ORE SORTING
gone from being mainly used in diamond and
industrial metal operations to potentially finding
new homes in uranium, iron ore, coal, copper,
gold, silver and phosphate mines, to name a few.
XRT
The ore sorting solution gaining most traction of
late is XRT.
Most of the XRT solutions on the market
recognise and separate materials based on their
specific atomic density, allowing a high level of
sorting purity irrespective of size, moisture or
surface pollution, according to the companies
manufacturing the machines.
In addition to a number of diamond case study
examples (see The ore sorting effect below),
tungsten and tin operations have also seen XRT
machines work wonders.
For instance, Minsur’s San Rafael tin operation
in Peru repaid the $24 million capital cost that
came with buying a TOMRA 3,000 t/d XRT ore
sorter within four months of installation.
The use of sensor-based XRT ore sorting
converted uneconomic waste material into
economic ore, according to TOMRA’s Robben,
meaning material below the cutoff grade for the
main plant, set at 0.9% Sn in 2014, was able to
be treated with lower specific operating costs,
thus bolstering reserves.
And, capacity of the San Rafael main plant
freed up after the installation of the ore sorter,
with nameplate rising to 3,600 t/d, compared
with 2,950 t/d, Robben said.
Companies are also starting to realise the
solutions can be applied at the front end of
operations, too.
For instance, at a bulk sample trial in Western
Australia for Novo Resources’ Karratha gold
project, a combination of XRT and
electromagnetic induction (EM) sensors were
recently used to concentrate gold.
The nuggety mineralisation witnessed at
Karratha had, up until this point, proven tricky to
separate from the waste materials, but recent
studies showed a combination of XRT and EM
sensor-based ore sorting could do exactly this.
Novo said the XRT identifies rocks containing
particles of high atomic mass such as gold, while
EM identifies rocks that become electrically
charged due to the presence of metallic particles.
At Vista Gold’s Mt Todd gold project in
Australia the combination of XRT and laser-based
ore sorting has shown a coarse fraction (+16 mm)
high pressure grinding rolled product can
efficiently be separated into waste and ore piles.
The laser sensor, in this case, detects quartz in
the ore after it has already gone through an XRT-
based process.
Robben was quick to dispel the opinion that
existing mines are reticent about using XRT ore
sorting in fear of underfeeding their follow-on
processing plants.
“As the application of sensor-based ore
sorting often leads to a reduction of total cash
costs, it turns resources into reserves and
actually increases the life of the mine. Marginal
resources are turned into reserves, for example,
at Wolfram’s Mittersill mine in Austria, Coeur
Alaska’s Kensington gold mine in the US or
Minsur’s San Rafael tin mine in Peru.”
Kroukamp, who with Cronimet has overseen
the installation and running of several XRT ore
sorting solutions, explains why there may be a
need to apply more than one sensor-based
solution at mining projects in the future.
“Once you start going into finely disseminated
tungsten ores, especially scheelite, you get these
little spots throughout the rock…When you start
Density separation
his month, W Resources is expecting the mechanical completion of an allmineral jig plant at its
La Parrilla tungsten-tin mine in Portugal.
With a throughput of 350 t/h, the two alljig ® jigs will provide grading, enrichment and cleaning
of the pre-ground ore.
The alljig process involves the feed material initially being fluidised with pulsed water, with the
grains graded according to density. The jigs separate the specifically lighter scheelite-poor yield
from the specifically heavier scheelite-rich ore from the stratified material bed, allmineral said.
The processing plant, with a capacity of 155 t/h connected to the pre-concentration stage
through the alljig jigs, will ensure tailings are separated from the valuable material, thereby
achieving a 65% WO 3 recovery, the company said.
The fines produced in several processing stages are to be recovered in a separate circuit.
allmineral also delivered a concentrator for the plant at La Parrilla, which provides the final
stage of beneficiation in the process to produce a 66% WO 3 concentrate and 62% Sn final
concentrate.
Following the first shipment of tungsten concentrate from the operation, announced in
December, W completed three further shipments as of February 5.
Cumulative production at the operation has exceeded 50 t of tungsten concentrate, with output
rates expected to grow in later months as production switches to higher-grade mined ore from
historic tailings feed.
T
going too small, the [XRT] machine fails as a
result of detection. The signal coming back is
negligible because of, let’s call it, background
noise,” he said.
Robben said TOMRA is continuously
developing the hardware and software to
improve detection efficiency, and that it is
tailoring its image processing to detect “specific
mineralogical features”.
This detection issue may not be addressed by
using more sensors, according to Guthrie.
“Multiple sensors can provide alternative ways
to categorise the ore or waste, and potentially
allow for more effective sorting, however the
heterogeneity of the ore is what will be the
deciding factor. It should also be noted that for
low concentration elements or minerals, the
sorters rely on associations with higher
concentration minerals rather than attempting to
detect the valuable component. This will rely on
identifiable and consistent associations, which
may not be possible with highly complex
orebodies.”
The value proposition
Even if a mining company has a heterogeneous
and distinguishable orebody suitable for ore
sorting detection, it doesn’t necessarily mean ore
sorting should be applied to it, according to
Guthrie.
There are, broadly, three cases for improving
project value through ore sorting, he said.
These include:
n Converting sub-grade waste (including
previously rejected material) to ore that can be
economic at modest recovery of values;
n Sorting of ore to increase grade and reduce
subsequent processing and transport costs,
and;
n Sorting of ore to bring forward cash flow.
Guthrie said: “The first case adds value to the
project by conversion of waste to ore. On this
basis, recovery is not critical and the value of the
recovered material needs to cover processing
costs plus margin. This can add significant value
to a project where the processing capacity exceeds
the ore supply rate and mine waste can be
converted to ‘ore’ by simple low cost processing.”
Such applications are common in the mining
space and where Cronimet normally starts its
conversation with mining customers, according
to Kroukamp.
Guthrie said the second case – sorting of ore
to increase grade and reduce subsequent
processing and transport costs – is generally
only economically favourable if the processing
and ore transport costs are high (in relation to
head grade) and the recovery of values is also
high (eg >95%) .
“This is particularly applicable to massive
orebodies where there is little disseminated
MARCH 2019 | International Mining