| HS Code | 840233 |
| Chemical Name | Sodium Hydroxide |
| Chemical Formula | NaOH |
| Cas Number | 1310-73-2 |
| Appearance | White Flakes |
| Grade | Technical Grade |
As an accredited Chemplast Caustic Soda Flakes factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Each 25 kg HDPE bag with inner liner protects Chemplast Caustic Soda Flakes from moisture and contamination. |
| Container Loading (20′ FCL) | Loading 20′ FCL with Chemplast Caustic Soda Flakes: palletized, sealed bags secured, moisture-protected, and blocked for safe transport. |
| Shipping | Chemplast Caustic Soda Flakes are shipped in sealed, moisture-proof bags, typically 25 kg or 50 kg, packed on pallets and shrink-wrapped. Transport is via covered trucks or containers to prevent moisture absorption. Proper labeling, segregation from acids, and dry, ventilated conditions are essential for safe handling and delivery. |
| Storage | Store Chemplast Caustic Soda Flakes in tightly sealed, moisture-proof containers in a cool, dry, well-ventilated area. Keep away from water, acids, and incompatible chemicals. Protect from humidity and physical damage. Use appropriate PPE when handling. Ensure containers are clearly labeled and stored off the floor on pallets. |
| Shelf Life | Shelf life is indefinite if stored tightly sealed in a dry, cool area, protected from moisture and air contamination. |
Replenishing free caustic in Bayer liquor after precipitation and mud washing requires caustic soda flakes to be dissolved in condensate before re-contact with milled bauxite at 145–245 °C, with digestion temperature selected for gibbsitic versus diasporic mineralogy; spent liquor exiting precipitation at 120–230 g/L Na₂O is fortified to restore free caustic and caustic-to-alumina molar ratio to 1.4–1.8 prior to slurry heating. Supplier assay is controlled by ASTM E291-18 for total alkalinity and ISO 3196:1975 for carbonate content, while refinery permit limits typically require chloride <0.05% and iron <20 mg/kg to protect nickel-rich autoclave liners. Make-up demand across refinery circuits is typically 0.08–0.15 t NaOH/t Al₂O₃; lime addition is set against reactive silica, and liquor caustic concentration is maintained through evaporation and condensate recycle. The downstream process routes digested slurry through atmospheric flash tanks, red mud sedimentation with high-molecular-weight anionic polyacrylamide flocculant at 90–105 °C, and seeded precipitation in continuous crystallizers with slurry density 600–1000 g/L. Terminal products are smelter-grade sandy alumina with median particle size 45–100 µm, alumina trihydrate, and filter-grade hydrate; operational boundaries exclude aluminum, zinc, and galvanized wetted surfaces, requiring nickel-based alloys or rubber-lined tanks for caustic liquor service.
In the recausticizing loop, sodium carbonate recovered from the black-liquor recovery boiler is reacted with slaked lime to regenerate white liquor; caustic soda flakes are introduced only as external make-up to close sodium and sulfur losses through dregs washing, electrostatic precipitator dust, and bleaching filtrate purges. The addition ratio in a continuous digester is expressed as active alkali on oven-dry wood, commonly 14–20% Na₂O for softwood furnish and 9–13% Na₂O for hardwood furnish, with sulfidity held at 18–35%; in the oxidative extraction stage after oxygen delignification, 1.0–2.5 wt% NaOH on OD pulp is applied at 70–85 °C through a twin-roll press to solubilize oxidized lignin fragments. Compliance measurements are anchored to ISO 302:2015 for kappa number and TAPPI T 236 om-13 for residual lignin after cooking. The downstream process operates at 160–175 °C and 8–10 bar in a downflow continuous digester with H-factor control, followed by blow-tank flashing, brown-stock washing in a pressure diffusion washer, oxygen delignification at 85–100 °C, and elemental chlorine-free bleaching stages. Terminal products include unbleached kraft pulp, bleached softwood or hardwood market pulp, and linerboard furnish; excessive caustic make-up beyond the target active alkali causes yield loss from carbohydrate peeling, so mills limit caustic flakes to 0.02–0.06 t NaOH/t air-dry pulp in sodium-and-sulfur balance make-up.
