SAG mill sizing errors are expensive. An undersized mill costs $5–20M/year in lost throughput. An oversized mill wastes $10–30M in capital. Here are the five mistakes we see most often.

1. Using Bond Work Index Alone for SAG Sizing

Bond's method was developed for ball mills. Applying it directly to SAG mills ignores the fundamentally different breakage mechanisms. SAG mills rely heavily on impact breakage, which is characterised by the A×b parameter — not the Bond Work Index.

Fix: Use the Morrell SMC method or the SPI/SGI approach for SAG-specific energy estimation.

2. Ignoring Ore Variability

Most designs use average ore properties. But SAG mills are far more sensitive to ore variability than ball mills. A 20% increase in ore hardness can reduce throughput by 30-40%.

Fix: Design for the 75th or 80th percentile ore hardness, not the average. Use our ore properties database to benchmark variability.

3. Incorrect Ball Charge Assumptions

The SAG mill ball charge (typically 8-15% by volume) significantly affects both throughput and specific energy. Many designs assume a fixed charge without optimising.

Fix: Model the effect of ball charge on specific energy. Our SAG mill sizing tool lets you vary ball charge and see the impact on mill dimensions.

4. Not Accounting for Pebble Crusher Recirculation

In SABC circuits, the pebble crusher recirculation load affects SAG mill feed rate, circulating load, and ultimately mill size. Ignoring this interaction leads to undersizing.

Fix: Model the complete circuit, including pebble crusher capacity and recirculation ratio. Our Total Circuit Energy tool handles multi-stage circuits.

5. Oversimplified Scale-Up from Pilot Data

Scaling from pilot SAG mill data (e.g., 1.8m diameter) to full scale (e.g., 12m diameter) requires careful application of scale-up factors. Direct linear scaling doesn't work.

Fix: Apply validated scale-up correlations. Our mill sizing tools use data from 2,500+ operating mills to validate predictions across the full range of mill sizes.

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