Heap leaching of copper ores – opportunity through science

Heap leaching of copper ores –

opportunity through science

Jochen Petersen

2

A typical heap leach operation

Heap leach process

?

What is going on inside a heap

Grain/Surface Scale

Particle Scale

Agglomerate Scale

Heap Scale

What is going on inside a heap

6

Solution flow downwards

Heat generationthrough reaction

Gas (humid air) transport up

Surface evaporation and radiation

The heap scale in heaps

gas flow through

pore network

Solution ‘seeps’ along preferential channels

on air-liquid contact surfaces

The agglomerate scale in heaps

inner particle pores

sessile and planktonic micro-organisms

individual

particle

stagnant solution space

mobile solution space

The particle scale in heaps

Key chemistry in copper heaps

Acid leaching of malachite

𝐶𝑢2𝐶𝑂3(𝑂𝐻)2+2𝐻2𝑆𝑂4 → 2𝐶𝑢𝑆𝑂4 + 𝐶𝑂2 + 3𝐻2𝑂

Ferric leaching of covellite

CuS + 2Fe3+ → Cu2+ + 2Fe2+ + So

Chloride facilitate leaching of chalcopyrite

CuFeS2 + 3Cu2+ − Cl− → 4Cu+ + Fe2+ + 2So

Ferrous re-oxidation (biologically accelerated)

4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H2O

Cuprous re-oxidation (rapid in chloride solution)

4Cu+ + O2 + 4H+ − (Cl−) → 4Cu2+ + 2H2O

Sulfur oxidation (chemically or biologically facilitated)

2S + 3O2 + 2H2O → 2H2SO4

Reaction network at particle surface

Fe2+

Fe3+

H+

O2,

CO2

acid migration or flow throughliquid film

solution phase solid phasegas phase

re-oxidation of ferrous (biological or direct)

Fe2+

Cu2+

reaction with gangue grains

𝐹𝑒𝐶𝑂3+ 𝐻2𝑆𝑂4→ 𝐹𝑒𝑆𝑂4 + 𝐻2𝑂 + 𝐶𝑂2

H+

Fe(OH)3 precipitation

𝐹𝑒3+ + 3𝐻2𝑂 → 𝐹𝑒 𝑂𝐻 3 + 3𝐻+

diffusion through micro-pores

reaction with mineral grains

𝐶𝑢𝑆 + 𝐹𝑒2 𝑆𝑂4 3 →

𝐶𝑢𝑆𝑂4 + 2𝐹𝑒𝑆𝑂4 + 𝑆0

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Is heap leaching a competitive technology?

• Slow process, residence time of months to years

• Poorer extraction (70-80%)

• Large physical footprint

• Long-term environmental burden uncertain

BUT

• Low energy requirements (crushing instead of milling)

• Simple, low-cost technology, can operate at remote sites

• Spent heaps comparable to tailings dams from minerals processing

Heap Leaching vs. Conventional Processing

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Heap Leaching vs. Conventional Processing

ROM ore

Primary crushing

Heap leachingComminution and

flotation

Atmospheric leaching

SX/EW

Copper cathode to market Concentrate to market

Route 1

Route 3

Route 2

13

In heaps, inventory cost is directly linked to tons of ore under leach/ leach time

Heap Leaching vs. Conventional Processing

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Why is heap leaching often so slow?

• Slow reaction kinetics at low temperatures

• Unliberated minerals in large particles

• Low aeration rates and poor gas-liquid mass transfer

• Solution channelling and stagnant zones

• re-adsorption of Cu onto precipitated Fe phases

Limitations of Heap Leaching

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Mineral extraction from large particles

• X-ray tomography shows shrinking shell and unreacted core

Limitations of Heap Leaching

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Gas-liquid mass transfer

• rate limited by low O2 solubility and interfacial area

• O2 solubility limited by temperature, solution salinity and air

pressure

0

2

4

6

8

10

12

14

0 20 40 60 80 100

Temperature ('C)

Ox

yg

en

so

lub

ilit

y [

pp

m]

pure water, dry air

0.15 M salinity, dry air

0.15 M salinity, sat. air

high salinity, high altitude

Limitations of Heap Leaching

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Solution Flow through Heaps

Drip point Drip point Drip pointa) b) c)

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Lag due to bio-colonisation

Metal attenuation due to

sorption on surface precipitate

Copper attenuation on Fe precipitates

• Progressive ‘cementation’ of heap ore by Fe-precipitates

Fresh Ore

After 1 month

of leaching

After 3 months

of leaching

Fe precipitation in column bioleaching

• Nano SEM studies of freshly precipitated Fe-OH

• Network structure of precipitate offers large surface formetal sorption and inhibits solution flow

Fe precipitation in column bioleaching

21

gas flow throughpore network

Solution ‘seeps’ through open pore spaces as a front

Transport-reaction modelling

Fe2+

Fe3+

Cu2+

H+

O2, CO2

Bulk flow diffusion through

Stagnant pores

Heap Modelling

HeapSim Model

Heap Modelling

Overall Mineral Conversions and Zinc Extraction

0%

20%

40%

60%

80%

100%

0 50 100 150 200 250

Leach time [d]

Con

ve

rsio

n

Sphalerite

Galena

Pyrite

Zn Extraction

Col 116 data

Zinc Concentration in Effluent

0

10

20

30

40

50

60

70

0 50 100 150 200 250

Leach Time [d]

Zn C

once

ntr

ati

on [

g/L]

Iron Concentration in Effluent

0

5

10

15

20

0 50 100 150 200 250

Leach Time [d]

Fe C

oncentr

ati

on [

g/L]

pH in Effluent

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 50 100 150 200 250

Leach Time [d]

pH

Potential (vs. Ag/AgCl) in Effluent

300

320

340

360

380

400

420

440

460

480

500

0 50 100 150 200 250

Leach Time [d]

Pote

nti

al (v

s. A

g/A

gCl)

[m

V]

Temperature with Depth and Time

20

25

30

35

40

45

50

55

60

65

70

0 1 2 3 4 5 6 7

Heap Depth [m]

Tem

pera

ture

[°C

]

Progression with each line: 9.2 days

Marmatite ZnS Conversion with Depth and Time

0%

10%

20%

30%

40%

50%

60%

70%

0 1 2 3 4 5 6 7

Heap Depth [m]

Zn

S C

on

versio

nProgression with each line: 9.2 days

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Key insights from modelling

• Complex distributions of reagent and heat distribution throughout

heaps determines local rates of leaching;

• Ambient conditions have significant impact on oxidative heap leaching;

• Optimal heap design varies strongly with ore characteristics;

• There are ‘speed limits’ to the maximum rate of leaching:

• Supply rate of acid (rare)

• Reagent distribution through stagnant (diffusion) zones

• Mineral liberation in large particles

• Supply rate of oxidant (air) – gas liquid mass-transfer

• Formation of adsorption buffers

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Conclusions

• Heap leaching is a technically simple process, but of immense physical

complexity at all scales.

• The interplay of these complexities is difficult to grasp, yet under-

standing them offers opportunities to conduct the process

efficiently.

• Economic analysis indicates if heaps can be made to operate reliably at

their optimum, they would be a serious technology of choice.

• Heaps do have a future!

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The National Research Foundation (NRF) of South Africa

is acknowledged for funding this research through their

Incentive Funding for Rated Researcher programme.

Thank You