# Ball mill work index adjustment

From SAGMILLING.COM

## Ball mill work index adjustment

The Bond ball mill work index test requires the operator to select an appropriate screen size to "close" the test. The procedure states that the closing screen size should be such that the product size of the test is the same as the expected product size of the operating plant. If the tests were done at the wrong closing screen size, then a ball mill work index adjustment procedure can be conducted as follows:

- In the laboratory, select a series of samples, each suitable for performing three or more work index tests (probably 30 kg of sample is the practical minimum).
- Conduct a series of work index determinations on each sample, but vary the closing screen mesh. At minimum, test two different closing mesh sizes (the closing size you did the original tests at, and the closing size that matches your new desired P80). Three of four different mesh sizes are better.
- Fit the results of each sample to an equation of the form Wi
_{BM}=*a*× P_{80}^{b}. Use logarithms to rearrange and solve for*a*and*b*using linear regression of the logarithms. - If there is a clear discontinuity in the work index at a particular size, this probably indicates a hard grain size is being encountered in the test. If this grain size is between your original test P80 and the desired, P80, just continue with the procedure. If this grain size is above or below the size range of interest, then exclude those sizes and repeat the regression.
- Similar ore types will usually have similar
*a*and*b*values. You can average these across those similar ore types and use the averages for the WiBM adjustment factors in the circuit models.