GOLD EXTRACTION
However, POX and roasting have high capital
intensity due to the extreme operating
temperatures and pressures, exotic materials of
construction and the ancillary equipment
required to provide reagents and environmental
controls.
Bioleaching operates at low slurry density and
has comparatively longer residence time, often
several days, which inflates the size of the
leaching circuit. Bio-oxidation is also very
sensitive to cyanide and thiocyanate as they are
toxic to bacteria, so it is necessary to keep the
pretreatment and cyanidation systems separate.
Pressure oxidation and roasting have high
processing costs. Although processing costs can
vary widely depending on power and reagent
consumptions associated with sulphide
oxidation, recent industry information indicates
POX ranges between $50 and $65/t and roasting
about $25-35/t. Woloschuk states that
processing costs are typically the highest
component of plant operating costs for refractory
deposits. This means that at a gold price of
$1,250/oz, these current methods need 0.6-1.6
g/t Au just to cover processing costs. Many
miners are faced with the dilemma that their
refractory gold deposits are either too small to
justify the capital outlay for current refractory
processing technologies, or too low grade to
cover the operational costs, or both.
FLSmidth is working closely together with
miners to unlock value, develop more productive
operati ons, improve energy efficiency, reduce
consumables consumption, and lower operating
costs, Woloschuk says. “The industry continues
to focus on increasing margins through
optimisation initiatives, and the ROL pre-
treatment is a technology we believe has the
potential to unlock significant value in the gold
space. When you couple low operating cost with
low capital intensity, it has significant impact on
the asset Net Present Value (NPV),” he explains.
“Due to the high capital intensity and high
processing costs of current refractory processing
methods, only assets with long mine life and
high grades are achieving investment hurdles.
There are a lot of undeveloped refractory gold
deposits that have less than 3 g/t gold head
grade, and some are coupled with small
resources which translates into short mine lives.
Currently those assets have little to no value as
greenfield deposits and they need a step change
in technology to unlock value,” he concludes.
Industry data on known refractory gold
deposits show that the amount of gold contained
in refractory Measured and Indicated (M&I)
resources is approximately double the contained
gold in refractory gold reserves. Woloschuk says,
some portion of those M&I resources did not
meet cut-off grade using current processing
methods. These are deposits that are sufficiently
38 International Mining | AUGUST 2017
drilled and some portion of the M&I resources
would convert to economically mineable reserves
at a lower cut-off grade.
Large deposits are becoming increasingly
scarce. Since 2012, there have been less than 10
major gold deposits discovered globally. Looking
at the five years prior to 2012, there were nearly
four times as many major discoveries. While not
all of these discoveries were refractory, this is an
indication that there isn’t the abundance of large
gold deposits left, so the industry needs to find
ways to treat smaller deposits going forward.
The ROL process is not merely applicable to
new deposits. According to Woloschuk, there
could be significant benefits to existing
operations as well: “Many existing refractory
operations have low grade stockpiles that are
waiting to be processed at the end of the mine
life. If you add ROL pretreatment to an existing
operation, it could become viable to process low
grade stockpiles earlier, rather than processing
them at the end of the mine life. By reducing the
cut-off grade, you can move those ounces
forward to generate cash flow earlier and this will
increase the asset NPV.”
FLSmidth’s ROL technology could be a game-
changer for the gold industry, displacing current
refractory processing methods, Woloschuk says.
“When we have proven this technology
successful, miners will be looking at a completely
different life of mine plan; by lowering the cut-off
grade, more ore will be converted into reserves,
extending mine life.”
Refining Albion
The Albion Process™ is a combination of ultrafine
grinding and oxidative leaching at atmospheric
pressure. The feed is base or precious metal
concentrates. The sulphides in the feed are
oxidised and liberated, allowing the wanted
metals to be recovered by conventional means.
FLSmidth’s ROL process is still under
development but initial batch studies have
shown very positive results. It is now working
with several gold customers, to develop the
process on a more significant scale
The successful commissioning and ramp up of
the Albion Process at GPM’s gold recovery plant in
Armenia relied on the successful scaling up of
the process from batch and continuous pilot
plant campaigns. Critical information about
reaction kinetics and residence time, grind size
and pulp density were determined at the
laboratory scale and successfully applied to the
commercial scale. A limitation of small scale test
work, is that some parameters cannot be
measured reliably and scaling up is a function of
the physical size of the equipment which isn’t
possible to test with laboratory scale equipment.
Oxygen mass transfer rate is one such
parameter since this is a complex interaction of
GT’s HyperSparge is a proven and cost effective
system for delivering air, oxygen or other
process gases into tanks or vessels for leaching
or oxidation processes. It is a gas sparging
technology that uses an alloy steel injection
lance fitted with a hard wearing ceramic nozzle
to inject a concentrated supersonic jet of gas
into the process solution or slurry. The
supersonic gas jet enters the process stream at
velocities in excess of 400 m/s, creating a region
of very high local shear, resulting in very efficient
mass transfer