| HS Code | 277811 |
| Product Name | Solvay Caustic Soda |
| Chemical Formula | NaOH |
| Cas Number | 1310-73-2 |
| Appearance | White solid, available as flakes, pellets, granules, or aqueous solution |
| Odor | Odorless |
| Solubility In Water | 1110 g/L at 20°C |
| Vapor Pressure | Negligible at room temperature |
| Typical Purity Percent | 99+ |
As an accredited Solvay Caustic Soda factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Solvay Caustic Soda is supplied in sturdy, double-lined 25 kg bags, ensuring safe handling, moisture protection, and secure industrial storage. |
| Container Loading (20′ FCL) | 20′ FCL of Solvay caustic soda: properly palletized, secured drums/bags loaded evenly, labeled, and container sealed for safe transport. |
| Shipping | Solvay Caustic Soda ships as a corrosive alkaline substance, typically as solid flakes, prills, or aqueous solution. Use UN1823 or UN1824, in approved bags, drums, or isotanks. Ensure proper labeling, segregation from acids, and spill containment. Transport via truck, rail, or sea with hazmat documentation and PPE. |
| Storage | Store Solvay Caustic Soda in a cool, dry, well-ventilated area away from moisture and incompatible substances like acids or aluminum. Keep containers tightly sealed, clearly labeled, and made of corrosion-resistant materials. Avoid exposure to humidity, as contact with water generates heat. Ensure secondary containment and proper safety equipment for handling. |
| Shelf Life | Solvay Caustic Soda has indefinite shelf life when stored in sealed containers, protected from moisture, humidity, and carbon dioxide. |
Bauxite digestion in the Bayer circuit depends on selective dissolution of gibbsite and boehmite with concentrated sodium hydroxide at elevated temperature and pressure. Solvay caustic soda is supplied as 50% membrane-cell liquid or as solid micropearls for redilution to 140–250 g/L Na₂O as NaOH equivalent; the liquid grade is preferred in logistics-intensive refineries because it avoids exothermic dissolution. Milled bauxite slurry is contacted with spent liquor in continuous tube digesters or agitated autoclaves at a solid-to-liquor ratio of 1:3 to 1:6. Gibbsite digestion is operated at 105–150°C and 0.3–1.5 MPa, while boehmitic ores require 200–250°C and 3.0–5.5 MPa pressurised digestion. The resulting pregnant liquor leaves digestion at an alumina-to-caustic ratio of 0.60–0.75 for gibbsitic ores; below this ratio the risk of premature gibbsite precipitation in flash tanks increases, and above it the liquor becomes unstable. Reactive silica in bauxite consumes caustic through sodalite formation; ores containing more than 5–7% reactive SiO₂ usually require pre-desilication at 90–100°C for 4–8 h to prevent severe scaling in digestion heaters. Digestion pressure vessels and slurry piping are maintained to ASME B31.3 and inspected under API 510. Clarified pregnant liquor is cooled and seeded with fine aluminium hydroxide in a series of continuous crystallisers, where the alumina-to-caustic ratio is reduced to 0.25–0.35 and gibbsite particles grow to 80–100 µm median diameter for smelter-grade alumina. Terminal product is calcined smelter-grade alumina meeting ISO 23201:2015 trace-element specifications and a loss-on-ignition below 0.8%.
Irrespective of whether a mill uses purchased Solvay caustic soda for alkali make-up or produces sodium hydroxide on site through recausticizing, the cooking liquor must be controlled for effective alkali, sulfidity, and carbonate. White liquor is prepared by slaking lime and causticizing green liquor; causticizing efficiency is normally 80–85%. In continuous Kamyr or Compact Cooking digesters, effective alkali as Na₂O on oven-dry wood is 16–20% for softwood and 12–16% for hardwood, with sulfidity 28–35% and 20–28% respectively. Digester temperature is maintained at 165–175°C, and the H-factor is 1200–1700 for softwood and 800–1200 for hardwood. Below 15% sulfidity the delignification rate drops and screen-room rejects increase; above 40% sulfidity the volatile sulfur load to the recovery boiler and TRS incinerators becomes difficult to control. White liquor NaOH concentration is typically 90–110 g/L Na₂O for softwood and 80–100 g/L Na₂O for hardwood, while carbonate should remain below 20–25 g/L Na₂CO₃ to avoid calcium carbonate scaling in the digester heating surfaces. Purchased sodium hydroxide make-up rates of 5–25 kg NaOH per air-dried tonne of pulp are common depending on sodium loss in dregs, knotter rejects, and bleach plant washing. The cooked pulp is blown to diffusion washing and screening; terminal unbleached pulp kappa number is 25–35 for softwood and 15–20 for hardwood measured by ISO 302:2015. White liquor composition is verified by TAPPI T 624 cm-21 on every batch or shift.
