Improving the Processing of Copper–Arsenic-Bearing Ores: Enhancing Separation and Extraction Methods Through Mediator Insights—A Brief Review

Abstract

The presence of arsenic-bearing minerals in ores, notably enargite (${Cu}_{3}As{S}_4$), remains an unresolved issue for copper beneficiation processes, including those for porphyry copper deposits. In particular, several operational challenges remain for the selective flotation of enargite from copper–sulphide ores, as well as the selective leaching of arsenic from enargite in copper concentrates. This study addresses these challenges from the standpoint of mediator science, where structures with specific elemental compositions observed by several authors at the surface of enargite and chalcopyrite, under different conditions and analytical techniques, are compiled and analysed. Most probable surface species, observed using technologies measuring the outmost surface layer and occurring onto the mentioned minerals, are identified and compared to species predicted by classic thermodynamic calculations. The results indicate that for chalcopyrite the major species formed in acidic conditions is elemental sulphur, while copper oxide and iron oxides and oxy-hydroxides species predominate with increasing pH. For the case of enargite, a similar situation is observed at low pH values, although slightly acidic conditions appear as a less examined condition for this mineral. Some of the observed species were found to be consistent with thermodynamic predictions, while others are notably absent. Particularly, for the case of enargite researchers have reported the formation of arsenic (III) oxide at pH values as high as 13, and observation not predicted by Pourbaix diagrams. Thus As₂O₃ could be considered a metastable species at highly alkaline conditions, which opens an option to beneficiation from froth flotation. Interestingly, at the same pH condition, iron oxide and oxyhydroxides species predominate at the surface of chalcopyrite. Therefore, applying the mediator concept, the initial alkaline flotation of sulphide ores turns into an oxide flotation case.

1. Introduction

The mining industry is continuously facing technical and operational challenges. In addition to the current steady decrease of valuable grades, as well as the increase of ore complexity, previous challenges are not fully resolved yet [1,2]. Among the latter, handling of arsenic-bearing sulphide minerals, notably the presence of enargite, is a problem without a proper standard solution which has been a matter of study for several years [3]. Enargite-bearing ores occur in several mine sites in Chile and the most common strategy to handle these ores entails the collective flotation of Cu-As minerals followed by a hydrometallurgical stage aimed at the selective leach of arsenic [4]. This strategy, although, of relative success is not free of challenges of various nature, which are summarized below.

1.1. Selective Separation of Enargite via Froth Flotation

Froth flotation of sulphide minerals is known to be selective [5]. The inherent instability of sulphide minerals in aerated aqueous solutions has prompted many reagent companies to develop more effective and specific surfactant molecules to profit from this situation. Since the reactions involved are electrochemical in nature, changes in the redox potential have been targeted as the most important variable to solve this challenge [6]. Indications such that chalcopyrite starts oxidizing at much lower potentials than enargite, or that the contact angle of enargite is less sensitive to xanthate-type collector dosages than enargite have delivered strategies, for instance, based on chalcopyrite depression at potentials above 0.2 V vs. SCE [7]. Nevertheless, these types of solutions are strenuous to implement at large scale in a standard manner. The possible reasons for this situation may be related to the variable water quality from one site to another, or the variability of redox potential measurements with time [8]. The above have led many mine sites to implement an initial stage of collective Cu-As froth flotation, followed by a selective leaching stage, where the details to reach operational efficiencies in both processes follow tailor-made decisions.

1.2. Selective Extraction of Arsenic via Leaching

Whenever mixed Cu-As-sulphide minerals are observed, particularly in the case of a copper concentrate, the implementation of leaching stages is preferred [9]. The smaller tonnage of concentrates allow for implementing reactor leaching operations which can be operated in either in acidic or alkaline conditions.

1.2.1. Leaching of Arsenic in Acidic Conditions

The leaching of arsenic in acid conditions have been tested using different pH modifiers such as sulphuric acid, hydrochloric acid, etc. [10]. Despite of promising results, several batch leaching stages are oftentimes required to achieve required efficiencies [11].

1.2.2. Leaching of Arsenic in Alkaline Conditions

In alkaline conditions, pH modifiers such as sodium hydroxide, and lime have been tested [12,13]. Sodium hydroxide exhibits higher extraction efficiencies, especially when dosing sodium sulphide [14]. The latter reagent has the capability of forming soluble complexes which enhance the extraction of arsenic; however, some studies highlight the possible formation of hydrogen sulphide gas, an ore-type driven mechanism, that may increase the risk behind the use of the reagent [15].

Both frotation and leaching processes have been thoroughly evaluated in terms of assessing the impact of different operating conditions on their efficiency. However, there is scarce scientific literature on the surface characteristics of minerals submitted to these processes, particularly in terms of the so-called mediators.

Indeed, understanding the reactivity of mineral surfaces and interphases is an unresolved challenge. Many case studies at industrial scale regarding extractive metallurgy or mineral processing involving the contact between minerals and another bulk phase (either aqueous or gaseous) depend on phenomena ranging from hydration interactions to electron-transfer reactions, and mediators are the major responsible for such interactions.

2. The Role of Mediators

During mineral processing and extractive metallurgy operations, the surface of ores actively interacts with different media [16,17]. These media are most often either an atmospheric phase, primarily air, or aqueous solutions containing a wide range of chemical species. The interaction between the surface of ores and these media triggers several physicochemical processes with varying degree of complexity, many of them not yet fully understood in fields related to natural materials such as minerals [18].

