NaHS

Journal Pre-proof Enargite leaching under ammoniacal media with sodium persulfate and consecutive precipitation of As/Cu with Na2S/ NaHS

A. Aracena, E. Rodríguez, O. Jerez PII:

S0304-386X(19)30609-7

DOI:

https://doi.org/10.1016/j.hydromet.2020.105290

Reference:

HYDROM 105290

To appear in:

Hydrometallurgy

Received date:

16 July 2019

Revised date:

9 November 2019

Accepted date:

31 January 2020

Please cite this article as: A. Aracena, E. Rodríguez and O. Jerez, Enargite leaching under ammoniacal media with sodium persulfate and consecutive precipitation of As/Cu with Na2S/NaHS, Hydrometallurgy(2019), https://doi.org/10.1016/j.hydromet.2020.105290

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© 2019 Published by Elsevier.

Journal Pre-proof ENARGITE LEACHING UNDER AMMONIACAL MEDIA WITH SODIUM PERSULFATE AND CONSECUTIVE PRECIPITATION OF As/Cu WITH Na2 S/NaHS A. Aracena1 , E. Rodríguez1 and O. Jerez2 1

Escuela de Ingeniería Química, Pontificia Universidad Católica de Valparaíso, Avenida Brasil 2162, Cod. Postal 2362854, Valparaíso, Chile. E-mail: [email protected] 2

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Instituto de Geología Económica Aplicada (GEA), Universidad de Concepción, Casilla 160-C, Concepción, Chile

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Abstract Enargite ore (Cu3 AsS4 ) is generally processed via pyrometallurgy, with high temperatures that release toxic arsenic (Asx O y ) and sulfur (SO x ) gases. In seeking alternatives that avoid production of noxious gases, this paper discusses batch leaching experiments of mineral mixtures of enargite and pyrite (Cu3 AsS4 and FeS2 ), agitated under ammoniacal media (NH4 OH) using sodium persulfate (Na2 S2 O8 ) as an oxidant. The results showed that almost complete leaching of enargite (98%) could be obtained due to the elimination of passivation (S°) by the action of the oxidant. With the help of a deep thermodynamic study and with DRX analysis, it was possible to establish the reaction mechanisms of dissolution of enargite with persulfate in an ammoniacal media. Leaching action selected for enargite, not affecting pyrite present. Enargite leaching can be represented under a heterogeneous kinetic model of reactant diffusion through a porous layer formed at reaction time. Orders of reaction 2.0 and 1.5, for persulfate and ammonium hydroxide concentrations, respectively, were established. Activation energy was calculated at 45.0 kJ/mol. Consecutive evaluations showed that metals present (Cu and As) could be precipitated with sodium sulfide and sodium hydrosulfide at 99% yields, thus generating a solution rich in ammonia that can be re-used in leaching.

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Key words: Enargite, sodium persulfate, ammonium hydroxide, leaching, passivation

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1. Introduction 1.1 Definitions Enargite (Cu3 AsS4 ) in porphyry deposits has been reported by Chilean mining companies at Chuquicamata Division, El Teniente Division, Escondida, Rio Blanco, and Doña Inés de Collahuasi [1], concurrent with chalcopyrite (CuFeS2 ), pyrite (FeS2 ), chalcocite (Cu2 S), covelline (CuS), and various sulfides. Traditional processing of arsenical minerals is similar to that of primary sulfides, beginning with comminution/milling/flotation stages. In this last stage, enargite are recovered in addition to primary copper concentrate (chalcopyrite) because they share similar physicochemical properties. Enargite are extremely difficult to remove by conventional processes [2], and increase the arsenic grade in copper concentrates. The problem becomes most apparent in subsequent pyrometallurgical treatment of arsenical concentrates. In Chile, both discontinuous process reactors (Teniente and Peirce-Smith Converters) and continuous processes (Mitsubishi and Flash) are used. These batch smelting technologies do not prevent emissions (uncontrolled gas flow) into the surrounding area. Exhaust, in the form of either volatilized compounds or metallurgical powder particulate matter, may contain arsenic, antimony, bismuth, sulfur, or other elements that constitute a hazard to the surrounding ecosystem. Because arsenic and sulfur have high vapor pressure (equation 1), they volatilize easily into forms highly harmful to the environment[3]. Enargite volatilizes at 648K in an atmosphere of 1.01 kPa of oxygen; i.e., it can volatilize to produce arsenic oxide (As4 O6 ) and sulfur dioxide (SO 2 ) at minimal oxygen concentrations (equation 2)[4]. Environmental restrictions in Chile require all Chilean foundries to fix more than 95% of As (in the form of Asx Oy ) and S (in the form of SOx ) content, which translates to high investment costs associated with new technologies. Furthermore, most copper smelters are significantly penalized for arsenic and are unwilling to treat copper concentrates when the arsenic content exceeds approximately 0.5%[5].

