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HEAP LEACH – SX/EW
drainage. These methods have also been used to
guide C&O practices and provide a level of
confidence in expected outcome.”
At the start of the project, the heap was
estimated to contain 50 Mlb of copper. The data
and findings from seven years of geophysical
investigation at Carlota generally show improved
drainage conditions through newer stacked ore,
but poor drainage persists in parts of the older
truck dumped material. “SSL has been shown to
push raffinate to areas that have been
underleached and favourably change the
Schematic illustration of heap instrumented with
TBT’s VMS. The VMS provides the heap
manager with continuous information on the
chemical composition of the pore solution as
well as online information on the flow
characteristics of the leachant through the heap
Data collected by the system allows direct
measurements of the temporal variations in
sediment water content with respect to the
irrigation cycles implemented on land surface. As
such the solution percolation velocity and fluxes
significant economic impacts on the operation,
profitability, and eventual closure of the mine.
Therefore, any inexpensive means to extract the
last bit of metal from the heap can be of great
value.
“Lately, we have been conducting targeted
subsurface leaching (SSL) on copper ore that has
historically underperformed due to high fines and
compaction. To this end, we recently hit a
milestone in our experience: a billion gallons of
barren solution injected through a number of
in the unsaturated sediments may be measured
on-line and provide direct feedback to the pad
managers on the efficiency of the infiltration
process. In addition, the monitoring system
wells that targets the underleached ore. Our
experience in this endeavour has provided much
insight into heap performance, effects of
localised phenomena, and the importance of
allows continuous sampling of the percolating
solution across the entire ore heap. As such the
chemical evolution of the percolating solution
with respect to the chemical properties of the
implemented leachant on land surface and the
hydraulic and metallurgical monitoring to
understand the economic impact of SSL.”
The Carlota mine in central Arizona includes a
fines-rich, 42 Mt run-of-mine heap that has
mineral dissolution in the heap ore may be
monitored across the entire heap cross section.
Accordingly, the leachant application cycles and
applied concentration may be managed
according to the actual chemical evolution of the
percolating solution in the heap for optimal
mineral extraction.
The VMS is a new addition to the family of
advanced hydrometallurgical technologies and
systems supplied by TBT, which include the
Bateman Settler™, Bateman Pulsed Columns
(BPC) and the Turbulent Technologies Mixing
System.
The VMS enhances monitoring of the heap
leach pad, and provides on-line temperature,
moisture, pressure measurement and continuous
solution sampling. It enables hydraulic and
chemical profiles to be drawn through the heap,
indicating the extent of percolation, leaching and
extraction efficiency. Its analytical sample
collection feature allows monitoring of the leach
profile in almost real-time, in-situ, instead of
waiting until the heap is dismantled.
Dale F. Rucker, Chief Technical Officer of
hydroGEOPHYSICS, explains that “many studies
have shown that surface leaching of low-grade
rubblised ore is generally an effective means to
extract nearly 70 to 90% of a heap’s metal
content. The last 10-30%, however, has
22 International Mining | NOVEMBER 2016
undergone many changes during construction
and operations (C&O)3. Most changes have been
driven by the need to increase production, which
is directly tied to the ability of the ore to drain
and the fines content has limited the mobility of
the leachate. Deviation from the original C&O
plan has concentrated on better stacking
practices and focused raffinate delivery.
“Specifically, the mine has undergone three
major C&O phases, listed in order from oldest to
newest: advanced truck dumping, retreat
conveyance stack ing, and deep well rinsing (or
SSL).
“Truck dumping was part of the original mine
design, but it became apparent that the traffic on
the heap’s surface increased the ore compaction
and density, which ultimately led to lower
irrigation rates. Construction then switched to
retreat stacking, which allowed for better
irrigation rates and little surface ponding. The
SSL program was initiated at the end of active
mining and is being used to target older areas
where copper is abundant and surface leaching is
unable to reach.
“Throughout the life of the heap, geophysical
mapping with electrical resistivity tomography
and ore samples obtained from sonic drilling
have been used to help spatially define
hydraulically troubled areas along with specific
hydraulic properties that give rise to the low
hydraulic conditions to allow for additional
drainage.”
Electrical resistivity is a fairly common
geophysical method to evaluate the internal
structure of heaps and understand the hydraulic
consequences of different C&O strategies. At
Carlota, this method was applied over many
years as C&O evolved.
“The resistivity acquired within the truck dump
material revealed heterogeneous material as
interpreted from the electrical data. A few areas
would be extremely wet, while others appeared
to remain dry. Significant ponding occurred in the
material because localised low permeability
zones prevented adequate drainage. When the
mine switched to retreat stacking, the newer
material atop the truck dumped ore was more
uniform in electrical resistivity, indicating uniform
wetting, but drainage was still an issue though
older truck dumped ore. Interlift drain pipes were
placed at the interface in anticipation of the
problem, with the drains stubbing out at the toe
of the heap. The drains were effective at
removing a portion of the PLS as they were
always full.
“At the last stage of C&O, SSL was initiated to
help with inventory drawdown. The electrical
resistivity data showed conclusively that the
raffinate could cover broad swathes of ore
uniformly and monitoring wells placed near the
rinse wells extracted solution with significantly
higher copper and lower free acid than the
raffinate. Over the next three to four years, SSL
and electrical resistivity monitoring will continue
the inventory drawdown. It is hoped that at least
40 to 50% of the remaining inventory can be
extracted prior to initiating closure.”
A pilot-scale injection test, four rinse wells,
was conducted in 2013. Work examined injection
scenarios and engineering parameters were
developed. The following year was the
operational-scale test with 21 rinse wells.
Monitoring continued. Full scale operation began
in 2015, with 31 additional rinse wells. 2016 sees
the drilling of 32 more rinse wells, with 28 more
planned. The draindown will begin in 2020.
Initial strategy was to start simply by
introducing raffinate to the rinse wells. The initial
well design was by Haley and Aldrich, with