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956 Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors)
Solution collection system for a ROM leach dump: Design
criteria to meet best available control techniques rules
Jorge Puell1, 2, Paloma Lazaro1, 3, Víctor Tenorio1
1Mining and Geological Engineering Department, University of Arizona, 1235 James E. Rogers Way, Tuc-
son, AZ 85719, United Sates; jpuell@email.arizona.edu
2Freeport-McMoran Copper & Gold, 4521 US-191, Morenci, AZ 85540, United States
3Rio Tinto Borates, 14486 Borax Road, Boron, CA 92342, United States
Abstract
Low grade ore dumps subject to leaching operations typically report the pregnant leach
solution (PLS) to a downstream collection point, which is subsequently pumped out
and processed for copper recovery. Proper design and operation of leaching collection
facilities are critical to prevent the run-of-mine (ROM) dump from seepage and unper-
mitted discharge of these solutions into the environment. Structures making up for the
solution collection system may include PLS impoundments, storm water diversions,
check dams, lined pre-stacking material, collection channels, ponds and other facilities.
is paper outlines the criteria to determine the speci c engineering design of such
facilities by meeting the use of the best available control techniques to minimize envi-
ronmental releases and comply with government regulations. Techniques and calcula-
tions should be performed to estimate: i) limit-equilibrium slope stability; ii) runo and
storages evaluation under historical storm event scenarios; iii) peak discharge values
and reductions, and iv) facilities size optimization. Finally, an application example in
support of a copper leaching dump exposed to extreme climate conditions in a surface
mine illustrates the proposed design criteria, methods, assumptions and outcomes.
Keywords: Solution collection system, leach dump, best available control techniques
Introduction
In the mining industry, leaching is a hydro-
metallurgical process that separates valuable
minerals from ore by dissolving the mineral
with a dilute cyanide solution in the case of
gold, or a sulfuric acid to dissolve copper
(Hearn RL & Hoye R 1998). Dump leaching
is a technique where run-of-mine low grade
material is stacked on prepared sites (pads)
and wetted with lixiviant chemicals under
atmospheric conditions. e metal content
is then recovered from the rich ‘pregnant
leach solution’ through mineral processing
(Zanbak C 2012). e main environmental
concern in permitting dump leaching facili-
ties is that of the pregnant leach solution and
its containments, and thus it is absolutely
imperative that no leakage takes place from
the solution collection systems (Van Zyl D et
al. 1998). Other ways of leaking solution can
result from dump sliding, broken pipes, dam
failure, and the occurrence of over ows due
to severe storm events.
Economic and sustainable management
of dump leaching operations implies that the
mining company should proactively adopt
the best available practices in the design, con-
struction, operation, maintenance, closure
and post-closure of every component of these
facilities. Modern environmental legislation
has introduced the concept of ‘best available
technology’, ‘best available control techniques’
or ‘best available demonstrated control tech-
nique’ which would ensure the elimination or
the greatest degree of discharge reduction of
pollutants in order to prevent groundwater
contamination (Singh MM 2010). While the
directives given by environmental authori-
ties must follow a de ned general pattern,
the applications of the best available practices
at particular mines will depend on several
site-speci c factors, such as the meteorology,
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Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors)
hydrogeology, topography, geology, and mag-
nitude of the mining operation (Hearn RL &
Hoye R 1998). A brief description of the gen-
eral guidance to engineering criteria and best
available practices is shown in Table 1.
Although all signi cant components in a
leach dump systems should be evaluated, the
present work will only focus on measure op-
eration ow rates, retention check dams, and
leach dump slope stability.
Case Study: ROM Leach dump
Final dump design can accommodate the
placement and leaching of approximately
212-Million tonnes of ROM material (Fig-
ure 1) located in the Western US. is paper
evaluates the design and construction of the
leach dump and associated leach collection
facilities directed to satisfy the long-range
mine plans and to meet environmental and
civil design requirements. Items within the
design criteria include regional design factors
(design storm events and ow rates), leaching
solution properties, application rate and de-
sign process ow rates. e base surface area
will be compacted and graded to conduct
pregnant leaching solution to the collection
area located in the Southeast side of the leach
dump. e dump design criterion has been
developed to meet the requirements for liner
systems, piping layout, and slope stability.
