Australia's junior explorers - The ASIA Miner

Prior to the early 1980s, if poor soil conditions
prevented a mining operation fr om safely accessing
a rich vein of or e, that company had
little recourse but to abandon the area in question
and lose the or e contained within. It was
generally accepted as a costly but unavoidable
facet of the business. Today, however, techniques
such as underhand cut-and-fill mining,
in which cemented mill tailings ar e used to
backfill the mined-out stope, allowing subsequent
stopes to be cut beneath pr eviously
mined areas, have established themselves as a
viable alternatives for working in such conditions.
While not as frequently relied upon—due
in large part to the added costs associated
with it—underhand cut-and-fill has pr oven to
be a valuable method and found its way into
mainstream mining practices. As a result, many
companies are pulling high-quality or e from
areas once viewed as unr eachable. One of
those, Montana’s Stillwater Mining Co., has
been successfully employing that approach for
more than a decade now and, with the help of
a KSP Series piston pump fr om Schwing
Bioset to deliver the paste for that process, has
literally “gone where no man has gone before.”
Big Sky Mining
Working a geological formation known as the
J-M Reef, Stillwater Mining extracts,
processes, smelts, refines and markets palladium
and platinum, as well as a limited
amount of copper, silver, rhodium and gold.
Located at the foothills of the Beartooth
Mountains in souther n Montana, the ar ea
represents the only known considerable
source of platinum group metals in the United
States, and is one of the most significant
sources outside Russia and South Africa.
According to Wayne Fallis, Stillwater’s stationary
equipment maintenance planner, the
ore is mined, crushed fine, and then run
through a flotation process. “At that point, it
is concentrated to about 30% to 35% metals
but there is still a good deal of dirt and
rock contained within,” he said. “So it is
sent to a refinery where it is run thr ough a
flash furnace to remove the base metals,
leaving a finished mineral product. The tail-
ings from that initial extraction process are
routed to the paste plant wher e, after processing,
they will eventually become the
mixture used in the backfilling process.”
To make that paste happen the mill tailings
are first mixed with a flocculent to settle out
the slimes. Excess water is extracted and the
residual material is pumped up and onto the
belt of a vacuum filter to a depth of about ¾ in.
(19 mm) where even more water is removed.
“The filter is critical in that it not only further
reduces water content, it also r emoves the
slimes that were not taken in suspension,”
said Fallis. “This is important because slimes
can have an adverse effect on the final product
by reducing the paste’s strength when the
cement is setting up; that simply can’t be allowed.
By the time material gets to the end
Paste Thickening
PUMP POWERS PASTE TO STILLWATER’S STOPES
Some of platinum producer Stillwater Mining’s more distant—and more difficult— stopes require expensive underhand cut-and-fill mining.
A sturdy piston pump has kept the paste flowing dependably to these sites for more than a decade.
By Larry Trojak
of the belt it is fairly dry. Water content is generally
only about 18% to 20%. We then send
it to a pug mill where the cement and any additives
are mixed. When it drops out the other
end it is finished product—paste ready to be
pumped to a waiting stope.”
Long Way to Go
Where that stope will be depends, of course,
on a host of factors. But it will generally be in
an area with poor soil conditions—ar eas
prone to rockbursts, for example. Stillwater’s
Mine Engineer, John Marjerison, said the decision
to go with underhand cut and fill starts
with the initial core sample.
“If we do the diamond drilling and geotech
assessment and the core shows the ground
to be very poor quality, we might decide that
Due to occasional poor ground conditions, PGM producer Stillwater Mining Co. finds it necessary to use underhand
cut-and-fill mining with paste backfill in about 10% of its stopes.
July/August 2012 | ASIA Miner | 59