Sodium Hydroxide, Pellets, Technical Grade, 50 kg

At kraft pulping and bleach-grade pulp mills, sodium hydroxide pellet make-up is introduced into the recausticizing loop to adjust white-liquor effective alkali before digester feed. Technical-grade pellets supplied in 50 kg bags are unloaded through bag-break hoppers with local dust extraction and screw-fed into a dissolving tank fabricated from 304L stainless steel or rubber-lined carbon steel. The pellets are mixed with weak wash at 60–90 °C and recirculated until density and conductivity targets are met. A typical pellet assay of ≥98.5% NaOH with ≤1.0% Na₂CO₃ and ≤0.5% NaCl allows mill operators to compute sodium balance across the recausticizing plant without exceeding chloride or carbonate tolerances that would otherwise depress liquor effectiveness. White liquor targets depend on wood species and cooking process; softwood lines normally maintain effective alkali at 85–110 g/L as NaOH, total titratable alkali at 100–140 g/L, and sulfidity at 25–35%, while hardwood lines operate at 75–95 g/L effective alkali and 20–30% sulfidity.

During kraft cooking, the charged effective alkali drives delignification through aryl ether bond cleavage while sulfidity controls selectivity and carbohydrate retention. Delignification follows temperature-dependent kinetics above 150 °C; softwood digester control uses H-factor 1400–1800 at 165–170 °C, and hardwood lines use 600–1000 at 155–160 °C. Continuous digesters such as Lo-Solids or Compact Cooking units with liquor-to-wood ratio of 3.2:1–4.5:1 require stable alkali concentration in the impregnation zone to prevent nonuniform cook and shive formation. A drop in effective alkali below the lower endpoint causes lignin condensation at black liquor pH below 12 and raises kappa number beyond target, while excess alkali reduces pulp yield by alkaline peeling of hemicellulose. Brown stock washing after blow-line discharge uses countercurrent washers operating with dilution factor of 2–3 m³/t pulp to recover sodium hydroxide and sulfide; sodium loss in unwashed pulp is measured as residual Na in black liquor solids. End product is unbleached softwood or hardwood kraft pulp with target kappa 25–32 for softwood and 15–20 for hardwood, moving to oxygen delignification and ECF or TCF bleach sequences. Compliance points include TAPPI T 236 om-13 for kappa number, ISO 302:2015 for pulp kappa determination, and ISO 5351:2010 for limiting viscosity in cupriethylenediamine solution.

Caustic Ratio and Digestion Matrix in Bauxite Refining

Bauxite digestion facilities replenish caustic lost to red mud residue by introducing 50 kg sodium hydroxide pellet bags into spent-liquor dissolving stations. Pellet addition is calculated from mass balance across the washer train, not from pH alone, because Bayer liquor is a concentrated sodium aluminate buffer with pH typically above 13.5 and pH does not respond linearly to alkali changes. Gibbsitic bauxite is digested at 140–150 °C with liquor containing 180–260 g/L Na₂O as caustic; boehmitic bauxite requires 220–280 g/L Na₂O at 230–250 °C; diasporic bauxite operates at 280–350 g/L Na₂O and 250–270 °C. Pregnant liquor alumina-to-caustic ratio typically falls at 0.60–0.70 as Al₂O₃/Na₂O mass ratio, while spent liquor returns at 0.30–0.40, and actual sodium hydroxide consumption ranges from 80–150 kg per tonne of alumina depending on bauxite mineralogy, liquor loss, and residue washing efficiency.

Digestion autoclaves with live steam injection and flash letdown are controlled to avoid localized overconcentration when pellet-derived caustic is injected. Scaling in digesters and heat recovery tubes is aggravated by carbonate and sulfate accumulation; therefore, technical-grade pellets with carbonate content above 1.0% Na₂CO₃ may require purge treatment in high-carbonate circuits. Red mud separation is carried out in thickeners with polymer flocculants, followed by filtration and precipitation in continuous or batch precipitators. Sodium hydroxide lost in red mud and disposed fines is the dominant make-up demand, and caustic pellets are dosed into spent liquor return lines before the digester feed tank. End product is smelter-grade alumina with Al₂O₃ typically above 98.3% and SiO₂ below 0.45%, used for aluminium smelting. Operational limits include protection against caustic embrittlement in carbon steel tanks at high temperature and avoidance of aluminium hydroxide precipitation in transfer lines when liquor A/C ratio drops below spent-liquor equilibrium.

