Bond Laboratory Procedure
The standard Bond test to establish an empirical work index for single stage ball milling, utilized a 12” x 12” ball mill to batch grind a dried sample of the mineral crushed to minus 6 mesh. A ball charge of approximately 20 kg together with a mineral sample of 700 cc are initially run for 100 revolutions. The weight of the sample is recorded before the test begins. The mill speed is 66 RPM. The undersize (i.e.. desired product size) portion of the sample is screened out, and fresh minus 6 mesh mineral added to the original sample’s oversize to bring its weight up to the original weight and the total returned to the mill. The number of revolutions for the second run is determined by a formula dependent upon the net product achieved in the second run is screened out, the sample restored to original size and the revolutions needed for the third run a gain determined by formula. Successive runs are made until the ratio of net product achieved to the number of run revolutions stabilizes. This stabilized ratio then is used in calculating the Bond work index by an empirical formula developed by Mr. Bond.
Limitations of the Bond Procedure
The Bond index is a widely used and respected tool. Experience gathered over many years has assisted mill application engineers in using the Bond index for reliable sizing of mills.
Whenever actual experience with an ore or an ore known to be comparable is not available, the engineer is well advised to keep in perspective:
The empirical nature of the test.
The relatively small splitting techniques and the human factors involve in the multiple steps of testing, and judgment relative to when “stability” has, in fact, been reached.
The possibility that natural grain boundaries may not reconcile with the test procedure. (In such cases it is not uncommon to arrive at an index higher than that needed for production size grinding).
A Bond rod mill test and other refinements are sometimes employed to obtain a better analysis for the expected operating conditions.
|
Material Size |
80 Percent Passing Equivalent |
|
|
|
Microns (F) or (P) |
formula-……. |
|
99% - 1 ½” |
25,000 |
158.0 |
|
99% - 1” |
18,000 |
134.0 |
|
99% - ¾” |
12,000 |
109.5 |
|
99% - ½” |
5,500 |
92.2 |
|
99% - 3/8” |
6,000 |
77.4 |
|
99% - 3 mesh |
4,200 |
64.8 |
|
99% - 4 mesh |
3,000 |
54.8 |
|
99% - 6 mesh |
2,100 |
45.8 |
|
99% - 8 mesh |
1,500 |
38.7 |
|
99% - 10 mesh |
1,000 |
31.6 |
|
99% - 14 mesh |
800 |
28.3 |
|
99% - 20 mesh |
550 |
23.4 |
|
99% - 28 mesh |
400 |
20.0 |
|
99% - 35mesh |
270 |
15.4 |
|
99% - 48 mesh |
150 |
12.25 |
|
99% - 65 mesh |
105 |
10.25 |
|
99% - 100 mesh |
72 |
8.48 |
|
99% - 150 mesh |
55 |
7.42 |
|
99% - 200 mesh |
23 |
6.00 |
|
99% - 325 mesh |
20 |
1.47 |
|
Correction Factor to Mathematical Sizing of Mills. |
|
Reference % Passing |
Correction |
|
50 |
1.035 |
|
CORRECTION FACTOR 3. CF3 (diameter) |
|
CORRECTION FACTOR 4. CF4 (oversized feed) |
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