Table 1 General guidance/requirements for leach dump facilities
Criteria General Guidance
Site characterization Appropriate when topography, soil properties, vadose zone, surface and subsurface hydrology
may in uence the dump design
Surface water control Identify all surface waters locations around the facility (lakes, springs, etc.). Information on 100-
year oodplains in the area. Control of runo and run-on.
Geological Hazards Identify actual and potential geologic hazards (soil collapse, landslides, subsidence and
settlement, liquefaction)
Solution/Waste/Tailings
characterization
Identify chemical and physical characteristics of solution, waste and tailings
Pad construction Site preparation for pad construction. Grubbing, grading and sub-grading the area.
Liner speci cations Design and installation of pad components. Appropriate speci cations for non-storm water
ponds, PLS impoundments, leaching dumps, tailings impoundments.
Stability design Provide stability under static and seismic loading conditions. Shear strength evaluation.
Recommended minimum factor of safety 1.3 (Non-storm water ponds, PLS Impoundments) and
1.5 static factor of safety for tailings impoundments. For leach dumps and engineered heap leach
dumps, te recommended FOS for static analysis is 1.5 (if geosynthetic components are not used),
and 1.3 otherwise
Closure /Post-Closure Present a Closure/Post-closure plan to prevent/control releases
Figure 1 Leach dump design for a total
capacity of 212-Million tonnes
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958 Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors)
Operational ow rates
Surface water hydrology analysis around the
proposed Leach Dump has been performed
to determine the approximate peak stormwa-
ter runo volume for the 100-year, 24-hour
storm event. Sub-basin areas are calculated
from the projection of the leach dump design
to the topography. It is assumed that all rain-
fall will percolate through the leach dump
and eventually will report to a contingency
pond for capacity considerations, and the
solution collection pipelines will discharge
into the pregnant leach solution pond, with
over ows reporting to the contingency pond
in case of upset conditions. Rainfall depth is
0.094 m and the profess solution ows were
calculated assuming nominal solution appli-
cation rate of 0.00489 m/h (Table 2)
Retention check dams
Check dams are structures installed perpen-
dicular to water channels and are aimed to
control wash o , trap sediments from run-
o and prevent discharge of pollutants to
groundwater (ADEQ 2005). Small check
dams can be used to reinforce the surface wa-
ter control systems in conjunction with ma-
jor dams or reservoirs. Check dams should be
sized to retain the maximum volume of run-
o attainable in considerations of site limi-
tations and access and will protect the work
during construction of the lined areas of the
leach dump infrastructure. Runo is calcu-
lated by amount of precipitation in the catch-
ment area and by in ltration properties of the
soil type and moisture (U.S. Depart, of Agri-
culture 1986). Check dams are placed within
the dump limits in locations where high vol-
ume precipitation ow could negatively im-
pact the leach dump foundation liners system
during construction and prior to leach dump
operation. e number of check dams, mate-
rial quantities and storage capacities are esti-
mated based upon storage requirements for
a 100-year, 24-hour design storm event so
that the impact on the dump foundation con-
struction activities and downstream system
due to precipitation are minimal (Figure 2)
Runo depths for the 100-year, 24-hours
storm for each watershed have been calcu-
lated using the TR-55 method (U.S. Depart,
of Agriculture 1986). Total estimated runo
depth, then, is the starting point to design the
check dams, so that the storage capacity cre-
ated by check dams can exceed or be equal to
Table 2 Summary of hydrology calculation results
Area
(sq.
miles)
Area
(m2)
Depth 100-
yr, 24-hr
(m)
Volume
(m3)
Leach
dump
Flow Rate
(m3/s)
0.2559 662,896 0.0940 62,299 54.0
Figure2 Check dam watershed site
plan
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Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors)
the estimated runo volume. e optimized
model is subject to successive iterations by
strategically placing each check dam to maxi-
mize its ability to retain stormwater. e op-
timization was performed iteratively using 5’
contours to estimate storage capacity for each
check dam at an upstream and downstream
check dam slope of 2H: 1V.
e results for the optimized stormwa-
ter check dam placement are shown in Table
3-4 below. Note that P is rainfall in inches
(NOAA 2018); CN is the curve number, S is
the potential maximum retention a er runo
begins (inches), and Q is runo . Values for
S and Q are calculated based on the TR-55
method (U.S. Depart, of Agriculture 1986).
