1. Main Classification Technologies Overview
Silica powder classification splits fine silica into different particle fractions per target D50/D97; two mainstream routes: dry air classification and wet hydraulic classification, matched with particle fineness (coarse >100 μm, medium 10–100 μm, fine 1–10 μm, ultrafine <1 μm).
1. Dry Classification (For dry silica powder, no water input)
(1) Turbo Air Classifier (Most widely used for industrial fine silica, D97:2–45μm)
- Working principle: High-speed rotating classification wheel generates centrifugal force; coarse silica particles overcome airflow drag and drop back to grinding chamber, fine particles follow updraft air into cyclone & bag collector for finished collection.
- Adjustable control parameters:
- Classifier rotor speed: Higher speed = finer cut size; reduce rotor RPM for coarser fraction.
- System air volume: Larger air intake carries more fine powder to increase cut point.
- Feed rate: Overfeeding causes agglomeration & oversize leakage.
- Typical application: Quartz/silica for filler, glass raw material, precipitated silica coarse grading.
(2) Vibrating Screen Classification (Coarse silica ≥45 μm, D97＞50μm)
- Mesh size determines cut particle size; dry silica passes through appointed screen aperture, oversize remains on screen.
- Screen options: Polyurethane screen (low iron contamination for high-purity silica) instead of metal woven mesh.
- Limitation: Poor efficiency below 45μm, agglomerated fine silica easily blocks screen holes.
(3) Gravity Air Separator (Coarse-medium silica 30–150μm)
Rely on natural settling speed difference in vertical airflow; low cost for low-precision rough classification.
2. Wet Classification (Slurry silica, D97:0.3–20μm, high precision for electronic/ultrafine silica)
(1) Hydrocyclone Classification (Core wet grading equipment for silica slurry)
- Principle: Pump silica slurry into cyclone tangentially; centrifugal field separates heavy coarse silica downward as underflow, ultrafine overflow from top outlet.
- Key tuning items for target cut size:
- Feed pressure: Higher inlet pressure → finer separation cut point.
- Spigot / vortex finder dimension: Shorten vortex finder or shrink spigot to collect finer overflow silica.
- Solid content of silica slurry: Normally controlled 20–45wt%; too high solid causes particle agglomeration & inaccurate grading.
- Combination mode: Multi-stage cyclone cascading for multi-fraction silica grading.
(2) Sedimentation Classification (Gravity settling, lab & small batch high-purity silica)
Based on Stokes’ law: Different-size silica particles sink at distinct settling velocity in static water; separate supernatant fine powder after scheduled standing time, collect settled coarse fraction.
Shortcoming: Low productivity, only for small-batch high-end electronic silica powder.
(3) Wet Spiral Classifier (Coarse wet silica＞75μm)
Screw pushes settled coarse silica upward for discharge; fine suspended silica flows out from overflow end, applied after wet coarse grinding of silica sand.
3. Classification Matching By Target Silica Fineness
| Target Particle Size | Recommended Classification Method |
|---|---|
| ≥100 μm coarse silica sand | Polyurethane vibrating screen |
| 30~100 μm medium silica powder | Dry air classifier + vibrating screen |
| 2~30 μm fine industrial silica | Turbo dry air classifier |
| 0.5~5 μm ultrafine electronic silica | Multi-stage hydrocyclone wet classification |
| <0.5 μm nano silica | Centrifugal sedimentation / high-efficiency wet ultra-fine classifier |
4. Critical Operation Tips To Improve Classification Precision of Silica
- Eliminate silica agglomeration: Add dispersant (low-ion dispersant for high-purity silica without metal impurity) in wet slurry; dehumidify raw dry silica below 1.5% moisture before dry classification to avoid particle sticking.
- Pre-remove tramp iron/impurity: Pass raw silica through magnetic separator ahead of grading, prevent hard iron fragments from damaging classifier rotors/screens.
- Closed-circuit grinding & classification: Combine classifier with silica mill; unqualified oversize silica returns back for regrinding to stabilize finished particle distribution.
5. Fineness Inspection Standard After Classification
Verify graded silica size via laser particle size analyzer (main industrial testing), sieve analysis for coarse grades, sedimentation method for ultrafine wet silica to confirm D50/D97 meets specification.
