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For the treatment of refractory
gold ores:
• Increased gold recovery rates
• Robust technology
• Environmentally friendly
• Commercially viable and cost
effective
BIOX®
Ashanti Goldfields, Ghana, the largest BIOX®
plant in the world.
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The BIOX® process, which pre-treats refractory sulphide
gold ores such as pyrite, arsenopyrite and pyrrhotite,
was developed to increase gold recovery rates during
the metallurgical extraction process. The gold in these
sulphide ores is encapsulated in sulphide minerals
which prevent the gold from being leached by cyanide.
The BIOX® process destroys the sulphide minerals and
exposes the gold for subsequent cyanidation, increasing
recovery rates.
The BIOX® process has many real advantages over
conventional refractory processes such as roasting,
pressure oxidation and nitric acid leaching. These
include:
• Improved rates of gold recovery
• Significantly lower capital costs
• Low running costs
• Robust technology that is suited to remote areas
• Low level of skills required for operation
• Environmentally friendly
• Ongoing process development and improvement
Rights to the process, which has been available
commercially for more than 15 years, are currently
held by Biomin Technologies SA, a subsidiary of Gold
Fields Limited. However, the initial research and
development into the process was conducted by Gencor
Process Research (now Billiton Process Research).
The BIOX® process
The process itself uses a combination of three bacteria
that occur naturally, thiobacillus ferrooxidans,
thiobacillus thiooxidans and leptospirillum ferroxidans,
to break down the sulphide mineral matrix in the ore
being treated, thus freeing the occluded gold for
subsequent cyanidation. The bacteria attach themselves
to the metal sulphide surfaces in the ore, resulting in
the accelerated oxidation of the sulphides.
The BIOX® process involves the continuous feeding
of the flotation concentrate slurry to a series of stirred
reactors.
Low pH levels and a high slurry temperature enhance
the efficiency of the process and it is important that
these parameters are controlled within narrow ranges
so as to maintain the right balance of bacteria in order
to achieve the optimum rate of oxidation.
The reactors are aerated and the slurry temperature
is maintained at the optimum level of 40-45ºC. As the
oxidation reactions of sulphide minerals are exothermic,
it is necessary to cool the tanks so as to maintain the
slurry temperature within the optimum range. This is
done by circulating cooling water and removing the
excess heat via a cooling tower.
The pH level is controlled by adding limestone or
sulphuric acid to the slurry. Since direct sulphide
oxidation requires high levels of oxygen, large volumes
of air have to be injected and dispersed in the slurry.
This is one of the main engineering challenges in the
design of a full-scale bio-reactor.
Furthermore, sufficient carbon dioxide is required for
the bacteria to maintain cellular growth. This is obtained
from the injected air as well as carbonate minerals.
Should the latter be absent, limestone is added.
The bacteria also require nutrients to sustain growth.
Nitrogen, phosphorous and potassium are added to
the primary reactors in various forms and quantities,
depending on the composition of the concentrate being
treated.
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A photomicrograph of the thiobacillus ferrooxidans bacteriawhich is a component of the bacterial culture used in the
BIOX® process.
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Flow diagram of the BIOX® process
Concentrate
Nutrients
Stock tank
Air
Coolingwater
BIOX®
reactors
CCD washthickeners
Cyanidationand CIP
Limestone
Residualdeposit
Neutralisationdeposit
Wash water
The overall residence time in the bio-oxidation reactors,
which is mainly a function of the mineralogy, typically
varies between four and six days. For an ore where the
gold is locked mainly in arsenopyrite, a shorter residence
time is expected to achieve optimum gold liberation
than with an ore where most of the gold is occluded
in pyrite. This is because the oxidation rate of
arsenopyrite is faster than that of pyrite.
Some ores require only partial sulphide oxidation to
liberate the gold. The circuit can be simplified for such
ores and the residence time reduced to two days or
less.
During the bacterial oxidation process, elements like
iron, sulphur and arsenic are dissolved. After oxidation,
the BIOX® product is washed in a counter-current
decantation circuit and the solution is neutralised in
a controlled two-stage process with limestone and/or
lime. The precipitates formed meet environmental
standards set in the United States and can be safely
deposited onto tailings dams. The BIOX® process is
thus a non-polluting, environmentally clean means of
treating refractory ore.
To save water, the neutralised effluent can be mixed
with flotation tailings and thickened. The overflow
solution can be recycled as dilution water in the milling,
flotation and BIOX® sections of the plant. This makes
the process ideally suited for arid regions.
