Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA19 / P31
AC-catalyzed enargite bioleaching using moderately thermophilic bacteria
Keishi Oyama, Naoko Okibe
Department of Earth Resources Engineering, Graduate School of Engineering, Kyushu University
Email: o.keishi8219@gmail.com
Abstract: The catalytic effect of activated carbon (AC) was evaluated in the bioleaching of enargite concentrate using
moderately thermophilic bacteria. Rapid pyrite dissolution began at the beginning of the experiment in the absence ofAC, which was increasingly delayed by addition of 0.1% and 0.2% AC. Suppression of pyrite dissolution was likely theresult of the lower Eh level. While final copper (Cu) recovery was improved by the addition of AC, this was not muchcontributed by the galvanic interaction between enargite and AC. A kinetic study using the shrinking core model revealed
that Cu dissolution continued steadily at lower Eh under suppression of pyrite dissolution, improving the final Cu
recovery. Addition of AC also facilitated the arsenic (As) immobilization. However, re-solubilization of once-immobilizedAs was observed coincided with rapid iron (Fe) dissolution, which might trigger the re-solubilization of As-precipitates.
Keywords: Copper; Arsenic; Bioleaching; Activated Carbon; Enargite
1. INTRODUCTION
Due to the decrease in the Cu grade of ores, thecontamination of As as an impurity increasingly becomesa serious problem. Although enargite (Cu3AsS4), known
as one of the major As-bearing copper sulfides, isconsidered as a hazardous material, its potential usage as
the Cu resource is not negligible, requiring a novel
process which enables selective extraction of Cu fromenargite. Bioleaching is expected to be one of thepromising techniques and high-temperature (70ºC)
bioleaching was indeed shown to be effective (>90% Cu
extraction; Takatsugi et al., 2011). On the other hand, it
still needs to be improved under low-temperature (2530ºC) conditions (<5% Cu extraction; Sasaki et al., 2010),
indicating the necessity of the reaction catalyst for furtherimprovement.
As a catalyst to solubilize refractory minerals, AC has
been tested by some studies. Its effectiveness on thebioleaching of chalcopyrite (CuFeS2) has already been
confirmed, achieving >70% Cu recovery (Liang et al.,2010). Basically, the mechanism of AC-catalyzing effecton chalcopyrite dissolution have been reported aselectrochemical galvanic reaction. However, its effect on
enargite bioleaching has yet been clarified. Some studies
have been carried out, which imply that the presence of
AC modify the surface of enargite, while the discussion
have yet been reached consensus.
The objective of this study was therefore set to evaluate
the effect of AC on the bioleaching of enargite and to
elucidate its mechanisms for further improvement of theCu extraction technique.

out in 500 mL Erlenmeyer flasks containing 200 mL ofheterotrophic basal salts media (Oyama et al, 2018; pH
adjusted to 2.0 with H2SO4) with 2.0% (w/v) enargite
concentrate and 5 mM Fe(II) (as FeSO4·•7H2O).
Different concentrations of AC (0-0.3% (w/v)) wereadded as a catalyst. The flasks were incubated shaken at
150 rpm and 45ºC for 60 days. Liquid samples wereregularly withdrawn to monitor cell density, pH, Eh (vs.
SHE), Fe(II) concentration (o-phenanthroline method),As(III) concentration (molybdenum blue method), andtotal soluble Fe, Cu and As concentrations (ICP-OES).
Leaching slurry sample was also obtained to clarify themicrobial diversity during the bioleaching test by RealTime PCR. Leaching residues were freeze-driedovernight and analyzed by XRD and EPMA. In order tounderstand the effect of galvanic reaction, anelectrochemical analysis was also carried out.
3. RESULTS&DISCUSSION
3.1 Suppression of Pyrite Dissolution by AC
In the absence of AC, rapid dissolution of Fe from pyrite
began at day 7, readily reaching >80% Fe solubilization
by day 40. Addition of 0.1 and 0.2% AC increasingly
delayed pyrite dissolution to day 25 and 35, respectively.
At 0.3% AC, pyrite dissolution was suppressed almost
completely until day 60. This trend was accompanied bylower Eh levels during bioleaching, and it took 7 (0%AC), 25 (0.1% AC), and 35 days (0.2% AC) to reach 700
mV: Since it has been reported that rapid dissolution ofpyrite begins at 700 mV in bioleaching cultures (Gu et al.,2012), AC likely played a role in this study to maintainthe lower Eh level to delay unwanted pyrite solubilization.
The preliminary experiment confirmed that sulfur (S)-ionspecies such as S4O62- are oxidized on the surface of AC
coupled with Fe(III) reduction. This suggests that lower
Eh was the result of Fe(III) reduction on the AC surface
coupled with the oxidation of RISCs (reduced inorganicsulfur compounds) deriving from sulfide minerals.
Microbial population analysis by Real-Time PCR
revealed that the abundance of S-oxidizer, At. caldus KU,
decreased from 71% (0% AC) to 62% (0.1% AC) and30% (0.2% AC), implying that the consumption of RISCs
on the AC surface led to depletion of the energy source