| Parameter | Softwood target | Hardwood target | Analytical method |
|---|---|---|---|
| Effective alkali as Na₂O on OD wood | 16–20% | 12–16% | TAPPI T 624 cm-21 |
| Sulfidity | 28–35% | 20–28% | TAPPI T 624 cm-21 |
| White liquor NaOH as Na₂O | 90–110 g/L | 80–100 g/L | TAPPI T 624 cm-21 |
| Carbonate as Na₂CO₃ | <25 g/L | <20 g/L | TAPPI T 624 cm-21 |
| Digester temperature | 165–175°C | 160–170°C | Continuous digester RTD |
| H-factor | 1200–1700 | 800–1200 | ISO 302:2015 |
In continuous fatty acid saponification for toilet-soap production, the sodium hydroxide dose is calculated from the oil blend saponification value rather than applied as a fixed percentage. For palm kernel olein with a saponification value of 230–250 mg KOH/g, the dry NaOH charge is 164–179 kg/t oil; for coconut oil with 248–265 mg KOH/g, the charge is 177–189 kg/t oil. Solvay caustic soda is diluted to 25–30% before injection into a high-shear loop reactor operating at 80–100°C with a recirculation ratio of 3:1 to 5:1. The reaction is stopped when a clear taffy trace forms in hot water; free NaOH in the finished superfatted soap base is then back-titrated according to ISO 684:2004 and must remain below 0.05% as NaOH for toilet bars. Industrial soap intended for flake or powder production may carry up to 0.1% free NaOH if subsequent neutralisation is programmed. Terminal products are vacuum-dried soap noodles, transparent toilet bars, or liquid soap bases. Fatty acid stocks with high unsaturation require nitrogen blanketing at 90–100°C to avoid oxidative darkening; sodium chloride from brine-contaminated caustic raises soap viscosity and slows phase separation. Sampling and chemical analysis follow ASTM D460-91(2014) for soap and soap products.
For municipal drinking water, 50% membrane-grade NaOH conforming to EN 896:2012, purchased to AWWA B501-19, and certified to NSF/ANSI/CAN 60 is metered into finished-water piping at 2–20 mg/L as neat product to shift the Langelier Saturation Index from negative to 0.0–0.5, with static mixers sized for 1.5–2.5 m/s pipe velocity and a post-injection contact time of 30–60 s before the first lead-service-line sample point; the end product is potable water meeting the Lead and Copper Rule action level for lead at 0.010 mg/L.
The production of sodium hypochlorite from chlorine and caustic soda is governed by the exothermic reaction Cl₂ + 2 NaOH → NaOCl + NaCl + H₂O. Solvay caustic soda at 32% or 50% is diluted to 15–25% NaOH before entering a packed absorption tower or vacuum eductor. To prevent product decomposition, the final bleach solution is maintained with 0.2–1.0 wt% free NaOH, corresponding to a pH of 11.5–12.5. The absorber liquor temperature is held at 25–30°C by a titanium or Hastelloy C276 plate exchanger; above 30°C chlorate formation increases and available chlorine decreases. Chlorine flow is controlled by oxidation-reduction potential at 700–800 mV and pH 11.5–12.5 to avoid free chlorine breakthrough. The final product contains 12–15% available chlorine and is stored in lined steel or fiberglass-reinforced plastic tanks with vented domes. Compliance with AWWA B300-18 and NSF/ANSI/CAN 60 is required for drinking-water-grade hypochlorite; disinfectant registration under US EPA FIFRA requires label-specific strength and contaminant limits. Metallic impurities such as nickel and copper at trace levels catalyse decomposition, so rubber-lined carbon steel, PVDF, or fibre-reinforced plastic are specified for piping. This process is incompatible with acid washdowns and ammonia-containing compounds because chlorates and chloramines are formed.
