Caustic Soda Flakes

Digestion of bauxite slurry in high-pressure autoclaves relies on sodium aluminate liquor preparation where caustic soda flakes serve as the primary source of reactive Na₂O. The flakes are first dissolved in weak process condensate within a continuous dissolution train fitted with a scraper cooler; dissolution enthalpy raises the exiting liquor temperature to 80–95°C. This concentrated stream—typically adjusted to 400–520 g/L total NaOH—is blended with recycled spent liquor to achieve an digestion feed having a caustic soda concentration of 140–250 g/L expressed as Na₂O. The molar ratio α_k (Na₂O/Al₂O₃) is maintained between 1.45 and 1.70, with lower ratios favoring higher liquor productivity but risking auto-precipitation of aluminum hydroxide in the feed line. Bauxite slurry preheated through a series of shell-and-tube heaters is contacted with the liquor in a multi-compartment autoclave or a tube digester operating at 240–270°C and 3.5–5.5 MPa. Under these conditions, gibbsite and boehmite phases dissolve within 15–45 min; diaspore-containing ores require residence times approaching 60–90 min and liquor NaOH levels exceeding 230 g/L. Process make-up requirement fluctuates between 60 and 100 kg of 98% NaOH per metric ton of smelter-grade alumina, depending on sodium loss via red mud entrainment and organic oxalate salt removal. Flake purity directly influences Bayer circuit chloride accumulation: a chloride content above 0.5 wt% (as NaCl) in the incoming caustic promotes pitting corrosion on duplex stainless steel digestor shells and elevates sodium inclusion in metallurgical alumina above the 0.4% Na₂O threshold specified in GB/T 4292-2017. Consequently, evaporator bleed streams and oxalate crystallization purge loops are integrated to cap circuit chloride below 3 g/L. Mercury and iron contaminants from sub-grade flakes partition into the product hydrate, for which the upper control limits in the precipitated Al(OH)₃ are <20 ppm Fe₂O₃ and <0.5 ppm Hg as per YS/T 803-2012 for aluminium fluoride feedstock. Continuous causticisation of sodium carbonate in green liquor with slaked lime recovers additional NaOH units, yet the primary alkalinity make-up remains flake-form soda for its ease of inventory control and minimal freight dilution compared to 50% membrane-grade liquid.

Alkali Charge Calculation via Saponification Value and the Lyotropic Behaviour of Neat Soap

Saponification of triglycerides in a continuous fatty acid neutralisation or direct kettle boiling process demands precise caustic soda dosing anchored to the saponification value (SV) measured under ISO 3657:2022. A refined tallow feedstock exhibiting an SV of 195–205 mg KOH/g requires a stoichiometric NaOH quantity of 0.714 × SV expressed as grams NaOH per gram of oil. Caustic soda flakes are dissolved in softened, de-aerated water to yield a 38–45 wt% stock solution; dissolution temperature is kept below 60°C to reduce carbonate formation from atmospheric CO₂ absorption. The calculated caustic dosage is increased by a deliberate excess of 0.5–1.5% of the stoichiometric mass—corresponding to a free alkali content in the finished neat soap of 0.08–0.12% as NaOH—to compensate for neutral oil that remains unsaponified during the short residence time of a high-pressure saponification loop. In a typical Alfa Laval “Saponiflex” line, the pre-heated oil and caustic solution are co-fed into a multi-stage static mixer maintained at 120–130°C and 0.3–0.5 MPa backpressure, where saponification reaches >99% conversion within 5–10 min. Phase separation of neat soap from the aqueous glycerine-rich lye is accomplished by exploiting the salting-out effect: a brine addition of 2–4% NaCl relative to the batch weight shifts the critical micelle concentration enough to yield a soap phase with 30–34% moisture and <0.5% glycerol. Residual electrolyte composition is verified against ASTM D460-21, with total alkalinity titration (Method 24A) and unsaponifiable matter determination forming part of the release protocol. Flake-sourced caustic must demonstrate nickel and copper contents each below 0.5 ppm because these metals catalyse oxidative rancidity during bar soap storage, producing discolouration detectable at a peroxide value above 1 meq/kg in accelerated shelf-life tests at 40°C/75% RH. Operational boundary: if the soap base is intended for removal via spray drying into powder detergents, the neat soap electrolyte level is pushed to a higher salt content of 4–6% NaCl to enable subsequent crutcher mixing with sodium tripolyphosphate without gelation.

