The impact of Grade Engineering on SAG Milling


In addition to minerals displaying variations in grade within an ore body, blasted rock often concentrates particular minerals preferentially into finer size fractions. Grade engineering strategies seek to exploit intrinsic heterogeneity and grade variability to improve the viability of operations experiencing declining grades and unit metal productivity. Examples may include processing coarse and fine material separately or selectively rejecting or sorting particular size classes. Since SAG circuits are almost universal in the industry, strategies are required for operating these circuits on altered feed size distributions and variable feed properties. Circuits should be operated to maximise productivity by leveraging variability with flexible strategies.

This paper evaluates the performance of SAG milling on scalped feeds and outlines operating and design processing strategies that can provide the necessary flexibility to maintain optimal performance in the context of varying size and ore properties. Grade engineering strategies that involve pre-screening and rejecting a coarse low-grade component may alter current SAG milling performance if the coarse rock content in the feed becomes too small. Although it is possible to increase the ball load to compensate for the reduction in large rocks, it can become difficult to maintain sufficient load within the mill, media costs increase and extra ball milling capacity is required. Therefore if preconcentration removes the coarse, competent fraction from the SAG mill feed, different operating strategies will be required.

In addition to natural grade by size responses grade engineering strategies highlight the bench scale heterogeneity. If measured for each processing ore type, variability of grade and competence can be used to control the process and maintain operation at peak performance. Potential solutions for SAG mill operation within grade engineered circuits will be presented, thus reducing the barriers to implementation of these novel strategies.

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