How to recycle oversized silica particles in the grinding process

Oversized silica particles are coarse fractions rejected by the dynamic classifier, which fail to meet D97 top‑cut requirements. In silica jet‑mill lines, effective recycling of these oversize particles is the core of closed‑loop grinding. Proper recycling eliminates grit residues, improves material yield, protects precipitated‑silica native aggregate structure, and avoids over‑grinding damage to BET and pore volume. Recycling strategies differ between integrated internal recirculation and external separate recycling workflows.

1. Internal closed‑loop recirculation (standard for fluidized‑bed jet mill)

This is the most widely‑adopted recycling method for precipitated silica. It is built into the mill body without extra conveying equipment.

• Working principle: After classification, centrifugal force throws oversized coarse silica particles toward the chamber wall. Under gravity and downward gas flow guidance, coarse particles fall directly back into the fluidized‑bed grinding zone, mixing with fresh incoming feed for another round of particle‑to‑particle collision and de‑agglomeration. No intermediate discharge occurs.
• Process tuning for recycling efficiency
  1. Adjust classifier rotor speed: Higher speed increases the amount of rejected oversize material and raises the recirculation ratio.
  2. Optimize grinding air pressure: Match pressure to break recycled agglomerates. Avoid excessive pressure that shatters silica primary aggregates.
  3. Control total system airflow: Too‑high airflow carries coarse particles through the classifier; too‑low airflow reduces throughput.
• Advantages: Compact layout, no extra handling equipment, fully enclosed negative‑pressure operation, low risk of dust escape or moisture‑induced re‑agglomeration.
• Limitations: Hard impurities such as iron‑rich mineral grains keep circulating inside the chamber. Continuous accumulation will hurt silica whiteness; upstream magnetic separation is required. Excessively high recirculation loading causes particle crowding and reduces grinding efficiency.

2. External recycling circuit

Applied when internal recirculation cannot satisfy production demands, or for retrofitted open‑circuit grinding systems. Oversized particles are discharged first, then transported back to the mill feeding inlet.

• Typical workflow
  1. Oversized fractions rejected by the classifier are collected via cyclone or special coarse‑discharge rotary valve.
  2. Recovered coarse silica is delivered by screw conveyor or pneumatic conveying back to the raw‑material silo or loss‑in‑weight feeder inlet.
  3. Mix with fresh raw feed and enter the grinding chamber for reprocessing.
• Key technical points for external recycling
  • Keep the whole recycling pipeline under negative pressure to prevent dust leakage and moisture absorption. Moisture will create sticky agglomerates that are hard to de‑agglomerate.
  • Install periodic sampling ports for recycled coarse powder, to monitor agglomerate status and impurity content.
  • Avoid continuous accumulation of coarse material in transfer equipment; prevent bridging and blockage.
• Suitable scenarios: Open‑circuit mill upgrade, high‑capacity production lines, situations requiring periodic partial discharge of hard impurities.
• Drawbacks: Extra equipment investment; more transfer points bring potential contamination risk.

3. Process parameters to optimize recycling performance

Recirculation ratio control

Recirculation ratio = mass flow of recycled oversized particles / mass flow of finished qualified product.

• High recirculation ratio: Strict D97 top‑cut, fewer residual grits. However, too high ratio increases internal powder loading, may trigger secondary re‑agglomeration and lower production throughput.
• Low recirculation ratio: Higher throughput, but some coarse particles may escape classification, worsening D97.
  For rubber‑grade precipitated silica, engineers balance recirculation ratio, feed rate and grinding pressure to preserve BET value.

Feeding rate matching

Over‑feeding leads to excessive total material inside the grinding chamber. Classification efficiency drops; recycled coarse fractions cannot be fully de‑agglomerated. Stable loss‑in‑weight feeding is essential for consistent recycling performance.

Compressed‑air quality

Dry, oil‑free compressed air keeps recycled silica from becoming sticky. Moist recycled agglomerates cannot be efficiently broken, and will keep circulating without size reduction.

4. Troubleshooting common recycling‑related issues

1. Oversized particles keep circulating and cannot be reduced in size
   Possible causes: Recycled fractions contain hard mineral impurities; grinding pressure is insufficient; nozzle wear reduces jet kinetic energy.
   Actions: Check nozzle condition; increase grinding pressure moderately; remove hard impurities via pre‑separation.
2. D97 still fails even with high recirculation ratio
   Possible causes: Classifier blade wear creates clearance gaps; system air leakage disturbs flow field; excessive internal powder loading.
   Actions: Inspect classifier wear parts; check sealing points; lower feed rate to reduce chamber loading.
3. Recycled material causes secondary agglomeration
   Possible causes: High moisture in process air; too‑much ultra‑fine powder accumulated in circulation loop.
   Actions: Improve compressed‑air dew‑point; adjust recirculation ratio to reduce internal fine‑particle buildup.

5. What to do with non‑recyclable oversize fractions

A small portion of oversized material may contain non‑breakable hard mineral impurities. Do not endlessly recycle them. Periodically bleed‑off a small stream of coarse material to purge impurities out of the loop, protecting final silica whiteness. This bleed flow can be sent to low‑grade filler production instead of discarding, improving overall material utilization.

Recycling oversized silica particles mainly adopts internal closed‑loop gravity return for fluidized‑bed jet mills, or external conveying recycling for modified open‑circuit lines. The core is reasonable control of recirculation ratio, matching feed rate and grinding parameters, while preventing accumulation of hard impurities. Effective recycling achieves tight D97 specification and high material yield, without sacrificing the critical porous aggregate structure of precipitated silica.