Where cotton warp yarn is processed on a continuous mercerizing range, caustic soda flakes are dissolved with softened water to a bath strength of 18–25% w/w NaOH at 15–25 °C, with alkali concentration verified by density reading of 23–30°Bé at 15 °C and titrated per ISO 3071:2005 after dilution. The process path integrates singeing, enzymatic or oxidative desizing, alkaline scouring, mercerization under controlled warp tension on a 10–40 m tenter frame, and open-width neutralization using 0.5–1.0% w/w acetic acid before drying; dwell time in the caustic bath is 45–90 s for woven fabrics with a heavy squeeze nip to maintain wet pick-up of 80–120%. Fabric tensile retention is assessed by ASTM D5035-19, and residual caustic on processed goods is limited to <0.15% NaOH by mass after neutralization to prevent dye migration and shade unevenness in subsequent reactive dyeing. Terminal finished products include mercerized cotton shirting, high-lustre cotton yarns, denim warp with improved alkali affinity, and flame-retardant treated cellulosic textiles. Quantitative alkali penetration depth in compact, high-twist yarns is not uniformly reported across mill trials; published data for that specific configuration is limited, so validation on the production range with fabric width and shrinkage measurements is required.
In batch saponification of refined palm olein, the stoichiometric addition of caustic soda flakes is derived from the saponification value of the fat charge; for coconut oil with SAP 258 mg KOH/g, the calculated NaOH demand is 0.184 g NaOH/g oil, and plant practice applies a 5–10% excess on that stoichiometric mass to compensate for hydrolysis losses and ensure complete fat splitting in the kettle. The flakes are metered as a 50% w/w aqueous solution into a jacketed kettle equipped with a high-torque anchor impeller and heated to 80–95 °C; saponification is followed by salting-out with sodium chloride at 8–12% w/w on neat soap, static separation of neat soap and spent lye, closed-loop glycerine recovery, and vacuum spray drying to soap noodles. Free caustic in the dried soap is maintained below <0.1% NaOH by titration per ISO 456:2001, while total fatty matter and moisture are controlled per ASTM D460-91(2014); finished product pH in 1% solution is held between 9.5 and 10.5 to prevent dermal irritation. Terminal product types include 80:20 soap noodles, milled toilet soap bars, laundry bar soap, and liquid castile soap with high lauric acid content. Excessive surcharge above 0.15% free alkali causes brittleness and surface crystallization in low-moisture bar formulations, so the final crutching step is adjusted with free fatty acid if the batch exceeds specification.
Operationally, pH correction in a surface-water treatment train with caustic soda flakes is selected over lime slurry where sodium residual increases are acceptable and where rapid dissolution avoids turbidity spikes in the settled water. The typical dose ranges from 5 mg/L to 50 mg/L as 100% NaOH to shift source water from pH 6.5–7.0 to a post-filtration target of pH 8.0–8.5, with a 0.5–2.0% w/w stock solution prepared in a fiberglass-reinforced plastic dissolving tank and injected through a positive-displacement diaphragm pump into a static mixer upstream of the flocculation basin. Compliance is governed by AWWA B501-19 for sodium hydroxide used in potable water and NSF/ANSI/CAN 60:2020 for drinking water treatment chemicals, while residual alkalinity and pH are monitored with EPA Method 150.1 or equivalent on-line analyzers. The treatment train includes rapid mix at velocity gradient 300–600 s⁻¹, three-stage flocculation at 10–60 s⁻¹, inclined plate clarification, dual-media filtration, and final chloramination. Terminal outputs are potable water, low-sodium boiler feedwater after ion exchange, and filter backwash reclaim water; operational boundaries require avoiding caustic feed through aluminum or brass fittings and isolating the caustic dosing line from coagulant injection points to prevent localized precipitation of aluminum hydroxide.
