High energy SAG mills
High energy SAG mills are used to grind powder material down to a particle size below one micron. Up to now, the detailed physical mechanisms of the grinding process inside the mill have not been fully understood. Hence, the design of these mills as well as their process parameters is based on empirical results. The SAG ball mill has been simulated by discrete event simulation (DES) using typical process parameters to monitor the balls motion and their collisions. The DES method – if applicable – strongly outperforms typical DEM methods.
Furthermore parameter variations of the main process parameters have been made and the distribution of the balls impact velocities in normal direction has been analyzed, which is responsible for the grinding effect. Based on the new knowledge about the ball’s motion inside the grinding chamber and the probability distribution of the impact’s velocity, we developed a new grinding chamber. Its shape has been optimized by a computer simulation. The new design of the grinding chamber allows higher impact velocities and therefore finer powder of new mechanical alloyed powders.
The selection of SAG and ball mill
The selection of SAG and ball milling sizes and configurations for new projects is complicated by the fact that grinding characterization data is typically limited at best. The use of this grinding character is action data by different grinding consultants, to specify totally different equipment and or configurations for the same project is becoming a major concern for the industry. This is highlighted by the fact that a number of new projects are following the concept of selecting the largest proven equipment available and then evaluating what the expected throughput will be using the largest SAG and ball mills available.
The task therefore shifts from SAG mill design, to how many tonnes will the given SAG mill grind in one hour. When the difference between design methods used by different consultants reaches a factor of 2, it is time to stop and review the techniques used. If the designers fail to do this, the possibility exists that clients will select alternative technology simply because clients have lost faith in the designer’s ability to design the correct mills or accurately predict throughput in a large SAG mill.
SAG milling and tonnage capacity
Cynicism about SAG milling and tonnage capacity is perhaps best explained by the comment from a senior mining consultant who recently said, "if a SAG mill can process design tonnage, it is overdesigned". In other words, some people are becoming so used to a SAG mill not being able to produce design tonnage that they do not expect it to happen.
From an engineering design perspective, it is unacceptable that a client be offered a design where the tonnage is uncertain, or cannot be achieved. The project could be rejected based on the mining economic equation which is driven by throughput. Contrary to common belief, one of the best investments in hardware today is the purchase of enough SAG mill capacity to not only achieve design tonnage all the time but also to reduce steel costs in SAG grinding. With this in mind this paper will examine new information that is relevant to designing and maximizing the profit ability of new mines.
Signifi cant differences between macro and micro grind ability relationships, and the true magnitude of SAG hardness variability have been discovered for SAG mill and ball mill design measurements. These new discoveries found using SAG Design testing allow for improved understanding of the uniqueness of each ore body and the measures that can be taken to achieve design throughput from the fi rst day of operation at a new plant.
