Liquid Sodium Metasilicate

Liquid sodium metasilicate is an aqueous solution of sodium metasilicate in which the molar ratio of SiO₂ to Na₂O is held near 1.0. The product is not a discrete molecule but a dynamic equilibrium of monomeric silicate anions, small silicate oligomers, and colloidal polysilicate species; the distribution shifts with concentration, pH, temperature, and dissolved divalent cation content. Industrial product designations are typically supplier-specific and may encode total solids, viscosity grade, or alkalinity reserve; no universal ISO model designation exists for liquid sodium metasilicate. Derivation from anhydrous sodium metasilicate CAS 6834-92-0 or sodium metasilicate pentahydrate CAS 10213-79-3 is common.

A representative clear grade is supplied with total solids between 38.0 and 45.0 wt%, Na₂O between 9.0 and 13.0 wt%, SiO₂ between 9.0 and 13.0 wt%, and specific gravity between 1.35 and 1.52 at 20 °C. Total alkalinity as Na₂O is commonly 9.0–13.0 wt%, while the weight ratio SiO₂:Na₂O is maintained at 0.9–1.1. Temperature-corrected viscosity is a primary specification because it controls transfer pump sizing and mixing energy. At 20 °C, a 40 wt% solids product may display 50–200 mPa·s; at 40 °C, viscosity commonly falls below 30 mPa·s. Iron content is commonly limited to ≤40 mg/kg for detergent and peroxide-bleaching applications because soluble iron accelerates peroxide decomposition and contributes to silicate gel discoloration. Silicate precipitation potential is checked by acid titration to pH 8.3 and pH 3.7 endpoints according to ASTM D501-03 or equivalent supplier methods.

Storage in carbon steel is acceptable at ambient temperature in closed tanks, but stress corrosion cracking has been reported when carbon steel contact occurs at continuous operating temperatures above 60 °C in the presence of chloride above 100 mg/L. Transfer systems should use 316L stainless steel or EPDM-lined positive-displacement pumps; high-shear centrifugal pumps with acid carryover can generate localized silica gel and plug wear rings. The product should be added to water rather than water to concentrated product because local high pH and concentration can form gel seeds, especially in water containing calcium and magnesium above 200 mg/L as CaCO₃.

What Separates Liquid Sodium Metasilicate from Higher-Ratio Sodium Silicate and Caustic Soda?

The molar ratio is the primary compositional variable. Sodium silicate 3.3 ratio water glass has an SiO₂:Na₂O weight ratio of approximately 3.2–3.4, whereas liquid sodium metasilicate has a ratio of approximately 1.0. This difference increases free alkalinity and reduces the degree of silicate polymerization. In aqueous dilution, metasilicate solutions remain clear at pH above 12.0, but acidification below pH 10.5 begins to convert silicate monomers into colloidal silica; below pH 8.5 the solution becomes unstable and silica gel may precipitate under quiescent conditions. Compared with 50% sodium hydroxide, liquid sodium metasilicate provides lower hydroxide alkalinity per unit mass but contributes soluble silica that can inhibit aluminum attack and provide antiredeposition activity. The buffering behavior is broader because silicate species hydrolyze over the pH range 10.0–12.5, whereas caustic soda is fully dissociated and exhibits a single high-pH plateau. Sodium orthosilicate, with a higher Na₂O:SiO₂ ratio, provides stronger alkalinity but is less storage-stable in liquid form and is typically supplied as solid; sodium sesquisilicate occupies an intermediate position.

When Reserve Alkalinity and Aluminum Inhibition Must Coexist in Cleaning Baths

In immersion cleaning of ferrous and aluminum components, liquid sodium metasilicate is used at active solids concentrations between 8 and 30 g/L. For steel stamping residues, a bath at 60–75 °C with 15 min immersion removes chlorinated paraffin and sulfonate soils; for aluminum parts, the metasilicate contributes soluble SiO₂ that forms a colorless passivating film, suppressing alkaline attack. Although the anticorrosion effect is widely reported in cleaning literature, published data for this specific configuration is limited; therefore, site-specific coupon exposure in accordance with ASTM G31-21 is required before full-scale deployment.

