0

5

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25

30

0:00:00 48:00:00 96:00:00 144:00:00 192:00:00 240:00:00

Dept

h di

ffere

nce

(cm

)

Time (h)

1 2 3 4 5 6 7 8 9 10

1/8th

1/16

th

1/4th

1/2th

(1h

on/1

h of

f)

1/2th

(4h

on/4

h of

f)

Full

rate

1 2 3 4 5

6 7

8 9

1 0 N

W E

S

The Effect of Initial Irrigation Conditions on Heap Leaching Efficiency - Preliminary Results

Ángel Briseño1, Tzung-mow Yao3, Michael Milczarek3, Mark L. Brusseau1,2 1Hydrology and Atmospheric Sciences Department, 2Soil, Water, and Environmental Science Department, 429 Shantz Building, #38,

The University of Arizona, Tucson, Arizona 85721. 3GeoSystems Analysis, Inc., Tucson, AZ

Many mine operators believe that slow initial irrigation and ramp up irrigation schemes can help minimize heap leach ore permeability loss and increase metal recovery. However, the mechanisms and solution flow processes are not well understood and have not been studied in detail. Laboratory saturated and unsaturated hydraulic conductivity tests and large column irrigation tests are being conducted on copper ore to better understand the flow mechanisms of different heap leach irrigation rates. The objective of the study is to test if lower initial irrigation rates and ramp up irrigation schemes can improve leaching efficiency. The results could then be used to optimize heap leach operations.

Heap leaching is a metal recovery process where a lixiviant is added to crushed rock (ore) to dissolve the target minerals and the solution is collected for extraction processing. Heap leaching needs unsaturated conditions to maximize metal recovery. Heterogeneities and other factors such as pore size distribution within the stacked ore can lead to uneven wetting and the formation of preferential flow pathways, which can reduce solution contact and lower metal recovery. Furthermore, mineral dissolution and “decrepitation” can cause alteration of the porous media structure, changing the ore’s physical and hydraulic properties, in particular, the loss of permeability and increase in solution holding capacity (Milczarek, et al. 2013)

Hydraulic Properties: • Significant increase in fines (decrepitation) for

acid addition and agglomeration of ores • Kunsat tests show incompetent ore hydraulic

behavior under high irrigation rates and marginal behavior under low irrigation rates.

Large Column Studies: • Fast solution advance in baseline test (less

than 6h) due to possible preferential flow. • Observed ore collapse/slump and dye test in

baseline test suggest broken down agglomeration, ore compaction, and possibly decrepitation and subsequent downward fine particles migration, causing loss of permeability.

• Less ore collapse in ramp-up test suggest that ore structure and permeability are less affected with slow irrigation ramp up.

• Neutron probe measurements show lateral spreading occurs from height 95 cm after 165 hrs in baseline test, but only detected when 1/8th of final rate is set in ramp-up test.

• Third test starting at 1/8th of baseline rate • Electrical resistivity tomography data processing

and analysis • Tracer (Boron) test analysis • Complete calibration of neutron probe

Sample Preparation: Prepare 1600 kgs of ½-inch crushed copper ore for copper leaching tests. Cured with sulfuric acid (6kg acid/ton of ore) and agglomerated with raffinate (7% moisture).

Hydraulic Properties: 6-inch diameter flexible-wall flow cells were used to test ore saturated hydraulic conductivity (Ksat) and unsaturated hydraulic conductivity (Kunsat) properties at various confining pressures (Dane & Topp, 2002, Milczarek, et. al., 2013 ).

Large Column Preparation: ─ 1.5 m high, 50 cm in diameter columns

for irrigation tests ─ Moisture sensors every 15 cm in

center of column ─ Electrical resistivity probes every 7 cm

at 13 locations on the edge of the column, and 1 at the top and 1 at bottom

─ Prepare dye (1g FD&C Blue/L and 100mg B/L tracer solutions with raffinate.

