Caustic Soda Flake 98 Manufacturers in China

Digestion-Liquor Caustic Concentration Control in the Bayer Circuit

In Bayer alumina refineries supplied from Chinese flake producers, sodium hydroxide dissolution is operated as a controlled make-down step to maintain the digesting liquor at 180 g/L to 260 g/L Na₂O and an alumina-to-caustic ratio of 0.50 to 0.75. Sodium hydroxide flakes with 98 wt% NaOH content are charged into a stainless-steel or lined dissolving tank with agitation and temperature control because the heat of solution can raise localized liquor temperature above 90°C and damage unprotected epoxy linings. The flake standard applicable to the raw material is GB/T 209-2018, with imported material also requiring REACH registration under (EC) No 1907/2006; refinery effluent and red mud disposal are typically aligned with IFC EHS Guidelines for Aluminum Manufacturing, which restrict tailings leachate pH to 9 or below after neutralization. Downstream, bauxite is milled in rod and ball mills to P80 0.075 mm to 0.150 mm and slurried in spent liquor before digestion. Gibbsitic bauxite is processed at 140°C to 160°C in atmospheric or low-pressure vessels, while boehmitic ores require tube digesters at 200°C to 260°C and pressures up to 3.5 MPa. Sodium hydroxide is consumed not only by gibbsite and boehmite dissolution but also by reactive silica, forming sodium aluminosilicate desilication product; this side reaction imposes a caustic loss in red mud of 60 kg to 150 kg Na₂O per ton of alumina depending on ore reactive silica content. Red mud separation includes thickeners, countercurrent decantation washers, and security filtration; the filtered pregnant liquor is decomposed in precipitation trains seeded with fine aluminium trihydroxide. The calcination stage operates at 950°C to 1250°C in rotary kilns or circulating fluidised-bed calciners to produce smelter-grade alumina, sandy alumina, or chemical-grade alumina trihydrate. Published data for specific Bayer liquor flake-dosage optimisation in Chinese refineries is limited, but operational limits are clear: caustic concentration must not fall below the threshold where gibbsite precipitation occurs prematurely in heat exchangers, and flake dissolution must avoid excessive free water addition that dilutes the liquor balance and increases energy consumption in evaporation.

What Constraints Effective Alkali Charge in the Causticizing Circuit?

Sodium hydroxide flakes are used in bleached kraft pulp mills primarily to fortify white liquor produced from the causticizing of green liquor rather than directly as digester alkali. The causticizing reaction between sodium carbonate and slaked lime yields a white liquor with effective alkali typically 100 g/L to 140 g/L Na₂O, but conversions are equilibrium-limited to 80% to 85%. For softwood cooking, the effective alkali charge ranges from 15% to 20% on oven-dry wood; for hardwood digesters the range is 13% to 18%. Sulfidity is maintained at 25% to 30% to protect cooking selectivity. Compliance for pulp sampling and fibre analysis references ISO 302:2015 for kappa number and ISO 5351:2010 for limiting viscosity number, while bleach plant effluent is managed under 40 CFR Part 430 Subpart B for AOX discharge limits. NaOH flakes after dissolution are added to the white liquor storage tank or directly into the digester recirculation line through a make-down skid with a dilution ratio of 0.30 kg flake per 0.55 kg demineralised water to produce a 50 wt% stock liquor. In continuous Kamyr digesters, the alkali profile is split into impregnation, bulk delignification, and residual delignification zones; in batch digesters, the cooking liquor-to-wood ratio is maintained at 3.5:1 to 4.5:1 L/kg. Process control measures include online effective alkali titration and residual alkali measurement in black liquor to avoid excessive carbohydrate peeling and yield loss. Downstream, screened brownstock enters oxygen delignification and bleach sequences such as D0-Eop-D1 or Op-Q-P depending on the target brightness. Terminal products include bleached softwood kraft pulp, dissolving pulp for viscose, unbleached linerboard, and sack kraft. Residual sodium hydroxide that is not consumed in cooking leaves with black liquor and is recovered through evaporation and chemical recovery, but high flake input without corresponding lime mud quality control can increase sodium carbonate dead load and reduce causticizing efficiency.

