Background
A coal export terminal in South Africa's KwaZulu-Natal province was experiencing significant carryback and spillage on the conveyor system transferring coal from the stockpile reclaim to the shiploader. The conveyor was 850 meters long, 1400mm wide, running at 4.2 m/s and handling approximately 2,500 tonnes per hour of export thermal coal.
The problem manifested as a continuous trail of fine coal along the return run β accumulating under the conveyor structure, on the support steelwork, and around the tail end. Cleaners were spending approximately 3 hours per shift removing coal buildup from under the conveyor. Material accumulated on return idlers, contributing to belt mistracking events.
Investigation
Carryback assessment: The existing belt cleaning system consisted of a single primary scraper at the head end. The scraper had not been adjusted in several months and blade inspection showed it was barely contacting the belt β the blades had worn down and the tensioning mechanism had not been advanced.
Belt surface condition: The belt cover showed glazing in the central zone β a smooth, polished surface resulting from fine coal particles being pressed repeatedly against the cover. A glazed belt surface has reduced friction and reduced scraper contact effectiveness.
Loading zone: At the reclaim feeder loading point, the skirt seals were worn and allowing fine coal to escape from both sides of the loading zone. Some of this material fell onto the return run directly.
Material characteristics: Thermal coal at this terminal contained significant fines (sub-10mm fraction). Fine coal with moisture content above 8% is notoriously difficult to clean from belt surfaces β it adheres tenaciously and a single scraper rarely achieves adequate cleaning.
Solution
Scraper upgrade and maintenance procedure: The single primary scraper was replaced with a new primary scraper (tungsten carbide blades) plus a secondary scraper (polyurethane blades) positioned 500mm further along the return run. A maintenance inspection schedule was established requiring monthly scraper blade inspection and tensioning adjustment β previously this had not been included in the planned maintenance schedule.
Belt surface reconditioning: The glazed belt surface was mechanically abraded using a belt surface preparation tool during a planned conveyor stop. This restored surface texture and improved scraper contact effectiveness.
Skirt seal replacement: Worn skirt seals at the loading zone were replaced with new 60 Shore A rubber skirting strips. The skirting adjustment was set to minimum effective contact pressure to prevent belt cover abrasion.
Return idler cleaning and replacement: Twelve return idlers with material buildup were replaced with rubber disc self-cleaning return idlers in the loading zone section.
Outcome
After the combined intervention, coal accumulation under the conveyor was reduced by approximately 80% β assessed by the cleaners' shift time for under-conveyor cleanup, which dropped from 3 hours to approximately 35 minutes per shift. Residual cleanup was from areas beyond the conveyor structure rather than carryback.
Belt mistracking incidents, which had been occurring weekly, stopped occurring over the subsequent 3-month monitoring period.
The secondary scraper required blade replacement after 4 months of operation β a planned maintenance event rather than an unplanned failure. The primary tungsten carbide scraper was still serviceable at the 6-month inspection.
Key Lessons
A scraper that isn't adjusted doesn't scrape. Belt scrapers are not install-and-forget components. They need monthly inspection and tensioning adjustment as blades wear. The most common reason a scraper system is ineffective is that maintenance has allowed the blade to wear away from contact.
Fine wet material needs two scrapers. Wet coal fines and similar sticky materials cannot be adequately cleaned by a single primary scraper. A primary removing 85β90% of the material plus a secondary removing most of the remainder is the correct system design.
Skirt seals are part of the carryback system. Material that escapes the loading zone through failed skirt seals and falls onto the return run looks like carryback but isn't. Inspecting skirt seals as part of carryback investigation is important.
Elephant Rubber supplied the replacement primary and secondary scrapers, rubber disc return idlers, and skirting rubber for this project.
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