In mineral processing, SAG (Semi-Autogenous Grinding) mill represents a rotating cylinder that employs the rocks themselves and steel balls for crushing of the stones. Throughput indicates ore tonnage, while feed grade and ore hardness are its mineral composition and ability of the rock to be fragmented. Sometimes, operators experience a peculiar contradiction when SAG mill throughput decreases although the ore hardness and feed grade remain the same. It is necessary to investigate the internal mechanics of milling process in order to provide an explanation.
The decrease of SAG mill throughput can be caused by changing feed size distribution, namely the accumulation of critical-sized rocks. Despite a constant rock hardness value, introduction of medium sized rocks makes it less efficient. The mid sized particles cannot serve as efficient grinding agents because they are too small; on the other hand, these particles are too big for being crushed with the help of existing steel balls (Delboni et al., 2022).
Moreover, the attrition of the internal steel ball charge also influences productivity. To have enough energy for high impact crushing processes, SAG mills need an accurate volumetric filling with steel media. Steel balls tend to wear out during operation and lose some mass. As a result, when the amount of added new material isn’t optimal, the machine doesn’t receive an energy source. The process of grinding becomes slower as a result. In order to avoid the overloading of the mill, it has to be reduced (Saldaña et al., 2023).
In addition, degradation of liners and lifters in the interior may also significantly impact the process. They have their geometry designed in such a way that a charge is raised up to a definite level with enough energy to fall and break rocks at the bottom. Because of continuous wear of these lifters, they become flattened and instead of falling forcefully to break the layer, it simply slides down the inner wall.
Mechanical issues related to discharge often lead to abrupt reductions in mill performance. The slurry is discharged from the mill using discharge grates, allowing large rocks to be retained inside and slurry to pass through. The discharge grate is blocked by the presence of rocks of an intermediate size in the openings—a process known as grate blinding. This reduces the cross-sectional area where slurry is discharged, making it accumulate inside and soften the grinding actions of the grinding media, requiring an urgent reduction in feed.
To sum up, the small changes in slurry rheological properties may affect performance. An increase in the internal slurry viscosity because of some changes in the clay may lead to its coating of the grinding media and weakening of collisions. Mill operators should monitor constantly slurry water, mill speed, and media charge. As soon as mill operators know how important, good internal health of the mill and discharge for the throughput is, they will be able to diagnose problems with mill performance in any case.
References
Delboni, H., Costa e Silva, E., Alves, V. K., & Chieregati, A. C. (2022). Pre-Processing to Increase the Capacity of SAG Mill Circuits—Case Study. Minerals, 12, 727. https://doi.org/10.3390/min12060727
Saldaña, M., Gálvez, E., Navarra, A., Toro, N., & Cisternas, L. A. (2023). Optimization of the SAG Grinding Process Using Statistical Analysis and Machine Learning: A Case Study of the Chilean Copper Mining Industry. Materials, 16, 3220. https://doi.org/10.3390/ma16083220