When Tension Mercerizing Requires Uniform Caustic Swelling Without Iron Contamination

Cotton yarn and woven fabric mercerizing uses sodium hydroxide flake to prepare a working solution at 18 wt% to 23 wt% NaOH, corresponding to approximately 24°Bé to 30°Bé at 20°C. The flake must meet low-iron specifications because dissolved iron above 50 mg/kg can create rusty staining on bleached cotton. For compliance, the processed textile is evaluated against GB 18401 for safety requirements, OEKO-TEX Standard 100 for residual chemical control, and ZDHC MRSL for effluent parameters; colour fastness to washing is tested by ISO 105-C10:2006 if required. In a continuous chain mercerizer, caustic soda solution is applied by impregnation through a saturator with a dwell time of 30 s to 60 s while the fabric is held under controlled tension through a series of clip chains or rollers to prevent shrinkage. After caustic impregnation, the fabric passes through countercurrent hot water washing at 60°C to 90°C and neutralization with dilute acetic or formic acid. The process recovers weak caustic from wash water through evaporation, returning strong caustic to the saturator. Caustic addition rate in a typical production line is governed by the fabric mass throughput and the carryover rate; a line running 80 m/min with fabric width 1.8 m and 200 g/m² basis weight consumes approximately 0.8 t to 1.2 t of NaOH flake per 24 h depending on washing efficiency. Terminal products include mercerized cotton yarn with improved tensile strength and dye uptake, high-lustre shirting fabric, and dimensional-stabilised knitted fabric. The caustic concentration must be continuously monitored with density meters or online refractometers; deviations below 17 wt% reduce the swelling effect and cause uneven dye absorption.

Caustic soda flake of 98 wt% assay enters kettle saponification as a 32°Bé to 36°Bé solution after dissolution in softened water. The addition ratio is set by the saponification value of the fat charge—palm stearin at 190 mg KOH/g to 210 mg KOH/g, coconut oil at 250 mg KOH/g to 264 mg KOH/g—plus a controlled free alkali excess of 0.1 wt% to 0.5 wt% in the finished neat soap. The fat blend is boiled with caustic soda solution in jacketed kettles at 90°C to 105°C for 3 h to 6 h under atmospheric pressure; brine graining then separates the neat soap from the spent lye containing glycerol and impurities. Compliance references AOCS Cd 3-25 for saponification value and EU Detergent Regulation (EC) No 648/2004 for final detergent safety; additional GMP requirements apply in exported oleochemical markets. Following graining, the soap is washed, settled, and dried in vacuum spray chambers to produce soap noodles with moisture 10 wt% to 14 wt%. Terminal products include laundry soap bars, soap noodles for plodding, and industrial sodium soap-based lubricants. Excess free caustic above 0.5 wt% must be neutralised or carbonated to avoid alkaline hydrolysis and skin irritation; high unsaponifiable matter in technical-grade fats can consume additional NaOH and alter the finishing curve.

Acidic Effluent Neutralization and Regeneration of Strong-Base Anion Resins

Demineralised water plants dissolve caustic soda flakes to a 25 wt% stock solution and blend to 4 wt% to 8 wt% for regeneration of strong-base anion exchange resins. The regenerant dosage ranges from 50 g to 100 g NaOH per litre of resin at a flow rate of 2 BV/h to 4 BV/h. Acidic effluent neutralisation from cation regenerant and acid spill basins uses metering pumps and inline static mixers to maintain discharge pH between 7.0 and 8.5. Compliance for potable water chemicals references NSF/ANSI/CAN 60 and AWWA B501; the neutralised effluent is discharged under local industrial sewer permits. The downstream terminal streams are demineralised boiler feedwater with conductivity below 10 µS/cm and pH-adjusted industrial wastewater. Operational boundaries include the use of stainless steel or lined day tanks, because concentrated caustic above 50 wt% can stress-crack carbon steel at welded joints.