When Technical-Grade Caustic Replaces Food-Grade in Industrial Soap Kettles

During fat saponification in 20–50 m³ jacketed kettles, caustic pellets from 50 kg bags are pre-weighed and mixed into a 25–30% w/w NaOH solution before being metered into heated triglycerides. The charge is derived from saponification value: NaOH required per kg oil equals saponification value in mg KOH/g multiplied by 40/56.1, expressed as grams per kg. For 1,000 kg coconut oil with saponification value 248–265 mg KOH/g, the pure NaOH requirement is 176.8–188.9 kg; with technical grade at 98.5% NaOH, the operator adds 179.5–191.8 kg pellets. A 0.5–2.0% free alkali excess over stoichiometric remains in finished neat soap, determined by titration with 0.1 N HCl using phenolphthalein indicator. Kettle temperature is held at 80–90 °C during lye injection over 45–90 min to avoid local caustic pockets that cause grain or darkening. Saponification value is measured by ISO 3657:2020 or AOCS Cd 3-25.

After saponification, the batch is boiled and salted out with 10–15 kg NaCl per 100 kg oil to separate neat soap from glycerine-bearing spent lye. Technical-grade pellets containing ≤1.0% Na₂CO₃ contribute carbonate alkalinity that can increase soap ash and reduce glycerine purity; iron above 20–30 mg/kg accelerates oxidative rancidity in tallow-based bases. Therefore, this grade is suited to industrial soaps, laundry bars, metal-stearate feedstock and emulsifier production, not to cosmetic-grade soap where contaminant budgets for Fe, Ni and Cr are stricter. The neat soap is washed, settled, and finished into noodles or bars via vacuum spray drying or roll mills. Compliance with EU Detergents Regulation (EC) No 648/2004 applies to final detergent formulations and labelling, while sodium hydroxide itself is handled under REACH registration and exposure scenario documentation.

Why Does Tension Control Matter More Than Caustic Concentration?

Mercerizing lines for cotton knit goods operate within 18–24% w/w NaOH, where cellulose crystal modification is controlled not by concentration alone but by the combination of alkali, temperature, and fabric tension. In a typical chain mercerizing range, 50 kg pellet bags are emptied into an automatic dissolver and mixed with recovered wash liquor to form a 28–32% w/w stock solution; the mix is cooled through a plate-and-frame exchanger to 15–20 °C before entering the padding trough. Wet pick-up at the mangle is set at 80–100% for woven cotton, with dwell time 35–60 s in alkali and controlled width shrinkage below 5% in tension mercerization. Slack mercerization for knitwear allows 15–25% dimensional shrinkage, producing soft handle and higher elongation rather than luster. Sodium carbonate accumulation above 5 g/L in the mercerizing bath, formed by atmospheric CO₂ absorption or evaporator carryover, reduces swelling and dye uptake.

The wash recovery system normally uses triple-effect evaporators to concentrate spent caustic to 400–600 g/L NaOH and precipitate sodium carbonate before reuse. Cotton processed at 18–24% w/w NaOH shows improved tensile strength and higher dye sorption compared with untreated fabric; testing uses ASTM D5035 for breaking strength and ASTM D3776 for mass per unit area. The end product is mercerized yarn or fabric for high-colour apparel, bed linen, and industrial cotton duct. Bath temperature above 25 °C reduces swelling and luster; therefore chilled water or jacketed troughs are required in tropical lines. Caustic recovery efficiency and tight width control are more important operating variables than marginal increases in caustic concentration because the cellulose decrystallization frontier depends on alkali uptake under tension.

Controlling Free Alkalinity in Continuous Sodium Hypochlorite Generation

Continuous sodium hypochlorite generators absorb chlorine gas into recirculating 15–20% w/w NaOH under backpressure of 5–10 kPa in FRP or PVC packed columns. The reaction follows Cl₂ + 2NaOH → NaOCl + NaCl + H₂O with high exothermicity; plate-type heat exchangers are sized to hold liquor temperature between 20–30 °C because chlorate selectivity climbs sharply above 40 °C and accelerates decomposition of available chlorine. The 50 kg pellet bags are emptied into a sealable caustic dilution vessel and made down to 15–20% w/w solution with softened water; hard water introduces Ca and Mg hydroxides that foul packing and hypochlorite storage. Final product is controlled to 10–15% available chlorine w/w with free NaOH residual 5–30 g/L and pH 12.0–13.5. If free caustic drops below 5 g/L, hypochlorous acid forms and disproportionation to chlorate proceeds, generating oxygen gas and heat.