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Vapor pressure of arsenic = exp ((−

20689 𝑇

) + 10.62)

4Cu3 AsS4 + 25O2(g) → 12CuO + As4 O6(g) + 16SO 2(g)

(1) (2)

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This research thus looks trade the problems inherent to pyrometallurgical Cu3 AsS4 treatment with a hydrometallurgy solution, completely eliminating the emission of harmful gases and is thus an environmentally friendly alternative. 1.2 Analysis of enargite leaching conditions On processing enargite in an oxidizing acid media (hydrochloric and sulfuric), the technical literature has indicated that this copper arsenical sulfide is even more refractory than chalcopyrite; results have established that high pressures (1013 kPa O 2 ) and high temperatures (493K) are needed to reach rapid dissolution rates (120 minutes)[6] and to completely leach enargite. Some studies have found that ferrous sulfate and pyrite incorporated in the leaching processes (207-1379 kPa O 2 , 413-473K) facilitates leaching the presence of pyrite increases the dissolution speed as a result of galvanic interactions and the constant generation of ferric ions[7]. That said, the economics behind high pressure and temperature conditions continue to be a concern [8]. In responding to this, the treatment of enargite in alkaline media has been reported the achieve almost complete leaching of Cu3 AsS4 , without high partial oxygen pressures or high temperatures, via systems of

Journal Pre-proof sodium hydroxide (NaOH) with sodium hypochlorite (NaClO), sodium hydrosulfide (NaHS), sodium sulfide (Na2 S), and ammonia (NH3 ) (Table 1). *****Table1***** In spite of research into alkaline leaching alternatives, there are no publications related to the dissolution of enargite with persulfate (S2 O8 2-) in ammoniacal medium. Noteworthy, given the results of bornite (Cu5 FeS4 ) and low grade copper mineral leaching under similar conditions[18-19].

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1.3 Thermodynamics of enargite leaching in S2 O8 2-/NH4 OH medium Sodium persulfate has a high solubility (73 g/100 g H2 O, 298K), and dissolves into sodium ions and persulfate (reaction 3). Furman [20] proposed persulfate anions as a potent oxidant that, in an alkaline media, decompose to produce gaseous oxygen (reaction 4). O 2(g) may then be reduced to generate hydroxide ions (reaction 5). In our case, gaseous oxygen generates electron transfer, and is the semi-reaction for the redox.

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Na2 S2 O8 → 2Na+ + S2 O8 2-

(4) (5)

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11/4O 2(g) + 11/2H2 O + 11e → 11OH-

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S2 O8 2- + H2 O → 1/2O2(g) + 2SO 4 2- + 2H+

(3)

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Next, enargite is a sulfur compound that oxidizes and dissociates itself, producing 11 electrons (reaction 6). Then, the enargite dissolution follows an electrochemical process with the gaseous oxygen (oxide/reduction pair) as shown in reaction (5). (6)

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Cu3 AsS4 → 3Cu2+ + 4S° + As5+ + 11e

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Ásbjörnsson [21] conducted an electrochemical study of natural enargite in a solution of 0.1 M HCl, observing that enargite has a dissolution potential ranging from -0.64 to 0.4 Volt (NHE), where copper and arsenic extraction increased significantly at levels above 0.8 volt. With these high potentials, it would be difficult to achieve Cu3 AsS4 leaching without the addition of a strong oxidant; thus, the oxygen gas from persulfate could achieve leaching. Furthermore, the same author [21] showed that mineral passivation (and thus resistance to electrochemical decomposition) occurred at 0.6 volt caused by the deposition of sulfide (S°) on the enargite surface. The thermodynamic conditions in our study thus presuppose that As5+ oxidizes to form HAsO 4 2-. Then, the reaction (6) is given by: Cu3 AsS4 + 4H2 O → 3Cu2+ + HAsO 4 2- + 7H+ + 4S° + 11e

(7)

Previous research suggests that while cupric ions may precipitate in high basicity conditions, they are maintained in solution in the presence of ammonia ions [22-23]. To corroborate the above, a predominance diagram of the Cu-NH3 -H2 O system was constructed to maintain the condition of 343K, a concentration of 0.001 M of copper, and to

Journal Pre-proof vary concentration of ammonium hydroxide from 0.3 to 0.6 M. The thermodynamic data used to construct the diagram was referenced from the HSC Chemistry database [24]. Fig. 1-A shows ammonia compounds stabilize with increased concentrations of NH4 OH, shifting pH limits to more basic regions for the copper tetra-amine species (Cu(NH3 )4 +2 ) and to more acidic regions for the copper amine species (Cu(NH3 )+2 ). Similarly, interactions between Cu(NH3 )4 +2 and copper bi-amine (Cu(NH3 )2 2+) is displaced to more reducing potentials, from a value of 0.15 to a value of 0.07 volt, resulting in an increase of the Cu(NH3 )4 +2 region in all directions.

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Notably, copper concentration increases from 0.001 to 0.004 M, i.e., maximum enargite leaching, as shown in Fig. 1-B (temperature = 343K, NH4 OH concentration = 0.6 M). Additionally, and unlike the previous diagram (Fig. 1-A) with pH range between 3.5 and 4.5, the stable copper oxide (Cu2 O) region now occupies a pH range between 3 and 6, between 0.1 and 0.4 volt.

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According to the reactions shown (Fig. 1-A), Cu2 O only has chemical equilibrium with Cu(NH3 )2+ and Cu(NH3 )2 + species, and no reactions involving thermodynamic equilibrium with Cu(NH3 )2 2+, Cu(NH3 )3 2+, or Cu(NH3 )4 2+. The problem is apparent: due to increased copper concentrations, the Cu2 O stability field increases and moves to the right, forcing limited equilibria among phases. However, because Cu(NH3 )2 2+ does not react with Cu2 O, the Cu(NH3 )2 2+ equilibrium phase is completely destabilized, causing the Cu(NH3 )2+ species to be transformed directly to Cu(NH3 )3 2+ at a temperature of 343K. Here the stability region of the copper bi-amine is completely removed, as shown in Fig. 1-B.

