Hydrometallurgy 96 (2009) 258–263

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Hydrometallurgy
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t

Arsenic leaching from a gold bearing enargite flotation concentrate

L. Curreli ⁎, C. Garbarino, M. Ghiani, G. Orrù
Department of Geoengineering and Enviromental Technologies (DIGITA), University of Cagliari, Italy

a r t i c l e i n f o a b s t r a c t

Article history: The high arsenic content of the flotation concentrate obtained from a gold-bearing enargite ore for
Received 17 September 2008 pyrometallurgical processing strongly diminishes its market value. An investigation has been carried out for
Received in revised form 24 October 2008 selectively leaching arsenic from enargite–luzonite minerals using alkaline Na2S solutions. By suitably
Accepted 26 October 2008
adjusting the main reaction conditions almost 98% arsenic was leached, as well as part of the gold,
Available online 11 November 2008
particularly with high Na2S concentrations. Copper was not lost from the solid phase in which the enargite is
Keywords:
converted into a new species with the chemical formula Cu1.5S.
Enargite © 2008 Elsevier B.V. All rights reserved.
Gold
Flotation concentrate
Arsenic leaching
Sodium sulphide

1. Introduction In this work, the influence of the most significant process variables
has been investigated, namely specific surface area of the solid,
The deeper portions of the Serrenti–Furtei gold-bearing deposit in temperature, pH and reagent concentration of the leach solution.
Southern Sardinia (Italy) are host chiefly to sulphide mineralisations
composed for the most part of enargite–luzonite and pyrite. Gold and 2. Methods and materials studied
subordinate amounts of tennantite, covellite, chalcopyrite and
arsenopyrite are associated with the pyrite (Garbarino et al., 1991; Tests were conducted on cores removed from exploration bore-
Boi et al., 1996; Fadda et al., 2004). holes drilled into the deeper part of the mineral deposit. After grinding
The gold is refractory to direct cyanidation but potentially to below 0.15 mm the product was subjected to flotation with ethyl
economic concentrations of enargite–luzonite are contained in the xanthate after pyrite depression with a mixture of lime and sodium
ores. Thus, the ores have been beneficiated by flotation, recovering the cyanide. As the chemical (Table 1) and diffractometric analyses (Fig. 4)
gold and copper in a collective concentrate destined for further show, the finished product contained high enargite and gold
processing via pyrometallurgy (Ghiani et al., 2000; Curreli et al., concentrations, making it suitable for the pyrometallurgical produc-
2005). Commercial application to the ore in question proved fairly tion of gold and copper.
efficient, achieving high Cu and Au recoveries in the bulk float. (Di To evaluate the influence of solid particle size on leaching results,
Giovanni et al., 2003). samples of the untreated concentrate were ground to different
Unfortunately, the fact that enargite prevails among the copper degrees of fineness. The specific surface area of the ground products
bearing minerals combined with the high As content of the concentrates (0.050, 0.020 and 0.010 mm) and of the concentrate itself (−0.150 mm)
strongly undermine their economic value because of the environmental were then determined using the BET method (Table 2).
issues associated with pyrometallurgical processing. Thus for the leaner Arsenic removal tests were conducted on the flotation concentrate,
ores direct roasting may well prove uneconomical. For this reason the using alkaline Na2S solutions, to determine the effect of the following
possibility of removing the arsenic contained in the concentrates prior to parameters: solid fineness, solution temperature and initial Na2S and
roasting has been explored by means of alkaline leaching with a mixture NaOH concentration. All experiments, each lasting 120 min, were
of sodium sulfide and sodium hydroxide (Nadkarni and Kusik, 1988; carried out on 5 g solid in 0.5 kg leach solution using a 0.75 L Mod.
Achmovičová et al., 1999; Baláž et al., 1999; Baláž et al., 2000; Delfini HPM-T Medimex Schlageter & Preuss high pressure laboratory reactor
et al., 2003; Viñals et al., 2003). (up to 300 × 105 Pa), provided with a variable speed mechanical stirrer
set at 220 rpm and a Medimex C-AE 3I control unit for temperature
regulation (up to 300 °C).
⁎ Corresponding author. Tel.: +39 070 675 5525; fax: +39 070 675 5523. The influence of each variable was determined by keeping all the
E-mail address: curreli@unica.it (L. Curreli). other variables constant. Leaching results were evaluated by means of

