Gujarat Alkalies GACL Caustic Soda Prills

In Bayer alumina refining, Gujarat Alkalies and Chemicals Limited (GACL) membrane-grade caustic soda prills are charged to liquor make-up tanks to replace sodium oxide losses from bauxite residue, desilication product, and product hydrate entrainment. The prills are typically dissolved in spent wash water or barren condensate inside agitated 316L stainless steel or nickel-alloy dissolving vessels with external cooling, because the enthalpy of solution is approximately 44.5 kJ/mol; an uncooled 10% w/w batch can rise from 25°C to above 50°C within minutes. Clarified Bayer liquor is maintained at 140–260 g/L Na2O caustic with an Al2O3/Na2O molar ratio of 0.65–0.75, depending on bauxite trihydrate or monohydrate feed and digestion temperature. Raising 1000 m3 of spent liquor by 1 g/L Na2O equivalent requires approximately 1.29 kg of pure NaOH per cubic metre, or about 1.30 t of 99% prills for the total volume, with the exact figure corrected for sodium carbonate and moisture. Chloride is the dominant impurity concern in Bayer circuits because accumulated chloride accelerates stress corrosion cracking in carbon steel and austenitic stainless steel digesters and flash vessels; membrane-cell prills with NaCl at or below 0.1% therefore reduce the rate of chloride concentration rise in closed liquor loops. Red mud settling and filtration are also affected by free caustic concentration, with excess caustic stabilising fine iron oxide particles and raising flocculant demand in high-rate thickeners. The final product pathway is smelter-grade alumina with typical particle size and purity targets governed by reduction plant specifications, but the caustic make-up point remains one of the principal chemical cost centres in the refinery.

What Governs Caustic Transfer in Continuous Pulp Bleaching Extraction?

In elemental chlorine-free and total chlorine-free bleaching sequences, extraction-stage caustic demand is controlled by hexenuronic acid hydrolysis, lignin phenolic-group ionisation, and residual peroxide stabilisation, not solely by incoming kappa number. GACL-type caustic soda prills are pre-dissolved to 10–15% w/w NaOH with low-iron process condensate before injection into a medium-consistency mixer, because dry prill addition to 10–12% consistency stock produces local pH spikes that degrade cellulose viscosity. The dissolved alkali is metered through a mass-flow system tied to the production rate of oxygen-delignified or Eop-stage feed stock. Residual alkali in the extraction filtrate is typically held at 0.02–0.05 mol/L NaOH, corresponding to pH 11.5–12.3, to solubilise oxidised lignin fragments while limiting alkaline peeling. The spent extraction liquor contains dissolved organic solids, sodium carbonate, and chloride; mills with closed recovery loops track chloride because chloride contributes to superheater tube corrosion risk in the recovery boiler, with site-specific limits derived from recovery boiler water chemistry models and tube metal temperatures. Carbonate in prills above 0.5% raises the dead-load of Na2CO3 in white liquor and reduces effective causticising efficiency. Mill performance is measured against ISO 302:2015 for kappa number and ISO 2470-1:2016 for ISO brightness, with extraction-stage alkali charge adjusted when final brightness deviates by more than 1.0 ISO point.

To produce tension-mercerised cotton yarn with increased strength and dye uptake, the preparation of caustic liquor from GACL prills uses soft water cooled to 15–18°C and a dissolution station in which the final alkali concentration is controlled at 20–24% w/w NaOH. A 20% w/w solution at reference temperature 20°C contains approximately 244 kg NaOH per 1000 L of final solution using solution density 1.219 g/mL; a 24% w/w solution contains approximately 303 kg NaOH per 1000 L using density 1.263 g/mL. The dissolution heat must be removed by plate-and-frame or shell-and-tube coolers before the liquor enters the impregnation padder, since temperatures above 20°C reduce sodium-cellulose I formation and lower the barium activity number below the mercerised range. In chainless or chain mercerising machines, fabric is treated under controlled tension for 30–60 s, then hot-stretched and washed stepwise with decreasing alkali concentrations, followed by neutralisation in acetic acid baths at 2–5 mL/L. Iron and chloride impurities in the caustic feed are critical; iron catalyses oxycellulose formation at the fibre surface and creates yellowness during later bleach or finishing, while chloride contributes to corrosion of stainless steel wash boxes. The treated cotton exhibits improved dimensional stability, higher dye uptake, and tensile strength increases typically in the range of 7–10% for long-staple cotton yarns. Barium activity number above 150 is commonly used as the mill verification threshold for full mercerisation. Operational limits include the need for soft water; calcium and magnesium in hard water form insoluble hydroxide sludge that deposits on padder rolls and reduces alkali penetration.

