GOLD EXTRACTION
double refractory ores without the use of
cyanide. This technology was first
commercialised in 2014 when Barrick Gold, in
partnership with Australia’s CSIRO, started up its
thiosulphate plant at the Goldstrike mine
(Nevada, US).
As part of the thiosulphate process at
Goldstrike, gold-bearing, sulphur-based ore is
heated as a thick slurry of ore, air, water and
limestone in large pressure chambers or
autoclaves and then pumped into the ‘resin-in-
leach’ circuit that takes place inside large
stainless steel tanks, according to CSIRO. Within
the tanks, the slurry interacts with thiosulphate
and a fine, bead-like material called resin that
collects the gold.
Barrick says the plant has allowed Goldstrike
to process 4 Moz of double-refractory ore that
would otherwise have been processed at the end
of the mine's lifecycle many years from now.
Despite the success achieved at Goldstrike,
the leach reagent system the mine adopted has
not yet found widespread appeal.
CSIRO, however, separately developed a
thiosulphate-based reagent system for gold
leaching that, according to the organisation, has
excellent stability and shown broad applicability
in the laboratory.
“The reagent system is an alternative to
cyanide and has particular application where
cyanide cannot be used and to unlock stranded
high-grade deposits,” CSIRO said.
CSIRO, in collaboration with Eco Minerals
Research Limited, commenced a project in July
2017 to undertake a demonstration at scale in
the field using the CSIRO reagent system. The
mobile demonstration plant setup on the
Menzies stamp battery site (in Western Australia)
used a low capital expenditure vat leach process
to recover gold from ores, having good gold
liberation at a p80 greater than 300 microns,
CSIRO said.
In under 10 months, the demonstration project
took a laboratory developed concept and
transformed it into a demonstration plant
involving design, build and commissioning
through to successfully producing gold doré
bars. The demo plant has since processed up to
30 t/d of ore by vat leaching and has operated
successfully for more than six months to validate
the reagent performance and stability. The leach
reagent consumption for the optimised
demonstration process was 1.6 kg/t, the majority
of this being entrained loss with the tails.
According to a paper presented at the ALTA
2019 conference, authored by CSIRO’s Paul
Breuer, Michael Jackson, Marlene Engelbrecht,
Amy Evans and Lauren Bourke, and Eco Minerals
Research/Clean Mining Ltd’s Jeff McCulloch, the
demonstration process was initially designed to
treat tailings from the gravity plant at Menzies
(less than 10 mm material with only 20-30%
passing 850 micron).
This coarse particle size distribution allowed
the adoption of the low capital cost vat leach
process, according to the authors.
To facilitate the short procurement and
construction time, the modular and mobile plant
design was also adopted. Additional advantages
of this design were the flowsheet could be easily
modified to treat different ores and the mobility
allowed easy relocation, if required, as opposed
to trucking in ore.
The plant treated old battery sands available
on the Menzies site, according to the authors,
who explained that this stockpiled material was
a conglomerate of many small parcels of ore from
the local area that had previously been treated
through the battery.
“The battery sands had variable mineralogy
and gold grade, and also contained significant
fines and clay materials, which were largely
removed using a 12 in cyclone (p80 of cyclone
overflow was 25-30 microns). The mill was used
to slurry the battery sands and remove any plus-
2 mm particles before being fed to the cyclone.
The cyclone underflow material was placed in the
vats and leached to recover the gold.”
The flowsheet for this process saw the
dissolved gold thiosulphate complex recovered
from the pregnant leach solution in the leach
solution processing plant using ion exchange
resins. This was subsequently eluted from the
resin under ambient conditions and a gold-
containing product obtained from the eluate.
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AUGUST 2019 | International Mining 47