Product stability depends on trace-metal exclusion: iron above 0.5 mg/L, nickel above 0.1 mg/L, and copper above 0.05 mg/L catalyse decomposition, producing visible oxygen at storage tank vents. Therefore, titanium or PVC-U piping is preferred, and storage tanks are equipped with scrubbers to retain off-gas. Product used for municipal wastewater disinfection must meet AWWA B300-22; European users specify EN 901:2013 for water treatment chemicals. Incompatibility with acids, ammonia, and reducing agents must be enforced by segregated storage and double mechanical seals on transfer pumps. The generated sodium hypochlorite is dosed into cooling water, wastewater effluent, or surface disinfection circuits after dilution, with final residual set by risk assessment and local discharge permits.

In metal finishing and semiconductor etch wastewater treatment, technical-grade caustic pellets are dissolved to 20–25% w/w solution and metered through progressive cavity or diaphragm dosing pumps into pH adjustment tanks ahead of clarification. A 50 kg bag of 98.5% NaOH pellets yields approximately 197 kg of 25% w/w NaOH solution when dissolved in 147 kg of water; the standard dissolution enthalpy for NaOH at infinite dilution is −44.5 kJ/mol, so injection water temperature can rise quickly. For pH correction of 5% w/w hydrochloric acid rinse water containing 1.406 mol/L HCl, the stoichiometric NaOH requirement is 56.2 kg pure NaOH per 1 m³ acid, equivalent to 57.1 kg technical pellets at 98.5% assay. pH control at the discharge point is set to 6.5–8.5, but automated cascades must account for the steep titration curve near pH 7, where overdosing of 0.1–0.3 L/min can shift pH from 4 to above 10 in small equalization tanks.

Neutralization of metal-bearing streams requires set points below 8.5 to avoid amphoteric resolubilization of zinc and lead; in printed circuit board etching, copper hydroxide precipitation is optimum at pH 8.5–9.5, but aluminium-bearing rinse water redissolves above pH 8.0 as aluminate. Sludge from caustic neutralization is dewatered in filter presses and classified under local hazardous-waste rules. Effluent streams containing cyanide or hexavalent chromium must not be mixed with acidic rinse water before treatment because caustic addition alone does not oxidize cyanide or reduce hexavalent chromium. United States metal finishing discharge is regulated under 40 CFR Part 433, which sets pH range 6.0–9.0 and metals limits; European users follow the local conditions of the Industrial Emissions Directive reference documents.

LPG and light naphtha processing units use fresh technical-grade sodium hydroxide pellets to prepare 10–25% w/w alkali solutions for prewash and Merox extraction of hydrogen sulfide and mercaptans. In a typical liquid-liquid contactor, 50 kg pellet bags are dissolved into demineralized water in a caustic mix drum, then recirculated through a fibre-film or packed column at a caustic-to-hydrocarbon volume ratio of 0.1:1–0.3:1. Hydrogen sulfide reacts to form sodium sulfide; mercaptans are extracted as sodium mercaptides into the aqueous phase and regenerated by air in a Merox unit using cobalt phthalocyanine catalyst on activated carbon. Spent caustic sodium sulfide concentration is typically 5–15% w/w and mercaptide sulfur 0.5–5.0% w/w depending on feedstock sulfur. Incompatibilities include sodium carbonate precipitation from CO₂ absorption; therefore mix tanks and transfer lines are nitrogen-blanketed and caustic solution is prepared daily to prevent carbonate levels above 3–5 g/L as Na₂CO₃, which otherwise fouls extraction packing and consumes alkali without removing sulfur.

Treated LPG and light naphtha are tested for mercaptan sulfur by ASTM D3227; refined product specifications may require below 10 mg/kg for jet fuel and below 30 mg/kg for motor fuel in certain regions. End products include sulfur-reduced LPG, light straight-run naphtha, and kerosene streams feeding hydrotreating or direct blending. The operational boundary for this application is spent caustic disposal: sodium sulfide and mercaptide-rich spent alkali requires neutralization, wet air oxidation, or licensed hazardous-waste processing before discharge, which often determines whether fresh pellet-derived caustic is economically feasible at small terminals.