Furthermore, whenever chemical reactions at any mineral surface are triggered, the traditional understanding of theories concerning energy activation, or that reactions follow mechanisms rather than proceeding as simple one-step processes is undoubtedly more reasonable. It is then expected that intermediate reaction products may form, which may or not be soluble. In the latter case, such intermediates may enhance, inhibit or act as neutral to subsequent steps of an overall process. This phenomenon has been referred to in the literature as the “mediator”.

In historical terms, the concept of mediator originates in the field of electrochemistry, and refers to intermediate species that facilitate electron transfer [19,20] (Figure 1).

The scheme presented in Figure 1 is observed in many situations related to minerals processing and extractive metallurgy such as the bioleaching of copper sulphide minerals when using Thiobacillus ferroxidans where the intermediate oxidation and reduction species are the Fe(III)/Fe(II) redox pair, with bacteria-catalyzed redox reactions taking place in a homogeneous phase coupled with oxygen and water [21,22]. In many cases, the solid intermediate structure of these interphases is independent of the chemical speciation of the aqueous solution in contact with the solid surfaces. However, significant changes in the aqueous solution may produce different structures exhibiting different reactivities, even as a function of time [21,23].

The applications of mediators are massive. For instance, the promotion of hydrophobicity during froth flotation by means of surface reactions, while simultaneously avoiding significant thicknesses above which flotation will not proceed, still remains a poorly understood phenomena [9]. Another application, this time in the field of extractive metallurgy, deals with the solubility ratio, which indicates the response the ore will exhibit to leaching strategies while the opposite response may be of value for froth flotation strategies [11]. In this context, the relative oxidation of minerals such as sulphides, does not produce oxidation products on a single step, while intermediate species being formed are not yet fully understood. Additional applications of mediators include ore aging in slurries, leaching of complex ores, etc. [11,12].

More importantly, the analytical technique used to study mediators in a certain system largely compromise the information to be obtained as different analyses not only will be conducted at similar conditions of the system being studied but will also provide information at different depths of analysis [13,14,15,24]. Moreover, even if the latter is resolved, such characterizations do not provide a clear understanding of the reactivity associated to the mediators found.

Applying the mediator concept to sulphide minerals, unstable in nature under atmospheric conditions, requires a broader perspective in terms of mineralogy and chemistry, thus extending this concept to interactions beyond electron transfer, as the elemental composition and structure of such mediators may evolve over time. In some cases, non-stoichiometric structures may form and explain the reactivity of minerals during flotation and leaching [16,17].

Across the challenges, there is one recurring key concept that may be responsible for inhibiting or enhancing some chemical reactions and physical sub-processes and that is the mediator. Indeed, whenever a chemical specie is difficult to dissolve or leach one of the possible ways to move forward is to understand how the surface structure is built up, and from there design a strategy to achieve its beneficiation. The latter also applies to solid phases during froth flotation. For instance, if two species have similar surface structures and react similarly to hydrophobization mechanisms, their flotation recovery should be similar: either they are going to float collectively or not float at all and their separation will then rely mainly onto non-selective mechanisms (entrainment or others).

Practical applications of this concept, as mentioned previously, has been published in the field of electrochemistry. For instance, the review article presented by Zeng et al. (2025) mainly devoted to the electrochemical organo-mediated oxidation (EOMO) and electrochemical organo-mediated reduction (EOMR) events, where the authors indicate that the use of this concept in much of the case studies leads to the implementation of a totally different mechanisms of reactions [25].

This study aims to critically review the species that can be classified as mediators across a wide range of pH values, focusing on two major copper-bearing sulphide minerals: chalcopyrite, and enargite. The reported mediator species are presented alongside the analytical techniques used to detect them, and the results are compared with stable species using classical thermodunamic approaches such as Pourbaix diagrams.

3. Methodology

A review of the surface structures and their elemental compositions observed by several authors at the surfaces of enargite and chalcopyrite under different conditions are compiled and analyzed (Figure 2). This information is classified according to the analytic technique used, and the most probable surface species occurring on the mentioned minerals, are identified and compared to classic thermodynamic calculations, such as Pourbaix diagrams.

The techniques were then assessed in terms of the depth of analysis and the information provided to identify the mediator structures. The analytical techniques used to describe the mediators formed along with the approximate depth of analysis. For instance, X-ray photoelectro spectroscopy (XPS) provides elemental and oxidation states coming from the first 10 nm of the sample [26]; Auger electron spectroscopy (AES) has been established to exhibit a depth of analysis a bit smaller (around 5 nm) than XPS and procures more detailed information about the associations between different elements in a specific structure [27]. Total electron yield (TEY) achieves depths of analysis up to 100 nm [28]. However, X-ray diffraction (XRD) has a depth of analysis reported of around 50,000 nm providing mainly structures rather than elemental compositions. The latter technique, although considered in our studies was later discarded due to the significant depth of analysis. In most cases, the depth of analysis depends on the chemical nature and other specific characteristics of the samples.

A preliminary analysis related to how this information may be used to improve or standardize current strategies for froth flotation and selective leaching is presented in the discussion. For instance, the conditions that enhance differences in physical and chemical properties between the two minerals under study are highlighted for forth flotation. The larger their differences, the greater the opportunity to profit from froth flotation operations. In the case of leaching operations, the analysis is oriented towards destabilization of the species formed. The analysis does not consider interactions of the minerals studied with others potentially present in the ore, which could lead, for instance, to galvanic effects.

