A method of preparing an aluminum metaphosphate (ALMP) particulate product includes charging a milling chamber of a ball mill with grinding media and loading an ALMP feedstock into the milling chamber. The ALMP feedstock is milled with the grinding media into ALMP particles at a particle reduction index in a range from 0.25 to 0.5. At a plurality of time steps during a period in which the milling is carried out, a fine fraction of the ALMP particles is removed from the milling chamber while a coarse fraction of the ALMP particles remains in the milling chamber for additional milling. An ALMP particulate product with a particle size distribution having a median particle size in a range from 100 μm to 700 μm is prepared from the ALMP particles removed from the milling chamber.