| HS Code | 233529 |
| Chemical Name | Sodium Hydroxide |
| Chemical Formula | NaOH |
| Cas Number | 1310-73-2 |
| Molecular Weight | 40.00 g/mol |
| Appearance | White flakes, prills, or pellets |
| Purity | 98% to 99% by weight |
| Melting Point | 318 °C (604 °F) |
| Boiling Point | 1388 °C (2530 °F) |
| Density | 2.13 g/cm³ at 25 °C |
| Solubility In Water | 1110 g/L at 20 °C |
| Ph 1 Solution | 13 |
| Grade | Commercial / Industrial Grade |
As an accredited DCW Caustic Soda factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | DCW Caustic Soda is supplied in 50 kg HDPE bags, ensuring safe handling, moisture protection, and secure storage. |
| Container Loading (20′ FCL) | 20′ FCL: Load DCW caustic soda in sealed, moisture-proof drums/bags, secure tightly, label hazards, and ensure ventilation. |
| Shipping | DCW Caustic Soda is shipped as a corrosive alkaline solution, typically in tank containers, ISO tanks, or drums. Transport requires proper UN 1824 labeling, corrosion-resistant equipment, and segregation from acids. Handling crews must use PPE, and spill protection protocols must be in place to ensure safe maritime, road, or rail transit. |
| Storage | DCW Caustic Soda (sodium hydroxide) should be stored in clearly labelled, corrosion-resistant containers, preferably stainless steel or suitable HDPE, in a cool, dry, well-ventilated area. Keep containers tightly sealed to prevent moisture absorption and contamination. Store away from acids, oxidizers, and reactive metals. Use secondary containment and ensure spill controls are available. |
| Shelf Life | Shelf life is approximately 24 months when stored sealed, cool, and dry, protected from moisture and carbon dioxide. |
In continuous Bayer circuits refining gibbsitic and boehmitic bauxite, a maintained free-caustic concentration of 180–250 g/L Na₂O at digestion temperature 140–265°C governs boehmite dissolution kinetics and aluminate liquor stability. Refinery-grade 50 wt% membrane or diaphragm cell caustic soda, as supplied under DCW material specification, is assayed for total alkalinity by ASTM E291-18, with chloride in digestion feed held below 0.020 g/L to limit closed-loop salt accumulation; bulk liquid transfer conforms to ADR UN 1824, Class 8, Packing Group II. Typical Bayer circuit consumption is 1.6–2.5 t NaOH per tonne of smelter-grade alumina, with the upper bound applying to diasporic bauxite or reactive silica above 5–8 wt%; spent liquor evaporation restores 150–220 g/L total caustic after mud washing. Bauxite slurry wet-ground in rod mills with recycled spent liquor is predesilicated at 95–105°C for 4–8 h, digested in tubular heaters at 240–265°C for boehmitic bauxite or 140–150°C for gibbsitic bauxite, sent to multi-stage counter-current decantation washers for red mud separation, seeded in air-agitated crystallizers, classified and calcined at 1000–1100°C in rotary kilns or gas suspension calciners. Terminal output is smelter-grade alumina at ≥98.3 wt% Al₂O₃ with residual 0.3–0.5 wt% Na₂O and bauxite residue directed to dry stacking; sodium aluminium silicate scale accumulation accelerates above 6 wt% reactive silica, driving non-stoichiometric caustic loss.
White-liquor sulfidity is maintained at 25–35% on active alkali while sodium hydroxide make-up compensates sodium loss from green liquor causticizing, bleach plant washing, and electrostatic precipitator dust removal. Compliance is anchored to EU IED 2010/75/EU BAT-AELs for COD and AOX, with closed-loop sodium/sulfur recovery above 98%; pulp bleachability and 1% alkali solubility are monitored to TAPPI T 212 cm-18. Effective alkali charge for softwood chips is 16–24% Na₂O on oven-dry wood, and extraction-stage caustic charge in elemental chlorine-free sequences is 2.0–4.0 wt% NaOH on oven-dry pulp after oxygen delignification. Wood chips are steamed and compressed through a continuous digester at 150–170°C and 7–10 bar, blown through pressure diffuser washers, reacted in two-stage oxygen delignification at 80–100°C and 4–7 bar, then bleached with ClO₂ and alkaline extraction towers at 60–75°C. Terminal product types are fully bleached softwood or hardwood kraft market pulp with ISO brightness 88–92%, integrated containerboard, and sack kraft grades. Excess sodium hydroxide beyond the recovery balance increases black liquor dead load, raises evaporator steam demand, and reduces throughput in the recovery boiler.