4. Specific Mediators Observed in Copper–Arsenic–Sulphide Minerals

The characterization of reactions occurring at the mineral-fluid interphase is of particular importance for the understanding and improvement of hydrometallurgical processes, as well as the characterization of biogeochemical processes of environmental importance [29]. In particular, the difference between surface and bulk mineralogy of copper sulphides highlights the importance of mediator phases, which ultimately control the rate and extent of mineral dissolution [30].

Several electrochemical techniques have been used to characterize mediator phases at the surface of copper sulphide minerals. However, it is important to note that voltametric potentials reported in the literature for the formation of mediators should be taken with caution, since such potentials depend on experimental conditions, including electrolyte composition, pH, dissolved oxygen concentration, degree of electrode polishing, and voltametric conditions (e.g., sweep rate) and ultimately peaks are normally assigned [31,32]. Thus, unless experimental conditions closely match between studies, the values reported in the literature are not directly comparable.

4.1. Chalcopyrite

Chalcopyrite (${CuFeS}_2$) is the major copper ore resource worldwide, as well as the most refractory mineral to hydrometallurgical processes owing to surface passivation occurring in both acidic and slightly acidic conditions [33,34]. Despite its refractiveness, chalcopyrite leaching has been extensively studies over the last 50 years [21,35,36,37,38,39], particularly given the amenability of hydrometallurgical processes to relatively low ore grades, as well as their lower environmental footprint compared to pyrometallurgy.

Table 1. Summary of mediators reported in the literature for oxidation and leaching of chalcopyrite. Abbreviations: X-ray photoemission spectroscopy (XPS), Auger electron spectroscopy (AES), X-ray absorption near-edge structure (XANES), reflection extended x-ray absorption fine structure (REFLEXAFS), total electron yield (TEY), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), Raman spectroscopy (Raman), Raman microscopy ($\mu$—Raman), scanning electron microscopy—energy dispersive spectroscopy (SEM-EDS), scanning photoelectron microscopy (SPEM), linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), small-angle X-ray diffraction (SAXRD), X-ray diffraction (XRD), N/R not reported.

Mediator	pH	Temp. [°C]	Experimental Conditions	Technique	Ref.
Atmospheric oxidation
FeOOH	-	90	Water vapor at 500 kPa	XPS	[40]
Fe2O3·xH2O CuS2 metastable CuSO4	-	22	Air, 2–120 days	XPS	[41]
Fe(OH)3 FeOOH Fe2O3 CuS2 metastable CuS	-	N/R	Air	LSV	[32]
FeOOH CuO Cu2O/Cu2S Cu/Fe sulphate	-	20	Air, 7 days	TEY-XAS, XES	[42]
Acid leaching
Cu0.75S2 CuS S0	~0	25	N2-purged solution 1 M H2SO4, 5·10−4 M CuSO4 −0.45–0.55 V vs. SCE	LSV, CV	[31]
CuS2 metastable S0	0	25	N2-purged solution 0.5 V vs. SCE 1 M HClO4	XPS	[32]
Cu1−xFe1−yS2 Cu1−x−zS2 CuS Fe2(SO4)3 Jarosite	~0.3	25	0.5 M H2SO4 0.1–0.75 V vs. MSE	EIS	[43]
S0	~0.3–0.5	95	0.1–2 M Fe2(SO4)3 0.3–0.5 M H2SO4	SEM-EDS	[44]
S0 Polymeric sulphur	~1.0	25	N2-purged solution 0.1 M HCl 0.57–0.69 V vs. Ag/AgCl	Raman	[45]
Cu5FeS4 (bornite) S0 Sulphide CuS	~1.0	25	N2-purged solution 0.1 M H2SO4	LSV, SAXRD µ-Raman	[46]
Cu2S S0	~1.0	30	0.1 or 0.5 M FeSO4 0.01 or 0.001 M CuSO4 Fe2(SO4)3 0.1 M H2SO4	Thermodynamic model	[47]
S22− (disulphide) Sn2− (polysulphide)	1.0	75	4 mM FeCl2 HClO4 0.65 V vs. SHE 5–10 days	SPEM, XPS	[48]
Sn2− (polysulphide) S0	1.0	85	0.1 M HClO4	SEM, XPS	[18]
S22− S0	1.5	65	culture medium 0.2 M KMnO4 50 mM FeSO4·7H2O 50 mM CuSO4·5H2O H2SO4	Redox titration XPS, XRD	[49]
S22− S0 Jarosite	1.5	65	culture medium Sulfolobus metallicus 0.2 M KMnO4 50 mM FeSO4·7H2O 50 mM CuSO4·5H2O H2SO4	Redox titration XPS, XRD	[49]
CuSn-like phase	1.8	30	9K basic salt medium mesophilic consortium H2SO4	XANES	[50]
CuSn-like phase Jarosite	1.8	45	9 K basic salt medium thermophilic consortium H2SO4	XANES	[50]
Sn2−/S0 S22−/CuS Jarosite/FeOOH	1.8	N/R	9 K basic salt medium 0.1–1.2 V vs. Ag/AgCl H2SO4	CV, XPS TEY-XANES Raman	[51]
S0 CuS FeOOH K-Jarosite	1.8	35	0 K basic salt medium with different ratios Fe2(SO4)3/FeSO4 0.4–0.6 V vs. Ag/AgCl H2SO4	XRD SEM-EDS	[52]
S0 FeOOH K-Jarosite	1.8	65	Norris nutrient medium with different ratios Fe2(SO4)3/FeSO4: 0.4–0.6 V vs. Ag/AgCl H2SO4	XRD SEM-EDS	[52]
Jarosite	1.83	N/R	Air-saturated 0.1 M NaNO3 HNO3—7 days	TEY-XAS, XES	[42]
Cu0.8S2	~2.88	N/R	Air-saturated 0.1 M CH3COOH 40 days	XPS	[41]
Fe2O3	6.53	N/R	Air-saturated 0.1 M NaNO3 HNO3—7 days	TEY-XAS, XES	[42]
Alkaline leaching
S22− Sn2−	9.0	N/R	KOH 100 ppm PAX	XPS	[53]
FeOOH Cu(OH)2	9.2	N/R	0.1 M Na2B4O7 1.5 V vs. SCE—7 min	REFLEXAFS	[54]
Fe(OH)3 Fe2O3 CuS2 metastable CuO S0	9.2	25	0.1 M Na2B4O7	CV, XPS, AES	[55]
Fe(OH)3 CuFe1−xS2	10.0	N/R	Aerated water KOH 25 min	XPS	[56]
FeOOH	10.67	N/R	Air-saturated 0.1 M NaNO3 NaOH—7 days	TEY-XAS, XES	[42]
Fe(OH)3 Fe2O3 CuS2 metastable CuO S0	12.7	25	0.05 M NaOH	CV, XPS, AES	[55]

