GOLD EXTRACTION
process and bacteria, and what equipment and design to use. The
generation one plants really proved the technical and economic viability of
the process. The success of the Obuasi plant in Ghana – a large 1,000 t/d
plant in a remote location – really established the process.
After 1995, the gold price fell and, over that period to the early-2000s,
we only commissioned one plant. It took until around 2003 for the market
to come back to life, when we started the design for two plants –
Fosterville and Suzdal – both commissioned in 2005. The generation two
plants were typically able to cope with higher sulphur levels and tonnages
than their predecessors. Our basic design had been established at this
point, so it was about finetuning the process. During that period to 2010,
we commissioned six plants.
We then reviewed the commissioning and operating experience gained
from those six plants to take what we learned into our generation three
plants. As we knew the technology basis and the design and selection of
equipment, we focused on operational issues to achieve ease of operation
and improve maintainability. For example, we addressed ore variability in
our design – very few plants operate at the design point for any period of
time. We looked at the robustness of some of the equipment and
developed our service offering to extend the life of the equipment. In the
20 years since we introduced BIOX, the industry, including the way
engineering companies execute projects, had changed and we needed to
adapt the way we worked with them accordingly.
Simultaneously, we started the development of our generation four
design to achieve a major reduction in the capital and operating costs
associated with the implementation and operation of the BIOX process.
We focused on the main capital cost items; blowers, BIOX agitators and
the construction materials of BIOX tanks. Regarding the operating costs,
we wanted to reduce power required for agitation and aeration and
cyanide consumption. This led to the development of a new agitation
system that provides about a 20% reduction in power through optimal
aeration and oxygen supply. We also developed our MesoTherm process
that combines mesophile and thermophile bacteria to get lower overall
cyanide consumption.
IM: Are there any generation four plants now running?
JvN: The Runruno plant, in the Philippines, was the first plant based on the
generation three design. Although not commercialised at that time, the
project team wanted to take advantage of the generation four agitation
system and the potential savings achievable and decided to include that
into the design of the plant. Fosterville has also replaced their older
generation BIOX agitators with the new generation four units to achieve
power savings.
IM: Has the industry requirements around cyanide changed and made you
adapt your offering over the past 30, or so, years?
JvN: Yes, certainly the pressure for plants to manage their water balance
has increased significantly. Part of our generation four developments –
reducing cyanide consumption – is focused on that, but we have also
having SART as part of the extraction process, even when base metal levels
are lower, and choose to live with reduced margins rather than adopt SART,”
he said.
“You do have significant costs with tolerating the levels of cyanide in
your process. It causes you to incur incremental costs, but people don’t add
these up and compare with the cost to implement SART.”
The Mexico project – Lluvia de Ore – is a case he points to: “They are
making money at levels that people did not think possible.”
BQE Water could soon have another plant to add to its reference list, with
GoGold Resources having recently contracted the company to build a SART
plant at its Parral silver-gold tailings operation in Chihuahua, Mexico.
developed the ASTER process for this, which offers biological thiocyanate
destruction to address the water balance and allow plants to recycle the
water to be used in a BIOX plant. BIOX bacteria are very sensitive to
cyanide and thiocyanate toxicity, but, whereas cyanide is relatively easy to
destroy chemically, thiocyanate is not. We had to come up with a process
to address that.
ASTER was developed as a biological means to do that. It is very
efficient at destroying thiocyanate down to less than 1 ppm at a very low
cost. There are three ASTER plants currently in operation, with a fourth
one due to be commissioned soon. Fairview has the first ASTER plant, and
the second one was installed for Suzdal; both of these plants treat tailings
dam return solutions. Suzdal has tested up to 4,500 ppm thiocyanate in
the feed to the ASTER plant that breaks that down to less than 1 ppm. The
third plant is at Runruno where we are detoxifying the CIL tailings
(destroying the thiocyanate) before it goes into the tailings dam to meet
their water licence requirements. Fosterville has also selected the ASTER
process to improve their water balance.
IM: How do you anticipate the process/technology continuing to evolve in
the next decade?
JvN: I think it is all about being able to react to the complexity of the
orebodies. Twenty years ago, a standard pyrite/arsenopyrite refractory
orebody was considered difficult to treat. Nowadays, if we get a standard
pyrite/arsenopyrite refractory orebody, we are very happy! Most of the
orebodies coming to us have got organic carbon, so are double refractory,
or has antimony, copper or other base metals within it. Orebodies are also
more and more variable, and you need to be able to adapt your process to
handle that.
Although usually outside our scope of delivery, we are investigating
alternatives to cyanide. I believe cyanide is probably going to be around
for a long time; it’s just so efficient and easy to use and, let’s be honest,
relatively safe compared with some of the alternatives. But, as a lot of our
customers are now asking for alternatives, a lot of our work will go into
that.
The HiTECC process that is used to mitigate preg-robbing and gold loss
when treating double refractory orebodies, originally developed by
Fosterville, is now in the Outotec business portfolio. We see more and
more need for that type of process where you combine BIOX and HiTECC
to address preg-robbing when treating double refractory orebodies.
A new focus area for Outotec is to supply BIOX-tailored equipment.
Historically, when BIOX was still part of Gold Fields and Biomin, we were
only able to supply the technology. As part of Outotec, we can now supply
the equipment as well. This can provide advantages for clients, such as a
single point of contact for the full package and, in house, we can build our
30-plus years of experience into the design of the equipment, as well as
the technology ‘island’. I think there is a need for that on smaller projects
– where clients are capital sensitive and don’t have the money for large
engineering companies – and equipment suppliers can provide a full
technology package for the entire plant, or part of the plant.
Construction is expected to be completed by the end of 2019 and, once
the plant is commissioned, BQE Water will provide operations support
services for a monthly fee for a period of three years.
Kratochvil concluded: “Hopefully, as more SART plants are built, the
upside of SART becomes better quantified. Unless there are benchmarks,
people can't really perform apples-to-apples comparisons or holistic
lifecycle costs analysis for projects with and without SART, respectively.”
We featured GreenGold Engineering’s ReCYN™ process earlier in the year
(IM March 2019), a process that, through an innovative resin-bead
absorbent, reduces cyanide consumption by 50%, capturing free cyanide
and recycling it back into the leach circuit, according to the company.
AUGUST 2019 | International Mining 53