Gujarat Alkalies GACL Caustic Soda Flakes

In Bayer digestion circuits, the primary function of caustic soda flakes is to reconstitute free alkali lost through red mud washing, desilication products, and evaporation-side carryover. The flakes are dissolved in spent liquor at 50–65°C in agitated predissolution tanks before transfer to digestion feed headers. The dissolution exotherm is 44.5 kJ/mol; uncontrolled water-to-flake addition can produce localized temperatures above 80°C, accelerating corrosion in carbon steel storage tanks and promoting carbonate pickup. The target free Na₂O concentration is set by bauxite mineralogy. Gibbsite plants operate with 140–160 g/L Na₂O at 140–160°C and 0.2–0.5 MPa; boehmite circuits require 180–240 g/L Na₂O at 200–240°C and 1.5–3.5 MPa; diaspore plants reach 220–260 g/L Na₂O at 250–270°C and 4.0–6.0 MPa. Addition ratio is therefore not a single batch percentage but a continuous free Na₂O control loop. The precipitation feed Al₂O₃/Na₂O mass ratio is maintained at 0.65–0.75, with spent liquor returning at 0.25–0.35. Caustic losses are typically 60–150 kg NaOH per tonne of smelter-grade alumina, depending on reactive silica and mud washing efficiency. The downstream process sequence includes flash letdown, sand removal, red mud thickening and washing, seed-hydrate precipitation in stirred vessels, and calcination. Terminal product types are smelter-grade alumina with Al₂O₃ not less than 98.5% and chemical-grade alumina. Compliance baseline includes pressure equipment under ASME BPVC Section VIII Division 1, process piping under ASME B31.3, environmental and energy management under ISO 14001:2015 and ISO 50001:2018, and worker exposure control under OSHA PEL 2 mg/m³ TWA and NIOSH IDLH 10 mg/m³. Operational limitations are specific: direct flake addition into high-temperature digestion liquor is avoided because localized caustic concentration above 300 g/L Na₂O can strip aluminosilicate scales and deposit them in downstream flash tanks. High-reactive-silica bauxite additionally requires lime dosing at 2–4 wt% of dry bauxite to stabilize desilication products and prevent heat exchanger scaling.

Within kraft bleach plants, caustic soda flakes are dissolved to 10–15 wt% sodium hydroxide and injected at the suction of a medium-consistency pump ahead of the extraction tower. The function is not bulk delignification; the white liquor loop already carries the pulping alkali. Instead, the flakes replace sodium hydroxide lost during recausticizing and maintain extraction-stage pH after chlorine dioxide delignification. Addition ratios are measured as percentage NaOH on oven-dry pulp: 1.5–2.5 wt% for oxygen-reinforced extraction, 2.0–3.0 wt% for oxidative extraction, and 1.0–2.0 wt% for pH adjustment in peroxide brightening. The extraction tower runs at 10–12% consistency, 60–80°C, and 60–120 min retention; bleached pulp is washed to residual sodium content below 0.1% on pulp to reduce brightness reversion. Compliance is defined by EPA 40 CFR Part 63 Subpart S for chemical pulp mill air emissions and the EU BAT conclusions under Commission Implementing Decision 2014/687/EU, with effluent AOX monitored after treatment. Terminal products include bleached softwood kraft pulp, bleached hardwood kraft pulp, dissolving pulp for viscose, and fluff pulp. Plant-scale equipment behavior shows that incomplete flake dissolution causes undissolved particles to accumulate in extraction tower bottom screens and accelerates washer blade wear; medium-consistency pumps with fluidizing rotors are therefore preferred over low-shear transfer pumps at the injection point.

For cotton yarn, the mercerization effect is concentration-dependent and becomes visually and tensile-recovery insufficient when the NaOH falls below 16 wt%. The accepted window is 18–26 wt% NaOH, with the lower limit set by incomplete cellulose I-to-soda cellulose transition and the upper limit by excessive fiber swelling that reduces tensile strength recovery. GACL flakes are dissolved to 20–24 wt% and maintained at 15–20°C. The addition ratio in yarn mercerizing is approximately 0.3–0.5 kg NaOH per kg cotton at a material-to-liquor ratio of 1:8 to 1:12. Tension is controlled at 0.3–0.6 cN/dtex during immersion, with dwell time 45–90 s. After caustic impregnation, the yarn passes through recovery washing with 5–10 g/L acetic acid or dilute sulfuric acid neutralization to a residual fabric pH of 6.5–7.5; incomplete neutralization leaves residual alkali above 0.05% on dry fiber, causing yellowing and reactive dye hydrolysis. Processing equipment includes pad/vacuum impregnators, chainless mercerizing machines for knitted goods, and twist-free roller cages for yarn. Compliance is assessed through AATCC TM89-2019 for mercerization evaluation and ISO 13934-1:2013 for tensile strength retention. Sodium hydroxide recovery typically reaches 85–90% reuse. Terminal products are high-luster cotton yarn, sewing thread, mercerized woven shirting, and mercerized knitted fabrics.