summarizes the principal surface mediators reported in the literature for the oxidation and lixiviation of chalcopyrite.

Upon contact with air, the surface of chalcopyrite develops an oxidation film that passivates further oxidation, and in turn diminishes the extent of later leaching in solution. Voltammetry and ex-situ XPS measurements indicate that the passivating layer formed during atmospheric oxidation is mostly composed of iron oxyhydroxides/oxides such as goethite (α-FeOOH) and hematite (Fe₂O₃), as well as copper/iron sulphates [32,41,42], suggesting an enhanced diffusion of Fe(II) ions from the mineral bulk to the surface of chalcopyrite. Interestingly, the solid-state diffusion of Fe(II) ions has been proposed as the rate controlling mechanism for chalcopyrite oxidative dissolution [29].

Chalcopyrite lixiviation in an acidic medium has consistently shown a greater dissolution of Fe(II) ions with respect to Cu(II) ions, with an Fe(II):Cu(II) aqueous molar proportion ranging from 4:1 to 5:1 [31,33]. This non-stoichiometric dissolution rate produces a film of metal-deficient polysulphide and elemental sulphur (S₈) at the surface of chalcopyrite, which eventually passivates further mineral dissolution [36,44]. The identification of the mediator phases responsible for surface passivation during acidic leaching of chalcopyrite has been a matter of debate over decades [31,35,39]. Proposed rate-inhibiting mediators include elemental sulphur S₈ [32,36,44,51], metal-deficient polysulphides (Cu_1−xFe_1−yS₂) [36,43,51,57,58], disulphides (Cu_1−x−zS₂) [31,32,33,43,51], ferric oxides/oxyhydroxides [51,55], and insoluble sulphates such as jarosites [43,52]. However, the passivating role of elemental sulphur has been challenged on the basis of electrochemical and kinetics studies [57,59,60].

Regarding medium composition, the acid leaching of chalcopyrite is well known to be enhanced in the presence of ferric ions in solution, which act as an oxidant agent in a process commonly known as ferric ion leaching [60,61]. Ferric ions can be provided by either ferric chloride or ferric sulphate, with the former being recognized as more effective for chalcopyrite lixiviation at temperatures above 50 °C. In this context, Equations (1)–(3) have been proposed to explain the kinetics of ferric ion leaching of chalcopyrite [36,44,47]:

$$CuFe{S}_{2(s)}+4{Fe}_{(aq)}^{3+}\to {Cu}_{(aq)}^{2+}+5{Fe}_{(aq)}^{2+}+2S_{(s)}^0$$

(1)

$$CuFe{S}_{2(s)}+4H_{(aq)}^{+}+O_{2(g)}\to {Cu}_{(aq)}^{2+}+{Fe}_{(aq)}^{2+}+2H_2{O}_{(l)}+2S_{(s)}^0$$

(2)

$$4{Fe}_{(aq)}^{2+}+4H_{(aq)}^{+}+O_{2(g)}\to 4{Fe}_{(aq)}^{3+}+2H_2{O}_{(l)}$$

(3)

With the proportions of reactions depending on the pH and availability of dissolved oxygen in the leaching medium. Despite the reaction route, most authors agree that a passivating layer forms on the surface of chalcopyrite during ferric ion leaching, whose nature has been postulated to be either bimetallic sulphide or copper polysulphides formed as an intermediate product of chalcopyrite oxidation [36], and jarosites formed during the hydrolysis and precipitation of ferric ions in acidic sulphate media [52] (Equations (4) and (5)):

$$CuFe{S}_{2(s)}+4{Fe}_{(aq)}^{3+}\to {Cu}_{(aq)}^{2+}+5{Fe}_{(aq)}^{2+}+2S_{(s)}^0$$

(4)

$$CuFe{S}_{2(s)}+4H_{(aq)}^{+}+O_{2(g)}\to {Cu}_{(aq)}^{2+}+{Fe}_{(aq)}^{2+}+2H_2{O}_{(l)}+2S_{(s)}^0$$

(5)

It is worth noting that biologically enhanced leaching under sulphuric acid has been reported to produce goethite and jarosites at the surface of chalcopyrite [49,50]. However, these mediators seem to be the result of ferric salt presence in the growth medium, rather than microbially induced activity at the mineral surface.

