Catalytic mechanism of activated carbon-assisted bioleaching of enargite concentrate

Highlights

• Activated carbon (AC) suppressed the E_h level by catalyzing Fe³⁺ reduction.

• Enargite mineral itself favored higher E_h for solubilization, so as pyrite.

• AC indirectly improved the final Cu yield by inhibiting pyrite dissolution.

• Pyrite dissolution triggered re-solubilization of ferric arsenate.

• E_h control was critical to enable durable Cu dissolution as well as stabilization of As precipitates.

Abstract

The catalytic mechanism of activated carbon-assisted bioleaching of enargite concentrate (enargite 37.4%; pyrite 47.3%) was investigated by employing microbiological, electrochemical and kinetic studies. By using moderately thermophilic microorganisms at 45 °C, the final Cu dissolution was improved from 36% to 53% at 0.2% (w/v) activated carbon. An excess activated carbon addition showed an adverse effect. The enargite mineral itself favored higher solution redox potential (E_h) for solubilization. However, the dissolution of co-existing pyrite, which also favors high E_h, immediately hindered enargite dissolution through the passivation effect. The surface of activated carbon functioned as an electron mediator to couple RISCs oxidation and Fe³⁺ reduction, so that elevation of the E_h level was controlled by offsetting microbial Fe³⁺ regeneration. As long as the E_h level was suppressed at <700 mV, the dissolution of pyrite was largely avoided, enabling a steady and continuous dissolution of the enargite mineral through the surface chemical reaction model. When the E_h-control by activated carbon becomes no longer sustainable and the E_h hits 700 mV, rapid pyrite dissolution was initiated and the surface chemical reaction of enargite dissolution came to an end. Arsenic species dissolved from enargite was constantly immobilized with an efficiency of 75–90% as amorphous ferric arsenate. However, the sudden initiation of pyrite dissolution also triggered the re-solubilization of ferric arsenate. Therefore, the sustainable E_h-controlling effect was shown to be critical to enable longer Cu dissolution from enargite as well as stabilization of As precipitates.

Introduction

Enargite (Cu^I₃As^VS^-II₄) is a highly refractory primary copper sulfide containing toxic arsenic. In order to scavenge the Cu value from enargite with the drawback of this toxic impurity, bioleaching is considered one of the most promising technologies from its environmental and economic advantages.

High-temperature bioleaching (65–70 °C) was generally shown to be effective in several studies, achieving the final Cu dissolution ranging 52–91% (Escobar et al., 2000; Muñoz et al., 2006; Lee et al., 2011; Takatsugi et al., 2011). However, the effect largely drops when low-temperatures (25–30 °C) are used (<15%; Escobar et al., 1997; Sasaki et al., 2010) and thereby necessity of reaction catalyst is stressed to improve the reaction. Hence, to overcome the refractoriness of enargite, several studies (incl. chemical leaching, electrochemical and bioleaching studies) aimed to search the effect of catalysts such as silver (Ag) and carbon materials.

The utility of the Ag catalyst has been recognized in the case of an another refractory primary copper sulfide mineral, chalcopyrite (CuFeS₂), in both bioleaching studies (Ahonen and Tuovinen, 1990; Muñoz et al., 2007) and chemical leaching studies (Hiroyoshi et al., 2002; Nazari et al., 2012a, Nazari et al., 2012b; Ghahremaninezhad et al., 2015; Nazari et al., 2011).

More recently, the catalytic effect of Ag was also confirmed with enargite in an electrochemical study, where the presence of Ag was suggested to promote the transformation of enargite into more amenable intermediate, chalcocite (Cu₂S), at the specific E_h range (Miki et al., 2016). Furthermore, Oyama et al. (2018) suggested that the mechanism of Ag-catalyzed bioleaching of enargite concentrate proceeds via at least two types of intermediates (i.e., Cu₂S and trisilver arsenic sulfide (Ag₃AsS₄)).

To alternate the expensive Ag catalyst, the effect of carbon has been also a focus of bioleaching as well as (electro)chemical leaching studies. In the case of chalcopyrite, the effectiveness of activated carbon in bioleaching was reported, where the Galvanic interaction between electrically nobler activated carbon and electrically poorer chalcopyrite, as well as the lowered E_h level, were suggested to be the driving force for the mineral dissolution (Nakazawa et al., 1998; Zhang and Gu, 2007; Liang et al., 2010; Ma et al., 2017; Hao et al., 2018).

As for the case of enargite, Olvera et al. (2013) performed electrochemical studies and proposed that the catalytic effect of activated carbon is caused by its role such as a conducting channel, an extension of the surface area for the reduction of O₂ and Fe³⁺ and as a modifier of the semiconducting properties of the mineral. Ahumada et al. (2002) reported that the catalytic effect of activated carbon on chemical enargite leaching includes direct Fe²⁺ oxidation to Fe³⁺ by its surface oxygen groups as well as indirect Fe²⁺ oxidation via generation of H₂O₂ from O₂, according to [Eqs. (1)] and [(2)]:

$$C_{\mathit{Red}}^{\ast }+\frac{1}{2}{O}_2+H_{2}O=H_2{O}_2+C_{\mathit{ox}}^{\ast }$$

$$2{\mathit{Fe}}^{2+}+H_2{O}_2+2H^{+}=2{\mathit{Fe}}^{3+}+2H_{2}O$$

Jahromi and Ghahreman, 2016, Jahromi and Ghahreman, 2018 conducted chemical enargite leaching studies and attributed the catalytic effect of CBCs (carbon-based catalysts) to the concentration of quinone-like functional groups.

Nonetheless, the catalytic effect of activated carbon is yet poorly understood in the bioleaching reaction of enargite. The fundamental difference between the two representative primary copper sulfides, chalcopyrite and enargite, is that unlike the case of chalcopyrite leaching which favors the controlled E_h level (Hiroyoshi et al., 2008), the dissolution of the enargite mineral itself favors the strong oxidizing condition (Lattanzi et al., 2008). Therefore, the effect of activated carbon may be expressed differently between the two cases. In the case of As-bearing enargite, the presence of activated carbon may also affect the oxidation/immobilization reaction of As species during bioleaching, as was observed by Jahromi and Ghahreman (2018). Thus, this study aimed to evaluate the effect of activated carbon on bioleaching of enargite concentrate and to elucidate its mechanism employing microbiological, electrochemical and kinetic modeling studies.