Dear Mr. Houdeh,

There are a number of reasons to select a variable speed drive for a SAG mill. If you seek optimal performance over the life of lifters in the mill, you need to modify mill speed as mill internal configuration wears. Your objective should be:

1. Optimize grinding performance - mfgrs don't know best mill speed. This is best speed is learned after installation.

2. Optimize grinding performance when lifter shape wears. Initial shape is usually configured for wear life and lifter comminution performance. As lifter wears, the level of cataryacting deceases which allows for increased speed.

3. New lifter geometries are comming that vastly improve SAG and AG mill performance. These shapes require changes in mill speed at inception and as mill wears. New modeling techniques show lifters with double present height and very significant change in shape to both increase general grind performance, but most significantly increase small particle volume. Goal is to maximize rock surface area. This is not presently the practice.

Aside from mill speed you need to determine whether a single direction or bi-direction is best. Cadia is still the world's largest mill and uses a single direction. For this you also need to configure the mill pan lifters for single direction. This will drop pan recycle by 40-50% and almost eliminate the pan recycle by 100% which otherwise inhibits flow and amplifies wear.

You need to consider the mill liner bolt pattern. Mfgrs. do not plan for a bolt pattern that can allow future lifter designs that will the need for fewer bolts, with larger pitch. Mfgrs. are way behind the best fit for purpose lifter configuration. Mfgrs. today do not know the best bolt designs and best location. S

You need to consider how to configure your mill for quick relines. Design the liner handler for minimizing time to change internals.

Time is money. How do you know the proposed mill design will give you best performance for the $$? Maybe you should ask for a performance sharing relationship. Presently, mill suppliers do not want to take risk on performance guarantees. So, you should structure your bid request according to best-hope.

At Cadia, we were able to increase mill through-put by about 6% and lifter life, in terms of tons ground/kg lifter metal, by over 40%. When liner replacements can exceed 1.5 million USD, the benefits can be considerable. This was 6 years ago. Today, much higher levels can be achieved. If you wish to know more, contact me.

You should obtain copies of SAG 2001 and 2006 proceedings to learn more. I was co-author on four 2001 and one 2006 papers.

These papers are all relevant to your need to optimize performance.

There are other useful publication to aid you. S.K. Kawatra's compendium, the published work of J.K. Tech (JKMRC), MEI online, etc. ■

Rewording Last Post:

Was:

This will drop pan recycle by 40-50% and almost eliminate the pan recycle by 100% which otherwise inhibits flow and amplifies wear

Should be:

This eliminates the 40-50% ore backflow into mill as ore rills over exit grates and eliminate the 100% exit pan recycling, which otherwise inhibits flow and amplifies pan and grate wear. ■

Can you tell us about the mill size you are considering?

Is this at Sarchesmeh? ■