Interestingly, the addition of ferrous salts has been observed to affect the dissolution efficiency of copper during ferric ion leaching of chalcopyrite, with increased dissolution at temperatures above 30 °C [62], and Fe(III):Fe(II) molar ratios below one, i.e., a corresponding redox potential below 413 mV vs. Ag/AgCl [52]. These results have been interpreted as an enhanced dissolution driven by either ferrous reduction of chalcopyrite to chalcocite ($C{u}_{2}S$) in presence of cupric ions [47], or ferric oxidation of chalcopyrite to covellite ($CuS$) [52], with both minerals more amenable to oxidative dissolution into cupric ions by either dissolved oxygen or ferric ions:

$$CuFe{S}_{2(s)}+3{Cu}_{(aq)}^{2+}+3{Fe}_{(aq)}^{2+}\to 2{Cu}_2{S}_{(s)}+4{Fe}_{(aq)}^{3+}$$

(6)

$$CuFe{S}_{2(s)}+2{Fe}_{(aq)}^{3+}\to Cu{S}_{(s)}+3{Fe}_{(aq)}^{2+}+2S_{(s)}^0$$

(7)

The proposed reactions are not mutually exclusive: it has been well established that chalcocite dissolution in sulphate solution progresses initially by diffusion of cuprous ions towards the surface, forming a series of copper deficient sulphides such as djurleite ($C{u}_{1.93}S$), and digenite ($C{u}_{1.8}S$) until the mineral transforms to covellite [63]. In a much slower second stage, the dissolution of covellite has been described by a shrinking core model, in which unreacted covellite is surrounded by a thickening layer of elemental sulphur [64]. The intermediate minerals during chalcocite dissolution are thus described by Equation (8):

$${Cu}_{2}S(s)\to {Cu}_{1.93}S(s)\to {Cu}_{1.8}S(s)\to CuS\left(s\right)\to S^0(s)$$

(8)

As seen in Table 1, several authors have reported the presence of covellite at the surface of chalcopyrite under acid lixiviation [46,52]. In addition, the electrochemical reduction of chalcopyrite under sulphuric acid has been shown to produce metal deficient sulphides, including djurleite, $C{u}_{2}S$, digenite, and bornite ($C{u}_{3}Fe{S}_4$) [65,66].

The dependence of chalcopyrite dissolution on redox potential has also been demonstrated by several electrochemical studies on polished chalcopyrite electrodes [31,32,43,51]. In particular, the initial surface oxidation of chalcopyrite under sulphuric acid produces small anodic voltametric peaks below 530 mV vs. Ag/AgCl, commonly referred to as prewave, which has been associated with formation of a passivating layer of either metal deficient disulphides [31] or metal deficient polysulphides [51]. The increasing the potential between 550 and 630 mV vs. Ag/AgCl produces a current associated with the formation of covellite, which increases the rate of chalcopyrite dissolution [43,51]. Finally, increasing the anodic potential above 650 mV vs. Ag/AgCl produces an additional passivating layer, which has been associated with either metal deficient polysulphides and elemental sulphur [51], or $F{e}_2{(S{O}_4)}_3$ and potentially jarosite [43].

Interestingly, the dependence of chalcopyrite dissolution on redox potential has also been observed during galvanic interactions with sulphide phases such as pyrite at solution potentials above 400 mV vs. Ag/AgCl [58,67], and bornite during bioleaching with a moderately thermophilic consortia, particularly at a potentials of 370–450 mV vs. Ag/AgCl [68].

Finally, the mediator phases evolving on the surface of chalcopyrite during lixiviation also depend on pH [32,42]: both acidic and alkaline media have consistently shown presence of metal deficient polysulphides, disulphides, and elemental sulphide, whose formation ratio depend on applied potential. On the other hand, acidic media additionally exhibits presence of iron oxyhydroxides and jarosite, while alkaline media exhibits additional presence of iron and copper oxides/hydroxides such as $CuO$ and $F{e}_2{O}_3$.

4.2. Enargite

Table 2. Summary of mediators reported in the literature for oxidation and leaching of enargite. Abbreviations: X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), X-ray absorption spectroscopy (XAS), Atomic force microscopy (AFM), contact angle (CA), electron probe micro-analyzer (EPMA), Raman spectroscopy (Raman), scanning electron microscopy–energy dispersive spectroscopy (SEM-EDS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X-ray diffraction (XRD), open circuit potential (OCP) measurements.

