INTEGRATING COMMINUTION TESTWORK INTO GEOMETALLURGICAL RECOVERY FRAMEWORKS: A CRITICAL REVIEW OF SPATIAL HARDNESS MODELLING FOR U.S. CRITICAL MINERAL OPERATIONS

Variability in ore hardness drives much of the gap between predicted and actual mill throughput, and accounts for a substantial share of downstream recovery shortfalls in critical mineral operations. Even so, many U.S. feasibility studies still treat hardness as a single average value across the deposit. The Bond Work Index (BWi) is still the industry's default for grindability, but the test consumes about 10 kg of sample and takes about eight hours per determination, which keeps the number of determinations low at the resource-estimation stage. This paper pulls together recent peer-reviewed work on three connected questions: How can a comminution testwork programme capture spatial variability without running thousands of BWi determinations? How can hardness domains be modelled in space once that data is in hand? And how do those domains feed into a geometallurgical recovery framework? The proxy methods reviewed include the point load index, Leeb rebound hardness, the Cerchar abrasivity index, and hyperspectral imaging. The throughput-prediction case study draws on operating SAG mill circuits, and the closing section ties the methodological picture to federal supply chain assessments under the 2025 List of Critical Minerals and Executive Order 14241.