Calculate the Operating Work Index

Operating work index has the same definition as “work index” (1) which states that “work index is the power required to break a material from a theoretically infinite feed size to 80% passing 100 micrometers.” Thus, by definition, work indices calculates from operating data always relate the operating data from which W_io is calculated to the same feed size and product size as given in the definition; namely, from a theoretically infinite feed size to 80% passing 100 micrometers.

Thus, operating work index can be used for in-plant grinding mill reporting and grinding studies such as:

1. Record mill performance on an hourly, daily, weekly or monthly basis, whichever is desired.
2. Compare current performance with past performance.
3. Compare circuits in a multi-circuit plant.
4. In plants with two or more grinding circuits, one or more circuits can be used as a standard, with others as test circuits for testing the effect of such variables as:

• mill speeds
• size of grinding media
• feed size
• product size
• amount of grinding media in mill
• liner designs
• liner wear
• changes in ores

As calculated, the operating work index includes motor, drive, and grinding inefficiencies and as such, is not directly comparable to work indices obtained form grindability test performed on the same mill feed.

Mill power, which as measured in most plants is motor input power, (that is, electrical energy going into the motor) has to be converted to power at the mill pinionshaft. This is done by applying the motor efficiency factor to obtain motor output power. If the plant does not have the motor efficiency data, it can be obtained from the motor manufacturer. When the motor is coupled direct to the pinionshaft, motor output power is mill pinionshaft power. If a speed reducer or other drive element is used between the motor and the pinionshaft, then the efficiency of the units used must be applied to the motor output power to obtain power at the mill pinionshaft.