Sodium Metasilicate Anhydrous

Sodium metasilicate anhydrous (CAS 6834-92-0, EINECS 229-912-9) is a non-hydrated crystalline sodium silicate with a molecular weight of 122.06 g/mol and a Na₂O:SiO₂ molar ratio of 1.00 ± 0.02. Commercial material is supplied as white granules, powder, or pellets under product codes that typically differentiate particle-size distribution, bulk density, and packaging type. Because it contains no water of crystallization, it delivers approximately 1.7 times the active Na₂O of sodium metasilicate pentahydrate on an equal-mass basis. The product is used as a high-alkaline builder in industrial cleaning, a source of silicate for corrosion inhibition, a peroxide stabilizer in textile and pulp bleaching, a concrete densifier, and a water treatment chemical. Its solution pH at 1 wt% is 12.5–13.0, which places it between sodium hydroxide and sodium carbonate in aggressive alkalinity while adding silicate functionality.

Thermal exposure above 1089 °C alters the anhydrous solid but not its packaged specification

The melting point of sodium metasilicate anhydrous is approximately 1089 °C, but commercial specifications are set for material as packaged at ambient temperature. A representative certificate of analysis for industrial-grade anhydrous metasilicate is shown in Table 1.

The loss-on-ignition limit of ≤1.0 wt% is critical because water pickup during storage forms a surface hydrate layer that slows dissolution and can promote caking in bulk bins. Particle-size distribution varies by grade; granular material in the 18–35 mesh range is typically selected for batch make-down, while 100–200 mesh powder is used where rapid dissolution is required but dust control is available. Bulk density values of 0.9–1.2 kg/L reflect tapping differences and affect silo capacity calculations. A true solid density near 2.6 g/cm³ is not used for batching because the product is handled as a bulk solid.

Dissolution of the anhydrous material is slower than the pentahydrate at ambient temperature. In field make-down, a 316L stainless steel tank with a high-shear disperser operating at 900–1,200 rpm is used; the product is screened through a 500 μm mesh and added to water at 40–50 °C to avoid clumping and local gel formation. Dust handling is conducted under local exhaust ventilation because the particles are hygroscopic and can form a slippery surface film on floors and equipment.

What separates anhydrous metasilicate from pentahydrate and sodium silicate liquids?

Three commercial sodium silicate forms are compared in Table 2 for alkaline cleaning and industrial use.

Anhydrous and pentahydrate metasilicates produce essentially the same solution chemistry once dissolved, but the anhydrous grade is preferred when formulation water must be minimized or when freight costs are significant. The pentahydrate is selected for small batch operations where ambient-temperature dissolution and lower dusting outweigh the mass penalty. Liquid sodium silicate of 3.22 ratio is not a direct alkalinity replacement; its higher silica-to-alkali ratio provides film-forming and binder properties but only about one-fifth of the Na₂O content of anhydrous metasilicate. Sodium hydroxide provides a higher pH, but it does not contribute silica, and its solutions are more aggressive to ferrous and non-ferrous substrates unless inhibitors are added. Sodium carbonate and sodium tripolyphosphate are also used as builders; sodium carbonate has lower pH and less alkalinity retention, while sodium tripolyphosphate sequesters calcium and magnesium but is undesirable where phosphate-free effluent is required. Metasilicate does not sequester hardness ions; it can precipitate calcium and magnesium silicates, so softened water or a chelating co-builder is used where hardness exceeds 150 mg/L as CaCO₃.

In automatic dishwashing detergents, anhydrous metasilicate is included at 5–15 wt% to provide alkalinity and reduce glass corrosion; silicate anions adsorb on glass and reduce etching. The product is blended in dry mixing equipment with low-shear plow or ribbon mixers to avoid excessive dust. Granular forms with low fines are selected because high fines content can stratify during pneumatic transfer. The bulk density specification of 0.9–1.2 kg/L is used for packaging fill volume and silo discharge design.

In ferrous metal soak cleaning, anhydrous metasilicate is dissolved to 0.5–2.0 wt% in a 316L stainless steel tank heated to 55–75 °C. The bath saponifies fatty soils via alkaline hydrolysis while silicate oligomers adsorb onto mild steel and reduce cathodic oxygen reduction at pH above 10.5. Agitated tank systems of 3,000–5,000 L commonly use low-pressure centrifugal circulation and oil-skimming to prevent redeposition. The same solution can be applied in spray washers at 0.3–1.0 wt% if nozzle pressure is 0.7–1.4 MPa and the water hardness is below 75 mg/L as CaCO₃. Hard water can form silicate scale on heating coils and nozzles; addition of 0.1–0.3 wt% phosphonate or gluconate is used to control scale. Rinse stages require counterflow deionized water; final-rinse conductivity above 50 μS/cm can leave a silicate film that interferes with electroplating adhesion or conversion coating uniformity. Mass-loss corrosion evaluations of AISI 1018 steel panels in 1 wt% metasilicate baths at 60 °C are typically conducted per ASTM G31-72; published data for specific process configurations is limited, so plant-specific coupon testing is used.

