Australia's junior explorers - The ASIA Miner

Paste Thickening
we have to take our licks on the first cut and
really over-support that r ock. In some instances,
we might even be shotcreting on the
way in. Once we get that first cut done and
backfilled with paste, however , we know
what our back integrity is because we already
know what that paste strength is.”
At Stillwater, the distance to the stope
being backfilled can be mor e than 7,000 ft
(2.13 km). To move the paste that distance, in
1998 the company installed a KSP 80 pump
from Schwing Bioset (Somerset, W isconsin,
USA) powered by twin 150-hp (112-kW)
electric motors. Marjerison said that, despite
the relatively limited role underhand mining
plays in Stillwater’s everyday operation—it is
used for roughly 10% of the stope backfilling—the
pump is the ideal tool for the job.
“To move a material with a high solids content
that distance would be a challenge for
any piece of equipment,” he said. “Granted,
in many of the deeper stopes, the material
has gravity helping it out. However, that’s not
always the case; we’ve pumped some really
long distances without much of a vertical
drop at all. But the combination of the stoutness
of the pump and our efforts to produce
a nice consistent product out of the mixer has
really paid off. When needed, we’ve been
getting steady pour rates in the 90 ton-anhour
range and holding to that rate for sixhour,
12-hour, even 18-hour continuous
pours. We’ve been very pleased with that
level of performance.”
60 | ASIA Miner | July/August 2012
Stillwater’s paste plant operation is monitored at an
operator station, and pump operation can be supervised
from its own control panel.
Designed for Strength
The mix design at Stillwater to which Marjerison
refers, includes 10% to 12% cement
content and a Euco-Fill 31 water -reducing
and plasticizing admixture that develops 85
psi (586 kPa) of unconfined compr essive
strength (UCS) after seven days. Though
lower in strength than paste used at other
comparable mines also doing paste backfilling
(a result of the large amount of –44 mi-
Dewatered and treated mill tailings are processed in a pug mill and transferred through a hopper, visible in the
background of the photo, to the Schwing Bioset KSP 80 pump, driven by dual 150-hp motors.
cron [–325 mesh] particles in the mill tailings),
the company benefits by being able to r educe
velocity in the pipelines to about 2.1
ft/sec (0.64 m/sec), less than one-thir d that
of the other mines.
The mine’s pristine location also puts it literally
at the end of the power grid, making
outages a distinct possibility, particularly in
the harsh Montana winters. T o deal with a
service interruption to the Schwing Bioset
pump—which could be catastrophic if it occurs
in mid-pour—the company maintains a
diesel-powered oil field mud pump.
“That is our only backup for getting the
pipes cleared if we have an outage,” said Fallis.
“If the paste hardens in the pipe it is a real
nightmare. More importantly, however, if the
pour is interrupted for any length of time, because
of the risk of cold joints we can’t simply
resume pouring. W e’ve had instances
where that’s been the case and we’ve had to
drill and blast a partially filled stope to start
over. It’s not what we like to do.”
Prior to backfilling, Stillwater first lays down
a bed of anywher e from 1–1.5 ft (30.5–46
cm) of broken rock or prep muck on the floor
of the mined stope. Doing so not only of fers
something of a buffer from subsequent blasting
which will take place below the newlyfilled
stope, but also acts as a base in which
to drive DYWIDAG r einforcing bolts and
plates for added str ength. The addition of
wire mesh on the floor and construction of a
7-ft-high (2.1-m) wooden r etaining wall to
hold the paste in place, complete the preparation,
making the stope ready for backfilling.
“There’s no denying that ther e is a lot
more work and additional cost to doing underhand
mining,” said Fallis. “It might take
ten days or so just to get a stope ready for
the pour, then the paste has to cure for another
seven days after the pour itself. That’s
two-plus weeks of lost pr oduction on that
stope. However, we are now safely able to
get to ore that we couldn’t before—and the
key phrase there is ‘safely.’ Stillwater Mining
places a tr emendous amount of emphasis
on doing things the right way and
has an enviable safety record within the industry
to show for it. Underhand mining
might only be used 10% of the time her e,
but we’re happy to be able to make that
10% happen.”
LARRY TROJAK is president of Trojak
Communications, Ham Lake, Minnesota,
USA; Email: trojakcom@gmail.com.