The grinding efficiency factors should be checked and applied as required to place the operating work index at the same level as the results from grindability tests. The operating work index so calculated is referred to as W_ioc. This operating work index divided by the work index from the grindability test give a measure of grinding efficiency in terms of actual performance compared to calculated or rated performance as a percentage.

The multipliers for the efficiency factors can be determined as discussed in the following.

• EF₁ – Dry grinding – form the same range of work, dry grinding requires 1.3 tiems as much power as wet grinding.