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Moreover, at decreased temperature or increased copper concentration (i.e., greater than 0.005 M), the Cu2 O region continues to displace to the right, inhibiting ammonium activity from competing against copper activity in an oxidizing media and resulting in a destabilization of the Cu(NH3 )2+ and Cu(NH3 )3 2+ phases. Since conditions are not propitious to the formation of ammoniacal complexes at lower thermodynamic stabilities, a precipitate of CuO forms occurs in the presence of an oxidizing media (O2 stability field), as shown in Fig. 1-C. *****Figure 1*****

The formation of copper tetra-amine is thus given by reaction (8): Cu3 AsS4 + 4H2 O +12NH3 → 3Cu(NH3 )4 2+ + HAsO 4 2- + 7H+ + 4S° + 11e

(8)

And so the oxide/reduction pair and electron transfer from reactions (5) and (8) results in an overall reaction given by expression (9). In this expression the original reactants (sodium persulfate and ammonium hydroxide) are taken up again: Cu3 AsS4 + 11/2Na2 S2 O8 + 12NH4 OH → 3Cu(NH3 )4 2+ + HAsO 4 2- + 11Na+ + 11SO 4 2- + 18H+ + 9H2 O + 4S° 2. Experimental Methodology 2.1 Mineral enargite

(9)

Journal Pre-proof Enargite mineral was obtained by visual/manual sorting of enargite crystals from an inert rock. Crystals were then crushed, ground and classified into average particle sizes: 203, 80, 46, and 38 microns (µm). X-ray fluorescence (XRF) analysis of all particle sizes showed that the percentage (grade) of copper, arsenic, iron and sulfur was 33.50, 12.80, 2.07 and 23.75%, respectively. The remaining percentage was silica (SiO 2 ). The 46 µm particle size sample was analyzed by X-ray diffraction (XRD) to confirm species present. Fig. 2 shows strong enargite peaks, as well as those of pyrite and silica. The amount of arsenic in the ore leaves the calculated enargite grade at 67.2%; while pyrite in the ore (calculated from % Fe) would be 4.4% FeS2 . Samples were also analyzed by BSE (Fig. 3), showing isolated enargite, pyrite, and silica particles, without any other compound. Note that no particles are occluded, i.e. high leaching exposure.

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*****Figure 2*****

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*****Figure 3*****

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2.2 Agitation leaching Leaching solutions were prepared with different concentrations of sodium persulfate (Na2 S2 O8 ) and ammonium hydroxide (NH4 OH). Ammonium persulfate ((NH4 )2 S2 O8 ) and potassium persulfate (K2 S2 O8 ) were prepared for comparison. As needed, pH was adjusted with sulfuric acid (concentrate, 98% purity) or sodium hydroxide (solid, ground, 98.5% purity). All reagents used (persulfates and ammonia) were of high purity.

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Leaching experiments consisted of Na2 S2 O8 and NH4 OH leach solution (1L) poured into a 2 L capacity reactor, with heating blanket, condenser (to decrease the evaporation rate), mechanical agitator, thermocouple, and a tube to extract the liquid samples, following the experimental setup in [23]. A mineral enargite sample (1×10-3 kg) was introduced. For experiments on temperature, leach solution was first heated. Liquid samples were extracted at set times, and analyzed by Atomic Absorption Spectroscopy (AAS) for Cu, As, and Fe content. In some cases, solid residues samples (washed and dried) were analyzed with Xray diffraction (XRD) and backscattered electrons images (BSE). 3. Results and Discussion 3.1 Enargite leaching region The optimal enargite leaching region as a function of solution pH was evaluated. The working conditions were sodium persulfate and ammonium hydroxide concentrations at 0.15 and 0.6 M, respectively; temperature, 333K; average particle size, 46 µm; agitation, 420 rpm; and solid/liquid ratio, 1/1000 g/mL. The pH values evaluated were 3.0, 6.0, 9.0, 11.0, and 13.0. Fig. 4 shows copper and iron recovery as a function of solution pH. ***** Figure 4**** Acidic pH (3.0) had minimal copper recovery (10%). As pH increased (between 6.0 and 11.0), copper recovery increased dramatically to 68%, 75%, and 70%. These results suggest that copper recovery is optimal at a pH value of 9.0. Indeed, recovery drops dramatically to 14% at pH 13.0. These results agree with the stability diagrams (Fig. 1), which establish a pH range of 5.0 to 11.0 for stable ammoniacal complexes like copper tetra-amine.

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To corroborate the proposed mechanism (reaction 9), solid samples (obtained at pH=9.0) were analyzed. Results (Fig. 5-A) present only enargite, silica and pyrite peaks (original compounds of the mineral), and no other copper or iron compound after leaching. No crystalline elemental sulfur (S°) was found either. Given the absence of this well-known copper sulfide passivation mechanism, this would indicate that enargite continues a leaching mechanism different from that proposed by the reaction (9). Liu et al. [18] studied bornite leaching with persulfate ions in an ammoniacal media. Within their mechanisms, they found that the elemental sulfur generated in the reaction was oxidized by reaction (10): *****Figure 5*****

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S° + 3S2 O 8 2- + 8NH3 + 4H2 O → 7SO 4 2- + 8NH4 +

(10)

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The samples obtained in our experiments (at pH=9.0) were analyzed by SEM (Fig. 6), showing fine enargite particles, with clean surfaces, without amorphous or crystalline sulfur residues.