In order to reduce the risk of damage to
the dump foundation, construction of check
dams is completed upstream of construction
activities. Once the leach dump has been es-
tablished and ready to receive run-of-mine
material from the mine, checks dams will be
covered. Because of the its temporary nature,
check dams are exempt from freeboard and
spillway construction
Leach dump slope stability
Leach dumps usually become these large
mining structures for which slope stability
studies must be considered in their designs.
Appropriate procedures for stability analysis
of leach dumps are determined by the type
of rock the dump is composed, whether it is
classi ed as hard or so rock, and other dump
design considerations that include: maximum
height, volume, slope angle, foundation ma-
terial and conditions, and berms at the edges
of li s (Marcus 1997). Leach dumps built
with hard, durable broken rock will be stable
under static conditions and will only require
evaluating slope stability if failure can occur
through potentially weak foundation. On the
other hand, for dumps built with so rock,
static and seismic stability analysis should be
performed. e factor of safety (FOS) is the
minimum ratio of available shear strength to
the shear stress required for equilibrium. e
recommended FOS is 1.3 for leach dumps
where site speci c testing and geosynthetic
material have been used, otherwise the rec-
ommended FOS is 1.5 (ADEQ 2005).
A limit-equilibrium analysis was per-
formed to assess the global stability of the
leach dump for the ultimate design. e slope
stability method calculates the minimum
shear stress to maintain the slope stable. e
maximum shear resistance is calculated for
the corresponding shear surface using rock
strength properties and pore water pressures.
One cross-section has been analysed that best
Table 3 Check dam analysis
P = Rainfall 100-yr (m) Curve number CN (m) S=1000/CN - 10 (m) Q=(P-0.25S)2/
(P+0.8S) (m)
Overexcavation
depth (m)
0.094 2.210 0.038 0.060 1.5
Table 4 Check dam storage volume for the Optimized check dam model (10 total locations)
Check
Dam #
Area Upstream of
Dam (m2 )
Estimated Runo
(m3)
Dam Height (m) Dam Length (m) Storage Volume
Available (m3)
C-01 19,974 1,199 1.5 167.6 2,370
C-02 38,090 2,286 4.6 174.3 8,104
C-03 65,961 3,959 4.6 576.1 26,759
C-04 56,950 3,418 4.6 306.3 14,297
C-05 42,364 2,543 3.0 249.3 6,957
C-06 100,335 6,022 7.6 216.4 21,178
C-07 82,962 4,979 3.0 319.4 8,945
C-08 45,151 2,710 1.5 278.0 3,899
C-09 23,133 1,388 1.5 188.1 2,676
C-10 15,979 959 1.5 134.4 1,911
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960 Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors)
represent the most adverse slope conditions
using a two-dimensional, limit equilibrium
modelling so ware Geo Studio Slope/W
(GeoSlope 2012). Morgenstern-Price method
of analysis was used to evaluate every cross-
section, considering both static and seismic
conditions. An e ective friction angle of
37degrees (angle of repose) that corresponds
with a slope of approximately 1.3H: 1V sus-
tains slope inclination of the ROM leach
dump. Prior to ROM material deliveries,
dump footprint will be covered by a liner pro-
tection system (liners and geomembranes).
A erwards and during the entire dumping
operation, a consistent leaching solution ow,
runo and rain water should be maintained
through the leach dump and collection chan-
nels.
Based on the mine plans, the dump con-
sists of three main components: a liner system
at the dump foundation, low grade ROM ma-
terial until a horizontal level is established to
place Oxide ROM material on top. e slope
geometries of the cross sections used in the
analysis is provided in Figure 3. Stratigraphy
and material properties used in slope stability
are summarized in Table 5. In addition, the
hydrostatic head in the proposed dump de-
sign is assumed to be 1 m.
Conclusions
Environmental quality standards may be vio-
lated around active leach dump facilities by
leachates discharges that can seep into the
groundwater. erefore, the applications of
e cient water management practices in min-
ing are mandatory for permitting approval
and renewal. Modern environmental regula-
tions that adopt the concept of ‘best available
technology’ work dynamically and are open
to using state-of-the-art technology that has
proven to be the best available in the industry.
Table 5 Material Properties
Material name Unit Weight (kN/m3) E ective Cohesion (KPa) Friction angle (degrees)
Foundation 22 900 35
Low Grade ROM 21 0 37
Oxide ROM 21 0 37
Figure 3 Slope Stability Analysis. Cross-Section A-A
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