CYANIDE REDUCTION/REPLACEMENT
oxidation was necessary before
proceeding to the CLEVR Process
leaching step, it noted.
“The direct CLEVR Process
leaching, without any pretreatment,
was successful and
demonstrated higher gold
extraction yields (>90%) when
compared with cyanidation (67.6%)
tests conducted on the same
material,” the company said.
“DST’s standard CLEVR Process
conditions were utilised, which
allowed for superior gold yields in a
fraction of the time (one-hour
leaching time).”
In addition, the solid tailings
residues produced by the CLEVR
Process met the US Environmental
Protection Agency’s Toxicity
Characterisation Leaching
Procedure meaning it may be
considered as “non-hazardous waste products”,
DST explained.
“DST is very excited with the results obtained
using the CLEVR Process and the collaboration
with the producer,” the company said. “The
corporation will continue its development efforts
with the producer to further develop this
promising opportunity and the technical and
economic parameters of DST’s technology.”
In March 2020, DST tested two distinct lots of
circa-50 kg and circa-30 kg of representative
material from a gold miner’s operating mine in the
Asia-Pacific region, meanwhile. The laboratory
metallurgical test program showed the direct
application of the CLEVR Process on oxidised
samples achieved gold extraction yields of 93.5%
and 94.2%, respectively, for the distinct lots
tested.
ATS emerges for Empire
Phoenix Copper has been planning for a lengthy
permitting process to extract precious metals at its
Empire Mine deposit, in Idaho, USA, but recent
test work with a non-toxic cyanide alternative has
the company hoping it might be able to launch
into production of gold and silver that much
sooner.
In late June, the company confirmed precious
metals tests performed on samples from the
Empire open-pit deposit using ammonium
thiosulphate (ATS) reagent had resulted in gold
and silver recoveries of 97.8% and 97.7%, and
69.8% and 78.2%, respectively.
Back-to-back bench scale tests carried out at
AuRIC Laboratories in Salt Lake City, Utah, showed
the reagent consumption and recoveries were
comparable to sodium cyanide, according to
Phoenix.
Test work on -80 mesh lab-scale (30-250 g) and
Testing of the new BIOX MesoTHERM process
has taken place at the Fairview operation in
South Africa
bench-scale (3,000 g) samples saw the
aforementioned gold and silver recoveries, with
ATS consumption of some 4% after six hours. By
comparison, the sodium cyanide gold recoveries
on splits of the same sample over the same time
were 97.8% and 97.7%, while silver recoveries
were 68.1% and 65.5%. The sodium cyanide
reagent consumption for both tests was 6%, the
company said.
Ryan McDermott, CEO of Phoenix Copper, said:
“The use of ATS, which is much less industrially
utilised in the US, but is praised for its non-toxic,
environmental friendly, and low cost nature when
compared to sodium cyanide, would allow us to
time the processing of the Empire precious metals
in parallel with the base metals given that we do
not need to enter into a lengthy permitting
process for the cyanide operation.”
The next step for the Phoenix team will be the
preparation of a pilot-scale test plant sited in the
AuRIC Laboratory scaled to run multiple tonnage
leach samples from various locations around the
Empire deposit using the ATS reagent.
Cutting consumption
Even in processes where cyanidation is a key
component of the metallurgical extraction process,
cyanide consumption is being examined.
This is the case for the recently merged Metso
Outotec’s proprietary bio-oxidation technology,
with Jan van Niekerk, Director: Gold Process
Solutions, Metso Outotec, convinced the latest
updates to the company’s technology will prove to
be a “step change in the applicability of the
process”.
The BIOX process, which has been in
commercial operation for over
30 years, was developed for
the pre-treatment of refractory
concentrates ahead of
conventional cyanide leaching
for gold recovery.
While BIOX has helped
produce circa-25 Moz of gold
since it originated, the amount
of cyanide it consumes has
been one of the inhibitors for
some of the projects adopting
this refractory gold ore
treatment technology.
The company explained:
“Traditionally, cyanide
consumption with
conventional bio-oxidation
residues is higher than with
residues produced through
other oxidative technologies.”
In late June, Outotec (prior
to the merger with Metso) introduced its new
MesoTHERM BIOX ® process to tackle this
problem.
Based on the existing mesophile process within
BIOX, MesoTHERM BIOX offers an easy, costeffective
upgrade path that can cut cyanide
consumption by as much as 50% compared with
conventional bio-oxidation, the company claimed.
“The Outotec MesoTHERM BIOX process
enhances the established mesophile BIOX process
by combining mesophile bio-oxidation technology
with a higher-temperature thermophile oxidative
stage to enable an even more effective overall
sulphide oxidation step,” van Niekerk said.
On top of cutting cyanide consumption by as
much as 50% compared with conventional biooxidation,
MesoTHERM BIOX significantly reduces
the formation of thiocyanate – a common and
stable cyanide species traditionally formed as a
further by-product, the company said.
Solubilised species prevalent in the mesophile
stage are decanted off in an inter-stage thickening
step between the two oxidative processes,
simplifying operation of the thermophile stage, it
explained.
This stage stabilises the thermophile process
so operators can better control the solution
chemistry, according to van Niekerk.
“By introducing the inter-stage step and
controlling the solution chemistry, you can get a
much higher bacterial activity and more stable
operation as well as improve recoveries following
the thermophile process,” he said.
For existing BIOX customers, upgrading to BIOX
MesoTHERM is a “relatively simple process”. It
involves reconfiguring the circuit with the addition
of Outotec High Rate Thickeners for inter-stage
thickening and OKTOP ® Atmospheric Reactors for
the thermophile step.
12 International Mining | JULY/AUGUST 2020