LEACHING & SX/EW
HPGR breakthrough
During the recent Vancouver Gold Conference,
Holger Plath, Vice President of thyssenkrupp
USA, provided an update on the successful use of
HPGR in gold leaching. Although HPGR
technology has been recognised since the late
1980s as a potential powerful tool for heap
leaching, operational applications did not
proceed until recently. Today, four gold
operations have implemented HPGRs. All of
those process operations confirm the early
assessments, ie using HPGR within the crushing
stages shows significant benefits for improving
the overall hydrometallurgical parameters.
The largest and most interesting such gold
heap leach HPGR project is at Golden Queen in
California. It has installed a tkIS Polycom HPGR
unit and operates under the following
parameters:
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HPGR model
Operating mode
Feed material
Throughput
Design throughput
Feed moisture
Feed size
Product size
Agglomeration
Other positive aspects of HPGR use in leaching
increasingly confirm the value of this technology
not only as a high(er) throughput, power- and
steel-wear saving comminution system but also
as a metallurgical tool. These include, Plath
reports, but are not limited to the following:
n Higher availability than any conventional
crusher
n Studded tyre wear protection achieved
extended lifetime of the rolls from 3,000 to
>10,000 h
n Possible rapid adjustments of pressure and
roll speed to match the geo-metallurgical
variance of the ore
n Possible recycling of edge material of the
HPGR discharge material for a finer product
n Indications that HPGR use will result in both
lower retained and saturated moisture in the
heap (depending on the ore/rock type)
POLYCOM ® 17/12-5
Continuous, open circuit (option for edge recycle)
heap-leachable gold ore
750 t/h (fresh feed, dry base) (Nominal)
900 t/h (with edge recycle)
3%
F100 <45 mm
P80 <6 mm
drum with cement addition
Historically, concerns were voiced in the
industry regarding the generation of excessive
amounts of fines when using HPGR as a tertiary
crusher. These were not substantiated. These
concerns have been addressed via a combination
of efforts including better ore characterisation,
optimal HPGR operation, agglomeration, heap
construction, leach practices and avoidance of
heap compaction. Although HPGR’s major benefit
to hydrometallurgy is its micro-fracturing of the
rock matrix, it does generate some finer product
sizes which are inherently beneficial for gold heap
leaching. Extensive testing between 1996 and
2016 has documented that HPGR technology,
when used under best-practice operating
conditions, including the adequate pressure
settings, will not introduce excessive amounts of
finer material.
The operational data confirm that the HPGR’s
performance contributes to substantially better
gold extractions (several percent) which are in
line with the bench- and pilot testing. In order to
assist in future gold leach operations in North
America for converting to or using HPGR,
thyssenkrupp has installed a Pilotwal HPGR unit
at the Kappes, Cassiday & Associates test
facilities in Reno, Nevada. This unit can handle
samples as small as 100 kg for trade-off and
feasibility studies but it can also process up to 20
t/h for larger scale testing and semi-commercial
purposes.
36 International Mining | NOVEMBER 2017
n Even with variable rock types and alteration,
HPGRs can generate discharge products
which (a) exhibit good agglomeration and (b)
show favourable PSDs for geo-technical heap
stability
n Faster leach kinetics
n Substantial increases of gold extractions with
up to 20% higher than achieved via
conventional crushing
n Recent publications suggest that HPGR may
even offer a good potential for heap leaching
Witwatersrand gold ores.
Re-thinking the standard approach of leaching
gold ores with new HPGR flowsheet and process
concepts may actually achieve metallurgical
results thus far believed to be out of reach. The
concept of de-sliming prior to leaching, has
become of considerable significance (again) with
the use of HPGR. Finally, larger HPGR units could
become a cutting-edge optimisation for bio-
leaching of low-grade refractory ores using a
combination of finer blasting and a primary
crusher followed by in-pit HPGR.
Best practice
A panel discussion was held immediately
following the Cyanide Alleviation/Alternatives
Forum during the Gold-PM Sessions at ALTA 2017
in Perth in May. The discussion centred mainly on
cyanide recovery/recycle and on-site cyanide
manufacture as key issues in satisfying the
concerns of regulators and the public and
allowing its use to continue.
Ralph Hackle, Rio Tinto (Australia), pointed out
that there was a panic about cyanide and a push
towards alternatives to cyanide in the late 1990s,
which perhaps is happening again. “It appears to
be similar to the sentiment around coal mining
and renewable energy. The fact is that cyanide is
technically and economically superior to any other
lixiviant. We’ve made some incremental
improvements with alternative lixiviants, but
cyanide is still ‘king’, and is used for most of the
world’s gold production. If cyanide is banned, gold
production will decline and the price will go up.”
He agreed with Malcolm Paterson, PT Green
Gold Engineering (Indonesia) (see August 2017
issue p45) that the best approach is to make
cyanide acceptable. Paterson believes that on-
site production of cyanide is an important issue
in changing the public perception about cyanide.
Xianwen Dai, CSIRO Mineral Resources
(Australia) reported that the Chinese government
has become tougher in approving plants using
cyanide, and Chinese companies have become
interested in thiosulphate. A cyanide sales tax
has been introduced and cyanide is not allowed
near population centres and environmental
protection zones. The trend is towards the
Chinese government becoming increasingly strict
on the use of cyanide.
Panayiotis Papacharalmbous, PT Kisangani
Boomi (Indonesia), commented that in Indonesia
it is not just an environmental issue, but also
about conservation of minerals. So, unless
operators can show that they are getting good
recoveries, it will be difficult to get a permit. The
trend is towards looking beyond processes such
as heap leaching with 70% recovery to better
processes with 95% or higher if possible. Some
of these alternative processes still have quite a
way to catch up with cyanide.
ALTA’s Alan Turner asked whether we are
heading for an increasing number of
governments banning cyanide, and increasing
community and media opposition. If so, is the
industry taking it seriously enough in developing
alternatives, or is the mining industry burying its
head in the sand?
Cyanide alternatives
Chairing the discussion, Stephen La Brooy,
Ausenco (Australia) opened the panel and the
floor for questions regarding cyanide
alternatives. He noted that the main sessions
included papers on chloride, bromide,
thiosulphate, thiourea, and that several of the
world’s experts on the application of
thiosulphate were present.
John O’Callaghan, Newcrest, (Australia), asked
whether there are alternatives being worked on
which were not covered during the main