# Relative Bond Work Index BWi | Comparative Grindability Test Procedure

The test is designed to give a reasonable indication of the grinding work index of an ore material relative to one or more known standards. It is not applicable to mill tailings material.

Basic Equipment

1. Set of screens @ 75 to 2440 micron sizes.
2. Ro-Tap Sieve Shaker
3. Ball mill rolls
4. CMS ball mill No.1 with approximately 10 kg of ball charge distributed as follows:

Procedure

1. Crush the known and test samples to -3350 micron using either the jaw or rolls crusher.
2. Riffle split the crushed sample. Weight out approximately 300 grams for feed sizing and exactly 1000 grams for grinding.
3. For sizing feed:
-wet screen at 70µm and dry the product.
– dry screen the oversize (+75µm) on 2440, 1680, 1016, 600, 300, 150 and 75µm.

For grinding:
– place 1000 grams of the known sample in the ball mill, introduce the ball charge, add 625ml of water. Secure mill lid tightly and grind for 10 minutes.

• Wash the pulp thoroughly from the mill and the charge, wet screen the pulp @ 75 microns and dry product.
• Dry screen the oversize (+75µm) product on 1680, 850, 600, 425, 210, 106, 75µm.
• Plot feed and product sizings on a log-normal graph sheet as cumulative weight % passing versus particle size. Each particle size is determined by calculating the geometric mean size (GMS) of two successive screens used. For example, GMS of –3350 + 2440µm size range equals to:

(3350 x 2440µm)^0.5  = 2830µm

Repeat above procedure for the unknown sample.
Read off F80(µm) and P80(µm) from the plots and calculate the comparative grinding work index (CWi) of the unknown.
Note: F80= the diameter in micron which 80% of the feed weight passes.
P80 = the diameter in micron which 80% of the product weight passes.

Calculation
The formula to use is:

where R refers to the reference standard sample of known Wi and C refers to the unknown sample.

For the comparison to be valid, the following rules of thumb should be noted:
– Feed size distributions of the reference sample and unknown should ideally be similar.
– Grinding conditions must be identical.
– Ideally, comparative ores should have roughly similar grinding characteristics; that is, similar slope and shape in size distribution curves.
For example, hard siliceous ore and soft talcose ore are not compatible.