Liquid Caustic Soda 48%

Liquid caustic soda 48% is an aqueous sodium hydroxide product concentrated to a nominal NaOH content of 48.0% by weight. The material is produced in chlor-alkali facilities by membrane or diaphragm electrolysis of sodium chloride and is supplied as a clear, odorless, corrosive liquid. The substance carries CAS registry number 1310-73-2 and European Community number 215-185-5; for transport it is consigned as UN 1824, Class 8, Packing Group II, with the proper shipping name Sodium hydroxide solution. In product coding systems, membrane-grade 48% caustic soda is commonly identified as LCS48-M, whereas diaphragm-grade product is designated LCS48-D. The distinguishing feature between these model designations is not sodium hydroxide concentration but the residual chloride and carbonate profile: membrane-grade material typically maintains a lower sodium chloride and sodium carbonate envelope. The concentrated solution has a density of 1.498–1.502 g/cm³ at 20 °C and is miscible with water in all proportions with a strongly exothermic heat of dilution. The product is packaged in bulk railcars, tank trucks, isotainers, and smaller polyethylene or lined-steel containers; packaging and unloading systems must exclude reactive metals.

What Distinguishes 48% Liquid Caustic Soda from 32% and 50% Solutions?

At 48.0% NaOH, each metric tonne of liquid caustic soda contains 480 kg of NaOH and 520 kg of water. By comparison, 32% material contains 320 kg NaOH and 680 kg water per tonne, while 50% material contains 500 kg NaOH and 500 kg water per tonne. On an equivalent alkali basis, the water burden of 48% material is approximately 1.08 kg water per kg NaOH; for 32% material the figure is 2.13 kg water per kg NaOH. This reduction in water ingress is the primary formulation difference when a plant evaluates 48% against 32% in a process such as alumina digestion, pulp liquor makeup, or wastewater neutralization with limited tankage. When evaluated against 50% material, 48% caustic soda has a lower density and lower crystallization onset in the 8–12 °C range, so it is frequently specified where 50% product would require aggressive tank heating or railcar thawing. However, 48% remains a viscous liquid; gravity flow from bulk storage is typically inadequate at ambient winter temperature, so pumps and lines are sized for viscosity rather than water-like behavior.

These differences have direct consequences for metering. A Coriolis mass flowmeter measuring 48% product gives a higher NaOH mass per unit volume than a volumetric meter calibrated for 32% material; if the density offset is not configured, the delivered alkali dose may be overestimated by the ratio 1.498/1.349 at 20 °C. The concentrated product also increases the exotherm relative to 32% under identical final dilution conditions.

Storage Envelope and Metallurgical Restrictions

Bulk storage tanks for 48% NaOH are fabricated from stress-relieved carbon steel, typically to API 650, with post-weld heat treatment specified to reduce the risk of caustic stress corrosion cracking. Service above 40 °C requires either a stress-relieved carbon steel tank with a suitable corrosion allowance or a lined/nickel-alloy design. The product is incompatible with aluminum, copper, zinc, tin, brass, bronze, and galvanized steel; contact with amphoteric or reactive metals can generate hydrogen and rapid heat. Wetted transfer lines and dosing skids at ambient temperature are commonly constructed of 316L stainless steel, PTFE-lined carbon steel, or polypropylene. EPDM and PTFE seals are acceptable; Buna-N and Viton are generally not acceptable in strong caustic service. Pump types used on production lines include magnetically driven centrifugal pumps with 316L or alloy C276 wetted parts and air-operated double-diaphragm pumps with PTFE bodies. Storage tanks are maintained above 15 °C because 48% NaOH starts to crystallize in the 8–12 °C range; heat tracing and insulation are required in locations where ambient air temperature falls below 10 °C. Open tanks are not recommended because atmospheric CO₂ absorption converts a portion of the product to sodium carbonate, producing turbidity and scale. The product is stored away from acids, ammonium salts, chlorinated solvents, and oxidizing liquids. Piping systems for 48% caustic are designed to ASME B31.3 with materials selected for caustic service. Published long-term corrosion data for mild steel in 48% NaOH at ambient are available, but field-specific rates vary with weld residual stress and temperature cycling; therefore baseline ultrasonic thickness scans are recommended after the first 12 months of service.

