High Pressure Grinding Rolls (HPGR)
were introduced as a new grinding technology in 1984. Since then,
they have been successfully installed in a large number of plants
throughout the world, mainly for cement and limestone. More
recently, HPGRs have also 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 AG
circulation loops, to final grinding of <100μm material to high
Blaine values in the preparation of pellet feed.
HPGR grinding significantly enhances
overall throughput. 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.
Click an image to enlarge
High Pressure Grinding Rolls (HPGR) offer several benefits to
the minerals industry, such as
- Low energy consumption, 0.8-3.0 kWh/t
- Ability to process moist ores, up to 10%
- Enhanced downstream recovery and
grindability
- Improved grade of downstream products
- Low maintenance requirements
- Low space requirements
- Low vibration and noise
- High availability >95%
- High wear surface life 6,000 - 36,000
hours
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 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.
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 (KHD patent).
