Sodium hydroxide, formula NaOH, molar mass 39.997 g/mol, CAS 1310-73-2, is delivered to detergent powder manufacturing sites as an aqueous solution at 32 wt% or 50 wt% strength. In normal production it functions as a neutralisation reagent for linear alkylbenzene sulfonic acid, as a saponification agent for fatty feedstocks, and as a source of free alkalinity in aqueous slurries and dry neutralisation routes. The solution begins to crystallise near 4 °C for the 32 wt% grade and near 12 °C for the 50 wt% grade, so storage tanks, unloading manifolds, and transfer lines require heat tracing or heated enclosures in temperate climates. Because sodium hydroxide corrodes aluminium, galvanised steel, and brass, wetted components are specified from stress-relieved carbon steel, nickel alloys, or austenitic stainless steels only within defined temperature and concentration limits. The dosing rate is tied to the stoichiometric demand of the sulfonic acid stream and to the free alkalinity specification of the finished powder, which is determined by potentiometric titration. The following unit operations describe where caustic soda exerts process control and where its concentration becomes a critical boundary.

Neutralisation Stoichiometry and Exotherm Control in Continuous Sulphonation Lines

In a continuous sulphonation plant, linear alkylbenzene is sulfonated with gaseous SO3 in a film reactor. The resulting linear alkylbenzene sulfonic acid has an active matter content of 96–97 wt% and must be neutralised before it can be blended into detergent powder. The reaction R–C6H4–SO3H + NaOH → R–C6H4–SO3Na + H2O is equimolar and strongly exothermic, releasing approximately 57 kJ per mole of water formed. For every 1000 kg of active LABSA, the theoretical NaOH demand is 122.5 kg, equivalent to 245 kg of 50 wt% sodium hydroxide solution. In a continuous neutralisation loop, the acid, pre-diluted water, and caustic solution are metered into a circulation stream. The reaction mixture is passed through a shell-and-tube cooler with cooling water at 25–30 °C to hold the discharge temperature at 40–70 °C. The pH target at the loop outlet is typically 7.5–9.0; lower pH leaves free sulfonic acid, while higher pH increases free alkali and can alter later slurry viscosity and additive stability. Inline pH probes with flat glass electrodes, automatic temperature compensation, and retractable holders are used for control. The neutralised LAS paste at 45–60 wt% active matter is then transferred to a buffer tank.

The neutralisation loop is operated with a circulation-to-feed flow ratio of 10–20 and a loop residence time of 5–15 min. The pH at the loop outlet is controlled at 7.5–9.0, and the neutralised LAS paste is buffered at 45–60 wt% active matter. Free sulfonic acid below pH 6.5 contributes to product colour and odour; free NaOH above pH 10.5 consumes acid-sensitive additives and raises the alkalinity of the finished powder. The measurement of free alkalinity in LAS paste follows ASTM D501-03, and anionic active matter is determined by two-phase titration according to ISO 2271. The neutralisation route is selected when the formulation requires high active matter without excessive sodium sulfate filler. A process conflict arises because the sulfonation step introduces residual free sulfuric acid, which also consumes caustic, so the NaOH dosing must account for total acidity rather than assumed LABSA active content. Membrane-grade caustic with a chloride concentration below 0.05 wt% dry basis is used to limit chloride carry-over into the powder, because chloride increases hygroscopicity and corrosion risk in packaging equipment. The shell-and-tube neutralisation cooler is specified with a cooling water supply at 25–30 °C and is interlocked to stop LAS acid feed if the caustic pump fails, if recirculation flow drops below a defined safety limit, or if the loop outlet temperature exceeds 85 °C. Static mixers are installed at the caustic injection point to disperse concentrated caustic into the acid stream rapidly; local pH above 12 at the injection point is kept short to limit colour development in LAS.

