed by desilication products such as sodalite and cancrinite. Scale formation on heat exchanger tube walls is a recurring production bottleneck, and the refinery must isolate individual tube digesters for acid wash with inhibited sulphuric acid at 60–70 °C when the heat transfer coefficient falls below operating limits. The terminal product, smelter-grade alumina with a residual Na₂O content below 0.5 wt%, is discharged through pressure filters and calcining kilns.

Red mud separation downstream of digestion is conducted in thickeners with a flocculant dose adjusted to achieve an underflow solids concentration of 35–45 wt%. The overflow pregnant liquor is passed through security filtration to remove fine solids before precipitation; any carryover of red mud into precipitation reduces the brightness and chemical purity of the alumina trihydrate. Caustic liquor losses occur through entrained red mud disposal and through the desilication product, and these losses are replaced by fresh 50 wt% caustic at a rate matched to the refinery's A/C balance. In high-carbonate bauxite operations, sodium carbonate impurities accumulate in the circuit and must be purged by soda ash recovery or by replacing a proportion of the caustic feed with filtered dilute liquor from the wash tanks. The accumulation of oxalate in the liquor is controlled by liquor polishing and by side-stream crystallisation, which avoids the occluded sodium oxalate crystals that otherwise disturb particle growth in the precipitation tanks.

What Limits Effective Alkali Charge in Continuous Kraft Digesters?

Kraft pulping of softwood chips operates with white liquor containing sodium hydroxide and sodium sulphide at an effective alkali charge of 18–22 wt% Na₂O on oven-dry wood. Continuous digesters with a hydraulic residence time of 4–8 h at 160–170 °C reduce the kappa number to a target of 15–30 when measured by ISO 302:2015. The causticizing loop converts recovered sodium carbonate to hydroxide using slaked lime in a series of three or more agitated causticizers followed by white liquor clarifiers. Sodium hydroxide sourced from membrane electrolysis is used as make-up in the recausticizing area, while spent pulping liquor is concentrated in multi-effect evaporators from 14–16 wt% dry solids to 65–80 wt% black liquor for the recovery boiler. In oxygen delignification, a subsequent alkaline charge of 15–25 kg NaOH/t pulp at a reactor top pressure of 5–8 bar and 90–100 °C removes approximately 50% of the residual kappa before bleaching. The terminal bleached softwood kraft pulp is then processed into linerboard, sack paper, and market pulp bales.

Causticizing efficiency depends on the sodium carbonate-to-sodium hydroxide conversion in the recausticizing plant, where the lime-to-sodium carbonate molar ratio is kept between 1.02 and 1.10. Under-liming leaves sodium carbonate in the white liquor, which increases dead load and reduces the effective alkali available for delignification; over-liming produces lime mud with high residual alkalinity and shortens the cycle life of the white liquor filter cloths. The lime mud is washed on belt filters to an alkali loss below 0.5 kg Na₂O/t pulp before it is returned to the lime kiln. In the bleach plant, sodium hydroxide is used in the alkaline extraction stage at a pH of 10.5–11.5 and a retention time of 45–60 min, followed by chlorine dioxide substitution; the terminal brightness of 88–90 ISO% is verified by ISO 2470-1. The finished bleached pulp is dewatered on a twin-roll press to 85–90% dryness and baled.

In continuous saponification lines processing refined bleached deodorized coconut oil, the sodium hydroxide feed is diluted from 50 wt% membrane-grade to 25–30 °Bé under high-shear mixing. The alkali tare is calculated from the saponification value of the oil batch using ISO 3657; for coconut oil the saponification value is typically 250–264 mg KOH/g, while palm kernel oil lies at 230–254 mg KOH/g and tallow at 190–205 mg KOH/g. A stoichiometric factor of 0.97–1.00 is applied to retain 0.5–1.5 wt% unsaponified matter in the soap phase, which controls the grainy texture and reduces free alkali in the finished bars. The reaction mass is held at 80–120 °C for 15–30 min in a multi-section continuous reactor, after which the glycerol-water phase is separated by decantation and refined to 99.5 wt% glycerol for technical or pharmaceutical grade. The soap phase is vacuum-dried to 12–14 wt% moisture, milled, plodded, and cut into soap noodles as the terminal bulk product for bar soap production.

