High Pressure Grinding Rolls (HPGRs)have been applied in mineral processing plants, largely in iron ore and diamond treatment. In these industries, the application of HPGR ranges from coarse grinding, e.g. the grinding of 65mm (2.5")size excess pebbles in circulation loops, to final grinding of <0.4mm material to high Blaine values in the preparation of pellet feed.
HPGR grinding significantly enhances over all through put. This results in the creation of a large proportion of finished product and the reduced Bond Work Index of the pressed material. This generally allows for a reduction in the projected number of equipment units in tertiary crushing, quaternary crushing and grinding.
Operating principle
High pressure grinding is achieved by an advanced type of grinding roll. Contrary to conventional crushing rolls, the particles are broken by compression in a packed particle bed, and not by direct nipping of the particles between the two rolls.
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 pre-crushed. Furthermore, a certain degree of particle rearranging occurs, filling the inter particle 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, micro-cracks 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.
The throughput of a HPGR depends on the ability of the rolls to pull the feed into the gap between the rolls (roll surface friction), on the feed material characteristics (e.g. internal cohesion, moisture), and on the operating conditions (e.g. roll speed, choke feed conditions).
The roll surface friction can be increased by applying an articulated surface texture to the rolls, such as a welded chevron pattern or inserted hard-metal studs which protrude a few millimeters above the roll surface