Flotation Separation of Chalcopyrite and Molybdenite Assisted by Microencapsulation Using Ferrous and Phosphate Ions: Part I. Selective Coating Formation

Abstract

Porphyry Cu-Mo deposits, which are the most important sources of copper and molybdenum, are typically processed by flotation. In order to separate Cu and Mo minerals (mostly chalcopyrite and molybdenite), the strategy of depressing chalcopyrite while floating molybdenite has been widely adopted by using chalcopyrite depressants, such as NaHS, Na₂S, and Nokes reagent. However, these depressants are potentially toxic due to their possibility to emit H₂S gas. Thus, this study aims at developing a new concept for selectively depressing chalcopyrite via microencapsulation while using Fe²⁺ and PO₄³⁻ forming Fe^(III)PO₄ coating. The cyclic voltammetry results indicated that Fe²⁺ can be oxidized to Fe³⁺ on the chalcopyrite surface, but not on the molybdenite surface, which arises from their different electrical properties. As a result of microencapsulation treatment using 1 mmol/L Fe²⁺ and 1 mmol/L PO₄³⁻, chalcopyrite was much more coated with FePO₄ than molybdenite, which indicated that selective depression of chalcopyrite by the microencapsulation technique is highly achievable.

1. Introduction

Porphyry Cu-Mo deposits are the most important sources of copper (Cu) and molybdenum (Mo), because they account for about 60% and 50% of the world’s Cu and Mo productions [1]. In porphyry Cu-Mo deposits, chalcopyrite (CuFeS₂) and molybdenite (MoS₂) are the predominant Cu and Mo minerals [2,3], and they are separately recovered as Cu and Mo concentrates via a two-step process: (i) bulk flotation in order to produce Cu-Mo bulk concentrates; (ii) the selective flotation of molybdenite from Cu-Mo bulk concentrates. Bulk flotation is conducted with the assistance of proper collectors, such as xanthate for chalcopyrite and insoluble nonpolar oily collectors (e.g., diesel, kerosene, or fuel oil) for molybdenite [4,5], and pH adjuster (e.g., quick lime (CaO) or slaked lime (Ca(OH)₂) to increase the pulp pH at >10 where iron sulfides (mostly pyrite (FeS₂)) are depressed due to the competitive adsorption of OH^– that inhibits xanthate adsorption on the surface of iron sulfides [6]. After this, the Cu-Mo bulk concentrates are treated with chalcopyrite depressant (e.g., sodium hydrosulfide (NaHS), sodium sulfide (Na₂S), and Nokes reagent (P₂S₅ + NaOH)) in order to depress chalcopyrite while floating molybdenite [5]. These reagents function as chalcopyrite depressants by producing HS^– that desorbs the adsorbed xanthate on the chalcopyrite surface and/or reduces the pulp potential, in which chalcopyrite does not float [7].

Although the conventional chalcopyrite depressants are effective in separating Cu and Mo minerals from bulk Cu-Mo bulk concentrates, these have the potential to generate toxic hydrogen sulfide gas (H₂S_(g)) when pulp pH is not properly controlled [3,5,8]. At a pH below 10, for example, H₂S_(aq) species starts forming and it is readily vaporized due to its high vapor pressure [7]. Thus, the flotation circuits should consist of covered flotation cells with an active ventilation system to avoid the accident that is caused by H₂S emission [5,7,9]. Moreover, the corrosive nature of H₂S that destroys pipelines and imperfect molybdenite recovery are other drawbacks of using conventional depressants [3,7,8,10].

Because of the above-mentioned limitations, there have been many attempts to replace the conventional chalcopyrite depressants with alternative depressants for molybdenite (e.g., dextrin [11,12], lignosulfonates [13], O-carboxymethyl chitosan [14,15], carboxymethylcellulose [16], humic acid [17], etc.) and chalcopyrite (e.g., sodium sulfite (Na₂SO₃) [3], pseudo-glycolythiourea acid (PGA) [18], 2,3-disulfanylbutanedioic acid (DMSA) [19], disodium bis (carboxymethyl) trithiocarbonate (DBT) [20], chitosan [21], etc.). Between these two approaches, the strategy of depressing chalcopyrite is more preferred than depressing molybdenite, because, in porphyry deposits, the amount of molybdenite is lower when compared to that of chalcopyrite (i.e., Cu grade, 0.44%; Mo grade, 0.018%) [2]. In the case that molybdenite depresses while chalcopyrite floats, the mechanical entrainment of molybdenite could occur within a large volume of froth products, resulting in the loss of valuable molybdenite, which is critical, because molybdenite recovery is of importance for Cu-Mo processing plants to be economically viable [22].

