Kutch Chemical Industries ltd Caustic Soda

Kutch Chemical Industries Ltd Caustic Soda is manufactured at the membrane-cell chlor-alkali plant in Gandhidham, Gujarat, India, and is released in two physical forms. Caustic Soda Lye 48/50% Technical or Rayon Grade is a clear-to-slightly hazy aqueous solution with a sodium hydroxide mass fraction between 0.48 and 0.50, a density of approximately 1.52 g/cm³ at 20 °C, and a freezing point near 10 °C; it is shipped in tankers or high-density polyethylene carboys. Caustic Soda Flakes 99.5% Technical is a white deliquescent solid with a minimum sodium hydroxide assay of 99.5% by mass, a bulk density of approximately 0.9–1.1 g/cm³, and a melting point near 318 °C. Both forms are mercury-free and asbestos-free because the ion-exchange membrane route is used; this route also lowers chloride, chlorate, and iron relative to diaphragm-cell grades.

The manufacturing sequence comprises treated brine preparation, electrolysis in a membrane cell, evaporative concentration of the catholyte, and flaking for the solid product. Release testing is performed by ASTM E291 for total alkalinity, chloride, and carbonate, with supplementary limits aligned to IS 252:2013 for technical and rayon grades. The lye is used in alumina refining, kraft pulping, textile mercerisation, viscose production, saponification, water treatment, and neutralisation. Compared with diaphragm-cell caustic soda, the membrane-cell product contains substantially lower sodium chloride and sodium chlorate; compared with mercury-cell caustic soda, it contains no detectable mercury and is therefore specified for wastewater discharge permits with a mercury limit below 0.001 mg/kg.

Indicative specification matrix for the two product forms appears below; the certificate of analysis for a specific lot governs shipment.

What Limits Chlorate and Chloride Carryover in Membrane-Cell Caustic Soda?

Chloride and chlorate carryover in membrane-cell caustic soda is governed by the cation-exchange membrane’s rejection efficiency and the dissolved impurity load of the brine feed. In a bipolar electrolyser running at 85–90 °C and 4–6 kA/m², hydrated sodium ions migrate across the membrane, while chloride anions are largely rejected; nevertheless, a finite chloride diffusion flux remains. For a conditioned membrane and a brine feed containing less than 10 mg/kg hardness and less than 0.3 mg/kg aluminum, chloride in 50% catholyte is typically held below 50 mg/kg, and sodium chlorate is held below 10 mg/kg. Diaphragm-cell caustic soda can contain 1.0–1.2% NaCl and 0.1–0.3% NaClO3 because the diaphragm is porous and does not provide equivalent anion rejection. Mercury-cell caustic soda can be low in chloride but carries mercury at trace levels and is restricted under Minamata Convention controls. For rayon-grade sodium hydroxide, IS 252:2013 applies lower chloride and iron thresholds because chloride attacks titanium spinneret surfaces and iron catalyses cellulose xanthate degradation.

At the liquid handling level, membrane-cell lye is transferred through nickel-free piping and titanium or duplex stainless steel heat exchangers. Storage tanks are specified as lined carbon steel or 304L stainless steel; nitrogen blanketing is used where atmospheric carbon dioxide pickup must remain below 0.1% Na2CO3. The freezing point of 48% lye is near 10 °C; storage at temperatures below 15 °C requires trace heating or dilution to 32% to avoid crystallisation of sodium hydroxide hydrates. For flake storage, ambient humidity above 60% RH accelerates caking and carbonate pickup, so the product is packed in moisture-barrier bags with a maximum stack height of 8–10 bags to avoid lump formation. Food-grade applications require compliance with 21 CFR 184.1763 and Food Chemicals Codex; the technical and rayon grades described here are not automatically food-grade.

Chlorate is not inert in downstream oxidation reactions. In sodium hypochlorite production, the reaction between chlorine gas and 30–35% NaOH at 20–30 °C proceeds through hypochlorous acid intermediates; chlorate in the feed caustic accelerates chlorate accumulation in the finished bleach. Membrane-cell caustic lye with a chlorate concentration below 10 mg/kg keeps the hypochlorite product within drinking-water additive limits when the bleach is dosed at 1–2 mg/L as Cl2. Diaphragm-cell caustic lye with 0.1–0.3% chlorate may require additional chilling and filtration of the finished bleach to remove chlorate crystals. Compliance with AWWA B300 for hypochlorite storage is therefore more readily achieved with low-chlorate caustic soda.

The Bayer process for alumina refining uses caustic soda at digestion temperatures of 145–175 °C and liquor strengths of 200–240 g/L NaOH expressed as Na2O. Low chloride is critical because chloride concentrates in the closed liquor loop and raises the chloride-induced stress corrosion cracking risk in austenitic stainless steel heater tubes and flash vessels. Caustic soda with high chlorate can contribute to oxalate degradation and red mud settling; membrane-cell lye avoids the chlorate load typical of diaphragm-cell material. In kraft pulping, caustic soda is mixed with sodium sulfide to produce white liquor; softwood cooking commonly uses an effective alkali charge of 12–16% NaOH on oven-dry wood at 155–170 °C. The low non-process element load of membrane-cell caustic soda reduces reactive soda loss in the recovery boiler and limits sticky particle deposition on superheater tubes.

