Thiosulfate leaching of gold from a mechanically activated

CuPbZn concentrate

Jana Ficeriovaa,*, Peter Balaza, Eva Boldizarovaa, Stanislav Jelenb

a Institute of Geotechnics of Slovak Academy of Sciences, Watsonova 45, 043 53 Kosice, Slovakiab Institute of Geology of Slovak Academy of Sciences, Severna 5, 974 01 Banska Bystrica, Slovakia

Received 11 April 2001; received in revised form 9 July 2002; accepted 19 August 2002

Abstract

The hydrometallurgical processing of complex concentrates represents an ecologically attractive alternative with respect to

classical pyrometallurgical technologies. The leaching of gold from a mechanochemically pretreated CuPbZn complex sulfide

concentrate of Slovak origin using ammonium thiosulfate was studied. Physicochemical transformations in the concentrate due

to mechanical activation have an influence on the rate of extraction and the recovery of gold. It was possible to achieve 99%

gold recovery within 45 min for a sample mechanically activated at an energy input of 403 kWh t� 1. Only 54% of gold were

recovered from the as-received concentrate in 120 min. Mechanical activation proved to be an appropriate pretreatment for this

CuPbZn concentrate before extraction of gold into thiosulfate leaching solution.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Mechanical activation; Complex sulfide concentrate; Gold; Thiosulfate

1. Introduction

Sulfides are a considerable natural resource of gold

that occurs in a wide range of forms. It may be

physically included in the ore, present within the

sulfides as finely dispersed submicroscopic particles

(invisible gold) or chemically bonded in both solid

solutions and compounds (Marsden and House,

1994).

Chemical, biological and physical pretreatments

are applied to the sulfide concentrate, with the aim of

changing the chemical composition and/or particle

size of the gold-bearing sulfides, thus facilitating the

subsequent leaching (La Brooy et al., 1994; Krizani,

1999). The relatively new process of mechanochem-

ical pretreatment (Fig. 1) is being successfully

applied in both fundamental research and plant oper-

ations (Balaz, 2000). In this process, which is also

called mechanical activation, the minerals are sub-

jected to high-intensity grinding. This grinding results

in particle size reduction and causes chemical or

physicochemical transformations, which significantly

affect subsequent hydrometallurgical processing

(Balaz, 2000; Tkacova, 1989; Balaz et al., 1995,

1996; Welham, 1997, 2001; Linge and Welham,

1997).

Alkaline cyanidation continues to be the dominant

method in hydrometallurgy for gold dissolution

0304-386X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0304 -386X(02 )00135 -4

* Corresponding author. Fax: +421-55-63-234-02.

E-mail address: ficeri@saskc.sk (J. Ficeriova).

www.elsevier.com/locate/hydromet

Hydrometallurgy 67 (2002) 37–43

(Ubaldini et al., 1996). Cyanide leaching has domi-

nated gold processing for over 100 years and will

probably continue to do so in the future, despite the

fact that the cyanide is now coming under the close

scrutiny of environmental legislators (Potter and Salis-

bury, 1974).

The use of thiosulfate as a gold leachant represents

an alternative method (Marsden and House, 1994; La

Brooy et al., 1994; Hiskey and Atluri, 1988; Abbruzz-

ese et al., 1995; Breuer and Jeffrey, 2000). Economic

and technical evaluation of plant tests of the Patera

and Newmont processes showed great promise for

thiosulfate leaching (Block-Bolten and Torma, 1986;

Wan and Brierley, 1997). Leaching of gold in thio-

sulfate solution results in the formation of a stable

complex and is described by the equation

Auþ 2S2O2�3 ! AuðS2O3Þ3�2 þ e� ð1Þ

The dissolution step in ammoniacal thiosulfate

solution is an electrochemical reaction and is pro-

moted by the presence of cupric ions (Aylmore and

Muir, 2001; Aylmore, 2001). The role of copper (II)

ions in the oxidation of metallic gold is shown in the

following reaction

Auþ 5S2O2�3 þ CuðNH3Þ

2þ4

! AuðS2O3Þ3�2 þ 4NH3CuðS2O3Þ5�3 ð2Þ

The aim of this work was to examine the possi-

bility of recovering gold from a CuPbZn concentrate

using ammonium thiosulfate leaching. A mechano-

chemical pretreatment was applied in order to deter-

mine its effect on the recovery of gold.

