Mine excursion
area, which resulted in management’s
decision not to use a central plant.
Instead, each of the processing plants is
the central unit around which mining
takes place in a radius of three to five
kilometres. This also allows for easier
relocation to new mining centroids,
as each plant is dismantled after three
to five years and moved to another
chromite-rich deposit identified during
the exploration programme.
According to Leon Richardson, chief
operation officer at ACF, some of the
chromite will be processed into low and
ultra-low carbon ferrochrome in the
aluminothermic facility, currently under
construction, to be used in the alloy and
stainless-steel industries. The company’s
main chrome processing facility is
located at a site near Chinyika in the
Lalapanzi Black Rhino Conservation
area, located north of the Sebakwe Dam,
south of the Munyati River.
In a country where the unemployment
levels are estimated to be between 90%
and 95% (depending on the source),
ACF’s project has been a saviour for
many families in the region. Kaka says
the mine employs more than 1 100 local
Zimbabweans and about 36 expatriates,
and with skills transfer programmes in
place, Zimbabweans are set to reap more
benefits in the future. “Our recruitment
programme specifically targets local
employees from the Midlands area and
only specialised skills are sought outside
the Midlands Province,” says Kaka.
Stripping the chrome
The big advantage of strip mining is
of course that there is no drilling and
blasting involved, so the impact on the
environment is minimal. Moreover, it
reduces the operating cost significantly.
The mine is really a large earthmoving
project and as a result, it requires
substantial infrastructure development
and maintenance. ACF has constructed
about 87km of gravel roads in the past
four years to access the mining units.
Negotiating harsh road conditions
impacts heavily on equipment and
haul trucks and requires a stringent
maintenance regime, but that cost is
offset by the straightforward mining and
processing methods and the fact that the
strip ratio is zero, although the grade is
not the best in the world.
ACF’s chrome is so close to surface
that it can be detected without digging.
“It is an alluvial deposit, and we do strip
mining in the enriched valleys based on
extensive geological prospecting. We
define the resource in terms of a split —
that means in two components, namely
a ‘washable’ portion and a ‘non-washable’
portion. The ‘non-washable’ portion
does not have economic value,” says
Richardson.
The vegetation in the area is normally
a good indicator of the quality of the
specific deposit. According to Barry
Jones, group geologist, tall grasses are a
sign of better splits: “Big trees indicate a
very poor split; in other words, most of
the material in that area is bigger than
1mm. In most cases though, the deposit
is heterogenous, where the split can
be anything from 20% ‘washable’ and
80% ‘non-washable’, to 85% ‘washable’
and 15% ‘non-washable’.” The chrome
content is defined at a minimum of 7%
chrome in some places, and more than
20% in others.
Basic mining methods
According to Richardson, the mining
cycle is reasonably straightforward: “It
starts with the definition of the resource,
a description of the vegetation type,
and then we do a complete evaluation.
After mine planning, we start with
bush clearing, primary extraction, pre-
screening, transportation to the washing
plant, return of the discard to the
mining site, and then the rehabilitation,”
says Richardson. Haul trucks load the
stripped material, it is processed in the
washing plants, and the slurry is then
stored in a dam and left to dry, after
which it is returned to the mining site,
where it is used again to fill the mined-
out area. This entire process takes about
18 months. “However,” says Richardson,
“the long time-lag is one of the reasons
why we introduced new thickeners.
We aim to improve the process with
each new plant we build, and through
good engineering and innovation,
Plant 6 is kitted out with the most
advanced technology, including the
Magra thickeners. The thickeners will
accelerate the process tremendously,”
says Richardson. (Ed’s note: Plant 6 was
started-up when Mining Mirror visited
the mine.)
At the moment, the tailings in ACF’s
slurry dams contain about 17% solids,
while the thickeners will increase that
figure to approximately more than 65%.
This will reduce the drying time of the
slurry dramatically, and as a result the
material will be ready for rehabilitation
sooner.
JULY 2018 MINING MIRROR
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