Ultrafine Grinding

[jwoods]

What exactly is meant by ultrafine grinding i.e. at what particle size does fine grinding end and ultrafine grinding begin?

What are the current practical limits to ultrafine grinding? What I mean is, even if it was economical to grind to, say 5 microns, would concentration processes such as flotation be able to cope with these fine particles?

[amainza]

Ultrafine is usually the subsieve particle size and more appropriately sub 10 microns. The measure of the fineness of grind is often taken as a percentage of material passing 75 microns and if this percentage is high then one would conclude that the grind is fine. However, the standard sieving arrangement allows you to go as low as 25 microns and this is still in the fine grinding range. Special cloths can be used to screen down to 10 microns and below this size cyclosizers would be the best method to obtain fractions. Cyclosizing depends on density so the particle sizes in the different fractions are highly dependent on the ore density .

If the mineral of value is in the sub sieve ultrafine grinding can be used to liberate it. However, sliming problems are encountered if these are treated in conventional flotation cells. If these are treated separately it is likely that the recovery of the valuable minerals will be high in the ultrafine. It is expensive to achieve ultrafine grinding for most ores and most concentrators extend the grind to these sizes only when it is necessary. Treating them separately is the key.

The practical limits is the number of grinding units required to achieve this. For example, depending on the ore hardness a number of say Isamills in series maybe required.

[amainza]

Liberation data can help to decide if it is necessary to grind to ultrafine. It is useful to do a theoretical grade-recovery curve to determine the practical limits for flotation.

[guest]

If one already have an ultrafine grinding circuit installed would it be beneficial to invest in an on-line psa. Most psa measures size distribution by volume and as such this reading may vary significantly from the mass size distribution. If all parameters remain equal viz. ore type, mineralogy etc. would it be safe to say that the error between volume size distribution and mass size distribution would remain equal and the results obtained from an on line psa can be used as a valuable input into the control logic of the u/f grinding circuit ?

Your comments

[mpowell]

A psa would work very well with ultrafine grinding, it is expected that the outputs would be consistent and the issue of coarser particles in the feed to the psa goes away. So it should be excellent in mill control.
A supplier may wish to make further comment.