The Bond Work Index and the Morrell SMC method are the two most widely used approaches for estimating comminution energy requirements. Both have their place, but choosing the wrong one can lead to significant design errors.

Bond Work Index

Fred Bond's method, published in 1952, remains the industry standard for ball mill sizing. It works well when:

- You have reliable Bond Ball Mill Work Index (BWi) test results

- The circuit is a conventional crush-rod-ball or crush-ball configuration

- Feed and product sizes are within the typical Bond range (F80 < 14mm, P80 > 45μm)

The Bond equation: W = 10 × Wi × (1/√P80 - 1/√F80)

Morrell SMC Method

Steve Morrell's Size-specific energy method (2004, updated 2009) addresses several limitations of Bond's approach:

- Uses the SMC Test (or DWi) to characterise ore at different size ranges

- Accounts for the different energy requirements of coarse and fine grinding

- Better prediction for SAG/AG mill circuits

- Uses Mia, Mib, and Mic parameters for different size ranges

When to Use Each

Scenario	Recommended Method
Conventional ball mill circuit	Bond BWi
SAG/AG mill sizing	Morrell SMC
Preliminary scoping (limited data)	Bond BWi
Detailed feasibility study	Both (compare)
HPGR circuit	Neither (use specific HPGR models)

The Bottom Line

For detailed design work, we recommend running both methods and comparing results. Our Circuit Design Suite includes both Bond and Morrell models, validated against 2,500+ operating mills.

The largest discrepancies occur with very hard ores (BWi > 20 kWh/t) and very soft ores (BWi < 8 kWh/t), where Bond's method tends to under-predict and over-predict respectively.