Mediator	pH	Temp. [°C]	Experimental Conditions	Technique	Ref.
Atmospheric oxidation
As2O3	-	25/100	Enargite concentrate, after milling with oxygen	XRD	[69]
CuSO4·5H2O	-	25/100		XRD
Tennantite as an intermediate phase	-		Enargite weathering (air)	Laser Raman microprobe, EPMA, XRD	[70]
Cu3(SO4)(OH)4	-	80	Enargite massive sample, after 28 days, in air at 85 °C and 80%, antlerite-like structures observed in cracks	XRD	[71]
S2−, S0, and S6+	-	Ambient temperature	Oxidized enargite, the surface is enriched also with arsenic in a thin layer of 0.5 nm with a inner layer of cooper depleted in sulphur. Cu appeared as Cu(I) and Cu(II) bonding oxygen atoms. As is mainly associated to oxygen.	XPS	[72]
Acid leaching
CuS	0	25/100	Enargite concentrate, in acid (0.5 M HCl, pH ~0) after 1 h	XRD	[69]
S0	$pH\le 1$	60–95	Enargite oxidized with Fe3+ ions	Soxhlet extraction	[73]
S0 amorphous	1	25	Enargite specimen, acid pH (close to 1), after 30 min contact with a NaClO/HCl mixture	SEM-EDS XRD	[74]
AsIII/AsV, with As(III)-Oxygen and CuII sulphate and chloride	1	Room temperature	After cyclic voltammetry tests, at potential higher than 0.2 V vs. SCE	XPS	[75]
S0	1	Room Temperature	In oxidation potentials of 0.3 V vs. SCE	In-situ Raman	[75]
S0	1	60–95	Synthetic enargite in presence of acid ferric sulphate solution after 80 h contact	Soxhlet extraction	[73]
${As}_{1-y-z}{S}_4$ and while increasing redox potential sulphide and polysulphide to sulphur	1	Room Temperature	Enargite electrode, dissolution in 0.1 M sulphuric acid, 450 to 750 mV Ag/AgClsat and then 750 to 900 mV.	EIS, CV, XPS, XANES, Raman,	[76]
${Cu}_{3-x}{As}_{1-y}{S}_4$ $x\gg y$	1	20	Electrochemical oxidation of enargite demonstrated an obvious passivation region from 500 to 750 mV (Ag/AgCl). The formed passivation film was found a n-type semiconductor behavior, which is different from the original enargite with a p-type behavior.	XPS, XANES, CV, Raman	[77]
As(III)-O increases in time, As(III)-S reduces in time, forming thiosulphate	1.8	Not reported (close to 30)	Enargite submitted to oxidation in presence and absence of the acidophilic microorganism Leptospirillum ferrooxidans.	XPS, SEM-EDS	[78]
S0	4	25	Crushed natural enargite, 30 min,	XPS, AFM, CA	[79,80]
$Cu{SO}_4$ ${As}_2{O}_3$	Around pH 7	Atmospheric temperature	Mechanochemical treatment using planetary ball milling in dry conditions speeds up oxidation in air. Arsenic bonds with oxygen and it is easily dissolved in water or alkaline solution	XRD, XAS	[81]
Three-layer structure: layer 1. thin metal deficient layer (0.7 nm) Cu, layer 2. 5–10 nm of As depleted and below a layer depleted in Cu and enriched in Sulphur–polysulphide structure	pH 2–4 (and 7.0 approx. for distilled water)	Room Temperature	Enargite dissolution distilled water, sulphuric acid at pH 4, and pH 2 ferric chloride/ferric sulphate mixture a mixed with 0.025 M Fe(III)	SEM-EDS, XPS with sputtering, OCP measurements	[82]
Alkaline leaching
${Cu}_3{\left(As{O}_4\right)}_2$	9.2	25	Natural enargite, electrochemical oxidation	CV	[83]
CuO, Cu(SO)4 Cu(OH)2	10	25	Crushed natural enargite, 30 min	XPS, AFM, CA	[79,80]
Cu2O CuO	10	35	Natural enargite, longer times of X-ray exposure (184 min)	XPS	[79]
CuO	10.5	Ambient temperature	Enargite microflotation tests controlled by hydrogen peroxide and sodium sulphide, conditioning times 2–5 min.	Thermodynamic studies	[84]
$FeAs{O}_4·2H_{2}O$ $CuO \mathrm{and} {As}_2{O}_3$ (alkaline conditions) $S^0$ (acidic conditions)	pH 1–11	Various	If iron is present in the system (for instance, in natural environments coming from pyrite) In presence of strong oxidation conditions copper oxide is predominant At redox potentials close to 0.56 vs. SHE	Various techniques	[85]
S0 at pH 2 f Cu3(AsO4)2 close to neutral pH Cu(OH)2 at pH 11	pH 2, 5 and 11	25	Copper ore containing Enargite oxidation with 0.013% H2O2 and O2 and microflotation tests conducted with prior oxidation for 1 h.	XPS	[86]
$CuO$	$4.6\le pH\le 11$	25	Natural enargite, electrochemical oxidation	CV, EIS and XPS	[83]
As2O3 As4S4 As2S3 Cu(OH)2 CuO Cu2O Sulphur-rich layer Sn (polysulphide structure)	11	20–22	Synthetic and natural enargite, nitrogen bubbling for 20 min and oxygen bubbling for 60 min, thin layer of oxidized species	XPS	[87]
CuO Cu(OH)2 Structure depleted in S in the form of polysulphide, As2O3	11.5–12.5	25–60	Natural enargite, particle sizes in three ranges 20–25 um, 40–53 um and 90–110 um, after leaching experiments with NaClO 0.07 M–0.47 M	XPS	[88]
$CuS$	$pH\ge 13$	60, 80, 90	120 min alkaline leaching after 15,30- and 60-min activation stirring ball millwith, with sodium sulphide, of enriched enargite concentrate	XRD an XPS	[89]
$Cu{SO}_4$ ${As}_2{O}_3$	Close to 13	Atmosphec temperature	Mechanochemical treatment using planetary ball milling in wet alkaline conditions speeds up oxidation in air. Arsenic bonds with oxygen and it is easily dissolved in water or alkaline solution for up to 50 h. Residue with higher crystallinity than dry conditions.	XRD, XAS	[81]
CuS-like structures S0 (pH9 up to 12) and longer conditioning times	pH8–13	Not reported (should be close to 25)	Enargite samples were used as electrodes, above −200 mV vs. SHE suggesting As-leaching.	In-situ Raman, CV, and thermodynamic computations	[90]
Cu2S	13.7	25/80	After 120 min leaching in presence of NaSH (0.68–1.35 M)	XRD	[91]
CuS	13.7	60/80/90	Enargite concentrate, activated with stirring mill using steel balls after 60 min maximum, and leached after 120 min leaching in presence of 100 g/L Na2S	XRD	[89]

