Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia StrategyP-25

Catalytic Effect of Activated Carbon on Bioleaching of Enargite Concentrate
Keishi OYAMA, Haruki NOGUCHI, Tsuyoshi HIRAJIMA, Keiko SASAKI, Hajime MIKI,
Naoko OKIBE
Department of Earth Resources Engineering, Kyushu University, Fukuoka 819-0395, Japan

Abstract
Catalytic effects of activated carbon (AC) on bioleaching of enargite concentrate wereevaluated. In bioleaching using moderately thermophilic mixed bacterial cultures at 45ºC,
addition of 0.1-0.2% AC positively affected Cu recovery (46-53% at day 60), compared toAC-free mixed cultures (36% Cu recovery). Whilst increasing the AC density to 0.3%
reversely affected the final Cu recovery (35%), its chemical leaching effect was noticeable
when compared to cell-free control cultures (11% Cu recovery). Increasing the bioleachingtemperature to 70ºC using a thermophilic archaeon promoted the baseline Cu recovery(AC-free cultures) to 67% at day 14 and a slightly positive effect of 0.1-0.2% AC addition
was seen with 69-77% Cu recovery. What was commonly found with As behavior at bothtemperatures was that addition of AC facilitates (at least temporarily 45ºC) As immobilization.
In order to stabilize As precipitates to prevent their re-solubilization, further condition
optimization is ongoing.
1. Introduction
Although enargite (Cu3AsS4), one of
the low-grade refractory copper sulfides, isconsidered as an important copper resource,
its refractoriness and As content necessitate
development of economically feasible andenvironmentally friendly techniques.
Bioleaching is considered as one of thepromising techniques to overcome thisproblemandpreviousstudies
ofhigh-temperature enargite bioleaching haveindeed achieved high Cu recoveries over
90% (Takatsugi et al., 2011). On the otherhand, under low-temperature conditions(25-30ºC), Cu recovery remained less than
5% (Sasaki et al., 2010). This large
difference in Cu recoveries suggests thenecessity of a reaction catalyst to realize
high Cu recovery from low-temperaturebioleaching.
Although its positive catalytic effect isknown for bioleaching of chalcopyrite(achieving > 70% Cu recoveries; Zhang etal., 2007), the effect of AC on enargite
bioleaching is yet unclear.
Hence, the utility of AC was examinedin this study as a possible catalyst for
low-temperature bioleaching of enargite.

2. Materials and Methods
2.1. Mineral

The enargite concentrate used for thisexperiment was from Peru, containingenargite (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%.
2.2. Bioleaching Experiments
Mixed cultures of three moderately
thermophilic bacteria and pure cultures of
the thermophilic archeaon were utilized forbioleaching experiments at 45ºC and 70ºC,respectively. Bioleaching tests were carriedout in 500 mL Erlenmeyer flasks containing
200 mL of heterotrophic basal salts media
with 2.0% (w/v) enargite concentrate and 5mM Fe(II) (as FeSO4∙•7H2O). Different
concentrations of AC (0-0.3% (w/v)) wereadded as a catalyst. The flasks wereincubated shaken at 150 rpm (at 45ºC or
70ºC for 60 or 14 days, respectively). Liquidsamples were regularly withdrawn tomonitor cell density, pH, solution redox
potential (vs. SHE), Fe(II) concentrationand total soluble Fe, Cu and As
concentrations.

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Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-25
3. Results and Discussion
3.1. Catalytic Effect of AC at 45ºC

resulting in lower Cu recovery.

While Cu recoveries remained 11-23%
in abiotic leaching both without and with
0.3% AC, respectively, enargite dissolution
was enhanced in AC-free bioleaching
cultures to obtain 36% of final Cu recovery
at day 60. This was further improved byaddition of increasing amount of AC (46%and 53% Cu recoveries at 0.1 and 0.2% AC,
respectively), contributed by the galvanicinteraction between enargite and AC.
Increasing the AC density also led tolower redox potential levels during
bioleaching, consequently hindering pyrite
dissolutionandpreventing
jarositepassivation on the enargite surface. The
lower redox potential level was likely
caused by Fe(III) reduction coupled withAs(III) oxidation on the AC surface as anelectron mediator. As a result of As(III)oxidation
facilitatedbyAC,As
immobilization was promoted up to 77% by
day 40). However, As precipitates were later
partly re-solubilized accompanied byinitiation of Fe dissolution; final As
immobilizations were 34% and 37% at 0.1
and 0.2% AC, respectively.
Although it was possible to prevent As
re-solubilization by increasing the ACdensity to 0.3% (90% of dissolved As wasstill immobilized at day 60), its Cu recovery
was only 35% (equivalent to AC-freebioleaching culture). This was caused byexcess reduction of Fe(III) coupled withAs(III) oxidation, which was faster than
microbial Fe(II) re-oxidation, leading todepletion of Fe(III) oxidant for enargite
dissolution.
Based on the above results, the catalytic
effects of AC can be summarized as follows:(i) chemically facilitated enargite dissolution
via galvanic interaction, (ii) suppressed Fe
dissolution via Fe(III) reduction coupledwith As(III) oxidation on AC surface, and(iii) enhanced As immobilization resulted
from promoted As(III) oxidation. However,excess addition of AC shows inhibitory
effect on bioleaching system, consequently

At 70ºC, AC-free bioleaching cultures
significantly improved Cu recovery (67% atday 14) compared to the AC-free abiotic
condition (11% at day 14). Addition of AC
to bioleaching cultures slightly enhanced
enargite dissolution: at day 14, 77 and 69%
at 0.1 and 0.2% AC, respectively). Arsenic
started to solubilize accompanied by Fe
dissolution to show final As immobilizationof 68 and 65% at 0.1 and 0.2% AC,respectively). The results showed that (i)bioleaching interaction more criticallyenhances enargite dissolution at 70ºC thangalvanic
interactionand(ii)Assolubilization cannot be prevented solely bya temperature increase. Based on theseobservation, further condition optimizationis ongoing to enhance enargite leaching at
the same time to maximize As
immobilization.

3.1. Catalytic Effect of AC at 70ºC

4. Conclusion
Catalytic effects of AC on bioleachingof enargite concentrate at 45ºC and 70ºC
were evaluated. At 45ºC, 0.1-0.2% AC
positively affected Cu recovery (46-53% atday 60), compared to AC-free mixed
cultures (36% Cu recovery). Bioleaching at
70ºC promoted Cu recovery to 67%(AC-free cultures) at day 14 despite slightly
positive effect of 0.1-0.2% AC addition.
Common As behavior at both temperatures
was that addition of AC facilitates As
immobilization.
Furthercondition
optimization is required to achieve selective
copper leaching with stably leaving arsenicin the solid phase.
Reference[1] Takatsugi, K. et al., Hydrometallurgy,
109, 90-96 (2011).
[2] Sasaki, K. et al., Hydrometallurgy, 104,424-431 (2010).
[3] Zhang, W. and Gu, S., Transactions ofNonferrous Metals Society of China, 17,1123-1127 (2007)

Email: o.keishi829@gmail.com

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