Mine excursion
designed to run at 2.5 million tonnes and
upgraded to 3.1 million tonnes per annum.
Solar reduces costs
Sion Kamati, plant metallurgist at Otjikoto.
Last year, Otjikoto installed a 7MW solar
plant to augment the supply of power
from its HFO power plant and that,
according to Barnard, has resulted in
significant cost savings. “Thirteen per cent
of our total costs were spent on HFO for
power generation.”
Before the construction of the solar
plant, the mine relied on 24MW of
installed generation capacity provided
by four heavy fuel oil (HFO) diesel
generators and three backup generators.
With a consistent demand of 12.5MW
per hour, the plant consumed about 21.7
million litres of HFO in 2017, at a cost
of close to USD10.5-million. According
to Dawe, the solar plant has made a big
difference. “The ‘green’ energy generated
by the solar plant has enabled us to shut
down one HFO generator during the day,
and that has resulted in substantial cost
savings,” says Dawe.
Blending to optimise
Peter Mawoyo, production manager at Otjikoto.
Eric Barnard, acting general manager at Otjikoto.
[18] MINING MIRROR FEBRUARY 2019
The grades in different areas of Otjikoto’s
two open pits — Otjikoto and Wolfshag
— vary substantially and therefore, an
efficient blending strategy is necessary
to optimise the processing plant’s
performance. Mawoyo explains that the
high-grade blocks contain gold at about
2 grams per tonne, but he emphasises
that these high-grade areas are few and
far between, and that medium-grade and
low-grade mining blocks dominate the
surface areas of both pits.
The high-grade ore needs to be blended
down to about 1.5 grams per tonne, as
these blocks are usually associated with a
high sulphur content, the grades of which
adversely affect the recoveries.
“Usually, high grades are associated
with ore containing high percentages
of pyrrhotite, which, in turn, has a high
sulphur content and impacts negatively
on the recoveries. So, if we process the
high-grade ore, the sulphur content would
increase, with the result that the cyanide
consumption and oxygen demand would
be higher; we wouldn’t have enough
oxygen and cyanide in the leach circuit;
and the leach efficiencies would drop. To
maintain the plus 98% recoveries, we have
to blend the high-grade ore on the ROM
pad and reduce the sulphur content in the
process from approximately 4% to 2%,
www.miningmirror.co.za