DESIGN AND CONSTRUCTION OF LIMESTONE BLENDED WASTE ROCK DUMPS - LESSONS LEARNED FROM A 10-YEAR STUDY AT GRASBERG 1

A ten-year acid rock drainage (ARD) mitigation program conducted by PT Freeport Indonesia at the Grasberg Mine, Papua Province, Indonesia has developed strategies to maximise the beneficial use of the mined limestone for ARD mitigation. Laboratory columns and field test pad investigations have examined limestone blends, layers and covers. Full-scale trial dumps comprising truck blends, conveyor/stacker blends and truck placed limestone covers have been constructed and monitored. Construction of an operational blended stacker built dump commenced in 2004 and is operated in accordance with blending specifications developed from the 10-year trial. Trials have demonstrated that run-of mine truck constructed blended dumps are unlikely to be effective because the finer size fractions do not received adequate limestone. Stacker-built dumps can be effective provided the blend ratio is based on achieving adequate acid neutralising capacity within the finer fractions. Operational experience with the large stacker-built dump indicates that there is no discernable segregation of sulfur or acid neutralising capacity down the slope. The scheduling of potentially acid forming waste and limestone to the in-pit crushers is determining the spatial variability within the dump rather than segregation of sulfur or acid neutralising capacity during the dumping process.