Mining_Methods_UnderGround_Mining - Mining and Blasting

Thickener
Binder cement and/or slag
Paste factory – principal flowsheet.
Planning considerations
Paste pump
Because the density of hydraulic fill is
only about half that of ore, a supplemen-
tary fill material will be needed when
less than half of the tailings can be recovered
from the mill circuit.
When planning a hydraulic fill
system, a major consideration is water
drainage, collection and disposal, particularly
on deep mines. Getting large
volumes of water back to surface can
be a costly exercise, and installing the
infrastructure may be difficult, expensive
and time consuming.
Portland cement added to hydraulic
fill as a binder also adds strength, and
this system of fill in normal and high
density is employed at many mines
around the world. A portion of the
cement may be substituted using fly
ash, ground slag, lime or anhydrite.
If cement is added in the ratio 1:30,
the backfill provides better support for
pillars and rock walls. If the top layer is
then enriched at 1:10, the backfill provides
a smooth and hard surface from
which broken ore can be loaded and re-
moved. Addition of cement reduces ore
dilution from the fill and facilitates se-
lective mining and greater recovery
from both stopes and pillars.
Water decanted from cemented fill
has to be handled appropriately to avoid
cement particles reaching the ore passes
Cyclone
Mixer
and sumps, where they can have great
nuisance value. One approach is to reduce
the amount of water in the fill,
increasing solids content to 65-75% and
more in a high-density fill. Additives
can also reduce the water decant from
fill.
Paste fill
Vacuum filter
Paste to the mine
Paste fill originally used non-cycloned
mill tailings mixed with cement at the
stope. Coarse tailings permit a very high
solids content of up to 88% to be pum-
ped at high pressure, and high setting
strengths were achieved. Paste is cur-
rently used as a replacement for hydraulic
fill, with the cement added at surface.
It exhibits the physical properties
of a semi-solid when compared to highdensity
fill, which is a fluid.
Because the slimes fraction of the
tailings forms part of the mix, cement
always needs to be added into paste fill,
with 1.5% as the minimum requirement
to prevent liquefaction. Very precise con-
trol of pulp density is required for gravity
flow of paste fill, where a 1-2%
increase can more than double pipeline
pressures.
Cemented rock fill
Tailings from concentrator
Cemented rock fill (CRF) originally
consisted of spraying cement slurry or
BaCkFilling
Paste fill plant at Garpenberg, Sweden.
cemented hydraulic fill on top of stopes
filled with waste rock, as practiced at
Geco and Mount Isa mines. Nowadays,
cement slurry is added to the waste rock
before the stope is filled. Where rock is
quarried on surface, it is normally gravitated
to the mining horizon through a
fill raise, from the base of which trucks
or conveyors are used for lateral transport
underground.
Advantages of CRF include a high
strength to cement content ratio, and
provision of a stiff fill that contributes
to regional ground support. CRF is still
selected for some new mines, and many
operators prefer this system.
Cement rich hydraulic fill was once
used for mats where poor ground conditions
dictated underhand cut and fill
mining. Since the major cost component
of backfill is the cement at a ratio of
1:2, this fill is not economical, and was
replaced with ready-mix concrete with
10-12% cement content for a standard
3,000 psi, or 20 Mpa, mix.
Ice fill has been used in Norway and
Russia in permafrost regions.
Hans Fernberg
underground mining methods 45