SEARCHING FOR THE OPTIMAL

HEAP LEACH APPLICATION RATE

Orlando Bernal

Crush Leach Superintendent

Sociedad Minera Cerro Verde S.A.A.

Obernal@phelpsd.com.pe

Guillermo Velarde

Senior Metallurgist

Sociedad Minera Cerro Verde S.A.A.

gvelarde@phelpsd.com.pe

ABSTRACT

Sociedad Minera Cerro Verde S.A.A, a subsidiary of Phelps Dodge Mining

Company, operates a copper mining facility located in southern Peru. The mine for leach

operation produces 90,000 mtpy of copper cathode from secondary sulfide ore utilizing

crushing, leaching, solvent extraction and electrowinning processes.

Over the years, the leach solution handling and application rates have undergone

constant development. This paper discusses the achievements in leach application design

to (1) satisfy the needs of the “lixiviant request rate”, which requires the use of

application rates as low as 3.5 l/h-m2 and as high as 15 l/h-m2 with variable rest periods

according to the leach kinetics, and (2) alleviate poor solution distribution on the surface

of the leach heaps inherent to drip emitters.

On one hand, too low, early application rates result in poor surface solution

distribution and the formation of upside down cones of dry, non-leached material in the

top meter of stacked ore, with heavy jarosite and iron salt precipitation; moreover, ferric

iron is lost as insoluble precipitates.

On the other hand, high unit flows associated with poor agglomerate quality can

lead to uneven solution flow patterns within the heap leading to solution short-circuits,

fines migration, and ponding with adverse effects on acid consumption and recovery.

INTRODUCTION

Cerro Verde, located in southwestern Peru, began its leach, solvent extraction and

electro-winning (LX/SX/EW) operation under Minero Peru in April 1977 producing at a

rate of 33 000 metric tonnes per year of high purity copper cathodes. The copper was

extracted using leach pads to leach copper oxide ore, principally chrysocolla and

bronchantite. The material was crushed to a size of two-inches then truck dumped onto

asphalt-lined pads where solution was applied with wigglers.

In the early 1980’s the oxide ore reserve was exhausted and the mining operation

began to exploit secondary sulfide ore. Cyprus Amax acquired majority of shares (82%)

the mine in 1994 during a national privatization process and undertook a leach process

improvement project with the aim of producing 48,000 mtpy of cathodes. The initial

project goals were quickly surpassed and continued improvements over the past seven

years enabled production to reach the current level of 90,000 mtpy. In 1999 Phelps

Dodge acquired Cyprus along with its participation in Cerro Verde.

Early on, in the 1970’s, copper heap leaching utilized wigglers and wobblers for

solution application. In the 1980´s some of the leach operations began to use drippers as

the irrigation systems improved, Cerro Verde adopted dripper technology in the 1990’s.

Improvements to these systems were made with hydraulic changes in the irrigation

designs (main pipes & grid irrigation) to obtain as uniform and constant flows as possible

and which were not sensitive to pressure variations. These changes were the result of a

team effort which investigated:

The Main Irrigation Pipes Redesign & Development.

Implementations of pressure regulating valves, gauges.

Use of different types and sizes of drippers

Use of new irrigation grids that reduce or eliminate the formation of cones within

whose limits (between borders) precipitation / lost of ferric iron takes place as a result

of the droplet incipient irrigation that characterizes dripper flow.

Size and drip emitter types varied from 8,0 to 2,2 liters per hour. The current

design is 1,0 liters per hour including a mini wobblers grid that operate between 0,08 to

0,14 l/s, with the purpose of eliminating the formation of cones from to the droplet flow.

The diameters of drip lines varied from 12 mm to 20 mm and finally settling on a

diameter of 16 mm.

HEAP LEACHING TECHNOLOGY

It is well-known that an initial guideline for leaching solution application is the

solution flow applied to a given area of the leaching module (L/h/m2) commonly

denominated "Flow Rate." In 1995, Cerro Verde, the flow rate design parameter for the

heap leaching of sulfides was 7.8 liters per hour per square meter as a continuous solution

application.

