High steel load in SAG mills and the implications on energy efficiency of this growing design trend

Semi-autogenous grinding (SAG) mills are commonly used in mineral processing plants to reduce ore particles so that the minerals of value can be extracted by downstream recovery processes. Ore introduced to SAG mills requires minimal processing after the mining stage, resulting in circuits with low capital and maintenance costs due to the absence of elaborate crusher plants. 

SAG mills can be operated as primary mills in multiple grinding stages or as single stage mills in closed circuit with classifiers to produce the final product for the recovery processes downstream. SAG mills are designed to operate with ball loads ranging from 4 to 15% volumetric filling, but high steel load SAG mills or Run of Mine ball mill designs have provision to accommodate ball loads in the range from 16 to 36%. 

The ball load in SAG mills plays a significant role in defining the throughput and product size for the circuit. For SAG mills operated at lower ball filling, the coarse component of the ore and its breakage characteristics play a significant role in establishing the energy efficiency that can be attained by the circuit while for SAG mills operated with a higher ball filling, the contribution from the steel media dominates. The rock/ball ratio is an important parameter in determining the energy efficiency of the SAG mills. Low energy efficiencies have been observed for mills that are operated with low ore/ball ratio. 

The significant role of ball filling in SAG mill operations for different feed attributes will be discussed by Prof. Aubrey Mainza in his keynote lecture at Comminution '23 in Cape Town next year. Opportunities to enhance circuit performance and reduce detrimental consequences that the ball filling degree can impose on the circuit will be explored and the paper will provide insights on the importance of considering implications of energy efficiency when designing plants to avoid oversizing the mills. If most mills draw only 70% of the motor power, what are the implications of this growing trend of using high steel load SAG mills? This should lead to an interesting discussion among the many experts on SAG milling from around the world who we expect to have in attendance at our first face-to-face comminution conference since Comminution '18. Any views you have now I am sure Aubrey would appreciate.

Aubrey heads comminution research at the Centre for Minerals Research of the University of Cape Town, and participates extensively in collaborative research with international universities, mining companies, and comminution and classification equipment manufacturers. For many years he has been a comminution consultant to MEI.

There is now a call for abstracts for Comminution '23, and we would like to thanks our current sponsors for their support.

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