Miki et al. [9] reported that the difference in electrical resistivity between chalcopyrite and molybdenite can be utilized for reducing the floatability of chalcopyrite selectively. The electrical resistances of chalcopyrite and molybdenite are 234 Ω and 1.2–1.5 MΩ, respectively, so electrochemical reactions occur more preferably on the surface of chalcopyrite than that of molybdenite [9]. Based on these distinctively different electrical properties of minerals, this study investigated the application of microencapsulation technique while using ferrous (Fe²⁺) and phosphate (PO₄³⁻) ions as a pretreatment for depressing the floatability of chalcopyrite. Due to the extremely high electrical resistivity of molybdenite, Fe²⁺ is hardly oxidized to ferric ion (Fe³⁺); however, chalcopyrite has a low electrical resistivity and, thus, Fe²⁺ can be oxidized to Fe³⁺, which then reacts with PO₄³⁻ and forms ferric phosphate (FePO₄) on the surface of chalcopyrite, rendering it hydrophilic. This is the first part of a two-part paper, the aim of which is to investigate whether microencapsulation while using Fe²⁺ and PO₄³⁻ can selectively create FePO₄ coating on the surface of chalcopyrite rather than that of molybdenite. Specifically, the overall scope of this part is as follows: (i) elucidating the mechanism of selective coating formation via an electrochemical technique (i.e., cyclic voltammetry (CV)) and (ii) confirming whether chalcopyrite is selectively coated with FePO₄ via shake-flask experiments that are coupled with surface characterizations (i.e., scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS)). A follow-up paper will discuss the effect of microencapsulation using Fe²⁺ and PO₄³⁻ on the selective depression of chalcopyrite in Cu-Mo flotation separation.

2. Materials and Methods

2.1. Mineral Samples

Chalcopyrite and molybdenite were obtained from Copper Queen Mine, Cochise County, AZ, USA and Spain Mine, Renfrew County, ON, Canada, respectively. The samples were crushed by a jaw crusher (BB 51, Retsch Inc. Haan, Germany), ground by a vibratory disc mill (RS 100, Retsch Inc., Haan, Germany), and then screened in order to obtain a size fraction of less than 75 µm. Mineralogical and chemical compositions of the samples were determined by X-ray diffraction (XRD; Figure 1) and X-ray fluorescence (XRF;

Table 1. Elemental compositions of chalcopyrite and molybdenite.

Chalcopyrite		Molybdenite
Elements	wt.%	Elements	wt.%
Cu	23.2	Mo	56.8
Fe	32.6	S	43.0
S	29.4	Others	0.2
Si	9.5	-	-
Others	5.3	-	-

). The chalcopyrite sample is composed of mainly chalcopyrite with pyrite and silicate minerals (e.g., quartz (SiO₂), amesite (Mg₂Al₂SiO₅(OH)₄), and actinolite (Ca₂(Mg,Fe²⁺)₅Si₈O₂₂(OH)₂)) as impurities, while molybdenite sample is highly pure and only contains trace amounts of impurities, as can be seen.

2.2. Stability of Fe²⁺ in the Presence of PO₄³⁻ vs. pH

The stability of Fe²⁺ in the presence of PO₄³⁻ was investigated as a function of pH in order to decide the suitable pH condition for FePO₄ coating formation on chalcopyrite to be selective. For this, a solution containing 1 mmol/L FeSO₄·7H₂O and 1 mmol/L KH₂PO₄ was prepared, and its pH was adjusted in the pH range of 2–6 while using dilute HCl and NaOH. All of the chemicals used in this study were of reagent grade (Wako Pure Chemical Industries, Ltd., Osaka, Japan). After pH adjustment, the solution was allowed to stabilize for 10 min. at room temperature under magnetic stirring (400 rpm). Afterwards, the solutions were filtered through 0.2 μm syringe-driven membrane filters (LMS Co. Ltd., Tokyo, Japan), and then analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES, ICPE-9820, Shimadzu Corporation, Kyoto, Japan) in order to identify the changes in dissolved Fe concentration.