How Does Caustic Soda Concentration Influence Cellulose II Crystallinity and Fibre Tensile Properties?

Cotton yarn or woven fabric is treated in an open-width mercerising range where caustic soda flakes are dissolved on-site to formulate a 22–30 °Bé sodium hydroxide bath, corresponding to a concentration of 20–28 wt% NaOH. The liquor temperature is tightly regulated between 15°C and 18°C because the exothermic swelling of cellulose I to alkali cellulose requires heat removal via plate heat exchangers or direct chiller injection; a temperature excursion above 22°C reduces cellulose II conversion efficiency and luminary reflectance by more than 8 gloss units as per AATCC TM109-2019. The fabric dwell time under controlled tension does not exceed 45–60 s in the caustic zone, after which a sequential hot-water wash at 70–90°C under gradually decreasing tension hydrolyses the soda-cellulose complex and restores fibre diameter. Bath hemicellulose accumulation—monitored by a calcium oxalate precipitation method—is capped at 8–12 g/L; above 15 g/L, competition for alkali reduces effective NaOH activity and generates uneven dye affinity known as “streaky” shade variation in reactive dyeing. Iron content in the flake NaOH must remain below 10 ppm, because soluble ferric hydroxide precipitates onto the fibre at alkaline pH and catalyses oxycellulose formation during subsequent peroxide bleaching, causing loss of tensile strength measured by ASTM D5034-09(2017) strip test. The mercerising line is typically equipped with a three-effect evaporation recovery system that reconcentrates weak wash liquor from 5–8% NaOH back to 25%, achieving a chemical recovery ratio exceeding 90%. Final product certification for tightly twisted combed cotton yarn demonstrates a single-end breaking force improvement of 18–25% and a lustre index (contrast ratio of diffuse reflectance at 45°) above 1.6 versus untreated grey cotton.

Make-up alkali introduced into the smelt dissolving tank of a kraft recovery cycle uses caustic soda flakes to correct the effective alkali (EA) deficit that develops through sulfidity drift and sodium loss in lime mud. White liquor EA—defined as NaOH + ½ Na₂S expressed as Na₂O—typically arrives from the causticising plant at 85–115 g/L, yet continuous pulping of softwood for linerboard at 20–22% effective alkali charge on oven-dry wood consumes sodium in the digester via neutralisation of acetic and hexenuronic acids, pulling the spent black liquor residual EA below 6–8 g/L. When recausticising efficiency drops below 78% or when the sulfidity falls outside the target band of 25–30%, supplemental NaOH is dosed into the dissolving tank at a controlled rate not exceeding 15 kg NaOH (100%) per tonne of green liquor dry solids to avoid local nucleation of pirssonite (Na₂CO₃·CaCO₃·2H₂O) that scales heating surfaces. Flake purity requirements for this application mandate iron below 50 ppm and silica below 100 ppm, because these metals accumulate in unbleached pulp; iron above 30 mg/kg in the pulp catalyses hexenuronic acid degradation during ECF bleaching, elevating the chlorine dioxide demand in the D₀ stage by 1.2–1.5 kg ClO₂ per tonne of pulp. The dissolved NaOH stream is filtered through a 10-micron polypropylene bag filter before injection to remove insoluble carbonates formed during flake hygroscopic weathering. Kappa number reduction after pulping is measured per ISO 302:2015 and targeted at 16–20 for linerboard and 6–8 for bleachable-grade softwood; insufficient EA in the digester increases screen rejects by up to 3% on wood feed, a condition traced by alkali profiling along the digester circulation screens.

When Sour Water Stripper Off-Gas Requires Residual Removal Before Thermal Oxidation