When cotton knit rolls are mercerized under tension to raise dye uptake and dimensional stability, the fabric is run through a 28–32°Bé caustic bath, corresponding to 20–24 wt% NaOH at 15–20°C, with wet pickup of 80–120%. Caustic recovery evaporators and wastewater discharge are operated within ZDHC MRSL Version 3.1 parameters, and neutralized fabric is verified to ISO 3071:2020 aqueous extract pH 5.5–7.5 before finishing. Batch scouring of greige cotton uses 3–6 g/L NaOH on fabric weight, 1–2 g/L nonionic wetting agent, and 0.5–1 g/L sequestrant at 95–98°C for 45–60 min in jet or overflow machines. The mercerizing line includes singeing, desizing, chainless or chain mercerization with width control, countercurrent wash recovery, lye evaporation to 30–45 wt% for reuse, acid neutralization, and final stentering at 130–150°C. Terminal product types are mercerized cotton knits, woven shirting, and high-yarn-dyed fabric with reduced shrinkage and lower dyestuff demand per kilogram; below 18 wt% NaOH, swelling collapses, while above 25 wt% and dwell times over 60 s, handle becomes brittle post-stenter.
Continuous saponification of refined tallow and coconut oil blends requires sodium hydroxide charge calculated from saponification value rather than a fixed weight proportion. Finished neat soap total alkali is analysed per ISO 685:2020, with free caustic alkali held below 0.1 wt% Na₂O for low-moisture toilet soap and 0.3 wt% for industrial laundry bars; EU Detergents Regulation (EC) No 648/2004 governs ingredient labelling where the finished article is placed on the market as a detergent. Actual sodium hydroxide dosing is set at 95–97% of the theoretical values tabulated below:
| Oil or fat feedstock | Saponification value | Theoretical NaOH charge |
|---|---|---|
| Coconut oil | 250–264 mg KOH/g | 0.18–0.19 kg NaOH/kg oil |
| Refined tallow | 192–202 mg KOH/g | 0.14–0.15 kg NaOH/kg fat |
| Palm kernel oil | 230–254 mg KOH/g | 0.16–0.18 kg NaOH/kg oil |
Fat/oil blends are heated to 70–80°C in plate heat exchangers, contacted with 30–32 wt% NaOH solution in high-shear homogenizers, saponified in a continuous reactor with neat soap recirculation, dried under vacuum evaporators to 65–75% total fatty matter, and extruded through a plodder. Terminal products include soap noodles, toilet bars, and laundry bars with 12–18% moisture and 0.5–1.0% sodium chloride content. Excess caustic above 0.2 wt% Na₂O increases irritation potential and rancidity risk, while undercharge leaves unreacted glycerides that soften bar structure.
Potable water post-filtration pH correction uses 25 wt% or 50 wt% sodium hydroxide diluted to 1–5 wt% and metered into finished water at 0.5–3.0 mg/L NaOH to maintain pH 7.6–8.0 in low-alkalinity surface water; site-specific dose is established by jar testing and continuous Langelier Saturation Index calculation. Treatment chemical compliance requires conformity to EN 896:2012 and AWWA B501-19, with health-effects certification under NSF/ANSI/CAN 60; chemical transfer piping is welded to ASME B31.3. The dosing process comprises day tank dilution, positive-displacement metering, static mixer injection, continuous pH analyser feedback, and finished water clearwell buffering. Terminal product is treated municipal potable water or food-plant process water within WHO pH 6.5–8.5 operational guidance.
Where degummed soybean and rapeseed oils report acid value below 2.0 mg KOH/g, sodium methoxide derived from caustic soda and methanol is metered at 0.4–0.6 wt% NaOCH₃ on oil, equivalent to 0.30–0.45 wt% NaOH. Finished B100 is tested against ASTM D6751-23 and EN 14214:2012+A2:2019, with sodium plus potassium below 5 mg/kg and total sulphur below 10 mg/kg. The process includes caustic-methanol mixing at 60–70°C to generate 25–30 wt% sodium methoxide solution, transesterification in a sheared reactor at 55–65°C, gravity separation of crude glycerine, methyl ester washing with 5–10 wt% soft water at 50–60°C, vacuum drying at 90–110°C and 20–50 mbar, then filtration. Terminal product is B100 fatty acid methyl ester for distillate fuel blending and crude glycerine at 80–88% purity. Oils with acid value above 2.0 mg KOH/g are routed through acid esterification before base transesterification to avoid excessive sodium soap formation that raises phase separation viscosity and centrifuge emulsion loads.
