From memory there is more shale in the deeper ore (fresh ore ) of Never Never , yes it is harder , (higher bond work index ) and probably higher MOHS , so as you rightly commented the extraction takes more energy to get a high recovery (over 90%)
I am guessing there are several ways to fix the issue , tilt the mill (guess it is a ball mill currently) at less of an angle to increase residency time that would be a cheap arse way of doing it and push up energy costs and probably go through mill liners faster
The better way would be to have a second circuit at the outlet of the main mill feeding via a cyclone to a small mill the oversize and regrind it to the right mesh size .
One thing looking at the photos of the plant it looks easy to put in a secondary circuit, (guessing they planned this initially ) and as it is virtually ancillary it could be second hand .
I doubt there is any preg robbing from shale in the tanks but I am not a Met .
Focusing on the shale seems a little myopic to me but I probably am wrong as usual .
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