Refinery sour water containing ammonium bisulfide and hydrogen sulfide is stripped with steam, but the overhead non-condensable gases still carry 2–5 vol% H₂S that cannot be routed directly to a Claus unit without ammonia removal. A pack-bed scrubber downstream of the stripper overhead condenser circulates a 12–20 wt% sodium hydroxide solution prepared from flakes in a dedicated blending skid equipped with a Hastelloy C-276 eductor. The caustic recirculation rate is set to maintain a liquid-to-gas ratio of 4–6 L/m³, achieving an H₂S removal efficiency above 99.5% and producing a bisulfide‑rich spent caustic with a sodium hydrosulfide (NaSH) concentration of 18–25%. This spent stream is either sold as heavy metal precipitator feedstock or injected into a wet air oxidation unit operating at 200–250°C and 4–6 MPa. Carbon dioxide pickup from the gas phase generates sodium carbonate, which precipitates as monohydrate in the scrubber sump when the NaOH concentration is allowed to fall below 6 wt%. To avoid column fouling and plugging of the ceramic random packing (typically 25 mm standard Pall rings), the circulating solution is continuously purged at a rate equivalent to 5–8% of the inventory per shift. The flake dissolution makeup tank is nitrogen-blanketed to prevent atmospheric carbonation and is constructed of stress-relieved carbon steel because the dilution exotherm is insufficient to raise the fluid temperature into the caustic stress corrosion cracking domain of non-stress-relieved welds. Analytical control of the scrubber liquor follows UOP Method 209-00 for free NaOH and sulfides, with an upper sulfide loading limit of 220 g/L as Na₂S equivalent before vapour-phase H₂S breakthrough becomes measurable above 10 ppmv at the scrubber exit.

Municipal drinking water plants handling low-alkalinity, low-pH surface water use caustic soda flakes to elevate pH for optimised coagulation with aluminium sulfate. The target settled water pH range of 6.8–7.2 is achieved by dosing a 3–5 wt% NaOH solution at a rate of 5–15 mg/L as NaOH, increasing the total alkalinity by 10–25 mg/L as CaCO₃. The feed system consists of a day tank of HDPE construction, a metering pump with PTFE diaphragm, and a static mixer positioned at least 10 pipe diameters upstream of the flocculation basin to avoid localised high-pH zones that can resolubilise precipitated aluminium floc. National regulatory compliance is demonstrated through NSF/ANSI/CAN 60 certification of the flake product for corrosion and scale control chemicals, which imposes a maximum impurity contribution of 0.002 mg/L mercury and 0.01 mg/L antimony at the drinking water tap. For heavy metal precipitation in industrial effluent—specifically removal of zinc and copper from electroplating rinse water—flake‑sourced caustic is preferred over hydrated lime because it generates minimal sludge volume. The pH is raised to 9.5–10.0 in a two-stage neutralisation clarifier; under these conditions soluble Zn²⁺ drops below 0.5 mg/L and Cu²⁺ below 0.1 mg/L, meeting GB 8978-1996 Class I discharge thresholds. The process limitation is that excess NaOH beyond pH 10.2 converts precipitated Zn(OH)₂ into soluble zincate ion Zn(OH)₄²⁻, causing effluent zinc to rebound above the compliance limit.

Kansui Alkaline Profile and Its Effect on Dough Extensibility in Alkaline Noodles

Traditional lamian and ramen-style noodles rely on an alkaline dough condition known as kansui, traditionally a mixture of sodium and potassium carbonates. Caustic soda flakes conforming to FCC 13 or Commission Regulation (EU) No 231/2012 are used to partially or fully replace carbonate salts to achieve a precise dough pH between 9.0 and 10.5. The flake is dissolved in the dough mixing water at a concentration equivalent to 0.25–0.55% of the wheat flour mass (14% moisture basis), representing a delivered NaOH dose of 2.5–5.5 g per kg of flour. At this addition level, the protein matrix is plasticised through thiol‑disulfide interchange, and the gelatinisation temperature of the starch granules is depressed by approximately 4–7°C relative to neutral dough, producing a rapid‑slurrying noodle that exhibits gel strength above 170 g·cm in a TA.XT Plus texture analyser with a 5-mm cylindrical probe. The flake must demonstrate a heavy metal specification of arsenic <1 mg/kg, lead <1 mg/kg, and mercury <0.1 mg/kg, and must be certified free of dioxins and PCBs under EC 1881/2006, as the alkalised dough is consumed directly after low‑temperature steaming or boiling. Commercial production uses a chilled‑water jacketed mixer to control dough temperature below 35°C during the 10–15 min kneading period; higher temperatures accelerate Maillard-type colour development and reduce the characteristic yellow hue contributed by endogenous flavonoids at alkaline pH values. The finished fresh noodle is sold with a moisture content of 30–35% and a shelf life of 3–5 days under refrigerated storage, with the pH evolution monitored to ensure it does not drop below 8.5, below which microbial spoilage from Bacillus cereus becomes a hazard.