GOLD EXTRACTION
matching leach strategies to source
characteristics’ paper at the recent ALTA 2019
conference, in Australia.
He said: “The gold industry has been
supporting research into alternative lixiviants
over a number of decades. However, it may
sometimes appear to the casual observer that
the industry is no closer now than it was 20 years
ago to implementing a feasible solution at
multiple sites.”
Eksteen continued: “Often the problem lies in
the fact that one-size-fits-all lixiviants are sought
and it is clear…such a panacea does not exist.
Ore mineralogy and chemistry, environmental
conditions and water availability, mobilisation of
toxic deleterious elements and creation of toxic
by-products remain challenges, over and above
the general required attributes such as
sufficiently fast kinetics, low reagent
consumption, low reagent price, reagent and
gold complex stability, and the need to recycle
reagents, which mostly involve expensive solid-
liquid separation equipment.”
DST believes it has devised a new technology,
the CLEVR Process™, that can have wide-scale
appeal. The cyanide alternative produces no toxic
liquid or gaseous effluents, and generates solid
residues that are inert, stable and non-acid
generating, according to the company.
The process uses sodium hypochlorite with a
catalytic amount of sodium hypobromite in acidic
conditions to put the gold into solution. “Contact
time is short, and the process operates in a
closed loop,” DST says, adding that all chemicals
are recycled within the circuit, and sea water is
also suitable where available.
DST has opened the number of applications
suitable for CLEVR by introducing, depending on
the nature of the mineral, a pre-treatment step
prior to gold extraction.
Howlett provides an example: “One of the
applications requiring
pre-treatment might
be if you have a high
sulphur content in
the concentrate. What
we would do is
remove the sulphur
first and create either
a sulphuric acid or
gypsum using the
sulphur – either as an
industrial product or
a benign tailing.”
Taking such a route
with high sulphur
ores allows companies to remove the sulphur
completely before gold extraction takes place,
leading to “non-acid leaching tailings”, Howlett
said; avoiding another potential environmental
liability.
This isn’t where the benefits to using CLEVR
end, according to the company.
“DST's CLEVR Process creates no liquid
effluent, which is unlike the cyanide process that
requires significant wastewater handling and
treatment,” DST said.
“We don’t have a liquid tailings pond that can
fail,” Howlett explained. This could be of huge
benefit to mining companies out there not only
struggling to seek permits for cyanide use but
also facing opposition to the use of wet tailings
dams.
CLEVR’s gold recoveries are, generally, in line
with those produced by cyanide, according to
Howlett, but have outperformed its fellow
lixiviant in certain tests. DST has been able to
validate these results not only in the lab, but also
at a 15 t/d demonstration plant in Thetford
Mines, Quebec.
Originally commissioned in 2015, a first
demonstration campaign, completed in early
DST’s CLEVR Process uses sodium hypochlorite
with a catalytic amount of sodium hypobromite
in acidic conditions to put the gold into
solution
2016, saw DST process a total of 170 t of a gold-
and copper-bearing refractory pyrite concentrate
from a region where the use of cyanide is
restricted. The program demonstrated that the
CLEVR Process had an average extraction yield
14% higher than cyanidation, with results of up
to 81% gold recovery, the company said.
And, the company has recently received a
mandate from a Chinese customer to continue
testing of the CLEVR Process, with a 30-kg
sample of mineralised material already delivered
to DST’s Quebec facility. “The goal of this work is
to demonstrate that its proprietary CLEVR
Process can extract gold at a rate of 95% or
better,” DST said.
DST previously completed analysis of smaller
samples from this client and was able to increase
recovery of the gold from concentrate to over
90% at a lab scale using the technology,
compared with approximately 71% using cyanide.
Thiosulphate leaching is another route often
highlighted in the discussion of processing
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ALMATY
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GOLD
Au
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