COPPER CONCENTRATE LEACHING IN CHLORIDE-SULFATE MEDIUM

J.P. Ibez, J. Ipinza, F. Guerrero, J.I. Gonzlez and J. Vsquez Departamento de Ingeniera Metalrgica y de Materiales

Universidad Tcnica Federico Santa Mara

Avda. Espaa 1680, Valparaso, Chile.

juan.ibanez@usm.cl

ABSTRACT

The mixed leaching of a Chilean copper concentrate (predominantly chalcopyrite) was

studied by using sulfate-chloride aqueous solution under normal pressure at different

temperatures at constant pH. The use of sea water to increase the chloride concentration

was studied as well. The leaching rate in sulfate-chloride solution was faster than in sulfate

solution by a factor of ten in 7 days. The use of sea water led to a dissolution of 28% of the

total copper in the same period of time at 60 C, when the chloride concentration was

around 60 g/L the dissolution was near to 36% at 60 C. The iron content in the concentrate

was around 30%, and remains in the solid after 7 days of leaching. The brown, reddish and

yellow color solutions resulting after experiments indicate the presence of different copper

chloride complexes ions. Additional experimental work is carrying out to improve the

copper recovery by analyzing addition of sodium chloride on the copper concentrate cured

with sulfuric acid, to generate in situ hydrochloric acid.

INTRODUCTION

Chalcopyrite (CuFeS2) is the most important copper mineral [1, 2] with the highest

concentrate production, which is traditionally treated by smelting technology [3]. This

copper sulfide has a high stability in aqueous systems and it is refractory to normal

hydrometallurgical processes [4].

The leaching of chalcopyrite has been widely investigated, seeking for parameters affecting

the kinetics of the dissolution [5, 2, 6] and supporting the best theoretical mechanism to

explain the dissolution [7], for both chemical and bacterial leaching [8, 9]. The dissolution

rate of chalcopyrite concentrate in sulfate media is slower to the leaching of secondary

copper sulfide [10]; being the passivation of the mineral surface at high solutions potentials,

like a dense layer of elemental sulfur or a polysulfide CuSn, one of the widely accepted

reasons although the nature of the passivation film is still controversial [7, 11].

The dissolution of the chalcopyrite with oxygen in acidic solutions can be represented by

reaction (1). When the leaching proceeds, a ferric oxidation of chalcopyrite can be carried

out by reaction (2), being dissolved by ferric ions, producing ferrous ions which are re -

oxidized by oxygen in acidic solutions according to reaction (3). This last step is slow at

ambient pressure [12].

CuFeS2 + O2 + 4H+ = Cu2+ + Fe2+ + 2S0 + 2H2O (1)

CuFeS2 + 4Fe3+ = Cu2+ + 5Fe2+ + 2S0 (2)

4Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O (3)

The use of chloride ions in the leaching solution involved the action of the second redox

couple Cu2+

/Cu+. It is advantageous due to the aggressive nature of the leaching and to the

stability of cuprous ions by the formation of chloro-complexes, being a more effective

process than a regular leaching in sulfate solutions with ferric ions as the oxidant agent.

Some possible reasons of these phenomena are the higher rates of electron transfer in

chloride solutions, as the reduction in chalcopyrite passivation [13, 14].

General reactions of dissolution of chalcopyrite under a sulfate chloride media are shown

by equation (4) and (5) [15]. The prevailing redox couple of copper chloride complexes are

still under discussion. It is suggested that cuprous ions are stable in the form [CuCln]1-n

, and

cupric ions in the form [CuCln] 2-n

[16]. The cuprous complexes are more stable than the

cupric group, being the reason for reaction (4) to occur [12].

CuFeS2 + 3Cu2+ = 4Cu+ + Fe2+ + 2S0 (4)

4Cu+ + O2 + 4H+ = 4Cu2+ + 2H2O (5)

The oxidative model presented above is not enough to explain a rate promotion by the

presence of ferrous and cupric ions. A reductive/oxidative dissolution model was proposed

to interpret this enhancement of chalcopyrite leaching, presented by reactions (6) (7) and

(8). The first step is the reduction of chalcopyrite by ferrous ions in the presence of cupric

ions, represented by reaction (6) and the final step is an oxidation of intermediate Cu2S by

ferric ions [17].

CuFeS2 + 3Cu2+ + 3Fe2+ = 2Cu2S + 4Fe

3+ (6)

Cu2S + 4Fe3+ = 2Cu2+ + S + 4Fe2+ (7)

Cu2S + 4H+ + 2O2 = 2Cu

2+ + S + 2H2O (8)

The intermediate Cu2S in reaction (6) is formed only at potentials below the critical

potential that is function of the ferrous and cupric ions concentrations. If that potential is

higher, Cu2S is not formed and reaction (1) or (2) occurs. Also, Cu2S is leached faster than

CuFeS2, increasing copper extractions at low potentials in the presence of these ions [18].