2.3. Electrochemical Behaviors of Fe²⁺ on Chalcopyrite and Molybdenite

The electrochemical behaviors of Fe²⁺ on chalcopyrite and molybdenite were investigated by CV while using an electrochemical measurement unit (SI 1280B, Solartron Analytical, Farnborough, UK) with a conventional three-electrode system consisting of a mineral (chalcopyrite or molybdenite) working electrode, a platinum (Pt) counter electrode, and an Ag/AgCl (saturated KCl) reference electrode. The mineral electrodes were prepared in an identical way that was illustrated in our previous works [23,24]. Two types of electrolyte solutions were prepared: (i) 0.1 mol/L Na₂SO₄ and (ii) 1 mmol/L FeSO₄·7H₂O and 0.1 mol/L Na₂SO₄, both of which were adjusted to pH 4. The solution was equilibrated at 25 °C and then deoxygenated by N₂ purging for 30 min. After this, three electrodes were inserted and equilibrated at open circuit potential (OCP), and then CV was measured under the following conditions: initial scan polarity, positive; potential scan range, −0.8 to +0.8 V; scan rate, 30 mV/s.

2.4. Microencapsulation Treatment

Prior to the microencapsulation treatment, mineral samples were washed in order to remove the effects that are caused by the presence of oxidized layers formed during sample preparation. The washing procedure is as follows: ultrasonic cleaning in ethanol, acid washing (1.0 M HNO₃), triple rinsing with de-ionized (DI) water (18.2 MΩ·cm), dewatering with acetone, and drying in a vacuum desiccator [25].

For the microencapsulation treatment, 1 g of washed mineral (e.g., chalcopyrite or molybdenite) and 10 mL of a solution containing 1 mmol/L Fe²⁺ and 1 mmol/L PO₄³⁻ (pH 4) were put into a 50-mL Erlenmeyer flask and then shaken in a constant temperature water bath at 25 °C and 120 min⁻¹ for 1–6 h. After predetermined time intervals, the suspensions were filtered through 0.2 μm syringe-driven membrane filters and then analyzed by ICP-AES. Treated minerals were thoroughly washed with DI water, dried in a vacuum dry oven (40 °C), and then analyzed by SEM-EDX (JSM-IT200, JEOL Ltd., Tokyo, Japan) and XPS (JPS-9200, JEOL Ltd., Tokyo, Japan). The XPS analysis was conducted using an Al Kα X-ray source operated at 100 W (Voltage, 10 kV; Current, 10 mA) under ultrahigh vacuum conditions (approximately 10⁻⁷ Pa). The narrow scan spectra were calibrated while using the binding energy of adventitious carbon (C 1s) (285.0 eV) for charge correction. For deconvolutions of the spectra, XPSPEAK version 4.1 (Raymond WM Kwok, Chinese University of Hong Kong, Hong Kong, China) was used with an 80% Gaussian–20% Lorentzian peak model and a true Shirley background [26,27].

3. Results

3.1. Stability of Fe²⁺ in the Presence of PO₄³⁻ vs. pH

The oxidation rate of Fe²⁺ to Fe³⁺ by dissolved oxygen (DO) is known to be pH-dependent [28,29]. Fe²⁺ oxidation rate under acidic conditions (i.e., pH < 4) is very slow and independent of pH, as shown in Figure 2a. However, at pH ≥ 5 Fe²⁺, the Fe²⁺ oxidation rate shows the second order dependence on [OH⁻], indicating that a 100-fold increase in the rate occurs for a unit increase in pH [28]. In the studied system, not only Fe²⁺, but also PO₄³⁻ coexist, so the stability of Fe²⁺ in the presence of PO₄³⁻ as a function of pH was investigated (Figure 2b). The concentration of Fe²⁺ was almost not changed between pH 2 and 4, but above which Fe²⁺ concentration decreases rapidly. This result is identical to that reported by previous works [28,29], indicating that Fe²⁺ is stable at pH < 4, even in the presence of PO₄³⁻. The oxidation of Fe²⁺ by DO is not preferred, because the selective coating formation could only be achieved when Fe²⁺ oxidation occurs on the surface of chalcopyrite. Thus, pH 4 was selected for microencapsulation treatment to be selective.

3.2. Electrochemical Behavior of Fe²⁺ on Chalcopyrite and Molybdenite

Figure 3 shows the cyclic voltammograms of chalcopyrite (Figure 3a) and molybdenite (Figure 3b) in the absence and presence of 1 mmol/L Fe²⁺. The current density was continuously increased as the applied potential increased, which means that chalcopyrite as well as its oxidation products (e.g., Fe²⁺) were oxidized (Equations (1) and (2)), as shown in the first anodic scan of chalcopyrite in the absence of Fe²⁺ (Figure 3(a2)) [30,31,32]. When 1 mmol/L Fe²⁺ was present, the current density was apparently increased as compared to that without Fe²⁺. This increase in current density most likely resulted from the oxidation of Fe²⁺ to Fe³⁺ (Equation (2)).