Continuous on-site sodium hypochlorite generation at a chlor-alkali satellite unit charges chlorine gas through a packed tower or eductor into recirculating caustic liquor prepared from flakes at 20–25% w/w NaOH; the reaction is maintained at 20–30 °C and above pH 12.5 to suppress chlorate and oxygen byproduct formation. Theoretical NaOH demand is 1.128 t NaOH per t Cl₂, while operating practice holds 1.24–1.30 t NaOH per t Cl₂ to leave 0.5–1.0% w/w free alkalinity in the finished liquor and to buffer decomposition during storage. The production process uses a titanium or nickel-alloy heat exchanger, an in-line oxidation-reduction potential probe for chlorine feed trim, a recirculation loop providing 10–15 turnovers per hour, and final filtration through 50–100 µm cartridge filters to remove metal precipitates. Finished bleach specifications are determined by AWWA B300-19 for hypochlorite storage and ASTM D2022-89(2014) for available chlorine and excess caustic; the output is held at 12–15% available chlorine with total iron <0.5 mg/L to reduce catalytic decomposition. Terminal products are sodium hypochlorite solution for drinking water disinfection, industrial bleaching in textile finishing, and food-contact surface sanitizers after dilution; pH excursions below 10.5 during chlorine absorption accelerate chlorate formation and must be prevented by interlocking the chlorine solenoid valve with the caustic recirculation pump and pH transmitter.
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Chemplast Caustic Soda Flakes is an anhydrous sodium hydroxide product supplied as white deliquescent flakes. The substance is identified by CAS 1310-73-2 and UN 1823, Class 8, Packing Group II. The flake form is produced from membrane-cell chlor-alkali liquor that has been concentrated to 50% NaOH and solidified on a flaker, yielding a nominal flake thickness of 0.8–1.5 mm and a bulk density of 0.9–1.1 kg/L. The solid form removes the water burden of 50% liquid caustic lye and allows gravimetric dosing in alumina refining, kraft pulping, viscose mercerising, soap saponification, industrial water treatment, and neutralisation operations. In comparison with prilled caustic soda, flakes expose a lower geometric surface area per unit mass, which reduces the rate of atmospheric carbonation and moisture uptake under identical storage conditions. In comparison with diaphragm-cell solid caustic, membrane-cell flakes contain a narrower chloride and chlorate fingerprint, a distinction that becomes process-critical in cellulosic and surfactant operations.
The representative specification for industrial-grade membrane-cell flake is batch-confirmed by certificate of analysis. Low-chlorate mercerising grade and low-carbonate soap grade are selected when the following impurity ceilings are reduced further: NaCl below 0.05% by mass, NaClO3 below 10 mg/kg, and Na2CO3 below 0.5% by mass. Published test certificates for specific Chemplast grade codes are limited in public literature; users should verify batch-specific certificate of analysis before setting control limits. Packing is typically in 25 kg or 50 kg laminated HDPE bags or 1000 kg FIBCs with inner polyethylene liners.
| Parameter | Representative limit | Test method |
|---|---|---|
| NaOH | ≥ 99.0% by mass | IS 252:2013 / ISO 979:2023 |
| Na2CO3 | ≤ 0.8% by mass | IS 252:2013 / ASTM E291-18 |
| NaCl | ≤ 0.1% by mass | IS 252:2013 / ASTM E291-18 |
| Fe2O3 | ≤ 50 mg/kg | IS 252:2013 |
| SiO2 | ≤ 50 mg/kg | IS 252:2013 |
| NaClO3 | ≤ 20 mg/kg for low-chlorate grade | Ion chromatography |
| Na2SO4 | ≤ 200 mg/kg | IS 252:2013 |
The dissolution heat of NaOH in water is -44.5 kJ/mol; preparation of a 25 wt% solution in an un-jacketed tank at ambient 30°C can exceed 70°C. Therefore, make-up tanks are vented and staged water addition is used to keep the adiabatic temperature rise below the softening point of PVC pipe and diaphragm pump components. Batch-to-batch variance in Na2CO3 is typically within ±0.05 wt% for membrane-cell flakes produced from a dedicated cell room. Flaker roll temperature and cooling water inlet temperature are the main controls for flake thickness; variations above 1.5 mm reduce dissolution rate and can raise the torque load in make-up agitators by 15–20% under constant speed.