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DCW Caustic Soda is a membrane-cell sodium hydroxide product supplied as aqueous lye and anhydrous solid. The product is identified by CAS 1310-73-2 and EC 215-185-5, with transport classifications UN 1824 for solution and UN 1823 for solid. Commercial forms include DCW Caustic Soda Lye 48% w/w, DCW Caustic Soda Lye 50% w/w, and DCW Caustic Soda Flakes with minimum NaOH assay of 99.5% w/w. The material is manufactured by ion-exchange membrane electrolysis of purified brine and is tested under IS 252:2013, ASTM E291, and ISO 979. End-use scenarios include Bayer alumina digestion, kraft pulp alkali makeup, textile mercerisation, soap saponification, sodium hypochlorite generation, boiler water pH adjustment, and acid-gas scrubbing.
Compared with diaphragm-cell caustic soda, the membrane route lowers chloride, chlorate, and transition-metal concentrations. Typical 48% w/w membrane-cell lye contains sodium chloride below 80 mg/kg; diaphragm-cell material may carry 7,000–12,000 mg/kg sodium chloride, and mercury-cell product, while lower in chloride, introduces mercury stewardship in waste streams. The DCW membrane product is specified with mercury not intentionally added and chlorate controlled to a maximum of 10 mg/kg in lye. Low chlorate content matters in hypochlorite synthesis and chlorine dioxide bleaching, where chlorate acts as a decomposition product and chlorine scavenger.
Representative acceptance limits are listed in Table 1. Values are for standard industrial grades and are verified by lot certificate of analysis; method detection limits vary with plant laboratory instrumentation.
| Parameter | DCW Lye 48% w/w | DCW Flakes 99.5% w/w | Test method |
|---|---|---|---|
| NaOH | 47.5–48.5% w/w | minimum 99.5% w/w | IS 252:2013, ASTM E291, ISO 979 |
| Na₂CO₃ | maximum 0.2% w/w | maximum 0.4% w/w | IS 252:2013 |
| NaCl | maximum 80 mg/kg | maximum 0.15% w/w | IS 252:2013 chloride |
| Iron (Fe) | maximum 5 mg/kg | maximum 20 mg/kg | ICP-OES |
| Nickel (Ni) | maximum 5 mg/kg | maximum 10 mg/kg | ICP-MS |
| Sodium chlorate | maximum 10 mg/kg | maximum 20 mg/kg | IS 252:2013 iodometric |
Solid flakes are hygroscopic and react with atmospheric carbon dioxide; open packages exposed to relative humidity above 60% form surface sodium carbonate. The flakes are handled in systems with dry-air purge at a dew point below -10 °C. Lye is delivered in tankers or carboys; the 48% w/w grade reduces freeze risk relative to 50% product. For food-grade or pharmaceutical auxiliary applications, the material is checked against Food Chemicals Codex and 21 CFR 184.1763 identity criteria; users must confirm lot-specific COA when trace metals affect final product colour.
In kraft pulp mills, sodium hydroxide is a makeup chemical for white liquor and oxidation-stage alkali. Chloride entering with purchased caustic accumulates in the recovery cycle because the main purge routes are electrostatic precipitator ash, recausticizing grit, and sodium sulfate disposal; in a closed-cycle mill the steady-state chloride can reach several grams per litre. At high chloride, digester and evaporator alloys face pitting and stress corrosion cracking, particularly above 150 °C in high-pressure steam service. Chlorate entering with caustic can increase chlorine dioxide demand in bleaching because it is an ineffective electrophilic substitution agent and contributes to dissolved organic halogen in final effluent. Purchasing membrane-grade caustic with residual chlorate at 10 mg/kg or lower is therefore a mass-balance control measure; the usual plant calculation treats the NaOH contribution as a chloride load in kilograms per tonne of air-dried pulp, with a maximum typically set by mill-specific corrosion audits rather than a single universal limit.
In Bayer alumina digestion, sodium hydroxide is the active leachant for gibbsitic, boehmitic, and diasporic bauxite. Digestion liquor is maintained at 150–250 g/L Na₂O, depending on feedstock mineralogy and autoclave temperature. Chloride from caustic makeup remains in the liquor and can accelerate corrosion of flash vessel inlet nozzles and plate heat exchangers. Membrane-grade caustic with sodium chloride below 80 mg/kg lowers the chloride accumulation term relative to diaphragm-grade material, particularly when bauxite itself introduces chloride and plant purge streams are constrained by red mud alkalinity limits. The product is also used in oxalate removal and precipitation stages, where low transition-metal content reduces iron hydroxide co-precipitation that can discolour aluminium hydroxide product.
