OFFICIAL 5.6 Conclusions and optimization direction ● Technical feasibility: Sublevel stoping with cemented backfill is feasible at a conceptual level, provided that sequencing and support requirements are managed. ● Primary optimization lever: Longer stopes (80 m) materially improve development per ton and extraction ratio, but will not significantly alter stress environment, pillar response, and rock support requirement. ● Production strategy: Meeting 5.0 Mt/y requires parallel mining in two horizons separated by a sill pillar, implying higher fleet and manpower requirements than a single-horizon plan. Thus, operational practicality is limited. ● Key constraints/risks: Large stope spans and strict subsidence constraints demand further work – particularly detailed pillar behavior assessments, calibrated rockmass/strength inputs, and confirmation that enlarged stopes remain stable under expected stress conditions. ● Relative positioning: Sublevel stoping entails substantial development and therefore requires stronger justification through scheduling optimization. 6. RAISE STOPE MINING WITH CEMENTED BACKFILL 6.1 Mining method concept Raise mining was originally developed for steeply dipping vein deposits and was historically hazardous and low-productivity. Modern implementations – enabled by Alimak systems and, more recently, raise boring – have significantly improved safety and efficiency. A well-documented early application involved extracting a crown pillar in a steep marble vein in Austria using surface-based raise-bore equipment and tunnel haulage, requiring only three operators. This success led to deployment at a marble mine beneath an open-pit highwall in southern Austria. Based on these experiences gained in Austria, raise mining is considered a strong candidate for the Fen rare earth project due to: low development requirements to start production, high mechanization/automation potential, minimal exposure of personnel to rockfall and radiation, adaptability to geological and stress variations, high recovery potential, and compatibility with cemented tailings backfill. Two access tunnels about 130 m apart provide the top working level and a bottom loading/haulage level. A pilot hole is drilled between levels, then enlarged bottom-up to form the raise. A headgear system on the upper level allows a multi-stage platform to be raised out of the opening during blasting to avoid air-blast damage. The platform is suspended on ropes, positioned by winches, centered by rubber wheels (no guide rails), and clamped in place with hydraulic jacks. A separate man cage provides personnel access. Stoping is performed with custom drill jumbos drilling ~20 m blast holes inclined 20–30° to improve confinement at the stope walls. A single blast may require up to ~120
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