A non-oxidative/oxidative process has been proposed for the dissolution of chalcopyrite

without any oxidizing reagent by reaction (9), with the formation of soluble cupric ions and

hydrogen sulfide which are metastable products, being covellite precipitated by reaction

(10). The rate of reaction (9) is governed by a rapid dissolution to assure the equilibrium

and a diffusion of the soluble species away from the mineral surface [19].

CuFeS2 + 4H+ = Cu2+ + Fe2+ + 2H2S (9)

Cu2+ + 2H2S = CuS + 2H+ (10)

The non-oxidative process was extended to include H2S removal, in the presence of oxidant

agents such as Fe+3

or Cu+2

and sustain the reaction (9). This is represented by reaction (11)

[19]. Assuming that the rate of reaction (11) is rapid compared to the rate of diffusion of

H2S from the surface, the equilibrium at the surface will be perturbed by the removal of

H2S by oxidation [18].

H2S + 2Fe3+ = S + 2Fe2+ + 2H+ (11)

As we saw on this review, there is no consensus about the nature of the rate determining

step and the mechanism involved in the dissolution. This work studies the effect of

parameters such as initial ions concentration in the leaching solution and temperature in the

dissolution of a chalcopyrite concentrate.

EXPERIMENTAL

Materials

Copper concentrate was obtained by flotation from operations in Chile, being classified into

narrow size fraction using a cyclo-sizer obtaining a 12.3 m average size. The chemical

analysis reported in the operation showed 26.8% Cu, 26.7% Fe, 31.3% S, 8.1% SiO2, 2.7%

Al2O3, 0.2% As and others elements. The chemical analysis was made by atomic absorption

spectroscopy (AAS) and the results were 24% Cu and 27.8% Fe. The salt supplied by

Sociedad Punta de Lobos (SPL) was 97.8 % NaCl and 1.4 % sulfate with a particle size of

100% -1/2".

Leaching experiments

Agitated leaching experiments were carried out in a 500 mL thermostatic jacketed glass

reactor with an effective volume of 200 mL of solution. The pulp was mechanically stirred

at 800 rpm. The lid contained ports for continuous measuring of temperature, pH and redox

potential using an Ag/AgCl reference electrode. The experimental setup is schematically

showed in Figure 1.

Figure 1 - Schematic diagram of the experimental setup. (1) vertical mixer; (2) temperature

controller; (3) glass reactor; (4) ORP & pH meter; (5) thermostatic bath; (6) plate heater.

One type of experiments consisted in to mix copper concentrate with 200 mL of acid

solution (0.2 M H2SO4). The percentage of solids in the concentrate pulp is considered

constant in 25 %. Another type of experiments consisted in to mix copper concentrate with

concentrated sulfuric acid and sodium chloride, with chemical curing times of 0 and 20 h.

The temperature was studied in the range of 40 to 70 C. The concentration of chloride ion

varied between 0 and 90 g/L using NaCl (provided by SPL).

The samples were withdrawn every 2 h, adding an equal volume of the leaching solution to

replace that removed. The filtered solutions were analyzed for copper and iron by AAS.

Measurement of the redox potential and pH, were made in the solution, immediately after

sampling of the mineral pulp.

RESULTS AND DISCUSSIONS

Effect of temperature

Figure 2 shows the results of leaching with 30 g/L Cl- at different temperatures of the

concentrate pulp. The copper extraction increases significantly by increasing the

temperature of the pulp. The difference of copper extraction between 50 and 70 C, is

highly significant in the control region by diffusion in the layer of product (sulfur) and less

important in the region of chemical control.

Figure 2 Effect of the temperature on the kinetic of copper extraction with 30 g/L Cl-.

Effect of sodium chloride concentration

The effect of different concentrations of chloride ion added directly in the mineral pulp to

70 C was studied. Figure 3 shows that the addition in the range of 40 to 80 g/L Cl-, does

not significantly change the extraction of copper, however the high temperature of the pulp.

This result shows that the concentration of chloride in the pulp does not affect the kinetic

behavior of the leaching of copper concentrate and only influences the thermodynamic

equilibrium and the distribution of the complex chloride-copper species.

Figure 3 Effect of chloride concentration on copper extraction from a copper concentrate in NaCl-H2SO4-H2O solution.

Effect of chemical curing and conditioning time

The chemical curing consists in mixing copper concentrate with sodium chloride and a

fraction of the acid consumption, added as concentrated sulfuric acid. This mixture is

conditioned for a certain period of time to promote the sulfating of the sulfide species

content in the concentrate. In this experiment, 30 kg NaCl/ton of concentrate is mixed with

20% of the stoichiometric consumption of sulfuric acid, without and with 20 days of

conditioning at room temperature.

Figure 4 shows that the conditioning produces a 20% increase in the copper extraction,

probably due to the "in situ" formation of HCl, whose pKa= -9 is significantly smaller than

the first dissociation of sulfuric acid (pKa= -6.6).

The reaction for formation of hydrochloric acid with a temperature under 50 C is given by:

NaCl + H2SO4 = NaHSO4 + HCl (12)

With temperatures over 50 C, the next reaction occurs:

2NaCl + H2SO4 = Na2SO4 + 2HCl (13)

The concentrate sulfating will be more efficient with reaction (13), which depends on the

heat generated by the exothermic reactions that occurs in the mixture, at expense of the

dissolution of the copper and iron sulfides.