provides a summary of the principal surface mediators reported in the literature for the oxidation and lixiviation of enargite. Enargite (${Cu}_{3}As{S}_4$), is an arsenic-bearing mineral commonly associated to primary sulphide copper ores. It is a mineral with an interesting economic value, as it contains a high relatively high copper grade, close to 44%, which is significantly higher than that of chalcopyrite, the primary copper sulphide mineral ubiquitously found in nature. The major drawback of this mineral is the simultaneous presence of copper and arsenic in its structure.

Enargite is known to produce cyanide and oxygen overconsumption in various metallurgical processes [90]. In most natural and mineral processing or extractive metallurgy operations, it is expected that enargite reactivity remains slow. The origin of such slow reactivity is uncertain, since the oxidation states of its elements are not fully resolved [92]. Nevertheless, it is likely that during exposure to an oxidizing medium, the release of arsenic from enargite will be further slowed down by at least temporary trapping in secondary phases. An adequate management of exposed surfaces and wastes should minimize the environmental impact of enargite-bearing deposits. According to previous information, bulk oxidation of enargite in air is slow at ambient temperature [85]. In addition, whenever this mineral is exposed to an aerated aqueous media, the release rate of copper is faster than that of arsenic in acid media [73,75]. The structures formed at the outmost surface layer depend largely on the water quality in contact with the mineral. For example, if pyrite (${FeS}_2$) is present, the formation of scorodite is likely to occur as shown in Equation (9) [85]:

$${Cu}_{3}As{S}_{4(s)}+{FeS}_{2(s)}+12.5O_{2(g)}+5H_2{O}_{(l)}\to {FeAsO}_4·2H_2{O}_{\left(s\right)}+3{Cu}_{\left(aq\right)}^{2+}+6{SO}_{4\left(aq\right)}^{2-}+6H_{(aq)}^{+}$$

(9)

The oxidation using oxygen may be described by Equation (10) [69], which indicates that oxygen gas does not produce strong oxidating conditions:

$$4{Cu}_{3}As{S}_{4(s)}+27O_{2(g)}\to {12CuSO}_{4(s)}+4S_{(s)}^0+2{As}_2{O}_3$$

(10)

On the contrary, the dissolution in sulphuric acid and Fe(III) leads to the reaction presented in Equation (11). In this case, probably due to the electrochemical reversibility commonly exhibited by ferric ions, the reaction exhibits production of anionic arsenic ions with higher oxidation states:

$${Cu}_3{AsS}_{4(s)}+11{Fe}_{(aq)}^{3+}+4H_2{O}_{(l)}\to 3{Cu}_{(aq)}^{2+}+{AsO}_{4(aq)}^{3-}+4S_{(s)}^0+11{Fe}_{(aq)}^{2+}$$

(11)

In fact, in sulphuric acid, the leaching rate of enargite depends on the Fe(III) concentration rather the acid concentration [73]. According to the latter equation, sulphur is the only species appearing in the residue. A similar behavior was observed using synthetic enargite [73]. If hydrogen peroxide is used as the major oxidant, the reaction follows a similar route (Equation (12)):

$$2{Cu}_3{AsS}_{4(s)}+11H_2{O}_{2(aq)}+10H_{(aq)}^{+}\to 6{Cu}_{(aq)}^{2+}+2H_{2}As{O}_4^{-}+14H_2{O}_{(l)}+8S_{(s)}^0$$

(12)

Again, the main residue is the sulphur. However, as previously mentioned, the electrochemistry of this mineral appears quite complex. Electrochemical studies using an enargite electrode between pH 4.2 and 9.2, indicated that major reactions occur according to Equations (13) and (14):

$${Cu}_{3}As{S}_{4(s)}\to {Cu}_{\left(3-x\right)}As{S}_{4(s)}+x{Cu}_{(aq)}^{2+}+2x{e}^{-}·pH\le 4.6$$

(13)

$${Cu}_{3}As{S}_{4(s)}+{xH}_2{O}_{(l)}\to {Cu}_{\left(3-x\right)}As{S}_{4(s)}+x{CuOH}_{(aq)}^{+}+{xH}_{(aq)}^{+}+2x{e}^{-}·4.6<pH\le 11$$

(14)

where the copper mono-hydrate species may react with water to form copper oxide as shown in Equation (15).

$${CuOH}_{(aq)}^{+}+H_2{O}_{(l)}\to {CuO}_{(s)}+H_3{O}_{(aq)}^{+}$$

(15)

These reactions show that, initially, the most relevant mediator formed during neargite oxidation is cupric oxide ($CuO$). In the same line of thought, voltametric studies have demonstrated that other species such as $CuS$ or even non-stoiquiometric species may also be formed mainly durin reduction reactions. In any case, as already reviewed in several research studies, elemental sulphur is one of the major species found in the residue.

In summary, the dominant process during enargite oxidation under acidic conditions is copper dissolution, forming a copper-depleted layer with polysulphide that oxidizes to elemental sulphur and further to sulphate, and, depending on the pH used the surface structure formed may be cupric acid. Interestingly, Davis et al. (1992) demonstrated that the leaching could be achieved closely to 100% at pH 2 [93].