The washed BIOX® product is treated in a conventional
cyanidation plant from which the gold is finally
recovered.
Operating parameters of the BIOX® process
Temperature: 40-45ºC
pH: 1.2-1.6
Percentage solids in feed: 20%
Dissolved oxygen: >2ppmRetention time: 4-6 days
Nutrients fertiliser type ammonium, potassium and phosphorus salts
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Plants in operationFour BIOX® plants are currently in operation – in South
Africa, Ghana, Brazil and Australia – with the most
successful of these being Ashanti’s Sansu plant near
Obuasi in Ghana. The Tamboraque plant in Peru is
currently being recommissioned.
Ashanti, Ghana
Ashanti Goldfields Company investigated several
refractory treatment options including roasting, pressure
oxidation and nitric acid leaching. The BIOX®
technology was selected for, among other reasons, its
lower capital and operating costs, reduced technical
risk, relatively benign environmental impact and its
ease of operation.
Designed with an initial capacity of treating 720 tonnes
of concentrate per day, the Sansu plant has since been
expanded and currently has four modules processing
960 tonnes per day in all. It is by far the largest bio-
oxidation plant in the world and its modular design
makes it possible to apply the technology to large
refractory deposits. The simplicity of the process also
makes it ideal for remote areas.
Furthermore, the metallurgical performance of theSansu BIOX® plant has been highly satisfactory in
coping with the local ore of which there are two types,
which differ widely regarding their mineralogical and
bio-oxidation characteristics. This demanded a plant
design with sufficient flexibility to treat both concentrates,
either individually or as a blend.
The capital cost of the plant totalled US$25 million
(1994 terms) and the operating cost is currently
US$17/tonne milled.
Fairview, South Africa
The BIOX® plant at the Fairview mine in Barberton,
which was the initial pilot plant, has been fully operational
for 15 years. It was originally designed to treat 10
tonnes a day but with the success of the project this
has been increased over time to 55 tonnes of concentrate
per day.
Much of the innovative research work on the bio-
oxidation of refractory gold ores conducted in the late
1970s and early 1980s was driven by the need to
replace the outdated Edward’s roasters at Fairview,
which at the time were seriously contributing to
atmospheric and water pollution in the environmentally
sensitive Barberton area of South Africa.
This plant has played a vital role in the ongoing
development of the BIOX® process as the scale of the
The Tamboraque plant in Peru is currently
being recommissioned.
The Fairview BIOX®
Plant in Mpumalanga province, South Africa,which currently treats 55 tonnes of concentrate a day.
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For further information on the BIOX® process, contact:
Pieter van Aswegen
Gold Fields Limited/Biomin Technologies SA
24 St Andrews Road
Parktown2193
South Africa
6
158 tonnes per day. Since the cost of power initially
amounted to 50% of the operating costs, a natural
gas-fired power station was built to replace the diesel
generation and limit costs.
Future developments
To date more than 250 concentrate and whole ore
samples have been tested for compatibility with the
BIOX® process and more than 15 integrated pilot
plants to monitor and refine the design of commercial
plants have been completed. One such pilot plant,
together with a detailed commercial process design,
was completed for Golden Star Resources Bogoso
Mine in Ghana.
Currently, several projects are at the feasibility stage
or awaiting financial backing. In addition, technology
licence agreements have been signed with gold mining
companies in Uzbekistan, Greece and Australia. Of
these, the largest project is that in Uzbekistan where
the agreement with the Navoi Mining and Metallurgical
Combinat is for a BIOX® plant with an ultimate capacity
of 2,055 tonnes of concentrate a day.
Project development framework
Requirements Duration Results
BIOX® amenability 10 kg concentrate 2.5 months Detailed report with:
testing 10 litre process water Rate of oxidation
Gold recovery vs oxidation
Pre-feasibility study Treatment rate 1 month Conceptual flow diagram
(optional) Concentrate grade Preliminary equipment list
Unit cost data Order-of-magnitude cost estimates
Pilot plant run Testwork agreement 4 months Detailed report with:
1,000 kg sample Design parameters
Reagent consumptions
Operating strategy
Process design Licence agreement 1 month Process design specifications
package Key design criteria Mass balances
Process flow diagrams
Basic engineering Approval of contractor 9 - 12 months Bacterial inoculum
design, construction Operating manual
and commissioning Operator training
Plant commissioning
On-going technical support
Tel: +27 11 644 2574Fax: +27 11 484 0631
E-mail:
Or visit the website:www.goldfields.co.za or
www.gold-fields.com
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