WINNING
interaction generates increased fines; although this
crusher has lower wear rates and produces improved
particle shape. Rock-on-metal interaction produces
fewer fines while maintaining a cubical product. The
main disadvantages of rock-on-metal interaction are
high wear of crusher components, and the loss in
crushing efficiency with increased fines production as
the anvils wear out.
In the plant
In a typical hard rock quarry, the process plants have
three- to five-stage crushing circuits with primary
gyratory crushers or jaw crushers, secondary cone
crushers or jaw crushers, tertiary cone crushers,
quaternary cone, or vertical shaft impact (VSI)
crushers. Soft rock quarries in turn have process plants
that have two- to four-stage crushing circuits with
primary jaw, gyratory or horizontal shaft impact (HIS)
crushers; followed by HSI crushers, cone crushers, or
hammer mills in the secondary and tertiary stages.
Crushing and sizing are the major sources of
particulates within the processing plant. Very fine-
grained material may be removed from the air in dry
processing plants by bag filters, cyclones, wet collectors
also known as scrubbers, or electrostatic precipitators
at dry processing plants. The choice of equipment will
be dictated in part by the size range of dust requiring
removal. Fabric filters and electrostatic separation
are most suitable for fine and ultra-fine particulates,
while cyclones and wet collectors are more useful for
coarser dust. Depending on the type of material being
produced, wet or dry screening is done. Dry screening
inherently produces more dust than wet screening,
with wet screening producing more slurry that needs to
be contained in settling dams.
Increasing the efficiency of screening can be achieved
by adjustments to the screening set-up. By increasing
the speed of screening and opening the screen slightly
more, decreases the death of bed during the process, but
increases the G-force, which decreases the bearing life.
Good quality grizzly bars for screening and crushing will
be able to handle much more difficult tasks and increase
equipment life. Changing the rotation can improve
the screen’s performance by increasing the material
retention time and action on the screen, subsequently
providing the particles more openings within the screen.
Adjusting the stroke as the coarseness of the transfer
material changes, will also increase screening efficiency.
Fine separation requires higher speeds and fewer
strokes, and coarse separation requires increased stroke
and slower speeds. Lastly, using tipping grids to increase
the inclination angle will result in significantly faster
material travel in dry screening applications. Caution
should however be taken not to increase the incline too
much because the rollover time could result in materials
missing openings. ■
QUARRY SA | JULY/AUGUST 2018 _ 27