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*****Figure 6*****

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Considering XRD and SEM results and that the elemental sulfur oxidation mechanism does not disappear (possibly formed in the enargite at some leaching stage), the leaching mechanism of Cu3 AsS4 with sodium persulfate in ammoniacal medium can be given by reaction (11):

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Cu3 AsS4 + 35/2Na2 S2 O8 + 44NH4 OH → 3Cu(NH3 )4 2+ + HAsO 4 2- + 35Na+ + 39SO 4 2- + 7H+ + 32NH4 + + 24H2 O

(11)

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Additionally, Fig. 4 shows minimal iron recovery between 6.0 and 13.0 pH, indicating that pyrite was not leached under persulfate oxidant in an ammoniacal system (corroborated by XRD, Fig. 5-A); in short, leaching selects for Cu3 AsS4 versus FeS2 . This study significantly demonstrates S2 O8 2- oxidant may be used with minerals or concentrates high in copperarsenic in the presence of pyrite without generating solid, highly impure residues. Given the above, the following experiments were at pH 9.0 3.2 Agitation velocity Agitation rate of the leach solution was tested at varying speeds (0, 220, 420 and 620 rpm), under similar working conditions as above. Results indicate increased stirring speed increased copper extraction percentage (0 rpm=20.1% Cu; 220 rpm=27.5% Cu), with diminishing returns for higher rpm. This behavior is mainly due to the low influence of the mass transfer between the bulk solution and particle surface area. The rest the experiments were set to 420 rpm. 3.3 Concentration of persulfate ions Differing concentrations of S2 O 8 2- (0.04 to 0.3 M) were analyzed with sodium persulfate as reagent and similar working conditions as above. Results are presented in Fig. 7.

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*****Figure 7***** At 240 minutes, persulfate concentration increases to 0.30 M, and maximum copper recovery (80%) is obtained. According to reaction (11), the equilibrium persulfate concentration for optimal enargite leaching was 0.04 M; for the same time, this experimentally yielded 13.5% recovery. This indicates that excess persulfate ions should be used to achieve elevated enargite leaching values in an ammoniacal media. Next, iron recovery using the maximum concentration of persulfate (0.30 M) only yielded 1.7% Fe in solution, thus demonstrating the selectivity of S2 O8 2- towards enargite.

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3.4 Ammonium hydroxide concentration effect Results for NH4 OH concentrations (0.03 to 0.75 M) on enargite dissolution rate are shown in Fig. 8. At concentrations above 0.30 M, copper recovery increases (75%); at concentrations below 0.03 M, recoveries were very low (<8%).

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*****Figure 8*****

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Behavior between 0.03 and 0.015 M NH4 OH is likely due to Cu(NH3 )4 2+ complex instability inside the reactor – here, decreases in NH3 /NH4 + (given by reactions at 343K [25]) decrease ammonia activity in solution. To analyze this as a possibility, residues obtained under different conditions (0.15 M NH4 OH; time=240 minutes) were analyzed by XRD (Fig. 5-B). Results showed enargite, pyrite, quartz, and small peaks of copper oxides in the form of tenorite (CuO). The latter oxides were obtained through reagent copper ammoniacal complexes dissociating towards equilibrium, explaining low concentrations.

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A speciation diagram of the Cu-NH3 system (Fig. 9) shows that increasing ammonia concentration is associated with more stable copper species, until reaching Cu(NH3 )4 2+ predominance. Compound stability is most favored at ammonia concentration of 0.1 M (??og[????3 ]=−1). However, it has been reported that equilibrium ammonia in solution slowly volatilizes as temperature increases (ammonia dissociation constant = 1.77×10-5 at 298K), and so concentration in solution decreases with leaching time. *****Figure 9*****

Following the series shown in the diagram in Fig. 9, Cu-NH3 dissociates from Cu(NH3 )4 2+ to Cu+2 as the (in solution) NH3 concentration decreases. From there, in-solution copper ions under high alkalinity conditions precipitate as oxides. 3.5 Influence of temperature Enargite leaching rate was analyzed at temperatures between 278 and significant effects on Cu recovery. For a temperature value of 293K, 24% was achieved; at 343K, recovery reached 85% Cu; though at conditions, close to the freezing temperature of the solutions (273K), still non-negligible (17%). *****Figure 10*****

353K (Fig. 10), with a copper recovery of very low temperature copper recovery was

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At 353K, the percentage of copper recovery decreased to a value of 24% after 120 minutes. Here, ammonia molecules are volatilized and unable to keep copper in solution. Next, iron recoveries reached values close to 2.3%; even at a maximum operating temperature, very little FeS2 is leached. 3.6 Average particle size assessment The effects of average particle size (203, 80, 46, and 38 µm) on enargite leaching at a temperature of 333 K are presented in Fig. 11. *****Figure 11*****

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At 240 minutes, copper extractions were 27% and 45% for average sizes of 203 and 80 µm, respectively. Average particle size 35 µm yielded almost complete Cu3 AsS4 leaching (98.3%). Thus enargite leaching rate is no stranger to the phenomenon of increased surface area improving leaching rate through greater interaction and electron transfer.