0304-386X/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.hydromet.2008.10.013
 L. Curreli et al. / Hydrometallurgy 96 (2009) 258–263 259

Table 1 Table 3
Chemical analysis of the flotation concentrate Results of leaching vs specific surface area of solid

Assay Specific surface area [m2/g] 0.54 1.04 1.75

3
Au [g/10 kg] Cu [%] As [%] Fe [%] As solubilised [%] 52.4 77.5 97.7

90.25 33.15 12.55 9.16 Temperature 115 °C; solids concentration 10 g/kg; Na2S 0.42 M; NaOH 2.5 M; leach time
120 min.

Table 2 indicate that arsenic leaching efficiency increases with the amount of
Specific surface area of the flotation concentrate ground to different degrees of fineness NaOH for both Na2S concentrations in the leach solution. Comparable
(BET method) leaching results can be obtained using different combinations of
D100 [mm] 0.150 0.050 0.020 0.010 Na2S–NaOH. In this case, the combination containing smaller
Specific surface area [m2/g] 0.54 1.04 1.75 2.37 quantities of the more expensive Na2S is preferable in principle.

3.4. Influence of Na2S concentration

chemical determinations on the leach products, diffractometric

Tests were performed under the most favourable temperature and
analysis (XRD) and electron probe micro-analysis (EPMA) of the
fineness conditions found for arsenic solubilisation, namely 115 °C and
solid residues.
specific surface area of 1.75 m2/g. Two series of tests were conducted
at two different NaOH concentrations, varying the quantity of Na2S.
3. Results and discussion
The results in Fig. 3 show As solubilization to increase with increasing
concentrations of Na2S in solution.
3.1. Influence of temperature
The results obtained by varying the combination of Na2S and NaOH
in the leach solution can be explained by the following reactions
Tests were conducted on the solid material ground to below
(Baláž et al. 1999; Achmovičová et al., 1999; Delfini et al., 2003).
0.020 mm (1.75 m2/g), with initial Na2S and NaOH dosage of 32.5 g/l
(0.42 M) and 100 g/l (2.5 M) respectively.
The influence of temperature is shown in Fig. 1. Arsenic removal Na2 S dissociation Na2 S↔2Naþ þ S2−
N50% is achieved only at temperatures N85 °C. At higher temperatures,
efficiency improves significantly, in a practically linear fashion up to As solubilisation AsO3− 2−
4 þ 4H2 O þ 4S ↔ðAsS4 Þ
3−
þ 8OH−
92.5% at 105 °C. At temperatures over 105°, the amount of soluble
arsenic continues to increase though less markedly. Throughout the Hydrolysis of S2− ions− S2− þ H2 O↔HS− þ OH− ; HS− þ H2 O↔H2 S þ OH−
tests, the autoclave pressure was around 200 kPa at 85 °C and 250 kPa
at 115 °C.
As the pH is increased, the fraction of hydrolysing S2− ions
3.2. Influence of solid specific surface area gradually decreases, resulting in increased availability of S2− ions
and other polysulfide aqueous species that can complex and solubilize
Tests were conducted at 115 °C again using 32.5 g/l (0.42 M) Na2S arsenic. Thus a strongly basic medium with high NaOH concentration
and 100 g/l (2.5 M) NaOH. The influence of specific surface area is is required to minimise consumption of the more expensive Na2S.
shown in Table 3. Clearly, arsenic solubilisation increases with Experimental results show favorable conditions for As leaching to
decreasing grain size because of the greater surface area exposed. be: fineness and specific surface area of the solid 0.020 mm and
1.75 m2/g; temperature 115 °C and initial overall concentration of
3.3. Influence of NaOH concentration leachants in the 160–200 g/l range with not less than 9.75 g/l Na2S
(0.125 M) and 150 g/l NaOH (3.75 M).
Tests were performed at 115 °C on the solid ground to below
0.020 mm (1.75 m2/g), using two different concentrations of Na2S 3.5. Gold solubilisation
leachant. In each of the two series of tests the NaOH concentration was
varied, holding the Na2S concentration constant. The results in Fig. 2 Analysis of the leach residue indicated a small reduction in the
quantity of gold originally contained in the concentrate. Further work

