Work Index and Sample Size

Work Index and Sample Size

In mineral processing, is the Bond work index of an ore different if the determination is made at a finer grind size? Is rock hardness is independent of testing size?

The answer to this question is that generally no, the size that the index is measured at does not effect the Bond work index value.  Bond work index (BWi) is a measure of the hardness of an ore.  This is a quality of the rock, which generally does not change over the size ranges encountered in ball milling.

The BWi  is used to predict the power required for grinding.  The equation used for calculating power required for grinding is:

BWi  in mineral processing



Where  W = KwH per short ton

  •  BWi =  Bond Work Index
  •  P = Product size in microns which 80% passes
  •  F = Feed size in microns with 80% passes

Sometimes the work index increases when determined a finer size.  There are two reasons why work index will increase when determined at a finer size.  For most hard rock ore the natural grain size is much finer than the grind size so breakage in ball milling is along grain boundaries. Occasionally the natural grain size of an ore is coarse enough that breakage at the ball mill grind size is across the grain rather than along the grain boundaries.  The Bond work index for cross-granular breakage is much higher than for breakage along grain boundaries.

Another reason Bond work index can be higher when determined at finer sizes is an artefact of the test procedure.  The accuracy of the dry screening procedure is better at 100 mesh as compared to 150 or 200 mesh.  Material left on the screen because of screening inefficiency or flat particles such as clay or mica particles that do not pass the fine screen result in a higher work index than the ore actually is.  To avoid the test procedure problems the preferred size to determine BWi is 100 mesh.

For design work done by Allis Chalmers the Bond work index was determined at 150 mesh (as the original Bond Procedure states).  Subsequent work done has been done at 100 mesh (coarser size) and has not shown a decrease in BWi.