Batch saponification of coconut oil, palm kernel oil, or tallow uses caustic soda flakes as the primary sodium hydroxide source for triglyceride saponification or fatty acid neutralization. The flakes are predissolved to 48–50 wt% NaOH and dosed into a jacketed kettle at 70–90°C under an anchor impeller with tip speed 3–5 m/s. Stoichiometric caustic demand is calculated from the saponification value: refined coconut oil at SV 250 mg KOH/g requires approximately 0.178 kg NaOH/kg oil, while tallow at SV 195 mg KOH/g requires 0.139 kg NaOH/kg oil. Industrial practice uses a slight excess of 0.5–1.5 wt% NaOH on oil mass to drive free alkali below 0.5% in finished soap. The process sequence includes slow caustic addition over 60–120 min, boiling to separate glycerin in spent lye, and brine washing with 10–15% NaCl solution to reduce glycerol content below 0.3% in neat soap. Compliance for finished soap is tested under ISO 685:2020 for total alkali and total fatty matter, and consumer detergents fall under Regulation (EC) No 648/2004. Terminal products are soap noodles with total fatty matter 63–80%, toilet soap bars, laundry soap, and glycerin as coproduct. Plant-scale failure data show that uncontrolled flake addition when the oil phase is below 65°C creates sodium soap gel layers on the agitator shaft, increasing motor load by 30–50% and requiring manual scraping. A recirculation loop with in-line caustic injection prevents localized saponification. Batch yield variance is typically ±1.5% total fatty matter when the caustic pump is calibrated against solution density at 20°C.

Low-alkalinity raw water, especially from upland reservoirs after snowmelt, lacks sufficient bicarbonate buffer to prevent coagulant pH depression. Caustic soda flakes are dissolved to 5–10 wt% sodium hydroxide and metered through a diaphragm pump into the rapid-mix basin at 1–10 mg/L NaOH to hold coagulation pH between 7.5 and 8.5 for alum or polyaluminium chloride. The addition ratio for acid neutralization is stoichiometric: 40 mg NaOH neutralizes 36.5 mg HCl, so an industrial waste stream with 50,000 mg/L acidity as HCl requires 54.8 kg NaOH per m³. Continuous pH control uses a three-term controller with static mixer injection, and flake dissolution tanks are sized for 24 h maximum storage to avoid atmospheric carbon dioxide absorption that raises sodium carbonate content above 0.5%. Compliance is defined by AWWA B501-19 for sodium hydroxide quality and NSF/ANSI/CAN 60 for drinking water treatment chemicals. The process reduces lime sludge handling when replacing hydrated lime, but caustic does not remove noncarbonate hardness; waters with sulfate or chloride hardness require parallel soda ash feed. Terminal products are coagulated and filtered drinking water, neutralized industrial effluent, and pretreated boiler makeup. The main operational boundary is that caustic feed above 15 mg/L in raw water with aluminum-based coagulants can form soluble aluminate species and increase dissolved aluminum in finished water above the 0.1–0.2 mg/L drinking water guidance range.

Because silica sand digestion is controlled by quartz particle size and sodium hydroxide concentration, caustic soda flakes are dissolved to 25–50 wt% NaOH before being charged to a stirred autoclave with sand milled to D50 45–75 µm. The SiO₂/Na₂O molar ratio is maintained between 2.0 and 3.2, and flake consumption is 0.55–0.85 kg NaOH per kg silica sand at reactor temperatures of 150–200°C and pressures of 0.5–1.8 MPa for 4–8 h. Unmilled sand above 150 µm reduces silica conversion to below 70%, leaving undigested quartz that accelerates autoclave impeller and discharge valve wear. The resulting sodium silicate liquor is pressure-filtered and adjusted to 38–55 wt% solids. Downstream neutralization with sulfuric acid precipitates silica to customer-specific grades; sodium silicate and precipitated silica placed on the European market are registered under Regulation (EC) No 1907/2006 and classified under CLP Regulation (EC) No 1272/2008. No single horizontal product standard applies across all detergent, tire, and personal care silica grades. Terminal products include sodium silicate liquor for detergent builders, precipitated silica for tire and toothpaste formulations, and zeolite precursors for catalysts. Process safety limits require completing the exothermic flake dissolution before sand charging to avoid localized hot spots above 200°C that accelerate stress corrosion cracking in austenitic stainless steel reactors.