Fig. 1. Influence of temperature on arsenic removal efficiency. Fineness and specific

surface area of the solid 0.020 mm and 1.75 m2/g; solids concentration 10 g/kg; Na2S Fig. 2. Influence of NaOH concentration on arsenic removal efficiency. Na2S = 0.21 M and
0.42 M; NaOH 2.5 M; leach time 120 min. 0.42 M. Other conditions as Fig. 1.
260 L. Curreli et al. / Hydrometallurgy 96 (2009) 258–263

analysis of the flotation concentrate (Fig. 5A and B) and the leach

residue (Fig. 5C, D, E, F, G, H) showed the formation of a new product
which did not match any compound in the JCPDS index. This product
is presumably the result of decomposition of the enargite phase — also
detected in the analyses. In particular, observation of polished sections
of the leach residues revealed a microporous structure of the new
solid phase often enclosing a nucleus of un-transformed enargite.
Quartz, pyrite and covellite on the other hand remain intact.
For the crystal chemical characterisation of the new mineral phase,
the leach residue was analysed by means of EPMA using an ARL SEMQ
microprobe. The analyses were performed at different points of the
polished sections, corresponding to the mineral phases of interest, in
order to quantify S, As, St, Cu and Fe contents. By so doing it was
possible to determine the average Cu, Fe, As, Sb and S assay of the
Fig. 3. Influence of Na2S concentration on arsenic removal efficiency. NaOH 2.5 M and enargite (Cu3AsS4), the newly formed mineral species and the
3.75 M. Other conditions as Fig. 1. covellite (CuS). The results provided the following average crystal
chemical formula for each phase:
is currently underway to quantify the gold leached and to determine
factors that affect the amount of gold leached. This will be the subject Enargite ðCu2:984 Fe0:017 ÞðAs0:951 Sb0:032 ÞS4 ⇒Cu3 AsS4
of a future paper. Gold dissolution may be attributed to the action of
the thiosulphates and polysulphides formed in the basic conditions
Unknown newly formed phase ðCu1:544 Fe0:009 ÞðAs0:0058 Sb0:0004 ÞS⇒Cu1:54 S
through reaction between OH− and sulphide ions with the free
sulphur released during enargite leaching. The following reactions are
proposed (Aylmore and Muir, 2001; Jeffrey and Anderson, 2003; Covellite ðCu1:029 Fe0:011 ÞðAs0:001 Sb0:0004 ÞS⇒CuS
Anderson, 2003; Senanayake 2004; Anderson and Twidwell, 2008).
The results of electron microanalysis are also schematically shown
Formation of thiosulphates 4S0 þ 6OH− ¼ S2 O2−
3 þ 2S
2−
þ 3H2 O in the triangular phase diagram in Fig. 6.
As can be observed from the triangular phase diagram, after
leaching the crystal chemical formula of enargite changes from
Formation of polysulphates S2− þ ðx−1ÞS0 ¼ S2−
x ðwhere x ¼ 2 to 5Þ Cu3AsS4 to Cu1.54S, a newly formed product with a micro-porous
structure that usually encloses a nucleus of un-transformed enargite.
Note that the covellite (CuS) maintains its position in the diagram,
Gold solubilisation:
indicating that the leach process has not altered its crystal chemical
−
– Polysulphides and sulphides 2Au + 2S2− + S2−
2 = 2AuS + 2S
2−
formula.
+ 2− 3−
– Thiosulphates Au + 2S2O3 = Au(S2O3)2 Further leaching tests on pure enargite crystals from the same ore
body confirmed the newly formed product to be unequivocally
3.6. Characterisation of leach residues attributable to the transformation of enargite and not of the other
species contained in the flotation concentrate. Analysis of the diffracto-
To identify the chemical–mineralogical changes brought about by grams for pure enargite (Fig. 7) and the leach product of the flotation
leaching, the flotation concentrate and solid residue were analysed by concentrate (Fig. 8) allowed the signals of the transformed enargite
means of XRD and EPMA and observed under the optical microscope. phase to be distinguished from enargite and the other mineral phases of
Diffractometric determinations (Fig. 4) and minero-petrographic the flotation concentrate.

