With respect to the field of mineral processing, the process of comminution is a capital and energy-intensive operation, with autogenous (AG) and semi-autogenous (SAG) mills being the principal reduction mechanisms employed for this purpose. Although the two techniques utilize the ore particles themselves as the grinding or crushing media, the distinction between the two is significant from an operational point of view. The main difference between the two techniques is the fact that, whereas the AG mill relies purely on the interparticle interactions between the ore particles, the SAG mill uses steel balls in addition to the natural media.
Mechanistically, this distinction between the two techniques is responsible for the operational stability and applicability of the two techniques. The AG mill is an entirely autogenous process, and the larger particles of the feed must act as the grinding media for the smaller particles. This renders the process very sensitive to the competency of the feed, and instability in the system may result if competent media are not present in sufficient quantities. In contrast, the addition of steel media in the SAG mill renders the system more stable and less sensitive to the hardness of the feed particles, since the steel media will always impart the same grinding energy.
Furthermore, the choice between the two techniques is significant from an economic point of view, since the elimination of the steel media in the AG mill reduces the operating costs considerably, although the feed must be carefully controlled for optimal efficiency. Recent investigations carried out with respect to itabirite iron ores, however, suggest the possibility of AG mills being more energy-efficient for certain applications.
In practice, it has been demonstrated that the optimization of the SAG mill with reduced steel media charge and utilizing the natural media for the purpose of grinding has resulted in considerable reductions in operating costs, coupled with an unexpected improvement in the size and grade of the metal obtained. Thus, the distinction between the two techniques extends beyond the mechanical differences and assumes a more strategic role with respect to the operation and efficiency of the system.