In viscose staple mercerising, cellulose is treated with NaOH in the range 18–25 wt% at 15–25°C. Residual chloride contributes to iron pickup from steel troughs, and residual chlorate can oxidise cellulose during alkaline degradation, lowering the degree of polymerisation. Membrane-cell flakes typically supplied as a low-chlorate grade carry NaCl below 0.05% by mass and NaClO3 below 10 mg/kg. Diaphragm-cell material may contain chlorate up to 0.2 wt% as NaClO3, making it unsuitable for viscose mercerising unless catalytic destruction is installed. In caustic recovery by evaporation and re-causticising, carbonate absorbed from atmospheric CO2 is controlled below 0.8 wt% Na2CO3; higher carbonate reduces wetting and produces uneven swelling. Process engineers monitor caustic strength after a plate-and-frame filter with a twin-oscillator density meter because iron oxide above 50 mg/kg deposits on filter cloth and reduces filtration rate.
In alumina refining, make-up caustic from flake is first dissolved in a dedicated dilution tank with an external heat exchanger and recirculation pump. Direct flake addition to hot spent liquor is avoided because localised adiabatic solution exotherms can exceed 90°C and induce aluminium hydroxide precipitation on the addition nozzle. Digestion of monohydrate bauxite at 240–260°C in multi-compartment autoclaves maintains free NaOH at 180–250 g/L Na2O and molar Na2O/Al2O3 at 0.62–0.72; flake make-up reduces water dilution compared with 50% lye and lowers evaporation load. Published data for specific bauxite deposits vary; bench-scale digestion tests on site-specific bauxite characterisation are required to set the exact charge. Carbonate in feed caustic consumes lime in causticisation; the stoichiometric requirement is 1 mol CaO per 1 mol Na2CO3, so an increase of 1 g/L Na2CO3 in liquor requires approximately 0.53 g/L CaO and increases solid waste in red mud.
In high-titre soap production, saponification of fatty acids or triglycerides with NaOH flakes is run as a continuous process in a high-shear reactor loop with a recycle ratio of 3:1 to 5:1 and temperature held at 80–95°C. Flake addition eliminates the water load of 50% lye and shortens the subsequent drying load, but the heat of neutralisation can exceed the cooling capacity of a shell-and-tube exchanger if the flakes are dosed too rapidly. The critical operational boundary is solids dissolution rate: in a baffled 5 m³ stainless steel crutcher with a pitched-blade turbine at 1.5 m/s tip speed, a 25 kg bag of flakes added over 10 min raises local caustic concentration near the impeller to above 35 wt% before bulk mixing, producing soap curd phase separation. The recommended arrangement is a rotary valve feeding a dedicated dissolving vessel, followed by transfer of 20–25 wt% NaOH solution to the saponification loop. Free alkali in the soap mass is maintained at 0.05–0.15 wt% as NaOH. In fatty acid routes, Na2CO3 is controlled below 0.5 wt% because higher carbonate generates sodium carbonate seeding and raises neat soap viscosity, increasing the power draw of the finishing screw.
Membrane-cell caustic soda differs from diaphragm-cell material in sodium chloride and sodium chlorate distribution because the perfluorosulfonic acid membrane rejects chloride ions at the cathode. Chlorate formed in the anolyte loop is not volatilised during evaporation and concentrates in the solid product. Low-chlorate grades require chlorate destruction in the cell room before evaporation. The following table compares the general envelope of flake, 50% membrane-cell lye, and prills for industrial dosing.
