GOLD EXTRACTION
s gold-bearing orebodies continue to become increasingly complex,
processing technologies such as bioleaching are, again, being looked
at to solve processing problems. BIOX is a process that has been
around for more than 30 years, over that time establishing itself as a proven
technology that can aid gold recovery through the pre-treatment of sulphide-
based refractory orebodies. Since 2015, the process has been further
developed by Outotec, a global leader in minerals and metals processing.
Dan Gleeson spoke with Jan van Niekerk, Senior Manager, BIOX,
Outotec, to find out a little more about the technology and how it has
helped produce over 22 Moz of gold since its launch in 1986.
A
IM: What are the main reasons customers are currently looking to
incorporate Outotec’s BIOX process into their flowsheet? Is it purely to
improve gold recovery?
JvN: If you have a refractory orebody, you need a pre-treatment process to
destroy the sulphides and liberate the gold. Otherwise you are not going
to get the required gold recovery.
Outotec is in a unique position in that it has all three of the main
oxidative pre-treatment processes as part of its offering – BIOX, pressure
oxidation (POX) and roasting. We can offer all three and evaluate them for
the client in an objective manner. This is one of the reasons Outotec is
becoming a leader in the treatment of refractory orebodies.
BIOX, specifically, offers some real advantages to clients, especially for
small to medium-sized projects, or projects in remote locations. That isn’t
to say BIOX cannot compete with other technologies for bigger projects,
but those are not in the sweetspot.
This is because BIOX offers lower capital costs than POX and roasting
due to its relative simplicity; it has less process controls and uses standard
atmospheric tanks operating at 40°C. It is also a simpler process to
operate and maintain for the same reason. For remote locations, it means
your spares holdings are lower and you can use local labour for plant
operation. The process is also safer as you are operating at lower
temperatures and pressures.
Mostly, BIOX is also environmentally favourable; it produces a stable
arsenic precipitate that can be deposited onto a tailings dam. We’ve
proven that over the past 30 years of operation. For example, at Fairview,
in South Africa, they are reprocessing an old tailings dam. They have
checked the stability of the old precipitates and proven that it’s as, or
more, stable than what was originally deposited onto the tailings dam. I
think the arsenic question has been answered in that respect.
IM: Out of the 13 BIOX plants successfully commissioned worldwide, which
one has seen the biggest improvement in terms of recoveries, cost,
consistency, etc? What do you put these results down to?
JvN: Plants like Fosterville, in Australia, and Fairview, in South Africa, have
dedicated metallurgical teams that are constantly looking at process
improvements. At those plants, you can really see the benefit of this – not
only in the BIOX process, but for the full plant flowsheet – in terms of
recovery and stability improvements.
“SART comes in because it avoids the negative interference of base metals
in the extraction of precious metals,” BQE Water’s David Kratochvil says
52 International Mining | AUGUST 2019
The Runruno plant, in the Philippines, was the first BIOX plant based on
the generation three design, Jan van Niekerk says
At most of the BIOX plants, you do see improvements over the years as
operators get to know the process and get a better understanding of how
to manage it.
We very often get asked the question, especially in the North American
markets, if BIOX bioleaching can operate in cold climates. We have a
reference – the Suzdal BIOX plant in Kazakhstan – that has been in
successful operation for almost 15 years. This proves that the process
generates enough heat, and, in fact, you still need to cool the BIOX tanks
during winter and when the tanks are located outside of the building. The
bacteria themselves are not exposed to the ambient conditions; they
operate in the BIOX reactors where the temperature is controlled at 40°C.
But it is important to understand the energy balance and ensure that it is
working out in your favour. We design the plant for potential power failures
where, in such extreme conditions, the pipelines or even the tanks may
freeze over.
IM: As high-grade, easy-to-treat ores continue to fall in volume, are you
finding more potential BIOX customers?
JvN: I think the gold price showing stability over the last few years at a
reasonable level is probably a bigger push for the industry. The bulk of
existing and new projects are still oxide projects – these are easier and
cheaper to develop – but, there are a lot of projects looking at treating
refractory orebodies and there are quite a few looking at going from an
existing oxide deposit to the sulphide orebody below. At a lower gold
price, they were not really looking at such developments.
IM: How has the technology and process evolved in the more than 30 years
since BIOX was introduced to the market?
JvN: We’re now on our fourth generation BIOX design.
The first generation was from the start of the technology, in 1986, up
until to about 1995. In that period, we commissioned five plants and were
still developing the technology. The focus was on understanding the
“Second, is the plant at LLuvia de Oro, which stood idle in the Sonora
desert (Mexico) for almost a decade exposed to elements and was
subsequently re-started with only a minor capital expense. This plant
moved the goal post by demonstrating profitable SART operation at copper
concentrations much lower than what the gold industry would think of as
the cutoff for SART applicability.”
BQE Water is on a mission to educate the wider community about SART’s
applicability in the mining sector, with Kratochvil saying, historically, SART
would only be considered in situations when cyanide-soluble base metals
such as copper and zinc are present at levels of around 10 kg/t of ore.
“What we see is that people underestimate the increased costs of not