Nano-silica is widely used in coatings, composite fillers, ceramics and electronic encapsulation. The sol‑gel route using quartz sand as raw material is a low-cost method to produce high-purity nano-silica. The whole process includes quartz sand pretreatment, alkali dissolution, purification, sol preparation, gelation, aging, washing, drying and calcination.
1. Raw Material Pretreatment of Quartz Sand
High-purity quartz sand is first crushed and wet-ground to fine powder, then purified through iron removal and mineral impurity separation. The particle size of quartz powder is usually milled to D50 below 10 μm to speed up alkali leaching. Remove surface organic contaminants and heavy metal impurities via acid washing and rinsing with deionized water, then dry completely. This step reduces impurities that will affect the final nano-silica purity.
2. Alkali Dissolution to Prepare Sodium Silicate Solution
Treat purified quartz powder with sodium hydroxide solution under heating and stirring. The reaction produces sodium silicate (water glass):
$$\ce{SiO2 + 2NaOH -> Na2SiO3 + H2O}$$
Control reaction temperature, solid-liquid ratio and stirring speed. Insoluble residues are filtered out to obtain crude sodium silicate liquor. Further filter and refine the solution to remove residual solid impurities.
3. Sol Preparation and PH Adjustment
Dilute the purified sodium silicate solution with deionized water to the target silica concentration. Add dilute acid (such as hydrochloric acid or sulfuric acid) slowly under continuous stirring to adjust pH. The silicate ions hydrolyze and form hydrated silica colloidal particles, forming a transparent silica sol.
Key control parameters: acid adding rate, stirring intensity and temperature. Fast acid addition causes uneven particle growth and broad particle size distribution.
4. Gelation and Aging
Keep the sol under constant temperature and static or mild stirring. The colloidal particles collide and crosslink to form a continuous silica hydrogel network.
Aging is critical: maintain temperature for several hours to days. Aging strengthens the gel network, improves particle uniformity and reduces pore shrinkage in later drying.
5. Washing and Impurity Removal
Wash the hydrogel repeatedly with deionized water to remove residual sodium ions, sulfate or chloride ions. High residual salt will cause hard agglomeration of nano-silica. Conduct conductivity tests to monitor washing endpoint until the filtrate conductivity meets requirements.
6. Drying and Calcination
Dry the washed hydrogel at low temperature to remove free water. Slow drying minimizes capillary stress that causes hard agglomeration.
Then calcine the dried silica xerogel at 400–600 °C to remove hydroxyl groups and organic residues. The product is loose nano-silica powder with particle size typically 10–100 nm.
7. Post-Treatment and Modification (Optional)
For applications in polymers or coatings, surface modification can be performed during or after synthesis using silane coupling agents. Modification reduces hydroxyl groups on the silica surface and improves dispersion in organic matrices.
Key Process Challenges
- Impurity control: Iron, aluminum and other impurities from quartz sand must be removed in the early stage to avoid discoloration and performance defects.
- Agglomeration prevention: Rapid drying and high salt content lead to irreversible hard agglomeration of nano particles.
- Particle size control: pH, temperature and silica concentration directly determine final nano particle dimension.
The sol-gel method converts low-cost quartz sand into high-value nano-silica via alkali extraction, hydrolysis, gelation and thermal treatment. Precise control of pH, temperature and washing quality is required to achieve uniform nano particle size and low agglomeration. This route is suitable for industrial batch production of high-purity nano-silica for filler and advanced material applications.
