High Pressure Grinding Rolls

Operating principle

High pressure grinding is achieved by an advanced type of grinding roll. The particles are broken by compression in a packed particle bed.This particle bed is created between two choke-fed, counter-rotating rolls. Between these rolls, a particle bed is pressed to a density of up to roughly 85% of the actual material density. This compression is achieved by applying high pressure of up to nearly 300Mpa, exceeding the compression strength of the feed material. During this compacting process the material is ground to a wide particle size distribution with a large proportion of fines, compacted into flakes.

The breakage process can be visualized as consisting of two distinct stages. In the first stage, the choke-fed material entering the space between the rolls is subjected to an acceleration to meet the peripheral roll speed. As a consequence of the narrowing gap between the rolls, the material is gradually compacted and the larger pieces and particles are precrushed. Furthermore, a certain degree of particle rearranging occurs, filling the interparticle voids.

In the next stage, the pre-crushed material enters a compaction zone.

This zone involves a gap between the rolls defined by a sector with an angle of about 7°. It is in this compression zone where the pressure occurs. The press force is acting principally on all particles passing the compression zone, through multiple point contacts between the particles in the compressing bed. This results in the disintegration of most particles.

During the process, microcracks are being generated within the particles, which results in the weakening of these particles for a subsequent grinding stage. Pressing in a particle bed reduces wear since the main grinding action does not take place between the roll surface and the material, but between the material particles in the particle bed.