Zinc-based die castings are outside the recommended substrate envelope. At pH above 12.0, zincate formation attacks the surface and produces visible darkening and dimensional loss. Mixed-metal immersion lines typically hold bath pH below 11.5 and limit immersion time to 5 min unless a zinc-specific inhibitor has been qualified in production trials. Spent cleaner neutralization with sulfuric acid generates amorphous silica gel at pH 7–9; this precipitation should be designed into wastewater solids removal rather than allowed to occur in drain lines. Lamella clarifiers or dissolved-air flotation units handle the resulting amorphous silica, and filter press throughput can decline when the gelatinous precipitate layer is highly compressible.

In-line monitoring of silicate as SiO₂ follows ASTM D859-16 after sample filtration. Fouling of sample lines with polymerized silica is a known maintenance point when bath pH drops below 11.0.

Spray-Drying Slurry Rheology and Tower Throughput

In detergent powder production, liquid sodium metasilicate is blended into aqueous crutcher slurries at dry-basis addition rates between 3 and 12 wt%. Higher additions can generate slurry viscosities above 3000 mPa·s at 60 °C, a range that reduces pressure-nozzle atomization quality in counter-current spray towers. Single-fluid pressure nozzles operating at 30–80 bar are particularly sensitive to viscosity swings; tower wall build-up and coarse-particle fraction increase when slurry viscosity exceeds the nozzle manufacturer’s recommended limit. The metasilicate provides alkaline reserve during crutcher mixing and complexes calcium and magnesium hardness, reducing free soap precipitation in tallow/coconut formulations. Slurry moisture, crutcher temperature, and metasilicate addition interact: at 60 °C, a slurry containing 65 wt% total solids and 8 wt% metasilicate on dry solids may exhibit viscosity of 1200–2500 mPa·s, while increasing total solids to 70 wt% can push viscosity above 4000 mPa·s. Spray tower operators commonly use Brookfield viscometry at 60 °C as a release criterion.

Mechanical pulp bleaching lines use liquid sodium metasilicate as a hydrogen peroxide stabilizer. The product is metered into bleach liquor at 0.5–3.0 wt% on oven-dry pulp, with target pH between 10.5 and 11.2. Manganese-catalyzed peroxide loss is suppressed by silicate sequestration, but the stabilization mechanism requires a minimum residual SiO₂ in the liquor; below 0.2 g/L SiO₂, peroxide decomposition accelerates. Process control is constrained by two competing failure modes: pH below 10.5 accelerates peroxide decomposition, while pH above 11.8 can increase anionic trash mobilization and pulp darkening. Sodium hydroxide is co-fed to trim pH, while the metasilicate supplies buffering and metal complexation. Excess silicate carryover can combine with alum at pH 4.5–5.0 to form colloidal silica-alumina deposits on forming fabrics. Alkaline peroxide mechanical pulp lines therefore monitor pressate silica and maintain washing efficiency above 90%; deposition risk increases when pressate SiO₂ exceeds 150 mg/L.

Condensate line corrosion protection is another documented use. Liquid sodium metasilicate is fed continuously at 2–15 mg/L as SiO₂ in boiler feedwater; silica forms a thin protective film on steel surfaces after several days of passivation. Film formation is most effective when feedwater pH is maintained between 8.5 and 9.5 and dissolved oxygen is scavenged to ≤20 µg/L. Overdosing above 20 mg/L SiO₂ in high-pressure boilers exceeding 40 bar creates risk of silicate deposition in high-heat-flux zones, where silica solubility is driven by pressure and pH. The residual silica is measured by ASTM D859-16 colorimetric analysis after filtration to avoid bias from precipitated silica.

Transport classification is not as severe as concentrated caustic; however, the product is alkaline and causes eye damage. GHS classification is Skin Corr. 1B, Eye Dam. 1, with neat-product pH above 13.5. In the European Union, the REACH registration requires exposure scenarios for industrial spraying and container cleaning. Compared with solid sodium metasilicate pentahydrate, the liquid product eliminates dust exposure and exothermic dissolution, but introduces a freight mass penalty: a 40 wt% liquid delivers approximately 40 kg active solids per 100 kg shipment, whereas the pentahydrate contains approximately 57 wt% active Na₂SiO₃ after water of crystallization. Model choice therefore turns on plant handling capacity, dust-control objective, and freight economics.