Later the concept designated as “Leach Rate” arose. This concept in many leaching

operations is commonly referred to as the solution requirement for the contained copper

dissolution and displacement. Leach Rate is generally defined as the amount of leach

solution applied per ton of ore during a given leach cycle. It is generally stated as cubic

meters per metric tonne (m3/mt) and is accomplished using a constant application rate

throughout the cycle. In 1998 Cerro Verde, in order to improve the inefficient use of the

leach solution with a continuous application, adopted the use of an intermittent

application with variable flow rate.

Figure 1 – Application of Variable Intermittent Flow Technology

Finally in 2001, Cerro Verde created a new guideline for the dosage of the

leaching solution that overcomes limitations of "Flow Rate" and "Leach Rate". It is

denominate “Lixiviant Request Rate”.

This rate is based on the kinetics of the mass transfer, diffusion, and reaction rate

processes which generate a variable solution demand expressed as the ratio of cubic

meters of solution per kilogram of copper extracted (m3/kg Cu). This ratio represents the

optimum volume of leach solution applied as a function of the extractable copper content

in the ore and expected kinetic performance (Equations 1 and 2). This has allowed the

application of intermittent and variable flows as a function of the variable demand for

leach solution which is a result of the variable leach extraction kinetics, as shown in

Figure 2.

Cu2S + 2 Fe3+

= Cu2+

+ CuS + 2 Fe2+

(1)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0 20 40 60 80 100 120 140 160 180 200 220 240

TIM E (days)

Flo

w R

ate

(l/

h.m

2)

0

10

20

30

40

50

60

70

80

Re

co

ve

ry C

uT

%

Current Recovery

Design Recovery

2 m3/ton 1 m

3/ton

CuS + 2 Fe3+

= Cu2+

+ 2 Fe2+

+ So

(2)

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280

Time (Days)

l/h.m

2

Theoretical Flow Rate Industrial Flow Rate

Figure 2 – Request Leach Rate Model Application

HANDLING SOLUTION DEVELOPMENT

Managing leach solution application to provide uniform wetting is undoubtedly

an important part of leaching. The solution to this problem required a team effort to

design the handling solution main pipes, drip line grid, install pressure regulating valves,

install pressure gauges, pressure compensation manifolds - flashing system, test various

types of drippers and drip lines, and ultimately install mini wobblers into the grid.

Development Irrigation Grid Design

The great lack of uniformity in the irrigation and the highly variable recoveries

obtained in the first lift of the Cerro Verde permanent pad led to the a revision of the

initial hydraulic design of the irrigation pipes as shown in Figure 3. Evidently there was

an inadequate design of the location of the main pipes in the irrigation unit (module) that

left areas without irrigation. This revision also identified that the dripper size of 8 liters

per hour in the grid of 0,80 x 0,80 was not compatible with the application rate of 7,8

L/h/m2, in fact it was closer to a rate of 12 L/h/m2 for which the main pipes were not

adequate.. This was also the reason the flow was low in the last drippers on the drip line

and the flow was higher in the first drippers. In order to correct this, in the last leach

modules of the 3rd

Lift, the dripper size was changed to 4 liters per hour. Both flow

conditions were corrected with a new design (the second) shown on Table 1.

Additionally this revision identified that the initial pipe design, failed to consider

the negative effect of the slope which works against the drip lines. This was corrected

utilizing a third design. (Table I).

CuT: 0, 79 %

Height: 7, 20 m Ratio: 0, 863

Request: 0, 45 m3/ kg Cu

2

m8

2 m

84

m

ZONE 1 ZONE 2 ZONE 3

4" 4" 4"

6" 6"

80 m 80 m 80 m

8"

Distance between lines: 0.80 m

Distance between drippers: 0.80 m

Drippers: 8 LPH

Area without rinse Area without rinse Area without rinse

Figure 3 - Leach Module Main Pipes & Drip lines - Project Design

In the fourth design (Table I) an important change was made in the drip line

diameter from 16 mm to 20 mm while decreasing the size of drippers to 2,2 liters per

hour since the main pipes were relocated to one side of the module requiring less

accessories. The installation of pressure compensation manifolds in the drip lines,

flushing systems and pressure gauges in the main pipes and in the drip lines improved the

handling of the pressures and achieved of application rates with irrigation uniformity

over 90%.

