Sodium Hydroxide Caustic Soda Flakes 98% China

Sodium hydroxide caustic soda flakes 98% manufactured in China are white deliquescent platelets of sodium hydroxide with a minimum assay of 98.0% by mass on an as-received basis. The material is identified by CAS 1310-73-2, classified for transport as UN 1823, and commonly cleared under import code 28151100. The active component is NaOH, with a molecular weight of 39.997 g/mol, a crystalline density near 2.13 g/cm³, and a melting point of approximately 318 °C. The industrial designation “98% flakes” indicates that sodium hydroxide constitutes at least 98.0% of the mass, while sodium carbonate, sodium chloride, iron oxide, and sulfate account for the balance. Flake geometry is produced by solidifying a concentrated sodium hydroxide melt on a cooled drum, yielding irregular platelets commonly 1–5 mm thick and 5–20 mm across. The material is hygroscopic and deliquescent; exposure to ambient moisture and carbon dioxide forms a surface brine film and sodium carbonate crust that can lead to caking and loss of free-flowing character. Standard export packaging includes 25 kg woven polypropylene bags with inner polyethylene liners and 1 t or 1.25 t flexible intermediate bulk containers.

Commercial model designation is not uniform across manufacturers. The product is typically labelled as “caustic soda flakes 98% min,” “NaOH flakes 98%,” or “industrial grade I” with reference to GB/T 209-2018. Buyers should confirm the actual certificate of analysis because the model name alone does not define the chloride, carbonate, or iron content for a specific shipment.

What Processing Differences Exist Between Flake, Pearl, and Liquid Sodium Hydroxide?

Flake morphology presents a lower surface area-to-mass ratio than pearl or prill material of the same chemical purity. During dissolution in an agitated tank, the flake particle wets more slowly than a pearl because the platelet shape reduces the solid–liquid interface available for water penetration. The practical consequence is a longer batch cycle unless the flakes are fed through a lump breaker or wetting cone directly into the impeller zone. Flakes generate fewer airborne respirable fines than granular or powdered caustic soda, but bag emptying can still release dust and requires local exhaust ventilation or a dust-tight tipping station. Liquid sodium hydroxide at 50% mass fraction eliminates the dissolution step and is preferred in continuous dosing loops where consistent concentration is required. The liquid product, however, has a freezing point near 12 °C, which necessitates heated storage tanks, trace lines, and insulated piping in cold environments.

The choice between flake and pearl is frequently driven by silo handling, metering accuracy, and bag residual loss. Pearls are generally free-flowing and bridge less frequently in conical silo hoppers; flakes may require a bin activator or vibratory discharge because interlocked platelets can form stable arches. Flake material, however, is more widely available in some export markets and is often priced below pearl-grade material for bulk neutralization and alkaline cleaning formulations.

Typical certificate-of-analysis limits for Chinese 98% flake are aligned to GB/T 209-2018, industrial sodium hydroxide, solid grade I. Total alkalinity is reported as NaOH, and the impurity profile is limited as follows.

Instead of relying solely on a certificate of analysis, a laboratory controlling incoming flake quality should determine total alkalinity by acid-base titration under CO2-free conditions because atmospheric carbon dioxide neutralizes dissolved NaOH and biases the free alkalinity result. The total alkalinity method in ASTM E291-18 is frequently used in import inspection, while GB/T 209-2018 remains the reference specification for Chinese domestic grade classification. Sampling should follow a recognized bulk solids scheme; the sample should be sealed immediately after collection and protected from humidity. When the flake is dissolved for quality control, the resulting solution should be filtered through a 0.45 µm membrane if turbidity from undissolved carbonate or silicate is present, because suspended solids interfere with spectrophotometric iron determination.

Batch-to-batch variability in 98% flake shipments from different Chinese manufacturers tends to appear in sodium carbonate and iron oxide values rather than in total alkalinity. Facilities that receive containers from multiple suppliers should compare certificates against the same GB/T 209-2018 grade designation and retain a sample from each lot. In high-volume alkaline cleaning compound production, flake from a single supplier may show lot-to-lot deviations in bulk density of approximately ±0.05 kg/L; such variation does not affect chemical assay but can alter volumetric feeder calibration on a twin-screw compounding line. The feeder should be calibrated with the actual flake lot rather than with a dry free-flowing reference material because interlocked flake particles and surface moisture change the fill factor of the feed screw.

When the Heat of Solution Is Released Into a Jacketed Dissolving Skid

Dissolution of solid NaOH in water is strongly exothermic. The integral heat of solution at infinite dilution is approximately −44.5 kJ/mol. In a batch dissolving system, uncontrolled flake addition can raise the liquid temperature above the atmospheric boiling point, generating steam and caustic aerosol. A standard industrial skid therefore consists of a jacketed or limpet-coil vessel, a retreat-curve impeller or axial-flow turbine, a rotary valve or screw feeder for flake addition, and a temperature interlock that stops the feeder when the solution exceeds 70–80 °C. Water is always charged first and agitation is run before solid addition begins. The 98% flake should be added into the vortex at a rate that maintains the batch below the specified maximum; for a 20,000 L vessel producing 50% NaOH, the cooling duty can exceed 1.0 MW during peak addition rates, although published data for this specific vessel size is limited and vendor-specific heat transfer calculations are required.

