This is pretty normal in the kaolin industry, classifying slimes...

This is pretty normal in the kaolin industry, classifying slimes to create a clay fraction. For example, desliming is a common procedure in heavy mineral sand deposits where clays are removed prior to dense media separation of HMS concentrates.

I think the 20um limit is however a little small as kaolin is usually reported in the -45um fraction. For reference check out ITech Metals Ltd (ITM) which is proposing this strategy for their REE clays in South Australia.

The challenge is going to be optimising the thickeners and screens to reject the +20um feed and concentrate the slimes. Plus finding enough water in the Cue area will be a challenge.

Otherwise, this is a reasonable bench scale metallurgical test result to demonstrate REE's can be extracted (theoretically) in order to allow the publication of a JORC resource.