Kettle saponification of refined tallow, palm stearin, or coconut oil with caustic soda prills is controlled by the saponification value of the fat charge rather than by a fixed oil-to-caustic volume ratio. For refined tallow with a saponification value of 190–200 mg KOH/g, the stoichiometric NaOH demand is 0.139–0.146 g NaOH per gram of fat on a dry basis, calculated using the factor 40.0/56.1, with an additional 0.2–0.5% excess avoided to prevent free caustic in finished soap. The prills are pre-dissolved to 32% w/w NaOH in softened water and metered into the fat melt at 70–90°C through a recirculating eductor or high-shear mixer, not as dry solids, because localised high caustic concentration can encapsulate fat in curd and entrain glycerine in the neat soap. The vessel may be a jacketed stainless steel kettle with slow ribbon or paddle agitation; saponification proceeds through a viscous emulsion phase before the mass breaks to translucent neat soap. Salt addition at 5–7% of the soap mass separates glycerine-rich spent lye from neat soap; spent lye strength is typically 8–12% glycerine. Washed neat soap is dried in vacuum spray or flash dryers to soap noodles with moisture 10–14% for laundry bars or toilet soap base. Saponification value determination is performed according to ISO 3657:2020. Carbonate in caustic prills above 0.5% is undesirable because sodium carbonate can crystallise as efflorescence on the final soap surface and reduce smoothness; it is neutralised, when present, with citric acid at 0.1–0.3% of the charge.

Sodium Methoxide Generation from Caustic Prills in Anhydrous Methanol

For biodiesel plants that prepare sodium methoxide on site, GACL caustic soda prills with moisture below 0.5% are preferred because the equilibrium NaOH + CH3OH ⇌ NaOCH3 + H2O shifts toward methoxide only when water is removed from the reaction mixture. A batch of 1000 kg prills at 99% NaOH represents 25.0 kmol NaOH; reaction with 3000 kg anhydrous methanol produces stoichiometrically 1350 kg sodium methoxide and 450 kg water, giving a nominal methoxide content of 33.8% w/w and water content of 11.3% before drying. The reactor is typically a 316L stainless steel or nickel 200 vessel with a reflux condenser rated for methanol, nitrogen blanketing, and a PTFE-lined circulation pump to suppress iron pick-up. Since transesterification of rapeseed or soybean oil tolerates only low water, the solution is pumped through a molecular-sieve column or a wiped-film evaporator to reduce water below 0.5%. The finished catalyst solution is stored under nitrogen and dosed to the transesterification reactor at 0.4–0.6 wt% of oil as sodium methoxide solution with methanol-to-oil molar ratio 6:1 and reaction temperature 60–65°C. Fatty acid methyl ester output is tested against ASTM D6751-20 or EN 14214:2012+A2:2019 for total glycerol, acid number, and oxidative stability. Operational limits are strict: free water in the catalyst above 1% increases soap formation in the oil phase, raising phase separation time and filter pressure. The addition of dry prills directly to methanol without temperature control can exceed methanol's boiling point and should be avoided by slow prill addition with jacket cooling.

For potable-water pH adjustment, corrosion control, and demineraliser regeneration, caustic soda prills are made down in day tanks to 10–20% w/w NaOH using softened or deionised water, then metered by diaphragm pumps into a quill injection system with carrier water velocity not less than 0.5 m/s. A surface water with total alkalinity of 30–60 mg/L as CaCO3 and pH 6.8 may require 3–8 mg/L NaOH to reach pH 7.5–7.8; the exact dose is determined by on-line pH analyser and jar testing, because raw-water alkalinity and temperature change with season. In separate demineralisation systems, Type I strong-base anion resin is regenerated with 4% NaOH solution at 40–50°C, applied at 4–6 bed volumes and contact time 45–60 min, to displace silica and weakly ionised anions from the resin. The prill grade must comply with NSF/ANSI/CAN 60 and, for municipal supply, verification against AWWA B501-17 is commonly referenced for assay, chloride, iron, and mercury. Membrane-cell prills with Fe below 15 ppm reduce the risk of coloured iron hydroxide deposits in filters and resin beds. Direct dry prill addition to low-alkalinity mains water is avoided because local pH excursions above 11 can precipitate calcium carbonate scale on downstream membranes and foul lime-softened media. Treated water is then re-carbonated or split-stream blended to maintain Langelier Saturation Index between -0.5 and +0.5 for distribution system corrosion control.

When Caustic Prills Are Metered Into Refinery H2S Scrubbers Without Pre-Dissolution, Chloride Accumulation Becomes Decisive

Refinery fuel gas and LPG sweetening with caustic scrubbing uses 10–25% w/w NaOH solution prepared from GACL prills in a dedicated dissolving skid. The absorber column is typically a packed tower with structured packing and a recirculation loop; the scrubbing reaction with hydrogen sulfide consumes 2 mol NaOH per 1 mol H2S for conversion to sodium sulfide, and methyl or ethyl mercaptans consume 1 mol NaOH per 1 mol thiol to form sodium mercaptides. Fresh caustic injection is coupled to caustic strength rather than flow alone, with the recirculating liquor pH maintained at 12.0–13.5; spent caustic is blown down when sulfide concentration reaches 5–10 wt% as Na2S to prevent under-deposit corrosion in the circulation piping. Dry prill addition directly to the absorber is not permitted because the hydration exotherm can form a concentrated caustic film on carbon steel surfaces, promoting caustic stress corrosion cracking; the prills must be dissolved in low-chloride water and cooled to below 40°C before injection. Chloride from the caustic prills adds to chloride absorbed from the refinery feed and can raise pitting susceptibility in 316L stainless steel sections above 60°C; membrane-cell prills with NaCl below 0.1% reduce this contribution. Materials selection and sour service limits follow NACE MR0175/ISO 15156:2020, and treated LPG is checked for residual sulfide by copper strip tarnish methods such as ASTM D1838. The sweetened fuel gas target is often below 10 ppmv H2S for gas turbine or refinery furnace service, with the precise limit set by burner manufacturer and environmental permit.