HEAP LEACHING AND SX
Keeping tabs on the pad
Isolating the ore from the rest of the 80 m high
leach pad on a HDPE liner, adding more
cyanide to the mix and increasing the leach
time from 90 days to 250 days has seen gold
recoveries step up at Eldorado Gold’s Kı ş lada ğ
heap leach operation in Turkey (credit:
Eldorado Gold)
Having dropped the mill construction plan,
Time, cost and complexity
continue to shape the heap
leaching and solvent
extraction space,
Dan Gleeson hears
roponents of heap leaching continually
talk up its potential to decrease capital
and operating costs, increase scalability,
reduce water use and process lower-grade ores.
P
Still, it remains one of the trickiest processing
methods to perfect.
The cost benefits and ability to process low-
grade deposits that heap leaching brings mean
metal recoveries can be that much lower than
other processes. Yet, to achieve these
recoveries, mining companies may need to wait
many months, or even years – hence the
process’ appealing and risky nature.
It’s worth providing an example here to
reinforce this point.
Kışladağ kicker
Eldorado Gold’s Kışladağ heap leach operation,
in Western Anatolia, Turkey, produced its first
gold in 2006, close to a year after construction
commenced.
Prior to this, a feasibility study had shown it
to be a large, low grade, predominantly
sulphide porphyry deposit. When the study was
carried out, the gold price was sitting at around
$300/oz and two processing options were on
the table – a milling route that came with an
80% gold recovery (there being a small
refractory component in the deposit) and a heap
leach option that had an average gold recovery
of 60%.
Paul Skayman, Eldorado Gold COO, told IM:
“Given the low grade and the capital and
operating costs that came with the two options,
we decided heap leach made the most sense.”
During the years the mine has been
operating, the company has revisited the
processing question – opting around 20 months
ago to pursue a milling option as the heap leach
The company had, until this point, leached
the material in 10 m lifts and, as it irrigated that
lift, the solution would flow through not only the
10 m of material, but the rest of the circa-80 m
pad.
“As a result, we’re often not seeing significant
solution flows for up to three months after we
commence irrigation,” Skayman explained. “It’s
like a massive flywheel – there is a lot of inertia
involved – and predicting the gold coming off
the pad gets increasingly difficult.”
During this time in 2017, Eldorado personnel
were not seeing any significant gold flows
coming off the pad.
“We were worried about the pad chemistry so
made the decision to adjust the cyanide dosing
the company has put some material back in a
leaching column, adjusting the cyanide
concentration and continued to see gold
recoveries increase with longer leach times.
“It seems to be a very slow leaching material
compared with what we have seen for most of
the mine life,” Skayman said.
The changing characteristics of ore can affect
recoveries in any metallurgical process, but it is
the time taken for the metal to be leached and
the “inertia” Skayman speaks of with heap
leaching that can potentially cause more harm
to the bottom line.
This means sampling and testing must
continually be carried out to ensure processing
is working as expected.
In the case of Kışladağ, Eldorado Gold was
able to avoid the $500 million milling project by
isolating the ore from the rest of the 80 m high
leach pad on a HDPE liner and increasing the
leach time from 90 days to 250 days.
This has enabled the company to achieve closer
metallurgical control of the material and get
back to the recoveries that made the mine one
of its most profitable operators – producing gold
expecting that to help with the pad chemistry,
but not expecting any improvement in gold
recovery for another couple of months as it got at an expected C2 total operating cost of $610-
660/oz of gold sold in 2019.
The refinements to Kışladağ might not end
through that material,” he said.
Eldorado expected similar recoveries at the
end of this period to what they were previously
achieving (around 60%). The recoveries never here. Skayman said the company is currently
looking into adding cyanide earlier in the
leaching process – to potentially reduce that
250-day leach cycle – and weighing up the use
met that mark.
So concerned was the company with these
results, it elected to stop mining at the end of of high pressure grinding rolls (HPGR) to
potentially replace the existing tertiary crushing
circuit at the mine and further improve
April 2018 and laid out its plans to build a mill
at the mine to ensure the operation remained recoveries.
profitable.
At this time, the final material it mined was
placed on a HDPE liner on top of the main leach Modelling mineralogy
pad and its normal irrigation process continued.
Skayman said the company was expecting a
50% gold recovery from that material following
laboratory test work. “After about 100 days, we
got about a 50% recovery, so we said, ‘yes,
that’s all fine. It’s behaving as expected’. But, as
we continued to leach it – we let it rest for three
or four weeks at a time and then put it back for
operation was struggling with recoveries.
Skayman explained: “In mid-2017, we noticed
we weren’t getting the flow of gold from the leach – we noticed the recovery just kept
coming up. Ultimately, after about 250 days of
leaching, we actually got to about 58%.” This
leach pad that we were expecting. The leach
pad, by now, is about 140 Mt of material and is
getting up to about 80 m in height.” compared favourably with the 40% recoveries
the company was getting from the original 90-
day column tests.
BHP’s Olympic Dam iron ore copper gold-
uranium deposit, in South Australia, is
renowned for its complex mineralogy and
processing requirements.
The process flowsheet includes grinding,
flotation, leaching, solvent extraction, smelting
and refining, with the concentrator processing
run-of-mine ore via bulk sulphide flotation to
produce a copper concentrate and flotation
tailings.
The flotation tailings contain the majority of
the uranium, while copper concentrate is treated
in a smelting complex, after which copper
cathode is produced by electrolytic refining.
Gold and silver bullion is, meanwhile,
recovered from the anode slimes and uranium is
NOVEMBER 2019 | International Mining 39