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*****Figure 12*****

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3.7 Analysis of different salts Sodium persulfate, ammonium persulfate, and potassium persulfate were tested as reagents with base concentration of 0.3 M (Fig. 12). Ammonium and potassium persulfate reagents yielded lower copper recovery than did sodium persulfate, perhaps due to the persulfate ligand selecting for either SO 4 2- or HAsO 4 2-. Further studies should be carried out to evaluate persulfate reagents (ammonium and potassium) for the alternative use of these salts.

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3.8 Precipitation with sodium sulfide and sodium hydrosulfide Given the efficient leaching processes, the researchers proposed to remove copper and arsenic from the leach solutions to achieve sulfurized or sulfated copper/arsenic salts; and a clean leach solution, contain sodium persulfate and ammonia, to be returned to the enargite leaching process. Experiments used sodium sulfide (Na2 S, 98.8% pure) and sodium hydrosulfide (NaHS 42.0 %w/w), both at 292K, 300 rpm, and 60 minutes. Experimental results for copper and arsenic precipitation are summarized in Table 2. Efficiency of both reagents in the precipitation process is above 98%, and in situ observation shows solid precipitates. *****Table 2***** Though the small amount of solid compounds precipitated (<5 mg) could not be identified, previous studies[22] precipitating copper from ammoniacal solutions with NaHS identified copper sulfate pentahydrate (CuSO 4 ·5H2 O), according to reaction (12). That study also found NH3 in solution, which can be reused to react with copper (reaction 8) and to oxidize the elemental sulfur (reaction 10) generated in the leaching of enargite with persulfate. Cu(NH3 )4 2+ + S2- + 2O 2(g) → CuSO 4 + 4NH3

(12)

Journal Pre-proof 3.9 Enargite leaching kinetics Temperature effects (Fig. 10) were not preponderant, suggesting the process is limited by the reactant diffusing through a porous layer formed on the surface. Supposing this, the velocity equation model for diffusion through a porous layer of radius ro for constant reactant concentrations can be described by: 1 − 3(1 − 𝑋𝑒𝑛 )

2⁄ 3

+ 2(1 − 𝑋𝑒𝑛 ) = 𝑘𝑎𝑝𝑝 t

(13)

Where Xen is the fraction of reacted enargite, t is leaching time, and k app is the apparent kinetic constant, given by the following expression: 𝑝

Ea ⁄RT

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𝑘𝑎𝑝𝑝 = 𝑘𝑖𝑛𝑡

[S2 O8 2−] [NH4 OH] ℎ 𝑒 −

(14)

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where k int is the intrinsic linear kinetic constant; [S2 O 8 2-] and [NH4 OH] are the molar concentrations of persulfate ions and ammonium hydroxide, respectively; p and h are their respective orders of reaction; ro , initial particle radius; Ea, activation energy; R, gas constant; and T, temperature. 2

***** Figure 13*****

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Figure 13 shows a graph of 1 − 3(1 − 𝑋𝑒𝑛 ) ⁄3 + 2(1 − 𝑋𝑒𝑛 ) as a function of time for experimental data (Fig. 10), between 293 to 343K and enargite particle sizes at 46 μm (similar to experimental conditions for persulfate ion and ammonium hydroxide concentration). The figure shows good linear fit of the kinetic data with regression coefficients, R2 , from 0.95 to 0.99 for the whole temperature range, indicating the applicability of equation (13).

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Orders of reaction p and h were calculated from kinetic data as shown in Fig. 7 (S2 O8 2concentration) and Fig. 8 (NH4 OH concentration between 0.3 and 0.6 M), respectively. Fig. 14-A shows experimental data for various concentrations of S2 O8 2-; and, Fig. 14-B, NH4 OH. Both figures were constructed using equation (13). Regression coefficients for all straight lines were greater than 0.98. The values obtained from k app (Fig. 14-A and B) were used to draw a graph of ln k app vs ln [S2 O8 2-] and ln k app vs ln [NH4 OH], as shown in Fig. 15. *****Figure 14***** *****Figure 15***** Figure 15 shows both lines with correlation coefficients above 0.96. Slopes of each line fit experimental data, demonstrating that order of reaction with respect to persulfate ion and ammonium hydroxide concentrations were 2.0 and 1.5, respectively. Next, reactant diffusion kinetics through porous layer, k app , generally vary linearly with the inverse of the square of the initial particle radius, as indicated in equation (14).

Journal Pre-proof Experimental results on particle size (Fig. 11) were adjusted according to equation (13), as shown in Figure 16. These k app values were then plotted as a function of the inverse square of the initial particle radii (Fig. 17). The good linear dependence of the data (R2 >0.99) supports the kinetic model used. *****Figure 16***** *****Figure 17*****

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Activation energy, a typical value for a diffusion control of the reactant through the porous layer formed during the reaction, was calculated using reaction orders with respect to the concentration of persulfate and ammonium hydroxide ions and average particle size of 46 m (eq. 14) and k app values (Fig. 10); and an Arrhenius graph constructed (Fig. 18), with good linear fit (R2 >0.96). The calculated activation energy was 45.0 kJ/mol for temperature 293 to 343K.

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*****Figure 18*****

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Other works have used the chemical surface reaction model to represent enargite leaching in different acid media [6, 26-27]; applying this model gives activation energies within the range of 55 to 69 kJ/mol (Table 3). In our work, the calculated activation energy was much lower than the empirical results. This important result indicates that basic systems require less energy to initiate the reaction between oxygen molecules and the enargite surface. It should be noted that the works cited above had problems with mineral passivation, while the present research saw no resistance to electron transfer, given the elimination of S° from persulfate and ammonia.

