LEACHING & SX/EW
There have been some
interesting breakthroughs
in the application of heap
leaching and SX/EW,
John Chadwick reports
Heaps and extracts
O
ne of the great challenges for the
industry has been to find an alternative
means to process the most abundant
type of copper ore on Earth, chalcopyrite. This is
particularly important now with global demand
for copper set to increase and world copper
resources dwindling and average grades falling.
As FLSmidth Director for the Rapid Oxidative
Leach (ROL) process, Gary Roy, states: “that
makes a compelling business case for
developing more effective processes to treat
lower grade copper ores. Leaching is the most
widely used low-cost, extractive metallurgy
technique for converting metals into soluble
salts in water.
“Until now, leaching has only been applied to
oxide ores and simple sulphide ores. Nobody
has been able to identify an economically viable
process to dissolve chalcopyrite (CuFeS 2 ). ”
In a dramatic breakthrough, FLSmidth has
cracked the code using a mechano-chemical
approach. It also won the global Top 100 R&D
award at the R&D 100s for the ROL process last
year.
Sally Rocks is Senior R&D Chemist with the
ROL process team in FLSmidth. After five years
of intensive research and laboratory work, she
and a team of chemical engineers, geologists
and minerals processing engineers has cracked
the code of primary copper sulphide leaching,
making it feasible to produce cathode copper on
site using existing equipment and bypass the
costly smelting process completely.
“We knew we were addressing a very difficult
challenge where countless other scientists have
failed and our satisfaction at finding a solution
has been immense,” Rocks states.
Leach reactions are highly complex systems
dependent upon interactions at the solid-liquid
interface. “We have succeeded by identifying a
new process that strains the atomic arrangement
of the minerals themselves, affecting the solid
below the interface,” she explains.
34 International Mining | NOVEMBER 2017
Chalcopyrite has presented the team with a
unique set of challenges. “It has an inbuilt
chemical defence,” she continues. “When
sulphide minerals start to leach, the resulting
elemental sulphur creates a passivating layer
consisting of a colloid film on the particle
su rface that slows the chemical reactions that
leach copper.”
Chemists have struggled for years with the
challenges of this defensive passivation layer
during processing - a critical issue has always
been the high energy required which has made
leaching uneconomical.
FLSmidth notes: “Energy intensive ultra-fine
grinding, catalysts, high temperatures and high
pressures have all been trialled. And some have
succeeded. But the recovery rates have been too
low and the energy consumption too high to
create a process that could be commercially
viable.”
Weatherly International’s Tschudi copper project
is an open pit copper mine located about 20 km
west of Tsumeb, Namibia, designed to produce
17,000 t/y of LME Grade A copper cathode. The
heap leach pad consists of 18 cells with an
overall pad 1,100 m by 500 m in plan. The pad
has a composite clay/HDPE liner overlain by a
granular drainage layer incorporating a network
of slotted drainage pipes. The solution ponds are
double lined with HDPE incorporating a seepage
interception layer over clay bedding. The storm
water and raw water ponds have a single HDPE
liner. The pad is stacked in 4 m lifts for
weathered oxide ore and 6 m lifts for unaltered
ore to a final height of 36 m. Knight Piésold
conducted a heap leach bankable feasibility
study, including geotechnical investigation and
laboratory testing of both construction materials
and leached ore. This was followed by detailed
design and construction monitoring of the leap
leach pad, drainage system and collection
channels, PLS, ILS, storm water, raffinate, SX
event, and raw water ponds
Rocks and her team took a mechano-chemical
approach coupled with pre-leach activation.
To activate chalcopyrite for faster copper
leach kinetics and improved copper recoveries,
the team needed to extract iron from the lattice
using copper ions. In order to initiate the process,
the research team needed to find a way to first
destabilise the structure of the chalcopyrite.
“We discovered that very small changes to
the structure of chalcopyrite could impact on
how rapidly it leaches. We designed an
'activation step', where we doped the copper-
rich chalcopyrite mineral with copper sulphate.
The copper sulphate quickly reacts with the
solid chalcopyrite to create a new solid structure
that produces strain throughout the entire
copper-bearing particle,” she explains.
That turned out to be a key process
component. The ‘doping’ was necessary to make
the chalcopyrite more reactive.
Long-term trends in processed copper ore grades.
Source:ALTA Keynote paper Process expertise: “The key to managing
hydrometallurgical project risk” by Mark Benz, President, MRB Business Services