Why Material Sticks to Conveyor Belts
Material carryback β?ore, coal or mineral fines that stick to the belt underside and carry past the head pulley β?is one of the most expensive and persistent problems in bulk material handling. It causes spillage under the return run, material buildup on return rollers (leading to mistracking and roller failure), and accelerated belt wear on the underside.
The Root Causes
1. Material Moisture and Clay Content
Wet, fine material develops adhesive properties at certain moisture levels. Clay minerals (kaolinite, montmorillonite) are particularly adhesive when wet. Coal fines mixed with moisture form a paste that bonds strongly to rubber belt covers. The adhesion force can exceed 5 kPa, meaning a standard wiper-type scraper cannot physically remove it without very high blade pressure.
2. Insufficient Scraper Coverage
Many conveyors have only a primary scraper at the head pulley. A single scraper typically removes 70β?5% of carryback. For sticky materials, this leaves significant material still adhering to the belt. Secondary and tertiary scrapers are needed to achieve 95β?9% removal.
3. Worn or Incorrect Scraper Blades
A scraper blade that has worn to a rounded profile loses its cleaning effectiveness dramatically. The blade must have a sharp leading edge to cut under the material film. Polyurethane blades last longer than rubber on abrasive ores but may be less effective on very sticky coal.
Scraper System Design
| Scraper Position | Type | Removes | Blade Material |
|---|---|---|---|
| Head pulley (primary) | Tungsten carbide tip or PU | Bulk carryback (70β?5%) | Tungsten carbide / PU 70 Shore |
| After head pulley (secondary) | Multi-blade return | Residual film (additional 10β?2%) | PU 60β?0 Shore A |
| Before tail pulley (tertiary) | V-plow or diagonal | Any remaining fines | Rubber or PU |
| Return strand (diagonal plow) | Passive V-plow | Material fallen onto return | Rubber |
Belt Washing Systems
For extremely sticky materials (bauxite, some coal types, mineral sands with clay), mechanical scrapers alone are insufficient. Belt washing systems use pressurised water jets to flush material from the belt surface, followed by rubber squeegees to remove water. The wash water is collected and pumped to a settling pond.
Water consumption: typically 2β? L/min per metre of belt width. A 1,200mm belt washer uses approximately 2.5β? mΒ³/hour of water.
π‘ Low-Friction Cover Compounds
For persistently sticky applications, consider specifying a low-surface-energy cover compound for the belt underside. These compounds (typically modified SBR with PTFE additives) reduce adhesion of wet fines by 30β?0% compared to standard cover rubber. The top cover remains standard Grade M or W for abrasion resistance.
Roller Self-Cleaning Designs
Even with good belt scrapers, fine material will accumulate on return rollers over time. Self-cleaning roller designs break up material buildup:
- Rubber disc rollers: Alternating rubber discs with gaps. Material falls through gaps rather than building up. Best for coarse, non-cohesive materials.
- Spiral (helix) rollers: Continuous spiral profile rotates material off the roller. Effective for sticky clay-bearing materials.
- Garland (catenary) rollers: Flexible roller string that vibrates during operation, preventing material adhesion.
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