Alkaline etch baths in architectural aluminium finishing lines are maintained with sodium hydroxide flakes to control free NaOH at 40 g/L to 60 g/L and dissolved aluminium at 75 g/L to 120 g/L. The addition ratio is not fixed; flakes are replenished to offset the reaction 2Al + 2NaOH + 6H₂O → 2NaAl(OH)₄ + 3H₂, consuming roughly 1.48 kg NaOH per kilogram of aluminium dissolved. Bath temperature is held at 50°C to 65°C with air agitation to generate a uniform matte finish; etch rate is 0.5 µm/min to 2.0 µm/min depending on zinc content of the 6000-series alloy. Compliance references ISO 7599:2018 for anodic oxidation coating specifications and ASTM B137-95 for coating mass measurement. Following alkaline etching, profiles pass through triple cascade rinses and a desmut step in sulfuric acid or mixed acid to remove smut, then anodising in 15 wt% to 18 wt% sulfuric acid at 18°C to 20°C and sealing in hot water or nickel acetate. Terminal products include anodized architectural profiles, automotive trim, and electronics heat sinks. Bath life is limited by rising sodium aluminate content; above 150 g/L dissolved aluminium the viscosity increases, etch rate decreases, and rinse water turbidity rises.

Catalyst Pre-Dissolution in Anhydrous Methanol for Triglyceride Transesterification

Biodiesel production uses sodium hydroxide flakes as a transesterification catalyst after conversion to sodium methoxide in anhydrous methanol. The addition ratio for refined low-free-fatty-acid oils is 0.3 wt% to 1.0 wt% of the oil mass, with a practical baseline of 0.5 wt% for soybean, rapeseed, or palm oil. Flakes are dissolved at 30 kg to 50 kg per 1000 L of methanol with methanol moisture limited to 0.1 wt% or less, and methanol-to-oil molar ratio is maintained at 6:1. Product compliance references EN 14214:2012+A2:2019 and ASTM D6751-20 for B100 FAME. The methoxide solution is transferred under nitrogen blanketing into a transesterification reactor operating at 55°C to 65°C for 1 h to 2 h; after reaction, the mixture is settled to separate crude glycerol and methyl ester. The ester phase is water-washed or dry-resin washed, dried at 85°C to 110°C under vacuum, and stored as B100. Terminal co-products are fatty acid methyl ester and crude glycerol at 80 wt% to 90 wt% purity. Operational limits are explicit: oil with free fatty acid above 1 wt% consumes sodium hydroxide to form soap, and water above 0.2 wt% triggers saponification and emulsion formation.

Spent caustic management in liquefied petroleum gas liquid-liquid caustic washing relies on sodium hydroxide flakes to prepare 5 wt% to 20 wt% caustic solution for extractive removal of hydrogen sulfide and mercaptans. LPG streams are contacted with recirculated caustic in a prewash tower or static mixer/settler train; the addition ratio is set by sulfur loading and spent caustic pH, which is held above 13.5 to maintain extraction capacity. Gasoline and naphtha sweetening operates with weaker caustic of 2 wt% to 5 wt% to reduce product carryover. Compliance for refinery caustic service includes NACE SP0403-2014 for avoidance of caustic stress corrosion cracking and ASME B31.3 for process piping design. Downstream, spent caustic is routed to wet air oxidation or sulfide oxidation units and then neutralised before biological treatment. Product streams after water washing are sweet LPG, naphtha, and jet fuel with copper strip corrosion rating 1a or 1b. The process boundary is temperature: carbon steel in caustic service should not exceed 50°C to 60°C unless stress-relieved, and welded joints require post-weld heat treatment.