CuFeS₂ → Cu_{(1 − x)}Fe_{(1 − y)}S_{(2 − z)} + xCu²⁺ + yFe²⁺ + zS⁰ + 2(x + y)e⁻

(1)

Fe²⁺ → Fe³⁺ + e⁻

(2)

During the cathodic scan in the absence of 1 mmol/L Fe²⁺, it exhibited four cathodic peaks (C₁, C₂, C₃, and C₄), as shown in Figure 3a,(a1). According to Holiday and Richmond [31], the first two cathodic peaks (C₁ and C₂) were due to the reduction of dissolved species, like Fe³⁺ and Cu²⁺. Holiday and Richmond [31] executed cathodic linear-sweep voltammetry while using stationary and rotating chalcopyrite electrodes, both of which were pretreated at 0.65 V vs. SCE for 2 min., and found out that two cathodic peaks at 0.38 and 0.15 V vs. SCE observed in the voltammogram using a stationary electrode were absent when the electrode was rotated. Thus, the appearance of C₁ and C₂ can be explained by the reduction of Fe³⁺ to Fe²⁺ at 0.3–0.4 V (Equation (3)) and the formation of covellite (CuS) at 0.1–0.2 V (Equation (4)), respectively. At the applied potential between −0.4 and −0.7 V, two additional peaks (C₃ and C₄) were observed (Figure 3(a1)), which resulted from the reduction of CuS to chalcocite (Cu₂S) and S⁰ to H₂S, as illustrated in Equations (5) and (6) [30,31,32].

Fe³⁺ + e⁻ → Fe²⁺

(3)

Cu²⁺ + S⁰ + 2e⁻ → CuS

(4)

2CuS + 2H⁺ + 2e⁻ → Cu₂S + H₂S

(5)

S⁰ + 2H⁺ + 2e⁻ → H₂S

(6)

Similarly, these four cathodic peaks (C₁, C₂, C₃, and C₄) were also observed in the voltammogram in the presence of 1 mmol/L Fe²⁺; however, it is important to note that the current density of C₁ was obviously increased. This indicates that more Fe³⁺ was generated during the anodic scan of chalcopyrite in the presence of 1 mmol/L Fe²⁺ as compared to that of control. On the other hand, cyclic voltammograms of molybdenite showed that there is no clear difference between the absence and presence of 1 mmol/L Fe²⁺ (Figure 3b). It is most likely attributed to the low electrical conductivity of molybdenite, making the electrochemical reactions of Fe²⁺/Fe³⁺ redox couple hard to occur on its surface. Therefore, the cyclic voltammetry results imply that the selective oxidation of Fe²⁺ on the chalcopyrite surface is highly possible.

3.3. Microencapsulation Treatment for Chalcopyrite and Molybdenite

Figure 4 shows the precipitation rates of dissolved Fe and P during microencapsulation treatment for chalcopyrite and molybdenite. The precipitation rate is calculated by the following equation:

Precipitation rate (%) = (Ci − Ct)/Ci × 100%

(7)

where C_i is the initial concentration of dissolved Fe or P in mg/L, and C_t is the concentration of dissolved Fe or P in mg/L at time t. As shown in Figure 4a, the precipitation of dissolved Fe considerably occurred in the presence of chalcopyrite; that is, around 60% of Fe²⁺ was precipitated after 1 h treatment and it reached 93% after 6 h. Similarly, dissolved P was also significantly precipitated with chalcopyrite (Figure 4b). When compared to the precipitation rate of dissolved P, the precipitated amount of dissolved Fe was a bit low. This is probably due to chalcopyrite dissolution that releases Fe^2+/3+, resulting in the lowering of the Fe precipitation rate. These results indicate that Fe²⁺ is oxidized to Fe³⁺ on the surface of chalcopyrite, then, the resultant product (i.e., Fe³⁺) reacts with H₂PO₄⁻, a dominant species of phosphate at pH 4, forming FePO₄, as shown in the following equation:

Fe³⁺ + H₂PO₄⁻ → FePO₄↓ + 2H⁺

(8)