2. Experimental

2.1. Materials

A gold-bearing copper–lead–zinc complex sulfide

concentrate from Banska Hodrusa (the Svetozar vein),

Slovakia, was selected as a model material for testing

the effect of mechanochemical pretreatment on the

subsequent thiosulfate leaching of gold. The chemical

composition of the concentrate was as follows: 353 g

t� 1 Au, 170 g t� 1 Ag, 0.93% Cu, 4.08% Pb, 3.57%

Zn, 20.06% Fe, 44.15% S, 0.2% Sb, 0.17% Hg, 0.02%

Bi, 0.12% As, 0.03% Mn, 0.02% Co, 0.07% Mg and

5.5% SiO2.

Mineralogical analysis (Fig. 2) showed the pres-

ence of chalcopyrite CuFeS2, galena PbS, sphalerite

ZnS, tetrahedrite Cu12Sb4S13, pyrite FeS2 and quartz

SiO2 in the concentrate (Balaz et al., 2000). Gold

occurs primarily free in the form of wiry, flat and

flaky aggregates filling up the intergrain space in

sulfides, carbonates and quartz. Some small gold

inclusions are also present in the sulfides, predom-

inantly sphalerite and galena (Jagersky, 1999). A

small amount of gold is associated with chalcopyrite,

while pyrite is regarded as barren in this respect. The

investigation of the presence of invisible gold was

beyond the scope of this paper.

2.2. Mechanical activation

Mechanical activation was performed in a stirred

ball mill (attritor) Molinex PE 075 (Netzsch, Ger-

many). The volume of the grinding chamber was

500 mL. The concentrate was separated into 50 g

samples, which were milled with 200 mL of water

using 2000 g of 2 mm steel balls as the grinding

media. The mill was operated at 600, 1000 and 1200

min� 1 for 15, 30 and 60 min at ambient temper-

ature.

2.3. Physicochemical characterization

The specific surface area SAwas determined by the

low-temperature nitrogen BET adsorption method

Fig. 1. The different methods of precious metal bearing ore

(concentrate) pretreatment (Balaz, 2000).

J. Ficeriova et al. / Hydrometallurgy 67 (2002) 37–4338

using a Gemini 2360 sorption apparatus (Micromer-

itics, USA).

The particle size distribution of the ground con-

centrate was measured by a laser beam scattering in a

Helos and Rodos granulometer (Sympatec, Germany).

The mean particle diameter was calculated as the first

moment of the volume size distribution function.

X-ray diffraction traces were measured using a

DRON 2.0 diffractometer equipped with an FeKa

source operating at 25 kV and 10 mA. Data were

collected every 2 s and the detector was moved at a

rate of 2j min� 1.

The effect of mechanical activation was assessed

using the increase in the X-ray-amorphous portion of

mineral compared with the nonactivated (reference)

sample, which is assumed to correspond to 100%

crystallinity X, calculated using

X ¼ Uo

Io

Ix

Ux

� 100 ½% ð3Þ

where Uo and Ux are the background counts for the

reference sample and activated sample and Io and Ix

are the integral intensities of the diffraction lines of

the reference sample and activated sample , respec-

tively. The extent of amorphization A is simply

calculated (Eq. (4)) using Eq. (3) and used for the

evaluation of degree of minerals disordering

A ¼ 100� X ½% ð4Þ

2.4. Thiosulfate leaching

The leaching was investigated using a 1000 mL

glass reactor into which 500 mL of leaching solution

(0.5 M (NH4)2S2O3 + 10 g L� 1 CuSO4) and 1 g of

concentrate were added. The stirred leaching was

performed at 70 jC and pH = 6–7 for up to 120 min.

Aliquots (5 mL) of the solution were withdrawn at

appropriate time intervals for determination of the

content of dissolved gold by AAS.

The leaching kinetics were least squares fitted to

the kinetic equation

�lnð1� eAuÞ ¼ kt ð5Þ

Fig. 2. Gold associations with sulfide minerals (Balaz et al., 2000).

J. Ficeriova et al. / Hydrometallurgy 67 (2002) 37–43 39

where eAu is fractional recovery of gold into solution,

k is the rate constant (s� 1) and t is the leaching time

(s).

3. Results and discussion

3.1. Physicochemical changes of mechanically acti-

vated concentrate

Mechanical activation induces significant changes

to the surface as well as the bulk structure of sulfide

minerals (Balaz, 2000). Increases in the fraction of

fine particles and specific surface area and a decrease

in the crystallinity of mineral components are changes

that are frequently observed as the consequence of

intensive grinding.

The particle size distribution for the as-received

concentrate and for a sample mechanically activated at

an energy input EM= 202 kWh t� 1 is shown in Fig. 3.