In cotton scouring and bleaching preparation, 20–40 g/L NaOH at 90–100 °C removes pectin, wax, and seed coat fragments; iron in the lye must be below 10 mg/kg Fe to prevent yellow-brown staining after hydrogen peroxide bleaching. For water treatment pH correction, caustic soda lye is dosed through a static mixer at concentrations of 5–10% to avoid localised pH overshoot and calcium carbonate scale; membrane-cell grade with low iron is used where clarified water is ozonated or passed through reverse osmosis membranes. In saponification, a 30–40% NaOH solution is reacted with fats and oils at 80–100 °C; low chloride is necessary when the resulting soap is formulated for metalworking or industrial cleaning because chloride promotes corrosion of ferrous parts after drying. Published data for the Kutch Chemical Industries product in these specific production windows is limited, but the impurity ceilings align with standard corrosion-control and fabric-quality requirements.

In acid gas scrubbing, caustic soda lye is sprayed through a packed column at 10–20% NaOH to absorb hydrogen sulfide and carbon dioxide; low iron and low chloride prevent fouling of packing by iron sulfide and reduce corrosion in downstream biological treatment. Mercaptan removal in hydrocarbon streams uses a caustic prewash at 15–25% NaOH followed by oxidation; membrane-cell caustic soda with low chloride reduces salt fouling in the prewash tower and lowers chloride carryover into the spent caustic oxidation reactor. Epoxy resin production uses caustic soda as a dehydrochlorination agent in the reaction between bisphenol A and epichlorohydrin at 60–80 °C; low chloride in the caustic soda reduces chloride contamination in the resin and lowers the wash-water load. Published data specific to the Kutch Chemical Industries product in acid gas scrubbing and epoxy resin production is limited; however, the impurity profile is consistent with refinery spent-caustic handling constraints.

Table 2 compares the impurity profile of membrane-cell caustic soda with diaphragm-cell and mercury-cell products.

When Caustic Soda Flakes Substitute Lye in Viscose and Dyehouse Operations

Solid caustic soda becomes necessary when liquid transport is uneconomic or when batch-weight verification is simplified by solid dosing. The flake product is dissolved in demineralised water in a jacketed tank with a turbine impeller tip speed of 3–5 m/s; the heat of solution is approximately −44.5 kJ/mol, and the temperature rise can exceed 30 K if flakes are added too rapidly to low-volume water. The dissolution tank is fitted with a cooling jacket capable of removing 1.0–1.5 kW per kilogram of NaOH dissolved per hour to keep the solution below 40 °C. For dyestuff reduction and oxidation baths, the solution is cooled to 20–25 °C before metering to avoid undesired hydrolysis of reactive dyes.

In viscose production, cellulose steeping uses 18–20% NaOH at 25–40 °C; chloride and iron in the steeping lye affect viscose filterability and final fibre colour. Diaphragm-based solid caustic soda may contain chloride above 500 mg/kg and carbonate above 0.6%, whereas the membrane-cell flake retains a chloride concentration below 100 mg/kg and a carbonate concentration below 0.4%. This difference reduces hypochlorite formation when the solution is used in chlorine-containing oxidation baths and reduces carbonate scale in dye jiggers. For cotton mercerising, flake-dissolved lye is filtered through a 10 µm cartridge before the saturator to protect pad rollers from abrasive fines. The filter housing is specified as polypropylene or fluoropolymer-lined steel because concentrated caustic soda degrades nylon and polyurethane components.

Caustic Soda Lye in Mercerisation and Aluminum Etch Bath Control

Cotton mercerisation operates in a narrow concentration and temperature window of 260–320 g/L NaOH and 15–20 °C, corresponding to 28–31 °Bé at 15 °C. A temperature rise to 30 °C reduces fibre swelling and changes tensile modulus, leading to non-uniform dye uptake across the fabric width. The mercerising range is therefore equipped with plate heat exchangers and chilled water at 10–14 °C; lye concentration is monitored continuously by density or refractive index. Membrane-cell lye with low sulfate and chloride permits spent lye recovery in a vacuum concentrator without excessive scaling or foaming; the recovered lye is adjusted to 28–31 °Bé and returned to the saturator. Sulfate levels below 50 mg/kg are required to avoid sodium sulfate crystal deposition on concentrator tubes.

In aluminum pre-anodising etch, sodium hydroxide is used at 80–120 g/L and 50–60 °C to produce a matte surface, with dissolved aluminum maintained at 40–80 g/L. The etch rate is approximately 1.0–2.0 µm/min per side under those conditions. Low-chloride caustic soda reduces localised pitting on 6063-T6 aluminum profiles; low-iron caustic soda avoids brown spotting in sealed anodic layers. Aluminum transfer piping and storage must not be used for concentrated lye; nickel-free plastic or stainless steel is required. The bath is titrated by acid-base titration after sodium gluconate or sorbitol addition to complex dissolved aluminum; free NaOH is maintained by dosing fresh lye while a slipstream is cooled and filtered through a 5 µm bag filter to remove smut.

In indigo denim dyeing, caustic soda maintains the reducing pH of sodium hydrosulfite baths at 11.5–12.0; low iron prevents ferric hydroxide precipitation on denim yarn guides. In reactive dyeing, caustic soda at 10–20 g/L is used for dye fixation at 40–60 °C; carbonate in the caustic increases carbonate hardness and can reduce dye penetration. Membrane-cell lye with carbonate below 0.2% reduces hard water interference and improves reproducibility in pad-batch processes.