Mercerization Bath Hydraulics and Caustic Recovery

Greige cotton fabric entering a chain mercerizer is impregnated under controlled tension with 20–24 wt% sodium hydroxide at 15–25 °C. The alkali forms sodium cellulosate and transforms native cellulose I to cellulose II, which reduces crystallinity and alters the cross-section of the fibre. A contact time of 45–60 s is maintained by roller speed and bath length before the fabric passes through a countercurrent washing section; the recovered weak alkali is directed to vacuum evaporators and re-concentrated to 35–40 wt% for reuse. The evaporation step is operated below 60 °C to suppress cellulose degradation products that would otherwise darken the recovered caustic. Caustic carry-over into the fabric is controlled by final hot-water wash tanks and a sour rinse with acetic acid at 1–2 g/L before stentering. The degree of mercerization is verified by the barium activity number procedure of AATCC TM 89, and the tensile strength of the treated fabric is measured by ASTM D5034. Terminal products include cotton poplin, twill, and denim fabrics where the treatment raises the barium activity number and wet tensile strength retention.

When Drinking Water Utilities Replace Soda Ash with Sodium Hydroxide

Utilities that switch from lime-soda ash softening to caustic soda pH correction conduct a buffer-capacity evaluation under ANSI/AWWA B501-19 and EN 896. The sodium hydroxide feed is metered by positive-displacement diaphragm pumps through injection quills into a static mixer, with a residual pH target of 7.5–8.2 at the distribution entry point. For pH adjustment of low-alkalinity surface water, the dose is typically 1–5 mg/L as 100% NaOH; for softening applications replacing soda ash, the dose can reach 15–40 mg/L depending on raw-water bicarbonate hardness. A caustic soda feed of 25 wt% or 50 wt% membrane grade is favoured because membrane electrolysis minimises chlorate and trace metal carryover, which is controlled by EN 896 limit values and by periodic lot certification under ANSI/AWWA B501-19. Heated traced lines are required in outdoor installations because 50 wt% sodium hydroxide thickens below 15 °C and can crystallise below 12 °C. The terminal product is potable water stabilized for corrosion control; however, operators avoid an excess hydroxide dose that raises the pH above 9.5, which can destabilise chloramine residuals and damage cement-mortar-lined ferrous pipe interiors.

Propylene Oxide via Chlorohydrin Saponification Requires Tight pH Control

In the chlorohydrin route to propylene oxide, the propylene chlorohydrin stream is mixed with dilute sodium hydroxide solution at pH 9.0–10.5 and 80–95 °C, producing propylene oxide by ring closure. The caustic-to-chlorohydrin molar ratio is held at 1.02–1.05; excess sodium hydroxide above this range hydrolyses propylene oxide to propylene glycol, while insufficient hydroxide leaves chlorohydrin unconverted and reduces propylene oxide yield. Online near-infrared analysers monitor the organic phase in a packed column, and the product is stripped with steam before distillation to 99.9 wt% propylene oxide. Terminal propylene oxide is stored under nitrogen and is used in polyether polyol production for flexible and rigid polyurethane foams.

The calcium-free membrane-grade caustic is preferred in this loop because residual calcium and magnesium ions accumulate in the brine recirculation and form insoluble hydroxide scale in the saponifier and downstream reboilers. Chlorate generated in older mercury or diaphragm cells can act as an oxidant in the organic phase, and the specification for the propylene oxide loop is set below 10 mg/kg chlorate in the fresh caustic feed. The byproduct sodium chloride brine is returned to the electrolysis plant at 200–250 g/L sodium chloride after organic stripping and activated-carbon polishing, which closes the chlor-alkali balance.

At root- and tuber-vegetable peeling lines, food contact sodium hydroxide meeting the Food Chemicals Codex identity specification and 21 CFR 184.1763 is applied as a 3–8 wt% solution at 60–90 °C. Immersion lasts 1–6 min, during which the hydroxide saponifies the pectic substances in the peel cell walls and loosens the outer layers; the peeled surface is then neutralized with citric acid to a residual sodium citrate content below 0.5 wt%. The process is used for potatoes, carrots, and root vegetables destined for canning, retorting, or frozen distribution. Caustic concentration and temperature are mutually adjusted so that heat penetration does not gelatinize starch more than 2–3 mm into the tuber core, which prevents shelf-life defects in the final canned product. The wastewater from the peeling line is screened for solids and neutralized to pH 6–9 before discharge under the site's industrial effluent permit.