Fumed Silica Production — Flame Hydrolysis of SiCl₄
title: "Fumed Silica Production — Flame Hydrolysis of SiCl₄" description: "How silicon tetrachloride is converted into fumed silica by flame hydrolysis, the role of primary particle size, and how production parameters determine grade properties." section: "upstream"
The Flame Hydrolysis Process
Fumed silica is produced by a single continuous reaction — flame hydrolysis of silicon tetrachloride (SiCl₄):
SiCl₄ + 2H₂ + O₂ → SiO₂ + 4HCl
SiCl₄ vapour is injected into a hydrogen-oxygen flame at 1000–1800°C. The extreme temperature causes immediate hydrolysis and oxidation, nucleating tiny amorphous SiO₂ particles of 5–50 nm primary diameter. These primary particles fuse irreversibly into aggregates (100–300 nm) and loosely entangle into agglomerates (1–100 µm). The resulting white powder is collected in cyclones and bag filters, then treated with dry HCl/air to remove residual chloride before bagging.
The entire process is gas-phase — no water contact, no milling — which yields the defining properties of fumed silica: extreme purity (SiO₂ ≥99.8%), high specific surface area, and amorphous structure.
From SiCl₄ to Fumed Silica: Key Parameters
| Process parameter | Effect on product |
|---|---|
| H₂/O₂ ratio | Controls flame temperature; affects particle size and hydroxyl density |
| SiCl₄ feed rate | Determines production rate; too high → coarser particles |
| Flame temperature | Higher T → lower surface area (larger primary particles) |
| Residence time | Longer → more aggregate fusion, harder to disperse |
| Post-treatment | Surface modification (e.g., HMDS → hydrophobic R972/R805 grades) |
Adjusting these parameters allows producers to dial surface areas from ~90 m²/g (coarse grades like Aerosil 90) to ~380 m²/g (ultra-fine Aerosil 380) with corresponding changes in rheology-control potency and moisture absorption.
Surface Area and Grade Range
The BET surface area (m²/g) is the primary grade descriptor:
| BET (m²/g) | Typical grade | Main applications |
|---|---|---|
| 90–130 | Aerosil 90, CAB-O-SIL M-5 | Silicone rubber reinforcement, coatings |
| 150–200 | Aerosil 200, HDK N20 | Adhesives, sealants, thickening |
| 250–300 | Aerosil 300 | High-performance coatings, pharmaceuticals |
| 380+ | Aerosil 380 | Ultra-fine thickening, specialty applications |
| Hydrophobic | R972, R805, R974 | Toner, masterbatch, waterproof coatings |
See the full grade comparison at Fumed Silica products.
Where SiCl₄ Comes From
The primary source of SiCl₄ for fumed silica is a by-product of the polysilicon and silane coupling agent industries. Fumed silica producers are therefore closely linked to the semiconductor/chemical supply chain — SiCl₄ availability and price directly affect fumed silica costs.
Applications where fumed silica is critical: silicone sealant and RTV reinforcement, coatings rheology, pharmaceutical excipient, inkjet paper coating, anti-caking in powder systems. See Coatings industry.