The Bond test is still surprisingly popular, despite the advances in modelling and computing power. However, it is well deomstrated that sizing SAG/AG mills on this basis is disastrous, so there are a number of different modelling techniques used for modelling and scale-up of these mills - all of which are rather empirical. Most designeres still do an across the circuit power check based on the Bond work index. There is some excellent work published by Steve Morrell on this topic.
Post by Adam Johnston on Jan 10, 2006 20:19:00 GMT
Computer simulation is only as good as the information that is the input. Rocks are usually a mixture of various minerals, hard and soft. An abreviated test that did not reach steady state as a Bond Work Index does would only be assessing the hardness of the soft minerals and would not be representative of the rock as a whole. Technology advances but the rocks stay the same. Adam
As I understand it the BWI calcs are used to determine mill sizing etc. How does the OWI relate to the BWI. Does the actual OWI as calculated from plant data serve as a check to see what the efficiency of the mill is compared to the original BWI ?
Secondly ,many plants look at the OWI of the -75µm fraction. This figure is presumably very different from the OWI index (the OWI looks at the energy required to reduce a feed particle of spesific size to a product of a spesific finer size -is this correct ?)
Indeed the OWI is often used to test the efficiency of a plant against its theoretical capability. However, as Morrell has demonstrated, the OWI is a stronger function of the degree of reduction than given by the standard Bond equation. Thus a plant that is operated well and produces a finer grind tends to be penalised relative to one that is cruising and producing a coarser grind at a consideraly lower energy input. The Bond equation is also insensitive to top feed size, but the real OWI is sensitive to this. Although poor operation wil increase the OWI, a simple across the circuit measure does not inform on this.
When quoting BWI or OWI, the product size should always be given. The energy to reduce to a % specific size, such as 75um, is the specific enerery, which is simply power across the cicuit divided by the tons per hour of -75um material that is produced. In its self this is a useful number, but it should be clarified whether the tons of -75um in the product are used (which is usual) or the tons of fresh -75um produced - which has the -75um that are in the feed subtracted from the product. The latter is more correct, but as the %-75um in the feed is often not known, this definition is not usually used.
Bond procedure gives very good results when you use it correctly and includes correction factors as proposed by F.C.Bond later (after 2 original papers). It still works good for rod and ball milling when your measure lab BWI close as possible to designed screen size, continue to pilot procedures and finally checks power consumptions vs work on plant mill.