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*****Table 3*****

Thus, modifying the apparent kinetic constant from (eq. 14), we have [S2 O8 2−]

2.0

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𝑘𝑎𝑝𝑝 = 1.2 × 109

[NH4 OH] 1.5 𝑒 −

45.0 ⁄ RT

r2o

(15)

where R is equal to the molar constant of the gases, given as 8.314 J/mol/K; ro is in µm; [S2 O8 2-] and [NH4 OH] are given in molars; t is in minutes; T, in K; and k int = 1.2×109 µm2 /M3.5 . 4. Conclusions Enargite (Cu3 AsS4 ) in the presence of pyrite was leached in ammoniacal sodium persulfate as oxidant. Extraction of Cu reached 98% without FeS2 , selective leaching. Preponderant enargite leaching variables were solution persulfate and ammonium hydroxide concentrations, temperature, and particle

medium with demonstrating pH, sodium size.

The expected reaction mechanisms of enargite suggest that elemental sulfur (S°) (otherwise causing mineral passivation) oxidizes in sodium persulfate and ammonia to be transformed into sulphate.

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The heterogeneous kinetic model that best represented enargite leaching was reactant diffusion through a porous layer. Orders of reaction 2.0 and 1.5 were obtained with respect to the concentration of persulfate and ammonium hydroxide, respectively. The calculated activation energy was 45.0 kJ/mol. The metals present (Cu and As) were precipitated by means of sodium sulfide and sodium hydrosulfide, yielding ~99% precipitation, and thus generating a rich solution of ammonia to be returned to enargite leaching.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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[7] R. Padilla, O. Jerez and M.C. Ruiz, Kinetics of the pressure leaching of enargite in FeSO 4 -H2 SO4 -O2 media, Hydrometallurgy 158 (2015), pp. 49-55

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[8] E. Domic, 2001, Hidrometalurgia, fundamentos, procesos y aplicaciones (in Spanish), Editorial Null, Santiago

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[9] I. Mihajlovic, N. Strbac, Z. Zivkovic, R. Kovacevic and M. Stehernik, A potential method for arsenic removal from copper concentrates, Minerals Engineering 20 (2007), pp. 26-33 [10] J. Viñals, A. Roca, M.C. Hernández and O. Benavente, Topochemical trasnformation of enargite into copper oxide by hypochlorite leaching, Hydrometallurgy 68 (2003), pp. 183-193 [11] L. Curreli, M. Ghiani, M. Surracco and G. Orrừ, Beneficiation of gold bearing enargite ore by flotation and As leaching with Na-hypochlorite, Minerals Engineering 18 (2005), pp. 849-854 [12] P. Baláž, M. Achimovičová, Z. Bastl, T. Ohtani and M. Sánchez, Influence of mechanical activation on the alkaline leaching of enargite concéntrate, Hydrometallurgy 54 (2000), pp. 205-216 [13] C.G. Anderson and L.G. Twidwell, Hydrometallurgical processing of gold-bearing copper enargite concentrates, Canadian Metallurgical Quarterly 47 (2008), 337-346

Journal Pre-proof [14] L. Curreli, C. Garbarino, M. Ghiani and G. Orrừ, Arsenic leaching from a gold bearing enargite flotation concéntrate, Hydrometallurgy 96 (2009), pp. 258-263 [15] F. Parada, M.I. Jeffrey and E. Asselin, Leaching kinetics of enargite in alkaline sodium sulphide solutions, Hydrometallurgy 146 (2014), pp. 48-58 [16] W. Tongamp, Y. Takasaki and A. Shibayama, Arsenic removal from copper ores and concentrates through alkaline leaching in NaHS media, Hydrometallurgy 98 (2009), pp. 213-218

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[17] M.C. Ruiz, L. Grandon and R. Padilla, Selective arsenic removal from enargite by alkaline digestión and wáter leaching, Hydrometallurgy 150 (2014), pp. 20-26

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[18] Z. Liu, Z. Yin, Y. Chen and L. Xiong, Leaching of calcareous bornite ore in amoniacal solution containing ammonium persulfate, Metallurgical and Materials Transactions B, 45B (2014), pp. 2027-2032

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[19] Z. Liu, Z. Yin, H. Hu and Q. Chen, Leaching kinetics of low-grade copper ore containing calcium- magnesium carbonate in ammonia-ammonium sulfate solution with persulfate, Transactions of Nonferrous Metals Society of China 22 (2012), pp. 2822-2830

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[20] O. Furman, A. Teel and R. Watts, Mechanism of base activation of persulfate, Environmental Science & Technology 44 (2010), pp. 6423-6428

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[21] J. Ásbjörnsson, G.H. Kelsall, R. Pattrick, D.J. Vaughan, P.L. Wincott and G.A. Hope, Electrochemical and Surface analytical studies of enargite in acid solution, Journal of the Electrochemical Society 151 (2004), pp. 250-256

