The Effect of Metallurgical Recovery Modelling on Net Present Value and Cut-Off Grade in Open-Pit Copper Mining: A Case Study from Eastern Türkiye

Abstract

Abstract

In open-pit optimization software, metallurgical recovery is commonly treated as a constant for every block, although it varies with ore type and grade. Here, recovery is modelled as a block-grade-dependent variable using 24 laboratory flotation tests on the sulfide ore of a copper deposit in eastern Türkiye, and its effect on net present value (NPV) and the cut-off grade decision is examined. The deposit is split by sulfur content into two routes: sulfide ore (S ≥ 9%) to flotation and oxide ore (S < 9%) to heap leaching. Across a feed grade of 0.22–6.99% Cu, the measured recovery increases with feed grade from about 61% to 94%; the linear correlation is only moderate (r = 0.60), but the relationship is well described by a bounded, saturating recovery–grade curve (R(g) = R_max•g/(g + k); R_max = 0.95, k = 0.12; R² = 0.87). For the leach route, where no test data are available, a fixed 80% recovery is retained throughout. Optimization I (fixed recovery) and Optimization II (variable recovery on the sulfide route) are compared over ten scenarios, using a slope-constrained ultimate pit (50° overall slope, 5% discount rate). Because the deposit's copper is concentrated in high-grade sulfide blocks with measured recovery of about 90–93%, the metal-weighted recovery of the sulfide ore is 88.9%, and the fixed 80% assumption underestimates both recoverable copper and NPV. Under a fixed pit and cut-off, variable recovery yields roughly 8% higher NPV and about 2.9 kt more copper. When the cut-off grade is instead determined economically, variable recovery reclassifies the marginal low-grade sulfide ore (~0.05 Mt, measured recovery below 80%) as uneconomic; even so, total copper output rises from 40.2 kt (fixed) to 43.0 kt (variable) — a slightly smaller gain, because this marginal ore is excluded. Grade-dependent recovery derived from laboratory data thus determines both the recoverable metal and the marginal-ore boundary more realistically than a fixed assumption, and materially affects NPV for this deposit.