Are SAG Mills Losing Market Confidence? ask P Staples, G Lane, R Braun, B Foggiatto and M P Bueno in this paper (now available through CEEC International) presented at the Sixth International Conference on Semiautogenous and High Pressure Grinding Technology, Vancouver, 20-24 September 2015.

AG and SAG grinding mills are considered a mature technology, “so,” they ask “why are we seeing a number of recent projects not achieving nameplate capacity and is SAG technology losing market confidence, particularly in financial circles?” This paper presents a critical review of underperforming AG/SAG mills based on public domain information comparing design expectations with actual operational performance. Actions taken in operations to mitigate the issues and optimise mill performance are listed, including a high level discussion on capital and operating costs.

The authors conclude that “the current scepticism within financial circles of the ability of consulting and engineering firms to predict SAG mill throughput [as evinced by Scotia Bank articles referred to in the paper], should not be considered as a lack of sophistication of the available testing methods or the maturity of SAG milling technology. Rather it reflects issues in properly apply the available techniques for prudent SAG mill design when processing competent ores, including benchmarking of performance.

“In some cases poorly compiled JKSimMet models have been used to support the data provided by other sources. The issues relating to JKSimMet modelling of competent ores have been previously discussed. There are several SAG mill specific design tests available to determine the amenability of an ore to SAG milling and the resultant SAG mill specific energy. Recent projects that have failed to achieve design throughput have typically failed to take sufficient heed of the indications of ore competency in the test work data or analysis of the data.

“If A x b values (breakage property) are below, say, 40 or DWI values are above 6.5, due consideration should be given to the inherent competency of the rock and benchmark data relating ore competency to SAG mill specific throughput. The scale-up of pilot plant mill specific energy to large scale operation requires careful consideration of the pilot mill discharge function and associated breakage rates external to the mill in SABC mode. As a minimum, careful benchmarking of design outcomes against available data in the public domain is strongly recommended.”