Under tension, cotton cellulose fibre undergoes partial crystalline lattice rearrangement when treated with sodium hydroxide within a defined concentration and temperature window. Mercerization is performed with 18–24 wt% NaOH at 15–25°C for maximum luster and tensile strength; hot mercerization with 28–30 wt% NaOH at 35–40°C is used when low-temperature cooling is not available. The fabric passes through a chainless mercerizing range with a dwell time of 30–60 s and tension equivalent to 3–5% stretch. Excess caustic is removed by vacuum slot extraction and countercurrent washing; the recovered weak lye at 5–7% NaOH is evaporated back to 30% or 50% for reuse, giving a modern range 85–95% recovery efficiency. Terminal products are mercerized cotton yarn and fabric with higher dye uptake, lower fibre rigidity, and increased equilibrium moisture regain. Residual pH of the finished fabric is measured by ISO 3071:2020 and should be 5.5–7.0; mercerization certification is commonly evaluated by AATCC 89-2019 barium activity number.
Chemical refining of crude soybean, palm, and canola oils uses dilute sodium hydroxide to convert free fatty acids into oil-insoluble soapstock. The theoretical NaOH dose for a crude oil with acid value expressed as oleic acid is 1.416 kg NaOH per tonne of oil for each 1.0% FFA; an additional excess of 0.02–0.05% NaOH on oil mass is added to complete neutralisation. The caustic solution is prepared at 12–20% NaOH and contacted with degummed oil at 70–85°C in a high-shear inline mixer, followed by 15–30 min retention in a stirred vessel and separation in a self-cleaning disc centrifuge. The neutralized oil is water-washed and vacuum-dried; residual soap after washing should be below 50 mg/kg. Terminal product is neutralized oil ready for bleaching and deodorization to RBD oil; the separated soapstock is acidulated with 1.0–1.2 mol H₂SO₄ per mole of sodium soap to recover acid oil. Food-grade sodium hydroxide must meet the Food Chemicals Codex monograph and FDA 21 CFR 184.1763 GRAS provisions. Crude oil acidity is determined by ISO 660:2020. Prolonged contact at temperatures above 85°C increases neutral oil loss through saponification; sodium hydroxide solution below 10% produces weak separation and excessive emulsions.
| Parameter | Typical range | Test/method |
|---|---|---|
| Theoretical NaOH dose | 1.416 kg/t per 1.0% FFA | ISO 660:2020 |
| Excess NaOH on oil mass | 0.02–0.05% | In-process acid–base titration |
| Caustic solution strength | 12–20% NaOH | ISO 979:2023 |
| Contact temperature | 70–85°C | Plate heat exchanger |
| Retention time | 15–30 min | Inline mixer and stirred vessel |
| Residual soap after water wash | <50 mg/kg | AOCS Cc 17-95 |
In dairy, brewery, and ready-to-eat food plants, 50% Solvay caustic soda is diluted to 1.0–2.0 wt% NaOH for organic soil removal in clean-in-place circuits. The wash solution is circulated at 75–85°C and 1.5–3.0 m/s pipe velocity to generate turbulent flow; spray balls are supplied at 1.5–2.5 bar differential pressure in tanks. Wetting agents and chelants are added to penetrate fat-protein deposits and to sequester hard-water calcium. The alkali stage is followed by a potable-water rinse until return-water conductivity is within 100 µS/cm of supply water, then an acid wash is applied to remove mineral scale. Terminal product is cleaned equipment surface ready for sanitization, with ATP swab results typically below 2 RLU in high-hygiene zones. Caustic concentration is monitored by conductivity; a 2.0% NaOH solution at 75°C reads approximately 60–80 mS/cm depending on soil load. Stainless steel 316L is specified over 304 for repeated high-temperature caustic exposure due to chloride and stress-corrosion risks; chemical suppliers must comply with FDA 21 CFR 178.1010 if the solution is used as a sanitizer and with plant HACCP residue monitoring.
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Solvay Caustic Soda is a commercial sodium hydroxide product family supplied as 50% w/w membrane-grade liquid, 32% w/w technical liquid, and anhydrous solid forms with NaOH mass fraction not less than 98.5%. The active alkali is NaOH, CAS 1310-73-2, molecular weight 40.00 g/mol. At 20°C the 50% w/w liquid has a specific gravity of approximately 1.53 and a crystallization temperature near 12°C, which imposes continuous heat tracing on storage tanks and transfer lines. The product is used in alumina refining, kraft pulping, chemical synthesis, water treatment, and food processing; grade selection is controlled by sodium chloride, sodium chlorate, carbonate, and iron residual levels. Unlike diaphragm-cell material, membrane-grade Solvay Caustic Soda exhibits chloride residuals below 100 mg/kg in liquid concentrates, and solid forms are specified with chloride and carbonate limits suitable for rayon, food, and pharmaceutical auxiliary use.