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[22] A. Aracena, F. Fernández, O. Jerez and A. Jaques, Converter slag leaching in ammonia medium/column system with subsequent crystallisation with NaSH, Hydrometallurgy 188 (2019), pp. 31-37 [23] A. Aracena, F. Pérez and D. Carvajal, Leaching of cuprite through NH4 OH in basic systems, Transactions of Nonferrous Metals Society of China 28 (2018), pp. 2545-2552 [24] A. Roine, 1999, HSC Chemistry 6.0, OutoKumpu Research Py, Pori, Finlandia [25] K. Emerson, R. Russo, R. Lund and R. Thurston, Aqueous ammonia equilibrium calculations: Effect of pH and temperature, Journal of the Fisheries Research Board of Canada 32 (1975), pp. 2379-2383 [26] R. Padilla, D. Girón and M.C. Ruiz, Leaching of enargite in H2 SO4 -NaCl-O2 media, Hydrometallurgy 80 (2005), pp. 272-279 [27] J.E. Dutrizac and R.J.C. MacDonald, The kinetics of dissolution of enargite in acidified ferric sulphate solutions, Canadian Metallurgical Quarterly 11 (1972), pp. 469476

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1.0

Cu(NH3 )3 2+

Cu(NH3 ) 2+

0.8

Cu(NH3 )2

Eh, [V]

0.6

Cu

(A)

2+

2+

CuO

Cu(NH3 )4 2+

0.4 0.2

Cu

0.0

Cu 2 O Cu(NH3 )2 +

0.6 M 0.3 M

Cu 2 O

of

-0.2 -0.4

Cu(NH3 )3 2+

0.6

-p

Cu(NH3 ) 2+

0.8

Cu 2+

Cu(NH3 )4 2+

0.2

Cu 2 O

0.0

Cu -0.4 1.0

ur

0.8

Cu 2+

Jo

0.6

Eh, volt

Cu(NH3 )2 + Cu 2 O

na

-0.2

(B)

CuO

re

0.4

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Eh, volt

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1.0

(C) CuO

Cu(NH3 )4 2+ CuO

0.4

Cu 2 O

0.2

Cu

0.0

Cu(NH3 )2 + 25°C

-0.2

Cu 2 O

-0.4 0

2

4

6

8

10

12

14

pH

Figure 1. Stability diagram of the Cu-NH3 -H2 O system. (A) Temperature=343K, [Cu]=0.001M; [NH4 OH]=0.3 and 0.6 M (B) Temperature =343K, [NH4 OH]=0.6M, [Cu] =0.001 and 0.004M (C) Temperature =343K, [NH4 OH]=0.6M, [Cu] =0.005M

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20000 Cu3AsS4

Intensity, cps

15000

SiO2 FeS2

10000

of

5000

10

20

30

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0 40

50

60

-p

2 Theta, degrees

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Figure 2. XRD analysis of the original enargite ore sample. Particle size of 46 µm

70

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En

Qz Qz

En

En

En

En

Py

En

Py Qz

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Py

Qz

Qz

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200 µm

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Qz

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Figure 3. BSE analysis of 200 and 50 µm mesh size. En=Enargite, Py=Pyrite, Qz=Quartz

50 µm

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80 Cu Fe

Metal recovery, %

60

40

0 4

6

8

-p

2

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20

10

12

14

pH

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Figure 4. Enargite leaching region for sodium persulfate in ammoniacal media at different pH values of the solution.

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18000 Cu3AsS4

16000

(A)

SiO2

14000

FeS2

Intensity, cps

12000 10000 8000 6000

of

4000

ro

2000

17500

-p

0 20000 CuO

re

12500

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10000 7500

na

5000 2500 0 10

Jo

0

ur

Intensity, cps]

15000

(B)

20

30

40

50

60

70

2 Theta, degrees

Figure 5. XRD analysis of enargite sample after leaching (240 minutes) (A) pH of 9.0 (B) NH4 OH concentration of 0.015 M.

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100 µm

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Figure 6. SEM images of leached enargite samples at 50 and 100 µm magnification.

50 µm

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100

80

Copper recovery, %

0.15 M 0.20 M 0.30 M 0.30 M Fe

0.04 M 0.06 M 0.075 M 0.10 M

60

40

0 0

50

100

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20

150

200

250

-p

Leaching time, min

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Figure 7. Effect of persulfate ion concentration on enargite leaching rate. Particle size of 46 µm

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80

0.03 M 0.15 M 0.30 M 0.45 M 0.60 M 0.75 M

60 50 40 30 20

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Copper recovery, %

70

0 50

100

150

-p

0

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10

200

250

Leaching time, min

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Figure 8. Effect of ammonium hydroxide concentration on enargite dissolution rate. Particle size of 46 µm

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Cu(NH3)2+2 Cu(NH3)3+2 Cu(NH3)4+2

4

3

Cu+2 Cu(NH3)+2

2

of

1

0 -5

-4

-3

-2

-1

0

-p

-6

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Concentration of copper, mM

5

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Log [NH3]

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Figure 9. Formation of copper and ammonia complexes as a function of NH3 concentration

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100

278 K 293 K 308 K 323 K 333 K 343 K 353 K 343 K-Fe

Copper recovery, %

80

60

40

0 0

50

100

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of

20

150

200

250

-p

Leaching time, min

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Figure 10. Influence of temperature on enargite leaching rate. Particle size of 46 µm

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100

203 m 80 m 46 m 38 m

60

40

of

Copper recovery, %

80

0 50

100

150

-p

0

ro

20

Leaching time, min

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Figure 11. Effects of average particle size on copper recovery

200

250

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50 Na2S2O8 (NH4)2S2O8 K2S2O8

30

20

of

Copper recovery, %

40

0 50

100

150

-p

0

ro

10

200

250

Leaching time, min

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Figure 12. Analysis of persulfates (ammonium, potassium, and sodium) on copper recovery.

