South African scientists are increasing the viability and
environmental friendliness of micro-organism technology
applications in mining.
The process of using micro- organisms, such as bacteria, to leach
out metals such as gold and copper from ore has a significant
impact on the industry, specifically its ability to extract
low-grade ore once thought worthless, a factor of increasing
importance for the industry as high-grade ores are being rapidly
mined out.
As a result, Randburg-based minerals technology and research
development group Mintek, has set up its HeapStar bioheap leaching
technology at the Sarcheshmeh plant in Iran, home to one of the
world’s largest copper deposits.
According to project leader Tony Pinches, the pilot plant valued at
$6,5-million was paid for by the National Iranian Copper Industries
Company (Nicico) and became fully operational in November last
year.
“It has exceeded all expectations and the plant is running
extremely well,” adds Pinches.
Interest in the technology was sparked as a result of worldwide
falling copper grades, explains Mintek technology manager Roger
Paul.
As a result, Nicico and Mintek signed an agreement in 2004 to
develop bioheap leaching for sulp-hide ore that has a grade of,
typically, 0,6% copper.
Through the application of the technology, Mintek has found it
possible to recover as much as 60% of the copper in the heap
material.
Professor Doug Rawlings of the University of Stellenbosch’s
microbiology department has been one of forces behind the
technology in South Africa.
“Biomining technology has become a very successful means of
extracting minerals such as gold and copper. In the past few years,
the technology has advanced and metals are now being leached out
more efficiently. The technology is being used in countries around
the world, including Australia, Ghana and Brazil,” adds
Rawlings There are two different biomining processes: leaching and
bio oxidation.
Leaching removes a soluble substance from a solid structure by
turning it into a liquid form easy for extraction. The process is
widely used to extract copper. About 25% of all copper worldwide,
worth more than $1-billion annually, is produced through
bio-leaching.
Bio-oxidation uses oxygen and micro-organisms as a pretreatment
process prior to the extraction of metals.
This method is mainly used in gold-mining.
Gold is usually found scattered in small particles in the ore
encased by minerals, which make extraction difficult.
Micro-organisms that eat away at the mineral coating are used to
pretreat the gold ores before they are extracted.
The advantage of biomining is that it has a far less polluting
effect on the environment.
Some traditional mining pro- cesses are especially toxic in that
the process involves the use of roasting and chemicals.
Biomining reduces the emission of poisonous gases and reduces toxic
liquid waste.
The technology is much slower than traditional mining and it can
not be used on a wide variety of ores, such as much of the gold
rock in South Africa. Local mining companies have, nevertheless,
exported the technology to countries with the appropriate
ore.
Biomining is effective on certain copper, cobalt, gold and uranium
ores.
Although it can be used to extract metals such as zinc and nickel
deposits that are economically workable, ways of using this
technology have still to be found.
Rawlings says that early innovative research work on biomining was
done in the late 1970s and early 1980s, driven by a need to replace
the outdated roasters at the Fairview mine near Barberton.
The initial research and develop- ment was conducted by Gencor
Process Research, which became Billiton Process Research and later
the Johannesburg Technology Centre of BHP Billiton.
“Back then, the mines used to roast ore by releasing arsenic,
a lot of which made its way into the environment,” explains
Rawlings.
“The mine needed to extend its roaster, but it decided to
experiment with a bioleaching plant. The system exceeded our
expectations considerably. “After operating for two to three
years, the bacteria grew more quickly. The retention time for the
metals was seven days then, but as the bacteria grew stronger, it
drop-ped to three-and-a-half days.
“It also became resistant to arsenic because it inherited DNA
from other organisms and acquired arsenic- resistant genes.”
Fairview, which had been creating atmospheric and water pollution
in the Barberton area, gradually became a far more
environment-friendly mine as biomining technology took hold.
Many important innovations in bio-oxidation technology have been
made at the Fairview mine. It has a plant that has been fully
operational for more than 15 years.
It was originally designed to treat ten tons a day but, with the
success of the project, this has gradually been upgraded so that it
now produces 55 t/d of concentrate.
At the moment, the rights to the Biox process in South Africa are
held by Biomin Technologies SA, a subsidiary of Gold Fields. The
largest application of Biox technology is at AngloGold
Ashanti’s Sansu plant near Obuasi, in Ghana. The Sansu plant
currently has four modules processing 960 t/d.
It is by far the largest bio-oxidation plant in the world.
Its success is largely due to the grade of the local ore of which
there are two types.
These differ widely, depending on their mineralogical and bio-
oxidation characteristics. This has demanded a plant design with
sufficient flexibility to treat both concentrates, either
individually or as a blend.
Biox plants are found in Brazil at the S