Under standard commercial specifications, grade designations for Solvay Caustic Soda are defined by nominal NaOH concentration, impurity ceiling, and physical form. The 50% membrane-grade liquid is typically specified with NaOH mass fraction 50.0 ± 0.5% w/w, sodium chloride below 100 mg/kg, sodium carbonate below 0.1% w/w, and iron below 2 mg/kg. The rayon-grade variant applies a lower sodium chlorate threshold because chlorate residues interfere with viscose dope oxidation and fiber tenacity. Solid micropearls and flakes are specified with NaOH mass fraction of 98.5–99.5% w/w, carbonate as Na₂CO₃ up to 0.5% w/w, chloride up to 0.03% w/w, and iron up to 10 mg/kg. These values are controlled by titration and photometric methods referenced in the compliance matrix below.
| Parameter | 50% membrane liquid | 32% technical liquid | Solid micropearls/flakes |
|---|---|---|---|
| NaOH mass fraction | 50.0 ± 0.5% w/w | 32.0 ± 0.5% w/w | 98.5–99.5% w/w |
| Sodium chloride as NaCl | ≤ 100 mg/kg | ≤ 150 mg/kg | ≤ 0.03% w/w |
| Sodium carbonate as Na₂CO₃ | ≤ 0.1% w/w | ≤ 0.2% w/w | ≤ 0.5% w/w |
| Iron as Fe | ≤ 2 mg/kg | ≤ 5 mg/kg | ≤ 10 mg/kg |
| Sodium chlorate as NaClO₃ | ≤ 5 mg/kg | ≤ 10 mg/kg | ≤ 10 mg/kg |
Because membrane electrolysis uses perfluorinated ion-exchange membranes that restrict chloride migration, the resulting 50% w/w liquid contains sodium chloride in the range of 10–100 mg/kg compared with 0.8–1.2% w/w for diaphragm-cell evaporator concentrates. Diaphragm-grade material also retains higher sodium chlorate and sulfate levels, and may require additional purification for rayon spinning or food-contact use. Mercury-cell caustic, where still available, is low in chloride but carries mercury process residuals and is increasingly restricted under the Minamata Convention. Solvay Caustic Soda membrane-grade is produced without mercury-cell conversion, so mercury is not part of the process-related impurity profile. This difference is material for ion-exchange demineralizer loading, where chloride contributes to regenerant demand, and for high-purity alumina and electronic-grade applications where metal and chlorate residues are critical. The lower chloride content reduces scaling in multiple-effect evaporators and lowers the concentration of sodium chloride in downstream sodium hypochlorite electrolysis cells.
In Bayer-process alumina refining, the product is dosed as 50% w/w sodium hydroxide to dissolve gibbsitic or boehmitic bauxite at digestion temperatures between 145°C and 260°C. The active Na₂O concentration is maintained between 150 g/L and 250 g/L depending on bauxite mineralogy; membrane-grade caustic with low chloride is preferred because chloride accumulates in closed liquor circuits and accelerates pitting corrosion in heat exchangers and flash trains. Production-scale equipment includes multi-pass shell-and-tube digesters, flash tanks, and spiral heat exchangers constructed from duplex stainless steel or nickel-based alloys. Caustic losses occur through red mud sodalite formation and liquor entrainment; sodium chloride in diaphragm-grade material raises chloride liquor load and increases the corrosivity of high-temperature circulating streams. The low carbonate content of membrane-grade product reduces sodium oxalate precipitation and scaling in precipitation vessels. No additional purification step is required when the product replaces diaphragm-grade material in existing Bayer liquor circuits, although storage tanks must be heat-traced because a 50% w/w liquid crystallizes at approximately 12°C.
