Pre-concentration—broadly defined as the rejection of waste before conventional processing—requires a tailored approach to economic evaluation. Multiple implementation strategies are possible, and effective evaluation must account for both these strategies and the outcomes of heterogeneity analysis and laboratory testing.
Typically, three strategies are commonly applied in pre-concentration:
- Upgrade feed by rejecting waste from mined ore, thereby increasing the grade of material sent to the mill.
- Extend reserves by processing marginal, sub–cut-off material that would otherwise be classified as waste.
- Maintain mill throughput by increasing mine production to offset material rejected by pre-concentration.
Strategy #1 can allow downsizing of the downstream processing facility—an advantage for greenfield projects where reduced capital expenditure improves project economics. For existing operations with spare mining capacity, however, Strategy #3 may be more compelling, as expanding production combined with pre-concentration can yield strong economic returns.
Strategy #2 lowers the processing cut-off grade, effectively turning waste into ore. In open pit mining, marginal material can be identified during grade control and sent to a pre-concentration plant. Alternatively, new pits, pushbacks, or underground stopes that would otherwise be uneconomic can be incorporated into the mine plan.
Pre-concentration evaluations use several distinct cut-off grades:
Mill Economic Cut-off Grade (COGE): The conventional threshold separating ore from waste.
Marginal Cut-off Grade (COGM): A lower threshold defining material that may become economic with pre-concentration.
Sort Target Cut-off Grade (COGT): The upper grade limit for pre-concentration; higher-grade ore bypasses sorting and thus is not subject to potential losses in the sorting process.
Evaluations typically vary the COGM and COGT to test the three strategies. The optimal approach defines the grade range fed to pre-concentration—bounded by COGM below and COGT above. Material below COGM is waste and that above COGT bypasses sorting for direct processing. In an underground mine, with well-defined stopes and abundant reserves, marginal material may be excluded by setting COGM equal to COGE.
Laboratory tests further refine the evaluation by relating mass pull (the fraction accepted as ore) to metal recovery (the fraction of valuable metal recovered). Because these relationships vary, different mass pulls correspond to different recoveries. In sensor-based sorting, higher sensitivity settings typically increase recovery but also mass pull. SRK also assesses fines bypass—material below the minimum sortable size, usually less than 10 to 25 mm—and metal deportment within that fraction. The fines bypass and metal upgrade (or downgrade) are inputs to the economics.
SRK’s economic evaluation follows two stages:
Size the Prize: A preliminary assessment conducted before laboratory results are available, using heterogeneity measures (Waste in Ore, Ore in Waste, and metal deportment) and assumed separation efficiencies to estimate potential value.
Detailed Evaluation: Incorporates laboratory test data and refines the analysis. This may involve only integrating test results or, in a more rigorous approach, re-optimizing the mine plan through pit optimizations or including additional underground stopes made economic by pre-concentration.
Both instances test the sensitivity of economic outcomes, typically NPV (Net Present Value), across ranges of COGM, COGT, and mass pull to identify optimal pre-concentration strategies.
Pre-concentration is not a one-size-fits-all solution but a flexible lever that can upgrade feed, extend mine life, or increase metal throughput depending on its application. A structured economic evaluation grounded in heterogeneity analysis and test data enables mining teams to identify the strategy that best fits their resource and operation. As analytical tools and test methods advance, these frameworks will continue to refine where pre-concentration delivers true step-change gains in project value.
