## BOND'S WORK INDEX: WHAT IT IS AND WHAT IT ISN'T

The reason for this variation in ball mill work index is because the ore at Mt. Milligan has an intrinsic exponent of –0.45 in the 100 µm to 200 µm size range, not the –0.50 exponent empirically measured by F. Bond. The work index increase between 100 µm and 76 µm is due to a porphyritic grain size that causes a spike in energy

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