2. MATERIALS&METHODSThe enargite concentrate used for this study was derived
from Peru, mainly containing enargite (Cu3AsS4; 37.4%)
and pyrite (FeS2; 47.3%). Elemental composition of theconcentrate is as follows: S 39%, Fe 22%, Cu 20%, As
7.1%, Zn 0.39%, Sb 0.32%, Al 0.22%.
Mixed culture of three moderately thermophilic bacteria(Acidimicrobium ferrooxidans ICP, Sulfobacillus
sibiricus N1, and Acidithiobacillus caldus KU) wasutilized for bioleaching experiments. The initial celldensity of each strain was set to 1.0 × 107 cells/mL (3.0× 107 cells/mL in total). Bioleaching tests were carried- 51 -

for this S-oxidizer.
3.2 Enhancement of Cu Dissolution by AC
In sterile controls, final Cu recovery at day 60 wasimproved from 11% to 23% by the addition of 0.3% AC.
A similar effect was observed in bioleaching cultures; Cu
recovery at day 60 was improved from 36% (0% AC) to46% (0.1% AC) and 53% (0.2% AC). In order to confirm
the possible galvanic effect on Cu dissolution,
electrochemistry of the pyrite-enargite system and theAC-enargite system were examined. At higher Eh (>800mV), a relatively high galvanic current was observed,
indicating that galvanic reaction could significantly
facilitate enargite dissolution (Fig. 1). However, underlower Eh conditions (<700 mV), where relatively fast Cu
dissolution was observed in the actual bioleachingexperiment, the galvanic current was less than 1 µA andonly minor difference between the pyrite-enargite systemand the AC-enargite system was seen (Fig. 1). These
result showed that contribution of the galvanic effect on
enargite dissolution is negligible in AC-assisted
bioleaching systems.
In order to explain the reason why final Cu recovery wasimproved by the addition of AC, a kinetic study using theshrinking core model was carried out. Increasing the ACconcentration led to a decrease in the rate constant value,
but prolonged the period of surface chemical reaction as
the rate-limiting step from 7 days (0%) to day 25 (0.1%),
35 (0.2%), and 60 days (0.3%). Since the surface
chemical reaction period ended by the initiation of rapid
pyrite dissolution, preventing pyrite dissolution by AC
addition was shown to be the key to support longer andmore stable Cu dissolution for greater final Cu recovery.

Fig. 1 Electrode potentials in the enargite-pyrite system andthe enargite-AC system (a), and the galvanic current in each of
the two systems (b) tested in various Eh solutions.

3.3 Immobilization of As and its Re-solubilization
The amount of As re-immobilization was facilitated byAC addition from 3.1 mM (0% AC at day 10) to 6.9 mM(0.2% AC at day 40, corresponding to 77% of dissolvedAs). However, once-immobilized As began to resolubilize at day 10, 30, and 40 in the presence of 0, 0.1,
and 0.2% AC, respectively. Since this As resolubilization was coincident with the initiation of rapid
Fe dissolution, it is possible that high dissolved Feconcentrations have the effect of triggering resolubilization of As-precipitates. EPMA analysis
revealed that ferric arsenate (Ferric arsenic) wasselectively precipitated on the surface of enargite grains
before the As re-solubilization started.
4. REFERENCES[1] Takatsugi, K. et al., Hydrometallurgy, 109 (2011)90-96.
[2] Sasaki, K. et al., Hydrometallurgy, 109 (2011) 153160.
[3] Liang, C. et al., Hydrometallurgy, 105 (2010) 179185.
[4] Oyama et al., Hydrometallurgy, 177 (2018) 197-204.
[5] Gu et al., Transactions of Nonferrous Metals Societyof China, 22 (2012) 1250-1254.

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