Testwork: Bond rod mill work index

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Testwork: Bond Rod Mill Work Index

The Bond rod mill work index is not as common as the Bond ball mill work index, and there are two different variations on the mill apparatus. The SAGMILLING.COM models expect a mill with a "wave liner", and is not compatible with mills using "smooth liners". Most laboratories outside of Australia use the wave liner style of machine – these results can safely be used in the models. Bond rod mill work index results from Australian laboratories with smooth liners should not be used in the SAGMILLING.COM models.

The test is a 'locked-cycle' test where ground product is removed from test cycles and replaced by fresh feed. The test must achieve a steady-state before completion.

Sample Requirements

The test requires about 15 kg of material. Although it can work on feed as fine as 10 mm, it is best to send material to the testing laboratory that is nominally at least 25 mm (including the natural fines that are part of the sample). The laboratories have a standard way of reducing the coarse material to the (roughly) 10 mm size used to feed the test that will not introduce excessive fines.

Test Inputs

When WiRM is used to size SAG mills, the Bond/Barratt model assumes a 14# Tyler (1180 µm) closing mesh size — the engineer should specify this closing mesh size to the laboratory performing the tests. When used to size rod mills, the engineer should specify the target product size for the industrial plant (the laboratory will choose an appropriate closing screen mesh size to achieve that product size).

Test Outputs

The laboratory will report the following information:

  • umclosing: The aperture size on the screen used to close the test, µm. (or the P100 size)
  • F80: Sample feed size in µm (and usually will provide a particle size distribution)
  • P80: Sample finished product size in µm (and usually will provide a particle size distribution)
  • gpr: The average grams per revolution of the last three cycles (sometimes is labelled GPB)
  • WiRM: The calculated work index (SAGMILLING.COM uses only metric units; if the laboratory reported work index in "short ton" units, multiply that value by 1.1023 and enter the result).

Extra field available for modelling

  • synthetic indicates whether this is a real test result, or just a synthetic one that should only be used for modelling. If this column contains a value of '1' (boolean=true) for a test, then that test is understood to not be a real test result and is therefore not shown on the testwork comparison charts. Synthetic values are available when running circuit model simulations and do show up in the list of model results.


Rod mill work index is used in the SAGMILLING.COM Bond/Barratt specific energy consumption model.

The work index is used to calculate the energy requirement to grind rocks in the medium size range, from 10 mm - 20 mm down to about 2 mm.