This invention relates to a supersonic center-feed nozzle system within the feed-inlet tube of a fluid-energy mill used for grinding particulate material such as titanium dioxide. Particularly, in the feed-inlet tube of the present invention, particulate material is introduced into the supersonic feed jet of primary grinding fluid in almost a perpendicular fashion, with the supersonic nozzle installed in the center of the particulate material core, imparting momentum to the particulate material. The momentum entrains the particulate into the main grinding chamber of the feed-inlet tube. In the main grinding chamber of the feed-inlet tube, a secondary stream of grinding fluid, introduced annularly, constricts the primary jet's divergent flow, enabling a higher turbulent mixing of the grinding fluids and the particulate material.