When 48% Caustic Soda Is Used in Alumina Digestion and Kraft Pulp Makeup

In Bayer alumina refining, 48% NaOH is metered into spent liquor or dilution tanks upstream of bauxite grinding mills. The addition point is usually a 316L or duplex stainless steel injection quill installed in a recirculation line; flow is measured by a Coriolis mass flowmeter, and the setpoint is derived from the process molar ratio of Na₂O to Al₂O₃. For gibbsitic bauxite digestion at 140–160 °C, liquor charge ratios commonly operate near 3.0–3.5 Na₂O/Al₂O₃. Replacing 32% material with 48% material reduces water load to the evaporator train by approximately 1.08 kg water per kg NaOH versus 2.13 kg water per kg NaOH, directly lowering live steam demand in the shell-and-tube evaporators and flash train. The product is added before the digesters, not into high-temperature flash lines, because localized caustic boiling can aggravate erosion-corrosion of unlined steel.

In kraft pulp mills, 48% NaOH serves as makeup alkali in white liquor systems and as pH adjuster in oxygen delignification and bleach extraction. White liquor total titratable alkali is commonly maintained in the 90–120 g/L Na₂O range; product addition is controlled by online conductivity and total alkali titration. Metering pumps for this service are often hydraulically actuated diaphragm pumps with PTFE heads and stainless steel wetted parts; the discharge line is fitted with a pressure relief valve set below the piping design pressure. The use of 48% rather than 32% material reduces water addition to the recausticizing circuit and preserves the heat balance of the recovery boiler and evaporator load.

Anion exchange systems in boiler feedwater and demineralizing trains use 48% caustic soda after dilution to 4%–8% w/w because direct injection of concentrated sodium hydroxide can shrink and crack Type II anion resin beads and accelerate silica polymerization within the bed. The regeneration skid normally includes a 316L stainless steel caustic branch, a water branch with a flow-ratio controller, and an in-line static mixer; the diluted regenerant is heated to 35–45 °C and introduced at 4–6 bed volumes per hour through a distributor plate. Effluent from the fast-rinse step is neutralized to pH 6–9 before discharge to industrial waste treatment. In wastewater pH control, 48% NaOH is selected over 32% where storage space is limited, but the dosing point must avoid brass or bronze valve trim and the feed line must be flushed after shutdown to prevent crystallization.

Controlling Exothermic Dilution and Exposure Thresholds

Dilution of 48% caustic soda is an exothermic operation. The heat of solution of sodium hydroxide is -44.5 kJ/mol. The product must be added slowly to water under agitation; adding water to the concentrated solution can produce local boiling and violent splatter. In-line dilution skids use flow-ratio controllers with a 316L or PTFE-lined static mixer and downstream conductivity or pH verification. At the point of use, the diluted solution may be heated; for example, anion regeneration systems heat the diluted caustic to 35–45 °C, while CIP operations use 2–3% NaOH at 60–80 °C. Occupational exposure limits for sodium hydroxide are set at a ceiling of 2 mg/m³ by OSHA PEL and ACGIH TLV-C. Dermal contact causes liquefactive necrosis; eye exposure requires immediate irrigation with tepid water for at least 15 minutes and medical evaluation. Spill containment systems are designed with pH-neutralizing capacity, but the neutralization reaction is also exothermic and must be conducted on diluted material in a controlled vessel.

Surfactant manufacturing uses 48% NaOH as the neutralizer for linear alkylbenzene sulfonic acid and other acid esters. The product is diluted in-line to 20%–30% NaOH before entering the neutralization loop; the loop is controlled at pH 7.0–9.0 and a temperature below 45 °C to limit color-body formation. In sodium hypochlorite production, 48% NaOH is diluted and reacted with chlorine gas in a packed column or ejector. The final bleach product is held at 2%–3% excess NaOH and below 25 °C during storage; elevated temperature and high ionic strength promote chlorate accumulation. The use of 48% instead of 50% caustic in hypochlorite service can simplify cold-weather handling without materially changing stoichiometric chlorine demand.