Detergent Powder Sifting

A big version of a flour sifter, with a screen (or graduated screens) matched to the size(s) of grains you want.

I´ve seen rotary mills also known as cone mills used succesfully in spray dryers and agglomerators. The oversize fraction is diverted into the cone mill of suitable screen size and continuosly recirculated back to sifter. One issue is that yóu´ll unavoidable get fines so you need to be careful selecting the proper screen size for the cone mill. This is done by trial and error so be prepared to purchase a few of different size.