Background
A coal washing plant in South Kalimantan, Indonesia was experiencing persistent blinding of the dewatering screen panels on its final product dewatering screens. The screens were dewatering washed coal fines (sub-6mm fraction) before the product went to the conveyor for stockpile and barge loading.
The plant used wire mesh panels with 1mm apertures for dewatering. Blinding β where fine particles wedge into the aperture openings and block them β was causing the screens to lose dewatering efficiency within 2β4 hours of operation. Operators were water-washing the screens every shift to restore throughput. Panel replacement was needed every 3β4 weeks.
Problem Analysis
Blinding of dewatering screens on fine coal is a well-documented problem. The mechanism is:
- Fine coal particles (especially the 0.5β2mm fraction) have dimensions close to the aperture size
- In wet conditions, surface tension of water holds particles in the aperture
- Wire mesh apertures are rigid β they cannot flex to dislodge wedged particles
- The result is progressive aperture blocking that reduces open area and dewatering efficiency
Wire mesh has inherently limited resistance to this blinding mechanism because the wire strands are rigid. Polyurethane panels offer an alternative because the PU material flexes with each screen stroke, and specifically designed anti-blinding panel geometries (crowned or wedge-wire profile panels) discourage particle wedging.
Trial with PU Dewatering Panels
Polyurethane dewatering panels with 1mm slot apertures (elongated slots rather than square apertures) and a crowned (domed) panel surface were trialed on one of the four dewatering screens.
The slot aperture geometry reduces blinding compared to square holes because particles are less likely to wedge in elongated slots. The crowned surface causes water and fine particles to flow toward the edges rather than pooling in panel centers.
Trial observations over 8 weeks:
| Week | Wire Mesh Screen (average open area %) | PU Trial Screen (average open area %) |
|---|---|---|
| 1 | 78% | 91% |
| 2 | 61% (washed) β 74% | 88% |
| 3 | 58% (washed) β 72% | 85% |
| 4 | Panel replacement | 84% |
| 6 | 65% (washed) β 71% | 82% |
| 8 | Panel replacement | 79% |
The PU screen maintained significantly higher open area without washing. Wire mesh required wash-down every shift. PU required a monthly light wash.
Panel Life Comparison
- Wire mesh: 3β4 weeks before replacement needed
- PU dewatering panels: estimated 6β8 months based on wear rate at 8-week inspection
Life improvement: approximately 6β8Γ
Full Conversion
All four dewatering screens were converted to PU slot panels. The manual shift wash-down of screens was eliminated β operators were able to redirect that time (approximately 30 minutes per shift across four screens) to other tasks.
Product moisture after the dewatering screens improved slightly (from approximately 14.5% to approximately 13.2% average) due to consistently higher screen open area. This had a minor positive effect on product quality for the customer.
Key Points
Aperture geometry matters for blinding. Slot apertures resist blinding better than square holes for elongated particles like coal fines. Specifying the right aperture shape for the material being screened is as important as specifying the aperture size.
Panel surface profile affects drainage. Crowned (domed) panel surfaces improve dewatering by directing water flow toward the edges. Flat panels allow water pooling. This detail is often not specified by purchasers but makes a meaningful difference in dewatering applications.
Blinding is a systems problem, not just a screen media problem. The fundamental cause β near-aperture-size particles in wet conditions β cannot be eliminated by screen media choice alone. PU mitigates it substantially through flexing and geometry but does not eliminate it entirely.
Elephant Rubber supplied the PU dewatering screen panels for this project.