8

4 m

ZONE 1 ZONE 2

80 m 80 m

4" 2" 4" 3"

6"

8"

4" 4" 3" 2"2"3"

Distance between lines: 0.40 m

Distance between drippers: 0.40 m

Nominal Flow Rate: 1 LPH

24 PSI24 PSI

Slope 0.5 - 1.5 %

Slope: 2 - 3 %

6" 4"

2"3"

6"4"

23 PSI

Figure 4 - Leach Module Main Pipes & Drip lines - Sixth Design

While maintaining the same diameter of main pipes header pipes, the area of the

leach module was reduced increasing the irrigation uniformity to 95%. This modification

was carried out in 2004 and is the sixth design that is shown in Figure 4. Additionally it

incorporated double pressure compensation manifolds - flushing system, drip lines of 16

mm of diameter with a dripper size of 1,0 liter per hour.

Actual Pipes Design

The application of Lixiviant Request Rate requires an application of variable

irrigation flows, initially high rates, and then diminishing until becoming alternating

cycles of irrigation-rest which also diminishes as the process advances the periods of

irrigation, everything in accordance with the particular kinetics of copper extraction of

the mineral contained within the leaching unit.

The conventional design of a single header was unable to respond to this wide

rangin demand of flows (from 15 L/h/m2 to 3,5 L/h/m2) that were needed. This demand

led to the development at the end of the year 2004 of a double header system. The two

headers also allowed the reduction of the irrigation grid and this was implemented

industrially in 2005 on the Lift 11 (Figure 5). The installation of mini wobblers in the

irrigation grid was also implemented for application flow rates between 0,08 to 0,14 L/s,

in order to reduce the precipitations of ferric iron on the border of the irrigation cones

whose occurrence will be explained later.

ZONE 1

Wobbler Feed

ZONE 2

102 m

16

0 m

4"3"

4"3"2"

Distance between lines: 0.80-0.40 m

Distance between drippers: 0.20 m

Nominal Flow Rate: 1 LPH

35 PSI35 PSI

Slope 0.5 - 1.5 %

Slope: 2 - 3 %

6" 4"

2"

6"

Feed Pipe

18 PSI 18 PSI

Pipe Flashing

6"

6"

6" 4"6"

4"

4"

6"8"

Pipe Flashing

Figure 5 - Main Pipes Leach Module Two Headers & Drip lines - Actual Design

Grid Design

In Table I, all the modifications in dripper design and drip line size are shown

resulting in dripper size changes from 8 liters per hour (L/h) to 4 then 2,2 and ultimately

1 L/h. The drip line diameters have changed as well from 12 millimeters (mm) to 20 and

finally to 16 mm, all for applied flow rates between 8,0 liters for hour per square meter

(L/h/m2) to 4,0 L/h/m

2 with one header, as shown in Figure 6 and the application rate

Equation 3.

Table I - Historical Irrigation Grid

Time LiftIrrigation Pipes

Design

Grid Spacing

(Line x Dripper)

m

Dripper

Q

LPH

Drip Line

Diameter

mm

Application

Rate (15 psi)

l/h/m2

1996 - 1997 1 Proyect 0.80 X 0.80 8.0 16 8.0 (*)

1998 - 1999 2 & 3 Second 0.80 X 0.80 8.0 16 8.0 (*)

1999 - 2000 4 & 5 Second 0.80 X 0.80 4.0 16 6.25

2000 - 2001 6 Third 0.80 X 0.40 4.0 16 12.5

2001 - 2002 7 Fourth 0.40 X 0.80 2.2 16 6.9

2002 - 2003 8 Fourth 0.80 X 0.40 2.2 20 6.9

2003 - 2004 9 & 10 Fifth 0.40 X 0.40 2.0 16 12.5

2004 10 (Cells 5 a 12) Sixth 0.40 X 0.40 1.0 16 6.3

1.0 16 6.2 - 12.52005 - 2006 11 & 12 Seventh / 2 Headers0.80 X 0.20

0.40 X 0.20 (*) Project Design Rate

Figure 6 – Dripper Selection Criteria in S.M.C.V.