| Parameter | Flake | 50% membrane-cell lye | Prill |
|---|---|---|---|
| NaOH mass fraction | ≥ 99.0% | 50.0% ± 0.5% | ≥ 99.0% |
| Water content | ≤ 1.0% | approx. 50% | ≤ 1.0% |
| NaCl | ≤ 0.1% | ≤ 50 mg/kg | ≤ 0.05% for low-salt grade |
| Bulk density | 0.9–1.1 kg/L | 1.53 kg/L | 1.1–1.3 kg/L |
| Dosing method | gravimetric screw, bag dump | metering pump | gravimetric screw |
| Dusting/caking tendency | moderate caking if exposed to moisture | none, but storage tank heating required | lower dusting, higher caking under humid air |
Unlike 50% membrane-cell lye, flake product does not require a crystallisation protection temperature above 12°C; however, it is not suitable for direct injection into a pressurised reactor without a dissolving vessel. Prilled caustic soda has lower dust when bag is emptied, but flakes exhibit less rolling movement and are often preferred where floor-contact caking must be minimised. In a continuous neutralisation process, a gravimetric screw feeder with a loss-in-weight controller and flexible hopper liner is used because caked material may bridge in the feed hopper.
Sodium methylate production from flakes and methanol is run in a stirred reactor or packed column with methanol reflux at 64°C. The reaction is equilibrium-limited and requires removal of water below 0.3 wt% in the feed flakes. Flake Na2CO3 is limited below 0.3 wt% because carbonate precipitates as sodium carbonate and blocks the methanol recovery column reboiler. In a continuous 10 m³ mild steel reactor with external circulation and an entrainer-based water removal loop, 99% NaOH flakes are fed by a rotary star valve into methanol; the sodium methylate product is maintained at 25–30 wt% and used as a catalyst in biodiesel transesterification. Water above 0.5 wt% in the final methylate reduces catalyst activity and increases soap formation in the downstream unit.
In boiler feedwater and industrial wastewater pH adjustment, flake caustic is preferred where liquid lye storage is not permitted due to secondary containment requirements. A 0.1 mol/L NaOH solution prepared from flakes and metered into a raw water line with static mixer raises pH from 6.5 to 8.5 at typical alkalinity 80 mg/L as CaCO3; the main process limit is the exotherm in the day tank. The dissolution tank is vented, and the solution is cooled below 40°C before metering to avoid damage to polyvinyl chloride piping and diaphragm pump components. In anaerobic digestion, caustic addition is controlled by a pH probe with a deadband of ±0.2 pH; flake-based caustic with Na2CO3 above 0.8 wt% can read as buffered alkalinity and delay digestion pH response.
In kraft pulping white liquor preparation, caustic soda flakes are used as make-up alkali only when oxidised white liquor requires sodium hydroxide addition to maintain effective alkali at 75–85 g/L as Na2O and sulfidity at 25–30%. Direct addition to the lime slaker is avoided because localised exotherm can flash steam and create caustic aerosol; the flakes are first dissolved to 30 wt% NaOH and then metered to white liquor storage. The flakes reduce sodium chloride input compared with diaphragm-cell caustic, which is important for chloride build-up in the recovery boiler. A mill evaporator train processing black liquor with chloride above 20 g/L in remaining liquor experiences increased sootblower nozzle corrosion and reduced electrostatic precipitator availability; membrane-cell flake make-up with NaCl below 0.1 wt% is specified to limit chloride accumulation.
Caustic soda flakes absorb water vapour and carbon dioxide simultaneously. Prolonged exposure at 60% RH and 30°C produces sodium carbonate crust and caking that reduces flowability. Warehouses with unheated concrete floors are problematic because moisture migrates through packaging and condenses on the bottom layer. Storage practice requires sealed HDPE bags with inner polyethylene liner, stacking no more than 3–4 pallets high, and a controlled environment below 35% RH. If caking has occurred, the material is not re-dried with direct steam because localised NaOH hydrate formation can elevate temperature above 100°C. The product should not be combined with acid dosing lines, ammonium salts, or reactive metals such as aluminium and zinc in the same bund, because exothermic neutralisation and hydrogen evolution create pressure hazards. Incompatibilities with chlorinated solvents and strong oxidisers are operational exclusions under EC 1272/2008 classification.
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