5. Most Probable Mediators Across Different pH Conditions

Examining the information provided by the reviewed studies, some of the data were obtained using analytic techniques exhibiting larger depths of analysis such as X-ray diffraction. Considering the studies associated more closely to the composition and structures of species at the outmost surface layer leads to the summary presented in Figure 3.

There is a wide pH range where the species reported by different studies are similar. For instance, at low pH values many authors reported that sulphur would be the major species present at the surface of both minerals. Strikingly, $A{s}_2{O}_3$ appears in Pourbaix diagrams across an approximate pH range of −1 to 10 [94]. As seen in this study, some researchers have reported the generation of similar species up to pH 13, as presented in Figure 4.

The latter finding disagrees with previous reports where the presence of arsenic (III) oxide would not appear as a relevant species [97]. This may be since the thermodynamic computations considered the stability of bulk structures, without taking into account metastable structures [98]. In Figure 4, the extent of the redox potential associated to stability and metastability of $A{s}_2{O}_3$ is assumed not negligeable, so as to enable the mixed potential between the arsenic (III) species and an oxidant (oxygen or another) to still fall into the pseudo-stability region.

In the case of iron, although not reported in the original Pourbaix diagrams where iron hydroxide are the major species considered, whenever the formation of iron oxy-hydroxide species are taken into account they appear stable across a wide range of pH values [99]. This way, the flotation operation may transform from flotation of sulphide minerals to oxide minerals, as suggested by other authors in previous reports [100]. However, to achieve this, the water quality, the reagent and the conditioning time used would play a more relevant role than original thought.

6. Discussion

There are two paths that may be examined to assess the separation of enargite from copper concentrates, or to improve the efficiency of selective arsenic extraction throughout leaching operations. These paths are separately discussed.

6.1. Selective Arsenic Leaching

The acid leaching of arsenic from mineral mixtures is difficult since the generation of sulphur and other solid species would reduce the dissolution rate of arsenic. In alkaline conditions, leaching looks more promising since arsenic (III) oxide is readily dissolved above pH 10 [94]. However, presence of copper oxides may interfere with the dissolution. One option may be the use of interchanged pH conditions where alkaline conditions lead to arsenic dissolution and acid dosages or milling may remove the copper oxides formed. Even if leaching of arsenic is successfully accomplished, another challenge must be faced down the line of the process: the stability of arsenic in a solid structure, for instance, a soil [101]. Scorodite ($Fe{AsO}_4·2H_{2}O$) is the preferred structure due to its low cost and relatively significant stability [102], however, it has been demonstrated that scorodite may not as stable as initially thought. For instance, reducing conditions may trigger its instability [103].

6.2. Selective Enargite Separation from Copper Concentrates (Governed by Chalcopyrite)

The major chemical composition differences are associated to the presence of iron and arsenic. It can be observed that at pH values, close to 10 and up to 13, the chalcopyrite surface builds up structures based on iron oxides and iron oxy-hydroxides while onto enargite arsenic (III) oxides and copper (I) and copper (II) oxides govern the surface composition.

Another way forward is to carry out controlled pH variations so that surface structures form first and then they remain stable enough to undertake subsequent pH variations. The latter permits the development of species with properties different from those originally created on the other mineral surface. For instance, the pH could first be increased to procure the formation of iron oxyhydroxide species, and then shifted towards acidic conditions to permit the formation and stabilization of sulphur and, eventually copper sulphide, even in presence of copper ions which may adsorb onto the latter mentioned surface. As a result. The two mineral species would present dramatically distinct surface composition, which may lead to greater selectivity in froth flotation plants. And this is precisely the lesson learned from this manuscript. Such differentiation in surface property may be built out of pH changes or changes in the activity of any other specific species.

Further analysis can be pursued by incorporating the ageing time of both species. Naturally, all these changes above mentioned at large scale will involve an increase of operational costs. In order to translate these conceptions into feasible solutions, the benefit of the overall process must be at least counterbalancing the associated operating cost.

7. Conclusions

The formation of surface structures, namely “mediators”, onto chalcopyrite and enargite have been studied. The most probable surface species are identified as a function of pH and compared against thermodynamic predictions. It has been observed that both chalcopyrite and enargite exhibit complex surface structures that vary with pH and time as well as on the conditions imposed. Many of these surfaces are expected to respond differently to surface-driven separation processes. Based on this, if froth flotation is selected, the most effective strategy would be to adjust water quality and incorporate specific conditioning times prior to separation. This would promote the formation of specific surface structures with distinct surface properties that can be exploited during flotation. Hydrometallurgical approaches are more complex, as they may require at least two stages: one to form a surface structure and another to induce its instability. The results indicate that for chalcopyrite, acidic conditions favor the formation of elemental sulphur, which transforms into copper oxide and iron oxides/oxyhydroxides as pH increases. Enargite shows a similar behavior in a low pH. At higher pH values, the situation is somehow different. For example, enargite has been reported to form arsenic (III) oxide at pH values as high as 13, a phenomenon not predicted by Pourbaix diagrams. This metastable species extends the apparent zone of predominance, offering opportunities to exploit in froth flotation. Interestingly, at the same pH range, chalcopyrite surfaces are dominated by iron oxide and oxyhydroxide species, effectively shifting the overall process initially thought as a sulphide flotation into an oxide flotation situation which changes completely the perspective of this process.