Application Rate = Q . (3)

(L * D)

Where: Q = Dripper Flow Rate

L = Distance between lines

D = Distance between drips

REDUCTION OF THE PRECIPITATIONS OF FERRIC IRON

In 2004, during the heap leaching of Pad 4 a large trench was cut, 6,5 meters deep

by 70,0 meters long. The figure 7 (photographs 1 and 2) sample the trench and the

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

0 5 10 15 20 25 30 35 40

Drip Pressure (PSI)

Flo

w R

ate

(L

PH

)

Design Limit Operation Limit

precipitation of the ferric iron that took place vertically inside of the heap along the edges

of the (borders) of the irrigation cones that is characteristic of the irrigation drip emitter.

This precipitation is evidenced on the walls of the trench and almost invariably took

place following a precipitation pattern.

This phenomenon was interpreted as an important and undesirable occurrence

because it implied loss of iron in the leaching solutions and as the ferric iron is the

oxidizing agent of the secondary sulfides of copper was the same as losing sulfuric acid

with economic and metallurgical consequences.

Figure 7 – Ferric Iron Precipitation in the Cones Borders - Dripper Irrigation

The interpretation of the mechanism of the precipitation was the following:

The droplet points of the dripping irrigation solution, form cones with the maximum flow

along the the axis of the cones and the minimum flow tending to zero on the borders of

these cones. The pH of leaching solution in the limits have an extremely little flow and

may increase to the point of producing iron precipitation in the form of jarosites.

Table II – Behavior of the Iron in Pad 4

Cu FeT Fe2+ Fe3+ Cu FeT Fe2+ Fe3+

1996 1.02 3.99 2.13 1.86 3.38 4.93 3.33 1.59

1997 1.10 4.60 2.07 2.54 2.83 4.99 3.32 1.66

1998 1.02 5.35 3.11 2.71 3.05 5.70 4.20 1.52

1999 1.11 5.20 2.32 2.89 3.50 5.50 3.95 1.54

2000 1.24 4.91 2.08 2.84 3.19 5.08 3.54 1.55

2001 1.16 4.96 2.32 2.64 3.03 5.20 3.85 1.37

2002 0.71 5.05 2.37 2.69 3.00 5.27 3.89 1.38

2003 0.51 4.53 1.93 2.60 2.83 4.72 3.67 1.05

2004 0.51 4.07 1.92 2.16 2.77 4.32 3.92 0.40

2005 0.50 4.36 2.09 2.26 2.62 4.64 4.25 0.39

2006 0.61 4.99 2.45 2.52 2.65 5.25 4.76 0.49

ILS + RAFF PLS

Year

It was evident these occurrences needed to be prevented. It was necessary to

avoid the occurrence of elevated pH. The irrigation grid was reduced so that the cone

limits cross closer to the surface of the pile and this action could be supplemented, with

an occasional irrigation with micro-wobblers especially when beginning the irrigation of

the heap. This was made possible with the double header design of 2005.

The figure 8 (photographs 3 and 4) shows the walls of another trench made in the

heap at the end of the leaching cycle, in late of 2006, to evaluate the results of the

prevention measures. It can be appreciated that the precipitation patterns disappeared and

a few, isolated spots of precipitate had taken place in points where the irrigation was not

satisfactory. It is considered that with the measures taken avoided at least 85% of the iron

loss by precipitation of the ferric sulfate.

Figure 8 – Elimination of Ferric Iron Precipitation by combining: Dripper Irrigation &

Non Continuous Aspersion (Only Begin Rinse and After Rest Periods)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

19

96

19

97

19

98

19

99

20

00

20

01

20

02

20

03

20

04

20

05

20

06

FeT

(g

/l)

&

Hei

gh

t x

10

(m

)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40F

e3+/F

eT

FeT Pad Height Ratio Fe+3/FeT

Figure 9 – Behavior of Iron in PLS Pad 4

Figure 9, shows the total iron concentration in the leaching solutions, evidently

affected by having strongly reduced jarosite precipitation inside the lift, as explained

earlier. Although the ferric iron concentration does not show the same tendency there are

other effects taking part such as the greater demand by the process.

INCREASE IN COPPER RECOVERY

The improvement in the superficial irrigation when reducing the irrigation grid in

a dripper system combined with a micro, non-continuous, spray (wobbler) and a variable

leaching regime, achieved more effective leaching, managing to leach a greater amount

of the mineral in the heap surface and reducing the losses of ferric iron by precipitations.

The higher concentration of ferric iron, oxidant of copper sulfides, enabled a more

favorable recovery. The quantification of its effect is very difficult to predict,

nevertheless we must accept the benefit qualitatively.