The as-received sample was 100% < 100 Am and d50was 25 Am. The mechanically activated sample

(energy input 202 kWh t� 1) was < 40 Am, d50 = 4.5

Am and 83% of particles were < 10 Am.

The trace of the copper–lead–zinc sulfide concen-

trate (as-received sample) is shown in Fig. 4 and

confirms the presence of pyrite, sphalerite, tetrahe-

drite, chalcopyrite, galena and quartz.

Fig. 3. Particle size analysis of the (1) as-received concentrate and (2) sample mechanically activated using 202 kWh t� 1.

J. Ficeriova et al. / Hydrometallurgy 67 (2002) 37–4340

The fractional amorphization of the mineral com-

ponents of the concentrate was calculated by Eq. (4)

and serves as an estimate of the bulk disorder in the

concentrate. In this concentrate, gold is predominantly

linked to sphalerite and galena (Jagersky, 1999) and it

is important to stress the dependence of the amorph-

ization of sphalerite and galena on the specific surface

area as presented in Fig. 5. For a sample with a

specific surface area of 3.1 m2 g� 1, sphalerite and

galena were 91% and 73% amorphized, respectively.

Chalcopyrite and pyrite were less resistant to bulk

damage and were less amorphized. Chalcopyrite binds

gold to a lesser extent and pyrite is regarded as barren

in this respect.

3.2. Thiosulfate leaching of gold from mechanically

activated concentrate

Fig. 6 shows the effect of leaching time on gold

recovery for various energy inputs during mechanical

activation experiments. In the as-received concentrate,

only 54% of the gold were recovered after 120 min

leaching (curve 1). The results for the mechanically

activated samples (curves 2–4) indicated that the

physicochemical changes of the gold-bearing minerals

brought about an acceleration of the process of

thiosulfate leaching. It was possible to achieve a gold

recovery of 99% within 1 h for activated samples

(curves 3 and 4).

Fig. 7 shows the relationship between the leaching

rate constant and the energy input of grinding of the

mechanically activated samples investigated. The

Fig. 4. X-ray diffraction pattern of as-received CuPbZn concentrate: pyrite (P), sphalerite (S), tetrahedrite (T), chalcopyrite (CH), galena (G) and

quartz (Q).

Fig. 5. Amorphization (A) versus specific surface area (SA): (1) ZnS,

(2) PbS, (3) CuFeS2 and (4) FeS2.

J. Ficeriova et al. / Hydrometallurgy 67 (2002) 37–43 41

results show that the extraction of gold from CuPbZn

concentrate strongly depends on energy consumption

by grinding. It is important to note that the values of

energy input were calculated for the batch attritor used

in this work and the actual energy input for continu-

ous operating attritors are usually 10 times lower

(Balaz, 2000).

The plot in Fig. 8 describes the effect of specific

surface area on the gold solubilization after 2 min

leaching. Clearly, the plot appears to be linear up to

~2.5 m2 g� 1, suggesting that the gold recovery was

simply due to the increased surface area. However,

there is a substantial increase in gold recovery for the

greatest surface area sample. Clearly, mechanical

activation was achieving more than simply increasing

the surface area and was enhancing the leaching of the

gold. The maximum value of recovery of gold after 2

min leaching was equal to 36%.

4. Conclusions

The physicochemical changes of CuPbZn concen-

trate due to mechanical activation have an influence

on both the rate of extraction and the recovery of gold

from this gold-bearing concentrate when leached with

ammonium thiosulfate. It was possible to obtain 99%

gold recovery after 45 min leaching of an activated

sample, which compares very favourably with 54%

recovery from the as-received concentrate in 120 min.

The leaching of gold from concentrate has shown

some dependence on the degree of amorphization of

PbS and ZnS. The consumption of energy during

grinding has an influence on the structural disordering

of the gold-bearing sulfides and this is evident in the

thiosulfate leaching.Fig. 7. Rate constant of gold leaching (k) versus energy input (EM).

Fig. 8. Recovery of gold after 2 min leaching (eAu) versus specificsurface area (SA).

Fig. 6. Recovery of gold (eAu) versus leaching time (tL) for

mechanically activated samples. Energy input (EM): (1) 0 kWh t� 1

(as-received sample), (2) 202 kWh t� 1, (3) 335 kWh t� 1 and (4)

403 kWh t� 1.

J. Ficeriova et al. / Hydrometallurgy 67 (2002) 37–4342

Thiosulfate leaching is nontoxic and has more

rapid kinetics for gold solubilization than classical

cyanide leaching.

Acknowledgements

This work was supported by the Slovak Grant

Agency for Science (grant no. 2/2103/22).

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