In tallow-based and palm stearin-based detergent powder formulations, sodium hydroxide is fed to the crutcher to saponify fatty acids and triglycerides. The reaction of a triglyceride with three moles of NaOH produces three moles of sodium carboxylate and one mole of glycerol. The stoichiometric caustic demand is calculated from the saponification value; for coconut fatty acid with a saponification value of 250 mg KOH/g, dry NaOH demand is 178 kg per 1000 kg of feed. The saponification is conducted at 80–110 °C for 30–90 min under agitation in a jacketed crutcher. Caustic solution is added slowly to the fatty phase to avoid local high-temperature exotherm and to prevent boiling of the water phase. The endpoint is not simply the disappearance of fat; the crutcher is titrated for free alkali and adjusted to leave 0.05–0.5 wt% NaOH in the slurry. Too little caustic leaves unsaponified oil, which can reduce detergency, promote rancidity odours, and deposit as a sticky film on the spray-dryer wall. Too much free caustic in the crutcher increases the alkalinity of the final powder and can destabilise sodium silicate and optical brighteners. The soap formed during saponification contributes foam regulation, soil suspension, and builder synergy with LAS. In a typical batch crutcher, the soap reaction is combined with sodium silicate, sodium sulfate, and polymer; the order of addition is arranged so that caustic is added before acid-sensitive optical brighteners and enzymes. The liberation of glycerol during triglyceride saponification increases the liquid phase volume and acts as a humectant in the dried powder. At ambient relative humidity above 60%, this humectant may increase moisture uptake and promote caking unless the total liquid phase is reduced or the base powder is surface-coated with zeolite or silica. The crutcher total solids are therefore maintained at 60–68 wt%. The water introduced with 32 wt% caustic is part of the batch water balance; using 50 wt% caustic reduces added water but increases local concentration gradients. The degree of saponification is monitored by titrating free alkalinity according to ASTM D501-03 and by extracting residual neutral oil with petroleum ether; the residual fat content is generally held below 0.5 wt% of the slurry solids. The soap-containing slurry is then transferred by a lobe pump to the spray-dryer feed tank. If the slurry is held too long at high temperature, the soap can react with dissolved calcium and magnesium to form insoluble lime soaps that reduce detergency and plug the spray nozzles.

What Happens to Slurry Rheology When Residual Free Alkali Drifts Above 0.8 wt%?

The aqueous detergent slurry leaving the crutcher is a non-Newtonian suspension with total solids of 60–68 wt% and temperature 60–80 °C. Free sodium hydroxide in the continuous water phase modifies ion activity and silicate equilibrium. If free alkali drifts above 0.8 wt%, sodium silicate in solution can polymerise and raise viscosity sharply. In extreme cases, the slurry loses pumpability and the spray-dryer high-pressure pump suffers cavitation and ring-line blockages. Slurry viscosity is commonly measured with a rotating spindle viscometer at 70 °C; typical acceptable ranges in process documentation are 5,000–15,000 mPa·s. Below 5,000 mPa·s, spray droplets can become too small and increase dust fines; above 15,000 mPa·s, atomisation is poor and rings build on the tower wall. Free alkali also stabilises sodium sulfate solubility but increases the risk of corrosion in carbon steel crutchers. The pH measured on a 10 g/L solution is usually held between 9.0–10.5. If pH is below 9.0, the finished powder may fail the alkalinity specification and have reduced acid soil neutralisation. If pH is above 10.5, the dust from the tower can cause irritation to workers and the powder may exceed the recommended total alkali limit for package compatibility. The pH measurement is performed according to ISO 4316 or ASTM D1172.

The cause of the viscosity increase is not a single species but a shift in the silicate polymerisation equilibrium. Detergent-grade sodium silicate has a SiO2:Na2O ratio of 2.0–2.5; at low free caustic, silicate species condense into colloidal silica and gel; at high free caustic, silicate depolymerises and no longer provides corrosion protection. Simultaneously, the soap carboxylate and LAS micelle structures respond to ionic strength. The crutcher motor amperage is trended against batch history; a rise without temperature or solids change indicates silicate polymerisation. Spray-dryer feed viscosity above 15,000 mPa·s at 70 °C reduces atomisation at a nozzle pressure of 30–80 bar and increases particle size. The result is high free moisture, wall deposits, and a bulk density above 0.65 kg/L. Below 5,000 mPa·s, the droplets shatter and increase fines below 180 µm, which can create dust explosion hazards and poor filling. The control of free alkali is therefore not simply a pH setting; it is a coupled variable with slurry viscosity, silicate stability, tower pressure, and dust alkalinity. Caustic soda is preferred over sodium carbonate in certain slurry formulations because it does not introduce carbon dioxide and provides a stronger pH response per unit mass, but this same strength means that the control band is narrower. Metering pumps for caustic are typically positive displacement diaphragm or peristaltic units with stroke adjustment and pH feedback; a dead time of 15–60 s is common in crutcher loops. Overshoot occurs when the pH setpoint is adjusted too quickly, and a 0.1 pH error can correspond to a meaningful change in free alkali due to the buffering of silicates and soap carboxylates.