On the other hand, the improvement in uniformity of the leaching solution

distribution inside the heap in both the horizontal and vertical planes is another

consequence of the changes which contributed to the increase in the recovery. Prior to

this, the quality of agglomerate was improved and another complementary aspect that

improved was the reduction of the effects of the high unitary flows which require larger

distribution grids (of greater volume/by area unit) which tend to generate channeling and

larger impermeable zones. Mores irrigation never compensates the deficient exposure of

the mineral to the leaching solutions.

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

0 30 60 90 120 150 180 210 240 270TIME (DAYS)

EX

TR

AC

TIO

N C

uT

(%

)

Lift 1 Lift 2 Lift 3 Lift4 Lift 5 Lift 6

Lift 7 Lift 8 Lift 9 Lift 10 Lift 11

Figure 10 – Extraction Curves Based on Pad 4 Ripios Sampling as Function of

Lixiviabilty Quality Ore (Chemistry Rate) and Lixiviant Request Rate

These three considerations have had effect on the recovery of copper and

Figure 10 shows the comparison of recoveries for the different lifts on the permanent pad

The greater recovery obtained on the Lift 12 can be appreciated which is when the

CuT Cpy Cc Cv Ratio m3/Tm m3/Kg Cu % Extrac.

Lift 1 0.90 24.04 60.61 13.95 0.84 4.69 0.75 69.53

Lift 2 0.80 15.47 67.43 15.00 0.88 3.51 0.58 75.72

Lift 3 0.82 9.78 72.44 12.92 0.91 2.76 0.44 77.20

Lift 4 0.80 11.46 72.07 14.69 0.86 2.70 0.48 70.88

Lift 5 0.74 9.60 67.14 18.77 0.90 3.36 0.64 70.82

Lift 6 0.64 9.33 64.53 21.99 0.90 2.76 0.58 73.63

Lift 7 0.67 14.31 60.80 20.22 0.85 2.53 0.55 68.54

Lift 8 0.68 13.65 60.89 20.72 0.85 2.59 0.56 68.70

Lift 9 0.75 13.03 68.04 15.28 0.87 2.60 0.49 70.63

Lift 10 0.80 12.30 49.41 37.32 0.86 2.87 0.51 70.88

Lift 11 0.75 4.88 61.48 33.61 0.86 2.65 0.50 76.26

changes described in this paper began. We must also consider that the comparison begins

with Lift 4, at which time the mineral processed came from the Cerro Verde Pit (the first

three first lifts came from Santa Rosa Pit). Nevertheless the recovery delta of 5,5 units %

obtained should not be considered as the exclusive consequence of these changes, as lift

forced aeration paralleled these improvements.

.CONCLUSIONS

The quality and measured quantity of solutions applied in heap leaching are without

a doubt the most important part of the recovery of metallic values and this has to be

applied in variable and highly uniform rates

The design of irrigation headers for variable rate application is the most critical part

of the irrigation to obtain high uniformities and the use of additional irrigation

headers must be considered.

As the leaching temperature is important for the reaction kinetics in the oxidation of

copper minerals, the sprays (wobblers) must be eliminated. In addition, grid irrigation

(distances between lines and drippers) must tend to ZERO to eliminate the effect of

ferric iron precipitation which occurs between the cone boundaries in dripping due to

the pH change.

The dripper size must be directly proportional to the drop size, according to columns

tests where rates of 4 - 5 cm3/minute were applied to approach the behavior of ideal

irrigation solution flows as described by the D' Alambert Theorem.

The three effects described were obtained by reduction of the irrigation grid

combined with micro wobblers applying the concepts explained in this paper

resulting in favorable effects on the recovery.

REFERENCES

1. Bernal, Orlando, “Solutions Management at Cerro Verde” - SME Annual Meeting,

paper SME/04-179, Denver, Colorado - USA, February 2004.

2. Bernal, Orlando / Velarde, Guillermo, “New Technology for Secondary Sulfide Heap

Leaching” Copper 2003, Chile December 2003.

3. Velarde, Guillermo “Criterios de Manejo de los Regímenes de Riego en la

Lixiviación de Sulfuros de Cobre” - Sociedad Minera Cerro Verde, Internal Report

CV/ MET-27041, Perú 2001.