Cotton and viscose cellulosic substrates are mercerised in sodium hydroxide solutions of 20–30% w/w NaOH, commonly held at 15–25 °C with a wetting agent to improve fabric penetration. Iron above 10–20 mg/kg can deposit on the fibre and form visible iron-hydroxide stains after subsequent oxidative bleach; nickel and copper act as Fenton-type decomposition catalysts for hydrogen peroxide in downstream finishing. DCW membrane-grade lye is specified with iron at or below 5 mg/kg in 48% w/w product, which supports optical whiteness requirements when the fabric is evaluated under ASTM E313. The lye is diluted with softened water to a target density of 1.22–1.33 g/cm³ at 20 °C, and inline Coriolis density meters are used to control the mercerising bath within ±0.5% of setpoint.
In continuous sodium hypochlorite manufacture, chlorine is absorbed into a circulating caustic stream at pH above 11.5 and residual excess NaOH of 5–10 g/L. The reaction requires 2 mol NaOH per 1 mol Cl₂ for stoichiometric conversion; excess alkali is held to avoid bleach decomposition and chlorate formation in the storage tank. Because the DCW membrane product starts with chlorate below 10 mg/kg, the final hypochlorite product can meet the chlorate limits of AWWA B300 and EN 901 when stored under controlled temperature below 20 °C.
Saponification of tallow and palm stearin with caustic soda proceeds at 80–100 °C, followed by sodium chloride addition to separate neat soap and spent lye. Although chloride in caustic soda is not a process disadvantage because salt is added intentionally, the low iron and nickel levels in DCW membrane product, specified at maximum 5 mg/kg Fe and 5 mg/kg Ni in 48% w/w lye, reduce the concentration of transition metals that catalyse oxidative rancidity during subsequent drying of soap noodles.
In demineralisation plants, caustic soda is injected as a 5–10% w/w solution downstream of cation exchange or reverse osmosis to raise pH without forming calcium carbonate sludge. The membrane product contains iron at or below 5 mg/kg, reducing precipitation on transfer pipes and protecting downstream membrane elements from metal-catalysed oxidative degradation. Published data for this specific configuration is limited, but plant evaluations generally monitor pH setpoint stability and differential pressure across the final filtration stage.
Table 2 consolidates the operating boundaries that drive impurity selection in each end use.
| Application | Typical operating condition | Critical impurity and boundary | Reference method/equipment |
|---|---|---|---|
| Bayer digestion | 150–250 g/L Na₂O, 145–265 °C | Chloride in NaOH <80 mg/kg | Digester flash vessel, plate heat exchanger |
| Kraft pulping | White liquor effective alkali 18–22% Na₂O | Chloride and chlorate control | Recovery boiler, falling film evaporator |
| Mercerising | NaOH 20–30% w/w at 15–25 °C | Fe <5 mg/kg, Ni <5 mg/kg | ASTM E313 |
| Hypochlorite production | Excess NaOH 5–10 g/L, pH >11.5 | Chlorate <10 mg/kg | AWWA B300, EN 901 |
| Demineralisation neutralisation | NaOH dilution 5–10% w/w | Fe <5 mg/kg | Reverse osmosis differential pressure |
Boiler water treatment represents another pH-control application in which low chloride content is critical. Caustic soda is dosed to maintain feedwater pH between 8.8 and 9.2 and to convert residual hardness to a non-scaling sludge. In high-pressure steam generators operated above 60 bar, free hydroxide alkalinity is typically held between 10 mg/L and 50 mg/L as CaCO₃; excess caustic can accelerate caustic gouging at tube-metal temperatures above 300 °C. The membrane product contributes sodium chloride below 80 mg/kg in 48% w/w lye, reducing the total corrosive ion inventory in the condensate return network.
In packed-tower acid-gas scrubbing, sodium hydroxide is diluted to 10–20% w/w before injection to avoid excessive heat of dilution. The low iron content below 5 mg/kg reduces the formation of metal-catalysed oxidation products that can foul packing; liquid distribution headers are typically stainless steel or polypropylene, and the pH in the quench sump is held between 7.5 and 8.5 when sodium bicarbonate buffering is used.
Aqueous 50% w/w NaOH freezes near 12 °C; 48% w/w lye remains liquid to a lower onset temperature, but storage tanks and outdoor transfer headers should still be heat-traced where ambient can fall below 5 °C. The viscosity of 50% w/w NaOH at 20 °C is approximately 78 mPa·s; the 48% w/w product is lower and is preferred for ambient transfer. Carbon steel tanks are used with stress-relief after welding; seal and gasket materials are EPDM or PTFE. Contact with aluminum, zinc, tin, brass, or galvanised steel generates hydrogen and exothermic heat, and such metals are prohibited in transfer systems. For solid flakes, storage requires dry-air purge because the surface reacts with atmospheric carbon dioxide to form sodium carbonate at relative humidity above 60%.
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