Kraft pulping and bleaching operations consume sodium hydroxide as a pH buffer, extraction chemical, and oxygen delignification alkali. In oxygen delignification, membrane-grade liquid is metered into medium-consistency mixers at 8–12% pulp consistency, with reactor temperature controlled between 90°C and 110°C and oxygen partial pressure between 400 kPa and 800 kPa. The alkali charge is expressed as NaOH on oven-dry pulp and typically ranges between 1.5% and 3.0% in the oxygen stage. Replacement of diaphragm-grade caustic with membrane-grade material reduces the introduction of sodium chloride and chlorate into the bleach plant. Chloride is not consumed in the alkaline extraction stage; it accumulates in the alkali recovery cycle and contributes to superheater tube corrosion and sticky ash deposition in the recovery boiler. Chlorate introduced into chlorine dioxide stages can form chlorate by-products; membrane-grade material with sodium chlorate below 5 mg/kg reduces this contribution. The relevant quality thresholds are specified in mill acceptance plans against ASTM E291-18 and ISO 979:1974; chloride and chlorate are monitored by ion chromatography on delivered lots. Operating data from continuous digesters and oxygen reactors indicate that substitution of diaphragm-grade product does not require modification of titration-based alkali control when the active NaOH content is kept within the specification band.
For chlor-alkali derivative synthesis and neutralization reactions, the choice between membrane-grade and diaphragm-grade material is governed by residual chloride and transition-metal limits. Epichlorohydrin production from glycerol hydrochlorination uses sodium hydroxide as the dehydrochlorination base; iron and chloride residuals influence by-product formation and catalyst fouling in continuous stirred-tank reactors. In polycarbonate melt transesterification, solid caustic is used in controlled stoichiometry to neutralize residual acid catalyst and must contain low iron to avoid polymer color shift. The solid forms are specified with iron below 10 mg/kg and carbonate below 0.5% w/w; high carbonate content reduces neutralizing efficiency and generates carbon dioxide gas during neutralization. Metering of solid caustic is performed with loss-in-weight screw feeders feeding jacketed dilution vessels, where the neutralization reaction is maintained below 80°C to avoid steam flashing and localized boiling. Compared with diaphragm-grade solid material, Solvay Caustic Soda micropearls exhibit lower caking tendency under humid storage because of controlled granulation and carbonate surface passivation. Published data for this specific configuration is limited for some specialty syntheses; however, the impurity ceilings are aligned with conventional chemical intermediate specifications.
Exothermic dilution of 50% w/w sodium hydroxide is the dominant process risk in water treatment and chemical dosing installations. When concentrated liquid is diluted to 10% w/w, the adiabatic temperature rise can exceed 85°C if the water and caustic are not mixed under controlled flow. High-density polyethylene tanks are rated for continuous service at 60°C, with allowable excursions only up to 80°C; therefore a dilution skid must inject caustic into a recirculating water stream at a rate that maintains bulk temperature below 80°C. Static mixers, temperature sensors, and automatic shutoff valves are used; the addition point must be downstream of the recirculating pump to prevent backflow. Carbon steel and 316L stainless steel are acceptable for ambient storage of 50% w/w liquid, but high-temperature heat exchangers require Alloy 400 or nickel 200 wetted surfaces because hot caustic induces caustic stress-corrosion cracking in stainless steels above approximately 100°C. Aluminum, zinc, galvanized steel, and tin are incompatible. EPDM and PTFE gaskets are preferred; natural rubber and nitrile gaskets are not recommended for continuous wetting. The 50% w/w liquid has a crystallization temperature near 12°C, so storage tanks, pump heads, and transfer lines must be heat-traced to 20–30°C in unheated containment areas. Viscosity increases from about 80 cP at 20°C to more than 200 cP near 0°C, which increases pump discharge pressure and reduces flow through load cells and flow meters.
In potable water treatment and food processing, Solvay Caustic Soda is supplied in food-grade form conforming to FDA 21 CFR 184.1763 and the Food Chemicals Codex sodium hydroxide monograph. The product is metered at 0.5–5% solution strengths to adjust pH, soften water, and regenerate weak acid cation exchange resins. In food processing, it is used for lye peeling of fruits and vegetables, cocoa processing, and pretzel alkali treatment; these applications require low chloride and iron to avoid product discoloration. The membrane-grade manufacturing route is the technical basis for low chlorate and chloride levels; material acceptance is verified by titration per ASTM E291-18 and chloride determination per ISO 979:1974. Storage in food plants uses 316L stainless steel or lined carbon steel tanks, and piping must be sloped and drained to prevent crystallization in dead legs. For water treatment plants operating under NSF/ANSI/CAN 60, the product is evaluated for trace metal contributions; the low transition-metal residual of membrane-grade material supports compliance with lead and copper leaching limits.
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