When anhydrous metasilicate replaces sodium hydroxide in a soak cleaner

Replacement of sodium hydroxide with anhydrous metasilicate is performed on the basis of total Na₂O equivalent. One part by mass of anhydrous metasilicate provides approximately 0.65 part of NaOH-equivalent Na₂O because 50.5–51.5 wt% Na₂O in metasilicate divided by 77.5 wt% Na₂O equivalent in sodium hydroxide equals 0.65. A formulation that previously used 100 kg sodium hydroxide beads would require approximately 154 kg anhydrous metasilicate to maintain the same Na₂O reserve. The bath pH will fall from about 13.3 to 12.7, which is often sufficient for oily carbonized soils while reducing caustic stress corrosion risk in stainless steel equipment. The substitution is not appropriate when maximum saponification rate is required on heavily polymerized grease or when the bath is used on aluminum; metasilicate remains aggressively alkaline, and silicate does not render aluminum safe. In high-pressure spray washers, substitution should be limited to 30–50 wt% of the original caustic charge because higher silicate levels can increase nozzle and coil scale in hard water. Dilution must follow the product-to-water sequence; adding water to concentrated slurry can create localized gel balls that are difficult to dissolve. Total alkalinity is verified by titration per ASTM D501-03 against a known acid standard, with results reported as wt% Na₂O.

In hydrogen peroxide bleaching of cotton knitgoods and mechanical pulp, anhydrous metasilicate is used at 0.5–2.0 wt% on material weight to buffer the bath and stabilize peroxide against transition-metal catalyzed decomposition. The silicate works with sodium hydroxide to hold pH at 10.5–11.0; this window balances bleaching rate and fiber damage. Silicate forms colloidal complexes with iron and manganese and reduces peroxide decomposition, but excessive silicate creates deposits on fabric, rollers, and pump seals. A hot rinse with 0.1–0.3 wt% nonionic wetting agent at 70–80 °C is used to remove residual silica before dyeing. Because anhydrous metasilicate dissolves more slowly than pentahydrate in cold bleaching baths, it is typically predissolved in a side tank at 40–50 °C. Published data for specific cotton varietal response is limited; pilot-scale trials with the target substrate are used to set the dosage.

Concrete densification and water treatment process parameters

For concrete surface densification, anhydrous metasilicate is dissolved to 5–15 wt% and applied at 3–6 m²/L to cured, carbonation-free concrete. The silicate reacts with calcium hydroxide in capillary pores to form calcium silicate hydrate, reducing surface dusting and water absorption. Application is not performed below 4 °C or on surfaces where carbonation has depleted available calcium hydroxide. In potable water treatment, sodium silicate is used for corrosion control at 1–10 mg/L as SiO₂, but anhydrous metasilicate is less common than liquid sodium silicate because low-concentration metering requires predissolution and filtration to remove gel specks. Compliance for drinking water additives is verified under NSF/ANSI/CAN 60; the specific grade and manufacturer must be listed for potable use. In closed cooling water systems, silicate treatment can reduce mild steel corrosion, but magnesium hardness above 50 mg/L as CaCO₃ can form magnesium silicate deposits; blowdown and dispersant addition are required.

Sodium metasilicate anhydrous is hygroscopic and corrosive to skin and eyes. Store in sealed multi-wall bags with a moisture-barrier liner at 10–35 °C and RH below 60%. Do not store with strong acids, ammonium salts, reactive metals such as aluminum and zinc, or organic peroxides. Use stainless steel 316L, polypropylene, or lined carbon steel for liquid handling; avoid glass vessels for concentrated solutions because silicate solutions slowly etch glass. Dust control is required at bag dump stations under OSHA 29 CFR 1910.1200; alkaline dust is irritating to the respiratory tract. Spills should be vacuumed dry before washing because the wetted material forms a slippery gel. The product is registered under REACH Regulation (EC) No 1907/2006 and supplied with a safety data sheet that contains CLP classification and specific concentration limits.