In dry neutralisation routes, a continuous turbo-agglomerator or plowshare mixer receives solid sodium carbonate, fillers, and polymer builder while LAS acid and liquid binder are sprayed onto the moving powder. The reaction 2 RSO3H + Na2CO3 → 2 RSO3Na + CO2 + H2O releases carbon dioxide and water. The gas release increases bed porosity and can reduce bulk density; the water of reaction initiates granule formation. Caustic soda is used when sodium carbonate neutralisation is partial or when residual free acidity from acid channelling requires trimming to meet pH specification. Aqueous NaOH at 32 wt% or 50 wt% is sprayed through a binary nozzle onto the pre-mixed solids. The liquid addition rate is set by the acid feed, the measured pH of a 10 g/L solution, and the target granule moisture of 8–14 wt%. The granulation temperature is maintained at 40–60 °C to avoid dehydration of sodium sulfate hydrates and to prevent sticky granules. High caustic loading increases wet granule strength and neutralises residual acid, but narrows the process window. Torque on a twin-screw extruder with an L/D ratio of 10–20 can rise sharply when the liquid-to-solid ratio exceeds 0.12–0.15, and the granule bed may consolidate into lumps. Published data for the exact torque threshold in this specific configuration is limited; equipment suppliers specify limits for liquid-to-solid ratio and wet mass rheology. Residual moisture is determined by loss on drying at 105 °C, and bulk density is measured according to ISO 697:1981. The dry neutralisation route is selected when high bulk density and low energy cost are required, but it has a narrower pH control window than wet neutralisation because the acid-base contact is imperfect. Caustic soda must not be sprayed undiluted onto localised dry sodium percarbonate or sodium hypochlorite adducts; it can decompose bleach precursors and release heat. In formulations containing percarbonate, caustic addition is therefore segregated or replaced by sodium silicate. The final powder pH is measured after the agglomerator and after the fluidised-bed cooler; the difference indicates acid migration and is used to adjust the caustic dosing. The total free alkali of the finished powder is kept below 1.0 wt% NaOH for household grades, while industrial presoak products may run higher only with additional hazard labelling and packaging controls.

Sodium Silicate Compatibility and Corrosion Boundaries in Caustic Storage and Metering Systems

Sodium silicate used in detergent powders is produced by dissolving silica in caustic soda or by fusing sand with sodium carbonate. In the crutcher, sodium hydroxide shifts the speciation of dissolved silicate and controls the SiO2:Na2O ratio that is stable in solution. Detergent-grade silicates with a ratio of 2.0–2.5 remain in solution only within an alkalinity window; if free caustic falls too low, colloidal silica forms and gels. If free caustic rises too high, the silicate depolymerises and no longer provides corrosion protection on aluminium and zinc washing-machine components. The total free alkali of finished powder is determined by acid titration and expressed as % NaOH. Household powders typically specify a maximum of 1.0 wt% NaOH, while heavy-duty industrial presoak formulations may exceed that value only with additional hazard labelling and packaging controls. In tower feed slurries, the free alkali level is one of the variables that determines whether the spray-dryer wall deposits are hard and glassy or soft and removable; high silicate plus high free alkali can create hard scale that requires mechanical cleaning.

Caustic storage and metering systems are engineered to avoid stress corrosion cracking of carbon steel. Concentrated caustic above 50 °C can crack welded carbon steel, particularly at heat-affected zones; therefore storage tanks and piping in neutralisation service use stress-relieved carbon steel below that temperature or nickel alloys such as Monel 400 for heating elements and thermowells. Gaskets are specified from EPDM or PTFE. Aluminium, galvanised steel, and brass are excluded from caustic wetted service because sodium hydroxide attacks these materials. The bulk storage of 50 wt% caustic requires heat tracing because the solution freezes near 12 °C; 32 wt% caustic freezes near 4 °C. Exposure to caustic mist is controlled by local exhaust ventilation; the occupational exposure limit for sodium hydroxide mist is typically a ceiling of 2 mg/m³. Dilution of concentrated caustic is carried out by metering caustic into water in an in-line static mixer, never by adding water to static concentrated caustic, because the heat of solution can cause local boiling and splattering. The metering skid is interlocked with the neutralisation loop so that caustic flow is stopped on loss of recirculation, high temperature, or pH sensor failure. The storage tank is equipped with a leak detection system and a containment bund sized for the full tank volume, with material compatibility for caustic and not simply general chemical service.