Fig. 4. XRD analysis of flotation concentrate (specific surface area 0.54 m2/g) and solid leach residue (leach conditions as Fig. 1).
 L. Curreli et al. / Hydrometallurgy 96 (2009) 258–263 261

Fig. 5. Optical micrographs; 20×. A, B: flotation concentrate, specific surface area 0.54 m2/g. C, D, E, F, G, H: solid leach residue from conditions in Fig. 1.

The average crystal chemical formulae of the solid residue 4. Conclusions

determined by means of electron microanalysis on the unattacked
enargite and on the transformed phase confirm the conversion of Leaching a gold and enargite concentrate obtained by bulk
enargite into a copper sulphide having stoichiometry ≅ Cu1.5S flotation with basic Na2S solutions is an effectual means of removing
arsenic prior to pyrometallurgical processing. Leaching selectively
solubilises the arsenic and some gold but does not affect the copper
Enargite crystal ðCu3:017 Fe0:001 ÞðAs0:968 Sb0:032 ÞS4 ⇒Cu3 AsS4 which transforms almost entirely in the leach residue as a new species
having the chemical formula ≅Cu1.5S. Process efficiency improves
with increasing specific surface area of the concentrate at tempera-
Reaction product ðCu1:52 Fe0:001 ÞðAs0:008 Sb0:000 ÞS⇒Cu1:52 S tures of over 100 °C. The concentrations of Na2S and NaOH, as well as
sulphur content in the enargite, influence the leaching process. In fact
they are responsible for the S2−, HS−, OH−, S2−
x and polythionates in the
There is practically complete solubilisation of arsenic and partial solution whose equilibrium governs arsenic and gold solubilisation.
solubilisation of sulphur, while the copper is concentrated almost The gold is partly solubilised in the form of various anionic Au–S
entirely in the solid residue. complexes, which is the subject of further studies. Gold and arsenic
262 L. Curreli et al. / Hydrometallurgy 96 (2009) 258–263

Fig. 6. Triangular diagram of formula numbers of solid phases analysed with electron microprobe.

Fig. 7. XRD analysis of enargite crystals. Specific surface area 1.75 m2/g (−0.045 mm).

Fig. 8. XRD analysis of solid residues of the enargite crystals (specific surface area 1.75 m2/g, temperature 115 °C, solids concentration 10 g/kg, Na2S 0.42 M, NaOH 2.5 M, leach time
120 min) and flotation concentrate.
 L. Curreli et al. / Hydrometallurgy 96 (2009) 258–263 263

would need to be recovered from the solution by standard techniques Convegno “Nuove realtà minerarie in Sardegna” Ente Minerario Sardo. Associazione
Mineraria Italiana, Iglesias, pp. 81–113.
used in plant practice that are currently under investigation. Curreli, L., Ghiani, M., Surracco, M., Orrù, G., 2005. Beneficiation of a gold bearing enargite
ore by flotation and As leaching with Na-hypochlorite. Minerals Engineering 18,
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Delfini, M., Ferrini, M., Manni, A., Massacci, P., Piga, L., 2003. Arsenic leaching by Na2S to
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for Education, Universities and Research), by CNR (Italian National Di Giovanni, S., Ghiani, M., Peretti, R., Serci, A., Zonnedda, G., Zucca, A., 2003.
Beneficiation of gold bearing ores associated with the Tertiary volcanism in south
Research Council) and by CINIGEO (Interuniversity Consortium for Sardinia (Italy). In: Lorenzen, L., Bradshaw, D.J. (Eds.), Proceedings of the XXII
Georesources Engineering). International Mineral Processing Congress, Cape Town, South Africa, September
29–October 3, 2003. The Southern African Institute of Mining and Metallurgy
(SAIMM), pp. 493–500.
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