Column Procedures: • Controlled inflow and outflow

monitoring • Moisture content and ER sensor

monitoring every 5 minutes • Neutron probe measurements

from four different directions • Conduct tracer test after steady

state conditions achieved TEST 1: Baseline test

• Irrigation rate: 9.1 L/m2/hr (38 mL/min (21.9 cm/day)

TEST 2: Ramp up scheme – at ratios of baseline irrigation rate (on going) • 1/16th: 0.5 hrs on, 7.5 hrs off. • 1/8th : 0.5 hrs on, 3.5 hrs off. • 1/4th : 1hrs on, 3 hrs off. • ½ :2 hrs on, 2 hrs, off. • Full rate (38 L/min)

TEST 3: Ramp up scheme - Start with 1/8th of baseline rate (next test)

The project is being funded by the Fulbright Program and Geosystems Analysis, Inc., with additional support from the NIEHS Superfund Research Program, the University of Arizona Center for Environmentally Sustainable Mining, KGHM and Freeport McMoRan Copper and Gold.

• J. H. Dane, C. G. Topp, editors, 2002. Methods of Soil Analysis: Part 4 Physical Methods. SSSA Book Ser. 5.4. SSSA, Madison, WI

• Milczarek, M., Yao, T. Y., Banerjee, M., & Keller, J. (2013). Ore permeability methods of evaluation and application to heap leach optimization. In Heap Leach (pp. 403–415). Vancouver.

BASELINE TEST

0.04

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9:36:00 21:36:00 33:36:00 45:36:00

Θ (c

m3/

cm3)

Time (h)

Depth

0.05 0.07 0.09 0.11Moisture (count ratio)

26:57:30

0.05 0.07 0.09 0.11Moisture (count ratio)

164:55:00

0

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25

300

5

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0:00:00 7:12:00 14:24:00 21:36:00 28:48:00 36:00:00

Diam

eter

(cm

)

Dept

h di

ffere

nce

(cm

)

Time (h)

Center Outside Center diameter

Steady state

20 cm 40 cm 50 cm 60 cm 80 cm 140 cm Depth

Dye test • Outer ring suggests lower

permeability near the center ring after steady state (more agglomeration failure and lost permeability at the center)

• Most flow occurs at edge when it arrives at the bottom of the column

---- Initial moisture ----Moisture at time t

Results of baseline test • Fastest collapse occurs in the first 4 h, from

then it all collapses at the same rate until 29 h • For inside the wetting area, initial solution

content 7%, Steady state water content 16% • Bulk solution content 0.098 cm3/cm3 (mass

balance) • Lateral spreading captured by neutron probe

at height 95 and below

Steady state after 29 h

TEST 2: RAMP-UP

Results of ramp-up test • Major collapse occurred during 1/8th scheme with

much smaller slumping compared to baseline test • Bulk solution content at ½ scheme = 0.060 cm3/cm3

(mass balance) • Bulk solution content at full rate scheme = 0.074

cm3/cm3 (mass balance) • Lateral spreading captured when 1/8th scheme was

set, during major collapse/slump

0.05 0.07 0.09 0.11Moisture

47:12:00

0.05 0.07 0.09 0.11Moisture

119:03:30 Time (h)

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24:00:00 72:00:00 120:00:00 168:00:00 216:00:00

θ (c

m3 /

cm3 )

Time (h)

135

120

105

90

75

60

45

30

151/16

th

1/8th

1/4th

1/2

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on/1

h of

f)

1/2

(4h

on/4

h of

f)

Full

rate

Depth 2.0

22.0

42.0

62.0

82.0

102.0

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142.00.05 0.07 0.09 0.11

Dep

th (c

m)

Moisture

0:00:00

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.00.05 0.07 0.09 0.11

Dep

th (c

m)

Moisture (count ratio)

5:17:00

---- Initial moisture ----Moisture at time t