GOLD EXTRACTION
GreenGold CEO, Malcolm Paterson, was
talking about applications in Indonesia back
then, but at ALTA 2019 he highlighted an
Australia case study where the ReCYN process
may lead to the re-start of a historic mine.
The Mt Morgan mine was an operation that, at
the turn of the last century, was the largest gold
and copper producer in the country, according to
GreenGold.
“The legacy is the usual hole in the ground
containing very low pH waters, which are also
contaminated with copper and other metals,”
Paterson said. These waters require continuous
detoxification before river discharge, at
significant ongoing cost to the Queensland
Government, he explained.
In 2018, Carbine Resources proposed a re-
treatment program for the tailings that would
solve the environmental problem and be
economically positive, according to Paterson. The
project, which incorporated the ReCYN process,
failed to attract funding, partly due to excessive
royalty obligation, he said.
Paterson says the rest of the flowsheet on this
project was not optimised to take “full
advantage” of the ReCYN Plant, resulting in high
capital and operating costs.
“GreenGold has reassessed the process
flowsheet and proposed a simplified version
(without a flotation circuit, for example),
optimising the use of the ReCYN process,”
Paterson told delegates. This saw costs drop and
a different royalty arrangement – one that makes
the project more financially attractive – proposed.
The project, which could produce 23,000 oz/y
of gold over a 20-year life according to studies, is
now undergoing a due diligence process,
according to GreenGold.
Arsenic penalties
The processing of gold-bearing sulphides is also
leading to the increased need to address the
arsenic that comes with extracting the gold from
these concentrates.
These sulphides contain arsenic in the form of
arsenopyrite and other complex arsenic sulphide
minerals, with the various forms released during
processing. This arsenic is potentially dangerous
to the environment and, in certain quantities,
viewed as ‘deleterious elements’ by smelters.
DST says it has a different patented process that
can address this problem: the GlassLock Process™.
“The penalty charged by smelters for taking
high arsenic concentrate is very expensive,”
Howlett explained. “Our GlassLock Process
comes in at a fraction of that price, so, if we can
give the miner a ‘value-add’ by selectively
removing and dealing with the arsenic, then it’s a
win-win for everyone.”
DST’s GlassLock Process deals with the
arsenic by integrating it, in various forms, in a
54 International Mining | AUGUST 2019
vitrification mixture of commonly available
reagents, such as silica or recycled glass and a
source of iron, such as hematite, according to
DST. “The mixture is then vitrified producing a
meta-stable oxide system of which can hold up
to 20% arsenic, depending on the form or arsenic
being vitrified.”
Briquetting is used to control the dust from
the mixed product going into the glass making
furnace. The briquettes also help to keep the
surface contacts between the arsenic product
and ingredients, after mixing, to create a
homogeneous glass product, DST said.
“Any arsenic that may happen to volatise can be
recirculated in the system,” the company added.
By using DST’s GlassLock Process, arsenical
compounds can be successfully and permanently
stabilised from “their amorphous states”,
providing greater process control latitude, DST
said. “The stability of glass offers a
sequestration solution for arsenic that will hold
over geological times, removing the need for
waste disposal site monitoring, ad infinitum.”
This has led to the arsenic content in the
concentrate being reduced at >95%, according to
DST, which, as Howlett said, can positively affect
the price companies receive for their product
from smelters and reduce their long-term
environmental burdens.
DST has put this process to the test having, in
2016, constructed an industrial GlassLock Plant
and demonstrated its technology on arsenic
trioxide flue dust wastes produced in a copper
smelting operation.
“During the tests, DST reached optimal
processing conditions for the successful
vitrification of arsenical material. The program
demonstrated the stability of the produced glass,
containing up to 20.4% arsenic while exceeding
the US EPA’s TCLP (toxicity characterisation
leaching procedure) guidelines,” DST said.
Since then, DST’s arsenic vitrification program
has continued to develop and reached the
detailed engineering phase, in 2017, before, in
March 2019, a 3,000 t/y plant was commissioned
at a miner’s processing facility in Africa .
“It’s running every day, 24 h/d,” Howlett said.
“This plant was meant to handle 10% of their
annual arsenic production needs. It was a proof
of concept but is working within the smelter.”
The plant has been performing well judging by
the fact DST is expected to begin engineering of
a full-scale installation by the end of the year,
according to Howlett.
Fine gold recovery
AuStar Gold has been looking to breathe new life
into some historic gold operations in Victoria,
Australia, recently re-commencing gold
processing at its Morning Star process plant.
At the same time as this, the company signed
an agreement with Gekko Systems to acquire a
custom-made intense leach reactor for the
processing of residual sulphide/gold
concentrate. This pact with Gekko should enable
increased overall gold recoveries from the
process plant, and was the last remaining
processing hurdle for AuStar, given the small
component of non-free milling gold present in its
ore, to maximise commercial gold recoveries, the
company said.
The process plant at the Morning Star mine
site uses standard gravity methods (enhanced by
the recent investment in a high-speed centrifugal
concentrator) to recover gold from the Morning
Star and Rose of Denmark ores. “Through the
gravity process, the gold room captures
approximately 65% to 75% of the gold directly to
bullion, with an additional 15% of fine free gold
residing in the middlings as concentrate and up
to 10% of fine free gold in the gold room table
rejects,” AuStar said.
The material containing the fine free gold
(concentrate) not captured directly into bullion at
the Morning Star plant is to be treated by Gekko
Systems offsite, with expected recoveries in
these two fractions of approximately 95% of the
contained gold, the company said. This is
expected to lift total gold recovered and sold
from ore supplied to the mill to better than 90%,
AuStar added.
A custom-made intense leach reactor made at
Gekko’s Ballarat facility and designed to suit
Morning Star concentrate, went into operation
earlier this year, with the company delivering 820
kg of concentrate for processing at the reactor as
of June 11.
Gekko’s line of intense leach reactors, branded
the InLine Leach Reactor (ILR), came to
prominence in 1997, with the introduction of the
world’s first intensive leach production unit,
Gekko says.
The ILR is designed to optimise the recovery of
gold and silver from high-grade gravity and
flotation concentrate streams, with the company
claiming the ILR has the highest unit recoveries
of any concentrate treatment option available.
“The heart of the ILR’s effectiveness is the
horizontal rolling drum. This high shear mixing
zone has been specifically designed to remove
the attrition layer from the gold by speeding up
cyanide access to the gold surface,” Gekko says,
adding that continuous removal of the diffusion
layer significantly improves leach kinetics. “The
Gekko ILR is the only intensive leach unit which
attritions this layer.”
Gekko continues: “The enriched oxygen zone
ensures optimum dissolved oxygen levels are
sustained, thus maintaining a high surface area
for the solution/oxygen interface. Finally, the
solution solid mixing zone allows for the most
efficient chemical reaction.” IM