The variability of ore hardness is a critical factor influencing the performance of fixed comminution circuits, primarily by dictating the maximum achievable throughput and energy efficiency. Because orebodies are inherently heterogeneous, they often consist of multiple lithologies with distinct physical properties (Deniz, 2022). This spatial variability creates significant challenges for maintaining stable plant operations.
Mechanisms of throughput reduction
In fixed comminution circuits, such as those employing Semi-Autogenous Grinding (SAG) and ball mills, ore hardness directly affects the rate of size reduction. When a circuit encounters ore with high resistance to breakage, the comminution process slows down (Kabemba et al., 2025). This leads to an increase in the internal load of the mill if the feed rate remains constant. If the mill is already operating at or near its nominal capacity, this accumulation can result in overloading (Deniz, 2022). To prevent mechanical damage or “bogging” of the mill, operators must reduce the feed rate, thereby lowering the total circuit throughput (Kabemba et al., 2025).
Operational instability and bottlenecks
Hardness variability also induces operational instability by altering the product size distribution and the “critical size” material accumulation. For instance, hard ores often build up as pebbles that are too large to be ground efficiently but too small to act as effective grinding media (Deniz, 2022). Furthermore, variability can shift the circuit’s bottleneck; while one ore type might be limited by the primary SAG mill’s power, a softer but more competent ore might shift the bottleneck to the secondary ball milling stage or the hydraulic transport capacity of the system (le Roux et al., 2016).
Mitigation through geometallurgy
To manage these fluctuations, modern operations utilize geometallurgical modeling and predictive control strategies. Integrating indices such as the Bond Work Index (BWI) into dynamic simulation models allows for better forecasting and tactical blending of ore to maintain a more consistent hardness profile (Kabemba et al., 2025; le Roux et al., 2016).
Ore hardness variability remains one of the principal drivers of throughput instability in fixed comminution circuits. Because these circuits are designed around average hardness assumptions, even moderate excursions in ore competence can cause measurable production losses. Integrating hardness characterisation into mine planning, through geometallurgical modelling and variability-aware testing, is essential to stabilise production and optimise circuit performance.
References
Deniz, V. (2022). Relationships between comminution and chemical, petrographic and mineralogical properties of ores, and their effect on concentration. Physicochemical Problems of Mineral Processing. https://doi.org/10.37190/ppmp/149892
Kabemba, A. M., Mutombo, K., & Waters, K. E. (2025). Towards a Dynamic Optimisation of Comminution Circuit Under Geological Uncertainties. Processes, 13(2), 443. https://doi.org/10.3390/pr13020443
le Roux, J. D., Olivier, L. E., Naidoo, M. A., Padhi, R., & Craig, I. K. (2016). Throughput and product quality control for a grinding mill circuit using non-linear MPC. Journal of Process Control, 42, 35–50. https://doi.org/10.1016/j.jprocont.2016.04.007
