Description
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 *5%of the
actual material density. This compression is achieved by applying
high pressure of up to nearly **0Mpa, 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
