How to Automate Particle Size Control in a Quartz Grinding Circuit

Manual operation of closed-circuit grinding-classification circuits leads to fluctuating D50, D97 and wide particle size distribution (PSD). For quartz powder used in epoxy resin and electronic packaging, unstable fineness directly causes inconsistent filling viscosity, reduced mechanical strength and batch-to-batch quality deviation.

Automated particle size control builds a closed-loop control system: online measurement → PLC algorithm adjustment → real-time modification of equipment operating parameters, maintaining stable target cut size (e.g. D97=10 μm).

1. Core System Architecture of Automatic Particle Size Control

A complete automation framework consists of five parts:

1. Online particle size analyser (real-time measurement terminal)
2. Central PLC & HMI control platform
3. Frequency converters for classifier, feeding fan and raw material feeder
4. Sensors: flow, pressure, temperature, vibration, power monitoring
5. PID / advanced multi-variable control algorithm

Control logic overview:
Online laser particle size data → compare with set target D97 → PLC automatically adjust classifier rotor speed, system air volume, raw feed rate to pull fineness back to set value.

2. Selection of Online Particle Size Detection Equipment

Lab laser particle size instruments cannot support continuous automatic control. Only industrial online analysers are suitable.

2.1 Dry online laser particle size analyser (Recommended for dry quartz grinding circuit)

Install position: finished powder outlet after dust collector or classifier fine product pipeline.

• Directly measure real-time D50, D97 and full PSD
• Continuous sampling without manual intervention
• Adapt to negative pressure dry powder airflow
• Output 4–20mA or Modbus signal to PLC

2.2 Key installation tips

• Install automatic purging nozzle to prevent quartz powder adhesion on optical window
• Avoid locations with unstable powder concentration or airflow turbulence
• Configure bypass pipeline for offline calibration without stopping production

Alternative indirect monitoring (only auxiliary, cannot be used alone):
Classifier motor load, mill power, return coarse material flow rate. These parameters have correlation with fineness but cannot give accurate particle size value. Must cooperate with online laser analyser for precise closed-loop control.

3. Adjustable Control Variables (Main tuning parameters for quartz circuit)

For typical configuration: Grinding Mill + Vertical Dynamic Air Classifier

Priority 1: Classifier rotor rotational speed (Primary control parameter)

Rule:

• Increase rotor speed → finer separation cut point (smaller D97)
• Reduce rotor speed → coarser finished powder
  This is the fastest response parameter for particle size adjustment.
  VFD frequency drive is mandatory to achieve continuous stepless speed regulation.

Priority 2: System circulating air volume (Secondary parameter)

Adjust main fan frequency to change airflow inside classifier:

• Higher airflow → more fine powder carried out, tendency to coarser D97
• Lower airflow → only ultra-fine powder escapes, tendency to finer D97

Priority 3: Raw material feeding rate

• Overfeeding: mill overload, incomplete grinding, finished powder becomes coarser
• Too low feed: over-grinding, excess ultrafine fractions, shifted PSD
  The system automatically limits feed rate when fineness deviation exceeds threshold.

Priority 4: Classifier secondary air volume

Stable secondary air ensures sharp classification curve. Automatic air damper control keeps secondary airflow constant to avoid fluctuation of PSD width.

4. Closed-Loop Automatic Control Strategy

4.1 Basic PID control (Standard configuration for most turnkey quartz plants)

1. Operator sets target value (e.g. D97 = 10.0 μm) on HMI
2. Online analyser sends measured D97 to PLC every 10–30 seconds
3. PLC calculates deviation: Actual D97 − Target D97
4. PID controller outputs signal to adjust classifier rotor speed
5. After parameter adjustment, wait for system delay, then re-measure particle size and loop again

4.2 Advanced multi-variable cascade control (High-end electronic-grade silica line)

Quartz grinding has obvious time lag. Single PID easily causes oscillation (fineness repeatedly swings between coarse and fine).
Cascade control logic:

• Inner loop: Stabilize mill load & classifier airflow
• Outer loop: Particle size closed-loop control
  When feed ore hardness or particle size fluctuates, the system first adjusts feeding quantity to stabilise mill working state, then fine-tunes classifier speed. Greatly reduces overshoot and oscillation.

4.3 Automatic interlock protection logic

Set upper and lower alarm limits for D97:

• If D97 continuously exceeds upper limit (too coarse): trigger alarm, automatically slow down feeding and raise classifier speed
• If D97 continuously below lower limit (over-fine): reduce classifier speed, avoid excessive energy waste
• Extreme deviation: automatic diversion valve switches off-spec powder into separate off-spec silo, preventing mixing into qualified finished product

5. Recommended Sensor Layout for Dry Closed-Circuit Quartz Line

1. Raw material silo: continuous loss-in-weight feeder with frequency control
2. Grinding mill: power transmitter, temperature sensor, vibration sensor
3. Classifier: rotor speed feedback, differential pressure sensor, secondary air electric damper actuator
4. Main exhaust fan: frequency converter + airflow monitoring
5. Coarse return conveyor: flow sensor (monitor circulating load)
6. Fine powder pipeline: online dry laser particle size analyser + automatic sampling purge unit
7. Finished powder silo: diversion valve for qualified / unqualified powder separation

6. Optimisation Tips for Quartz Silica Production

1. Compensate system lag
   Material needs 60–180 seconds to travel from classifier to particle size measuring point. Add lag compensation in PLC program to prevent over-adjustment.
2. Set different parameter recipes
   Store multiple preset formulas on HMI:

• Recipe 1: D97=10 μm (epoxy electronic filler)
• Recipe 2: D97=25 μm (general coating silica)
  One-click switching of target fineness, without manually resetting all parameters.

3. Regular automatic calibration
   Schedule daily automatic zero calibration of online particle analyser during short production breaks to eliminate drift caused by dust contamination.
4. Combine circulating load monitoring
   High circulating load means many qualified fine particles return to mill, low efficiency. The automation system can balance fineness target and grinding energy consumption to reduce power cost.

7. Common Failures & Avoidable Mistakes

1. Relying only on classifier power or mill current without online particle size instrument
   → Cannot achieve true particle size closed-loop control, only semi-automation.
2. Overly aggressive PID parameters
   → System oscillates; D97 fluctuates up and down continuously.
3. Poor sampling condition for online analyser, frequent optical window fouling
   → Measurement data distortion, wrong automatic adjustments. Must configure automatic air purging.
4. No off-spec diversion design
   → Out-of-standard powder mixes with qualified products, leading to customer complaints for epoxy formulation.

8. Typical Automatic Control Workflow Example (Target D97=10 μm Silica)

1. Set target D97 =10 μm on HMI
2. System runs at initial stable parameters
3. Online analyser feeds back measured D97=11.3 μm (too coarse)
4. PLC PID increases classifier rotor speed
5. After 2 minutes delay, re-detection D97=10.2 μm
6. Minor fine-tuning until value stabilises within tolerance ±0.3 μm
7. If incoming quartz hardness rises and D97 drifts repeatedly, the system slightly reduces raw feed rate as auxiliary adjustment

Fully automatic particle size control for quartz grinding circuits relies on dry online laser particle size analyser + PLC closed-loop PID / cascade control. The system takes classifier rotor speed as primary regulating variable, supplemented by airflow and feeding rate adjustment.

For manufacturers producing consistent epoxy-grade silica powder, this automation solution minimises manual operation errors, stabilises PSD, improves powder yield and reduces waste of off-spec material. When designing a turnkey quartz plant, reserve signal interfaces and installation position for online particle size detection in advance.