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0.12

293 K 308 K 323 K 333 K 343 K

0.10 0.08 0.06

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0.04 0.02 0.00 50

100

150

-p

0

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1 - 3 (1- Xen)

2/3

+ 2(1-Xen)

0.14

200

250

Leaching time, min

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Figure 13. Leaching kinetics of enargite mineral samples leached with sodium persulfate in ammonium hydroxide medium using experimental data from Figure 10.

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0.14 0.04 M 0.06 M 0.075 M 0.10 M 0.15 M 0.20 M 0.30 M

0.10 0.08 0.06

(A)

0.04

of

1-3(1-Xen)

2/3

+2(1-Xen)

0.12

ro

0.02

0.04 M 0.06 M 0.075 M

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(B)

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0.015

na

0.010

0.005

0.000

Jo

0

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1-3(1-Xen)

2/3

+2(1-Xen)

0.020

-p

0.00

50

100

150

200

250

Leaching time, min

Figure 14. Leaching kinetics of enargite samples for: (A) Several concentrations of S 2 O8 2(data obtained from Fig. 7) (B) Several concentrations of NH4 OH (data obtained from Fig. 8)

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-7.0 -7.5

ln kapp

-8.0 -8.5 -9.0

Data for S2O82-10.0

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-9.5

-10.5 -3.0

-2.0

-1.5

-p

-2.5

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Data for NH4OH

-1.0

-0.5

ln []

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Figure 15. Reaction orders with respect to the concentration of persulfate ions and ammonium hydroxide. α: S2 O8 2- or NH4 OH, all in Molar

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0.20

0.16

203 m 80 m 46 m 38 m

0.14 0.12 0.10 0.08 0.06

of

1-3(1-Xen)

2/3

+2(1-Xen)

0.18

0.04

0.00 50

100

150

200

250

-p

0

ro

0.02

re

Leaching time, min

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Figure 16. Leaching kinetics of enargite ore samples for different particle sizes. Experimental data obtained from Fig. 11.

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0.0008

kapp

0.0006

0.0004

0.0010

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0.0005

0.0015

-p

0.0000 0.0000

of

0.0002

0.0020

0.0025

0.0030

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1/r2, 1/m2 Figure 17. Dependence of the velocity constant on the inverse to the square of the initial particle size for the mineral enargite leaching.

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-7

ln kint

-8

-9

-10

-12 2.8

3.2

3.4

3.6

-p

3.0

ro

of

-11

re

3 10 /T, 1/K

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Figure 18. Arrhenius graph for calculation of activation energy of enargites leaching with persulfate in ammoniacal medium.

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Table 1. Bibliographic review of enargite leaching processes under alkaline conditions found since 2000.

Sodium hypochlorite – Sodium hydroxide

M

M-C

0.3 M NaClO 5.0 g/L NaOH 0.07 – 0.47 M NaClO 0.003 – 0.03 M NaOH 0.05 – 0.28 M NaClO 0.0003 – 0.03 M NaOH

NR – 80%

No

[9]

NR – 100%

No

[10]

No

[11]

Yes

[12]

NR – 93.5%

Yes

[13]

NR – 70%

Yes

[14]

NR – 98%

Yes

[15]

NR – 99%

Yes

[16]

NR – 70%

Yes

[17]

NR – 70%

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100 – 1000 g/L Na2 S 100 – 1000 g/L NaOH

2.66 – 5.31 M Na2 S 2 – 4 M NaOH a M: Mineral, C: Concentrated, NR: Not reported Alkaline digestion

90 – 65%

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100 g/L Na2 S 10 g/L NaOH 0.05 – 0.42 M Na2 S 1.25 – 3.75 M NaOH 0.5 – 1.0 M Na2 S 1.5 – 3.5 M NaOH

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M-C

M Sodium hydrosulfide (NaSH)– Sodium hydroxide

Reference

100 g/L Na2 S 50 g/L NaOH

C

C Sodium sulfide– Sodium hydroxide

Selective separation

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Machanical pretreatment Sodium sulfide– Sodium hydroxide

Recovery Cu – As (%)

of

M

Reagents

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Materiala

-p

Leaching media

M

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Table 2. Working conditions for copper and arsenic precipitation using Na 2 S and NaHS [As], ppm

[Cu], ppm

Na2 S, gr

N°1 N°2

98.4 98.4

228 228

13.6 -----

NaHS volumen, mL ----32.5

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Experiment

% Precipitation Copper Arsenic 98.3 99.5 98.6 99.7

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Table 3. Different calculated activation energies for different leaching media Leaching media

Equipment used

Reference

H2 SO4 -NaCl-O2(g) H2 SO4 -O2(g) FeSO 4 -- H2 SO4

Atmospheric pressure Autoclave Atmospheric pressure

[26] [6] [27]

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Activation energies, kJ/mol 65.0 69.0 55.6

Journal Pre-proof Highlights 

It was possible to selectively leach enargite in an ammoniacal medium with sodium persulfate



The presence of persulfate and ammonium help to eliminate the passivation of the compound. Copper recoveries reached very high values (about 98%) and impurities were minimal (below 2%)



The copper and arsenic in pregnant solution in contact with Na2 S and Nash ) could be precipitated with sodium sulfide and sodium hydrosulfide at 99% yields



Enargite leaching can be represented under a heterogeneous kinetic model of reactant diffusion through a porous layer formed at reaction time

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