It is interesting to note that, even during microencapsulation treatment for molybdenite, around 20–40% of dissolved Fe and P were precipitated (Figure 4a,b). Although the CV results indicated that Fe²⁺ oxidation could not occur on the surface of molybdenite (Figure 3b), the precipitation of dissolved Fe and P apparently occurred in the presence of molybdenite. These opposite results are attributed to the different electrical resistivity of different sides (e.g., basal and edge planes) of molybdenite [9]. Miki et al. [9] investigated electrolysis oxidation treatment while using basal- and edge-plane-oriented molybdenite electrodes and confirmed that electron transfer was actively pursued through the edge plane when compared to that through the basal plane. At an applied potential of 1.0 V vs. SHE, for example, the current density of the edge-plane electrode was around 0.1 mA/cm², lasting for 800 s, while it was closed to 0 mA/cm² in the case of the basal-plane electrode. The particle size of molybdenite used in this study is less than 75 μm. As the size of molybdenite particle reduces, the ratio of basal-plane/edge-plane also decreases [33]. This means that electron transfer reactions, like Fe²⁺ oxidation on fine molybdenite, become easier to occur when compared to coarse molybdenite. Thus, approximately 20–40% of dissolved Fe and P were precipitated during the reaction with molybdenite (Figure 4).

Figure 5 shows the SEM-EDX results of untreated- and treated-chalcopyrite. As can be seen in the SEM image of untreated chalcopyrite, its surface was smooth and clear; however, after microencapsulation treatment, the surface morphology dramatically changed the secondary precipitates that covered the surface of chalcopyrite. Untreated chalcopyrite exhibited strong signals of Cu, Fe, and S, while treated chalcopyrite showed that the signals of Cu and S were decreased, but signals of Fe and O were increased, as shown in the EDX spectra of these samples. In addition, the P signal appeared in the spectrum of treated chalcopyrite. These increased (i.e., Fe, P, and O) and decreased (i.e., Cu and S) signals were also found in the elemental maps of treated chalcopyrite. These results indicate that, after microencapsulation treatment, Fe–P–O-containing coatings were formed on the chalcopyrite surface. On the other hand, the SEM-EDX results of molybdenite with and without treatment (Figure 6) showed that there is no clear difference between the two samples, although 40% of dissolved Fe and P were precipitated (Figure 4). It could be speculated that the signals of Fe and P are almost noise levels due to the small amount of precipitate present on the molybdenite surface or the precipitates do not exist on the molybdenite surface.

In order to further characterize the surfaces of untreated and treated minerals, XPS analysis was adopted, which can analyze a very thin layer of coating (around ~6 nm) and give the information on the chemical state of the element. Figure 7a shows the XPS P 2p spectra of untreated and treated chalcopyrite. As it can be seen, treated chalcopyrite exhibited a broad peak that was centered at around 133.5 eV composed of adsorbed PO₄³⁻ (130.0, 131.4 and 132.8 eV) [34] and P^(V) of FePO₄ (133.7 eV) [34,35], which support that FePO₄ coating was formed on the chalcopyrite surface by microencapsulation while using Fe²⁺ and PO₄³⁻. In the case of the XPS spectrum of treated molybdenite (Figure 7b), a weak and gentle peak of FePO₄ was observed. The peak area of FePO₄ in the XPS spectrum of treated chalcopyrite was three times higher than that of treated molybdenite, which indicated that more FePO₄ is present on the chalcopyrite surface, as confirmed by Figure 4, Figure 5 and Figure 6. All of the results that were obtained in this study imply that Fe²⁺ oxidation, followed by FePO₄ precipitation, preferably occurs on the chalcopyrite surface rather than on the molybdenite surface, so the selective depression of chalcopyrite in the flotation of bulk concentrates is highly achievable, which will be evaluated in detail in Part 2 of this study.

4. Conclusions

This study investigated microencapsulation while using Fe²⁺ and PO₄³⁻ in order to selectively coat chalcopyrite with FePO₄ with the aim of improving Cu-Mo flotation separation. The findings of this study are summarized, as follows:

• In the presence of phosphate ion, Fe²⁺ was stable at pH ≤ 4, above which, however, Fe²⁺ became unstable due to its rapid oxidation to Fe³⁺, which was then precipitated as FePO₄ and/or FeO(OH). Thus, microencapsulation treatment using Fe²⁺ and PO₄³⁻ is recommended to be conducted at pH 4 in order to achieve the selective Fe²⁺ oxidation on chalcopyrite surface.
• The CV results indicated that Fe²⁺ oxidation can occur on the chalcopyrite surface, but not on the molybdenite surface, due to their different electrical properties.
• After microencapsulation treatment using Fe²⁺ and PO₄³⁻, SEM-EDX and XPS analyses confirmed that chalcopyrite was more coated with FePO₄ than molybdenite.