Precipitated Silica Production
title: "Precipitated Silica Production" description: "How sodium silicate reacts with sulfuric acid to produce precipitated silica, the difference from fumed silica, and key applications in tires, rubber, and food." section: "upstream"
Wet Chemistry Route
Unlike fumed silica (gas-phase process), precipitated silica is made entirely in aqueous solution through a neutralisation reaction:
Na₂SiO₃ + H₂SO₄ → SiO₂↓ + Na₂SO₄ + H₂O
Sodium silicate (waterglass, Na₂SiO₃) — itself made by fusing quartz sand with sodium carbonate — reacts with dilute sulfuric acid in a stirred reactor. Precipitation conditions (temperature, pH, acid addition rate, agitation) govern the primary particle size, aggregate structure, and porosity of the resulting silica cake.
After precipitation, the slurry is filtered, washed to remove sodium sulfate, then dried by spray drying or flash drying. The final powder is classified and, for rubber grades, surface-treated with silane to improve compatibility with the polymer matrix.
Process Variables and Grade Outcomes
| Variable | Low value → | High value → |
|---|---|---|
| Precipitation temperature | Finer particles, higher surface area | Coarser particles, lower surface area |
| Final pH | More aggregated, higher structure | More primary particles, lower structure |
| Acid addition rate | Slow → more controlled structure | Fast → broad PSD, higher surface area |
| Drying method | Spray → granular, free-flowing | Flash/filter cake → irregular aggregates |
Key Applications and Grade Requirements
| Application | BET (m²/g) | CTAB (m²/g) | Key spec |
|---|---|---|---|
| Green tire tread compound | 160–200 | 155–185 | Low rolling resistance (CTAB/N₂ ratio) |
| Mechanical rubber goods | 120–160 | 110–150 | Reinforcement + process viscosity |
| Oral care / toothpaste | 40–120 | — | RDA abrasion ≤200, tasteless, food-grade |
| Food anti-caking (E551) | 150–300 | — | High absorption, free-flowing |
| Polymer carrier / masterbatch | 170–220 | — | High oil absorption |
The green tire application is the largest and fastest-growing use — silica-filled tread compounds reduce rolling resistance (better fuel economy) versus carbon black while maintaining wet grip. CTAB surface area (measuring rubber-accessible surface) is the key technical metric here, not just N₂ BET.
Fumed vs Precipitated Silica
| Property | Fumed silica | Precipitated silica |
|---|---|---|
| Process | Gas-phase (SiCl₄ flame hydrolysis) | Wet chemistry (precipitation) |
| Purity | ≥99.8% SiO₂ | 94–98% SiO₂ |
| Surface area | 90–380 m²/g | 40–250 m²/g |
| Structure | Low porosity, non-porous | Mesoporous (2–50 nm pores) |
| Primary cost driver | SiCl₄ price | Sodium silicate + H₂SO₄ price |
| Cost | Premium | Commodity to mid-range |
| Key use | Coatings rheology, silicone reinforcement | Tires, rubber, oral care |