How to Prevent Product Agglomeration in ACM Grinding Systems

Agglomeration is one of the most common quality defects during ultrafine grinding on ACM mills. Fine particles stick together via van der Waals forces, electrostatic attraction, liquid bridging (moisture) or thermal adhesion. Agglomerates behave like large particles during laser particle testing, cause wide PSD, residual coarse fractions, poor dispersibility in coatings/plastics, and increase internal recirculation load & energy consumption.

Agglomeration control requires a systematic approach covering raw material conditions, mechanical configuration, process parameters, airflow design, temperature management and post-collection handling. All guidance is based on industrial ACM operation experience referenced from acm-mill.com.

1. Understand Root Causes of Agglomeration inside ACM

1. High specific surface area of fine particles → strong inter-particle van der Waals attraction (worse for submicron & D97<10 μm powders)
2. Excess moisture in feedstock: water forms liquid bridges binding particles
3. High grinding temperature: heat-sensitive materials soften/melt and fuse together
4. Over-concentrated particle cloud in classification zone; frequent particle collision promotes adhesion
5. Static charge accumulation on powder surfaces
6. Flow turbulence creates low-speed stagnant zones on classifier blades, duct walls and cyclones
7. Over-grinding: excessive ultrafine fraction amplifies agglomeration tendency

2. Raw Material Pre-Treatment (First Line of Defence)

2.1 Strictly control feed moisture

• General mineral powders (GCC, talc, kaolin, silica): Moisture ≤0.3~0.4%
• Organic powders, resins, plant extracts: moisture target ≤0.25%
  If raw material moisture exceeds limits: install upstream drying equipment. Moisture-induced agglomerates are the hardest to break apart inside the mill.

2.2 Remove sticky impurities

Eliminate wax, humus, viscous organic contaminants. Impurities act as binders to form permanent hard agglomerates.

2.3 Maintain consistent feed particle size distribution

Extremely fine incoming material instantly forms agglomerates after entering the grinding chamber. Avoid excessive pre-fineness of feedstock.

3. Process Parameter Optimisation (Zero-Cost Adjustment)

3.1 Reduce particle concentration inside grinding & classification zone

Agglomeration accelerates sharply under high particle loading:

• Operate at 30% ~ 60% of ACM nominal feed capacity for ultrafine grades; avoid maximum throughput
• Prevent overfeeding: particle crowding increases collision frequency and adhesion

Principle: Dilute particle suspension improves dispersion before classification.

3.2 Rational matching of rotor speed and classifier speed; avoid over-grinding

Do not run rotor continuously at maximum speed. Excessive impact energy generates massive ultrafine particles that readily agglomerate.
Best practice:

1. Set rotor speed to the minimum tip speed required to achieve target particle fracture
2. Adjust fineness mainly via classifier wheel speed
3. Reduce unnecessary recirculation load: optimise airflow to extract qualified fines promptly, avoid long residence time inside the mill

3.3 Precisely stabilise airflow and utilise controlled secondary air

Secondary dry air inlet is highly effective against agglomeration:

• Low-volume, dry secondary air injects into the classification zone to disperse particle clusters
• Secondary air dilutes particle concentration and breaks loose agglomerates before separation
  Important limits: Do NOT inject excessive secondary air — it breaks system pressure balance and triggers coarse bypass.
• Secondary air must be dehumidified; wet secondary air worsens agglomeration.

3.4 Optimise airflow pressure to eliminate flow stagnation

Stagnant eddies allow fine powder to accumulate and form agglomerates on classifier blades and chamber walls:

• Seal all air leaks to maintain stable negative pressure
• Avoid extremely low airflow which causes powder sedimentation inside the mill
• Optimise fan VFD setting to prevent airflow fluctuation

4. Temperature Control (Critical for Heat-Sensitive & Ultrafine Powders)

Frictional collision generates heat inside ACM. High temperature promotes thermoplastic adhesion and intensifies static agglomeration.
Solutions:

1. Install water cooling jacket on ACM main grinding chamber
2. For organic flammable powders / low-melting resins: adopt closed-loop nitrogen circulation with gas cooling
3. Avoid long-time no-load operation; empty mill generates high temperature without material cooling effect
4. Limit continuous maximum rotor speed to cut frictional heat generation

Target operating temperature guidance:

• Mineral fillers: outlet powder temperature ＜65°C
• Organic resin, powder coating raw materials: outlet temperature ＜45°C

5. Hardware & Airflow Circuit Modifications

5.1 Optimise classifier aerodynamic design

• Use properly set backward-inclined airfoil blades (22°–26° baseline angle) to minimise inter-blade vortex stagnation zones
• Install labyrinth seals; eliminate local low-pressure dead zones where powder accumulates
• Smoothly polish classifier wheel surface to reduce powder adhesion points

5.2 Optimise pipeline layout

• Use long-radius elbows; eliminate sharp bends, sudden pipeline expansion
• Keep ducts inclined ≥12° to prevent powder deposition
• Avoid horizontal long duct sections where fine powder settles and forms agglomerates

5.3 Static elimination design

Static electricity is a major driver of fine-particle agglomeration:

1. Full continuous grounding of ACM host, cyclone, ductwork, dust collector and airlocks
2. Install static elimination bars at discharge and classification inlet zones
3. Closed-loop inert gas systems maintain controlled humidity to reduce static charging
4. Avoid insulated plastic piping in powder flow paths

5.4 Optimise powder collection system

Conventional cyclones cannot fully disperse agglomerates. Supporting improvements:

• Select membrane-coated filter bags in pulse dust collector; reduce agglomerate adhesion on filter surfaces
• Optimise pulse cleaning frequency to prevent thick fine-powder layers building up on bags

6. Optional Auxiliary Technologies for Severe Agglomeration Scenarios

1. Minor dispersant injection (for mineral filler production): Metered gaseous or low-dose liquid dispersant sprayed into grinding zone to weaken inter-particle attraction. Widely used for coating-grade GCC and kaolin.

Note: Confirm dispersant compatibility with end-use formulation.

2. Cryogenic air-assisted grinding: Inject chilled dry air for polymers, wax and heat-sensitive organics to suppress fusion agglomeration.
3. Two-stage classification: Primary separation inside ACM + external secondary classifier to re-disperse and remove agglomerates.

7. Preventive Maintenance Practices

1. Regularly clean classifier wheel blades, chamber walls and cyclone inner surfaces. Thin powder layers on surfaces act as seed sites for agglomerate growth.
2. Check for uneven blade angle: inconsistent angles create local turbulence and stagnation zones.
3. Monitor dust collector differential pressure; blinded filter bags cause airflow drift and unstable internal flow field.

8. Common Troubleshooting Checklist

1. Laser particle test shows large false coarse particles, but microscope observes loose agglomerates
   → High feed moisture OR insufficient cooling OR excessive particle concentration
   Solution: Dry feedstock, reduce feed rate, activate cooling jacket, introduce dry secondary air
2. Agglomerates stick heavily on classifier wheel
   → Inter-blade vortex due to improper blade angle or airflow imbalance
   Solution: Adjust blade angle; stabilise system negative pressure
3. Agglomeration worsens when producing finer grades
   → Increased specific surface area; over-grinding generating excess ultrafines
   Solution: Lower rotor speed moderately, optimise airflow to extract fines quickly
4. Agglomeration only appears under low humidity conditions
   → Static charge accumulation
   Solution: Install static eliminators; adjust minor gas humidity inside closed circuit

9. Conclusion – Priority Implementation Sequence

1. Control raw material moisture (fundamental requirement)
2. Reduce feed concentration; stabilise system temperature
3. Utilise dry secondary air to disperse particle clusters
4. Optimise classifier blade angle and airflow field to eliminate stagnant vortex zones
5. Implement static elimination and full equipment grounding
6. Avoid over-grinding by balanced rotor/classifier speed matching

If agglomeration cannot be eliminated via process tuning alone, introduce hardware upgrades (cooling jackets, static eliminators) or auxiliary dispersion technologies.