There are two separate autoignition metrics for sulfur dust, which apply to different scenarios inside grinding equipment: dust layer autoignition temperature (MAIT) and suspended dust cloud autoignition temperature. Particle size strongly shifts these values.
1. Minimum Autoignition Temperature of Sulfur Dust Layers (MAIT – Highest Practical Risk)
This is the temperature of a hot metal surface that ignites settled, stacked sulfur powder (caked deposits on liners, classifier rotors, duct walls, dust collectors).
- Ultra-fine sulfur (1–10 μm, mill finished powder): 220 ~ 248 °C
- Medium sulfur (30–75 μm, rubber grade): 250 ~ 270 °C
- Coarse granular sulfur (>100 μm raw feed): 280 ~ 310 °C
Key risk note:
Sulfur melts at 112–119 °C. Molten sulfur adheres tightly to hot surfaces, forming insulated thick dust layers that trap heat. The trapped heat slowly climbs toward the 220 °C autoignition threshold for fine sulfur, creating delayed smoldering or ignition even after the mill stops running.
2. Autoignition Temperature of Suspended Sulfur Dust Clouds
For airborne sulfur dust-air explosive mixtures flowing through ducts or chambers:
- Standard range: 330 ~ 360 °C
This value is much higher than the layer ignition temperature. Hot surfaces are a far greater ignition hazard than hot gas clouds under normal milling operating temperatures.
3. Smoldering Pre-Ignition Threshold (Critical Warning Marker)
When caked sulfur deposits reach 180 °C and above, slow exothermic smoldering begins, releasing sulfur dioxide gas. Smoldering material will instantly flare into a dust explosion once airflow vibration disturbs the layer and creates a suspended dust cloud.
4. Operational Safe Temperature Limits for Sulfur Mills
To avoid approaching autoignition risks:
- Normal continuous running chamber temperature: ≤90 °C (cooling system required)
- High-temperature alarm interlock: 95 °C (automatically reduce feed)
- Emergency shutdown trip point: 100 °C (prevent sulfur melting)
- Any surface over 180 °C requires immediate equipment stop for inspection and cleaning.
Quick Reference Summary
| Sulfur Form | Dust Layer Autoignition Temp | Suspended Dust Cloud Autoignition Temp |
|---|---|---|
| Ultra-fine 1–10 μm sulfur | 220–248 °C | 330–360 °C |
| Medium 30–75 μm sulfur | 250–270 °C | 340–360 °C |
| Coarse >100 μm sulfur | 280–310 °C | 350–370 °C |