In continuous neutralization, a flake-derived 50% NaOH solution is metered by a positive displacement pump into a recirculating acidic stream. The injection point is followed by a static mixer, and the recirculation loop returns a slipstream to the metering pump suction to reduce concentration gradients. A temperature element downstream of the static mixer is interlocked with the alkali pump; if the mixed-stream temperature exceeds 60 °C because of acid–base neutralization heat, the alkali pump stops and the acid feed remains open. This control prevents localized boiling, vapor-phase corrosion, and deposition of sodium salts in the injection quill. The quill is fabricated from PTFE-lined carbon steel or a suitable nickel alloy because the wetting zone experiences alternating acidic and alkaline conditions. Sodium hydroxide solutions react with carbon dioxide from air or wastewater to form sodium carbonate; in hard-water neutralization, calcium carbonate scaling on the static mixer can occur when the local pH exceeds 10.0. The dosing skid is therefore flushed with soft water after each batch to prevent carbonate scale accumulation.

In alumina refineries, 98% flake is dissolved with recycled spent liquor to produce Bayer digestion liquor. Low-temperature digestion of gibbsitic bauxite is typically carried out at 140–150 °C with caustic concentrations in the range 140–160 g/L Na2O, while boehmitic ores may require 200–250 °C and higher caustic loadings. The flake product’s low iron oxide specification contributes to controlling iron contamination in the alumina product, although refinery-grade Bayer liquor is also managed through filtration and precipitation steps. In kraft pulp mills, the flakes are mixed with sodium sulfide and sodium carbonate in the recausticizing area to produce white liquor. Cooking of softwood or hardwood chips is normally performed with effective alkali charges of 15–25% on oven-dry wood and cooking temperatures of 150–170 °C. The chloride content of the flake can concentrate in the recovery cycle; a limit near ≤0.05% NaCl is generally acceptable for many mills, but bleach plants and closed-cycle mills may require tighter chloride control. Textile mercerization uses NaOH in the 18–25 °Bé range at 15–25 °C. Flake-derived mercerizing liquor is filtered or settled before contact with cotton because undissolved carbonate particles and iron-bearing fines can deposit on the fabric and reduce luster uniformity.

Storage, Moisture Uptake, and Corrosion Boundaries

Sodium hydroxide flakes 98% should be stored in a dry, covered area with floor containment. The solid absorbs water and CO2 from air; partially emptied bags should be folded and secured to minimize the formation of a sodium carbonate crust. Aluminum, magnesium, zinc, tin, and galvanized steel are incompatible with sodium hydroxide solutions because the metal is attacked with evolution of hydrogen. Concentrated NaOH service at ambient temperatures commonly uses carbon steel tanks and piping, but carbon steel may require stress-relief and careful control at temperatures above 60 °C because of caustic stress corrosion cracking risk in some conditions. For pumps, seals, and small-bore lines, PTFE-lined steel, polypropylene, or a high-nickel alloy is used depending on temperature and concentration. The product does not burn, but its heat of solution can raise the temperature of wet paper, wood, or other organic material to ignition in a confined and wetted bulk spill. For this reason, spills should be recovered dry with a polypropylene shovel or vacuum system and placed into a closable caustic-resistant container. Water flushing of large spills is appropriate only with pH neutralization and drain control; uncontrolled flushing can release high-pH water that violates discharge limits for aquatic systems.

Occupational exposure to caustic soda is controlled by containment and personal protective equipment. The NIOSH IDLH for sodium hydroxide is 10 mg/m³. Eye protection should meet ANSI Z87.1 or equivalent splash goggle standards, and protective gloves should be tested against alkali permeation under EN 374. Contaminated clothing should be removed immediately and affected skin flushed with water for at least 15 min. Emergency showers and eyewash stations should be located within 10 s travel of a bag dumping station or dissolving skid.

Compared with 96% sodium hydroxide flakes, the 98% grade carries lower sodium carbonate and sodium chloride loads. This difference matters in closed-loop pulping liquors where chloride accumulation accelerates corrosion, and in organic synthesis where iron oxide can shorten catalyst life or darken product color. Compared with 99% flakes produced from membrane-cell feedstock, the 98% grade is typically used in bulk neutralization, extraction, cleaning, and process pH control where the extra cost of a lower-chloride specification is not justified. The 98% industrial flake is not automatically food-grade or pharmaceutical-grade; FCC and USP monographs impose separate limits on heavy metals, mercury, and chloride that a generic industrial certificate may not address. For alkyd resin catalysts, epoxide ring-opening reactions, or trace metal-sensitive polymerization systems, a 99% grade or liquid membrane-cell caustic soda should be specified after reviewing the actual chloride and iron certificate values. The product also differs from liquid 50% caustic soda in transport and unloading: solid flake reduces freight mass but requires on-site dissolution, while liquid caustic reduces labor and exposure risk but increases heated storage and freeze-protection requirements.