Figure 4 Effect of chemical curing and conditioning on copper extraction from a copper concentrate in NaCl-H2SO4-H2O solution, 30 kg NaCl/ton of concentrate,

without and with 20 days of conditioning at room temperature.

Figure 5 shows the effect on the kinetics copper extraction of sodium chloride addition

during curing and concentrated conditioning. The experiment was performed with a

conditioning time of 30 days and with acid leaching at 70 C.

It is noted that without f NaCl, the copper extraction is less than 5% after 50 h of leaching.

However, the addition of 30 kg NaCl/ton produces a complete extraction of the copper

content in the concentrate in the same period of time. By decreasing the addition to the half

reduces the copper extraction to around of 70% after 50 h, which is indicating a non-linear

relationship between dosage for curing and the leaching answer of the concentrate.

Figure 5 - Effect of sodium chloride addition during the curing and conditioning of the

concentrated, pH=2 at 70 C and conditioning time of 30 days.

Figure 6 shows the effect of conditioning time on the iron extraction from their sulfides

contained in the concentrate, with a conditioning time of 20 days and addition 15 kg NaCl/

ton for the curing. It can be seen that this leaching medium produces a significant extraction

of iron from the copper concentrate. In effect, the iron extraction increases from 5% in the

sample without conditioning time until around 50% in the sample with conditioning, in a

time period of 50 hours.

Pyrite, principal iron species in the copper concentrate, is generally not attacked during

leaching and as a consequence, the ferrous iron in solution appears by the dissolution of the

chalcopyrite. This ferrous ion is oxidized to ferric ion in presence of cupric ion,

contributing to the dissolution of the chalcopyrite. For this reason, some studies have

focused on the catalytic effect on the leaching of the initial addition of copper sulfate in the

pulp.

Effect of chloride concentration on the redox potential

Figures 7 shows comparatively the evolution of the redox potential and the copper

extraction for 15 and 30 kg NaCl/ton added for curing the concentrate for 20 days. The

agitated leach was carried out at pH = 2 and 70 C.

Figure 6 Effect of conditioning time on iron extraction from a copper concentrate in NaCl-H2SO4-H2O solution; 15 kg NaCl/ton concentrate at 70 C and conditioning time of 20 days.

The results show that the addition of sodium chloride in the curing has a positive and

significant effect on the kinetics of the process. In effect, with 30 kg/ton is dissolved 80%

of the copper in 45 h, whereas with 15 kg/ton reach the same copper extraction with more

than 100 h.

In both cases, the redox potential is under 700 mV, significantly less than the value

normally found in the bacterial leaching (about 750 mV). At time zero, the redox potential

is 500 mV and increases as the concentration of chloride in the pulp decreases. This

demonstrates that it is possible to control the redox potential of the pulp, adding sodium

chloride continuously during the leaching.

This condition of less oxidizing redox potential in the pulp significantly enhances the

leaching of chalcopyrite. The lower value of redox potential reached in the pulp should be

probably to the domain of the redox couple Cu2+

/Cu+ on the Fe

3+ /Fe

2+.

Figure 8 shows the leaching process with sea water and distilled water, at pH 2 and 70 C.

Both samples have the same kinetic behavior, increasing their redox potential according the

leaching proceeds. Other ions present in sea water, apparently, do not affect the leaching

process, being feasible the use in copper concentrate leaching.

Figure 7 - Evolution of redox potential and copper extraction as a function of time for a

conditioning time of 20 days at pH= 2 and 70 C. (a) 30 kg NaCl/ton; (b) 15 kg NaCl/ton.

Figure 8 - Ionic force water effect on the copper extraction and redox potential in the copper

concentrate leaching.

500

550

600

650

700

0

20

40

60

80

100

0 10 20 30 40 50

Red

ox p

ote

nci

al

[mV

]

Co

pp

er e

xtr

act

ion

[%

]

Time [h]

Copper ext.ORP

500

550

600

650

700

0

20

40

60

80

100

0 20 40 60 80 100 120

Re

do

x p

ote

nci

al [

mV

]

Co

pp

er e

xtr

act

ion

[%

]

Time [h]

Copperext.

(a)

(b)

CONCLUSIONS

The chemical curing and the conditioning time are the most relevant variables in the

kinetics of the leaching process. The addition of 15 kg NaCl/ton concentrate with 20 days

of conditioning time allow a copper extraction higher than 80% with 50 h of leaching.

The addition of sodium chloride increases significantly the kinetics of copper extraction

from the chalcopyrite concentrate, probably due to the lower redox potential of the pulp

(near to 550 mV).

The sea water does not affect the kinetics of leaching of copper concentrate. However,

recirculation of water recovered could affect the kinetics of the process.

ACKNOWLEDGMENTS

We thank InProMet (Innovacin en Procesos Metalrgicos) form the Departamento de

Ingeniera Metalrgica y de Materiales of the Universidad Tcnica Federico Santa Mara

for supporting this work.

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