Testwork: Bond ball mill work index

Testwork: Bond Ball Mill Work Index

Test name

Bond Ball Mill Work Index

Abbreviation Alternate Forms WiBM BWI

Typical F80 size Typical P80 size 2500 µm 75-300 µm

Models

Bond models

The Bond ball mill work index is one of the most commonly used grindability tests in mining, and is often referred to as the Bond work index.

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

Sample Requirements

The test requires about 8 kg of material. Although it can work on feed as fine as 2.5 mm, it is best to send material to the testing laboratory that is nominally at least 8 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) 2.5 mm size used to feed the test that will not introduce excessive fines.

Test Inputs

It is necessary for the engineer to specify the desired product size of the test so that the laboratory can choose the appropriate closing mesh screen for conducting the test. This product size is typically the feed size to flotation or leaching. Example product sizes are 200 µm for copper porphyries (select a 212 µm screen), 100 µm for gold cyanidation (select a 150 µm or 125 µm closing screen) or 75 µm for complex sulphides (select a 105 µm screen).

Test Outputs

The laboratory will report the following information:

• 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)
• WiBM: 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).

Modelling

Ball mill work index is used in the SAGMILLING.COM Bond/Barratt specific energy consumption model. It is also used in the CEET2 model (distributed by SGS) and is commonly used by people running the JK SimMet population balance model (distributed by JK Tech).

The work index is used to calculate the energy requirement to grind rocks in the fine size range, below 2.5 mm into the range of a few hundreds of micrometres. Heterogeneous ore types typically are sensitive to the product size of the test, and the work index value changes if the test is performed to a coarser or finer product. Always specify the desired product size of the test to the laboratory, or conduct a ball mill work index calibration to determine the effect of changing the product size on work index.