Every grinding circuit has an economic optimum. Grind finer and recovery improves — until the extra power, liner wear and slime losses cost more than the metal you liberate.
What P80 actually tells you
P80 is the size at which 80% of the material passes. For most sulphide flotation circuits the useful window is 75–150 µm. Gravity and magnetic circuits often run coarser, 150–300 µm, because the separation method does not need full liberation.
The three variables that move your product
Circulating load
A closed circuit with a classifier or cyclone is how you get a narrow product without over-grinding. Push circulating load from 200% to 350% and the mill passes more material per tonne of product, at the cost of more pumping and wear. Below about 150%, the circuit becomes unstable.
Ball charge and ball size
Total charge volume of 35–45% is normal. Ball size should match the feed: too large and you crush the fines you already made; too small and the top size never gets reduced.
Classification efficiency
This is the most commonly ignored lever. A classifier that returns fines to the mill causes exactly the over-grinding you are trying to avoid.
- Check the sharpness of the cut.
- Check the water split in the cyclone cluster.
- Check whether the apex is worn or roping.
How to run the test that decides it
- Sample the current feed and product.
- Run a grindability test at three target P80 values.
- Convert to a power curve and add liner and media cost.
- Compare with the recovery curve at the same three grinds.
- Pick the P80 where the sum of cost is lowest.
This is a week of lab work and it routinely saves more than it costs, because most plants discover they are 15–20 µm finer than they need to be.
Measure where the tonnes are lost before you buy more horsepower. In most plants, the mill is doing exactly what it was asked to do.
